1

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

Preliminary Pollinator Assessment to Support the 

Registration Review of Imidacloprid

January 4th, 2016

PC Code: 129099. IUPAC Name: N-{1-[(6-Chloro-3-pyridyl)methyl]-4,5-dihydroimidazol-2-yl}nitramide

Prepared by:

US EPA -

Justin Housenger, M.S., Biologist

Keith G. Sappington, M.S, Senior Science Advisor

Mohammed A. Ruhman, Ph.D., Senior Agronomist

California Department of Pesticide Regulation:

Richard Bireley, Sr. Environmental Scientist (Specialist)

John Troiano, Ph.D., Research Scientist III

Denise Alder, Sr. Environmental Scientist (Specialist)

Approved by

Mah T. Shamim, Ph.D. Branch Chief 

United States Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch V

1200 Pennsylvania Ave.

Mail Code 7507P

Washington, D.C. 20460
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Contents

Contributors, Reviewers, and Acknowledgements.....................................................................................12

1. Executive Summary.............................................................................................................................13

1.1. Background and Scope................................................................................................................13

1.2. Use Characterization...................................................................................................................13

1.3. Environmental Fate and Transport.............................................................................................13

1.4. Exposure Assessment..................................................................................................................14

1.5. Effects Assessment......................................................................................................................14

1.6. Pollen Route of Exposure............................................................................................................16

1.7. Non-Apis Bee Characterization...................................................................................................16

1.8. Additional Lines of Evidence .......................................................................................................17

1.9. Risk Conclusions..........................................................................................................................17

2. Problem Formulation..........................................................................................................................24

2.1. Registration Review Background ................................................................................................24

2.2. Nature and Scope of Assessment ...............................................................................................24

2.3. Pesticide Type, Class, and Mode of Action .................................................................................25

2.4. Overview of Imidacloprid Uses...................................................................................................26

2.5. Overview of Physicochemical, Fate, and Transport Properties..................................................26

2.6. Stressors of Toxicological Concern .............................................................................................26

2.7. Protection Goals and Assessment Endpoints .............................................................................27

2.8. Conceptual Models and Risk Hypotheses...................................................................................28

2.8.1. Foliar Spray................................................................................................................................28

2.8.2. Soil Application .........................................................................................................................30

2.8.3. Seed Treatment.........................................................................................................................31

2.9. Analysis Plan................................................................................................................................32

2.9.1. Risk Assessment Methodology .................................................................................................32

2.10. Measures of Exposure.............................................................................................................37

2.11. Measures of Effects.................................................................................................................37

3. Use Characterization...........................................................................................................................39

3.1. Agricultural Uses.........................................................................................................................39

3.1.1. Foliar Applications.....................................................................................................................40

3.1.2. Soil Applications........................................................................................................................42

3.1.3. Seed Treatments.......................................................................................................................43
3

3.1.4. Multiple Application Types (e.g. combinations of seed, soil, and/or foliar).............................45

4. Exposure Assessment..........................................................................................................................46

4.1. Physical/chemical and fate and transport properties ................................................................47

4.1.1. Physical/Chemical properties ...................................................................................................48

4.1.2. Environmental Fate and Transport Properties .........................................................................48

4.2. Imidacloprid Plant Up-take .........................................................................................................55

4.2.1. Imidacloprid applied to soil including seed treatment.............................................................55

4.2.2. Imidacloprid applied to foliage and fruits.................................................................................56

4.2.3. Imidacloprid: soil versus foliage applied...................................................................................57

4.3. Plant Metabolism of Imidacloprid ..............................................................................................58

4.3.1. Imidacloprid metabolism in various plants.........................................................................59

4.3.2. Imidacloprid metabolism profile in plants..........................................................................60

4.3.3. Imidacloprid metabolism profile in plants..........................................................................62

4.4. Potential for Exposure to Bees ...................................................................................................63

4.5. Screening-level Exposure Estimation..........................................................................................68

4.6. Experimental Residue Studies.....................................................................................................71

4.6.1. Rationale for Residue-based EEC Selection for Refined Tier I ............................................71

4.6.2. Rationale for Comparing Residue Data With Tier II Endpoints...........................................73

4.6.3. Foliar Application Residue Studies – Registrant Submitted................................................73

4.6.4. Soil Application Residue Studies – Registrant Submitted...................................................77

4.6.5. Soil Application Residue Studies – Open Literature............................................................83

4.6.6. Seed Treatment Application Residue Studies – Registrant Submitted...............................86

4.6.7. Seed Treatment Application Residue Studies – Open Literature .......................................91

4.6.8. Combined Application Method Residue Studies ................................................................93

4.7. Carry-over of Imidacloprid Residues in Soil ................................................................................97

4.8. Observational Residue Monitoring Studies................................................................................98

4.8.1. Agricultural crop studies.....................................................................................................99

4.8.2. Hive monitoring studies......................................................................................................99

5. Effects Assessment............................................................................................................................101

5.1. Tier I ..........................................................................................................................................101

5.1.1. Adult Acute Contact Toxicity.............................................................................................103

5.1.2. Adult Acute Oral Exposure................................................................................................110

5.1.3. Adult Chronic Oral Toxicity (Apis and non-Apis)...............................................................115
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5.1.4. Larval Acute Oral Toxicity..................................................................................................117

5.1.5. Larval Chronic Oral Toxicity...............................................................................................118

5.1.6. Acute and Chronic Toxicity of the Degradation Products of Imidacloprid .......................120

5.2. Tier II .........................................................................................................................................123

5.2.1. Registrant-Submitted........................................................................................................123

5.2.2. Open Literature Studies....................................................................................................127

5.3. Tier III ........................................................................................................................................154

5.3.1. Registrant Submitted ........................................................................................................154

5.3.2. Open Literature.................................................................................................................154

5.4. Reported Pollinator Incident Information ................................................................................160

6. Risk Characterization ........................................................................................................................168

6.1. Risk Estimation..........................................................................................................................168

6.1.1. Tier I - Screening-level RQs (On-field Contact – Foliar Uses Only)....................................174

6.1.2. Tier I - Screening-level RQs (On-field oral)........................................................................175

6.1.3. Screening Level RQs (Off-Field).........................................................................................177

6.1.4. Refined RQs (On-field Oral)...............................................................................................183

6.1.5. Refined RQs (Off-field Oral) ..............................................................................................204

6.1.6. Uncertainties at the Tier I Level........................................................................................205

6.2. Risk Description.........................................................................................................................206

6.2.1. Characterization of Tier I Risks and Tier II Analysis...........................................................207

6.2.2. Tier III analysis...................................................................................................................271

6.2.3. Examination of the pollen route of exposure...................................................................272

6.2.4. Risk Characterization of Non-Apis Bees............................................................................277

6.2.5. Additional Lines of Evidence .............................................................................................283

6.2.6. Higher Tier and other General Uncertainties....................................................................284

7. Conclusions.......................................................................................................................................287

8. References ........................................................................................................................................293

Table of Tables

Table 1-1. Summary of the toxicity endpoints to be used in risk estimation for individual bees. ............15

Table 1-2. Summary of risk findings for honey bees (Apis mellifera) for the registered use patterns of 

imidacloprid ................................................................................................................................................20

Table 2-1. Protection goals and examples of associated assessment and measurement (population and 

individual) endpoints for bees. ...................................................................................................................28
5

Table 3-1. Summary of labeled use information for foliar applications of imidacloprid...........................40

Table 3-2. Summary of labeled use information for soil applications of imidacloprid...............................42

Table 3-3. Summary of labeled use information for seed treatment applications of imidacloprid...........44

Table 4-1. Chemical profile of imidacloprid................................................................................................48

Table 4-2. Fate and transport properties for imidacloprid.........................................................................48

Table 4-3. Imidacloprid root up-take/distribution and resultant concentrations in cotton, potatoes, corn 

and eggplant (%= up-take in % of the applied radioactivity and numbers in brackets are resultant 

concentrations in mg/kg)............................................................................................................................55

Table 4-4. Imidacloprid up-take/distribution and resultant concentrations in various parts of the potato 

plants and only in the fruits of apples and tomatoes (%= up-take in % of the applied radioactivity and 

numbers in brackets are resultant concentrations in mg/kg). ...................................................................57

Table 4-5. Observed estimated concentrations of the stressor in parts per million= ppm) (parent 

imidacloprid + IMI-olefin and IMI-5-OH compounds) in varied crops, plant parts and application 

procedures based on radioactivity data .....................................................................................................62

Table 4-6. Attractiveness of crops for the registered foliar uses of imidacloprid to bees (as indicated by 

USDA, 2014). Note, the potential for off-field exposure is indicated from all foliar uses.........................64

Table 4-7. Attractiveness of crops for the registered soil uses of imidacloprid to bees (as indicated by 

USDA, 2014)................................................................................................................................................65

Table 4-8. Attractiveness of crops for the registered seed treatment uses of imidacloprid to bees (as 

indicated by USDA, 2014) ...........................................................................................................................66

Table 4-9. Summary of contact and dietary exposure estimates for foliar applications, soil treatment, 

seed treatments, and tree trunk injections of pesticides for Tier I risk assessments. ...............................69

Table 4-10. Summary of estimated food consumption rates of bees. ......................................................70

Table 4-11. Summary of available registrant submitted foliar application residue studies......................75

Table 4-12. Summary of available registrant submitted soil application residue studies.........................79

Table 4-13. Summary of the soil application residue studies evaluated from the open literature..........84

Table 4-14. Summary of the registrant submitted seed treatment application residue studies..............87

Table 4-15. Summary of residue data from imidacloprid-treated seed studies evaluated from the open 

literature. ....................................................................................................................................................92

Table 4-16. Summary of the registrant-submitted combined application method residue studies (soil 

application + foliar spray) ...........................................................................................................................95

Table 4-17. Summary of the registrant submitted combined application method residue studies (seed 

treatment + foliar spray).............................................................................................................................96

Table 4-18. Distribution of samples from corn fields according to their concentration of imidacloprid 

(Bonmatin, 2005) ........................................................................................................................................99

Table 4-19. Distribution of residues from corn and sunflower pollen according to their concentration of 

imidacloprid (Bonmatin, 2007)...................................................................................................................99

Table 5-1. Summary of endpoints to be used in screening-level and refined Tier I risk estimation .....103

Table 5-2. Summary of registrant submitted adult acute contact toxicity studies (all studies tested Apis 

mellifera)...................................................................................................................................................104

Table 5-3. Summary of adult acute contact toxicity studies to Apis bees evaluated from the open 

literature ...................................................................................................................................................105

Table 5-4. Summary of registrant submitted adult acute contact toxicity studies for non-Apis bees 

(Note: both studies concern Bombus terrestris).......................................................................................106
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Table 5-5. Summary of adult acute contact toxicity studies to non-Apis bees evaluated from the open 

literature ...................................................................................................................................................107

Table 5-6. Summary of registrant submitted adult acute oral toxicity studies (Note: All studies tested 

Apis mellifera). ..........................................................................................................................................110

Table 5-7. Summary of adult acute oral toxicity studies for Apis bees evaluated from the open literature

..................................................................................................................................................................111

Table 5-8. Summary of registrant-submitted and evaluated open literature studies assessing the chronic 

oral toxicity of imidacloprid to Apis and non-Apis adults.........................................................................115

Table 5-9. Summary of results from Abbott et al., 2008 examining the effects of imidacloprid TGAI on 

larval development of blue orchard bees (Osmia lignaria) .1 (Note: Study classified as qualitative).......119

Table 5-10. Summary of acute oral toxicity studies testing the degradates of imidacloprid in the open 

literature ...................................................................................................................................................120

Table 5-11. Summary of chronic adult oral toxicity studies with urea metabolite and 6-CNA (all studies 

conducted with Apis mellifera).................................................................................................................122

Table 5-12. Summary of semi-field (feeding) studies available from the open literature (Apis)

1 ...........130

Table 5-13. Summary of semi-field (Tunnel) studies available from the open literature (Bombus) .......141

Table 5-14. Summary of semi-field (feeding) studies available from the open literature (Bombus)......143

Table 5-15. Summary of Tier III (full field) studies available from the open literature for Apis bees .....156

Table 5-16. Summary of Tier III (full field) studies available from the open literature for Bombus bees.

..................................................................................................................................................................159

Table 5-17. Summary of reported pollinator incident reports that are either associated with 

confirmatory residue analysis or registrant submitted ............................................................................163

Table 6-1. Attractiveness of crops to bees for the registered foliar uses of imidacloprid.......................170

Table 6-2. Attractiveness of crops to bees for the registered soil uses of imidacloprid. ........................171

Table 6-3. Attractiveness of crops to bees for the registered seed treatment uses of imidacloprid......172

Table 6-4. Summary of Tier I screening-level RQs for contact exposure resulting from foliar uses of 

imidacloprid (screening-level contact on-field)........................................................................................174

Table 6-5. Summary of Tier I screening-level RQs for oral exposure resulting from foliar uses of 

imidacloprid (based on model-generated exposure values on-field).4 ....................................................176

Table 6-6. Summary of Tier I screening-level RQs for oral exposure resulting from soil uses of 

imidacloprid (based on model-generated exposure values on-field).4 ....................................................176

Table 6-7. Summary of labeled use information for seed treatment applications of imidacloprid 

(screening-level oral on-field) 4 .................................................................................................................177

Table 6-8. Imidacloprid Use Patterns for Crops with or without Specific Application Restrictions ........178

Table 6-9. Distance from the edge of the field associated with LOC exceedance, for citrus and pome 

fruits, calculated using AgDRIFT v.1.1.1, the Tier I Orchard/Airblast module, and app rate of 0.25 lbs. 

a.i./A..........................................................................................................................................................179

Table 6-10. Citrus and Pome Fruits: Tier II aerial applications, boom height 10 ft, wind speed 15 mph 

(label required), non-volatile rate 0.25 lbs./A, spray volume 5 gal/A ......................................................180

Table 6-11. Citrus and Pome Fruits: Tier II aerial applications, boom height 10 ft, wind speed 10 mph 

(label required), non-volatile rate 0.25 lbs./A, spray volume 5 gal/A ......................................................180

Table 6-12. Globe artichoke (only ground apps allowed): Tier I ground applications, high boom height 

(50 inches), application rate 0.126 lbs. a.i./A, 90th percentile results.....................................................181
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Table 6-13. Globe artichoke (only ground apps allowed): Tier I ground applications, low boom height 

(20 inches), application rate 0.126 lbs. a.i./A, 90th percentile results.....................................................181

Table 6-14. Stone fruit, Tree nuts: Tier II aerial applications, boom height 10 ft, wind speed 15 mph 

(label required), non-volatile rate 0.10 lbs./A, spray volume 25 gal/A (label required for these crops).181

Table 6-15. Stone fruit, Tree nuts: Tier II aerial applications, boom height 10 ft, wind speed 10 mph, 

non-volatile rate 0.10 lbs./A, spray volume 25 gal/A (label required for these crops). ...........................182

Table 6-16. Tuberous & Corm Vegetables and Certain Other Crops: Tier II aerial applications, boom 

height 10 ft, wind speed 15 mph (label required), non-volatile rate 0.04 lbs./A, spray volume 5 gal/A.182

Table 6-17. Tuberous & Corm Vegetables and Certain Other Crops: Tier II aerial applications, boom 

height 10 ft, wind speed 10 mph, non-volatile rate 0.04 lbs./A, spray volume 5 gal/A...........................183

Table 6-18. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for foliar applications on citrus fruits (oranges) based on measured residue data..................................184

Table 6-19. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from foliar-applications to oranges.1,4 .................................185

Table 6-20. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for foliar applications on cherries based on measured residue data .......................................................186

Table 6-21. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from foliar-applications to cherries1,4 ..................................186

Table 6-22. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for foliar applications on cotton based on measured residue data..........................................................187

Table 6-23. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in nectar from foliar-applied cotton1,4 ..................................................................188

Table 6-24. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for soil applications on tomatoes based on measured residue data........................................................189

Table 6-25. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen from soil-applications to tomatoes1,4 .....................................................189

Table 6-26. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for soil applications on melons based on measured residue data. ..........................................................190

Table 6-27. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from soil-applications to melons.1,4 .....................................191

Table 6-28. Summary of the refined acute and chronic estimated environmental concentrations (EECs) 

for soil applications to citrus based on measured residue data...............................................................192

Table 6-29. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in nectar from soil applications to citrus1,4............................................................193

Table 6-30. Summary of the refined acute and chronic EECs in pollen and nectar following soil 

applications to blueberries based on measured residue data. ................................................................194

Table 6-31. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured 

total imidacloprid residues in pollen and nectar following soil applications to blueberries1,4 ................194

Table 6-32. Summary of the refined acute and chronic EECs for soil applications to strawberries based 

on measured residue data. .......................................................................................................................195

Table 6-33. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured 

imidacloprid residues in pollen from soil applications to strawberries1,4 ................................................196

Table 6-34. Summary of the refined acute and chronic EECs for soil applications on cotton based on 

measured residue data. ............................................................................................................................197
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Table 6-35. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured 

total imidacloprid residues in pollen and nectar from soil-applications to cotton.1,4 ..............................197

Table 6-36. Summary of the refined acute and chronic EECs for seed treatment applications on corn 

based on measured residue data..............................................................................................................198

Table 6-37. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from seed-treated corn1,4.....................................................199

Table 6-38. Summary of the refined acute and chronic EECs for combined (soil + foliar) applications to 

tomato based on measured residue data.................................................................................................200

Table 6-39. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured 

imidacloprid residues in pollen from combined soil + foliar-applications to tomatoes.1,4.......................201

Table 6-40. Summary of the refined acute and chronic EECs for combined (soil + foliar) applications to 

cotton based on measured residue data..................................................................................................202

Table 6-41. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from soil + foliar-applied cotton1,4 .......................................202

Table 6-42. Summary of the refined acute and chronic EECs for combined (seed + foliar) applications on 

cotton based on residue data ...................................................................................................................203

Table 6-43. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured 

imidacloprid residues in pollen and nectar from combined seed + foliar applications to cotton.1,4 .......203

Table 6-44. SLUA data for imidacloprid and citrus fruits (2004-2013) ...................................................210

Table 6-45. SLUA data for imidacloprid and stone fruits (2004-2013) ...................................................214

Table 6-46. SLUA data for imidacloprid and oilseed crops (2004-2013)1 ...............................................219

Table 6-47. SLUA data imidacloprid and use patterns registered for additional foliar and soil use 

patterns (2004-2013) with no available residue data...............................................................................224

Table 6-48. Summary of risk findings for the foliar applied use patterns of imidacloprid with available 

residue data. .............................................................................................................................................225

Table 6-49. SLUA data for imidacloprid and fruiting vegetables (2004-2013).........................................226

Table 6-50. SLUA data for imidacloprid and cucurbit vegetables (2004-2013) ....................................230

Table 6-51. SLUA data imidacloprid and berries/small fruit (2004-2013) ..............................................239

Table 6-52. SLUA data for imidacloprid on soil only registered use patterns (2004-2013). (Note: Data are 

available for soil treated cucurbit vegetables and therefore is not presented below since its SLUA data is 

shown in the soil treated melon discussion..............................................................................................250

Table 6-53. Summary of risk findings for the soil applied use patterns of imidacloprid with available 

residue data. .............................................................................................................................................251

Table 6-54. SLUA data imidacloprid and cereal grains (2004-2013)1

.......................................................252

Table 6-55. SLUA data imidacloprid and other non-cereal grain seed treatment uses (2004-2013)1

.....257

Table 6-56. Summary of risk findings for the seed treatment use patterns of imidacloprid. .................258

Table 6-57. Summary of risk findings for the combined method use patterns of imidacloprid..............270

Table 6-58. Comparison of oral exposure to pollen and nectar for adult Apis and Non-Apis bees1 ......279

Table 6-59. Comparison of oral exposure to pollen and nectar for larval Apis and Non-Apis bees1.......279

Table 6-60. Comparison of imidacloprid acute contact toxicity to Apis and non-Apis bees .................280

Table 6-61. Comparison of imidacloprid acute oral toxicity to Apis and non-Apis bees.........................280

Table 7-1. Summary of risk findings for honey bees (Apis mellifera) for the registered use patterns of 

imidacloprid ..............................................................................................................................................289
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Table of Figures

Figure 2-1. Conceptual model for risk assessment of foliar spray applications of imidacloprid to honey 

bees. Red depicts systemic pathways. Dashed lines represent routes of exposure that are not 

considered major. .......................................................................................................................................30

Figure 2-2. Conceptual model for risk assessment of soil applications of imidacloprid to honey bees. Red 

depicts systemic pathways. Dashed lines represent routes of exposure that are not considered major.31

Figure 2-3. Conceptual model for risk assessment of planting of imidacloprid-treated seeds to honey 

bees. Red depicts systemic pathways. Dashed lines represent routes of exposure that are not 

considered major. .......................................................................................................................................32

Figure 2-4. Tiered approach for assessing risk to honey bees from foliar spray applications....................35

Figure 2-5. Tiered approach for assessing risk to honey bees from soil/seed applications.......................36

Figure 4-1. Imidacloprid application and processes involved in bee exposure..........................................46

Figure 4-2. Imidacloprid expected accumulation in 10 years of repeated application at a rate of 0.5 lb. 

a.i/A (This rate is equal to 0.245 ppm distributed on the top 6” of the soil); Noting that the graph was 

constructed based on yearly application coupled with daily degradation based on the aerobic soil 

degradation rate constant. .........................................................................................................................50

Figure 4-3. Expected degradation profile for imidacloprid in compartments of the terrestrial 

ecosystems. Imidacloprid parent, and the IMI-olefin and IMI-5-OH degradates are considered residues 

of toxicological concern. .............................................................................................................................53

Figure 4-4. Comparison of up-take data obtained for imidacloprid applied to soil and that applied to 

foliage in potatoes......................................................................................................................................58

Figure 4-5. Suggested Imidacloprid degradation profile in plants (based on submitted plant metabolism 

data)............................................................................................................................................................61

Figure 5-1. Scatterplot of adult acute contact toxicity of Apis and non-Apis bees from registrantsubmitted and open literature sources conducted with technical grade active ingredient (TGAI) and 

formulated typical end product (TEP) imidacloprid. Red circle denotes endpoint used for Tier I risk 

estimation purposes. ................................................................................................................................109

Figure 5-2. Scatterplot of adult acute oral toxicity of Apis and non-Apis bees from registrant-submitted 

and open literature sources conducted with technical grade active ingredient (TGAI) and formulated 

typical end product (TEP) imidacloprid. Red circle denotes endpoint used for Tier I risk estimation 

purposes....................................................................................................................................................114

Figure 5-3. Queen Production Data from Whitehorn et al (2012) in Controls (A), Low (B) and High 

Exposure Treatments (C). Legend numbers indicate the concentration in pollen and nectar, respectively.

..................................................................................................................................................................152

Figure 6-1. Summarization of the potential scenarios warranting a Tier I on and/or off-field risk 

assessment................................................................................................................................................169

Figure 6-2. Summary of acute and chronic RQ values using totality of pollen and nectar residue data 

from foliar-applied citrus residue study (MRID 49521301)......................................................................211

Figure 6-3. Imidacloprid nectar residues in nectar from foliar-applied citrus study (MRID 49521301, 

trials NT005 and NT006 only) as compared to effect levels in registrant submitted colony feeding study 

(MRID 49510001)......................................................................................................................................212

Figure 6-4. Imidacloprid average residues in pollen from foliar-applied citrus study (MRID 49521301, 

trials NT005 and NT006 only). ..................................................................................................................213
10

Figure 6-5. Summary of acute and chronic RQ values using totality of pollen and nectar residue data 

from foliar-applied cherry residue study (MRID 49535601).....................................................................215

Figure 6-6. Imidacloprid residues in nectar from foliar-applied cherry study (MRID 49535601) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................216

Figure 6-7. Imidacloprid average residues in pollen from foliar-applied cherry study (MRID 49535601).

..................................................................................................................................................................217

Figure 6-8. Summary of acute and chronic RQ values using totality of residue data from foliar-applied 

cotton residue study (MRID 49103301)....................................................................................................220

Figure 6-9. Imidacloprid residues in foliar-applied cotton residue study (MRID 49103301) as compared 

to effect levels in registrant submitted colony feeding study (MRID 49510001).....................................221

Figure 6-10. Summary of acute and chronic RQ values using totality of bee-collected pollen residue data 

from soil-applied tomato residue study (MRID 49665201)......................................................................227

Figure 6-11. Imidacloprid average residues in pollen from soil-applied tomato study (MRID 49665201).

..................................................................................................................................................................228

Figure 6-12. Summary of acute and chronic RQ values using totality of pollen residue data from soil 

melon residue study (MRID 49090501)....................................................................................................231

Figure 6-13. Imidacloprid residues in nectar in soil-applied melon residue study (MRID 49090501) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................232

Figure 6-14. Imidacloprid average residues in pollen from soil-applied melon study (MRID 49090501).

..................................................................................................................................................................233

Figure 6-15. Summary of acute and chronic RQ values using totality of nectar residue data from the soilapplied citrus residue study (MRID 49090505). .......................................................................................236

Figure 6-16. Imidacloprid residues in nectar in soil-applied citrus residue study (MRID 49090505) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................237

Figure 6-17. Summary of acute and chronic RQ values using totality of the bee collected-pollen and hive 

nectar residue data from soil-applied blueberry residue study (MRID 49535602)..................................240

Figure 6-18. Average total residues in nectar from soil-applied blueberry study (MRID 49535602) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................241

Figure 6-19. Imidacloprid average residues in pollen from soil-applied blueberry study (MRID 

49535602).................................................................................................................................................242

Figure 6-20. Summary of acute and chronic RQ values using totality of pollen residue data from soil –

applied strawberry residue study (MRID 49090502). (Note: Sampling interval not provided)................244

Figure 6-21. Imidacloprid average residues in pollen from soil-applied strawberry study (MRID 

49090502).................................................................................................................................................245

Figure 6-22 Summary of acute and chronic RQ values using totality of pollen and nectar residue data 

from soil-applied cotton residue study (MRID 49665202). ......................................................................247

Figure 6-23. Imidacloprid residues in nectar in the soil-applied cotton study (MRID 49665202) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................248

Figure 6-24. Imidacloprid average residues in pollen in the soil-applied cotton study (MRID 49665202).

..................................................................................................................................................................249

Figure 6-25. Summary of acute and chronic RQ values using totality of pollen residue data from seed 

treatment corn residue study (MRID 49511701)......................................................................................253

Figure 6-26. Imidacloprid residues in pollen in the seed treated corn study (MRID 49511701).............254
11

Figure 6-27. Summary of acute and chronic RQ values using totality of pollen residue data from the 

combined soil + foliar treatment tomato residue study (MRID 49665201). ............................................259

Figure. 6-28. Imidacloprid residues in pollen in the soil + foliar applied tomato study (MRID 49665202).

..................................................................................................................................................................260

Figure 6-29. Summary of acute and chronic RQ values using totality of pollen and extra-floral nectar 

data from the combined soil + foliar-applied cotton residue study (MRID 49665202). ..........................262

Figure 6-30. Imidacloprid residues in nectar from the soil + foliar cotton study (MRID 49665202) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................263

Figure 6-31. Imidacloprid average residues in pollen from the soil + foliar cotton study (MRID 

49665202).................................................................................................................................................264

Figure 6-32. Summary of acute and chronic RQ values using totality of pollen and floral nectar data from 

the combined seed treatment + foliar-applied cotton residue study (MRID 49511702).........................266

Figure 6-33. Imidacloprid floral nectar residues in seed + foliar cotton study (MRID 49511702) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001). ..................267

Figure 6-34. Imidacloprid averages residues in pollen from the seed + foliar cotton study (MRID 

49665202).................................................................................................................................................268

Figure 6-35. Box and Whisker Plots of Total Imidacloprid Measured in Hive-Collected Uncapped Nectar 

(A) and Bee Bread (B) from the Sucrose Colony Feeding Study (MRID 49510001)..................................276

Figure 6-36. Comparison of effect levels from qualitative Tier II feeding studies on B. terrestris obtained 

from the open literature (numbers in parentheses refer to the magnitude of effects and/or additional 

exposure to pollen)...................................................................................................................................283
12

Contributors, Reviewers, and Acknowledgements 

USEPA

Contributors:

Office of Pesticide Programs:

Christina Wendel, Biologist 

Christine Hartless, Wildlife Biologist

José Meléndez, Chemist

Office of Research and Development/National Health and Environmental Effects Research Laboratory/ 

Atlantic Ecology Division:

Glen Thursby, Chief, Watershed Diagnostics Branch

Reviewers:

Thomas Steeger, Ph.D., Senior Science Advisor

Michael Wagman, Biologist

California Department of Pesticide Regulation:

Contributors:

Alexander Kolosovich, Environmental Scientist, Ecotoxicology

Brigitte Tafarella, Environmental Scientist, Ecotoxicology

Russell Darling, Environmental Scientist, Reevaluation
13

1. Executive Summary 

1.1. Background and Scope 

Imidacloprid, along with the other nitroguanidine-substituted neonicotinoid insecticides, clothianidin, 

thiamethoxam, dinotefuran, are currently undergoing Registration Review by the USEPA. With 

imidacloprid, the EPA published a final registration review Work Plan in 2009 and issued a Generic Data 

Call-in in 2010 to obtain data required for assessing risks to bees and other taxa. This 2015 Preliminary 

Ecological Risk Assessment evaluates the risk of the registered agricultural uses of imidacloprid to bees 

alone. Consistent with the EPA 2014 Guidance for Assessing Pesticide Risks to Bees (USEPA et al. 2014), 

risks are quantified for the honey bee, Apis mellifera, and to the extent that data are available, 

characterized qualitatively for other bee taxa relative to the honey bee to the extent that data are 

available. Following the receipt of public comments on this Preliminary Pollinator Risk Assessment, the 

EPA plans to issue a revised Preliminary Ecological Risk Assessment at the end of 2016 that will: (i) consider 

any comments or information submitted in response to this bee-only preliminary risk assessment; (ii) 

incorporate additional data EPA anticipates to receive that is relevant to bees; and, (iii) assess the 

potential risks of all registered uses of imidacloprid to all taxa.

1.2. Use Characterization 

Imidacloprid is registered for a variety of agricultural crops, including (but not limited to): root and tuber 

vegetables, bulb vegetables, leafy, brassica, cucurbit, and fruiting vegetables, beans and other legumes, 

citrus fruit, pome fruit, stone fruit, berries, tree nuts, cereal grains, herbs, oilseed crops (e.g. canola, 

cotton) and other use patterns not associated with a crop group such as peanuts and tobacco. It has been 

registered for use in the United States since 1994. Maximum application rates vary by crop and method, 

but typically do not exceed 0.5 lbs. a.i./A (single application or per year). Imidacloprid may be applied to 

crops via a variety of methods including aerial and ground foliar sprays, soil drench, chemigation, soil 

injection, in furrow sprays, and seed treatment, including multiple application methods within the same 

growing season so long as the 0.5 lbs a.i/A rate is not exceeded. There are a wide variety of nonagricultural uses, some examples of which include tree trunk injection, forestry, pet spot-on treatments, 

turf, and applications to ornamentals; non-agricultural uses will be assessed in the Preliminary Risk 

Assessment scheduled for 2016. Additionally, there are a number of use patterns that specifically prohibit 

applications during the pre-bloom or blooming period or whenever bees are foraging. 

1.3. Environmental Fate and Transport 

Imidacloprid is a systemic insecticide that is associated with a high water solubility and low volatility. 

These properties, combined with low propensity to partition to organic carbon suggest that imidacloprid 

will be highly mobile in the terrestrial environment (i.e., subject to leaching in soils and runoff). The 

dominant transformation processes for imidacloprid are photolysis (very fast in the presence of water) 

and aerobic soil degradation. However, aerobic soil metabolism for imidacloprid is very slow (half-lives 

range from 200 days to more than one year) and therefore, imidacloprid is expected to persist in the soil 

system. Based on their occurrence as the primary degradates identified in plant metabolism studies and 
14

comparable toxicological properties with respect to bees relative to parent imidacloprid, the primary 

stressors of concern include parent imidacloprid and its metabolites IMI-olefin (IMI-olefin) and IMI-5-OH 

(5-OH-IMI). As a systemic chemical, in plants, imidacloprid is absorbed via the roots, stems and foliage 

and is considered xylem mobile, with dominant uptake routes following the transpiration stream (i.e., no 

downward transport from leaves to roots). Additionally, numerous field studies have demonstrated that 

imidacloprid applied via foliar, soil or seed treatment methods can result in residues in pollen and nectar 

of blooming plants. 

1.4. Exposure Assessment 

Exposure of bees through direct contact by foliar spray of imidacloprid (i.e., interception of spray droplets 

either on or off the treated field) and oral ingestion (e.g., consumption of contaminated pollen and nectar) 

represent the primary routes of exposure considered in this assessment. Bees may also be exposed to 

imidacloprid through other routes, such as contaminated surface water, plant guttation fluids, honey dew, 

soil (for ground-nesting bees), and leaves; however, there is high uncertainty regarding the importance of 

some of these exposure routes, and the Agency lacks information to understand the relative importance 

of these other routes of exposure and/or to quantify risks from these other routes. With respect to 

potential exposure via drift of abraded seed coat dust, the Agency is working with different stakeholders 

to identify best management practices and to promote technology-based solutions that reduce this

potential route of exposure. Finally, the “carryover” of imidacloprid residues in soil (i.e. the potential for 

year-to-year accumulation in soil leading to higher residues in pollen and nectar) was considered as a 

potential route of exposure in this assessment. This potential for carryover was evaluated using multiple 

lines of evidence. While model results and some empirical data from multi-year applications in soil 

suggest possible year-to-year accumulation in soils, available residue data in pollen and nectar are not 

indicative of imidacloprid carryover in treated crops. Furthermore, imidacloprid residues in succeeding 

crops (e.g. white clover following seed treatment applications to corn) are low when detected, such that 

risks to honey bees is not expected.

In accordance with the 2014 Guidance for Assessing Pesticide Risks to Bees (USEPA et. al. 2014), the 

exposure assessment considered Tier I (model-generated/screening-level) exposures of bees via contact 

and oral routes. Prior to this step, a determination was made on the potential for exposure based on 

indications of crop attractiveness to bees and cultural practices (e.g. whether the crop is harvested before 

bloom) referenced in the United States Department of Agriculture document, Attractiveness of 

Agricultural Crops to Pollinating Bees for the Collection of Nectar and/or Pollen (2014). For foliar sprays, 

off-field exposures via spray drift were also considered. These modeled/screening-level, exposure 

estimates were then refined using available information on measured imidacloprid residues in pollen and 

nectar of representative crops to assess risks to individual bees. This same residue information was also 

used to characterize risks at the colony level. 

1.5. Effects Assessment 

As with other neonicotinoid insecticides, imidacloprid acts on the insect nicotinic acetylcholine receptors 

(nAChRs) of the central nervous system via competitive modulation. At the individual organism level, a 
15

number of molecular, cellular, physiological, histopathological and behavioral effects of imidacloprid to 

bees have been reported from laboratory tests at varying levels of exposure for adult and larval bees. 

A robust registrant-submitted dataset was available to characterize the acute and chronic toxicity of

imidacloprid to adult and larval honey bees at the Tier I (individual) level. Additionally, the EPA, through 

a joint review effort with Health Canada’s Pest Management Regulatory Agency (PMRA) and the State of 

California’s Department of Pesticide Regulation (CDPR) evaluated over 75 studies from the open literature 

that investigated the toxic effects on Apis and non-Apis bees at the individual and colony level. Consistent 

with the aforementioned 2014 guidance, the focus for this assessment was on apical endpoints, that is, 

those related to growth, development, survival, and reproduction known to impact bees at the colony 

and population/community level. 

There are a number of data designed to evaluate the toxicity of acute and chronic exposures to individual 

bees including data for adults as well as larvae. Based on these data, imidacloprid is classified as very 

highly toxic to adult honey bees (Apis mellifera) with acute oral and acute contact LD50 values of 0.0039 

and 0.043 μg a.i/bee, respectively. For larval toxicity, there was no acute oral study available, and a 21-

day chronic toxicity test did not show significant effects (p>0.05) up to and including the highest 

concentration tested, 40 μg a.i/L (equivalent to 0.00183 μg a.i/bee). For chronic oral toxicity to adults, 

while a 10-day registrant-submitted study did not achieve a No Observed Adverse Effect Concentration

(NOAEC), based on significant effects (p<0.05) on food consumption at all concentrations, a 10-day study 

evaluated from the open literature (Boily, 2013, MRID 49750601), determined a definitive NOAEC at 

0.00016 μg a.i/bee based on significant effects (p<0.05) on mortality and body weight. Table 1-1 below 

shows the endpoints to be used for risk estimation for adult and larval honey bees at the individual level. 

Table 1-1. Summary of the toxicity endpoints to be used in risk estimation for individual bees.

Study Type Endpoint1 Reference Classification

Adult Acute Contact 

Toxicity 96-hr LD50: 0.043 μg a.i/bee MRID 49602717 Acceptable

Adult Acute Oral Toxicity 48-hr LD50: 0.0039 μg a.i/bee MRID 42273003 Acceptable

Adult Chronic Oral 

Toxicity

10-day NOAEC/LOAEC (mortality, body 

weight): 0.00016/0.00024 μg a.i/bee

Boily, 2013, 

MRID 49750601 Quantitative

Larval Acute (single dose) No data available

Larval Chronic (repeat 

dose)

21-day NOAEC/LOAEC: 0.0018/>0.0018 μg 

a.i/larva MRID 49090506 Supplemental

1Represents most sensitive (i.e. lowest) of all endpoints within a particular study type for studies for which raw data (to allow for 

independent statistical verification of the endpoint) are available.

Currently available data (registrant-submitted and from the open literature) suggest that colony level 

effects of imidacloprid on honey bees may result for some uses through multiple mechanisms including

(but not limited to) reduction in number of worker bees available for foraging or maintaining hive 

temperature (during over-wintering), reduction in foraging efficiency via sublethal effects on workers, 

decreased number or delayed development of brood either from direct exposure or indirectly from 

reduced brood feeding and maintenance by hive bees, and reduced fecundity and survival of queens. The 

colony level effects assessment (Tier II) is based on a registrant-submitted colony feeding study that 

assessed a 6-week exposure through nectar (spiked sucrose). This study was subjected to a tri-agency 

review by EPA, PMRA, and CDPR that included a comprehensive statistical re-analysis of the raw data. 
16

Although there were other evaluated colony studies conducted with colonies of Apis mellifera, only this 

study was considered acceptable for quantitative use in this risk assessment. Based on a tri-agency 

analysis of the statistical and biological considerations of the data, a NOAEC and LOAEC of 25 and 50 μg 

a.i./L in nectar were determined based on reductions of the number of adult workers, numbers of pupae, 

pollen stores and honey stores which persisted across much of the study duration. The level of 

imidacloprid in nectar at or below which no effects would be expected to the colony is determined to be

25 μg a.i./L.

1.6. Pollen Route of Exposure 

Honey bees are exposed to both pollen and nectar, which serve as the protein and carbohydrate sources 

in the diet, respectively. The risk assessment for individual bees assumes an equal contribution of these 

two food sources in the diet and equal potency at the individual level. No information was identified that 

enabled these assumptions to be directly evaluated at the individual organism level. At the colony level, 

the risk assessment is based on comparisons of imidacloprid residues measured in nectar in various crops 

to the sucrose-based dietary endpoints from the registrant-submitted colony feeding study. Comparison 

of imidacloprid residues in pollen to the sucrose-based dietary endpoints from the colony feeding study 

was not considered appropriate due to the differential utilization of pollen by the colony relative to nectar, 

and the subsequent differences in exposure of bees to dietary imidacloprid via pollen and nectar. 

Although this represents a limitation in the risk assessment, several lines of evidence suggest this 

uncertainty is not likely to substantially alter the risk conclusions for several reasons. First, while colonies 

were not fed spiked pollen in the colony feeding study, bees were nonetheless exposed to imidacloprid in 

pollen in the form of bee bread (a combination of stored pollen and honey) at concentrations that 

approximated 20% of those measured in uncapped nectar. Therefore, from an in-hive exposure 

perspective, the effects observed from the sucrose colony feeding study actually reflect a combination of 

exposure to contaminated nectar and pollen in the form of bee bread. Second, nectar is the dominant 

food source for adult foragers and hive bees whereas pollen is not consumed directly by adult bees and 

is processed into bee bread for feeding to developing larval bees. Third, the assessment for individual 

bees indicates that larval bees are at least one order of magnitude less sensitive than adult honey bees 

on a chronic basis. Finally, although not definitive, the available suite of higher-tier studies resulting from 

the pollen route of exposure suggest that colony-level effects on honey bees via contaminated pollen 

occur at higher residue levels than those in nectar. Taken together, these lines of evidence suggest that 

the lack of pollen consideration in the assessment of colony effects is not likely to substantially alter the 

risk conclusions except when exposure via pollen is extraordinarily high relative to nectar. The latter is 

noted for certain crop groups where pollen is expected to be the dominant route of exposure (e.g., corn),

and additional consideration is given to available data in the open literature on colony-level effects 

associated with spiked pollen. 

1.7. Non-Apis Bee Characterization 

The risk profile of imidacloprid to non-Apis bees (e.g., bumble bees, solitary bees) may differ relative to 

honey bees due to differences in their exposure and sensitivity to imidacloprid. Therefore, uncertainty 

exists in extrapolating the risk findings of this assessment to non-Apis bees. The relative importance of 
17

this uncertainty was evaluated by first considering the relative differences in exposure (e.g., oral 

consumption rates) and sensitivity (e.g., acute toxicity) individual level. Although data were very limited 

for non-Apis bees, results suggest oral exposure and effects of imidacloprid on the honey bee are 

reasonably representative (protective) of available data on adult non-Apis bees (primarily bumble bees). 

It is also noted that there are limited data on the toxicity of imidacloprid to non-Apis bees. At the colony 

level, however, a review of studies published in the open literature suggests that bumble bees may be 

adversely affected at the colony level at concentrations in sucrose considerably lower than those observed 

for the honey bee. These effects are primarily associated with reproduction (i.e. worker and queen 

production) in sucrose and/or pollen feeding studies. These studies were considered for qualitative use 

in thee risk assessment, primarily from a lack of analytical verification of the test substance and lack of 

raw data, and therefore additional data with Bombus would benefit the risk characterization for non-Apis

bees.

1.8. Additional Lines of Evidence 

The agency evaluated available wildlife incidents for bees and for most incidents, there was not a strong, 

established association between individual bee or colony losses to imidacloprid as indicated by 

confirmatory residue analysis. In the cases where a link between imidacloprid exposure and individual or 

colony losses was made, these reports generally concerned residential uses or other uses pesticide control 

operators (PCOs). Additionally, there were studies available from the open literature that surveyed 

imidacloprid residues in agricultural fields as well as various hive matrices. Results from the agricultural 

monitoring studies, where pollen samples originated from corn and sunflower fields seed treated with 

imidacloprid, indicated that while imidacloprid was detected frequently (ranging from 36 – 58% of the 

total samples), the mean values of quantifiable residues ranged from 0.6 – 3.0 ppb, which is just above 

the limit of quantitation for these studies. The hive monitoring studies included surveys across the United 

States and Europe where imidacloprid residues were investigated in pollen, nectar, bee, and wax samples. 

These studies indicated that while imidacloprid was detected in various matrices, the frequency of 

detection was generally below 10% and where the frequency exceeded 10%, the mean values were 

generally marginally above the limit of quantitation (varies depending on the study). Although there was 

one study in which the mean residue of imidacloprid in pollen samples reached as high as 39 ppb, this 

mean originated from 10 detections out of 350 analyzed samples (2.9%). The studies suggest that despite 

widespread use of imidacloprid on crops through multiple application methods, the magnitude and 

frequency of detection in hive matrices is relatively low. 

1.9. Risk Conclusions 

The agency conducted a screening level assessment (Tier I) for the various uses of imidacloprid utilizing 

the toxicity endpoints in Table 1-1 above and either conservative (modeled) exposures or, as a more 

refined assessment, actual residue values from pollen and/or nectar (where data were available) to 

determine if there are risks to individual bees. If these analysis indicated that the level of concern is not 

exceeded, the agency concluded that there is not a risk and that there is not a concern at the colony level. 

In these instances, no further analysis was necessary. However, if the analysis demonstrated a risk to 

individual bees, the agency did, when data were available, conduct a risk assessment to determine 
18

whether there were risks posed to the colony. As mentioned above and further described in Section 2

(Problem Formulation), the risk assessment approach to honey bees proceeds in a stepwise, tiered 

process evaluating risks to individual bees first and, if needed, risks to the colony. After the initial step in 

determining the potential for exposure of bees to agricultural uses of imidacloprid, risk quotients (or levels 

of concern) are estimated to evaluate the risk to individual bees using modeled/screening-level exposure 

estimates and the acute and chronic laboratory toxicity endpoints (i.e. adult acute contact LD50, adult 

acute oral LD50, adult chronic oral NOAEL, and larval chronic oral LOAEL). For all crops and application 

methods where on-field exposure, is expected, values exceeded risk levels of concern. Even in cases 

where on-field exposure was not expected, an off-field spray drift assessment was conducted and 

indicated that there could be risk for all foliar uses (depending on what crop is adjacent to the field, 

whether the crop is in bloom, whether the crop is pollinator attractive, etc). Additionally, a refined 

analysis was conducted using available measured residue data to supplant the modeled/screening-level 

estimates of exposure that were mentioned above. These refined values were compared to the hazard 

endpoints tabulated above. For all use patterns where residue data were available, LOCs were exceeded

based on refined estimates of exposure. 

Table 1-2 summarizes the agency’s preliminary risk findings on a crop group-based approach. The table 

presents the findings for groups of crops that have similar use patterns and application methods and are 

further split out into three categories of risk findings. When residue data are available, the crop is 

identified parenthetically within the table along with the respective crop group. 

Crop groups/use patterns where either on-field exposure is not anticipated due to attractiveness or the 

crop is harvested before bloom, or the tiered process indicates a low potential for on-field risk, are listed 

in the green group in Table 1-2. These include all application methods of root/tuberous, bulb, leafy 

greens, and brassica vegetables, globe artichoke, and tobacco (harvested before bloom) as well as soil 

applications to blueberries (berries and small fruits) and seed treatment applications to corn (cereal 

grains). Additional members of the cereal grain group (which is registered for seed treatment uses only) 

including wheat, barley, oats, rye, and millet are either not attractive to honey bees or primarily wind 

pollinated. Finally, members of the fruiting vegetable group (of which soil and soil + foliar residues data 

for tomato are available) are largely unattractive to honey bees with the exception of okra. Therefore, a 

low potential for on-field risk is determined for all members of this group, except okra, for all application 

methods based on a lack of exposure.

The yellow/gold group represents crop groups/use patterns for which the assessment for individual bees 

indicates that the LOCs have been exceeded; however, uncertainty exists in the assessment of risk to the 

colony. These include uses where either no data are available (with indications of the potential to bridge 

to other neonicotinoid chemicals where data are expected for that same use pattern and application 

method) or where there is uncertainty in the nectar and pollen residue data originating from uncertainties 

in the available studies. For several crop groups including legumes, tree nuts, and certain application 

methods of stone fruits, berries/small fruits, and oilseed, residue data are unavailable but there is the 

potential to bridge from data for other neonicotinoid chemicals with forthcoming data for certain 

application methods. In other cases, data are not available and there are no data expected for the other 

neonicotinoid chemicals such as certain application methods for legumes, tree nuts, berries/small fruits, 
19

nectar producing cereal grain members, and herbs and spices. In the case of cucurbit vegetables (soil 

applications to melons data available), citrus fruits (soil applications to oranges and grapefruits data 

available), and berries/small fruits (soil applications to strawberries data available), there are limitations 

with the residue studies that create uncertainty in the risk determinations with these use

patterns/application methods. This uncertainty is generally associated with these studies having an 

unknown timing of application relative to bloom (strawberry), no nectar data available (strawberry), no 

pollen data available (citrus fruits), and no available residue data from coarse soils, which are shown 

through several studies to yield residues in nectar and pollen up to an order of magnitude higher as 

compared to medium and fine soil types. Furthermore, the soil-applied citrus study was conducted with 

a post-bloom application while the label does not restrict pre-bloom or during bloom applications and 

therefore the residues from this study are likely underestimated. For soil applications to cucurbits and 

citrus fruits, there is a potential to bridge with forthcoming data for other neonicotinoid chemicals. In the 

case of cucurbit vegetables, a full field study (Tier III) on pumpkins is expected in 2016 to further refine 

the risk picture. Additionally, although foliar applications to stone fruits resulted in pollen residues 

exceeding a threshold that is indicated in the open literature to cause colony level effects, the bloom 

duration of stone fruits is markedly shorter than the exposure duration employed in from those studies 

that determined these effects and therefore there is uncertainty with this determination. Finally, while 

data are unavailable for pome fruits, residue data for imidacloprid are expected in 2016.

Lastly, the red grouping within the table indicates use patterns with associated application methods that 

present a risk to individual bees as well as a risk in nectar or both nectar or pollen. These include foliar 

applications (with a 10-day pre-bloom interval) to citrus fruits and foliar, soil, soil + foliar, and seed 

treatment + foliar applications. (with no bloom restrictions) to cotton. A full field study with cotton is 

expected in 2016 to further refine this risk determination. 
20

Table 1-2. Summary of risk findings for honey bees (Apis mellifera) for the registered use patterns of imidacloprid

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

Crop Groups/Use Patterns that Present Low On-Field Risk

Root/Tuber 

Vegetables4

Foliar N Y

No further analysis conducted

Low On-Field Risk (all uses, lack of exposure)1

; Off-Field

Risk (Tier I, foliar uses only) Soil N

Seed N

Bulb 

Vegetables

Soil N Low On-Field Risk (all uses, lack of exposure)1

; Off-Field 

Seed N Risk (Tier I, foliar uses only)

Leafy Greens 

Vegetables

Foliar N Y Low On-Field Risk (all uses, lack of exposure)1

;

Soil N Off-Field Risk (Tier I, foliar uses only)

Brassica 

Vegetables

Foliar N Y

Low On-Field Risk (all uses, lack of exposure)1; Off-Field

Risk (Tier I, foliar uses only) Soil N

Seed N 

Fruiting 

Vegetables 

(Tomatoes)

Foliar Y Y

Y No data2 N

Low On-Field Risk (Tier II, pollen; nectar not produced, 

lack of exposure)

Off-Field Risk (Tier I, foliar uses only)

(Determinations apply to all members except okra due 

to unattractiveness of group to honey bees, Bombus 

used for pollination services in greenhouse)

Soil Y

Berries/Small 

Fruits 

(Blueberry)

Soil Y Y N N Low On and Off-Field Risk (Tier II, nectar and pollen)

Cereal Grains

(Corn) Seed Y Y No data2 N

Low On and Off-Field Risk (pollen; nectar not produced)

(Other members such as wheat, barley, oats, millet and 

rye are either not attractive to bees)

Tobacco, globe 

artichoke

Foliar N Y

No further analysis conducted Low On-Field Risk (all uses, lack of exposure)1

;

Soil N Off-Field Risk (Tier I, foliar uses only)

Crop Groups/Use Patterns with Uncertainty in Colony (Tier II) Assessment

Legumes

Foliar Y Y No data No data No data On Field Risk (Tier I, all uses); Tier II Risk unknown

Off Field Risk (Tier I, foliar uses only)

(Honey bee attractive; no bloom restrictions; seed 

treatment of soybean = highest usage of all registered 

crops (400,000 lbs a.i/year). 

Soil Y No data No data No data

Seed Y

No data

(Potential 

bridging)

No data

(Potential 

bridging)

No data

(Potential 

bridging)
21

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

Cucurbit 

Vegetables 

(Melons)

Soil Y Y

Uncertain 

(Potential 

bridging)

Uncertain 

(Potential 

bridging)

On-Field Risk (Tier I); Tier II Risk uncertain 

(Long [6 weeks +] bloom duration; uncertainty of lower 

than maximum annual rate used and one sampling 

interval, no residues in coarse soils, unknown as to 

whether application closer to bloom would yield higher 

residues; Tier III full field study [pumpkins] expected for 

2016 assessment)

Citrus Fruits 

(Oranges/ 

grapefruits)

Soil Y Y

Uncertain 

(Potential 

bridging)

No data

(Potential 

bridging)

On-Field Risk (Tier I); Tier II Risk uncertain

(6 week + bloom duration; uncertainty of no residues in 

coarse soils and residues do not reflect worst case 

scenario as current labels permit pre and during bloom 

applications where these applications were made postbloom)

Pome Fruits

Foliar Y Y Y No data No data On-Field Risk (Tier I); Off-Field Risk (Tier I, foliar uses 

only)

(Residue data expected in 2016) Soil Y Y No data No data

Stone Fruits 

(Cherries) Foliar Y Y Y N Possible

Low On-Field Risk (Tier II, Nectar;), Tier II Risk possible 

(Pollen); Off-Field Risk (Tier I) 

(Stone fruits associated with short bloom duration [2-3 

weeks] relative to exposure duration in open literature 

pollen feeding study [12 weeks] which likely mitigates 

the potential for colony level from pollen route of 

exposure)

Stone Fruits Soil Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I); Tier II Risk unknown

Berries/small 

fruits Foliar Y Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I); Tier II Risk unknown

Off-Field Risk (Tier I)

Berries and 

small fruits

(Strawberries)

Soil Y Y No data Possible 

On-Field Risk (Tier I); Tier II Risk possible (pollen)

(Long [6 weeks +] bloom duration; uncertainty of one 

sampling interval, no residues in coarse soils, unknown 

timing of application relative to bloom
22

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

Tree nuts

Foliar Y Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I, all uses); Tier II Risk unknown

(Variable bee attractiveness within group); 

Off-Field Risk (Tier I, foliar uses only) Soil Y Y No data No data

Cereal grains Seed Y Y No data No data

On-Field Risk (Tier I); Tier II Risk unknown 

(Nectar producers within the group (i.e. sorghum, 

buckwheat).

Herbs/Spices

Foliar Y Y Y No data No data On-Field Risk (Tier I); Tier II Risk unknown

Off-field Risk (Tier I, foliar uses only)

(Variable attractiveness within group)

Soil Y Y No data No data

Seed Y Y No data No data

Oilseed5 Seed Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-field Risk (Tier I), Tier II Risk unknown

Crop Groups/Use Patterns with Colony (Tier II) Risk Indicated

Citrus Fruits

(Oranges) Foliar Y Y Y Y Possible

On-field Risk (Tier I), Tier II Risk (nectar), Tier II Risk 

possible (pollen)

Off-field Risk (Tier I) 

(10-d pre-bloom restriction for foliar uses; 6 week + 

bloom duration; used for honey production)

Oilseed5

(Cotton)

Foliar Y Y Y Y Possible On-field Risk (Tier I), Tier II Risk (nectar), Tier II Risk 

possible (pollen), Off-field Risk (Tier I, foliar uses only)

(Tier III full field study [cotton] expected for 2016 

assessment.

Soil Y Y Y Possible

Hash marks represent no off-field exposure expected for soil and seed treatment uses. 1 Crop is harvested before bloom (except for small acreage for seed production; nectar and pollen residue data were not required as minimal on-field exposure is expected. 2Nectar is not produced by representative crop where residue data are available

3Possible Tier II Risk for pollen indicated when residues in pollen from a residue study exceed 100 ppb, which is indicated in the literature to be a level where colony 

overwintering survival is potentially impacted.

4Two members of this group, potatoes and sweet potato, are noted to be harvested after bloom, although potatoes are not honey bee attractive and in the case of sweet 

potato, require pollination only for breeding, which is a small percentage of the total acreage. 

5Cotton is registered for all application methods. All other members of the oilseed group including canola and sunflower are registered only for seed treatment use
23

1.10. Major Assumptions and Uncertainties 

There are several assumptions and uncertainties associated with both the effects and exposure 

assessments for imidacloprid. While these assumptions and uncertainties are described in further detail 

throughout this assessment, a list of the major assumptions and uncertainties is provided below:

• Pollen and nectar are assumed to be the dominant routes of exposure for bees. Potential 

exposure via abraded seed coat dust is being addressed through separate ongoing development 

of best management practices.

• Model-predicted, screening-level EECs serve as a conservative estimate for predicting exposure 

to individual adult and larval honey bees resulting from foliar, soil, and seed treatment 

applications and therefore may over-estimate exposure. 

• It is assumed that pollen and nectar are equally potent routes of exposure when assessing the risk 

to individual bees.

• Extrapolation of individual bee risk findings to risks at the colony-level is uncertain due to the 

complexities of exposure and effects at the colony level. 

• Off-field estimates of risk are based on screening-level exposure estimates which cannot be 

refined with available residue data and are assumed to be to pollinator friendly crops at the time 

of bloom. Therefore, potential off-field risks may be overestimated.

• Available data from crop residue studies may not fully capture variation in temporal and spatial 

factors (e.g., weather patterns, soil type) that affect imidacloprid residues in pollen and nectar for 

the tested crop.

• Except for citrus where multiple crops are represented by residue information, most crop groups 

are represented by residue studies for one or two crops. It is therefore assumed that residue 

information for the tested crop(s) are representative of other crops in the same crop group. 

• Interpretation of Tier 2 risks based on the 6-week, sucrose colony feeding study assumes that 

bees forage on the treated crop nearly 100% of the time to represent the nectar needs of the 

colony. In the field, bees may forage for significantly shorter periods of time particularly for crops 

such as cherries and blueberries that have a 2-3 weeks blooming duration. Bees may also forage 

on alternative (untreated) plants. Conversely, bees associated with migratory colonies used for 

pollination services may feed on treated crops for similar or possibly longer periods of time over 

the course of a growing season.

• Available full field data (Tier III) for sunflower, canola, and corn are considered inadequate to 

evaluate risk for use patterns where further refinement of risks to the colony are indicated.

However, full field studies for pumpkin and cotton are expected in 2016.
24

2. Problem Formulation 

Problem formulation serves as the first step of a risk assessment and it provides the foundation for the 

entire ecological risk assessment. In addition to identifying the risk assessment scope and objectives, the 

problem formulation includes three major components: (1) assessment and measurement endpoints that 

reflect management goals and the ecosystem they represent, (2) conceptual models that describe key 

relationships between a stressor (i.e., pesticide) and assessment endpoint or between several stressors 

and assessment endpoints, and (3) an analysis plan that summarizes the key sources of data and methods 

to be used in the risk assessment (USEPA 1998).

2.1. Registration Review Background 

As articulated by the Agency’s Registration Review Schedule, the nitroguanidine-substituted 

neonicotinoid insecticides (imidacloprid, clothianidin, thiamethoxam, dinotefuran) are currently 

undergoing Registration Review. With imidacloprid, the first installment of the Registration Review 

process was the publication of the Problem Formulation and Preliminary Work Plan documents in 2008, 

(USEPA 2008a; 2008b). With respect to assessing ecological risk, these documents summarized the 

available data on ecological effects and environmental fate of imidacloprid, identified key data gaps, and 

set forth a schedule for obtaining these data and completing the ecological risk assessment. Following its 

receipt and response to public comment comments, the Agency published a Final Work Plan in 2009 

(USEPA 2009), which was subsequently amended in 2010 to request additional data related to assessing 

risks to bees (USEPA 2010a). Also in 2010, a Generic Data Call-In (GDCI) was issued (USEPA 2010b) that 

required registrants to submit certain types of environmental fate and effects data in preparation for the 

forthcoming Preliminary Ecological Risk Assessment document.

2.2. Nature and Scope of Assessment 

Unlike most of the Agency’s Preliminary Ecological Risk Assessment for pesticides which focus on multiple 

taxa of aquatic and terrestrial non-target organisms, this preliminary assessment focuses solely on the risk 

of registered imidacloprid uses to bees. The decision to focus on imidacloprid’s potential risk to bees

(honey bees [Apis mellifera] and non-Apis bees) reflects that Agency’s desire to evaluate potential risks 

and appropriate mitigation measures earlier in the Registration Review process relative to other taxa. It 

also reflects the large volume of information related to environmental exposure and effects of 

imidacloprid to bees which has been generated over the past decade. Following receipt of public 

comments on this Preliminary Pollinator Assessment, the Agency plans to issue a revised Preliminary Risk

Assessment (PRA) at the end of 2016. The revised 2016 assessment which will include all taxa traditionally 

considered in Agency pesticide ecological risk assessments (e.g., fish, aquatic invertebrates, mammals, 

birds, amphibians, reptiles, plants) and update the bee risk assessment with additional information that 

may have become available.

Several other aspects related to the scope of this assessment are important to note. First, this assessment 

includes a quantitative estimate of risk (i.e., derivation of risk quotients) for the honey bees. Other types, 
25

i.e. non-Apis bees, are also considered in this assessment, e.g., bumble bees (Bombus spp.) and solitary 

bees), but risks are evaluated qualitatively (i.e., without derivation of risk quotients) due to limitations in 

available data and suitably vetted risk assessment methods for these species. This approach is consistent 

with the Agency’s Guidance for Assessing Pesticide Risks to Bees (USEPA/PMRA/CDPR, 2014) which 

recognizes that methods and data for assessing pesticide effects (and exposure) to bumble bees and 

solitary bees are still evolving and lack standardized regulatory guidelines. 

Second, this assessment is limited to registered agricultural uses of imidacloprid and therefore, does not 

include evaluation of risks associated with non-agricultural uses (e.g., residential, forestry uses). The 

revised assessment to be published at the end of 2016 will include all registered uses of imidacloprid 

(agricultural and non-agricultural) as there is additional information expected to be incorporated for the 

non-agricultural uses including ornamentals.

Finally, the effects data considered in this assessment are centered on the Agency’s protection goals and 

their associated assessment endpoints previously identified for bees (USEPA et. al. 2014). As described 

further in Section 2.5, the assessment and measurement endpoints used to support these protection 

goals are those that closely relate to survival, growth and reproduction of individual (solitary) bees and 

overall colony strength and survival (for eusocial bees). A large body of literature has been generated on 

effects of imidacloprid on bees at lower levels of biological organization (e.g., molecular, organ-level 

effects) in addition to endpoints relating to behavioral aspects of individual bees. While such data serve 

as additional lines of evidence in risk assessment and understanding the mechanisms of toxicological 

effects, they were formally evaluated in this assessment only when they were quantitatively linked to 

Agency assessment endpoints described in Section 2.5. 

2.3. Pesticide Type, Class, and Mode of Action 

Imidacloprid (IUPAC name: N-[1-[(6-chloropyridin-3-yl)methyl]-4,5-dihydroimidazol-2-yl]nitramide) is a 

systemic, neonicotinoid insecticide which acts on the insect nicotinic acetylcholine receptors (nAChRs) of 

the central nervous system via competitive modulation (IRAC 2015). Imidacloprid in the N-nitroguanidine 

group of neonicotinoids (IRAC subclass 4A) along with clothianidin, thiamethoxam and dinotefuran.1

 Its 

mode of action on target insects involves out-competing the neurotransmitter, acetylcholine for available 

binding sites on the nAChRs (Zhang et al. 2008). At low concentrations, neonicotinoids cause excessive 

nervous stimulation and at high concentrations, insect paralysis and death will occur (Tomizawa and 

Casida 2005). Imidacloprid is a xylem-mobile systemic compound that is readily taken up by the roots of 

the plant and translocated throughout the plant via the transpiration stream2

. 

 1 http://www.irac-online.org/ 2 Sur, R. and Stork, A. (2003). Uptake, translocation, and metabolism of imidacloprid in plants. Bulletin of 

Insectology. 56 (1), 35 – 40.
26

2.4. Overview of Imidacloprid Uses 

Imidacloprid is a registered on a wide variety of agricultural crops, including (but not limited to): root and 

tuber vegetables, bulb vegetables, leafy, brassica, cucurbit, and fruiting vegetables, cereal grains, citrus 

fruit, pome fruit, stone fruit, berries, tree nuts, beans and other legumes, herbs, oilseed crops (e.g. canola, 

cotton) and other use patterns not associated with a crop group such as peanuts and tobacco. It has been 

registered for use in the United States since 1994. Maximum application rates vary by crop and method, 

but typically do not exceed 0.5 pounds of active ingredient per acre (lbs a.i/A; single application or per 

year). Imidacloprid may be applied to crops via a variety of methods including aerial and ground foliar 

sprays, soil drench, chemigation, soil injection, in-furrow sprays, and seed treatment. There are a wide 

variety of non-agricultural uses, some examples of which include tree trunk injection, forestry, pet spoton treatments, turf, and applications to ornamentals. Additionally, there are a number of use patterns 

that specifically prohibit applications during the pre-bloom or blooming period or whenever bees are 

foraging. However, as described in Section 2.2, the focus of this preliminary risk assessment for bees is 

on agricultural uses only. A detailed summary of registered agricultural uses of imidacloprid is provided 

in Section 3. 

2.5. Overview of Physicochemical, Fate, and Transport Properties 

As described in Section 4.1, imidacloprid is a highly water soluble chemical with low vapor pressure and 

Henry’s Law Constants. These properties suggest that the chemical will be readily soluble for movement 

with water and that it is unlikely to volatilize to a meaningful degree. Furthermore, the organic carbon: 

water partitioning coefficient (KOC) for imidacloprid is low. 

The dominant transformation process for imidacloprid are photolysis (very fast in the presence of water) 

and aerobic soil degradation. However, aerobic soil transformation for imidacloprid is very slow (half- life 

values range from 200 days to more than a year) and therefore, it is expected to persist in the soil system. 

Photodegradation may occur on soil surfaces via soil application and on wet foliage in case of foliar 

application, although photolysis on dry soil appears to be slow. Several metabolites of imidacloprid may 

be formed in the terrestrial soil/plant system and are of toxicological concern with respect to bees. These 

include IMI-olefin (IMI-olefin) and IMI-5-OH (5-OH-IMI). In plants, imidacloprid may be taken up via the 

roots or across plant stems and leaves. Imidacloprid is considered xylem mobile, with dominant uptake 

routes following the transpiration stream3

. Details of imidacloprid fate and transformation pathways are 

provided in Section 4.1.

2.6. Stressors of Toxicological Concern 

As discussed in Section 4.1, imidacloprid is considered persistent in the terrestrial environment with the 

exception of conditions that favor aqueous photolysis. Metabolites identified from aerobic soil 

metabolism studies include IMI-olefin, nitrosamine, guanidine, and 5-keto urea isomers. Based on plant 

metabolism studies submitted to the Agency, metabolites of imidacloprid detected in various plants 

 3 Ibid
27

include guanidine, IMI-5-OH, IMI-olefin, IMI-4,5-OH, 6-chloronicotinic acid (6-CNA), 6-chloropicolylalcohol (6-CPA), nitrosamine and urea. Data on the relative toxicity these metabolites are discussed 

in Section 5. These data indicate that two metabolites (IMI-olefin and 5-OH-IMI) are of similar toxicity as 

parent imidacloprid to the honey bee, while other metabolites are much less toxic (e.g. 6-CNA and 

urea). Therefore, based on relative toxicity of various imidacloprid metabolites to bees and their 

occurrence in pollen and nectar, the primary stressors of toxicological concern for this assessment are:

• Imidacloprid (parent)

• IMI-olefin, and 

• IMI-5-OH.

2.7. Protection Goals and Assessment Endpoints 

The Agency has recently defined protection goals for assessing pesticide risks to bees which include: 1) 

maintenance of pollination services, 2) hive product production (e.g., honey, wax, propolis), and 3) bee 

biodiversity (Table 2-1; USEPA/PMRA/CDPR 2014). These goals do not apply uniformly across Apis and 

non-Apis bees; however, they are considered relevant for both social and solitary bees, and honey bees 

are generally used a surrogate for non-Apis bees. Protection goals dictate assessment endpoints for 

which specific measurement endpoints are identified. As EPA regulates pesticides under the Federal 

Insecticide, Fungicide, and Rodenticide Act, which directs EPA to weigh ecological risks associated with a 

pesticide product against the benefits of that product, protection goals serve to clarify the potential risks 

against which benefits can be balanced. 

The management goals, assessment endpoints and measurement endpoints depicted in Table 2-1 reflect 

the Agency’s use of honey bees as a surrogate for other bee pollinators. Although this approach has

limitations, it is assumed that data on individual organisms as well as colony-level data can provide 

relevant information on the potential effects of a pesticide on both solitary bees as well as social bees. In 

addition, protection of honey bees would contribute to pollinator diversity indirectly by preserving the 

pollination and propagation of the many plants species pollinated by honey bees, which also serve as food 

sources for other pollinating insects. In evaluating potential risks specific to honey bees, the protection 

goals of preserving pollination services and production of hive products (e.g., honey, wax) are readily 

assessed through the assessment of population size and the stability (e.g., presence of a queen, uniform 

brood pattern) of the colony and through direct and indirect measures of the quantity and quality of hive 

products4

. As such, the sensitivity of individual larval or adult honey bees based on laboratory-based 

acute and chronic toxicity studies serve as reasonable measurement endpoints for screening-level 

assessments of potential adverse effects on colony strength, survival and capacity of the colony to 

produce any products. While these measurement and assessment endpoints are tested using managed 

 4 USEPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees. Submitted to the FIFRA 

Scientific Advisory Panel for Review and Comment September 11 – 14, 2012. Office of Chemical Safety and Pollution 

Prevention Office of Pesticide Programs Environmental Fate and Effects Division, Environmental Protection Agency, 

Washington DC; Environmental Assessment Directorate, Pest Management Regulatory Agency, Health Canada, 

Ottawa, CN; California Department of Pesticide Regulation

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-0543-0004
28

honey bee colonies, they apply to feral honey bee colonies and, in the absence of data specific to other 

bees, these measurement endpoints provide useful information for assessing the survival and 

development of solitary bees and potential effects on bee species richness and biodiversity. To the extent 

that data are available for other species such as the bumble bee (e.g., Bombus terrestris), blue orchard 

bee (Osmia lignaria), and the alfalfa leafcutting bee (Megachile rotundata), the effects of imidacloprid on 

these species are also considered in this risk assessment. 

Table 2-1. Protection goals and examples of associated assessment and measurement (population and 

individual) endpoints for bees.

Protection Goal Assessment Endpoints

Example Measurement Endpoints

Population level and 

higher Individual Level

Contribution to Bee 

Biodiversity

Species richness1 and 

abundance

Individual bee survival 

(solitary bees) and 

colony strength and 

survival (social bees) 

Species richness and 

abundance1

Individual worker and 

larval survival assays;

larval emergence;

queen 

fecundity/reproduction 

Provision of Pollination 

Services

Population size2 and 

stability of native bees 

and commercially 

managed bees 

Colony strength and 

survival; colony 

development

Individual worker and 

larval survival assays;

queen fecundity;

brood success;

worker bee longevity

Production of Hive 

Products

Quantity and quality of 

hive products

Quantity and quality of 

hive products; 

including pesticide 

residue levels on 

honey/wax

Individual worker and 

larval survival assays;

queen 

fecundity/reproduction; 

larval emergence 1 Use of honey bees as a surrogate for other insect pollinators has limitations; however, it is assumed that as with all surrogates, data on individual 

organisms as well as colony-level data would provide some relevant information on the potential effects of a pesticide on both solitary bees as 

well as “eusocial” taxa. In addition, protection of honey bees would contribute to pollinator diversity indirectly by preserving the pollination and 

propagation of the many plants species pollinated by honey bees, which also serve as food sources for other pollinating insects. 2 For managed honey bees, population size can include numbers of colonies.

2.8. Conceptual Models and Risk Hypotheses 

The risk hypothesis and conceptual model are used to depict the hypothesis in terms of the source of the 

stress, route of exposure, receptor, and changes in the receptor attribute(s) of concern (USEPA, 1998). 

With imidacloprid, the conceptual models are depicted separately for each method of application to 

agricultural crops (i.e. foliar spray, soil application and seed treatment).

2.8.1. Foliar Spray

There are many factors that determine the exposure of bees to a pesticide, including methods and timing 

of application, application rate, attractiveness of the crop to bees, and agronomic practices such as 

harvesting crops prior to bloom. In general, however, foliar application of systemic pesticides such as 
29

imidacloprid are expected to result in exposure of bees via two dominant routes: 1) direct contact with 

the bee via interception of pesticide spray droplets and newly-sprayed vegetation, and 2) oral ingestion 

through contaminated pollen and nectar (Figure 2-1). With foliar sprays, these routes of exposure may 

occur on the treated field or adjacent to the treated field in the case of spray drift. With honey bees, 

nectar and pollen foragers are expected to receive high exposure via their frequent interaction with 

blooming crops. Dominant exposure routes of in-hive bees (e.g., nurse, queen, drone bees) include 

ingestion and processing of pollen and nectar and exposure through production. Stored honey is expected 

to be an important exposure route for over wintering bees. Processed bee bread, brood food, and royal 

jelly are major routes of exposure for developing larvae and the queen, although limited evidence 

suggests pesticide levels in royal jelly are orders of magnitude below those found in pollen and nectar 

(USEPA 2012). 

Exposure through the vapor phase is not expected to be a significant route of exposure for imidacloprid, 

regardless of application method. Exposure of honey bees through contact with contaminated soil is also 

not expected to be a major route of exposure, although this may be important for ground-nesting bees 

on or near the treated site. Other routes of exposure are also possible, including consumption of plant 

guttation fluids, water from dew droplet formation on leaves, puddles, and other surface water. Although 

relatively high concentrations of neonicotinoid insecticides have been reported in plant guttation fluid

(e.g. Girolami et al. 2009), recent reviews of honey bee exposure routes indicate high uncertainty in the 

importance of guttation fluid ingestion relative to other oral ingestion sources of pesticides (e.g., nectar 

and pollen). This uncertainty is partly due to the availability of guttation fluid at times of the year when 

crops are generally unattractive to pollinators and there are other sources of water (Godfray et al. 2014; 

USEPA 2012). Furthermore, there is presently a lack of robust information on water intake rates by bees 

from surface water and multiple factors that affect these rates. Therefore, this pathway is not currently 

considered for quantitative estimation of risk to bees. 

Changes in the assessment endpoints (e.g., size and stability of bee colonies, production of hive products, 

pollinator species richness and abundance) as a result of the aforementioned pesticide exposure routes 

may occur through various means, including reduction in number of worker bees available for foraging or 

maintaining hive temperature (over wintering), reduction in foraging efficiency via sublethal effects on 

workers, decreased number or delayed development of brood either from direct exposure to pesticide or 

indirectly from reduced brood feeding and maintenance by hive bees, and reduced fecundity and survival 

of queens. Changes in these assessment endpoints are directly related to impacts on protection goals of 

maintaining pollination services, production of hive products and contribution to pollinator biodiversity.
30

Figure 2-1. Conceptual model for risk assessment of foliar spray applications of imidacloprid to honey 

bees. Red depicts systemic pathways. Dashed lines represent routes of exposure that are not considered 

major. 

2.8.2. Soil Application

Exposure of honey bees to imidacloprid via soil applications (e.g., drench, injection, in-furrow sprays and 

chemigation) are expected to follow the same routes of exposure as shown previously with foliar sprays, 

except that contact exposure (on-field and off-field) is not expected to be significant since applications 

are typically made close to soil surfaces where the likelihood of drift is reduced (Figure 2-2). Furthermore, 

the nature of these applications is not expected to result in substantial spray drift to adjacent sites relative 

to foliar sprays. Depending on the timing of rainfall events, there is some potential for exposure via 

imidacloprid runoff and subsequent translocation into plants adjacent to the treated field. Also, given its 

persistence in soil, there is potential for soil applications of imidacloprid to be taken up by rotational plants 

(e.g., cover crops) that are planted after crop harvest. Some of these rotational crops may be attractive 

to bees as sources of pollen and/or nectar (e.g., clover).
31

Figure 2-2. Conceptual model for risk assessment of soil applications of imidacloprid to honey bees. Red 

depicts systemic pathways. Dashed lines represent routes of exposure that are not considered major. 

2.8.3. Seed Treatment 

Potential exposure routes of honey bees to imidacloprid used as seed treatments include pollen, nectar, 

exudates (e.g., guttation fluid), and honey dew resulting from translocation from the seed to growing 

plant tissues (Figure 2-3). Another route of exposure includes contact with abraded seed coat dust during 

planting. The latter pathway has been associated with incidents of honey bee mortality (Pistorius et al.

2009, Forster et al. 2009) and is the focus of considerable research (e.g., Tapparro et al. 2012, Krupke et 

al. 2012). The extent to which honey bees are exposed via contact with abraded seed coat dust is 

determined by many factors including the physico-chemical properties of the seed coating, seed planting 

equipment, use of fluency agents (e.g., talc), environmental conditions (wind speed, humidity), and hive 

location in relation to sowing. Off-site drift of contaminated seed coat dust also may contribute to 

residues on plants, soil, and surface water to which bees may be exposed through direct contact and 

ingestion of surface water, pollen, and nectar. This is further described in Section 2.10 (Measures of 

Exposure). One important attribute of the seed treatment exposure pathway is that exposure to pesticides 

may occur over a wide time scale (e.g., at seed sowing, during plant growth and flowering, and potentially 

at plant harvest from exposure to contaminated plant dust). 

Imidacloprid: Soil Application
32

Figure 2-3. Conceptual model for risk assessment of planting of imidacloprid-treated seeds to honey bees. 

Red depicts systemic pathways. Dashed lines represent routes of exposure that are not considered major. 

2.9. Analysis Plan 

The analysis plan provides a rationale for selecting and omitting risk hypotheses in the actual analysis. As 

with any risk assessment process, the analysis plan also articulates data gaps, the methods used to 

evaluate existing and anticipated data, and the assumptions that will be made where data may be missing. 

The analysis plan also identifies the specific measures of exposure (e.g., estimated environmental 

concentrations; EECs) and effect (e.g., median lethal dose for 50% of the organisms tested; LD50) which 

will be used to develop risk estimates.

2.9.1. Risk Assessment Methodology

For assessing the risks of registered agricultural uses of imidacloprid to bees, this assessment follows the 

Agency’s guidance entitled: “Guidance for Assessing Pesticide Risks to Bees” (USEPA et al. 2014). The risk 

assessment consists of an iterative, tiered process that considers multiple lines of evidence related to 

Imidacloprid: Planting of Treated Seeds
33

exposure and effects of pesticides to bees. The overall risk assessment framework for foliar spray 

applications and soil/seed applications are shown in Figure 2-4 and Figure 2-5, respectively.

Assessing the Potential for Exposure. The first step of this process is to determine whether exposure to 

adult and larval bees is of concern. This determination is made based on information about the application 

methods, application timing, attractiveness of crops to bees, and agronomic practices for the treated 

crops. This process also considers the potential for bees to be exposed both by foraging on the treated 

field (i.e., on-field exposure) and from foraging at sites adjacent to the treated field (i.e., off-field 

exposure). With foliar spray applications of pesticides such as imidacloprid, it is presumed that off-field 

exposure would occur due to spray drift to adjacent areas regardless of the attractiveness or agronomic 

practices pertaining to the treated crop.

Tier I Assessment (Screening-level). The next step in this process is to conduct a Tier I risk assessment 

based on estimated exposure via contact and oral routes and effects on individual bees tested in the 

laboratory. The (EECs) are first calculated at a screening-level using conservative (high end) assumptions 

of potential exposure. For foliar sprays, these screening-level EECs are calculated for both “on-field” and 

“off-field” exposures. The screening-level EECs are then compared to acute and chronic toxicity endpoints 

for adult and larval bees (oral exposure) and acute toxicity endpointsfor adult bees (for contact exposure) 

for the purposes of calculating risk quotients (i.e. the ratio of the EEC to toxicity endpoints). 

Tier I Assessment (Refined). If the screening-level tier I RQ values exceed the acute or chronic risk level 

of concern (LOC), then refinements to the Tier I screening-level RQs are considered. These refinements 

include additional information on the potential exposure of bees to the pesticide, such as field studies 

that quantify the pesticide residue in pollen and nectar of treated crops, i.e. using measured rather than 

estimated exposure levels. The Tier I RQ values are then recalculated using the refined EECs and again 

compared to the acute (0.4) and chronic (1.0) LOCs. If the acute or chronic risk LOCs are again exceeded 

using the refined Tier I, then mitigation options may be considered and/or a higher tier assessment may 

be conducted. 

Tier II Assessment. The Tier II assessment is based on effect studies that characterize pesticide effects at 

the whole-colony level and therefore, reduce uncertainty associated with extrapolating effects on 

individual bees under laboratory conditions (Tier I toxicity studies) to effects on the colony. It is important 

to recognize that Tier II effect studies are conducted under semi-field conditions where the high-end 

exposure at the colony level is generally expected. Often, Tier IIsemi-field studies are conducted in which 

whole colonies are exposed to the pesticide of concern, either in enclosed mesh tunnels or via the diet, 

such as through feeding spiked sucrose. In Tier II studies other stressors may be present and potential 

compensatory mechanisms of the colony may occur. Unlike Tier I, characterization of risk in Tier II does 

not involve the calculation of RQ values per se. Rather, risks at the colony level are usually characterized 

in relation to pesticide application rate and/or measured residue levels in their diet. Interpretation of 

such whole-colony effects studies is often much more complex than Tier I studies, and relies on 

comprehensive considerations of the extent to which adverse effects are likely to occur at the colony 

level. Based on the risks identified at lower-tier assessments, their associated uncertainties, and other 
34

lines of evidence, the risk assessor considers the impact of any risk mitigation options identified for the 

pesticide of concern.

Tier III Assessment. The need for more refined information conducted at the Tier III level is determined 

depending on the nature of the estimated risks, the associated uncertainties, and available risk mitigation 

options. Tier III studies are full-field studies that are designed to mimic actual pesticide applications and 

exposure of bees encountered in the environment. Tier III full field studies are usually highly complex and 

require a high level of effort to design and conduct so as to address specific sources of uncertainties and 

potential risks identified in lower risk assessment tiers. Similar to risk characterization at Tier II, risk 

characterization at Tier III considers multiple lines of evidence available from lower Tiers and other 

information sources (e.g., open literature) that meet the respective Agency’s standard for inclusion in risk

assessments. Risk assessment conclusions are made based on the weight of evidence, available risk 

mitigation options, and uncertainties in the available data and methods. 
35

Figure 2-4. Tiered approach for assessing risk to honey bees from foliar spray applications
36

Figure 2-5. Tiered approach for assessing risk to honey bees from soil/seed applications
37

2.10. Measures of Exposure 

The primary routes of exposure being assessed quantitatively in this assessment are the contact and oral 

routes. These are considered the dominant exposure routes for imidacloprid. Measures of contact 

exposure include the estimated contact dose on a per bee basis (e.g., μg a.i./bee). Contact exposure is 

also incorporated into Tier II semi-field (tunnel) studies; however, it is not quantified on a per bee basis. 

Oral exposure is also determined on a mass a.i. per bee basis and considers ingestion of contaminated 

pollen and nectar. Detailed methods for estimating contact and oral exposure to honey bees are 

described later in Section 4. 

Bees may also be exposed to pesticides via other routes of exposure such as through plant guttation fluid, 

surface water, soil (for ground nesting bees) and drift of abraded seed coat dust. As noted previously, the 

extent to which bees are exposed via plant guttation fluids and surface water is considered uncertain. 

Furthermore, the Agency currently lacks reliable methods for evaluating these exposure routes in a 

quantitative manner (i.e., derivation of Tier I EECs). Therefore, consistent with the Agency’s 2014 risk 

assessment guidance, this risk assessment will focus on quantitative estimates of exposure via contact 

and ingestion of, pollen and nectar only. Although exposure and effects to bees via abraded seed coat 

dust has been documented, obtaining quantitative estimates of this route of exposure is also considered 

highly uncertain. Rather than assess the risks of abraded seed coat dust, the Agency is focusing its 

resources on mitigating risks from this exposure pathway through best management practices and 

working with the regulated community in the development of alternative technologies to reduce dust-off 

during planting (e.g., alternative fluency agents, equipment modifications, etc.)5

 

An additional potential route of exposure that is assessed to some extent but certain available data is the 

carryover of imidacloprid residues in soil from one planting season to another. As will be discussed, 

environmental fate data suggest a high persistence in the soil and to the extent that residue studies allow, 

it will explored the magnitude of residues in pollen and nectar following planting in a field where 

imidacloprid applications were made in previous years.

2.11. Measures of Effects 

The primary species of focus in this risk assessment is the honey bee and reflects the dominant role this 

species maintains in providing managed pollination services for agricultural crops throughout the U.S. It 

also reflects the availability of standardized methods for estimating exposure and effects on A. mellifera. 

As such, this assessment will consider a variety of measures of effects for quantifying risk to honey bees 

which differ according to the level of biological organization being assessed. At the Tier I (organism) level, 

measures of effects include:

• The acute contact LD50 to adult worker bees,

• The acute oral LD50 to adult worker bees

• The chronic (10-d) oral NOAEL6 for adult worker bees, and

 5 http://www2.epa.gov/pollinator-protection/2013-summit-reducing-exposure-dust-treated-seed 6 No Observed Adverse Effect Level
38

• The chronic (21-d) NOAEL for larval bees.

The acute contact and oral endpoints are derived from standardized laboratory toxicity tests conducted 

according to EPA Office of Chemical Safety and Pollution Prevention (OCSPP) and the Organization for 

Economic Cooperation and Development (OECD) guidelines and consider lethality as its primary test 

endpoint, although sublethal effects are commonly noted. Currently, standardized test guidelines do not 

exist for the 10-d adult chronic oral test or the chronic larval test, but draft guidance have been developed 

by the OECD7

. This test measures lethality and food consumption of adult bees during a 10-d oral 

exposure. For larval honey bees, measures of effect at the Tier I level include the acute oral LD50, 

conducted by OECD Test Guideline 237 and the chronic oral NOAEL following draft OECD guidance or 

other testing protocols currently in development. Acute effects on honey bee larvae are based on lethality 

while chronic effects include larval bee mortality and the percent emergence of adult bees following 

pupation. While, the acute LD50 for larval bees is also commonly included as a measure of effect, the

acute data for honey bee larvae were not available for imidacloprid. 

At the Tier II and Tier III levels, measures of effect at the colony level typically include:

• forager bee mortality, 

• fecundity (e.g., eggs production), 

• brood development and survival, 

• hive weigh, strength and survival, 

• foraging activity, and

• the quantity and quality of food provisions. 

These effects may be expressed in terms of a particular pesticide application rate (e.g., lbs. a.i./A) or the 

concentration of the active ingredient in the diet (e.g., μg a.i./L in sucrose). As discussed in USEPA et al. 

2014), other sublethal endpoints such as proboscis extension reflex (PER), histopathological effects, and 

behavior anomalies are not considered as regulatory endpoints by themselves. However, to the extent 

that these effects contribute to impairment of the aforementioned colony level effects, they are indirectly 

incorporated into Tier II and Tier III measure of effect and the ensuing risk assessment.

Although the focus of this risk assessment is on the honey bee, the Agency recognizes that numerous 

other species of bees occur in North America and that these non-Apis bees have ecological and in some 

cases, commercial importance. For example, several species of non-Apis bees are commercially managed 

for their pollination services, including bumble bees (Bombus spp.), leaf cutting bees (Megachile 

rotundata), alkali bees (Nomia melanderi), and blue orchard bees (Osmia lignaria), and the Japanese hornfaced bee (Osmia cornifrons). Importantly, a growing body of information indicates native bees play an 

important role in crop and native plant pollination, besides their overall ecological importance via 

maintaining biological diversity. Although standard methods are currently not available to quantitatively 

assess exposure and effects to non-Apis bees, this assessment will include data on the effects of 

imidacloprid to non-Apis bees and qualitatively assess risks to non-Apis bees. 

 7 Available at: 

http://www.oecd.org/chemicalsafety/testing/Draft_GD_honeybee_larval_tox_repeated_exposure_25_February_2014.pdf
39

3. Use Characterization 

As noted in the problem formulation, Imidacloprid is registered for the control of sucking insects on a 

large variety of agricultural and non-agricultural sites, including vegetable crops, tree nuts, tree fruits, 

stone fruits, cotton, tobacco, grapes, citrus, turf, and ornamentals. Target pests include aphids, thrips, 

whiteflies, termites, turf insects, soil insects and some beetles. Imidacloprid formulations are available as 

wettable powder, granular, seed dressing (flowable slurry concentrate), and soluble concentrate. 

Overall agricultural use of imidacloprid includes a large component as a seed treatment where 

approximately 520,000 pounds are used, and main seed-treatment uses include soybean, followed by 

cotton, then corn and potato. Use of imidacloprid appears to have increased, where approximately 5 

million acres received an imidacloprid treatment in 1998, and approximately 30 million acres received an 

imidacloprid treatment in 2012. Part of this usage increase (as a foliar or soil treatment) has occurred on 

a number of specialty crops such as on apples, carrots, cauliflower, cherries; other usage increase, as a 

seed treatment, has occurred on crops such as soybean and wheat. 

3.1. Agricultural Uses 

Table 3-1 shows the maximum application rates and maximum number of applications for the different 

crops for imidacloprid with foliar applications, as well as other labeled use information. Each of the tables 

provides additional comments where there are caveats to the federal labels. Table 3-2 shows use 

information for the different crops for imidacloprid with soil applications, and Table 3-3 shows use 

information for different crops for seed-treatment applications.

It is noted that several crops have restrictions on applications made during the pre-bloom and bloom 

period. These include use patterns that have either a pre-bloom interval associated with them or prohibit 

applications made pre-bloom, during bloom or when bees are foraging (i.e. only post-bloom applications 

are permitted). 

Those use patterns and associated application methods that require a 10-day pre-bloom interval include:

• Foliar applications to strawberries

• Foliar applications to citrus fruits

Those use patterns and associated application methods that prohibit applications made during the prebloom or during bloom period, or when bees are foraging include:

• Soil applications to strawberries (annual and perennial varieties)

• Soil and foliar applications to bushberries (e.g. blueberry)

• Soil and foliar applications to caneberry (e.g. blackberry and raspberry)

• Soil (containerized) applications to citrus fruits

• Soil and foliar applications to coffee

• Soil applications to cranberry
40

• Soil and foliar applications to pome fruits

• Soil and foliar applications to stone fruits

• Soil and foliar applications to tropical fruits

• Soil and foliar applications to tree nuts

3.1.1. Foliar Applications

Table 3-1. Summary of labeled use information for foliar applications of imidacloprid

Crop Group 

(Use Pattern)

Max. Single 

Appl. Rate 

(lbs a.i/A)

Max # 

of Appl. 

Appl. 

Interval

Annual 

total (lbs 

a.i/A)

Appl. Method Appl. Timing Comment

1 (Potato) 0.05 4 7 0.2 Ground/Aerial

From emergence 

to 7 days prior to 

harvest

--

1(Tuberous 

and corm 

vegetables

0.04

3

(1 only 

on 

radish)

5 0.13 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

4A (Leafy 

greens 

vegetables)

0.046 5 NA 0.23 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

5 (Brassica 

(Cole) Leafy 

vegetables)

0.046 5 5 0.23 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

6 (Legume 

vegetables 

(except 

soybean)

0.04 3 7 0.13 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

6 (Soybeans) 0.05 3 7 0.14 Ground/Aerial

At bloom to 21 

days prior to 

harvest

--

8 (Fruiting 

vegetables) 0.08 3 5 0.24 (per 

season) Ground/Aerial

After planting up 

to 0 days prior to 

harvest.

--

10 (Citrus

Fruits) 0.25 2 10 0.5 Ground/Aerial

Anytime up to 0 

days prior to 

harvest.

--

11 (Pome 

fruits) 0.25 2 10 0.5 Ground/Aerial

Anytime up to 7 

days prior to 

harvest.

0.25 lbs./A is 

only for pear, 

other crops 

have max. of 

0.1 lbs./A

12 (Stone 

fruits) 0.10 5 7 or 10 0.5 Ground/Aerial

Anytime up to 0-7 

days prior to 

harvest.

Yearly 

maximum 0.3 

lbs./A for 

Apricot, 

Nectarine, 

Peach; 0.5 

lbs./A for 

Cherry, Plum, 

Plumcot, 

Prune.
41

Crop Group 

(Use Pattern)

Max. Single 

Appl. Rate 

(lbs a.i/A)

Max # 

of Appl. 

Appl. 

Interval

Annual 

total (lbs 

a.i/A)

Appl. Method Appl. Timing Comment

13A 

(Caneberry) 0.1 3 7 0.3 Ground/Aerial

After bloom up to 

3 days prior to 

harvest.

13B 

(Bushberry) 0.1 5 7 0.5 Ground/Aerial

After bloom up to 

3 days prior to 

harvest.

--

13 (Grape) 0.1 1 14 0.1 Ground/Aerial

Anytime up to 30 

days prior to 

harvest.

--

13 

(Strawberry) 0.047 3 5 0.14 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

14 (Tree nuts) 0.10 3 6 0.36 Ground/Aerial

Anytime up to 7 

days prior to 

harvest.

--

19A (Herbs) 0.04 3 5 0.13 (per 

season) Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

20 (Cotton) 0.06 5 7 0.31 Ground/Aerial

40 days after 

planting up to 14 

days prior to 

harvest

--

No group 

(Banana and 

plantain)

0.1 5 14 0.5 Ground/Aerial

Anytime up to 0 

days prior to 

harvest.

--

No group 

(Coffee) 0.1 5 7 0.5 Ground/Aerial

Anytime up to 7 

days prior to 

harvest.

--

No group 

(Globe 

artichoke)

0.126 4 14 0.5 Ground/Aerial

After planting up 

to 7 days prior to 

harvest.

--

No group 

(Hops) 0.10 3 21 0.3 Ground/Aerial

Anytime up to 28 

days prior to 

harvest.

--

No group 

(Peanut) 0.04 3 5 0.13 Ground/Aerial

From emergence 

to 14 days prior 

to harvest

--

No group 

(Pomegranate) 0.10 3 7 0.3 Ground/Aerial

Anytime up to 7 

days prior to 

harvest.

--

No group 

(Tobacco) 0.05 5 7 0.28 Ground/Aerial

From emergence 

to 14 days prior 

to harvest

--

No group 

(Tropical fruit) 0.10 5 10 0.5 Ground/Aerial

Anytime up to 7 

days prior to 

harvest.

--

NA = not applicable; lbs a.i./A = pounds of active ingredient/acre
42

3.1.2. Soil Applications

Table 3-2. Summary of labeled use information for soil applications of imidacloprid

Crop Group 

(Use Pattern)

Max. 

Single 

Appl. 

Rate 

(lbs 

a.i/A)

Max 

# of 

Appl. 

Appl. 

Interval

Annual 

total 

(lbs 

a.i/A)

Appl. Method Appl. Timing Comment

1 (Sugar beet) 0.18 1 NA 0.18 In-furrow Prior to or at planting --

1B (Root 

vegetables) 0.38 1 NA

0.38 

(per 

season)

In-furrow / band 

/ chemigation

At or after planting 

up to 21 days prior to 

harvest.

--

1 (Potato) 0.31 1 NA 0.31

in-furrow / band 

/ subsurface 

side-dress

At Planting --

1 (Tuberous 

and corm 

vegetables)

0.38 1 NA

0.38 

(per 

season)

In-furrow / 

shank / sidedress

At or after planting 

up to 3 days prior to 

harvest.

--

3 (Bulb 

Vegetables) 0.5 1 NA 0.5 (per season)

In-furrow / band 

/ chemigation / 

drench

Prior to, at, or after 

planting up to 21 

days prior to harvest.

Generally applied at 

planting for greatest 

benefit

4A (Leafy 

greens 

vegetables)

0.38 1 NA

0.38 

(per 

season)

In-furrow / band 

/ chemigation / 

drench

At or after planting 

up to 21 days prior to 

harvest.

--

4B (Leafy 

petiole 

vegetables)

0.38 1 NA

0.38 

(per 

season)

In-furrow / 

band/ 

chemigation / 

drench

At or after planting 

up to 45 days prior to 

harvest.

--

5 (Brassica 

(Cole) leafy 

vegetables)

0.38 1 NA

0.38 

(per 

season)

In-furrow / band 

/ chemigation 

/drench

At or after planting 

up to 21 days prior to 

harvest.

--

6 (Legume 

vegetables 

(except 

soybean)

0.38 1 NA

0.38 

(per 

season)

In-furrow / 

band/ 

chemigation / 

drench

At or after planting 

up to 21 days prior to 

harvest.

--

8 (Fruiting 

vegetables) 0.5 1 NA 0.5 (per 

season)

In-furrow/band/ 

chemigation / 

drench

At or immediately 

following planting

- 0.5 lbs./a for pepper 

and okra, 0.38 lbs./a 

for other crops

9 (Cucurbit

vegetables) 0.38 1 NA 0.38

In-furrow /band 

/chemigation 

/drench

At Planting --

10 (Citrus

fruits) 0.5 1 NA 0.5 Chemigation / band/drench

Anytime up to 0 days 

prior to harvest. --

11 (Pome 

fruits) 0.38 1 NA 0.38 Chemigation Anytime up to 21 days prior to harvest. --

12 (Stone 

fruits) 0.38 1 NA 0.38 Chemigation Anytime up to 21 days prior to harvest. --

13A 

(Caneberry) 0.5 1 NA 0.5 Chemigation /drench

After bloom up to 7 

days prior to harvest. --

13B 

(Bushberry) 0.5 1 NA 0.5 Chemigation / band

After bloom up to 7 

days prior to harvest. --

13 (Grape) 0.5 1 NA 0.5

Chemigation / 

sidedress/drench

Anytime up to 30 

days prior to harvest. --
43

Crop Group 

(Use Pattern)

Max. 

Single 

Appl. 

Rate 

(lbs 

a.i/A)

Max 

# of 

Appl. 

Appl. 

Interval

Annual 

total 

(lbs 

a.i/A)

Appl. Method Appl. Timing Comment

13 (Cranberry) 0.5 1 NA 0.5 Chemigation / 

direct app.

Anytime up to 30 

days prior to harvest. --

13 (Strawberry 

(annual and 

perennial)

0.5 1 NA 0.5 (per 

season)

Chemigation / 

band

Prior to, at, or after 

planting up to 14 

days prior to harvest.

--

13-07G 

(Strawberry 

(perennial and 

post-harvest)

0.38 1 NA 0.38 Chemigation / 

band

During renovation up 

to 14 days prior to 

harvest.

--

14 (Tree nuts) 0.50 1 NA 0.5

Chemigation / 

sidedress/drench

Anytime up to 7 days 

prior to harvest. --

19A (Herbs) 0.38 1 NA

0.38 

(per 

season)

in-furrow / 

shank / drench 

/chemigation

At or after planting 

up to 14 days prior to 

harvest.

--

20 (Cotton) 0.33 1 NA 0.33 In-furrow / band 

/chemigation At Planting

Labels allow both at 

planting AND foliar 

applications

No group 

(Banana and 

plantain)

0.5 1 NA 0.5 Chemigation Anytime up to 0 days 

prior to harvest. --

No group 

(Coffee) 0.5 1 NA 0.5

Chemigation / 

sidedress/drench

Anytime up to 7 days 

prior to harvest.

- Basal treatment 

available on 264-827, 

264-758)

No group 

(Globe 

artichoke)

0.5 1 NA 0.5 In-furrow 

/chemigation

Prior to, at, or after 

planting up to 7 days 

prior to harvest.

--

No group 

(Hops) 0.3 1 NA 0.3

Chemigation / 

sidedress/drench

Anytime up to 60 

days prior to harvest. --

No group 

(Peanut) 0.38 1 NA 0.38 In-furrow / chemigation At Planting --

No group 

(Pomegranate) 0.50 1 NA 0.5 Chemigation Anytime up to 0 days prior to harvest. --

No group 

(Tobacco) 0.04 NA 0.5

In-furrow / tray 

drench 

/chemigation

Prior to or At 

Planting

Single app rate is based 

on lbs. a.i/1000 plants. 

Optimum plant 

population = 6200 to 

7200 plants per acre.

No group 

(Tropical fruit) 0.50 1 NA 0.5 Chemigation

Anytime during the 

year up to 6 days 

prior to harvest.

--

NA = not applicable; lbs a.i./A = pounds of active ingredient/acre

3.1.3. Seed Treatments
44

The maximum single application rate in (lbs a.i/A) was estimated based on the amount of product applied 

to seeds coupled with the number of seeds planted per acre. The number of seeds per acre were either 

provided or calculated from parameters listed in Acres Planted per Day and Seeding Rates of Crops Grown 

in the United States. (US EPA, March 24, 2011).

Table 3-3. Summary of labeled use information for seed treatment applications of imidacloprid

Crop Group (Use pattern)

Max. Single 

Appl. Rate 

(lbs a.i/A)

Comment1

1A (Sugar beet) 0.293

Calculated from labeled application rate of 0.2 lbs a.i/2.2 lbs 

seed. Application rate = ((1/2.2) * 0.2 * 3.24)). 3.24 is the 

number of pound of seed per acre

1A, 1B (Carrot) 0.036

Calculated from labeled application rate of 0.003 lbs a.i/lbs seed 

and lbs of seed/acre (11.95). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

1C (Potato) 0.50

Calculated from labeled application rate of 0.001 lbs a.i/lbs seed. 

As the rate that was calculated from number of seeds per acre 

and number of seeds per pound exceeded the maximum single 

application rate (in lbs a.i/A) of all imidacloprid uses, the rate was 

capped at 0.5 lbs a.i/A.

03-07A, 03-07B 

(onions/leeks/scallions) 0.15

Calculated from labeled application rate of 0.002 oz./1000 seed 

and lbs seed per acre. Rate = ((0.002 / 16) * 1,229,929 seeds / 

1000))

5A (Broccoli) 0.18

Calculated from labeled application rate of 0.014 oz./1000 seed 

and lbs seed per acre. Rate = ((0.014 / 16) * 210,845 seeds / 

1000))

6A (Soybean) 0.210

Calculated from labeled application rate of 0.0013 lbs a.i/lbs seed 

and lbs of seed/acre (167). Pounds of seed/acre calculated from 

number of seeds per acre and number of seeds per pound.

6 (Beans and peas) 0.50

Not permitted in California. Calculated from labeled application 

rate of 0.001 lbs a.i/lbs seed. As the rate that was calculated 

from number of seeds per acre and number of seeds per pound 

exceeded the maximum single application rate (in lbs a.i/A) of all 

imidacloprid uses, the rate was capped at 0.5 lbs a.i/A

15 (Barley) 0.130

Calculated from labeled application rate of 0.00094 lbs a.i/lbs 

seed and lbs of seed/acre (138). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

15 (Buckwheat)

0.017

Calculated from labeled application rate of 0.00023 lbs a.i/lbs 

seed and lbs of seed/acre (72). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

15 (Corn, field) 0.118

Calculated from labeled application rate of 0.004 lbs a.i/lbs seed 

and lbs of seed/acre (29.57). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

15 (Corn, pop) 0.056

Calculated from labeled application rate of 0.003 lbs a.i/lbs seed 

and lbs of seed/acre (22.04). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

15 (Corn, sweet) 0.189

Calculated from labeled application rate of 0.006 lbs a.i/lbs seed 

and lbs of seed/acre (31.52). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

15 (Millet) 0.12

Calculated from labeled application rate of 0.004 lbs a.i/lbs seed 

and lbs of seed/acre (30). 

15 (Oats) 0.081 Calculated from labeled application rate of 0.0009 lbs a.i/lbs seed and lbs of seed/acre (90). 

15 (Rye) 0.436

Calculated from labeled application rate of 0.004 lbs a.i/lbs seed 

and lbs of seed/acre (109).
45

Crop Group (Use pattern)

Max. Single 

Appl. Rate 

(lbs a.i/A)

Comment1

15 (Sorghum) 0.092

Calculated from labeled application rate of 0.004 lbs a.i/lbs seed 

and lbs of seed/acre (23). 

15 (Wheat) 0.176

Calculated from labeled application rate of 0.0009 lbs a.i/lbs seed 

and lbs of seed/acre (188). Pounds of seed/acre calculated from 

number of seeds per acre and number of seeds per pound.

15 (Triticale) 0.10

Calculated from labeled application rate of 0.0009 lbs a.i/lbs seed 

and lbs of seed/acre (109). 

19B (Mustard) 0.070 Calculated from labeled application rate of 0.01 lbs a.i/lbs seed 

and lbs of seed/acre (7).

20 (Canola/Rape) 0.082

Calculated from labeled application rate of 0.01 lbs a.i/lbs seed 

and lbs of seed/acre (8.23). Pounds of seed/acre calculated from 

number of seeds per acre and number of seeds per pound.

20 (Cotton) 0.095

Calculated from labeled application rate of 0.005 lbs a.i/lbs seed 

and lbs of seed/acre (18.89). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound.

20 (Sunflower) 0.02

Calculated from labeled application rate of 0.005 lbs a.i/lbs seed 

and lbs of seed/acre (4). Pounds of seed/acre calculated from 

number of seeds per acre and number of seeds per pound.

20 (Safflower) 0.175

Calculated from labeled application rate of 0.005 lbs a.i/lbs seed 

and lbs of seed/acre (35). Pounds of seed/acre calculated from 

number of seeds per acre and number of seeds per pound.

No group (Peanuts) 0.141

Calculated from labeled application rate of 0.00062 lbs a.i/lbs 

seed and lbs of seed/acre (228). Pounds of seed/acre calculated 

from number of seeds per acre and number of seeds per pound. 1

Number of seeds per acre either provided or calculated from parameters listed in “Acres Planted per Day and Seeding Rates of Crops Grown in 

the United States.” (US EPA, March 24, 2011).

3.1.4. Multiple Application Types (e.g. combinations of seed, soil, and/or foliar)

As indicated above, the maximum annual application rate for several use patterns of imidacloprid is 0.5 

lbs a.i/A. Several use patterns stipulate that a variety of application methods (i.e. foliar, soil, and seed 

treatment) can be used so as long as the applied rate does not exceed 0.5 lbs a.i/year. As will be discussed, 

there are residue studies available for combined application methods of soil + foliar treatments and seed 

+ foliar treatments.
46

4. Exposure Assessment 

Imidacloprid is a systemic insecticide that is associated with multiple use patterns. Exposure of bees to 

imidacloprid is determined by many factors that are expected to affect the concentration of the chemical 

in plant parts visited by bees. Two main procedures are used for applying this pesticide: soil and foliar 

applications. 

(1) Direct Soil application including in-furrow, drench, chemigation (through drip irrigation), band, shank 

injection and planting treated seeds: These types of applications deliver most of the pesticide mass 

into the soil system with the potential for relatively low amount of drift such as seed drilling dust; and 

(2) Foliar application by ground and air equipment. These types of applications deliver the pesticide on

to the plant foliage (target) with a percentage being deposited or drifting to the soil upon application 

(also, later from plant wash-off). 

Figure 4-1 depicts important processes governing exposure of bees to imidacloprid through the plant. In 

this figure, it is assumed that imidacloprid alone is systemic as it is uncertain if any of imidacloprid 

metabolites are systemic.

Figure 4-1. Imidacloprid application and processes involved in bee exposure

 Soil System

Imidacloprid

Pool for Plant 

Foliage Uptake Foliar Applied

Drifted

Run-Off

Imidacloprid

Pool for Plant 

Root Uptake

Flowers

Carried-over/Accumulated
47

As shown in Figure 4-1, determining the extent of bee exposure to imidacloprid through the plant requires 

knowledge of the source and movement of the compound from the point of application into the point of 

plant entry (roots and/or foliage). To help in the analysis, two virtual imidacloprid pools are assumed: a

pool for plant root up-take and a pool for plant foliage up-take. In case of the “root up-take pool”, sources 

of imidacloprid are from direct soil application including seed treatment, indirect spray following foliar 

application, in addition to that reaching the soil later by plant wash-off. Other sources could be 

imidacloprid carried-over from applications to previous rotational crop(s) (accumulated in the soil and/or 

added from treated plant material left in the field after harvest). In case of the “foliage up-take pool”, the 

source of imidacloprid is from direct foliar application noting that part of the foliage-applied chemical is 

expected to reach the soil during application and later through wash-off. The dynamic nature of these 

two virtual pools is expected because chemical species present in these pools and their concentrations 

will vary with time following application. Therefore, it is important to understand factors that govern the 

characteristics of these imidacloprid virtual pools including important factors such as: mode of application 

(e.g., soil, foliar), procedure (e.g., ground, aerial), rate, and timing in relation to the crop growth stage;

and, imidacloprid physical/chemical and fate and transport properties(solubility, mobility and persistence 

in the soil system in case of the “root up-take pool” or persistence within the plant foliage in case of the 

“foliage up-take pool”). Furthermore, it is equally important to understand factors related to crop 

including: root and foliage up-take properties, imidacloprid physical distribution, and metabolism within 

various plant parts.

Ahead of this discussion, it is noted that this exposure section includes several studies that characterize 

the residues of imidacloprid and its metabolites in plant parts other than those frequented by pollinating 

insects, such as honey bees. While these studies do not allow for the assessment of residues in the pollen 

and nectar for a given crop, they will be used to provide further characterization of exposure in terms of 

availability of resides in plant parts (e.g. stems, leaves, fruits) available for consumption by other taxa such 

as birds and mammals in the subsequent assessment for imidacloprid expected by the end of 2016.

4.1. Physical/chemical and fate and transport properties 

Table 4-1 contains a summary of the chemical profile of imidacloprid. These data indicate that 

imidacloprid is highly soluble with low vapor pressure and Henry’s Law Constant. These properties suggest 

that the chemical will be readily soluble for movement with water and that it is unlikely to volatilize to a 

meaningful degree. Furthermore, the Kow for imidacloprid is low, and this property along with the high 

solubility are known attributes ofsystemic pesticides although the systemic nature of the pesticide should 

be based on residue and fate analyses to reduce uncertainties (Bonmatin et al, 2015).
48

4.1.1. Physical/Chemical properties

Table 4-1 contains a summary of the chemical profile of imidacloprid.

Table 4-1. Chemical profile of imidacloprid

Property Value

Chemical Structure: Name

1-(6-chloro-3-pyridin-3-ylmethyl)-N-nitroimidazolidin-2-

ylidenamine

Molecular Formula C9H10ClN5O2

Molecular Weight (CAS No.) 255.7 g/mole (13826-41-3)

Water Solubility @ 20 oC 580 mg/L (ppm)

Octanol: Water Coefficient Kow 3.7 @ 21 oC

Vapor pressure (Henry’s Law 

Constant) 1.5 x 10-9 torr (9.9 x 10-13 atm m3 mol-1 @ 20 oC

Data in Table 4-1 indicate that imidacloprid is highly soluble with low vapor pressure and Henry’s Law 

Constants. These properties suggest that the chemical will be readily soluble for movement with water 

and that it is unlikely to volatilize to a meaningful degree. Furthermore, the octanol: water coefficient 

(Kow) for imidacloprid is low, and this property along with the high solubility are known attributes of

systemic pesticides. 

4.1.2. Environmental Fate and Transport Properties

The environmental fate and transport characteristics of imidacloprid are summarized in Table 4-2. These 

data suggest that the compound is relatively stable to multiple routes of degradation other than 

photolysis in water and is therefore likely to be persistent in soils. Given the persistence in soil and the 

mobility of imidacloprid, the compound has the potential to leach into ground water and to run-off into 

surface waters for extended periods of time depending on soil and climatic conditions. 

Table 4-2. Fate and transport properties for imidacloprid

 

 Imidacloprid

N+ O

N

CH NH Cl 2 N

ON

Property Values MRID Reference

Hydrolysis t 1⁄2 Stable @ pH 5, 7 and hydrolyzed slowly (Extrapolated 

t 1⁄2= 355 d) in sterile alkaline solutions @ pH 9 420553-37

Environmentally 

Relevant Direct 

Aqueous 

Photolysis t 1⁄2

(Two hours 

study)

0.2 days

Major Metabolites: Guanidine or desnitro compound (NTN-38014): Max 17% 

and urea compound (NTN-33519): Max 10% @ End of study= EOS

Additionally, three major un-knowns reached Maximums of 8-13% @ EOS.

Minor Metabolites: Several un-knowns with a total Max of 13% @ EOS

Important Notes:

(1) UV spectra of the chemical has a maximum absorption at 269 nm, therefore 

degradation by sunlight is expected

422563-76
49

(2) Under natural sunlight, in a dilute aqueous solution in the greenhouse: 60%

of the chemical degraded within 4 hours supporting the results of the study

Environmentally 

Relevant Soil 

Photolysis t 1⁄2 

(15-d study)

171 days in a sandy loam soil from Kansas (pH= 5.2; O.C= 1.4% and CEC= 22 

meq/100 g)

Major Metabolites: None

Minor Metabolites: 5-hydroxy compound (WAK-4103): Max 6%; Nitosimine 

compound (WAK-3839): Max 1% and a mixture of urea compound (NTN33519) and Olefin compound (NTN-35884): Max 3%; and 6-Cloronictonic acid: 

Max 2%; All Maximums @ EOS. Additionally, two unidentified reached 

Maximums of >>5% @ EOS

Un-extracted Residues (UER): Max 11% @ EOS

422563-77

Aerobic soil t 1⁄2 

@ 20 ± 2 oC 

(End of study 

“EOS”= 366 day; 

Pyridinyl14Cmethylene 

imidacloprid)

>>Year (Parent reached only 71% @ EOS) in a sandy loam soil from Kansas (pH= 

4.8; O.C= 1.4% and CEC= 16 meq/100 g). 

Note: Levels of metabolites were insufficient to permit their identification 

(Needed 20x to 100x the rate) 

Major Metabolites: None

Minor Metabolites: Olefin compound (NTN-35884), WAK-4230-1, Nitosimine 

compound (WAK-3839), Guanidine or desnitro compounds (NTN-33014) and 

the two isomers of 5-keto-urea compounds. Additionally, one unidentified 

reached Maximums of nearly 1% @ EOS

Un-extracted Residues (UER): Max 10-15% @ 30 d-EOS after additional reflux 

extraction yielding parent

Mineralization to CO2: Max 7.4% @ EOS

420735-01

Aerobic soil t 1⁄2 

@ 20 ± 2 o

C 

(End of study 

“EOS”= 100 day; 

Pyridinyl14Cmethylene 

imidacloprid)

289 d (Extrapolated value because parent reached 71% @ EOS) in BBA 2.2, a 

loamy sand soil from Germany (pH= 5.5; O.C= 2.2% and CEC= 10 meq/100 g). 

Note: Levels of metabolites were insufficient to permit their identification 

(Needed 20x to 100x the rate) 

Major Metabolites: None

Minor Metabolites: Same as in the soil, above

Mineralization to CO2: Max 10% @ EOS

Un-extracted Residues (UER): Max 13-16% @ 30 d- EOS after additional reflux 

extraction yielding parent Un-extracted Residues (UER): Max 13-16% @ 30 dEOS after additional reflux extraction yielding parent

452393-01

Aerobic soil t 1⁄2 

@ 22 ± 2 o

C 

(End of study 

“EOS”= 100 day; 

Pyridinyl14Cmethylene 

imidacloprid)

210 d (Extrapolated value because parent reached 75% @ EOS) in Hoefchen, a 

loamy soil from Germany (pH= 5.3; O.C= 1.2% and CEC= 11 meq/100 g). 

Major Metabolites: None

Minor Metabolites: Several metabolites occurred at very low levels: total= 7% 

(not identified nor quantified)

Mineralization to CO2: Max 6.4% @ EOS

Un-extracted Residues (UER): Max 11-13% @ 35 d- EOS after additional reflux 

extraction yielding parent

452393-02

Aerobic soil t 1⁄2 

@ 22 ± 2 oC (End 

of study “EOS”= 

366 day; 

Pyridinyl14Cmethylene 

imidacloprid)

>Year (Parent reached 73% @ 125 days to EOS) in Monheim 1, a sandy loam 

soil from Germany.

Major Metabolites: None

Minor Metabolites: None were tracked, if any

Mineralization to CO2: Max 5% @ EOS

Un-extracted Residues (UER): Max 12-22% @ 100- EOS after additional reflux 

extraction yielding parent

452393-03 

Terrestrial Field 

Dissipation 

All studies were unacceptable

GA: 422563-79

MN: 422563-80

CA: 422563-81

Koc ( (L Kg -1) Parent 425208-01
50

Persistence

Available environmental fate data suggest that the main route of imidacloprid, transformation in 

terrestrial ecosystems, is abiotic aqueous photolysis with very low metabolism in the aerobic soil. 

Aerobic soil transformation of imidacloprid is expected to be slow with degradation half-lives ranging from 

305 to >2,000 days and 90th percentile t1⁄2 of 1,669 days (n=4). Based on this route of degradation alone, 

imidacloprid is expected to be highly persistent in the soil system. This persistence in soils may lead to 

accumulation over time with repeated applications. For example, if it is assumed that imidacloprid 

dissipates in the soil by aerobic soil metabolism alone with the shortest half-life of 305 days or the 90th

percentile half-life of 1,669 days, an accumulation of about five times the yearly rate is possible, with the 

long half-life, within 10 years of repeated yearly applications (Figure 4-2). In reality, the magnitude of 

accumulation is expected to be highly affected by other important routs of dissipation including: leaching, 

run-off and plant up-take which is expected to reduce this accumulation. 

Figure 4-2. Imidacloprid expected accumulation in 10 years of repeated application at a rate of 0.5 lb. 

a.i/A (This rate is equal to 0.245 ppm distributed on the top 6” of the soil); Noting that the graph was 

constructed based on yearly application coupled with daily degradation based on the aerobic soil 

Average= 318 (n=5) with a range from 277 to 411 L Kg -1 in soils differing in 

texture (sand, loamy sand, silt loam “replicated” and loam), cation exchange 

capacity (4-16 meq/100 g), organic carbon content (0.4-2.6%) and pH (4.5-6.5); 

Found no relation with O.C, Clay or pH

Guanidine Compound (a metabolite)

Average= 742 (n=4) with a range from 327 to 942 in the same soils used for the 

parent, above

and 

425208-02
51

degradation rate constant.

Photo-degradation may occur on soil surfaces in the case of direct/indirect soil application and on foliage 

in the case of foliar application. Photolysis on soil data suggest that dissipation of imidacloprid through 

this process is expected to be slow (t1⁄2 = 171 days). In contrast, aqueous photolysis is expected to be a 

significant process for imidacloprid transformation on wet foliage. The significance of this process is 

dependent on the presence of light and moisture (rain and/or irrigation) on foliage and factors that 

determine how much of the chemical is taken up by the plant (application rate, formulation, tank mixes, 

timing, application procedure and plant foliage density/characteristics) and for how long (affected by 

plant wash-off by rain and/or irrigation). Although many factors are required for dissipation of the 

chemical by aqueous photolysis, laboratory data suggest that abiotic photolysis may play an important 

role in imidacloprid dissipation (t1⁄2 = 0.2 days). Unfortunately, this parameter was not measured in the 

field although measurement of imidacloprid decline and formation of the urea metabolite on foliage may 

be used as an indication of the importance photolysis. These factors will be examined later in this 

assessment.

For persistence of imidacloprid in the field, available terrestrial field dissipation studies are all classified 

as invalid for several reasons including application rates not confirmed, and metabolites were not tracked. 

However, it could be stated that the chemical showed relative stability in the field. Additionally, available 

rotational crop studies confirmed occurrence of soil carry-over from application to one crop to the 

following crop based on data obtained for magnitude of residues in rotational crops (MRIDs: 432459-01 

and 440637-01). In these studies, detectable residues of imidacloprid were found in variable quantities in 

rotational crops planted after 1, 4, 8 and 11 months rotational intervals following a single granular 

application of 0.29-0.32 lb. a.i/A. Measured average residues of imidacloprid plus its metabolites (parent 

plus metabolites containing the 6-chloropyridinyl moiety) were observed in California: wheat 

forage/straw (0.12-0.19 (ppm), turnip tops (0.58 ppm), and spinach leaves (0.32 ppm) all planted-back 

after 8 months. It is noted however, that residues were much lower in other parts of the plant such as 

roots and grain (e.g., grains: <0.05 ppm) and that the magnitude of residues varies within a given crop 

depending on the planting location (i.e., CA vs. KS or MS). It is noted that the list of degradates containing 

the 6-chloropyridinyl moiety includes the two degradates of concern (IMI-olefin and IMI-5-OH) plus 

guanidine, 4-5-hydroxy, nitrosimine and urea compounds.

With the exception of the soil applied blueberry residue study (MRID 495356-02), field soil residues data 

were not obtained for studies conducted to measure pollen, nectar and/or leaves residues in several other 

crops. In the blueberry study, imidacloprid residues (imidacloprid, IMI-olefin, and IMI-5-OH) were 

measured in three locations (Site 1: Loam soil in NY, Site 2: Silt loam soil in IL and Site 3: Sand soil in MI). 

In each site, nine samples, from the top 6” of the soil, were analyzed (after the first application of 0.50 lbs 

a.i/A) at two separate sampling intervals (245 and 361 days after the 1st application at site 1 and 275 and 

357 days after the 1st application at site 2). At 366, 360 and 366 days after the 1st application a 2nd

application of 0.50 lbs. a.i/A was applied to sites 1, 2 and 3, respectively. After this 2nd application, the 

same scheme of sampling/ analyses were performed after 588, 611 and 608 days following the 1st

application at sites 1, 2 and 3, respectively (Appendix H). Parent was the major constituent of the tracked 

residues (on the average 72% of the applied after nearly a year following the 1st application at sites 1 and 
52

2). The IMI-olefin metabolite constitutes 2 to 5% of the applied after same period at site 1 and 2. Within 

the year after the first application, each of the three sites received a second application of 0.5 lbs a.i./A. 

Residue analyses was not performed just before and just after this 2nd application but rather within a year 

after the application (nearly two years after the 1st application). Again, residue data indicate that parent 

was the major constituent of the tracked residues (on the average 50, 69 and 48% of total applied “% of 

the 1st plus the 2nd applications” after nearly two years following the 1st application at sites 1, 2, and 3, 

respectively). The IMI-olefin metabolite constitutes 2, 5 and 2% of the applied after same period at sites 

1, 2 and 3, respectively. The IMI-5-OH metabolite was not detected at sites 1 and 2 within the first year 

from the first application but was sporadically detected, after two years in the three sites, at very low 

level (<0.4% of the total two applications). A wide range of concentrations were observed and some were 

even higher than what is expected from the amount applied (further details in Appendix H). This may be 

a reflection of the small width of the band application (18” on each side) in relation to the larger area of 

sampling (100 x 200 ft and 200 x 400 ft). However, the large number of samples may reflect the real 

concentration present resulting from application followed by dissipation (degradation and movement). 

Mobility

Based on laboratory batch equilibrium studies, parent imidacloprid is expected to be moderately mobile 

(Koc = 318 L Kg -1

, n=5; FAO Classification). Persistence/mobility data suggest that imidacloprid has the 

potential to leach into groundwater and to move into surface waters through run-off for long periods of 

time. The mobility of imidacloprid was confirmed in the field by two prospective ground water (PGW) 

studies. One of the studies was conducted in Montcalm County, Michigan (0.34 lbs. a.i/A to potatoes; 

MRID 458582-01) and the other in Monterey County, California (0.45 lb. a.i/A to broccoli; 458787-01). In 

both studies, the registrant monitored for imidacloprid parent, imidacloprid guanidine, imidacloprid 

olefin, and imidacloprid urea in the vadose zone (area between ground surface and where groundwater 

is at atmospheric pressure) and in shallow ground water. In both studies, the predominant compound 

detected in soil, soil-pore water throughout the vadose zone, and in ground-water (when detectable) was 

parent imidacloprid. Of the three degradates analyzed for (guanidine, olefin, and urea compounds) only 

the urea compound leached at concentrations that were frequently detectable in the shallow ground 

water. It was noted that detections in ground water (i.e., breakthrough) started after 500 days from 

application and continued five years after application. Residues of imidacloprid in ground water were 

most frequently observed under use conditions which promoted greater ground-water recharge and/or 

when imidacloprid was used in multiple growing seasons at the same site.

Degradation Profile

Based on various laboratory fate studies (Table 4-2), abiotic direct photolysis appears to be the major 

degradation pathway for imidacloprid. In contrast, the chemical is expected to resist biotic metabolism in 

the aerobic soil. Based on this data, Figure 4-3 is suggested to represent the degradation pathways for 

imidacloprid in terrestrial ecosystems.
53

Maximum: 17%

 

Maximum: 10%

Maximum: 6%

Maximum: 

2%

 

(Maximums for Metabolite, above: <1-2%)

+UNE (Un-Extracted Residues)

(10-16%)

+ CO2 (6-10%)

Figure 4-3. Expected degradation profile for imidacloprid in compartments of the terrestrial ecosystems. 

Imidacloprid parent, and the IMI-olefin and IMI-5-OH degradates are considered residues of toxicological 

concern.

 

 Imidacloprid

N+ O

N

CH NH Cl 2 N

ON

 

 Guanidine NH

CH NH Cl 2 N

N

 Olefin N NO2

CH NH Cl 2 N

N

 Nitrosimine N NO

CH NH Cl 2 N

N

 

 Guanidine NH

CH NH Cl 2 N

N

 Urea O

CH NH Cl 2 N

N

5 hydroxy

OH

N NO2

CH NH 2 N

 

 6-chloronicotinic acid

Cl COOH

N

 

 5-Keto – Urea Isomers

O

CH NH Cl 2 N

N

O

O

CH NH Cl 2 N

N

O

 WAK-4230-1 N

Cl CH2 NH NH

N

NO2

Aqueous Photolysis: 

Rapid (t 1⁄2= 0.2 days)

Soil Photolysis:

Slow (t 1⁄2 = 171 days)

Aerobic Soil System: Slow Metabolism

(t 1⁄2 = 200 days - > Year)
54

Data in Figure 4-3 show the following:

• Direct aqueous photolysis: photolysis is expected to be rapid (t1⁄2 = 0.2 days) producing the following 

metabolites: guanidine/desnitro compound (NTN-38014) at a maximum of 17% and imidacloprid urea 

(NTN-33519) at a maximum of 10%. Both maxima occurring at the end of the study suggesting their 

stability to further photo-degradation. Many other metabolites were not identified with only three of 

them at levels ranging from 8-13% of the applied residues. Availability of water and sunlight is 

necessary for this process to occur and therefore is expected to be important in clear shallow surface 

water exposed to sunlight and could be important on wet foliage exposed to sunlight; and

• Biotic aerobic soil metabolism: this process is expected to be very slow producing metabolites at very 

low concentrations. Although it is very slow, aerobic soil degradation is the only degradation pathway 

that is expected to affect applied parent reaching the soil directly upon soil or seed application or 

indirectly from foliar applications and later from wash-off. Limited, biotic degradation in aerobic soil 

systems is expected to produce the following minor metabolites: olefin (NTN-35884), WAK-4230-1,

nitrosimine (WAK-3839), guanidine/desnitro (NTN-38014) and the two isomers of 5-keto-urea 

compounds.

The degradation profile of imidacloprid suggests the following:

 Imidacloprid parent is expected to be the major species present in the soil system of the terrestrial 

eco-systems because it resists aerobic soil bio-degradation and abiotic photolysis on soil. Therefore, 

parent is expected to be the dominant species in both imidacloprid pools (root and foliage), noting 

that metabolites are also expected to be present but at low concentrations. In addition to parent, the 

only other two metabolite that are reported to be of toxicological concern for bees are the olefin and 

5-hydroxy imidacloprid; and

 Although, aqueous photolysis is expected to be a significant process for imidacloprid transformation 

on wet foliage during daylight (t1⁄2 = 0.2 days). However, the importance of this parameter was not 

demonstrated in the examined field trials herein.

In terrestrial ecosystems, there are two dissipation processes that appear to be of varying importance in 

imidacloprid exposure: degradation and movement. Degradation in the soil system is expected to have 

minimal effects on parent imidacloprid and available field data do not show that photolysis is important 

in degradation of imidacloprid reaching foliage. In contrast to degradation, imidacloprid exposure is 

expected to be highly affected by its movement including leaching down the soil profile, movement into 

the plant (plant up-take) and with surface water run-off. Imidacloprid mobility is necessary for its 

movements towards the root system from the point of application for root up-take but it may also reduce 

its availability for the same root up-take, by leaching downwards, as it reduces the available pool of 

imidacloprid from which the roots could up-take the pesticide. In this respect, it should be noted that 

dense/deep plant root systems may overcome effects of imidacloprid leaching. This factor depends on 

the plant type and the stage of growth in relation to application timing of the pesticide. Although the 

compound is expected to leach and no longer be available for root uptake for crops with shallow/thin root 
55

systems, it would still be available for those plants with deep/dense root systems. Similarly, reduction of 

the available pool of imidacloprid for root-uptake is expected as a result of its movement away from the 

application site in run-off waters. This dissipation pathway is highly dependent on many factors such as 

soil type/slope and rainfall intensity/timing in relation to the time of pesticide application. 

4.2. Imidacloprid Plant Up-take 

Several studies were evaluated in order to understand root and foliage up-take. These studies were not 

specifically designed for this purpose but rather for determining the nature of imidacloprid residues in 

varied raw agricultural commodities (apples, corn, tomatoes, potatoes and eggplants). In this section, 

plant up-take will be examined for imidacloprid applied to soil (including seed treatment) as well as 

applied to foliage. 

4.2.1. Imidacloprid applied to soil including seed treatment

In four studies, radio-labeled compound (14C-imidacloprid) was soil applied to cotton, potatoes, corn and 

eggplant (MRIDs: 425561-05/06/10 and 11, respectively). Only a summary of the results obtained from 

these studies is included herein and more details are included in Appendix H.

Results from these four studies are summarized in Table 4-3. In this Table 4-3, observed up-take in percent 

of applied and resultant concentrations are included. It is noted, that data for foliage were combined 

from stems and leaves but most of the radioactivity assigned for foliage in Table 4-3 is present in leaves 

rather than stems. 

Table 4-3. Imidacloprid root up-take/distribution and resultant concentrations in cotton, potatoes, corn 

and eggplant (%= up-take in % of the applied radioactivity and numbers in brackets are resultant 

concentrations in mg/kg)

Cotton

Timing 1 211 days

Type 2 Foliage Seeds

Data 4.7% (0.11) 0.2% (0.007)

Potatoes

Timing 1 129 days

Type 2 Foliage Tubers

Plant 2.2% (5.76) 0.3% (0.091)

Soil 98.4% (0-20 cm: 0.98-0.47; 20-50 cm: 0.007-0.002)

Corn

Timing 1 33 days 61 days 134 days

Type 2 Foliage Foliage Foliage Husks Cobs Grain

Plant 4.2% (5.84) 10.2% (1.52) 19.7% (3.08) 0.12% (0.21) 0.15% (0.12) 0.14% (0.04)

Eggplant

Timing 1 14 days 35 days 69 days

Type 2 Foliage Foliage F/FC/IMMF Foliage F/FC/IMMF Calyx Fruits
56

Plant 2.7% (5.89) 2.7% (3.63) 0.03% (0.73) 1.6% (1.47) 0.04% (0.74) 0.01% (0.17) 0.03% (0.04)

Soil 79% (1.67) 74% (1.43) 78% (1.60)

1 Timing: Timing in days from imidacloprid application which coincides with planting time noting that it was transplanting time for 

eggplant plantlets which were transplanted at the eight leaves stage 

2 Type: Type of sample noting that Foliage= Stems and leaves/vines; F/FC/IMMF= Flowers, flower clusters and immature fruits 

Data in Table 4-3 suggest that the total root uptake for soil-applied imidacloprid, appeared to take place 

upon application reaching equilibrium in the early growth stage of the plant. Uptake was generally very 

low (ranged from 2-5% of the applied in cotton, potatoes and eggplant). In corn, much higher uptake was 

observed with quantities increasing towards maturity (from 4 to 20% of the applied). In all cases, 

radioactivity that was up taken through the roots concentrated in foliage (leaves and stems) with minor 

amounts reaching the productive parts of the plant at maturity (ranged from 0.1 to 0.5% of the applied). 

As a result of the skewed distribution of radioactivity within the plant, concentrations in the foliage ranged 

from 0.1 to 5.89 ppm compared to 0.007 to 0.2 ppm in the reproductive parts of the plant. 

Radioactivity left in the soil was measured in only two studies in eggplant at 14, 35 and 69 days showing 

that radioactivity left in the soil were almost constant (74 to 79% of the applied). The same was observed 

in the soil planted with potatoes in which 98.4% of the applied radioactivity left in the soil. Loss of 

radioactivity may be related to leaching, in addition to expected analytical errors. Transformation of 

imidacloprid was observed in the soil planted in eggplant as observed parent concentrations were 

between 62 and 82% with not more than 2% of the metabolites 5-hydroxy and nitrosimine compounds 

and 6-CNA.

Data, not shown in Table 4-3, suggest that the presence of high concentrations of imidacloprid in the soil 

lead to high root up-take. This was demonstrated in cotton by applying an additional soil drench of 

imidacloprid to some of the cotton plants (60 X of the seed treatment amount applied in the main 

experiment). Application of 60 X the rate to cotton resulted in 379 to 1908 fold increase in concentrations 

related to root up-take.

4.2.2. Imidacloprid applied to foliage and fruits

In three studies, 14C-imidacloprid was applied as formulated liquid spray to the foliage of potato plants 

and to the fruits of apple and tomato plants planted in the greenhouse (MRIDs: 425561-07/08 and 09, 

respectively). These studies represent application of the chemical directly to foliage and depending on the 

growth stage of the plant, it may also be directly applied to flowers and fruits. In this case, plant up-take 

is determined by the amount of chemical inside the fruits in the case of application to fruits (tomato and 

apple experiments). However, uptake can be only confirmed in the case of foliage (the potato experiment) 

by the occurrence of plant metabolism inside leaves and stems (when metabolism on the surface can be 

discounted) and by translocation to other plant parts that are not directly sprayed by the chemical (such 

as tubers when no chemical is present in the soil). Only a summary of the results obtained from these 

studies is included herein and more details are included in Appendix H. Data obtained from the three 

studies for radioactivity distribution and resultant concentrations are summarized in Table 4-4.
57

Table 4-4. Imidacloprid up-take/distribution and resultant concentrations in various parts of the potato 

plants and only in the fruits of apples and tomatoes (%= up-take in % of the applied radioactivity and 

numbers in brackets are resultant concentrations in mg/kg).

Potatoes

Timing 1 7/90 days 28/111 days 64/147 days

Type 2 Vines Tubers Vines Tubers Vines Tubers

Plant 40.1% (2.51) 0.02% (0.01) 48.5% (1.97) 0.02% (0.01) 49.0% (1.35) 0.20% (0.01)

Soil

50.75% (0-15 cm: 0.006-0.004; 15-55 cm: 0.001-<0.001); Samples for 64/147 days only and the depth of 

sampling, in cm, is indicated

Tomatoes

Timing 1 4 days 7 days 14-21 days

Type 2 Fruit Surfaces Fruit Pulp Fruit surfaces Fruit Pulp Fruit surfaces Fruit Pulp

Fruits 88% (0.89) 12% (0.12) 77% (0.64) 23% (0.19)

76-60% (0.65-

0.39)

24-40% (0.2-

0.25)

Apples

Timing 1 Zero Day (Just After) the Last of 3 Applications 14 Days After the Last of 3 Application

Type 2 Fruit Surfaces Fruit Peel Fruit Pulp Fruit Surfaces Fruit Peel Fruit Pulp

Fruits 74.2% (1.31) 15.9% (0.28) 9.9% (0.17) 64.9% (0.94) 21.1% (0.31) 14.0% (0.2)

1 Timing: Timing in days from imidacloprid application; For potato 7/90 days mean that (vines)/tubers were sampled 7 days after 

application on plants at age of 90 days)

2 Type: Column: Type of sample noting that Potato Vines= Stems and leaves 

Data in Table 4-4 indicate that only half of the chemical reached/stayed on/in the foliage of the potato 

plants (40-49% of the applied radioactivity), the other half reached the soil and only 0.2% reached tubers 

presumably by direct up-take from the contaminated soil or by translocation from the foliage. In the case 

of tomato and apples fruits, most of the applied radioactivity stayed on the surface of the fruits (60 to 

88%) with relatively substantial amounts entering the fruits (12 to 40% in tomatoes and 26-35% in apples). 

Additionally, increasing residence time on the fruit surfaces appears to increase radioactivity that enters 

the fruits (an increase from 26 to 35% in apple peel+ pulp after a resident time of 21 days and from 12 to 

40% in tomato pulp after a resident time of 14 days. The results suggest the likelihood of an increase in 

residues in fruits sprayed at younger age compared to those sprayed at older age. Data suggest that 

important translocation may occurred from the surfaces fruits to the fruits inside. In contrast, no apparent 

transport of radioactivity occurs from plant leaves into fruits in both apples and tomatoes in a separate 

imidacloprid translocation experiments (refer to Appendix H).

4.2.3. Imidacloprid: soil versus foliage applied

As expected, the chemical residues present on/in foliage from foliar applied imidacloprid was much higher 

than that resulting from soil application (A total of 49.2% on mature plants, 64 days after application 

compared to a total of 2.5% on mature plants, 129 days after application ). In the first case, imidacloprid 

reaches foliage directly and the soil indirectly while in the second case, only a comparatively small fraction 
58

of the chemical reaches foliage by root up-take. In this respect, it is noted that the level of chemical ending 

up in the foliage, from foliar application, depends on many factors that were not investigated here such 

as photolysis on wet foliage, use of stickers and levels of wash-off (weather dependent). 

Soil (at planting) and foliar applications were investigated in parallel in potatoes. Data obtained from 

previously stated experiments are summarized in Figure 4-4. It is important to note that in the soil applied 

part of the graph: radioactivity in the soil is from application and that radioactivity in foliage is from root 

up-take and radioactivity in the tubers is from soil/root up-take; however, in the foliar applied part of the 

graph radioactivity in BOTH soil and foliage are from direct/indirect application and ONLY the amount in 

tubers is from soil/foliage up-take. These data indicate that a large percentage (49%) of imidacloprid was 

present on foliage due to direct application while a relatively low percentage (2.2%) reached the foliage 

from the soil by root up-take. However, the amount of radioactivity moving from the foliage into the 

tubers was almost the same (0.3% in the case of soil application and 0.2% in case of foliar application). 

Although radioactivity reaching tubers was the same, it is noted that measured concentrations in tubers 

from soil-applied imidacloprid is relatively higher than that present in the tubers from foliar applied 

imidacloprid (0.09 mg/kg compared to 0.01 mg/kg). A possible explanation to the observed may be 

related to higher yield of tubers in the foliar applied experiment compared to that in the soil applied 

experiment.

Figure 4-4. Comparison of up-take data obtained for imidacloprid applied to soil and that applied to 

foliage in potatoes

4.3. Plant Metabolism of Imidacloprid 

In the experiments discussed in the preceding section, radioactivity that was applied and/entered into the 

plant, as parent, was examined to obtain data on imidacloprid plant metabolism during the period from 

0%

20%

40%

60%

80%

100%

Soil Applied Foliar Applied

Soil

Soil

Tubers:

0.3% (0.09 mg/kg)

Tubers

0.2% (0.01 mg/kg)

Foliage

Foliage

% of Applied Radioactivity
59

initial exposure (application time) to fruit maturity. Only a summary of the results obtained from these 

studies is included herein and more details are included in Appendix H. 

4.3.1. Imidacloprid metabolism in various plants

In contrast to the persistence of imidacloprid in the soil system, plant metabolism appears to play an 

important role in its degradation within various plant parts. Biotransformation occurs as a result of 

changes in in moieties associated with the imidazole ring with the backbone structure of the chemical 

staying intact in addition to cleavage of the chemical structure between the imidazole and chloropyridinyl 

rings. The following is a summary of the data obtained for metabolism of imidacloprid in various plants:

(1) In cotton (seed treated), almost all parent (99 to 97%) was transformed into mainly guanidine and 

glucoside in the leaves and 6-CNA in seeds with relatively high percentages of un-identified 

compounds (extracted and un-extracted);

(2) In potatoes (soil applied), high percentage of parent persisted in both leaves and tubers (25 and 48%). 

Transformation in potatoes produced the major metabolites 5-hydroxy, guanidine and 6-CNA and the 

minor metabolites nitrosimine, IMI-olefin and 6-CPA with relatively high percentages of un-identified 

compounds (extracted and un-extracted) in the leaves compared to the tubers;

(3) In corn (seed treated), relatively high percentages (65 to 47%) of imidacloprid parent appear to persist 

in the whole plant throughout the plant growth stages up to maturity as it then decreases to 22%. 

High concentrations were observed in corn husks and cobs (43 to 47%) with lower concentrations in 

grains (24%). Imidacloprid appears to be transformed primarily into IMI-5-OH and guanidine with 

minor amounts of IMI-olefin, 4, 5-hydroxy, nitrosimine, 6-CNA, 6-CPA and open ring guanidine. Higher 

percentage of un-identified compounds (extracted and un-extracted) appear to form as the corn plant 

matures;

(4) In transplanted eggplant (soil applied), Imidacloprid parent appears to decrease (i.e., degrade) in 

foliage as the plant matures (decrease from 33 to 9% of the total residues) with relatively higher 

percentage of parent persisting in the fruits (22% of the residues). In all cases, parent degradation 

resulted in formation of imidacloprid transformation products and substantial amounts of unidentified compounds (extracted and un-extracted). Transformation products found in foliage 

included the major metabolite guanidine with minor amounts of IMI-olefin, IMI-5-OH, nitrosimine, 

glucoside and 6-CNA but glucoside and 6-CNA were the major metabolites in fruits;

(5) In potatoes (foliar applied), parent imidacloprid dominated the percentage of radioactive residues in 

the vines at Day 90 but declined (i.e., degraded) as the plant matured. Metabolite residues in young 

and immature vines consisted primarily of the metabolites guanidine, 4,5-hydroxy and IMI-5-OH and 

minor amounts nitrosamine, IMI-olefin and glucoside. Residues in the tubers were primarily 6-CNA 

with a large percentage of un-identified compounds (extracted and un-extracted).

(6) In potatoes, comparison between two cases: Case 1 in which the chemical entered potato plant 

through the leaves from foliar application (root up-take from soil may not be discounted as 
60

imidacloprid was present in the soil during foliar application), and Case 2 in which the chemical 

entered the leaves from soil through root up-take following a soil application suggested the following:

(a) Plant transformation of imidacloprid in the leaves, following foliar application in Case 1 (38% of the 

applied persisted and 62% metabolized), is less pronounced than in Case 2 in which imidacloprid 

was applied as a soil treatment (25% of the applied persisted and 75% metabolized). This might be 

resulting from the longer resident time of imidacloprid in the plant in Case 2 compared to Case 1

(129 days compared to 64 days) giving more time for metabolism to occur; and

(b) Chemical residues reaching the tubers in Case 1 (0.2% of the applied) contain 11% as parent and 

those reaching the tubers in Case 2 (0.3% of the applied) contain 48% as parent. Residues in both 

cases are at least partly translocated from other parts of the plant, therefore, no conclusions can be 

drawn on possible imidacloprid transformation in the tubers. The high amounts of parent in tubers 

in Case 2 compared to Case 1 (48% compared to 11%) appear to suggest the tubers are affected by 

direct up-take of parent from the soil. It is noted however, that imidacloprid parent was available, 

in the soil, for tuber up-take in both cases. 

(7) In apple and tomato Fruits, parent imidacloprid was applied to the surfaces of immature fruits. Data 

indicated the following:

(a) Parent dominates residues in both outside and inside apple and tomato fruits with no major 

transformation products present in either outside or inside the fruits;

(b) Minor metabolites were observed on the surface of the apple fruits including: Guanidine, 4-5-

Hydroxy, Urea and Nitrosimine. The same minor metabolites were present on the surface of 

tomatoes with 5-Hydroxy replacing 4-5-hydroxy. Authors suggested minimal abiotic 

transformation (assume minimum photolysis possibly due to lack of moisture (plant in 

greenhouse irrigated through the soil);

(c) Minor metabolites were observed inside the apple fruits including Guanidine, 4-5 & 5-Hydroxy, 

Olefin and Glucoside Minor metabolites identified inside tomatoes included: Guanidine, 5-

Hydroxy, Nitrosimine, Olefin and Glucoside.

Finally, it is generally noted that the importance of plant metabolism in the fate of imidacloprid 

appears to differ from one species of plant to another. Additionally, un-identified compounds 

(extracted and un-extracted) appear to form as the plant matures due to association of residues with 

natural plant compounds resulting in compounds that are difficult to identify.

4.3.2. Imidacloprid metabolism profile in plants

Based on data presented earlier, parent imidacloprid appears to be metabolized in the plant through two 

main processes: 
61

(1) Changes occurring in moieties associated with the imidazole ring with the backbone structure of the 

chemical staying intact. This includes reduction and loss of the nitro group as well as hydration of the 

imidazole ring and subsequent loss of H2O; and

(2) Breakage of the backbone of the chemical structure between the imidazole and chloropyridinyl rings 

resulting in the formation of the metabolite 6-CPA followed by either association with glucose forming 

glucoside or oxidation into 6-CNA. 

Figure 4-5 contains a summary of the plant metabolism profile of imidacloprid based on submitted data. 

It is noted that not all of the plant metabolism radioactive residues were extracted/identified (due to 

possible incorporation into the natural plant constituents) or were extracted but not identified. The first 

fraction of the residues is termed herein as un-extracted residue (UER) while the second is termed as 

unidentified residue (UN-ID).

Figure 4-5. Suggested Imidacloprid degradation profile in plants (based on submitted plant metabolism

data).

+

Un-Extracted Residues 

(UER)

+

Un-Identified Residues 

(UN-ID)

 Urea O

CH NH Cl 2 N

N

 

 Guanidine NH

CH NH Cl 2 N

N

 

 Imidacloprid

N+ O

N

CH NH Cl 2 N

ON

 

 6-CPA: 6-chloropicoly1 alcohol

Cl CH2 OH

N

 

 Glucoside

Cl CH2 O-Glucose

N

 

 6-CNA: 6-chloronicotinic acid

Cl COOH OH

N

 4,5 hydroxy

OH OH

N NO2

CH NH Cl 2 N

N

 5 hydroxy

OH

N NO2

CH NH Cl 2 N

N

 Olefin N NO2

CH NH Cl 2 N

N

 Nitrosimine N NO

CH NH Cl 2 N

N

 

 Imidacloprid

N+ O

N

CH NH Cl 2 N

ON 

 Ring opened guanidine 

Cl CH2 NH C

N

NH2

NH
62

4.3.3. Imidacloprid metabolism profile in plants

As a result of plant metabolism, the quantity of parent entering the plant through root or foliage/fruit uptake is expected to decrease with time. However, plant metabolism produces two metabolites that are 

considered of concern: IMI-5-OH and IMI-olefin and olefin. Table 4-5 contains a summary of the estimated 

stressor concentrations in various plants and plant parts. 

Table 4-5. Observed estimated concentrations of the stressor in parts per million= ppm) (parent 

imidacloprid + IMI-olefin and IMI-5-OH compounds) in varied crops, plant parts and application 

procedures based on radioactivity data

Soil Applied

Cotton

Timing 1 211 days

Type 2 Foliage Seeds

Data 0.004 <0.001

Potatoes

Timing 1 129 days

Type 2 Foliage Tubers

Plant 2.016 0.054

Corn

Timing 1

33 

days 61 days 134 days

Type 2 Foliage Foliage Foliage Husks Cobs Grain

Plant

4.497 0.851 0.893 0.118 0.074 0.019

Eggplant

Timing 1

14 

days 35 days 69 days

Type 2 Foliage Foliage F/FC/IMMF Foliage F/FC/IMMF Calyx Fruits

Plant

2.533 0.436 0.088 0.206 0.163 0.037 0.009

Foliar Applied

Potatoes

Timing 1 7/90 days 28/111 days 64/147 days

Type 2 Vines Tubers Vines Tubers Vines Tubers

Plant

2.033 0.000 1.162 0.000 0.635 0.001

Tomatoes

Timing 1 4 days 7 days 14-21 days

Type 2 Surface Pulp Surface Pulp Surface Pulp

Fruits

0.748 0.012 0.461 0.036

0.0455- 0.0700 0.068- 0.1040

Apples Timing 1

Zero Day (Just After) the Last of 3 

Applications 14 Days After the Last of 3 Application

Type 2 Surface Peel Pulp Surface Peel Pulp

Fruits

0.865 0.031 0.019 0.526 0.040 0.026

1 Timing: Timing in days from imidacloprid application;

2 Type: Column: Type of sample noting that Potato Vines= Stems and leaves ; Foliage= Stems and leaves/vines; F/FC/IMMF= Flowers, 

flower clusters and immature fruits 
63

4.4. Potential for Exposure to Bees 

As described in the Problem Formulation (Section 2), the first step in the tiered pollinator risk assessment 

process is assessing the potential for exposure to adult and larval honey bees for a given use pattern. 

Tables 4-6 to 4-8 below summarize potential exposure pathways for each of the registered use patterns 

for imidacloprid, organized by application method. The determination for potential on-field exposure is 

based on whether the crop is attractive to bees and the agricultural practices, such as whether the crop 

is harvested prior to or after the bloom period. The potential for on-field exposure is presumed for crops

harvested after bloom and which are attractive to visiting honey bees, while off-field exposure is pertinent 

only for foliar uses, whether the crop is attractive to bees or not, as a result of spray drift. 
64

Table 4-6. Attractiveness of crops for the registered foliar uses of imidacloprid to bees (as indicated by USDA, 2014). Note, the potential for offfield exposure is indicated from all foliar uses.

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive? 

(Y/N)

Bumble Bee 

Attractive? 

(Y/N)

Solitary Bee 

Attractive? 

(Y/N)

Notes

Potential for OnField Exposure? 

(Y/N)

1 (Root and Tuber Vegetables)1 Y (Pollen and 

Nectar)

Y Y Bees important for seed production, 

typically harvested prior to bloom. 

Potatoes noted to be harvested after 

bloom

Y

4A (Leafy Green Vegetables) Y (Pollen and 

Nectar)

Y Y Bees important for seed production, 

crop harvested prior to bloom when 

not used for seed production.

N

5 (Brassica Leafy Vegetables) Y (Pollen and 

Nectar)

Y Y Harvested prior to bloom N

6 (Legume Vegetables) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

8 (Fruiting Vegetables) Y (pollen and 

nectar)4

Y Y May be grown in glasshouses, with 

bumble bees for pollination

Y

10 (Citrus Fruit) Y (Pollen and 

Nectar)

Y Y -- Y

11 (Pome Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

12 (Stone Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

13 (Berry and Small Fruit)2 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

14 (Tree Nuts) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

9 (Herbs) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y

20 (Oilseed)3 Y (Pollen and 

Nectar)

Y Y Y

Non-crop group uses (Globe artichoke, 

banana and plantain, peanut, 

pomegranate, tobacco, coffee, hops, 

tropical fruit)

Y (Pollen and 

Nectar)

Y Y Globe artichoke harvested before 

bloom, tobacco deflowered as part of 

the harvest process

N (globe 

artichoke, 

tobacco) Y for all 

others 1Refer to members of subgroups 1C (potato) and 1D (yams, ginger, others) only
65

2Includes 13A, 13B, 13-07D, 13-07F, 13-07G 3Cotton is sole member of this group with registered foliar uses 4

Okra nectar and pollen indicated to be attractive to honey bees (USDA, 2014)

Table 4-7. Attractiveness of crops for the registered soil uses of imidacloprid to bees (as indicated by USDA, 2014)

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive? 

(Y/N)

Bumble 

Bee 

Attractive? 

(Y/N)

Solitary 

Bee 

Attractive? 

(Y/N)

Notes

Potential for 

On-Field 

Exposure? 

(Y/N)

Potential for 

Off-Field 

Exposure? 

(Y/N) 

1 (Root and Tuber Vegetables) Y (Pollen and 

Nectar)

Y Y Bees important for seed production, 

typically harvested prior to bloom. 

Potatoes noted to be harvested after 

bloom

Y N

3 (Bulb Vegetables) Y (Pollen and 

Nectar)

Y Y Typically harvest prior to bloom. N N

4 (Leafy Vegetables) Y (Pollen and 

Nectar)

Y Y Crop harvested prior to bloom when 

not used for seed production.

N N 

5 (Brassica Leafy Vegetables) Y (Pollen and 

Nectar)

Y Y Harvested prior to bloom N N 

6 (Legume Vegetables) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

8 (Fruiting Vegetables) Y (Pollen and 

nectar)

Y Y May be grown in glasshouses, with 

bumble bees for pollination

Y N

9 (Cucurbit Vegetables) Y (Pollen and 

Nectar)

Y Y -- Y N

10 (Citrus Fruit) Y (Pollen and 

Nectar)

Y Y -- Y N

11 (Pome Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

12 (Stone Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N 

13 (Berry and Small Fruit)1 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N 

14 (Tree Nuts) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

19 (Herbs) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N
66

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive? 

(Y/N)

Bumble 

Bee 

Attractive? 

(Y/N)

Solitary 

Bee 

Attractive? 

(Y/N)

Notes

Potential for 

On-Field 

Exposure? 

(Y/N)

Potential for 

Off-Field 

Exposure? 

(Y/N) 

20 (Oilseed)2 Y (Pollen and 

Nectar)

Y Y -- Y N

Non-crop group uses (Globe 

artichoke, banana/plantain, peanut, 

pomegranate, tobacco, coffee, hops, 

tropical fruit)

Y (Pollen and 

Nectar)

Y Y -- Y N

1Includes 13A, 13B, 13-07D, 13-07F, 13-07G, 13-07H 2Cotton is sole member of this group with registered soil uses.

3Okra nectar and pollen indicated to be attractive to honey bees (USDA, 2014)

Table 4-8. Attractiveness of crops for the registered seed treatment uses of imidacloprid to bees (as indicated by USDA, 2014)

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive? 

(Y/N)

Bumble 

Bee 

Attractive? 

(Y/N)

Solitary Bee 

Attractive? 

(Y/N)

Notes

Potential for 

On-Field 

Exposure? 

(Y/N)

Potential for 

Off-Field 

Exposure? 

(Y/N) 

1 (Root and Tuber Vegetables)1 Y (Pollen and 

Nectar)

Y Y Bees important for seed 

production, typically harvested 

prior to bloom. Potatoes noted to 

be harvested after bloom

Y N

3 (Bulb Vegetables)2 Y (Pollen and 

Nectar)

Y Y Typically harvest prior to bloom. N N

5 (Brassica Leafy Vegetables)3 Y (Pollen and 

Nectar)

Y Y Requires pollination only when 

grown for seed; small % of acreage; 

harvested prior to bloom

N N 

6 (Legume Vegetables)4 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom N N

15 (Cereal grains)5* Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

19 (Herbs)6 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

20 (Oilseed)7 Y (Pollen and 

Nectar)

Y Y -- Y N
67

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive? 

(Y/N)

Bumble 

Bee 

Attractive? 

(Y/N)

Solitary Bee 

Attractive? 

(Y/N)

Notes

Potential for 

On-Field 

Exposure? 

(Y/N)

Potential for 

Off-Field 

Exposure? 

(Y/N) 

Non-crop group uses (peanut) Y (Pollen and 

Nectar)

Y Y -- Y N

1Labels specify sugarbeet (1A), carrot (1B), and potato (1C)

2Labels specify onions/leeks and scallions (03-07A, 03-07B) 3Labels specify broccoli (5A)

4Labels specify soybean (6A) and beans/peas (6) 5

Labels specify buckwheat, triticale, wheat, barley, oats, millet, sorghum, rye, and corn (pop, sweet, field)

6Labels specify borage (19A) and mustard (19B)

7Labels specify flax, sunflower, safflower, cotton, canola, and crambe
68

4.5. Screening-level Exposure Estimation 

As described above in Section 2, the pollinator risk assessment process is a tiered approach that begins 

with model-generated (based on consumption rates of pollen and nectar and application rate) or default 

estimates of exposure and laboratory toxicity data at the individual level (Tier I). These estimates are also 

based on the bee’s life stage (i.e. adult vs larvae) and the method of application (i.e. foliar, soil, or seed 

treatment applications). 

In Tier I, pesticide exposures are estimated based on honey bee castes with known high-end consumption 

rates. For larvae, food consumption rates are based on 5-day old larvae, which consume the most food 

compared to other days of this life stage. For adults, the screening method relies upon nectar foraging 

bees, which consume the greatest amount of nectar of all castes while nurse bees consume the greatest 

amount of pollen. It is assumed that this value will be comparable to the consumption rates of adult 

drones (males) and will be protective for adult queens as well. Although the queen consumes more food 

than adult workers or drones, the queen consumes “processed” food (i.e., royal jelly produced by the 

hypopharyngeal glands of nurse bees) that is assumed, based on currently available data, to contain 

orders of magnitude less pesticide than that consumed by adult workers. 

Nectar is the major food source for foraging honey bees as well as nurse bees (young, in-hive females). 

Therefore, pesticide residues in nectar likely account for most of the exposures to bees, and may 

represent most of the potential risk concerns for adult bees. However, if residues in pollen are of concern, 

exposures to nurse bees, which consume more pollen than any other adult honey bees, should be 

considered. This is the case especially when pesticide concentrations in pollen are much greater than in 

nectar, or for crops that mainly provide pollen to bees and would be assessed on a case-by-case basis. In 

fact, the screening level Tier I risk estimation model for honey bees (Bee-Rex; v.1.0) allows calculation of

exposure and resulting risk quotients (RQs) for all types of bee castes. As described in the 2012 White 

Paper (USEPA et al. 2012) presented to the FIFRA Scientific Advisory Panel and the final Guidance 

Document for Assessing Risk to Bees (USEPA et al. 2014), for dietary exposure from foliar applications, it 

is assumed that pesticide residues on tall grass (from the Kenaga nomogram of T-REX which is 

incorporated into Bee-REX) are a suitable surrogate for residues in pollen and nectar of flowers that are 

directly sprayed. The Bee-REX model is a screening level tool that is intended for use in a Tier I risk 

assessment to assess exposures of bees to pesticides and to calculate risk quotients. This model is 

individual-based, and is not intended to assess exposures and effects at the colony-level (i.e., for honey 

bees).

The Tier I exposure method is intended to account for the major routes of pesticide exposure that are 

relevant to bees (i.e., through diet and contact). Exposure routes for bees differ based on application 

type. In the model, bees foraging in a field treated with a pesticide through foliar spray could potentially 

be exposed to the pesticide through direct spray as well through consuming contaminated food. For 

honey bees foraging in fields treated with a pesticide through direct application to soil (e.g., drip 

irrigation), through seed treatments, or through tree injection, direct spray onto bees is not expected. For 

these application methods, pesticide exposure through consumption of residues in nectar and pollen are 

expected to be the dominant routes. 
69

Table 4-9 below (extracted from Guidance for Assessing Pesticide Risks to Bees, USEPA et al. 2014) 

summaries the exposure estimates for contact and dietary exposures for adult and larvae resulting from 

foliar, soil, seed treatment and tree injection application of pesticides.

Table 4-9. Summary of contact and dietary exposure estimates for foliar applications, soil treatment, 

seed treatments, and tree trunk injections of pesticides for Tier I risk assessments.

The consumption of nectar and pollen vary depending on the bee’s life stage and caste within the hive. 

The consumption rates tabulated below inform the exposure estimates and resultant RQs in the default 

Tier I and refined Tier I analyses that are presented in Section 6. Table 4-10 below is extracted from

Guidance for Assessing Pesticide Risks to Bees, USEPA et al. 2014, and additional detail of the derivation 

of these consumption rates can be found in the White Paper (USEPA et al. 2012).
70

Table 4-10. Summary of estimated food consumption rates of bees.
71

4.6. Experimental Residue Studies 

In cases where the screening-level Tier I RQs exceed the level of concern (LOC, discussed below), estimates 

of exposure may be refined using measured pesticide concentrations in pollen and nectar of treated crops, 

and further calculated for other castes of bees using their food consumption rates (see Table 4-10).

As discussed above in Section 4.2, the most conservative (highest) exposure estimates for contact and/or 

diet exposure routes are selected for the Tier I screening-level assessment. These exposure estimates are 

based on adult and larval bees with the highest food consumption rates among bees. The Bee-REX tool 

also calculates dietary exposure values and associated RQs for larvae of different ages, adult workers with 

different tasks (and associated energy requirements) and the queen. This is accomplished using the food 

consumption rates provided in Table 4-10. Those food consumption rates are based on work described 

in the White Paper8 and updated to reflect comments from the Scientific Advisory Panel (SAP). Exposure 

values for other groups of bees within a hive along with their RQs can be used to characterize risks of 

dietary exposures of different bees within the hive. Empirical data can be used to refine conservative 

exposure estimates and reduce uncertainties associated with the Tier I exposure assessment by providing 

direct measurements of pesticide concentrations resulting from actual use settings. Studies investigating 

pesticide concentrations in pollen and nectar should be designed to provide residue data for crops and 

application methods of concern. The available residue studies for imidacloprid for foliar, soil, and seed 

treatment applications from both registrant and open literature sources are summarized below. For 

detailed summaries of the methods and findings of each study, please see Appendix E.

4.6.1. Rationale for Residue-based EEC Selection for Refined Tier I

The Agency has a long standing practice of deriving estimated environmental concentrations (EECs) using 

model-derived exposure data (USEPA 2000). For example, acute EECs for aquatic organisms are based on 

the maximum peak (daily) concentration with an estimated return interval of 1-in-10 years while chronic 

EECs are based on the 21-d average (invertebrates) or 60-d average (fish) concentration with the same 

return interval. Generally speaking, these EECs are considered “high-end” estimates of exposure within 

the context of the available model output. Terrestrial EECs produced by the T-REX model similarly reflect 

high-end estimates of exposure to birds and mammals. The general rationale behind the Agency’s 

selection of EECs from its exposure models relates to the desire to achieve an EEC that is sufficiently 

protective given the temporal and spatial variability in exposure concentrations that can be expected to 

occur across the United States. 

Unlike EEC selection from its standard exposure models, the Agency does not yet have a standard process 

for selecting EECs in pollen and nectar obtained from field residue studies. This partly reflects the wide 

diversity of residue study designs from which residue data are obtained and the relatively recent adoption 

of a quantitative risk assessment process for bees. Nonetheless, the conceptual approached used by the 

 8 USEPA, PMRA, CDPR (2012) White paper in support of the proposed risk assessment process for bees. United States 

Environmental Protection Agency, Office of Pesticide Programs, Washington DC. Pest Management Regulatory 

Agency, Health Canada, Ottawa. California Department of Pesticide Regulation, Sacramento, CA.
72

Agency for selecting model-based EECs appropriate to other taxa (e.g., fish, birds) is used here to guide 

the selection of pollen and nectar EECs obtained from field residue studies. 

In selecting the acute and chronic EECs from field residue data in pollen and nectar, the following factors 

were considered:

1. Field residue data typically have relatively coarse resolution with respect to capturing the 

temporal variability in pollen and nectar residues that would be expected to occur for a given crop 

in the U.S. This reflects the technical and resource constraints associated with the conduct of 

these studies. Specifically, pollen and nectar residue trials sample residues at discrete times 

following pesticide application, usually 5 or fewer sampling intervals. Often, these sampling 

intervals may span one or more weeks such that the pattern of residues in between the sampling 

events is not known. Furthermore, data are usually available for 1 or 2 growing seasons, which 

likely underestimates the temporal variation associated with pesticide residues in pollen and 

nectar over multiple growing seasons.

2. From a spatial variability perspective, field residue data for pollen and nectar generally reflect a 

limited number of sites in the U.S. (commonly 3 or less). Where a substantially greater number 

of sites have been included in pollen and nectar residue studies (e.g., 10), these tend to be located 

in one specific region or State for practical reasons. Therefore, available field residue data sets 

currently available to the Agency likely underestimate the extent of spatial variation that exists in 

in pollen and nectar residues in the U.S. 

3. From a toxicological perspective, the averaging period associated with a given EEC should reflect 

the time period necessary to elicit adverse effects in the toxicity studies to which it is being 

compared. In the case of honey bee toxicity studies, acute toxicity endpoints obtained from Tier 

I studies reflect a single oral or contact dose to the bee. Therefore, the EEC averaging period 

appropriate for comparing to acute toxicity endpoints should be relatively short (e.g., 1 day). 

Chronic toxicity endpoints derived from Tier I toxicity studies reflect exposure durations of 10 

days (adult) and 21-days (larvae). However, the actual dosing in chronic larval tests last only for 

3-4 consecutive days, after which larvae undergo pupation and emergence. Based on these 

considerations, it seems appropriate for the chronic EEC to reflect several days at most, given that 

toxicological effects may be manifest from exposure periods that are shorter than the duration of 

a chronic test.

4. Most of the residue studies available to the Agency with imidacloprid contained multiple sample 

replicates for a given sampling period. Therefore, some variation due to sampling and pesticide 

application methods was captured in these cases. 

Tier I Acute EEC. Given the limitations of residue trial data to account for temporal and spatial variability, 

the Agency defines the field residue acute EEC as the overall maximum residue value measured for each 

matrix (pollen, nectar). If replicate data are reported (i.e., multiple samples on a given sampling day), 

then the acute EEC would be the maximum of the replicates. These field residue acute EECs are then used 

to calculate the acute RQ for adult and larval bees (caste and life stage/task specific). 
73

Tier I Chronic EEC. Given the short exposure windows of chronic adult and larval toxicity tests and 

relatively coarse temporal resolution associated with the field residue data, the Agency defines the field 

residue chronic EECs as high average residue value determined from a given sampling event (usually a 

daily average). 

Additional characterization of RQ values derived from the aforementioned EECs will be conducted using 

the entire pollen and nectar data set obtained for each representative crop where the totality of the data 

will be compared to the Tier I endpoints to yield a set of resultant RQs. This will be expressed as a 

percentage of the RQs which exceed the respective LOC.

4.6.2. Rationale for Comparing Residue Data With Tier II Endpoints

According to the 2014 Guidance for Assessing Pesticide Risks to Bees, RQ values are not determined in 

evaluating risks at higher tiers. Rather, risks are evaluated qualitatively and consider multiple lines of 

evidence. In the case of the Tier 2 colony feeding study, consideration is given not only to the magnitude 

of the residue in nectar relative to the NOAEC and LOAEC, but also the duration and frequency that 

residues exceed these Tier 2 endpoints. Additionally, information regarding the duration that the crop 

remains in bloom is also factored into the Tier 2 risk characterization to characterize the potential for longterm exposure of bees to contaminated pollen and nectar. The quality and quantity of available residue 

data are also carefully considered at the Tier 2 level. For example, available information suggests that soil 

applications of imidacloprid in coarse soils results in substantially greater residues in pollen and nectar 

compared to fine/heavy soils. Thus, if no data are available for coarse soils for a particular soil application, 

this information will be considered when evaluating the uncertainty associated with the Tier 2 risk 

characterization.

4.6.3. Foliar Application Residue Studies – Registrant Submitted

There are three registrant submitted studies available to characterize the total residues of parent 

imidacloprid and the metabolites IMI-olefin and IMI-5-OH in pollen and nectar. There were no studies 

that were available from the open literature that examined residues on crops following foliar applications 

of imidacloprid. Table 4-11 below summarizes the key elements of the available registrant submitted 

foliar application residue studies. Further details of each residue study are provided in Appendix E. 

Available studies on oranges, cherries, and cotton were conducted at rates that represent 20% (cotton) –

100% (orange and cherry) of the maximum permitted annual rate for these crops (and respective crop 

groups, noting that no foliar applications are permitted for other members of the oilseed group, of which 

cotton is a member). In the case of the foliar cotton study, an additional 4 foliar applications of 0.06 lbs 

a.i/A are permitted during the indeterminate bloom period, but the available study only assessed on 

application. Cotton represents one of the few use patterns of imidacloprid where there are no restrictions 

for foliar applications associated with the bloom period given the protracted period of time over which 

cotton blooms. 

In the study on oranges conducted in Florida in 2012-2013 (MRID 49521301), imidacloprid as the 

formulated product Admire® Pro SC (42.9% a.i) was applied twice at 0.25 lbs a.i/A with a reported 8 – 10 
74

day reapplication interval and the last application prior to the 10-day pre bloom interval, in accordance 

with labeled parameters and represents the highest single and annual rate on oranges and other citrus 

fruits. Maximum residues across all individual replicates (i.e. acute EEC) and the maximum average 

concentration among all individual sampling events (i.e. chronic EEC) were noted to be an order of 

magnitude higher in pollen as compared to nectar (acute and chronic EEC of 4,100 and 3,000 ppb, 

respectively in pollen compared to 430 and 324 ppb, respectively in nectar). 

In the cherry study (conducted in New York and Oregon in 2013 - 2014), 5 applications of Admire Pro® 

(42.9% a.i) at 0.1 lbs a.i/A were made post-bloom after harvest in the first year of the study and preharvest in the second year of the study (MRID 49535601). This scenario is in accordance with labeled 

parameters and represents the highest single and annual rate of foliar application on cherries and other 

stone fruits. Acute and chronic EECs in pollen were noted to be two orders of magnitude higher than in 

nectar (1000 and 545 ppb, respectively in pollen as compared to 10 and 5.6 ppb, respectively in nectar). 

It is noted that the label permits foliar applications to stone fruits only after the bloom period.

As indicated previously, the available foliar-applied cotton study represents approximately 20% of the

permitted maximum annual application rate. For this study (conducted in California from 2008 – 2010, 

MRID 49103301) one application of imidacloprid (as Provado® 1.6 F, 17.4% a.i) of 0.06 lbs a.i/A was made 

during the bloom period. It was noted that previous applications of Admire® Pro (42.9% a.i) were made 

as soil application in 2008 and 2009 to other crops with rates ranging from 0.18 – 0.38 lbs a.i/A in the 

same fields as the cotton. Due to the lower annual application rate and lack of pollen data, the acute and 

chronic EEC of 66 and 56 ppb, respectively, are considered underestimates of the potential risk associated 

with foliar applications on cotton. 
75

Table 4-11. Summary of available registrant submitted foliar application residue studies

Crop 

Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4 (days) Study Notes Classification

(Reference)

Citrus

Fruits –

10 

(Orange)

3 sites (FL)

2 years 

(2012, 

2013)

Gaucho® 600 FL Admire®

Pro SC

2 x 0.25 lbs. a.i/A @ 8-10d 

interval (0.5 lbs. a.i/A total)

Ground applied ~10d prebloom

Pollen

Nectar

4,100

430

3,300

324

7, 4

4

• Experimental trials (sandy 

soils); 

• Data are from trials NT005 

and NT006 only; 

• Nectar residues declined 

with time; pollen usually 

remained constant or 

declined (one trial/year);

• Year-to-year residue 

carryover uncertain

• LOQ and LOD for total 

imidacloprid residues were 1 

and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable 

(NT005 & NT006 

only) 

(Murphy et al.

2014, MRID 

49521301)

Stone 

Fruit – 12

(Cherry)

4 sites, 

(NY, OR)

2 years

(2013-

2014)

Gaucho 600® FL Admire®

Pro SC (airblast) post bloom

5 x 0.1 lbs. a.i/A @ 8-11d 

interval (0.5 lbs. a.i/A total)

Year 1: Post harvest (fall)

Year 2: Pre-harvest 

(summer)

Pollen

Nectar

1000

10

545

5.6

208

208, 212

• Experimental trials

• Sandy loam soils

• NY sites 10X higher pollen 

residues vs. OR, 

• Post-harvest (fall) appl. > 

residues vs. Pre-harvest 

(summer) appl.

• Year to year residue carry 

over is uncertain

• LOQ and LOD for total 

imidacloprid residues were 1 

and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable

(Miller et al. 

2014, MRID 

49535601)
76

Crop 

Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4 (days) Study Notes Classification

(Reference)

Oilseed –

20

(Cotton)5

5 sites (CA)

2-3 years

(2008-

2010)

2010: Provado® 1.6F

1 x 0.06 lbs. a.i/A during 

bloom (aerial) 

2008-2009: Admire® Pro: 

0.18-0.38 lbs. a.i/A 

(chemigation to other 

crops)

Nectar 66 56 6

• Commercial fields; 

• Heavy (clay) soils; 

• Field portion of study was 

non-GLP

• Only 1 sampling event post 

application (nectar only)

• Less than max seasonal rate 

tested

• LOQ reported to be 1 ppb in 

nectar

Supplemental

(Beedle and 

Harbin 2011, 

MRID 49103301)

NR: Not reported; LOQ: limit of quantitation; LOD: limit of detection; DAA: Days after application

1 Refers to hand collected pollen and nectar

2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application, refers to parent + IMI-olefin and IMI-5-OH 3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application, refers to parent + IMI-olefin and IMI-5-OH 4 DAA = Days after the last application of the pesticide

5Cotton represent sole member of oilseed group with registered foliar uses.
77

4.6.4. Soil Application Residue Studies – Registrant Submitted

There are seven registrant-submitted studies available to characterize the total residues of parent 

imidacloprid and the IMI-olefin and IMI-5-OH metabolites in pollen and nectar. Table 4-12 below 

summarizes the key elements of the available registrant-submitted soil application studies. Detailed 

methods and findings of each study are provided in Appendix E. 

Studies on tomatoes, melons, citrus fruits, blueberries, strawberries, and cotton are available that 

represent 47% (tomatoes) – 100% (tomatoes, citrus, melons, blueberries, strawberries, and cotton) of the 

maximum permitted annual rate for these crops (and associated groups). 

There are two studies available for tomato with one study (California, 2009 – 2010; MRID 49090503) 

testing 47 – 66% of the maximum annual rate permitted for tomatoes and other fruiting vegetables while 

a more recent study (California, 2013 – 2014; MRID 49090503) assessed the highest annual rate of 0.38 

lbs a.i/A. Both studies tested the formulated product Admire® Pro (42.9% a.i) and applications ranged 

from at being made at transplant to 25 days after transplant, depending on the trial. Both studies 

employed a drip irrigation method of soil application. It is noted that tomato does not produce nectar, 

and therefore only pollen data is available. In the case of the more recent study testing the higher 

application rate, the residues in pollen collected by bumble bees was assessed. The higher acute and 

chronic EECs resulted, as expected from the more recent study and were 242 and 198 μg a.i/L (parts per 

billion; ppb), respectively.

In the available melon study (California, 2008 – 2011; MRID 49090501), cantaloupe and unidentified 

varieties of melons were treated with Admire® Pro (42.9% a.i), Alias® (40.6% a.i), and an unidentified 

formulation of imidacloprid at application rates ranging from 0.23 – 0.38 lbs a.i/A, representing 60 – 100% 

of the maximum annual rate permitted for melons and other cucurbit vegetables. Applications were 

made via soil drip or seed line drench at transplant depending on the trial. Bee-collected (trapped) pollen 

and hive (comb) nectar were sampled as opposed to hand-collected nectar and pollen directly from the 

melon flowers. Acute and chronic EECs in trapped pollen were 32 and 19 ppb, respectively, and 8 and 4.9 

ppb in hive nectar, respectively. 

In a soil-applied citrus study (California, 2009 – 2011; MRIDs 49090504 and 49090505), orange, tangerine, 

and grapefruit orchards were treated in multiple trials at applications rates ranging from 0.25 – 0.50 lbs 

a.i/A which represent 50 – 100% of the maximum labeled annual rate of soil application to citrus fruits. 

The trials were conducted either in tunnels or open fields, all with Admire® Pro (42.9% a.i). Only one field 

trial assessed the residues in pollen, which was the sole trial in the study that assessed the lower 0.25 lbs 

a.i/A. Maximum residues in nectar from individual replicates (i.e. acute EECs) were similar (29.1 – 35.5 

ppb) across the three trials that tested the highest maximum annual rate of 0.5 lbs a.i/A. In the trial that 

tested the half rate of 0.25 lbs a.i/A, maximum residues were approximately 50% reduced, at 18.3 ppb. 

The magnitude of residues in pollen from this study are uncertain given the trials with the higher 

application rate did not sample residues in pollen.

For the blueberry study (New York, Illinois, Michigan, 2012 – 2013; MRID 49535602), one Admire® Pro 

600 SC (42.9% a.i) at 0.5 lbs a.i/A (representing the highest permitted soil-applied rate for blueberries and 
78

other bushberries) was made 3 days post-harvest. Honey bee hive nectar and bee-collected (Apis and 

Bombus) pollen were assessed with acute and chronic EECs of 16 and 8.8 ppb, respectively in hive nectar 

and 42 and 16.5 ppb, respectively in bee-collected pollen (i.e honey bee and bumble bees that were within 

a flight cage during the course of the nectar and pollen collection period.). The highest concentrations 

were noted to have been determined in coarser soils. 

In the strawberry study (California, 2010 – 2011; MRID 49090502), although the highest application rate 

of 0.5 lbs a.i/A to strawberries was made (Admire® Pro [42.9% a.i] or Alias® 4F [40.6% a.i]), its timing in 

relation to bloom as well as the interval between application and sampling of residues is unknown. Labels 

prohibit soil applications to strawberries prior to bud opening, during bloom, or when bees are foraging. 

Additionally, this study did not investigate the residue levels in nectar, which is considered to be attractive 

to honey bees (USDA 2014). The acute and chronic EECs in pollen were 320 and 280 ppb, respectively, 

and due to the absence of residue data for nectar, the exposure to residues in pollen alone is considered 

to be an underestimation of the potential exposure to imidacloprid for foraging honey bees.

Finally, in the cotton study (California, 2013 – 2014, MRID 49665202), a single soil application of Admire®

Pro SC (42.9% a.i) application was made at planting at the maximum single and annual application rate for 

cotton at 0.33 lbs a.i/A. This study was one part of another component that tested the combined residues 

of this soil application and three foliar applications (to be discussed later in the combined method 

applications section). Residues in pollen, nectar, and extra-floral nectar were assessed, with the maximum 

nectar residue samples being roughly 3 - 3.5 fold higher than those in pollen or the extra-floral nectar 

(acute EEC of 127, 43.4 and 35.9 ppb in floral nectar, pollen and extra-floral nectar, respectively).
79

Table 4-12. Summary of available registrant submitted soil application residue studies

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. 

Rate, Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Classification

(Reference)

Fruiting 

Vegetable –

8 

(Tomato)

9 Sites

Kings & Kern 

Co, CA

2 years 

(2009-2010)

Admire® Pro

3 sites: 1 x 0.18 lbs.

a.i/A per year, 2-25d 

post transplant (drip 

chemigation)

6 sites: 2 x 0.13 lbs.

a.i/A per year; at/near 

transplant & during 

bloom (drip 

chemigation)

Pollen 

(including 

anthers)

54 46 100

• Commercial fields; heavy 

and medium soils

• Residues from 2 composites 

from a single sampling time 

in 2010

• Tested rates reflect 47-66% 

of maximum single 

application rate

• Field sampling not GLP

Supplemental

(Freeseman and 

Harbin, 2011; 

MRID 49090503)

Fruiting 

Vegetables 

– 8 

(Tomato)

9 sites

CA

2 years

(2013-2014)

Admire® Pro Systemic 

Protectant SC

0.38 lbs. a.i/A @ 7d 

post- transplant (soil 

drip/ drench)

Pollen (b) 242 198 36-38

• Experimental fields; fine, 

medium, and coarse soils

• Year 2 ongoing for 5 sites

• 1-2 replicates from bumble 

bee-collected pollen 

• Most residue data reflect 

coarse soils

• Limited data indicates no 

year-to-year carry over 

(leaves)

• LOQ and LOD for total 

imidacloprid residues were 1 

and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable 

(Gould and 

Jerkins, 2015, 

MRID 49665201)
80

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. 

Rate, Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Classification

(Reference)

Cucurbit 

Vegetable -

9

(Cantaloupe 

& unknown 

melons) 

10 sites

CA

2-4 years 

(2008-2011)

Admire® Pro, Alias®, 

and unknown 

formulation

0.23-0.38 lbs. a.i/A per 

yr.

soil drip or seed line 

drench at transplant 

(2011)

Pollen (t)

Nectar (h)

32

8

19

4.9

Approx. 

90-120

• Commercial fields; heavy & 

Medium soils

• LOQ in nectar and pollen 

were 1 and 10 ppb, 

respectively

Supplemental

(Beedle 2012 

MRID 49090501)

Citrus – 10

(Orange)

3 Tunnels

Exeter, CA

1 year

Admire® Pro

1 x 0.5 lbs. a.i/A; 

post bloom via soil 

drench

(Sept 3, 2009),

Nectar 

Nectar (b)

Nectar (h)

34.6

37.1

95.2

21.2

17.5

72.8

~230 

(4/22/10)

• 3 trees/tunnel; 1 hive/tunnel

• Loam soil, weekly irrigation

• Higher conc. in hive nectar 

may be partly due to water 

loss

Supplemental

(Byrne et al.

2011, MRID 

49090504; 

Fischer and 

Bowers, 2012, 

MRID 49090505)

Citrus – 10

(Orange, 

Tangerine)

Multiple 

Open fields, 

CA (1-2 mi 

radius 

around 

hives)

1 year 

(2009-2010)

Various formulations 

(unspecified); 

1 x 0.25 lbs. a.i/A @ 

post bloom (Fall 2009)

Presumed soil drench 

Nectar 

Nectar (b)

Nectar (h)

Pollen (t)

18.3

16.0

15.5

10.2

9.4

7.6

11.6

9.4

~230 

(April 

2010)

• Commercial citrus fields 

• Loamy soil

• Small # of pollen samples 

could be collected

• Half of maximum single 

application rate

Supplemental 

(Byrne et al. 

2011, MRID 

49090504; 

Fischer and 

Bowers 2012, 

MRID 49090505)
81

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. 

Rate, Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Classification

(Reference)

Citrus – 10

(Orange)

2 sites

Open fields

Lindcove 

Research 

and 

Extension 

Center 

(LREC) and 

Bakersfield, 

CA

Admire® Pro

1 x 0.5 lbs. a.i/A @ 

post-bloom via soil 

drench

(Sept 3 & 8, 2009)

Nectar 29.1 19.3 Spring 

2010

• 3.9 ac commercial field and 

LREC site (size unspecified) 

2X (1 lbs. a.i/A) also tested. 

• Loamy soil

• Residues scaled with 

application rate

Supplemental 

(Byrne et al. 

2011, MRID 

49090504; 

Fischer and 

Bowers 2012, 

MRID 49090505)

Citrus – 10

(Grapefruit)

Multiple 

fields: 

Helmet, 

Temecula, 

LREC, 

CA

Admire® Pro

1 x 0.5 lbs. a.i/A per yr.

Helmet & LREC = postbloom appl.

(2 years; Fall ‘08/09)

Temecula = summer ’08 

& spring ’09)

Presumed soil drench

Nectar 35.5 23.8 Spring 

2010

• Study designed to evaluate 

carry over (1 x 1X rate shown 

here)

• Helmet = commercial 

orchard, sandy loam soil, 

weekly irrigation

• Temecula = 6 commercial 

fields (soil type not 

specified)

• LREC = 5 citrus blocks, loamy 

soil

• Residues generally reflect 

most recent appl.

Supplemental 

(Byrne et al. 

2011, MRID 

49090504; 

Fischer and 

Bowers 2012, 

MRID 49090505)

Berries – 13

(Blueberry)

3 sites

NY, IL, MI

2 years

(2012, 2013)

Gaucho® 600 FL 

Admire® Pro 600 SC

1 x 0.5 lbs. a.i/A 

3-d post-harvest (Fall)

Banded soil appl.

Pollen (b)

Nectar (h)

42

16

16.5

8.8

240

233

• Experimental fields, irrigated

• Sandy, silt loam, loam soils; 

Highest conc. in sandy soils

• Residues steady/ increase 

during sampling

• No obvious year-to-year 

carryover

• LOQ and LOD for total 

imidacloprid residues were 1 

Acceptable

(Gould et al. 

2014; MRID 

49535602)
82

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. 

Rate, Interval, Timing Matrix1

Residuebased 

Acute 

EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Classification

(Reference)

and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Berries – 13

(Strawberry)

7 sites, 

CA

2 years

(2010, 2011)

Alias® 4F,Admire® Pro, 

or unknown 

formulation

1 x 0.5 lbs. a.i/A in 

2010 & 2011

Presumed soil appl.

Bloom timing unknown

Pollen 320 280 Not 

known

• Commercial fields, light 

(sand) and medium (loam) 

soils; 

• Field portion non-GLP; 

application method and 

timing unknown

• Residues from sandy soils 

higher than loam (<LOD)

• LOD and LOQ in pollen were 

2.6 and 10 ppb, respectively

Supplemental

Gould et al 2012

MRID 49090502

Oilseed – 20

(Cotton)5

9 sites

CA

2 years 

(2013-2014)

Admire® Pro SC

0.33 lbs. a.i/A per yr. @ 

plant

In furrow spray

Pollen

Nectar

Exfl. 

Nectar

43.4

127

35.9

41.1

83.1

35.9

78

78

78

• 2 fine, 1 medium and 6 

coarse soils

• 3 trials = 1 yr. only; 6 trials = 

2 yr.

• No indication of carryover

• LOQ and LOD for total 

imidacloprid residues were 1 

and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable

(Fischer and 

Jerkins, 2015; 

MRID 49665202).

NR: Not reported; LOQ: limit of quantitation; LOD: limit of detection; DAA: Days after application

1Refers to hand collected pollen and nectar unless otherwise specified: “h” (hive collected), “b” (bee collected), or “t” (trapped pollen), 2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application, refers to parent + IMI-olefin and IMI-5-OH 3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application, refers to parent + IMI-olefin and IMI-5-OH 4 DAA = Days after the last application of the pesticide

5Cotton represents sole member of oilseed group with registered soil uses.
83

4.6.5. Soil Application Residue Studies – Open Literature

Additionally, there were 3 studies available from the open literature that investigated the residues of 

imidacloprid in pollen and nectar following soil applications (i.e., 2 studies on cucurbit vegetables and 1 

study on the carryover of imidacloprid residues from soil applications to potatoes). These studies 

generally reported the range of residues determined as well as an average. Table 4-13 below summarizes 

the key elements from each of the studies. Summaries of each study including methods and results are 

provided in Appendix B. 

In a study that assessed the residues of imidacloprid in clover (Rogers and Kemp, 2003 MRID 49719626), 

soil applications to potatoes were made in one year, followed by underseeded grain in the following year 

(treated with imidacloprid), and finally clover in the following year (not treated with imidacloprid). 

Applications to potatoes and underseeded grain were 0.18 lbs a.i/A for each of potatoes and underseed 

grain. Residues in clover pollen and nectar were determined to be below the LOQ (2 ppb) although were 

noted to be as high as 32 ppb in soil (underseeded grain fields).

In one study assessing residues in pollen and nectar from squash (Stoner and Eitzer, 2012; MRID 

49719616), applications of Admire® Pro (42.9% a.i) were made at 0.32 lbs a.i/A (slightly lower than the 

maximum single application rate of 0.38 lbs a.i/A) in two consecutive years, with one year having 

application at one day pre-plant and the other year at 5-days post-transplant in a green house. The pollen 

and nectar residues from both trials were pooled which resulted in residues as high as 28 ppb in pollen 

and 14 ppb in nectar. As these data were pooled, it could not be ascertained the potential differences in 

year-to-year results as well as the potential effect of differing applications regimens on the magnitude of 

residues.

In another residue study with pumpkins (Dively and Kamel, 2012; MRID 49719612), various treatment 

regimens of imidacloprid were tested with applications rates ranging from 0.027 to 0.38 lbs a.i/A 

(representing 7 – 100% of the maximum annual application rate for soil application to cucurbit 

vegetables). The lowest residues (6.7 ppb in pollen and 0.5 ppb in nectar) resulted from the bedding 

drench method at 0.027 lbs a.i/A while the highest (101 ppb in pollen and 13.7 ppb in nectar) were 

associated with a split application of 0.19 lbs a.i/A as a transplant water treatment followed by 0.19 lbs 

a.i/A as a drip irrigation treatment. In a subsequent trial the following year, the maximum residues in 

pollen and nectar (associated with the split application method described above) were 44 and 16 ppb, 

respectively.
84

Table 4-13. Summary of the soil application residue studies evaluated from the open literature

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Max 

Value

(ppb)2

Average

Value

(ppb)3

DAA 

(days) Study Notes

Classification

(Reference)

Root & 

Tuber 

Vegetables –

1 (Potato), 

Cereal Grains 

– 15

(unspecified)

, Non-grass 

animal feed 

– 19 (Clover)

23 sites

(18 in Prince 

Edward 

Island; 5 in 

New 

Brunswick) 

3 years

(1999-2001)

Admire® Pro 240F

0.18 lbs a.i/A, 1999 (Year 3 

– clover), 2000 (Year 2 –

under seeded grain), 2001 

(Year 1 – potato) (All 

applications made in 

Spring)

Pollen (b) 

– clover 

only)

Nectar (b) 

– clover 

only)

<LOQ

<LOQ

NR

NR

NR

NR

• Study examined carryover 

of residues primarily in soil 

over the course of three 

years and different crops. 

• Pollen and nectar 

measurements only in 

clover. 

• LOD: NR; LOQ: 2 ppb

Qualitative

(Rogers and 

Kemp, 2003 

MRID 

49719626)

Cucurbit 

Vegetable –

9

(Squash)

2 sites, 

Connecticut, 

2 trial years 

(2009/2010)

Admire® Pro, 0.32 lbs a.i/A

@ 1 d pre-plant (soil spray) 

Admire® Pro, 0.32 lbs a.i/A

@ 5 d post-transplant in 

greenhouse (drip irrigation)

Pollen

Nectar

28

14

14

10

Variable

Variable

• Residue values pooled 

across appl. methods (effect 

of appl. method unknown) 

• Pollen and nectar samples 

obtained at varying times 

depending on the treatment 

regimen and trial year,

• LOD: 0.5 – 2 ppb depending 

on the matrix (no further 

information provided),

LOQ: NR

Qualitative

(Stoner and 

Eitzer, 2012

MRID 

49719616)

Cucurbit 

Vegetable –

9

(Pumpkin)

1 site, NC, 2 

trial years

(2009, 2010)

Admire® Pro, 0.027 lbs 

a.i/A bedding drench 

(2009)

Admire® Pro, 0.25 lbs 

a.i/A, transplant water 

treatment (2009)

Admire® Pro, 0.38 lbs 

a.i/A, transplant water 

treatment, (2009)

Pollen

Nectar

Pollen

Nectar

Pollen

Nectar

Pollen 

6.7

0.5

40.1

7.3

86.6

11.9

101

4.9

0.4

36.7

5.7

60.9

7.4

80.2

NR

NR

42-45

42-45

42-45

42-45

42-45

• Soil characteristics not 

provided

• The metabolites IMI-olefin,

IMI-5-OH, desnitroimidacloprid, urea 

metabolite, and 6-CNA 

were 14-31% of the values 

of parent (based on means) 

in pollen and 25 – 57% in 

nectar (breakdown of 

metabolite not provided)

Qualitative

(Dively and 

Kamel, 2012

MRID 

49719612)
85

Crop Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Max 

Value

(ppb)2

Average

Value

(ppb)3

DAA 

(days) Study Notes

Classification

(Reference)

Admire Pro, 0.19 lbs a.i/A x 

2), transplant water / drip 

irrigation (2009)

Nectar 13.7 11.2 42-45 • LOD and LOQ of 0.2 and 

0.66, respectively

Admire® Pro, 0.027 lbs 

a.i/A, bedding drench 

(2010) 

Admire® Pro, 0.25 lbs 

a.i/A, transplant water 

treatment, (2010)

Admire® Pro, 0.19 lbs a.i/A 

x 2, transplant water / drip 

irrigation (2010)

Pollen

Nectar

Pollen

Nectar

Pollen

Nectar

<LOD

<LOD

23.9

6.7

44.0

16.0

<LOD

<LOD

18.2

6.1

31.8

9.1

NR

NR

42-45

42-45

42-45

42-45

• Soil characteristics not 

provided

• Weather/more frequent 

irrigation in 2010 may 

contribute to lower residues 

in 2010 vs. 2009. 

• The metabolites IMI-olefin, 

IMI-5-OH, desnitroimidacloprid, urea 

metabolite, and 6-CNA were 

not detected in nectar and 

pollen

• LOD and LOQ of 0.2 and 

0.66, respectively

NR: Not reported; LOQ: limit of quantitation; LOD: limit of detection 1

Unless delineated as “h” (hive collected), “b” (bee collected), or “t” (trapped pollen), nectar and pollen refer to hand collected pollen and nectar 2If study provided a low to high range of residues, the high end value is reported here 3Value reflect the reported mean value of all residues within the provided scenario. Studies generally did not provide information on the numbers of sampling intervals from which the average was 

derived and therefore it is assumed to be one sampling period unless otherwise noted.
86

4.6.6. Seed Treatment Application Residue Studies – Registrant Submitted

A registrant-submitted study is available to characterize the total residues of parent imidacloprid and the 

metabolites IMI-olefin and IMI-5-OH in pollen in seed-treated corn followed by a subsequent planting of 

clover as a rotational crop to examine the uptake of imidacloprid from soil. Additionally, there are several 

other registrant-submitted studies that were either a semi-field tunnel or full-field study design that had 

a residue component in addition to characterizing the effects of imidacloprid on honey bee colonies. 

While these studies will not be individually discussed, it is noted here that they generally reported no 

residues in pollen and nectar (hand collected from plant, bee-collected, and hive sources) above the LOD 

or LOQ, which depending on the study, ranged from 1.5 to 10 ppb (inclusive of LOD and LOQ). Due to 

several deficiencies associated with each study (which are summarized in Appendix A), these studies are 

designated as supplemental from an exposure (i.e. residue information) standpoint and invalid with 

respect to effects. Table 4-14 below summarizes the key elements of the available registrant-submitted 

seed-treatment residue information. 

In the available seed-treatment corn study (conducted in Kansas and Nebraska, 2012-2013; MRID 

49511701), imidacloprid (as Gaucho® 600 ST) was applied at a rate of 1.34 mg a.i/seed (equivalent to 0.12 

lbs a.i/A) which is the highest labeled equivalent application rate for seed-treated corn). Residues were 

only available in pollen as corn does not produce nectar. Acute and chronic EECs for pollen were 39.7 and 

22.3 ppb, respectively. Notably, while the average and maximum residues values were similar in two of 

the three trials (i.e. sites), residues were generally higher in the third trial. The percent sand in soils from 

the third trial (36%) is about 2X (16%) and 30% greater (28%) from that of the other two trials, respectively 

suggesting that the higher imidacloprid residues in pollen for this trial may be the result of its greater 

fraction of sand in soil.

Additionally, this study planted clover as a rotational crop to investigate the residues in pollen and nectar 

following seed treatment applications to corn the previous year. The majority of samples were below the 

LOD; however, in samples with detectable levels, the maximum measured residues in pollen and nectar 

were 3.8 and 1.3 ppb, respectively. A more detailed description of the methods and results of this study 

can be found in Appendix E. 
87

Table 4-14. Summary of the registrant submitted seed treatment application residue studies

Crop Group 

Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA 

(days) Study Notes Classification(Reference)

Cereal Grain –

15

(Corn/Maize)

3 sites, 

KS, NE

2 years

(2012, 2013)

Gaucho 600 ST

1.34 mg a.i./seed 

(0.12 lbs. a.i/A)

Pollen 39.7 22.3 84

• Experimental fields

• Loam, silty loam, silty 

clay soils

• Residues increase 

during sampling time

• LOD and LOQ in pollen 

were 0.5 and 1 ppb, 

respectively

Acceptable

(Miller et

al. 2014, MRID

49511701)

Rotational Crop 

(Clover)

3 sites KS, NE 

(2013)

White clover planted on 

fields with prior year 

planting of seed-treated 

corn @ 1.34 mg a.i./seed

Pollen

Nectar

3.8

1.3

2.3

1.1

461, 456

401

• Vast majority of 

residues were < LOD

• Residues at 1 ppb 

reflect assumptions of 

1⁄2 the LOD for nondetects.

Cereal Grain -

15

(Corn/Maize)

1 site, 

(tunnel)

Germany

1 year

Gaucho 70 WS, 

1 mg a.i/seed, 

seeds sown on 5/10/2000

Pollen <LOQ NR 70-77

• Semi-field tunnel 

study

• Pollen from seed 

treated corn fed to 

bees for 38 day 

exposure 

• No soil information

• LOD: NR, LOQ: 5 ppb

Supplemental

(exposure only)

(Maus et al. 

2000, MRID 

47699416)

Cereal Grain -

15

(Corn/Maize)

1 site, 

(tunnel)

Germany

1 year

Imidacloprid FS 600, 

1 mg a.i/seed, 

seeds sown 11/23/2000 

(in Brazil)

Pollen <LOQ NR 63

• Semi-field tunnel 

study Pollen from 

seed treated corn fed 

to bees for 45 day 

exposure

• Soil characterized as: 

3.1% coarse sand, 

7.3% fine sand, 37.6% 

clay, 51.9% silt)

Supplemental

(exposure only)

(Maus et al. 

2002, MRID 

47699414)
88

Crop Group 

Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA 

(days) Study Notes Classification(Reference)

• LOD: NR, LOQ: 5 ppb

Oilseed – 20

(Sunflower)

1 site, 

Germany 

1 year (1998)

Gaucho 70 WS, 

0.7 mg a.i./seed 

(0.05 lbs a.i./A) 

seeds sown on 5/8/1998

Nectar (b) <LOQ NR

14 

(exposure 

duration)

• Full field study, 

• % foraging on treated 

crop not quantified

• Control field had 

sandy, gravelly soil (no 

data on treatment 

field soil)

• LOD: NR, High LOQ: 

(10 ppb)

Supplemental

(exposure only)

(Schmidt et al. 

1998 – MRID

49766206)

Oilseed – 20

(Rapeseed/ 

canola)

4 sites, 

(tunnels)

Germany

1 year (1999)

Poncho FS 500 

(formulated with betacyfluthrin)

0.03 lbs a.i./A

seeds sown 5/12/1999

Nectar (h)

Nectar 

Pollen (b)

Pollen

<LOD

<LOD

<LOD

<LOD

NR

NR

NR

NR

10

54 - 59

10

54 - 59

• Semi field tunnel study

• Silty clay soil type 

• Sites had various prior 

regimens of 

imidacloprid use

• LOD: 1.5 ppb, LOQ: 5 

ppb

Supplemental

(exposure only)

(Schmuck, 

Schöning,

Schramel 

1999,

MRID 

47699417)

Oilseed – 20

(Sunflower)

4 sites,

(tunnels)

Germany

1 year (1999)

Gaucho WS 70

0.05 lbs a.i/A

seeds sown 5/12/1999

Pollen 

Pollen (h)

Nectar (h)

<LOD

<LOD

<LOD

NR

NR

NR

NR

10

NR

Cereal Grain -

15

(Corn/Maize)

4 sites

(tunnels)

Germany

1 year (1999)

Gaucho WS 70

0.08 lbs a.i/A

seeds sown 5/12/1999

Pollen <LOD NR NR

Oilseed – 20

(Rapeseed/ 

canola)

4 sites

(tunnels) 

Germany

1 year (1999)

Poncho FS 500 

(formulated with betacyfluthrin

0.06 lbs a.i./A

Nectar 

Pollen (b)

<LOD

<LOD

NR

NR

NR

10

• Semi field tunnel study 

loamy silt soil type

Supplemental

(exposure only)
89

Crop Group 

Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA 

(days) Study Notes Classification(Reference)

seeds sown 05/11/1999 Pollen 

Pollen (h)

<LOD

<LOD

NR

NR

59 – 69

10

• Sites had various prior 

regimens of 

imidacloprid use

• LOD: 1.5 ppb, LOQ: 5 

ppb

(Schmuck, 

Schöning, 

Schramel 

1999,

MRID 

47699422, 

47699425, 

47699423)

Oilseed – 20

(Sunflower)

4 sites 

(tunnels) 

Germany

1 year (1999)

Gaucho® WS 70

0.04 lbs a.i./A

Seeds sown 5/10/1999

Pollen (h)

Nectar (h)

Pollen 

<LOD

<LOD

<LOD

NR

NR

NR

4

2-8

NR

Cereal Grain -

15

(Corn/Maize)

4 sites 

(tunnels) 

Germany

1 year (1999)

Gaucho® WS 70

0.08 lbs a.i/A

Seeds sown 05/09/1999

Pollen <LOD NR NR

Oilseed – 20

(Rapeseed/ 

canola)

1 site

(tunnel) 

Sweden

(1999)

Poncho® FS 500 

(formulated with beta 

cyfluthrin)

0.05 lbs a.i/A (Planting 

date not specified, 

exposure period July 2-6)

Nectar (b)

Nectar

<LOQ

<LOQ

NR

NR

4

NR

• Semi-field tunnel 

study 

• LOD: NR, High LOQ: 

(10 ppb)

Supplemental

(exposure only)

(Schmuck, 

Schöning, 

1999, MRID 

47699418)

Oilseed – 20

(Rapeseed/ 

canola)

2 sites, 

Ontario, 

Canada, and 

Minnesota, 

USA

(2000)

Gaucho® + Vitavax 

(carboxin and thiram), 6-7 

lbs product/A (Ontario), 

planting time not 

reported

Gaucho + Vitavax 

(carboxin and thiram), 4.5 

lbs product/A (Ontario), 

planting time not 

reported

Nectar (b)

Pollen (b)

Nectar (b)

Pollen (b)

<LOQ

<LOQ

0.81 

7.6

NR

NR

NR

NR

8

8

8

8

• Full field study, 

Ontario Loam soil 

(Ontario), no soil 

information for MN 

component

• Vitavax (carboxin and 

thiram) + Lindane was 

used as negative 

control

• % foraging on crop not 

quantified 

• LOD: 0.3 ppb, LOQ: 1 

ppb

Supplemental

(exposure only)

(Scott-Dupree 

et al, 2001 –

MRID

45422435)
90

Crop Group 

Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix1

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA 

(days) Study Notes Classification(Reference)

Oilseed – 20

(Rapeseed/ 

canola)

1 site, 

Germany

(1999)

Imidacloprid + beta 

cyfluthrin FS

0.03 lbs a.i./A

Seeds sown 08/23/1999

Nectar <LOD NR NR

• Full field study

• Soil type not reported

• % foraging on crop not 

quantified

• LOD: 1.5 ppb, LOQ: 5 

ppb

Supplemental

(exposure only)

(Schuld, 2002 

MRID 

49073605)

Oilseed – 20

(Rapeseed 

/canola)

1 site

France

(1998)

Poncho® (formulation 

with beta-cyfluthrin)

0.05 lbs a.i./A

Seeds sown 03/19/1998

Nectar (b)

Nectar

<LOQ

<LOQ

NR

NR

NR

NR

• Semi field tunnel 

study

• LOD: NR; LOQ: (10 

ppb)

Supplemental

(exposure only)

(Schmuck, 

Schöning, 1999

MRID 

47699419)

NR: Not reported; LOQ: limit of quantitation; LOD: limit of detection

1Unless delineated as “h” (hive collected), “b” (bee collected), or “t” (trapped pollen), nectar and pollen refer to hand collected pollen and nectar 2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application, refers to parent + IMI-olefin and IMI-5-OH (applies only to Miller et al. 2014, MRID 

49511701). 

3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application refers to parent + IMI-olefin and IMI-5-OH (Miller et al. 2014, MRID 49511701)
91

4.6.7. Seed Treatment Application Residue Studies – Open Literature

Additionally, there were 4 studies available from the open literature that investigated the residues of 

imidacloprid in pollen and nectar following seed treatment applications either as a targeted residue study 

or as part of a semi-field or full-field study design. As these studies originated from the open literature, 

only the maximum and average residue values were available and not the entire dataset to verify the 

findings. Table 4-15 below summarizes the key elements from each of the studies. Summaries of each 

study including methods and results are provided in Appendix B. 

In a study by Donnarumma et al. (2011, MRID 49719614 ), seed-treated corn (Gaucho® 350 FS at 1.0 

mg/seed, insufficient information to provide rate in terms of lbs a.i/A) were planted and samples were 

collected at 30, 45, 60, 80, and 130 days after initial sowing. Analysis of pollen residues 130 days after 

sowing indicated residues below the LOQ of 1. The study also indicated that residues in the soil declined 

steadily as the trial progressed, i.e., 652 ppb at 30 days after sowing to 11 ppb 130 days after sowing. 

In a study by Laurent and Rathahao (2003; MRID 48077902), the uptake and distribution of seed treatedimidacloprid in sunflowers was examined under controlled conditions in the laboratory and uncontrolled 

conditions using an outdoor lysimeter. Sunflower seeds were dressed with Gaucho® 70 WS (1 mg 

a.i/seed). The dressed seeds were also radiolabeled with 14C (radiochemical purity of >97%). Pollen 

residues were collected when approximately 2⁄3 of the florets on the treatment plots were blossoming and 

indicted a mean residue level of 13 ppb and a maximum of 36 ppb. It was also determined from the 

radiolabeling analysis that between 3 – 10% of the total applied radioactivity was taken up by the plant 

depending on whether the plant was grown under controlled laboratory conditions or within an outdoor 

lysimeter.

Schmuck et al. (2001), conducted an uptake and metabolism study of imidacloprid-treated sunflower 

seeds in a greenhouse as well a residue component of imidacloprid-treated sunflower seeds in a honey 

bee field study. For the greenhouse component, sunflower seeds were dressed with labeled [methylene- 14C]imidacloprid formulated as the commercial 700g/ kg WS (Gaucho® WS 70) at a rate of 0.7 mg a.i/seed. 

The field residue component was conducted with a rate of 1 mg a.i/seed. Parent imidacloprid, IMI-olefin

and IMI-5-OH were assessed. The LOD and LOQ were reported to be 1.5 and 5 ppb, respectively. Pollen 

and nectar residues in both study components were reported to be below the LOD with sampling interval 

of 62 – 66 days after application. 

In Stadler 2003 (MRID 47796301), while the primary focus was to evaluate the effects of imidacloprid on 

honey bee colonies exposed to imidacloprid-treated sunflower seed, the residues in bee-collected nectar 

and pollen as well as hive wax were quantified with an LOD of 1.5 ppb in all matrices. The honey bee 

colonies were exposed to seed-treated sunflower for 10 days, monitored through an overwintering 

period, and after which nectar and pollen samples were taken. Parent imidacloprid, IMI-olefin and IMI-5-

OH were below the LOD in all matrices assessed. The interval between samples being collected and 

analyzed was approximately 216 days.
92

Table 4-15. Summary of residue data from imidacloprid-treated seed studies evaluated from the open literature.

Crop Group (Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. 

Rate, Interval, Timing Matrix

Max 

Value

(ppb)2

Average 

Value

(ppb)3

DAA 

(days) Study Notes Classification(Reference)

Cereal Grain - 15

(Corn/Maize)

1 site

Italy

(Year of 

study not 

reported)

Gaucho® 350 FS

1 mg/seed

Sowing time not 

reported

Pollen <LOQ NR 130

• Soil composition was 

54.3% clay, 43.4% silt, 

and 2.3% sand

• LOD: NR, LOQ: 1 ppb

Qualitative

(Donnarumma, 

2011

MRID 49719614)

Oilseed – 20

(Sunflower)

1 site, 

Argentina

(2000 –

2001)

Gaucho® 60 FS

0.26 mg a.i/seed, 

Seeds sown 

12/7/1999

Honey (b)

Pollen (b)

Wax 

<LOD

<LOD

<LOD

NR

NR

NR

13 days

13 days

13 days

• Full field study

• No soil characterization 

Pollen analysis showed 

20-30% of pollen 

collected was from 

sunflower

• LOD: 1.5 ppb, LOQ: 5 ppb

Qualitative

(Stadler, 2000

(also part of 

open lit effort as 

Stadler 2003, 

MRID 47796301)

Oilseed – 20

(Sunflower)

1 site, 

France

(Year of 

study not 

reported)

Gaucho® 70 WS

1 mg a.i/seed

Grown in controlled 

conditions for 4-5 

days until emergence 

then transferred to 

outdoor lysimeter

Pollen 36 13 NR

• Reported that uptake of 

radiolabeled 

imidacloprid into plant 

from treated seeds 

ranged from 3-10%

• LOD: NR, LOQ: 0.5 ppb

Qualitative

(Laurent and 

Rathahao, 2003

MRID 48077902)

Oilseed – 20

(Sunflower)

1 site, 

Germany

Gaucho® WS 70

0.7 mg a.i/seed 

(greenhouse 

component)

Gaucho® WS 70

1 mg a.i/seed

Pollen

Nectar

Pollen

Nectar

<LOD

<LOD

<LOD

<LOD

NR

NR

NR

NR

62-66 

days

• Greenhouse component: 

LOD: 1 ppb; LOQ: NR

• Full field study 

component: LOD: 1.5

ppb; LOQ: 5 ppb

• Full field component 

seeds also treated with 

carbendazim, metalaxyl

and copper oxyquniolate

Qualitative

(Schmuck 2001, 

MRID

47812303)

1Unless delineated as “h” (hive collected), “b” (bee collected), or “t” (trapped pollen), nectar and pollen refer to hand collected pollen and nectar 2If study provided a low to high range of residues, the high end value is reported here 

3Value reflect the reported mean value of all residues within the provided scenario. Studies generally did not provide information on the numbers of sampling intervals from which the average was 

derived and therefore it is assumed to be one sampling period unless otherwise noted.
93

4.6.8. Combined Application Method Residue Studies 

There are three registrant-submitted studies available to characterize the total residues of parent 

imidacloprid, IMI-olefin, and IMI-5-OH in pollen and nectar following applications made via two different 

methods (i.e. a combination of two of applications via seed treatment, soil, or foliar methods). It is noted 

that labels stipulate maximum annual rate of 0.5 lbs a.i/A for several use patterns and allow for a 

combination of methods to get to that maximum rate. Two studies in tomato and cotton examine a soil 

application followed by multiple foliar applications while one study in cotton involves a seed treatment 

application followed by foliar spray applications. None of the residue studies evaluated from the open 

literature combined application method design. Table 4-16 and Table 4-17 below summarize the key 

elements of the soil + foliar and seed treatment + foliar residue studies. A more detailed description of 

each study is provided in Appendix E. 

In a study assessing residues from the combined soil + foliar applications to tomatoes (conducted in 

California, 2013 – 2014; MRID 49665201; same study as that discussed in the soil-applied section), 2 foliar 

applications of 0.06 lbs a.i/A each were made at bloom following the a soil application of 0.38 lbs a.i/A for 

a total rate that approximates the highest annual application rate for imidacloprid on fruiting vegetables. 

Tomatoes do not produce nectar and therefore only pollen data (bumble bee-collected) are available9

. 

The acute and chronic EECs were 1521 and 1268 ppb, respectively, and are approximately 6-fold higher 

than acute and chronic EECs for the soil-applied component alone.

For the combined soil + foliar study on cotton (conducted in California, 2013 -2014; MRID 49665202; same 

study as that discussed in the soil-applied section), 3 foliar applications of 0.06 lbs a.i/A each were applied 

during bloom after a 0.33 lbs. a.i/A soil application for a total rate that approximates the highest annual 

application rate for imidacloprid on cotton. As with the soil-applied alone study component, residues 

were assessed in pollen, floral nectar and extra-floral nectar. While in the soil-alone component found 

floral nectar residues above those of extra-floral, when the foliar component was added, the extra-floral 

nectar residues were an order of magnitude higher than floral nectar (i.e., acute and chronic EECs were 

2775 and 1952 ppb, respectively in extra-floral nectar as compared to 171 and 152 ppb, respectively in 

floral nectar). Acute and chronic EECs in pollen were similar at 328 and 324 ppb, respectively.

Finally, in the seed treatment + foliar study on cotton (conducted in Missouri, 2012 – 2013; MRID 

49511702), 5 applications of 0.06 lbs a.i/A (Admire® Pro – 42.9% a.i) followed a seed treatment with 

Gaucho® 600 Flowable equivalent to an application rate of 0.05 lbs a.i/A. This scenario represents the 

highest annual application rate of foliar applications made to cotton (0.31 lbs a.i/A). Residues in pollen, 

floral nectar, and extra-floral nectar were assessed. Similar to the soil-applied alone component of that 

study, residues in floral nectar were higher than that of extra-floral nectar (acute and chronic EEC of 40 

and 29 ppb, respectively for floral nectar as compared to 30 and 16.2 ppb, respectively for extra-floral 

nectar). These residues were also approximately 3.5-fold lower than those determined in the soil-applied 

alone study component of the soil + foliar study discussed above, despite 5 foliar applications at bloom. 

Acute and chronic residues in pollen were 57 and 25 ppb, respectively. Additionally, this study 

 9 Greenleaf, S. and Kreman, C. (2006). Wild bee species increase tomato production and respond differently to 

surrounding land use in Northern California. Biological Conservation, 133. 81-87.
94

investigated residues in white clover as a rotational crop planted following foliar applications the previous 

year. Total residues were near or below the level of detection (0.7 ppb) in the majority of samples 

analyzed (detection frequency = 38% for clover nectar and 53% for clover pollen). The maximum 

concentrations of total IMI measured in clover nectar in trials NT014 and NT015 are 1.6 and 2.7 ppb, 

respectively. The maximum concentrations of total IMI measured in clover pollen in trials NT014 and 

NT015 are 8 and 8.6 ppb, respectively.

 
95

Table 4-16. Summary of the registrant-submitted combined application method residue studies (soil application + foliar spray)

Crop 

Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix 1

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Classification (Reference)

Fruiting

Vegetables 

– 8 

(Tomato)

9 sites

CA

2 years 

(2013-2014)

1 x 0.38 lbs. a.i/A 

Admire® Pro SC

(soil @ transplant + 

2 x 0.06 lbs. a.i/A 

Admire® Pro SC

(foliar, at bloom)

Pollen 

(b) 1521 1268 2-8 

• Experimental fields; fine, 

medium, and coarse soils

• Year 2 ongoing for 5 sites

• 1-2 replicates from bumble 

bee-collected pollen 

• Most residue data reflect 

coarse soils 

• Limited data indicates no 

year-to-year carry over 

(leaves)

• LOQ and LOD for total 

imidacloprid residues were 

1 and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable 

(Gould and 

Jerkins, 2015, 

MRID 49665201)

Oilseed –

20

(Cotton)5

9 sites

CA

2 years 

(2013-2014)

1 x 0.33 lbs. a.i/A

Admire® Pro SC @ plant 

(in furrow spray) +

3 x 0.06 lbs. a.i/A

Admire® Pro SC (@ 

bloom)

Pollen

Nectar

Exfl. 

Nectar

328

171

2775

324

153

1952

4

4

5

• 2 fine, 1 medium and 6 

coarse soils

• 3 trials = 1 yr. only; 6 trials 

= 2 yr.

• No indication of carryover 

• LOQ was 1 ppb in pollen, 

nectar and extra-floral 

nectar

Acceptable 

(Fischer and 

Jerkins, 2015; 

MRID 49665202)

1Refers to hand collected pollen and nectar; “b” refers to bee-collected sample 2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application, refers to parent + IMI-olefin and IMI-5-OH 3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application, refers to parent + IMI-olefin and IMI-5-OH 4 DAA = Days after the last application of the pesticide

5

Cotton represents sole member of oilseed group with registered soil and foliar uses.
96

Table 4-17. Summary of the registrant submitted combined application method residue studies (seed treatment + foliar spray)

Crop 

Group 

(Crop)

No. Sites/

Location/ 

Duration

Formulation, Appl. Rate, 

Interval, Timing Matrix

Residuebased 

Acute EEC2

(ppb)

Residuebased 

Chronic 

EEC3

(ppb)

DAA4

(days) Study Notes Reference

Oilseed –

20

(Cotton)5

3 sites

MO

2 years

(2012, 2013)

Gaucho® 600 FL Admire®

Pro SC

5 x 0.06 lbs. a.i/A x 5 

(foliar), 5-8 d int. @ 

bloom +

Gaucho® 600 Flowable

0.05 lbs. a.i/A (seed trt) 

@ planting

Pollen

Nectar

ExNectar

56.7

39.5

30

25.2

29

16.2

26, 14

21, 14

14, 29

• Experimental fields, sand, 

sandy loam, silty loam soils

• General decline in nectar 

and extra floral nectar 

residues during 10-20 DAA

• Unclear whether higher 

residue in nectar (year 2) is 

due to carryover. 

• LOQ and LOD for total 

imidacloprid residues were 

1 and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively

Acceptable 

(Gould et al. 

2014, MRID 

49511702)

Rotational 

Crop 

(Clover)

3 sites

MO

(2013)

Untreated white clover 

planted on fields with 

prior year planting of 

seed-treated cotton @ 

0.05 lbs. a.i./A and foliar 

spray of 5 x 0.06 lbs. 

a.i./A

Pollen

Nectar

8.6

2.7

4.8

1.3

439

405, 411

• Vast majority of residues 

were < LOD

• Residues at 1 ppb reflect 

assumptions of 1⁄2 the LOD 

for non-detects.

• LOQ and LOD for total 

imidacloprid residues were 

1 and 0.7 ppb in nectar, 

respectively, and 1 and 0.5 

ppb in pollen, respectively 1 Refers to hand collected pollen and nectar

2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application, refers to parent + IMI-olefin and IMI-5-OH 3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application, refers to parent + IMI-olefin and IMI-5-OH 4 DAA = Days after the last application of the pesticide

5

Cotton represent sole member of oilseed group with registered foliar uses
97

4.7. Carry-over of Imidacloprid Residues in Soil 

The carryover of imidacloprid residues in soil (i.e. year-to-year accumulation in pollen and nectar) was 

considered as a potential exposure route. As discussed in Section 4.1, imidacloprid is persistent in the 

soil, with half-ranging from 305 days to several years in studies that were terminated after one year and 

up to 71% of the applied imidacloprid was still present in the soil. Several lines of evidence were 

considered in evaluating the potential impact of this exposure route including modeling results, rotational 

crop studies, and field trials in pollen and nectar, with a subset of these latter studies exploring the 

residues of pollen ad nectar in a rotational crop (white cover) in fields that were previously treated with 

imidacloprid. 

The modeling of potential residues present after carryover accumulation in soil indicated an accumulation 

of about 5 times the annual rate is potential within 10 years of repeated annual applications. This 

simulation does not take into account important routes of dissipation including leaching, run-off, and 

plant up-take of imidacloprid residues which are expected to reduce to the potential magnitude of this 

accumulation. 

Available rotational crop studies confirmed occurrence of soil carry-over from application to one crop to 

the following crop based on data obtained for magnitude of residues in rotational crops. In these studies, 

detectable residues of imidacloprid were found in variable quantities in rotational crops planted after 1, 

4, 8 and 11 months rotational intervals following a single granular application of 0.29-0.32 lb. a.i/A. While 

residues reached as higher as 0.58 ppm in the edible portions of various crops, residues in pollen and 

nectar were not available from these studies. Furthermore, these studies considered the total residues 

of imidacloprid as parent plus 7 other degradation products including those that are not identified as 

being of toxicological concern (IMI-olefin and IMI-5-OH). 

Additionally, the available field trials in pollen and nectar were evaluated. In several studies that were 

conducted in one growing season, where only one sampling interval was included (as was the case with 

the foliar-applied cotton study, soil-applied melon study, and soil-applied strawberry study) the potential 

for carryover could not be assessed due to limited data from one year only. Additionally, the foliar applied 

studies with citrus fruits (oranges) had uncertainties associated with it that confound the ability to 

ascertain a carryover effect. These include inadvertent applications of imidacloprid to the trial field and 

differing nectar and pollen sampling measurements across trials. In other cases (soil + foliar applied 

tomato and soil + foliar applied cotton) there was insufficient information present to determine whether 

a carryover effect was present. Finally, 3 studies (soil-applied blueberry, seed treatment corn, and seed 

+ foliar-applied cotton) included sufficient information to assess whether a carryover effect was present 

across the multiple trial years within a study. For 2 of these studies (seed treatment corn seed + foliarapplied cotton) a rotational crop (white clover) was planted in the season directly following the trial years 

to investigate the residues in pollen and nectar resulting from plant uptake of imidacloprid residues from 

the soil in the previous season. These studies are further discussed below.

In the soil applied blueberry study conducted across two trial years, there was no indication of carryover 

as nectar residues decreased from year 1 to year 2 (7.25 ppb vs 1.8 ppb) while residues in pollen remained 
98

essentially the same (13.7 vs 14.0 ppb) from year 1 to year 2. In the seed treatment corn study, there did 

not appear to be a consistent increase or decrease in pollen residues in year 2 values relative to year 1. 

This finding is despite the fact that residues in soil measured prior to planting in year 2 (9-80 ppb) are

elevated compared to those measured prior to planting in year 1 (2-4 ppb) which suggests a year-to-year 

carryover in soil. Finally, the seed + foliar treatment cotton study indicated that year 2 mean residues in 

floral and extra floral nectar increase by 1.2X to 2.7X over year 1 mean residues. With cotton pollen, yearly 

averages of mean residuesincrease by 1.5X to 2.9X from year 1 to year 2. Interestingly, the two trials with 

the highest coarseness in soils show the greatest relative increase in yearly average residues from year 1 

to year 2 in nectar and pollen (1.7X to 2.9X) compared to the trial where the soil type was described as 

mostly silt (1.2-1.5X). It is not certain whether this differential increase is related to differences in soil 

composition, but all three trials had similar amounts of IMI in soil prior to the 2nd year planting (24-42 

ppb). 

In the rotational crop (clover) component of the seed treatment corn study, the mean residues in pollen

and nectar planted following planting and harvesting corn the previous year were near or below the 

combined limits of detection (1.24 ppb for pollen; 1.33 ppb for nectar) in the majority of samples analyzed 

(detection frequency = 28% for clover pollen and 0% for clover nectar). The maximum concentrations of 

imidacloprid residues in clover pollen in three trials is 3.8 ppb. Similarly, in the rotational component of 

the seed + foliar treatment cotton study, mean residues of imidacloprid were near or below the level of 

detection (0.7 ppb) in the majority of samples analyzed (detection frequency = 38% for clover nectar and 

53% for clover pollen). The maximum concentrations of imidacloprid in nectar were 1.6 and 2.7 ppb, 

respectively. The maximum concentrations of imidacloprid residues in pollen were 8 and 8.6 ppb, 

respectively.

Based on the available data for which sufficient information is present to indicate an effect, there is limited 

indication of a carryover effect from year-to-year accumulation of imidacloprid resides in soil that 

translates to increased residues in pollen and nectar, even in the case where a year-to-year build up in 

soil residues was present (as with the seed treatment corn study). Additionally, the two studies that 

investigated the residues in pollen and nectar in a rotational crop (white clover) did result in a widespread 

occurrence of residues that were substantially above the limit of quantitation for these studies. 

4.8. Observational Residue Monitoring Studies 

In addition to the registrant submitted and open literature field residue trials discussed previously which 

characterized the residues in pollen and nectar following a specific application regimen and sampling 

schedule, there are several monitoring studies available from the open literature to characterize the 

residues of imidacloprid. Rather than a targeted study as those described above, these studies surveyed 

residues of pollen and nectar in crops on agricultural fields with known imidacloprid use as well as samples 

from various matrices (nectar, pollen, wax) from honey bee hives. 
99

4.8.1. Agricultural crop studies

The studies by Bonmatin 2005 (MRID 47523411) and 2007, investigated the residues in various plant parts 

from fields known to have been planted with imidacloprid-treated seed. As a result, it is not possible to 

tie a particular application rate or sampling interval relative to the application timing to the residues of 

imidacloprid that were determined. The work in 2005 investigated the residues in corn pollen and 

trapped pollen originating from corn fields while the study in 2007 assessed corn and sunflower pollen. 

The findings of the 2005 and 2007 studies are summarized below. Full study summaries with a discussion 

of the methods are provided in Appendix C. 

Despite the aforementioned uncertainty of an unknown application rate or sampling interval, Tables 4-18 

and 4-19 below indicate low mean residues of imidacloprid in sampled corn and sunflower pollen with 

values either below the LOQ or a maximum of 3-fold above it. 

Table 4-18. Distribution of samples from corn fields according to their concentration of imidacloprid 

(Bonmatin, 2005)

Sampled 

Matrix

Number of 

samples

Number of 

samples 

below LOD1

Number of 

samples between 

LOD and LOQ1,2

Number 

(Percent) of 

samples 

above LOQ2,3

Mean 

concentration 

(ppb ±SD)3

Corn 

pollen 47 6 18 23 (49%) 2.1 ± 2.7

Trapped 

pollen 11 5 2 4 (36%) 0.6 ± 1.0

1 LOD = 0.3 ppb 

2 LOQ = 1 ppb

3Refers to samples above the LOQ

Table 4-19. Distribution of residues from corn and sunflower pollen according to their concentration of 

imidacloprid (Bonmatin, 2007)

Sampled Matrix Number of samples Percentage of samples 

exceeding LOQ1

Mean concentration 

(ppb)2

Corn pollen 47 2 2.0

Sunflower pollen 24 58 3.0 1LOQ: 1 ppb 

23Refers to samples above the LOQ

4.8.2. Hive monitoring studies

In addition to the crop monitoring studies discussed above, several studies are available from the open 

literature that survey residues in in-hive pollen, wax, nectar , and dead bee samples, for various chemicals, 

including imidacloprid. These studies were not part of the suite of studies that received a review for their 

utility in terms of quantitative or qualitative use for this assessment for the exposure and effects 

assessments. Rather, these studies serve to characterize the potential extent to which bees are exposed 

to imidacloprid in the field. What follows is a summary of these studies while more detailed summaries 

are provided in Appendix C.
100

The available studies that survey various matrices for pesticide contamination, including hive pollen (bee 

bread), trapped pollen (pollen collected from bees as they enter the colony), honey, beeswax, and honey 

bee samples provide a broad picture of the overall in-hive residues that result from use of imidacloprid 

and other chemicals. While the studies differed in the location of sampled hives, as well as the condition 

of the colony from which the samples originated (with only Mullin 2010 and Kasiotis 2014 indicating that 

healthy and known diseased colonies were sampled), all studies had similar sampling procedures for a 

given matrix and appropriately low LOQ values reported for the analytical methods used (LOQs varied 

from study to study, see Appendix C for further information). 

In the several of available studies, regardless of whether they were conducted in the United States or 

Europe, imidacloprid was generally detected in 10% or less of pollen, honey, wax, or honey bee samples. 

For those studies (Mullin 2010, Wiest 2011, Kasiotis 2014, Johnston 2014), the highest imidacloprid 

concentration was detected in trapped pollen at 149 ppb. For the remaining studies (Chauzat 2006, 

Chauzat 2009, Stoner and Eitzer 2013, and Lu 2015), imidacloprid was detected in at least one matrix with 

a frequency of 10% or more While there were high frequencies (nearing or above 50%) of detections of 

imidacloprid in pollen and honey samples in the Chauzat studies, as well as Lu 2015, the mean 

concentrations were generally at or slightly above the reported LOQ. The Chauzat studies in particular 

claim that although certain pesticide residues were frequently detected across various hive matrices, that 

there did not appear to be relationships between the abundance of brood and adults and the presence of 

a particular residue. Stoner and Eitzer 2013 found 12% frequency of detections in pollen.

An additional point to be made from these studies is that, for all studies except Lu 2015 (which screened 

only for neonicotinoid pesticides), multiple pesticides were found in the same samples, with some 

samples containing up to 12 pesticides (Johnston, 2014). In the majority of these cases, the Varroa mite 

(Varroa destructor) treatment miticides fluvalinate, coumaphos, and amitraz (DMA and DMPF degradates) 

were detected, in some cases in up to 98% of the assessed samples, depending on the matrix (Mullin, 

2010). Additionally, fungicides, particularly those of the sterol biosynthesis inhibitor class that include the 

triazole fungicides were detected with high frequency. There are reports in the literature that these 

chemicals may exhibit a greater than additive (e.g., synergistic) effect on toxicity when bees are exposed 

simultaneously with neonicotinoid chemicals like imidacloprid. While the extent of this relationship is 

beyond the scope of this assessment, it highlights the complex nature of interactions of different stressors 

that exist in the hive.
101

5. Effects Assessment 

5.1. Tier I 

At the Tier I (screening) level, effects to individual bee are considered. This is achieved through a suite of 

laboratory studies that assess different life stages (i.e. adults and larvae) and different durations of 

exposure, i.e., acute (single dose) and chronic (repeat dose). The adult acute contact, adult acute oral, 

and larval acute oral toxicity studies have formal protocols published from at least one regulatory entity 

and these protocols are generally adhered to with registrant-submitted data. While test methods 

originating from the open literature can be more varied, the adult acute contact and adult acute oral tests 

evaluated from the open literature for imidacloprid were also generally conducted in accordance with one 

or more published guidelines. The most sensitive endpoints from the Tier I studies (from which findings 

can be statistically verified) inform the Tier I default and Tier I refined RQs, using screening level estimates 

and residue data in pollen and nectar (where available), respectively.

Sources of Data

Registrant-Submitted Studies

For registrant-submitted studies, the distinction between acute/chronic adult and acute/chronic larval is 

made as these guidelines are either already released or in development and are in line the with the 2014 

Guidance for Assessing the Risk of Pesticides to Bees (USEPA et al. 2014). For the acute contact toxicity, 

registrant-submitted studies adhered to either the Office of Chemical Safety and Pollution Prevention 

(OCSPP) guideline 850.302010, the Organization for Economic Cooperation and Development (OECD) Test 

Guideline 21411, or the European and Mediterranean Plant Protection Organization (EPPO) guideline 17012

for adult honey bees. For acute oral toxicity to adult honey bees, studies generally adhered to OECD TG 

21313 and EPPO 170. Acute oral toxicity studies with honey bee larvae were conducted in accordance 

with OECD TG 23714. Finally, the chronic oral larval toxicity tests and chronic adult (10-day) oral toxicity 

test protocols are currently in development but the available studies were conducted in accordance with 

methodology determined to be sufficient for quantitative risk assessment purposes. 

As guidelines are well established for most Tier I data requirements (particularly the acute contact and 

acute oral toxicity tests for adult honey bees), the methodology for each submitted study is not discussed 

extensively but rather only when major guideline deviations are noted. As distinguished from the open 

literature studies, registrant-submitted studies that are designed to satisfy a guideline requirement are 

 10 USEPA. 2012a. “Honey Bee Acute Contact Toxicity” Ecological Effects Test Guidelines OCSPP 850.3020. EPA 712-C-019 Web: 

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0154-0016

11 OECD.1998b. OECD Guidelines for the Testing of Chemicals. Test Number 214, Acute Contact Toxicity Test. http://www.oecdilibrary.org/environment/test-no-214-honey bees-acute-contact-toxicity-test_9789264070189-en;jsessionid=43gvto47wnue9.delta

12 EPPO. 2010. Efficacy Evaluation of Plant Protection Products: Side-effects on Honey bees. PP 1/170 (4). OEPP/EPPO Bulletin 40: 313–319

13OECD. 1998a. OECD Guidelines for the Testing of Chemicals. Honey bees, Acute Oral Toxicity Test. 213. 

http://lysander.sourceoecd.org/vl=5988235/cl=12/nw=1/rpsv/cgi-bin/fulltextew.pl?prpsv=/ij/oecdjournals/1607310x/v1n2/s14/p1.idx

14 OECD. 2013. OECD Guidelines for Testing Chemicals. Honey bee (Apis mellifera) larval toxicity test, single exposure. http://www.oecdilibrary.org/environment/test-no-237-honey-bee-apis-mellifera-larval-toxicity-test-single-exposure_9789264203723-en
102

classified as acceptable (suitable for quantitative use in risk estimation), supplemental (some deviations 

noted that render the study useful for either quantitative or qualitative use), or invalid (not suitable for 

use in risk assessment due to guideline deviations that affect the scientific soundness of a study). 

Typically, open literature studies are designated as for quantitative, qualitative, or invalid for risk 

assessment purposes.

Open Literature Studies

Through a joint collaborative effort by the EPA, Canada’s Pest Management Regulatory Agency (PMRA), 

and the state of California Department of Pesticide Regulation (CDPR), over 30 studies in the open 

literature were evaluated to further characterize the toxic effects of imidacloprid at the Tier I (individual) 

level. These effects include effects on mortality, food consumption, brood production, and behavioral 

responses on several subspecies of Apis, as well as non-Apis bees including bumble bees (Bombus spp.) 

and several solitary bee species including blue orchard bees (Osmia lignaria) and alfalfa leafcutting bees 

(Megachile rotundata). 

While the Guidance for Assessing Pesticide Risks to Bees (USEPA, 2014) stipulates that data from non-Apis

species can be considered in the risk assessment, it does not provide a process to estimate risk as it does 

for honey bees (Apis). This is due in part to the fact that there are different exposure estimates that 

would be needed for non-Apis species that at the present time have not been sufficiently explored by the 

Agency. For example, bumble bee workers and drones are larger than their honey bee counterparts, in 

addition to having higher food consumption rates that would necessitate different contact and oral 

exposure estimates, respectively. For the sake of discussion of the Tier I data, due to the exposure and 

test durations varying so greatly as compared to the more standardized registrant-submitted studies 

(which generally follow established regulatory guidance), those studies with a 5-day or less exposure/test 

duration will be considered acute while those 6 days or longer will be considered chronic. 

To obviate the need to state it for every open literature study discussed, it is noted here that generally all 

open literature studies (with the exceptions noted in the individual discussions) did not provide raw data 

in order to conduct an independent verification of the statistical results. This limitation was one of the 

primary reasons that open literature studies were considered to be qualitative in their utility; those that 

were evaluated and considered invalid for utility in this risk assessment are tabulated in Appendix 1. The 

studies from the open literature not only serve to broaden the database of species for which effects of 

imidacloprid can be characterized, but also expand on the suite of effects that are investigated in the 

registrant-submitted studies, which is generally limited to observations of mortality and clinical signs of 

toxicity (sublethal effects). Additionally, studies from the open literature serve to examine any differential 

toxicity that may be present in Apis vs. non-Apis bees, particularly as it relates to effects on individual bees 

at the Tier I level. 

What follows is a summary of the available registrant-submitted and open literature studies to 

characterize the acute and chronic effects to Apis and non-Apis adult bees and larvae. The studies are 

organized by species (e.g. Apis vs. non-Apis), duration (acute or chronic), route of exposure (contact or 

oral) and source (registrant-submitted and open literature). Unless otherwise stated, in the section 
103

dealing with Apis studies, all studies concern A. mellifera. It is also noted here that a limitation to all Tier 

I data is the uncertainty as to the extent to which the lethal and sublethal effects described in these studies 

translate to an adverse effect(s) at the colony level. 

Table 5-1 below summaries the most sensitive endpoints from each of the Tier I study types with further 

discussion of all studies providing Tier I endpoints provided below. Endpoints in this table originate from 

registrant-submitted studies conducted with A. mellifera with the exception of a chronic (10-day) oral 

toxicity test reported by Boily et al. (2013; MRID 49750601) where raw data were made available by the 

study author to statistically verify the results.

Table 5-1. Summary of endpoints to be used in screening-level and refined Tier I risk estimation

Study Type Endpoint1 Reference Classification

Adult Acute Contact 

Toxicity 96-hr LD50: 0.043 μg a.i/bee MRID 49602717 Acceptable

Adult Acute Oral Toxicity 48-hr LD50: 0.0039 μg a.i/bee MRID 42273003 Acceptable

Adult Chronic Oral Toxicity

10-day NOAEC/LOAEC (food consumption): 

<0.004/0.004 μg a.i/bee (<10/10 μg a.i/L) MRID 49511703 Supplemental

10-day NOAEC/LOAEC (mortality, body 

weight): 0.00016 μg a.i/bee

Boily et al., 

2013;

MRID 49750601

Quantitative

Larval Acute (single dose) No data available

Larval Chronic (repeat 

dose)

21-day NOAEC/LOAEC: 0.0018/>0.0018 μg 

a.i/larva MRID 49090506 Supplemental

Toxicity of Residues on 

Foliage2 (OSCP 850.30303

)

2-hr residues of 0.025 lbs a.i/A: 20% mortality

2-hr residues of 0.05 lbs a.i/A: 19% mortality

2-hr residues of 0.1 lbs a.i/A: 28% mortality

MRID 42480503 Supplemental

Bolded value to be used in risk estimation if more than one endpoint present for a study type. 1

Represents most sensitive (i.e. lowest) of all endpoints within a particular study type for studies for which raw data (to allow for independent 

statistical verification of the endpoint) are available. 2

Although cited in 40 CFR Part 158 as an EPA testing requirement, the results of this study are not used for risk estimation. 3 USEPA. 2012b. “Honey Bee Toxicity of Residues on Foliage.” Ecological Effects Test Guidelines OCSPP 850.3030. EPA 712-C-018. Web. 

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0154-0017

5.1.1. Adult Acute Contact Toxicity

Apis – Registrant-Submitted Studies

There are five available contact studies to characterize the acute toxicity of imidacloprid to adult honey 

bees with technical grade active ingredient (TGAI, purities range from 98.6 - 99.8%) and one study 

conducted with a formulated typical end use product (TEP, 200 g/L, 20% a.i, assuming density of 1 g/L). 

As indicated above, these studies were conducted in accordance with one or more recognized protocols 

for testing the acute contact toxicity to honey bees. The observation period (i.e. study duration) ranged

from 48 – 96 hours and the resultant LD50 values ranged from 0.043 – 0.104 μg a.i./bee. Clinical signs of 

toxicity were noted in the majority of studies. Table 5-2 below summarizes the available registrant 

submitted acute contact toxicity studies to adult honey bees. Summaries for each study are provided in 

Appendix D. It is noted here, as above in Table 5-1, that the most sensitive adult acute contact toxicity 

endpoint is 0.043 μg a.i/bee (MRID 49602717).
104

Table 5-2. Summary of registrant submitted adult acute contact toxicity studies (all studies tested Apis 

mellifera)

Test Substance (% 

a.i)

Study 

Duration

Endpoint (95% 

CI)

(expressed in 

terms of μg 

a.i/bee)

Comments

Classification

(Reference, 

MRID)

TGAI (99.8) 48-hr LD50: 0.078 

(0.068 – 0.090)

No observations (if any) of clinical 

signs of toxicity were noted to be 

present in the study report

Acceptable

(42273003)

TGAI (98.6) 72-hr LD50: 0.104 

(0.080 – 0.131)

- Clinical signs of toxicity include 

paralysis, spasms, or frozen behavior, 

and were observed at all treatment 

groups. 

Acceptable

(49766209)

TGAI (98.6) 72-hr LD50: 0.048 

(0.041 – 0.057)

- Clinical signs of toxicity included 

bees observed to have been 

incapacitated and uncoordinated 

(stumbling) at all treatment groups

Acceptable

(49602715)

TGAI (98.6) 96-hr LD50: 0.043

(0.026 – 0.055) 

- Lying on back/difficulty standing and 

coordination issues reported at all 

treatment groups

Acceptable 

(49602717)

TGAI (98.6) 96-hr LD50: 0.069 

(0.056 – 0.085)

- Clinical signs of toxicity include 

lethargy, lack of coordination, and 

immobility (not specified which 

treatment groups)

Acceptable

(49602714)

TEP (Imidacloprid 

200 SL)

(200 g/L, 20% 

purity assuming 

density is 1 g/L)

96-hr

LD50: 0.045 

(0.034 – 0.060) 

0.0246 μg 

product/bee

- Clinical signs of toxicity noted were 

uncoordinated movement in the 4 

highest treatment groups

Acceptable

(49602707)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product

Bolded value represents endpoint to be used risk estimation 

Apis – Open Literature Studies

There were six studies evaluated from the open literature that investigated the acute contact toxicity to 

honey bee adults. These studies generally followed at least one of the protocols available for the acute 

contact toxicity testing to honey bees. The observation period (i.e. study duration) ranged from 24 – 72 

hours and tests assessed multiple subspecies of A. mellifera. The acute contact LD50 values ranged from 

0.018 – 0.24 μg a.i/bee. As noted previously, these studies were classified as qualitative primarily due to 

their absence of raw data provided to statistically verify the results. In contrast to the suite of registrantstudies, clinical signs of toxicity were generally not reported in the open literature studies. Summaries of 

each study including methods and other limitations and uncertainties are provided in Appendix D. 
105

Table 5-3. Summary of adult acute contact toxicity studies to Apis bees evaluated from the open 

literature

Test 

Species

Test 

Substance 

(% a.i)

Study 

Duration

Endpoint (95% CI)

(expressed in terms of μg 

a.i/bee)

Comments

Classification

(Reference, 

MRID)

Apis 

mellifera TGAI (>99) 24-hr LD50: 0.018 (0.009 – 0.032)

- Also tested piperonyl 

butoxide, triflumizole, 

and propiconzole with 

imidacloprid to assess 

potential synergistic 

effects (no significant 

differences in all

combined LD50 values 

relative to imidacloprid 

alone.

Qualitative

(Iwasa 2004,

47523404)

Apis 

mellifera 

carnica

TGAI (>98)

48-hr

LD50: 0.081 (0.055 – 0.119)

- No mention of whether 

dose response was 

present

Qualitative 

(Schmuck

2001, 

47812303)

LD50: 0.23 (NA)

TEP (70) LD50: 0.24 (0.17 – 0.35)

TEP (200 

g/L, 20% 

purity 

assuming 

density of 1 

g/L)

LD50: 0.060 (0.039 – 0.093)

Apis 

mellifera 

carnica

TGAI (>98) 48-hr

LD50: 0.061 (0.026 – 0.090)

- No mention of 

whether dose response 

was present

Qualitative 

(Schmuck 

2003, 

47796304)

LD50: 0.050 (0.009 – 0.071)

LD50 0.075 (0.062 – 0.091)

Apis 

mellifera 

mellifera TGAI

(98) 48-hr

LD50: 0.024 (0.022 – 0.027)

- Mortality rates 

increase at low doses, 

decrease at 

intermediate doses, and 

increase again at higher 

doses. 

Qualitative

(Suchail 2000, 

47800513)

Apis 

mellifera 

caucasia

LD50 0.013 (0.010 – 0.016)

Apis 

mellifera

TGAI

(99.9) 48-hr LD50: 0.067 (0.044 – 0.102)

- Also tested 

myclobutanil, 

propiconazole,

flusilazole, and 

tebuconazole.

Qualitative

(Thompson 

2014a, 

49750606)

Apis 

mellifera

TEP 

(Provado® 

1.6F)

(17.4)

48-hr

LD50: 0.03 μg a.i/bee (0.017 –

0.05) 

0.15 μg product/bee; 0.05 –

0.32)

- Range of actual doses 

tested was not provided.

- There was no mention 

of whether dose 

response was present.

Qualitative

(Biddinger 

2013, 

(49719605)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product
106

Non-Apis – Registrant-Submitted studies

There are two available registrant-submitted contact studies to characterize the acute toxicity of 

imidacloprid to adult bumble bees; one study each with TGAI (98.6% purity) and one study with 

formulated product (30.4%). These studies are limited in their utility as the study with TGAI could not 

determine a LD50 due to excessive mortality in the majority of concentrations tested by 24 hours after 

treatment and the formulated product study not indicating a clear dose response in the results. 

Summaries of each study are provided in Appendix D. 

Table 5-4. Summary of registrant submitted adult acute contact toxicity studies for non-Apis bees 

(Note: both studies concern Bombus terrestris)

Test 

Substance 

(% a.i)

Study 

Duration

(Type)

Endpoint (95% CI)

(expressed in terms 

of μg a.i/bee)

Comments

Classification

(Reference, 

MRID)

TGAI (98.6) 72-hr Could not be 

determined

- Test concentrations were evidently 

too high as all but lowest treatment 

group had at least 90% mortality after 

24 hours.

- Definitive LD50 could not be 

determined. There was 90 – 100% 

mortality in the 4, 8, 31, 65, and 101 μg 

a.i/bee and 47% mortality at the lowest 

dose (0.1 μg a.i/bee).

Supplemental 

(49766208)

TEP

(Imidacloprid 

FS 350)

(30.4)

96-hr LD50: 85.3 (N/A)

- There was no clear indication of a 

dose response provided (percent 

mortality was 0, 20, 33, 27, 53, and 47% 

in the control, 1.23, 3.70, 11.1, 33.3, 

and 100 μg a.i/bee)

- There was 46.7% mortality in the 

highest treatment group (100 μg 

a.i/bee)

Supplemental

(MRID 49532101)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product

Non-Apis – Open Literature Studies

There were 5 studies evaluated from the open literature that characterize the acute contact toxicity to 

non-Apis bees including bumble bees (B. impatiens), Japanese orchard bees (Osmia cornifrons), blue 

orchard bees, alfalfa leaf cutting bees, and a species of stingless bee (i.e., Melipona quadrifasciata). The 

key elements of these studies are summarized below with full summaries provided in Appendix D. Some 

studies did not estimate endpoints in terms of dose (i.e., μg a.i/bee) and did not provide sufficient 

information for estimating dose per bee. 
107

Table 5-5. Summary of adult acute contact toxicity studies to non-Apis bees evaluated from the open 

literature

Test Species

Test 

Substance 

(% a.i)

Study 

Duration

Endpoint (95% CI)

(expressed in terms 

of μg a.i/bee unless 

otherwise noted)

Comments

Classification

(Reference, 

MRID)

Bumble bee 

(Bombus 

impatiens)

TGAI (>95) 72-hr

No endpoint 

calculated; there was 

72, 96, and 100% 

mortality for the 

0.05, 0.5, and 5 lbs 

a.i/A treatment 

groups, respectively.

- Contact administration 

to bees was via a Potter 

Spray Tower 

- Notably high test 

concentrations, 

particularly the highest 

dose

Qualitative

(Gradish 

2009, 

48194902)

Bumble bee 

(Bombus 

terrestris)

TGAI (purity 

not 

reported)

72-hr LD50: 0.02 (NA)

- Doses that bees were 

exposed to not provided

- The purity of 

imidacloprid was not 

reported

- There was no

information on the 

performance of the control

although it was stated that 

trials in which over one 

control individual had died 

were not considered.

- There was no indication 

on whether a doseresponse was present

Qualitative

(Marletto

2003, 

47796306)

Japanese 

orchard bee

(Osmia 

cornifrons)

TEP (17.4) 48-hr LD50 0.66 (0.30 –

2.19)

- The study also exposed 

Japanese orchard bees to 

imidacloprid with 

fenbuconazole (mixture 

was 2-fold less toxic 

relative to imidacloprid 

alone.

- The doses that the bees 

were exposed to were not 

provided

Qualitative

(Biddinger 

2013, 

49719605)

Bumble bee

(Bombus 

impatiens-

(females only)

TGAI (>95) 48-hr

LD50: 32.2 μg/kg test 

solution (NA)

- The test groups were 

presented in terms of 

percent active ingredient 

in solution as opposed to 

actual treatment 

concentrations. These 

concentrations were 

converted to μg/kg by 

assuming the density of 

the test solution was 1 

g/mL 

Qualitative

(Scott-Dupree

2009, 

48191904)

Alfalfa 

leafcutting 

bee

(Megachile 

rotundata)

LD50: 1.7 μg/kg test 

solution (NA)

Blue orchard 

bee (Osmia 

lignaria)

LD50: 0.7 μg/kg

solution (NA)
108

Test Species

Test 

Substance 

(% a.i)

Study 

Duration

Endpoint (95% CI)

(expressed in terms 

of μg a.i/bee unless 

otherwise noted)

Comments

Classification

(Reference, 

MRID)

Melipona 

quadrifasciata

(stingless bee)

700 g a.i/L

(70% purity 

assuming a 

density of 1 

g/L)

24-hr LD50: 0.023 (NA)

- No mention of control 

mortality but data in 

treatment groups were 

corrected for control 

mortality

- Relatively short (24-

hour) observation period

-this species of stingless 

bee does not have a range 

that extends into North 

America and its 

appropriateness as a 

surrogate for other species

of stingless bees is 

unknown

Qualitative

(Tomé, 2015

49719633)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product

There were additional studies evaluated from the open literature that assessed the effects of acute 

contact exposure to adult honey bees that were determined to be unsuitable for discussion in this 

assessment due to various uncertainties and limitations. These studies, along with their respective 

associated uncertainties and limitations, are provided in Appendix 1.

Summary of Adult Acute Contact Exposure Route to Apis and non-Apis Bees

From the suite of Tier I registrant-submitted studies, the most sensitive Apis adult acute contact toxicity 

endpoint (which could be verified by provided raw data) was a 72-hour LD50 value of 0.048 μg a.i/bee 

(MRID 49602715). In total, there were ten studies (from both registrant-submitted and open literature 

sources) that tested the acute contact toxicity of imidacloprid to adult honey bees, inclusive studies listing 

A. mellifera as the test species as well as studies testing two subspecies (A. mellifera caucasia and A.

mellifera carnica). 

There was not a clear trend, based on the available studies, of the impact of the study observation period 

(e.g., 48, 72, 96 hours), on the determined LD50. Additionally, there was not a clear pattern in the data to 

ascertain whether one subspecies of A. mellifera is differentially more or less sensitive than another. For 

these reasons, data concerning different subspecies of A. mellifera and varying study durations are 

grouped together in Figure 5-1 below, separated by whether the data were registrant-submitted or were

evaluated from the open literature. Additionally, registrant-submitted and open literature studies that 

tested formulated imidacloprid do not indicate (albeit with a notably limited dataset) an increased or 

decreased sensitivity as compared to technical grade imidacloprid. Three open literature studies testing 

formulated imidacloprid on non-Apis species (B. terrestris, O. cornifrons, and M. quadrifasciata), show a 

range of values spanning over an order of magnitude. It is noted that for two non-Apis studies (Gradish, 

2009, MRID 48194902 and Scott-Dupree, 2009, MRID 48191904) endpoints were not expressed in μg 

a.i/bee; therefore, these values are not represented in Figure 5-1 below.
109

Figure 5-1. Scatterplot of adult acute contact toxicity of Apis and non-Apis bees from registrant-submitted 

and open literature sources conducted with technical grade active ingredient (TGAI) and formulated

typical end product (TEP) imidacloprid. Red circle denotes endpoint used for Tier I risk estimation 

purposes.

As depicted in the scatterplot above, the acute contact LD50 values span over an order of magnitude 

(inclusive of all studies). The most sensitive (i.e. lowest) acute contact toxicity LD50 value originating from 

a registrant-submitted study (allowing for an independent verification of the statistical analysis based on 

the raw data) is 0.043 μg a.i/bee. The duration of Apis studies conducted with TGAI (registrant submitted 

and open literature sources) range from 24 hours to 96 hours and the LD50 values range from 0.013 –

0.104 μg a.i/bee, with a median LD50 of 0.061 μg a.i/bee and a mean LD50 of 0.068 μg a.i/bee. The duration 

of Apis studies conducted with TEP (registrant submitted and open literature) range from 48 – 96 hours 

and the LD50 values range from 0.030 – 0.243 μg a.i/bee, with a median LD50 of 0.052 and a mean LD50 of 

0.094 μg a.i/bee. It is noted that these ranges do not include endpoints that were non-definitive., nor 

does Figure 5-1 depict endpoints that were non-definitive.
110

5.1.2. Adult Acute Oral Exposure 

Apis – Registrant-Submitted Studies

There are nine available acute studies to characterize the oral toxicity of imidacloprid to adult honey bees 

with TGAI (purities range from 98.6 - 99.8%) and one study conducted with a formulated TEP (200 g/L, 

20% a.i., assuming density of 1 g/L). These studies were generally in line with OECD TG 213 and LD50 values 

ranged from 0.0039 μg a.i/bee – 0.151 μg a.i/bee. Clinical signs of toxicity were noted in most tests that 

included observation of bees being incapacitated and/or uncoordinated activity/movements and were 

similar to those reported for the acute contact toxicity tests. Summaries of each study are provided in 

Appendix D. From the suite of registrant-submitted Tier I adult acute contact toxicity studies (from which 

raw data were provided), the most sensitive Apis acute oral toxicity endpoint was 0.0039 μg a.i/bee (MRID 

42273003 study). 

Table 5-6. Summary of registrant submitted adult acute oral toxicity studies (Note: All studies tested 

Apis mellifera).

Test 

Substance 

(% a.i)

Study 

Duration

Endpoint (95% CI)

(expressed in terms 

of μg a.i/bee)

Comments

Classification

(Reference, 

MRID)

TGAI (99.8) 48-hr LD50: 0.0039 (0.0027 

– 0.0054) --

Acceptable

(42273003)

TGAI (98.0) 48-hr LD50: >0.036 (NA)

- Clinical signs of toxicity included 

coordination problems, lethargy, and 

agitation were observed in the four 

highest treatment concentrations

Acceptable

(49766202)

TGAI (98.6) 48-hr LD50: >0.020 (NA)

- Clinical signs of toxicity included 

paralysis and spasm observed in bees at 

the four highest treatment groups

Acceptable 

(49766205)

TGAI (98.6) 48-hr LD50: >0.045 (NA)

- Clinical signs of toxicity include bees 

observed being incapacitated or loss of 

coordination in the 3 highest treatment 

groups

Acceptable 

(49602716) 

TGAI (98.6) 48-hr LD50: >0.070 (NA)

- Clinical signs of toxicity included

lethargy, loss of coordination , and 

immobility in the two highest treatment 

groups

Acceptable 

(49602714)

TGAI (98.6) 96-hr LD50: >0.035 (NA)

- incapacitated/loss of coordination

reported at the highest test concentration

- Percent food uptake decreased in a dose 

dependent manner as concentration 

increased.

Acceptable 

(49602717)

TGAI (99.4) 96-hr LD50: 0.151 (0.078 –

1.86)

- Clinical signs of toxicity included loss of 

coordination, lethargy, agitation, and 

incapacitation were observed in all but the 

lowest treatment groups

Supplemental

(49766203)
111

Test 

Substance 

(% a.i)

Study 

Duration

Endpoint (95% CI)

(expressed in terms 

of μg a.i/bee)

Comments

Classification

(Reference, 

MRID)

TGAI (99.4) 96-hr LD50: 0.041 (13.5 -

3980173)

- Clinical signs of toxicity included 

lethargy, loss of coordination problems, 

agitation, and inactivity were observed in 

the five highest treatment groups.

Supplemental

(49766204)

TEP

(200 g/L, 

20% purity 

assuming 

product 

density of 1 

g/L)

96-hr

LD50: 0.053 (0.038 –

0.074) 

0.290 μg 

product/bee

- Clinical signs of toxicity that were noted 

were uncoordinated movement in the 2 

highest treatment groups

Acceptable

(49602707)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product

Bolded value represents endpoint to be used risk estimation

Apis - Open Literature Studies

Discussed below are those studies from the open literature that investigated the toxic effects to honey 

bees following oral exposure. All studies discussed below are a single oral exposure to 5 or more 

concentrations follow by a 48-96 hour observations period that generally follow OECD TG 213 with the 

exception of the study reported by Ramirez-Romero et al., 2005 (MRID 47796305), which exposed bees 

to only a single concentration. As a result, this study did not estimate an endpoint (i.e. an LC50 and does 

not appear in Table 5-7 summarizing the adult acute oral exposure studies from the open literature (study 

summary provided in Appendix D. It is noted that for some studies, multiple trials were conducted, 

yielding several estimates of toxicity within the same study (e.g. Schmuck 2001 [MRID 47812303] and 

Schmuck 2003 [MRID 47796304]).

Table 5-7. Summary of adult acute oral toxicity studies for Apis bees evaluated from the open literature

Test 

Species

Test 

Substance 

(% a.i)

Duration

Endpoint (95% CI)

(expressed in terms of μg 

a.i/bee)

Comments

Classification

(Reference, 

MRID)

Apis 

mellifera 

carnica

TGAI (>98)

48-hr

LD50: 0.0037 (0.0027 –

0.0053)

- There was no mention of 

whether a dose response 

was present.

Qualitative

(Schmuck 

2001,

47812303)

LD50: >0.021 (NA)

LD50 0.041 (NA)

TEP (as 

WG 70)

(70)

LD50: 0.012 (0.007 – 0.018)

TEP (as SC 

200) (200 

g/L)

LD50: 0.021 (0.015 – 0.030)

Apis 

mellifera 

carnica

TGAI (>98) 48-hr

LD50: 0.041 (NA)

- There was no mention of 

whether a dose response 

was present.

Qualitative

(Schmuck 

2003,

47796304)

LD50: >0.020 (NA)

LD50 >0.081 (NA)

LD50 >0.081 (NA)

LD50 >0.081 (NA)
112

Test 

Species

Test 

Substance 

(% a.i)

Duration

Endpoint (95% CI)

(expressed in terms of μg 

a.i/bee)

Comments

Classification

(Reference, 

MRID)

LD50: >0.081 (NA)

LD50: >0.081 (NA)

Apis 

mellifera 

mellifera TGAI

(98) 48-hr

LD50: 0.0048 (0.0045 –

0.0051) - Analytical confirmation 

of imidacloprid in the 

treatment concentrations

was not conducted

Qualitative

(Suchail 2000

47800513)

Apis 

mellifera 

caucasia

LD50 0.0065 (0.0047 –

0.0083)

Apis 

mellifera

TGAI

(97) 96-hr LD50: 0.037 (NA)

- Analytical confirmation of 

imidacloprid in the 

treatment concentrations 

was not conducted

- No mention of whether a

dose response was present

Qualitative

(Suchail 2001,

47523402)

Apis 

mellifera

TGAI

(99.9) 48-hr LD50: 0.536 (0.339 – 1.18)

- Also tested myclobutanil, 

propiconazole, flusilazole, 

and tebuconazole in 

separate combinations 

with imidacloprid (no 

endpoint significantly 

lower than imidacloprid 

alone

- No mention of any 

presence of control 

mortality

Qualitative 

(Thompson 

2014a,

49750606)

Apis 

mellifera

TEP 

(Confidor 

– 17.8%)

72-hr

LD50: 0.194 (NA) - Study states that 42% of 

the data presented is from 

another source making this 

study both a primary 

source and review 

(secondary source) article

(no way of discriminating 

the primary and secondary 

source data from the 

available information in 

the study

- The actual number of 

exposed bees per 

treatment group is not 

specified.

- There was no mention of 

whether a dose response 

was present.

Qualitative

(Laurino 2013,

49719620)

Apis 

mellifera 

ligustica

LD50: 0.030 (NA)

LD50: 0.065 (NA)

LD50: 0.025 (NA)

LD50: 0.035 (NA)

NA: not available; TGAI: technical grade active ingredient; TEP: typical end use product
113

Non-Apis – Registrant Submitted Studies

In an acute oral study, 30 bumble bees (Bombus terrestris) per group were exposed to nominal 

concentrations of TGAI (98.6%) imidacloprid at 0.110, 0.330, 0.530, 0.720, and 0.960 μg a.i/bee (including 

a negative control). Clinical signs of toxicity included paralysis, and spasms at all treatment 

concentrations. It was reported that most of the bumble bees that ingested 0.330 μg a.i/bee or more 

died within 24 hours. The 72-hour LD50 was determined to be 0.170 μg a.i/bee; the study is classified as 

acceptable.

Non-Apis – Open Literature Studies

In Marletto, 2013 (MRID 47796306, discussed above regarding adult acute contact toxicity results), 5 

bumble bee (B. terrestris) workers were placed in each cage, although it is not known how many replicates 

per treatment group were used. Additionally, the actual test concentrations to which the bees were 

exposed was not reported. The 72-hour oral LD50 was determined to be 0.02 μg a.i/bee (95% confidence 

intervals not available). Limitations in addition to those listed above include the purity of imidacloprid 

not being provided, no information on the performance of the control available, and no analytical 

verification of the test substance in the provided sucrose. 

In a study by Thompson et al. 2014b (MRID 49719632), TGAI (>99% purity) was administered to bumble

bees (B. terrestris) in a 30% sucrose solution at nominal imidacloprid concentrations of 0 (control), 1.0, 

10, and 100 μg a.i/L. After 3 days of exposure, mortality was 15, 5, 15, and 15% for the control, 1.0, 10, 

and 100 μg a.i/L groups, respectively. Additionally, it was reported that a significantly (statistical results 

not provided) lower spiked sucrose was consumed in the 10 and 100 μg a.i//L groups. Limitations in this 

study include that the discussion of certain results are not present. For example, mortality data were

excluded if 100% mortality was reached before the end of the experimental period. Without raw data to 

confirm any of the statistical findings, the results of this study are uncertain. Also, despite concentrations 

spanning two orders of magnitude, mortality did not show evidence of a dose response. 

Summary of Adult Acute Oral Exposure Route to Apis and non-Apis Bees

In total, there were 15 studies (from both registrant-submitted and open literature sources) that tested 

the acute oral toxicity of imidacloprid to adult honey bees, inclusive of studies indicating A. mellifera as 

the test species as well as studies testing two subspecies (A. mellifera caucasia and A. mellifera carnica). 

Similar to the dataset for the acute contact toxicity to adult bees, the available studies do not show a clear 

trend in differential sensitivity of one subspecies of A. mellifera as compared to others, nor do the data 

allow for making inferences of changes in toxicity associated with duration of the post-exposure

observation period. Of the 15 acute oral studies for adult honey bees that tested TGAI (inclusive of both 

registrant-submitted and open literature sources), less than half (47%) yielded endpoints that were 

definitive LD50 values. The non-definitive endpoints were not plotted in Figure 5-2 depicting registrant 

and evaluated open literature Apis and non-Apis studies conducted with TGAI and formulated TEP of

imidacloprid. 
114

Figure 5-2. Scatterplot of adult acute oral toxicity of Apis and non-Apis bees from registrant-submitted 

and open literature sources conducted with technical grade active ingredient (TGAI) and formulated

typical end product (TEP) imidacloprid. Red circle denotes endpoint used for Tier I risk estimation 

purposes.

As depicted above, the acute oral LD50 valuesspan over two orders of magnitude, ranging from 0.0039 –

0.536 μg a.i/bee (inclusive of registrant-submitted and open literature studies testing TGAI imidacloprid, 

observations periods of 48 – 96 hours). From the suite of Tier I registrant-submitted studies, the most 

sensitive Apis adult acute oral toxicity endpoint conducted was a 48-hour LD50 value of 0.0039 μg a.i/bee 

(MRID 42273013). The mean and median for TGAI studies (registrant-submitted and open literature) are 

0.087 and 0.039 μg a.i/bee, respectively, and for formulated TEP imidacloprid studies are 0.054 and 0.033 

μg a.i/bee, respectively. It is noted that these measures of central tendency do not include endpoints that 

were non-definitive. 
115

5.1.3. Adult Chronic Oral Toxicity (Apis and non-Apis)

There are 5 studies available from combined registrant-submitted and open literature sources that 

examine the chronic toxicity of imidacloprid through dietary exposure for honey bee and bumble bee 

adults (results combined into one summary table and discussion due to low number of studies relative to 

the acute data). These effects are summarized in the Table 5-8 below. Where available, the no observed 

adverse effect concentration (NOAEC) and the lowest observed adverse effect concentration (LOAEC) are 

provided; otherwise, a description of the report’s effects is tabulated. An endpoint more sensitive than 

the registrant-submitted study was available from the open literature (Boily 2013 – MRID 49750601) and

for which raw data were obtained from the primary author to verify the statistical findings. Therefore, 

this study is suitable for quantitative risk assessment purposes. Summaries of each study, including 

methods and full findings, are provided in Appendix D.

Apis – Registrant-Submitted Studies

Table 5-8. Summary of registrant-submitted and evaluated open literature studies assessing the chronic 

oral toxicity of imidacloprid to Apis and non-Apis adults.

Test 

Species

Test 

Substance 

(% purity)

Exposure 

Period

Exposure 

concentrations Reported Effects Comments

Classification

(Reference / 

MRID)

Apis 

mellifera

TGAI 

(99.4%) 10 days

0 (control), 10, 

20, 50, and 

100 μg a.i/L

NOAEC/LOAEC 

(mortality): 

100/>100 μg 

a.i/bee

NOAEC/LOAEC 

(food 

consumption: 

<10/10 μg a.i/L 

(equivalent to 

<0.0039 μg 

a.i/bee)

-

Concentrations 

in terms of 

mean intake 

over study 

course 

(excluding 

control) were 

0.0039, 0.0063, 

0.016, and 

0.028 μg 

a.i/bee)

- No definitive 

NOAEC 

established 

based on food 

consumption

Supplemental

(49511703)
116

Test 

Species

Test 

Substance 

(% purity)

Exposure 

Period

Exposure 

concentrations Reported Effects Comments

Classification

(Reference / 

MRID)

Apis 

mellifera

Admire 240 

(24.0% a.i 

assuming a 

product 

density of 1 

g/L)

10 days

0 (control), 

0.08, 0.16, 

0.24, and 0.30 

ng a.i/bee

NOAEC/LOAEC 

(mortality and 

body weight): 

0.16/0.24 ng 

a.i/bee 

(0.00016/0.00024 

μg a.i/bee)

- Test 

conducted with 

formulated 

product 

(Admire 240F) 

but 

concentrations 

provided in 

terms of active 

ingredient

- Clinical signs 

of toxicity 

included 

tumbling and 

trembling at all 

doses

Quantitative

(Boily 2013, 

49750601)

Apis 

mellifera

Imidacloprid 

(purity not 

clear from 

citation but 

stated to be

obtained 

from 

Cluzeau

(Analytical 

Chemistry 

Materials

Provider, 

France)

10 days

0 (control), 

0.70, 7.0, 70 

ppb

Mortality: 5.6, 

10.4, 16.3, and 

17.4%, 

respectively

- Mortality for 

all treatment 

groups was 

significantly 

increased from 

control (p<0.05)

- No significant 

effects 

reported for 

food 

consumption 

(p>0.05)

Qualitative

(Alaux 2010, 

48077922)

Apis 

mellifera

Analytical 

standard 

(exact 

purity not 

provided)1

6 days

0 (control), 

0.08, 0.2, 0.51, 

1.3, 3.2, 8.0, 

20, 50, and 

125 μg a.i/L

No reported 

effects up to and 

including the 

highest 

concentration 

- Mortality, 

locomotory 

activity, and 

food 

consumption 

were assessed

- Results not 

reported as 

percent 

difference from 

control

Qualitative

(Cresswell 

2012, 

Bombus 497196610)

terrestris

Analytical 

standard 

(exact 

purity not 

provided)1

38% reduction in 

food consumption 

when ingesting 

4.9 ng a.i/bee 

(0.0049 μg 

a.i/bee) (percent 

effects reported 

only for certain 

concentrations)

Apis 

mellifera

Analytical 

standard 

(exact 

purity not 

provided)1

3 day 

exposure

(followed 

by 5 days 

of 

0 (control) and 

125 μg a.i/L 

(0.0022 μg 

a.i/bee/day)

No reported 

effects in 3-day 

and 8-day 

exposures

- Food 

consumption 

and locomotory 

activity were 

measured

Qualitative

(Cresswell 

2013, 

49719611)
117

Test 

Species

Test 

Substance 

(% purity)

Exposure 

Period

Exposure 

concentrations Reported Effects Comments

Classification

(Reference / 

MRID)

Bombus 

terrestris

Analytical 

standard 

(exact 

purity not 

provided)1

untreated 

sucrose), 

or 8 day

continuous

exposure

(treated 

sucrose)

Significant 

reductions from 

control (p<0.05) 

in food 

consumption and 

locomotion

- Bumble bees 

on 3 days 

exposure 

showed 

recovery of 

these effects 

1As confirmed by study author, email communication 03/16/15

Bolded value represents endpoint to be used risk estimation 

5.1.4. Larval Acute Oral Toxicity

Apis - Registrant Submitted Studies

There are no registrant-submitted studies or studies that were surveyed as part of the open literature 

effort that concern the acute oral exposure to honey bee larvae.

Non-Apis – Open Literature Studies

In a study conducted by Tomé et al. (2012), the larvae of the stingless bee Melipona quadrifasciata 

anthidiodes were exposed to varying concentrations of imidacloprid TEP (reported as 700 g a.i/L, 70% 

purity assuming product density of 1 g/L), for up to 5 days, depending on when adults emerged. 

Observations were made for mortality, body mass, and developmental time. Colonies were collected in 

the field and maintained in an experimental apiary within a laboratory at Federal University, Vicosa, Brazil. 

Brood chambers containing eggs were removed from the hives and transferred to artificial cells containing 

larval diet (also obtained from the field) that was either untreated or spiked with 18 varying 

concentrations of imidacloprid ranging from 0.0056 to 56 μg a.i/bee. Upon emergence, the adult workers 

were marked with different colors to facilitate age monitoring and were fed with untreated honey and 

pollen syrup (not further described in the article). For each treatment concentration, there were five 

replicates of 24 larvae each. There was 97% control survival and survival was above 50% only at the lowest 

treatment concentration (0.0056 μg a.i/bee). There was a negative correlation between imidacloprid dose 

and median survival time (TL50). According to the study authors, body mass and development time, by 

contrast, were not significantly affected; however, statistical results were not provided. The authors

noted that stingless bee larvae ingested the entire dose, irrespective of the treatment concentration. 

While a definitive endpoint was unclear from the study, none of the larvae reached pupation at treatment 

concentrations higher than 0.28 μg a.i/bee. Limitations from the study include no analytical verification 

of the concentrations of imidacloprid in the treatment groups and the fact that no explicitly stated 

endpoint was available from the study. Additionally, it is noted that the bees were wild collected as well 

as their diet and it is an uncertainty of the prior exposure to pesticides. Finally, it is noted that the 

geographic distribution of this species of stingless bee does not extend into North America, and it is 

unclear to what extent this species is representative of other species of stingless bees.
118

5.1.5. Larval Chronic Oral Toxicity

Apis – Registrant-Submitted Studies

In a chronic toxicity test, a repeated dose of TGAI (99.4%) was administered to honey bee larvae which 

were then monitored through pupation and emergence over the course of the 21-day study (MRID 

49090506). This study followed the test protocol recommendations of Aupinel et al. (2009)15 with 

modifications. Four independent test runs were conducted. At Day +1 (Day 0 was the anticipated day of 

larval hatching), first instar bee larvae (A. mellifera carnica) were transferred from their bee hive into an 

artificial in vitro testing system. The bee larvae were fed with standardized amounts of untreated artificial 

diet at Day +1 and Day +3. On Day +4, +5 and +6, the bee larvae in the test item treatment groups were 

fed with standardized amounts of diet spiked with imidacloprid. Additionally, beginning on Day +4, the 

bee larvae in the reference item treatment group were fed with standardized amounts of diet spiked with 

dimethoate TGAI (98.5%) at 3.0 μg a.i/larva. Concurrently, the bee larvae in the control group (on Days

+4, +5 and +6) and in the reference item group (on Days +5 and +6) received untreated standardized diet, 

respectively. The nominal concentrations of imidacloprid of treatment groups in the diet was 5, 10, 20 

and 40 μg a.i/kg diet. Percent recovery of imidacloprid in the treatment concentrations was determined 

to be 98-115% of the nominal concentrations. 

The study authors set a control mortality validity criteria of Day 0 to Day 22 mortality of equal to or less 

than 30%. In runs 2, 3, and 4 of the definitive test, this was met with control mortality ranging from 15 –

19%. The first run yielded a control mortality of 37% by Day 22 of the study. Consequently, the combined 

results of runs 2, 3, and 4 (yielding a Day 22 mean control mortality of 16.7%) were used to verify the 

results of the study. There was generally no dose response observed either in each individual trial or

when the results of the trials were combined. Specifically in runs 2 and 3, the percent mortality in the 

highest treatment concentration (40 μg a.i/kg diet) was lower than that of the lowest treatment 

concentration (5 μg a.i/kg diet). In run 4, although the percent morality in the highest treatment 

concentration exceeded that of the lowest concentration, percent mortality at the two middle treatment 

concentrations (10 and 20 μg a.i/kg diet) was roughly half of that observed at the lowest and highest 

concentrations. The percent mortality of runs 2, 3, and 4 combined were 16.7, 25.4, 10.0, 24.6, and 16.7 

for the control, 5, 10, 20, and 40 μg a.i/kg diet groups, respectively. Due to the variability in the response 

across runs, as well as the absence of a monotonic dose response in the study, there is uncertainty in the 

NOAEC derived from this study. However, as the control mortality criterion was met for 75% of the runs 

tested and the study generally followed the protocol recommendations described above, this study is 

classified as supplemental and suitable for quantitative risk assessment purposes. The NOAEC and LOAEC 

for this study were determined to be 40 and >40 μg a.i/kg diet, respectively or 0.00183 and >0.00183, 

respectively when expressed on a μg a.i/bee basis.

Non-Apis – Open Literature Studies

 15 Aupinel, P., Fortini, D., Michaud, B., Medrzycki, P., Padovani, E., Przygoda,D., Maus, Ch., Charriere, J.D., Kilchenmann, V., 

Riessberger-Galle, U.,Vollmann, J.J., Jeker, L., Janke, M., Odoux, J.F., Tasei, J.N. 2009. Honey bee brood ring-test: method for 

testing pesticide toxicity on honey bee brood in laboratory conditions; published in: Hazards of pesticides to bees: 10th 

International Symposium of the ICPBR Bee Protection Group, Bucharest (Romania), October 8-10, 2008. Julius-Kühn Archive 423: 

96-102 
119

In a study by Abbott et al. (2008, MRID 47812301), the eggs of blue orchard bees (Osmia lignaria) were 

exposed to varying concentrations of TGAI (97.5% purity) into microwell plates and until adulthood, 

representing a duration of approximately 30-40 days. The bees were obtained as over-wintering adults 

in cocoons and kept in storage until the start of the experiment. Parameters that were assessed included 

the timing and completion of larval development, the number of days between the egg stage and the 

beginning of each larval stage, and the start of cocoon formation and its completion, including darkening. 

Time was also recorded from the date of first observation (egg stage) to the date each larva finished 

spinning a thin white cocoon around itself, and to the date the darkened cocoon was completed. 

Confirmation of the concentrations of the final pollen provisions were performed by Bayer CropScience 

and yielded levels for the low, medium, and high treatments of 2.7, 35, and 276 ppb, respectively for 

imidacloprid. 

Further details on the method and limitations of this study are provided in Appendix D.

Table 5-9. Summary of results from Abbott et al., 2008 examining the effects of imidacloprid TGAI on 

larval development of blue orchard bees (Osmia lignaria) .

1 (Note: Study classified as qualitative)

Response variable 3 ppb 30 ppb 300 ppb

Lab Component – Own Pollen

Time to reach last larval stage (days) NS NS NS

Time to spin a cocoon (days) NS NS NS

Time to finish darkening a cocoon (days) NS NS Males: NS

Females: ↓

Time to emerge from cocoons (days) NS NS NS

Weight of bees after emergence from cocoon (grams) Males: ↑

Females: NS

Males: ↑

Females: NS

Males: ↑

Females: NS

Lab Component – New Pollen

Time to reach last larval stage (days) NS NS NS

Time to spin a cocoon (days) NS NS NS

Time to finish darkening a cocoon (days) NS NS NS

Time to emerge from cocoons (days) NS NS Males: NS

Females: ↓

Weight of bees after emergence from cocoon (grams) NS NS NS

Field Component

Time to reach last larval stage (days) NS Males: NS

Females: ↑

Males: ↑

Females: NS

Time to spin a cocoon (days) NS Males: ↑

Females: NS NS

Time to finish darkening a cocoon (days) NS Males: ↑

Females: ↑

Males: ↑

Females: NS

Time to emerge from cocoons (days) NS NS NS

Weight of bees after emergence from cocoon (grams) NS NS NS 1 Means not presented. Arrow up or down denotes significant (p<0.05) increase or decrease from control, respectively; NS = not significant 

(p>0.05)
120

5.1.6. Acute and Chronic Toxicity of the Degradation Products of Imidacloprid

As discussed in Section 4, imidacloprid can degrade into various products both within the plant as well as 

in the soil. Specifically, imidacloprid is metabolized within the plant to 4,5 IMI-OH, which then degrades 

to 5-OH-IMI, and subsequently to IMI-olefin. In the aerobic soil metabolism pathway, the olefin-IMI 

metabolite is formed at a minor (i.e. less than 10% of the applied residues) rate.

While there are no registrant-submitted studies to characterize the toxicity of these metabolites, there 

are studies available from the open literature that investigated the acute and chronic oral toxicity of these 

metabolites to adult honey bees. As described in Section 4, parent imidacloprid undergoes metabolism 

to several degradates that include IMI-olefin, IMI-5-OH, 4,5-OH-IMI, desnitro-IMI, 6-CNA, and a urea 

metabolite. Table 5-10 below summarizes the studies assessing the acute oral toxicity of the various 

degradation products of imidacloprid. 

There are registrant-submitted and open literature studies available to characterize the acute and chronic 

toxicity of the various degradation products of imidacloprid. The registrant-submitted studies primarily 

concern the chronic oral toxicity to honey bee adults of the urea metabolite and 6-CNA metabolites. 

Additionally, there are two studies from the open literature that assess the acute oral toxicity to honey 

bee adults to several degradates including IMI-olefin, IMI-5-OH, 4,5-OH-IMI, desnitro-IMI, 6-CNA, and the 

urea metabolite.

Table 5-10. Summary of acute oral toxicity studies testing the degradates of imidacloprid in the open 

literature

Species

Test 

Substance 

(% a.i)

Duration Endpoint1 Comments

Classification

(Reference,

MRID )

Apis 

mellifera

IMI-olefin

(>98)

72-hr

LD50: >0.036 μg 

a.i/bee (NA) - No mention of whether dose 

response was present

- Unclear from methods 

section whether analytical 

confirmation of the test 

substance in the treatment 

groups was conducted

- Failure to capture sufficient 

dose response to enable 

calculation of LD50 values 

Qualitative

(Schmuck 

2003, 

47800520)

IMI-5-OH

(>98)

LD50: 0.159 μg 

a.i/bee (NA)

4,5-OH 

imidacloprid

(>98)

LD50: >0.049 μg 

a.i/bee (NA)

6-CNA (>98) LD50: >121 μg a.i/bee 

(NA)

Urea 

metabolite

(>98)

LD50: >99.5 μg a.i/bee 

(NA)

Apis 

mellifera

IMI-olefin

(>97)

96-hr

LD50: 0.023 μg 

a.i/bee (NA) - Analytical confirmation of 

imidacloprid in the treatment 

groups was not conducted

- No mention of whether a 

dose response was present

Qualitative

(Suchail 2001,

47523402)

IMI-5-OH

(>97)

LD50: 0.222 μg 

a.i/bee (NA)

4,5-OH 

imidacloprid

(>97)

LD50 >1 μg a.i/bee 

(NA)
121

Species

Test 

Substance 

(% a.i)

Duration Endpoint1 Comments

Classification

(Reference,

MRID )

Desnitroimidacloprid

(>97)

LD50 >1 μg a.i/bee 

(NA)

6-CNA (>97) LD50 >1 μg a.i/bee 

(NA)

Urea 

metabolite

(>97)

LD50 >1 μg a.i/bee 

(NA)

1

Numbers in parentheses for acute endpoints refer to 95% confidence intervals, listed as NA if not available

The available suite of chronic oral studies assessing the toxicity of the urea metabolite and 6-CNA, indicate 

that that these two degradates do not elicit a lethal effect significantly increased from that of controls up 

to and including a dietary concentration of 10 μg a.i/L. The test designs of all studies were generally the 

same although one key difference was the level of control mortality across studies, which ranged from 0 

– 44% for the chronic urea metabolite studies and from 0 – 54% for the 6-CNA studies. While there is, at 

present, no formal guideline for a chronic adult 10-day oral toxicity test with honey bees, a control 

mortality level of above 20% suggests that husbandry conditions or general procedures in conducting the 

test may not have been optimal and therefore the ability to discern a true treatment-related effect may 

be compromised. As a result, the studies with greater than 20% mortality are not tabulated below.

As the study designs were so similar, the major elements of each study are summarized in Table 5-11 

below as opposed to a summary discussion of each study. All studies were conducted with A. mellifera, 

were 10 days in duration (i.e., 10 days of exposure), and tested either the urea metabolite or 6-CNA at 

concentrations of 0.1, 1.0, and 10 μg a.i/L in the diet (sucrose solution). Additionally, there was generally 

no evidence of analytical verification of the concentrations of the urea metabolite and 6-CNA in the 

treatment solutions. Finally, there were generally no observations of clinical signs of toxicity recorded for 

these studies (i.e. studies did not report whether such effects were examined).

The results for the urea metabolite studies indicate that concentrations up to and including 10 μg a.i/L do 

not have an increased mortality effect to adult honey bees as compared to the control group. Two studies 

(MRIDs 49602711 and 49602713) also included food consumption as a response variable, but this 

endpoint was not subjected to statistical analysis by the study authors. 

Similarly, the results of the studies conducted with 6-CNA generally indicate (when not confounded by 

excessive control mortality; these studies not tabulated below) that concentrations up to and including 

10 μg a.i/L do not result in an increased level of mortality when compared to controls after a 10-day 

exposure. As with the urea metabolite studies, for certain tests (MRIDs 49602710 49602720), food 

consumption was included as a response variable, although not statistically analyzed in the original study 

reports. 

Interestingly, for MRID 49602711 (urea metabolite study), the bees that were reported to be older (i.e.

22-45 days) had a 3-fold higher level of control mortality than the same study that included a component 
122

that tested 12-17 day old bees (results not tabulated below for this component). Additionally, while the 

age of the bees was not reported, MRIDs 49602713 and 49602710 tested bees characterized as “house” 

bees and “field” bees that showedmarkedly different rates of control mortality despite both cohorts being 

subjected to the same methodology within each test (results not tabulated below for this component). It 

is an uncertainty whether the age of the test bees had an effect on the results of these studies. 

Table 5-11. Summary of chronic adult oral toxicity studies with urea metabolite and 6-CNA (all studies 

conducted with Apis mellifera)

Experimental Design

Results (presented for the 

control, 0.1, 1.0, and 10 μg a.i/L 

in ascending order)

Comments

Classification

(MRID)

Urea metabolite (99.4% purity for all studies)

3 reps/trt, 10 

bees/rep

Mortality: 10, 37, 3, and 63% 

Food Consumption (per 10 

bees): Mean food uptake was 

4.87, 4.64, 3.93, and 3.57 g

- Bees were 12-17 days old

- Statistical analysis of data not 

conducted by study authors

Acceptable

(496027111

)

5 reps/trt, 10 

bees/rep

Mortality: 4, 10, 8, and 12%

Food Consumption (per 10 

bees): 7.30, 7.27, 7.27, and 7.54 

g

- Test bees characterized as 

“house” bees, with no age reported

- No significant differences in 

mortality, food consumption not 

statistically analyzed by study 

authors

Acceptable

(496027132

)

Mortality: 0, 8, 6, and 0%

- Statistical analysis of the data not 

conducted by the study authors

- Age of bees up to 5 days old (postemergence).

Acceptable

(496027211

)

6-CNA (99.6% purity for all studies except MRID 49602720 where purity not reported)

Mortality: 4, 10, 4 and 6%

Food Consumption (per 10 

bees): 7.29, 7.24, 7.34, and 7.19 

g

- Test bees characterized as 

“house” bees, with no age reported

- No significant differences in 

mortality, food consumption not 

statistically analyzed by study 

authors

Acceptable

(496027102

)

3 reps/trt, 10 

bees/rep

Mortality: 7, 10, 7, and 7%

Food consumption (per 10 

bees): 5.94, 5.87, 5.50, and 5.8

- Test bees were 12-17 days old

- Statistical analysis of data not 

conducted by study authors

Acceptable

(496027201

)

5 reps/trt, 

unspecified number 

of bees/rep

Mortality: 0, 2, 4, and 0% - Age of test reported to be up to 5 

days old

Acceptable

(496027223) 1Results are the same as those provided in Schmuck 2004 (“Germany II” testing facility)

2Results are the same as those provided in Schmuck 2004 (“Germany III” testing facility) 3

Results are the same as those provided in Schmuck 2004 (“Germany I” testing facility)
123

5.2. Tier II 

As discussed in the Pollinator Risk Assessment Guidance (USEPA et al. 2014), Tier II encompasses studies 

that characterize effects at the colony level. The need for these studies depends on whether Tier I LOCs 

are exceeded, the availability of data, and the nature of uncertainties that warrant further testing. Tier II 

studies can include those characterized as “semi-field” studies where small colonies are enclosed in 

tunnels, along with pesticide-treated crops. Additionally, these studies may be a feeding study design in 

which whole colonies are provided pesticide-treated sucrose or pollen and the colonies are not confined 

to enclosures (i.e., the bees are free-foraging). Typically, semi-field studies are conducted under 

conditions that represent the worst-case exposure scenario of proposed uses to the entire colony or 

designed to address specific uncertainties with respect to the effects of the colony. Tier II study designs 

may be amenable to additional treatment levels and replication, this facilitating quantification of an 

application rate-response (semi-field tunnel study) or dose-response (feeding study) relationship at the 

colony level and determination of a NOAEC. 

For imidacloprid, both registrant-submitted and open literature Tier II-type studies are available. The 

registrant-submitted Tier II study (MRID 49510001) employed a feeding design in which 84 hives were 

provided either untreated sucrose solution within the hives or sucrose spiked with one of 5 concentrations 

of imidacloprid. Additionally, there are a number of Tier II-type studies (inclusive of tunnel and feeding 

study designs) that were evaluated by EPA, Health Canada’s Pest Management Regulatory Agency 

(PMRA), and the California Department of Pesticide Regulation (CDPR) (referred to as “tri-Agency”) open 

literature review effort. While the registrant-submitted study exposed honey bee colonies to varying 

concentrations of imidacloprid spiked in sucrose solutions, the suite of open literature studies concern 

both Apis and non-Apis species (B. terrestris and B. impatiens) exposed to imidacloprid through diet (i.e., 

both spiked sucrose and spiked pollen). 

5.2.1. Registrant-Submitted

Colony Feeding Study

The registrant-submitted colony feeding study was conducted with honey bees to assess the potential for 

long-term effects, including colony overwintering survival, resulting from exposure to imidacloprid. The 

study was conducted in 12 test areas (Apiaries A – L) reported to be of low agricultural cultivation in North 

Carolina from June 21, 2013 to March 24, 2014. Eighty-four hives were divided according to hive strength 

(number of brood frames) with the strongest 7 hives assigned to Apiary A and the weakest 7 hives assigned 

to Apiary L (i.e., the study design was stratified to account for differences in colony strength). Within 

each apiary, the 7 hives were randomly assigned to treatment groups. At each apiary, five test hives were 

artificially fed with 50% sugar solution spiked with imidacloprid at 12.5, 25, 50, 100 or 200 μg a.i/L for six 

weeks continuously in the field, with two hives at each apiary serving as controls. The 8th colony at each 

apiary served as a monitoring hive to characterize the alternative sources of forage (pollen/nectar) of the 

test colonies as well as to monitor for the potential contamination with other pesticides. Assuming the 

density of a 50% sugar solution is 1.23 g/ml, the reviewer calculated that the test concentrations at 12.5, 
124

25, 50, 100, and 200 μg/L are equivalent to 10.2, 20.3, 40.7, 81.3, and 162.7 ppb (μg/kg), respectively. 

Eight Colony Condition Assessments (CCAs) were conducted during the study. Three CCAs (CCA1 - 3) were 

conducted prior to feeding (i.e., pre-exposure phase) to determine hive strength (number of adult and 

developing bees) and initial hive conditions. A CCA was conducted during exposure with another one 

conducted one week after termination of exposure (CCA4 and CCA5, respectively) which characterize hive 

condition during exposure (i.e., exposure phase). Two more CCAs were conducted at 5 and 10 weeks after 

exposure (CCA6 and CCA7, respectively) to assess the chronic effect following exposure to imidacloprid 

and to characterize pre-overwintering hive conditions (post-exposure phase). A final CCA was conducted 

after overwintering in March 2014 (CCA8) to assess potential exposure impact on survival and chronic 

colony-level effects. Multiple parameters, such as hive weight, number of individuals at different life 

stages in the hive, hive honey and pollen stores, and hive overwintering survival, were measured during 

the course of the study.

A joint review effort of this study was conducted by the United States Environmental Protection Agency 

(EPA), Canada’s Pest Management Regulatory Agency (PMRA), and the State of California Department of 

Pesticide Regulation (CDPR). As part of that effort, a separate statistical analysis was conducted by each 

regulatory entity as an independent verification of the results from the analysis provided by the registrant. 

These analyses were distinct in approach but generally yielded similar statistical results. It is noted here 

that when weighing the statistical results as well as biological concerns, particularly as they relate to honey 

bee biology at the colony level, EPA, PMRA, and CDPR arrived at the same overall conclusion and are 

therefore harmonized in terms of the determination of an overall NOAEC and LOAEC yielded by this study. 

For further details on the methodology and a more detailed discussion of the results of this study, please 

refer to Appendix G.

Conclusions

While there were uncertainties that were generally related to inherent aspects of any semi-field or full 

field study design (such as dilution of the test chemical through alternative sources of forage, detection 

of other chemicals in the monitoring hives), this study still provides information on a number of colony 

condition parameters about the long-term effects (including overwintering) from dietary exposure to 

imidacloprid at the colony level. 

As indicated in the results section above, the PMRA, EPA, and CDPR analyses determined significant 

effects (at both the 0.05 and 0.1 alpha levels) in the 50, 100, and 200 μg/L groups across multiple CCAs for 

the majority of response variables. Specifically, for the 100 and 200 μg/L treatment groups, significant 

effects (p<0.05) were determined for every response variable and persisted across at least 2 CCAs, along 

with very high overwintering mortality. While the 50 μg/L group had overwintering mortality similar to 

the controls, colony condition effects were different from controls with an early onset of effects which 

tended to persist, and notably poorer colony condition in surviving hives after overwintering in 

comparison to controls. 

Conversely, there was not a strong indication from the PMRA, EPA, and CDPR analyses of an impact at the 

colony level at the 12.5 and 25 μg/L treatment groups. This is evidenced not only by a general lack of 
125

statistical findings (p>0.1) at these treatment levels but in cases where significant effects were 

determined, they either did not show strong dose-responsiveness, did not persist across multiple CCAs, 

or were considered potential transient effects (e.g., at CCA6) which did not persist after overwintering. 

This latter point was the case for the total life stage and pupal cell findings in which the PMRA analysis 

determined significant effects at all treatment levels at CCA6 (EPA also determined a significant reduction 

in pupal cells at the lowest treatment group of 12.5 μg/L at CCA6). As well at CCA6, PMRA determined 

significant effects on the proportion of eggs and larvae at 25 μg/L treatment (but not at the 50 μg/L). For 

these two lowest treatment groups (12.5 and 25 μg/L), the colony condition of surviving hives at CCA8 

following overwintering was similar to controls, indicating the effects observed at CCA6 were likely 

transient and the colony was able to compensate for these effects. 

When examining the effects on food stores (pollen and nectar), the PMRA, EPA, and CDPR analyses did 

not determine any consistent and significant reductions in pollen and nectar stores at the 12.5 and 25 

μg/L treatment groups. This is distinguished from the 50 μg/L group where effects on nectar in particular 

were apparent when compared alongside the response of the control in terms of the level of nectar 

buildup before hive preparation for overwintering at CCA7. This finding was also confirmed statistically 

in all three agency analyses with significant reductions in honey stores at CCAs 6, 7, and 8 (CCA8 data 

excluded from the EPA analysis for the 100 and 200 μg/L groups). Significant (p<0.05) reductions in pollen 

stores were also confirmed at CCAs 4 and 5 at the 50 μg/L treatment during the exposure phase.

Specifically, when considering the adult and honey and pollen stores response variables, the differences 

from control were apparent both visually and statistically, particularly in the three highest treatment 

groups. For the proportion of adults, the onset of a decline in numbers occurred one CCA earlier in these 

groups than in the control, 12.5 and 25 μg/L treatment groups. For honey stores, the buildup that 

occurred starting at CCA5 in the 50 μg/L treatment group, reached only half the level reached in the 

control, 12.5, and 25 μg/L treatment groups by CCA7. Pollen stores were also reduced at CCA4 and CCA5 

compared to controls for the three highest treatment groups, as well as at CCA6 and CCA7 at the highest 

treatment group. These effects were statistically significant (p<0.05) and indicate that the 50 μg/L 

treatment group was associated with trends and proportions of abundance for life stages and food stores 

not observed in the control, 12.5, and 25 μg/L treatment groups. 

Therefore, when weighing biological significance and the natural seasonal changes of honey bees colonies, 

as well as supporting conclusions from the statistical approaches used in PMRA, EPA, and CDPR, the 

NOAEC and LOAEC for this study is determined to be 25 and 50 μg/L, respectively. 

Strengths and limitations

It is important to recognize the inherent strengths and limitations of this study as results are considered 

in this risk assessment. 

In the context of available field studies involving honey bees and imidacloprid, this study contains a 

number of strengths including: 

• Use of a high degree of replication (n=12) to achieve a reasonable level of statistical power;
126

• Demonstration of a generalized concentration-response relationship with respect to the 

concentration of imidacloprid in sucrose solution and the magnitude and duration of adverse 

effects;

• Quantification of exposure to parent (imidacloprid) and toxicologically-relevant metabolites in 

diet and in hive matrices (uncapped nectar, pollen, honey, bee bread);

• Use of a 6-week exposure duration to represent a “high end” exposure scenario;

• Inclusion of multiple colony-level endpoints reflecting hive strength, brood development and food 

stores;

• Detailed quality assurance/quality control (QA/QC) regarding quantification of chemical residues 

in various matrices; and,

• Availability of raw data for conducting/verifying statistical analysis.

A number of limitations are also noted with this study, including:

• Exposure of bees through nectar (sucrose) alone, whereas bees in the field are likely exposed 

through both pollen and nectar routes. While exclusion of the pollen route is expected to reduce 

overall exposure, the impact of this exclusion on the study results is uncertain and will likely 

depend on the life stage/caste of bee. 

• Imidacloprid was found in both hive nectar and hive pollen (beebread), at concentrations lower 

than the feeding solutions. Dilution compared to the treatment feeding solution is expected since 

bees could also forage on outside nectar and pollen sources. As well, hive pollen contains only 

some hive nectar, thus would not be expected to have a concentration equivalent to nectar alone, 

and it is mixed with pollen which comes from outside [untreated] sources. Therefore exposure 

through both hive pollen and nectar occurred via exposure to the sucrose feeding solution, but 

how this compares to exposure through contaminated pollen directly is not known. It is also noted 

that nectar is considered the dominant exposure route for forager bees; other hive bees and 

larvae consume both nectar and pollen. 

• The quantity of nectar provided to hives (2 L per week per hive) likely did not fulfill the complete 

carbohydrate needs of the colony, as indicated by colony bioenergetics and the lack of remaining 

sucrose solution upon their renewal. This suggests that bees could be exposed to a greater dose 

of imidacloprid in nectar had a greater volume of spiked sucrose been provided. Although the 

dosing regimen may have underestimated exposure through sucrose relative to 100% 

contaminated diet, it is noted that bees had to supplement their spiked sucrose by foraging on 

their own for other sources of nectar. 

• Overwintering success of controls was impacted (36% hive mortality). This may have reduced the 

ability to detect adverse effects related to hive loss following overwintering. Although 

comparable to overwintering losses of commercial beekeepers (32% based on a 5-yr average16), 

 16 White House. 2015. National Strategy to Promote the Health of Honey Bees and Other Pollinators. Pollinator 

Health Task Force. May 19, 2015. 

https://www.whitehouse.gov/sites/default/files/microsites/ostp/Pollinator%20Health%20Strategy%202015.pdf
127

it is possible that elements of the study design may have contributed to this loss (e.g., lack of 

supers to allow for colony growth, delayed supplemental feeding during fall).

• Hive contamination with pesticides from food sources other than the artificial feeding was 

detected during the exposure period and post-exposure periods through collection of pollen from 

pollen traps. Although the study was deliberately conducted in an area where minimal potential 

for pesticide contamination from other sources was expected, the bees still appeared to be 

foraging on contaminated pollen and possibly nectar. During both exposure and post-exposure 

periods, multiple pesticides such as spiromesifen (maximum at 961 ppb) and piperonyl butoxide 

(maximum at 591 ppb) that may cause concern for bees were detected in most monitoring hives,. 

Trace amounts of other bee-toxic pesticides, such as chlorpyifos (LOD = 1.0 ppb) and malathion 

(LOD = 4.0 ppb) were also detected. The test chemical imidacloprid was found at 12.1 ppb in 

pollen from one (apiary L) of the total of six sites analyzed. 

• Residues of imidacloprid in uncapped nectar and bee bread within the hives at CCAs 4, 5, and 8 

represent a single sample per hive on a single frame rather than a composite sample from multiple 

portions of the comb within a hive. This means that residue results may reflect “hit or miss” 

scenario with respect to detecting residues in nectar laid down from spiked sucrose diets (fed) vs. 

outside sources. 

• The exposure, based on residues measured in the hive (hive nectar and hive pollen) indicated 

that, overall, higher measured hive residues correlated with higher nominal residues in feeding 

solutions. However, individual hive residue values varied, and there was some overlap in 

measured values, particularly among the three lowest doses. Given the limited spatial and 

temporal sampling methodology (as mentioned above), there is uncertainty in whether these 

residues represent actual in hive residues across all portions of the frame. Specifically, one sample 

of one area of the comb on one side of the frame to represent the nectar or pollen residues of an 

entire hive may not reflect the true nature of the residues across all portions of a given hive. 

In addition to the colony feeding study, there are several registrant-submitted Tier II studies employing a 

tunnel design that were previously mentioned in Section 4 regarding their residue information. These 

studies generally involved exposure to honey bee colonies foraging on seed-treated corn, canola, or 

sunflower within a netted enclosure (i.e. tunnel). These studies, while serving as a line of evidence in 

terms of the residue information provided, have several deficiencies that limit their utility from an effects 

standpoint. The limitations associated with each study can be found in Appendix 1.

5.2.2. Open Literature Studies

This section summarizes the available Tier II (i.e., tunnel and feeding study design) studies that were 

evaluated from the open literature as part of the aforementioned joint review between EPA, PMRA, and 

CDPR. At the Tier II (and Tier III) levels, effects on the colony as a whole are assessed as distinguished 

from Tier I studies that characterize the effects of imidacloprid at the individual bee level. Where 
128

sufficient information is available, a table summarizing the results is provided within the discussion of 

each study. Additionally, the limitations of each study are provided within each summary. It is noted here 

that all studies are determined to be of qualitative utility for characterization purposes in this assessment. 

As with the Tier I data, this is primarily due to the fact that raw data were not available to allow for an 

independent verification of the statistical results but, as will be discussed, other uncertainties contribute 

to this classification of utility. 

Apis

Summary of Tier II Apis Studies from the Open Literature

While many of the evaluated studies from the open literature do not have a robust experimental design 

(i.e., lack of replication of colonies or plots within treatment groups), lack of an overwintering component 

to provide insight on the long-term effects of imidacloprid at the colony level, and only one treatment 

concentration, these studies provide several additional endpoints not captured in the registrantsubmitted colony feeding study. For example, foraging observations dealing with behavior and success 

are examined in the majority (90%) of the higher-tier Apis studies. Additionally, several higher-tier studies 

were evaluated that investigate the colony-level effects of imidacloprid on bumble bees and, as will be

discussed, the concentrations in pollen and nectar at which effects are observed vary from that of Apis

colonies. 

There were a total of 3 Tier II studies (all feeding design; 1 with spiked sucrose, 2 with spiked pollen) 

evaluated from the open literature to characterize the colony-level effects of imidacloprid to honey bees. 

These studies employed study design elements that were akin to the registrant-submitted colony feeding 

study discussed above such as replication of hives among treatment groups, monitoring for pests and 

pathogens, multiple colony condition assessments to monitor the hives during and after exposure, and an 

overwintering component to assess the long-term effects of colony health as a result chronic exposure to 

imidacloprid. There were an additional 4 studies (Bortolotti 2003, Eiri and Nieh 20120, Schneider 2012, 

and Tan 2014), that exposed honey bees to a single oral dose, but examined endpoints that could 

potentially lead to colony-level impacts. These studies, while not traditional colony-level feeding study 

designs provide information on foraging behavior and success endpoints not provided in other studies 

that were evaluated in the open literature. Besides being differentiated by the other colony feeding 

studies in their exposure duration, they are also distinct in that they do not include other measures of 

colony health such as information on mortality, proportions of various life stages, and overwintering 

survival. As noted in the Problem Formulation, while these studies do not provide information on 

regulatory endpoints by themselves, they provide additional information when characterizing the 

potential impact on the hive by measures of foraging success and behavior.

Table 5-12 below summarizes key elements and findings from the Apis high-tier studies that were 

evaluated from the open literature. While the studies were varied in their exposure duration, 

concentrations tested, and endpoints assessed, the following is a discussion of each subset of endpoints 

that aims to put into context the results of these studies with those of the registrant-submitted Tier II 

colony feeding study described above. Summaries of each of these studies including the methods and full 

results are provided in Appendix D.
129

There are several points worth noting with regards to this discussion:

1. While the registrant-submitted colony feeding study had raw data provided to allow for an 

independent verification of the statistical results, the higher tier studies from the open literature 

did not have this information available. For some studies, the responses of certain endpoints 

were estimated from the graphs provided in the original article and are therefore not without 

some uncertainty in their precision. For other studies, the means of the responses were not 

provided, but rather only the direction of the effect as compared to the control and whether or 

not the results were statistically significant. 

2. The studies evaluated from the open literature do not demonstrate exposure to the extent 

provided in the registrant-submitted colony feeding study. Specifically, this refers to a general 

lack of information regarding analytical confirmation of imidacloprid in treatment solutions, as 

well as a general lack of in-hive residue data for stored pollen, honey, and honey bee samples. 

3. While the registrant submitted study assessed numerous colony condition parameters, it did not 

include any response variables regarding foraging behavior or success. By contrast, a number of 

the Tier II studies from the open literature include some measure of foraging behavior or success 

as a response variable. 

4. Only 3 of the 7 available Tier II studies included an overwintering component. This design element 

is critical in evaluating the long term success of a colony following exposure to imidacloprid since, 

as was suggested by the results of the registrant-submitted colony feeding study, the honey bee 

colony can be a resilient entity that is capable of recovery of certain effects when exposure ceases. 
130

Table 5-12. Summary of semi-field (feeding) studies available from the open literature (Apis)

1

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure Dur.

(Observ. Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations2

Classification

Citation 

(MRID 

Number)

Confidor - 

not 

reported

(Apis 

mellifera)

Sucrose

(0, 100, 

500, and 

1000 ppb)

Not reported –

although 

assumed to be 

single dose 

given that 

observations 

were made at 

given intervals 

after exposure 

study duration 

was 24 hours 

(24 hours)

- Single 

colony 

isolated from 

other 

colonies and 

bees trained 

to forage on 

feeder for 

observations

Percentage of bees 

returning to hive 

after treatment, 

percentage of bees 

returning to feeding 

sites 

--

(No)

Return to hive: ↓25% 

(100 ppb, 0-2 hours

post-exposure), ↓31% 

(100 ppb, 4-5 hours post 

exposure), ↓5.1% (100 

ppb, 24 hours post 

exposure)

Return to feeder:

↓90% (100 ppb, 0-2 

hours and 4-5 post 

exposure)

- Purity of test substance 

not reported

- Bees in 500 and 1000 

ppb appeared to avoid 

feeders and were not 

observed for the duration 

of the test;

- One hive per group 

precluded statistical 

analysis;

- Unknown impact of 

these effects to other 

colony health parameters;

- Unknown amount of 

time that the bees spent at 

the feeders.

Qualitative

Bortolotti, 

20034

47800505

IMI - not 

reported

(Apis 

cerana)

Sucrose (0, 

10, 20, and 

40 ppb)

Single dose 

(not reported)

- 90 bees per 

group in 

feeder 

component

- 21 bees per 

group in 

nectar 

collection 

component

- 20 bees per 

group for 

predator 

avoidance 

component

Proportion of bees 

returning to feeder, 

average volume of 

nectar collected, 

predator avoidance

--

(Yes)

Proportion of bees 

returned to feeder: 

↓23% (40 ppb)†; 

average volume of 

nectar collected: ↓46% 

(20 ppb)†, ↓63% (40 

ppb)†; 

- Majority of the sugar 

solution was stated by the 

study authors as having 

been regurgitated 

suggesting an unknown 

dose level the bees were 

exposed to.

- The purity of 

imidacloprid was not 

stated

- Test species (Apis 

cerana) is not distributed 

in North America.

Qualitative

Tan, 20144

49719631
131

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure Dur.

(Observ. Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations2

Classification

Citation 

(MRID 

Number)

IMI - 

technical3

(Apis 

mellifera)

Sucrose (0, 

0.15, 1.5, 

3.0, and 6 

ng a.i/bee)

Single dose (48 

hours)

- Subset of 

bees from a 

colony were 

fitted with 

RFID tags to 

monitor 

foraging 

behavior after 

single oral 

dose

- 12 bees per 

dose group

- 2 identical 

trials 

conducted 

consecutively

Number of feeder 

trips, length of 

foraging trip, time to 

feeder, time at 

feeder, time to hive, 

interval between 

foraging trips, time of 

stay inside the hive 

immediately 

following treatment

--

(Yes)

Number of feeder visits: 

↓47% (1.5 ng)†, ↓98% 

(3 ng)†

- 12 bees per group and 

two total trials results in 

limited sample size from 

which results are based.

- High variability for 

certain endpoints that is 

likely the result of limited 

sample size and 

replication.

- Unknown impact of 

these effects to other 

colony health parameters 

particularly since these 

effects were noted to have 

been observed 

immediately after 

treatment were not 

present 24 and 48 hours 

after dose administration.

Qualitative

Schneider, 

20124

49719629

Length of foraging trip: 

↑50% (1.5 ng)†, ↑130% 

(3 ng)†

Time to feeder: ↑65% 

(1.5 ng)†, ↑241% (3 

ng)†

Time at feeder: ↑25% 

(1.5 ng)†, ↑46% (3 ng)†

Time to hive: ↑20% (1.5 

ng)†, ↑210% (3 ng)†

Interval between trips: 

↑33% (1.5 ng)†, ↑993% 

(3 ng)†

Time inside hive (1st

stay): ↑972% (3 ng)†

Time inside hive (2nd

stay): ↑33% (1.5 ng)†, 

↑1077 (3 ng)†
132

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure Dur.

(Observ. Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations2

Classification

Citation 

(MRID 

Number)

IMI –

technical

(Apis 

mellifera 

ligustica)

Sucrose 

(0, 24, and 

241 ppb 

for sucrose 

response; 

0 and 24 

ppb for 

dancing 

behavior)

Single dose (1 

hour for SRT, 

24 hours for 

dancing 

behavior)

- 314 nectar 

foragers and 

209 pollen 

foragers used 

for SRT 

component

- 65 bees 

used for 

dancing 

behavior 

component 

- SRT (lowest sucrose 

concentration that 

bees would elicit 

complete PER)

-Dancing behavior 

(number of dance 

circuits) 

--

(Yes)

Nectar forager SRT: 

↑78% (24 ppb)†, ↑81% 

(241 ppb)†

Pollen forager SRT: 

↑206% (241 ppb)†

- It was unclear whether 

the entire dose was 

consumed;

- Mean % sucrose 

response threshold 

(minimum percent sucrose 

to elicit a proboscis 

extension response) noted

to be highly variable with 

%CV values of 143% and 

224% for control nectar 

and pollen foragers, 

respectively. 

- Although the authors 

state that the objective of 

the dancing behavior 

component was to 

examine the effects of 

imidacloprid metabolites, 

residues are not identified 

and/or measured. 

- Unknown impact of 

these effects to other 

colony health parameters.

Qualitative

Eiri and 

Nieh4

, 2012
133

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure Dur.

(Observ. Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations2

Classification

Citation 

(MRID 

Number)

IMI - not 

reported

(Apis 

mellifera 

mellifera)

Sucrose 

(0-unfed, 

0-fed), 0.5, 

and 5 ppb)

34 days (7 

months, 

including 

overwinter)

- Unfed 

control group 

relied 

exclusively on 

forage

- 8 

colonies/trt

- 7 colony 

condition 

assessments

Mortality, egg laying, 

activity index(bees 

per min entering 

hive), capped brood 

area, hive weight, 

adult population, 

disease and parasite 

incidence

--

(Yes)

Number of frames of 

capped brood area : 

↑14% (0.5 ppb)†; ↓34% 

(5 ppb)†

- Purity of imidacloprid not 

reported 

- Results refer to after 

overwintering period and 

comparisons made to fed 

(sucrose) control (not clear 

from study whether 

different from unfed 

control)

Qualitative

Faucon, 2005 

47523406
134

AdmirePro 

– not 

reported 

(not 

reported)

Pollen 

(0, 5, and 

20 ppb)

12 weeks (5 

months from 

exposure start 

to last CCA 

(mid-October, 

colonies were 

then overwintered)

- 10 replicate 

colonies per 

group

- October 15 

was last CCA

- Limited 

space in 

nucleus hives 

and 

supplemental 

feeding was 

not provided

Egg-laying activity, 

larvae development, 

food consumption, 

amount of pollen 

collected, total 

foragers returning to 

hive, percentage of 

foragers with pollen 

pellets, nectar 

station visits

--

(Yes)

Nectar station visits: 

↓35.7% (5 ppb)†, 

↑13.1% (20 ppb)

- Unknown confounding 

effect of queen 

replacement and 

food/brood removal frame 

removal

- For the foraging trials, 

numbers of marked bees 

are provided but this 

represents a small 

(approximately 2-4% of the 

total numbers exposed) 

- Unknown confounding 

effect of queen 

replacement and 

food/brood removal frame 

removal;

- For the foraging trials, 

numbers of marked bees 

are provided but this 

represents a small 

(approximately 2-4% of the 

total numbers exposed); 

- No way to confirm the 

lack of potential for 

exposure to other sources 

of imidacloprid or 

neonicotinoids in the 

surrounding area or where 

the imidacloprid in the 

control colonies came 

from. The study area was 

noted to have been 

surrounded by corn that 

may have been seed 

treated with 

neonicotinoids.

Qualitative

Dively, 2009

(47775502)
135

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure Dur.

(Observ. Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations2

Classification

Citation 

(MRID 

Number)

Admire Pro 

– 42.8% 

(not 

reported)

Pollen (0, 

5, 20, and 

100 ppb

12 weeks

(10 months 

from exposure 

start to last 

CCA (midMarch)

- 10 replicate 

colonies per 

group, each 

blocked in 

groups of 2 

across 5 

apiary 

locations

- Pollen 

traps installed 

at each 

entrance to 

direct bees to 

feed on 

spiked pollen 

patties

-Residue 

analysis 

conducted 

confirmed 

presence in 

pollen patties 

(5.5, 19.8, 

and 97.5 

μg/kg for the 

5, 20, and 100 

μg/kg groups, 

respectively)

Queen event (manual 

replacement or 

natural supersedure), 

disease presence and 

incidence (Varroa

and Nosema), mean 

colony size, mean 

amount of pollen 

collected, percentage 

of total frame 

covered for bees, 

capped brood cells, 

capped honey, 

beebread, drawn out 

cells (defined by 

study as empty, 

cleaned out cells), 

overwintering 

survival

--

(Yes)

2009 Trial:

Overwintering survival: 

↓25% (100 μg/kg)†

Percentage of total 

frame coverage by 

capped honey: ↑65% 

(100 μg/kg)

2010 Trial:

Percentage of total 

frame coverage by 

capped honey: ↑125% 

(100 μg/kg)

- In the 2009 trial, mean 

Varroa mite levels were 

7.1, 8.8, 6.6, and 13.3 

mites per 100 bees in the 

control, 5, 20, and 100 ppb 

treatment groups, 

respectively but there was 

no mention of treatment. 

It is noted however that 

overwintering success in 

this trial was 100% in the 

control group.

- In the 2010 trial, queen 

events were collapsed for 

the control and 5 ppb 

levels in one metric and for 

the 20 and 100 ppb groups 

in another. It is unclear if 

more queen events 

occurred in the control or 5 

ppb group.

- Control overwintering 

survival in the 2010 trial 

was 57% which renders 

results from this trial 

uncertain in discriminating 

treatment related effects

- Survival data were 

pooled in the 2009 and 

2010 trial despite 100% 

control survival in 2009 

and 57% in 2010.

Qualitative

Dively, 2015

IMI: imidacloprid; PER: Proboscis extension response; RFID: radiofrequency identification; CCA: colony condition assessment, SRT: sucrose response threshold
136

†Indicates effect was statistically significant (p<0.05). 1Most studies not associated with NOAEC/LOAEC values. Reported is the most sensitive statistically derived or otherwise observed difference relative to the control. 2Only subset of limitations are listed here. Others associated with this study can be found with the study summaries in Appendix D. 3Although not explicitly stated in the article, personal communication with the author (email dated 01/29/15) indicates that imidacloprid was of technical grade. 4 These studies, while assessing endpoints that could potentially impact colonies, are not of a feeding design in the traditional sense in that bees are exposed to a single dose
137

Mortality (inclusive of worker/forager and colony overwintering):

There were 3 studies which included either worker or colony mortality (or survival) as a response variable. 

With these studies, exposure to imidacloprid through spiked nectar, pollen, or following seed treatment 

applications generally did not appear to have an overall impact on worker or overall colony survival when 

compared to the control with the exception of colonies provided imidacloprid in spiked pollen at 100 

μg/kg (Dively 2015). The registrant-submitted colony feeding study (technical imidacloprid, 6 week 

exposure, 12.5, 25, 50, 100, and 200 μg a.i/L groups, overwintering component) showed an erratic dose 

response (18, 9, 36, 91, and 82% overwintering mortality for the treatment groups, respectively) for 

overwintering mortality and when compared to 36% overwintering mortality in the control group, 

inferences made about treatment-related overwintering mortality at the lower groups (12.5, 25, and 50) 

are uncertain.

In Faucon, 2005, (sucrose feeding study design, 0.5, and 5 ppb treatment groups, 34-day exposure, 

overwintering component), the study authors did not subject the mortality data to statistical analysis as 

it was stated that daily mean mortality was low with means of 3.1, 4.4, 4.3, and 3.3 for the unfed control 

group, untreated sucrose control, 0.5 and 5 ppb imidacloprid groups, respectively. These data 

corresponded to a period that included the entire 34-day exposure phase as well as the 16-day interval 

just after exposure ended. For overwintering success, colonies were scored by the number of frames with 

brood combined with frames of adults. The colonies were scored the March after exposure began 

(previous July) and there was no significant differences (p>0.05) in the treated groups from that of both 

control groups. 

Dively, 2015 (formulated imidacloprid, pollen feeding study design, 5, 20, and 100 ppb treatment groups, 

12-week exposure period, overwintering component) included two years of feeding study trials (2009 and 

2010) with separate colonies and exposures taking place each year. In the 2009 trial, there was a 

significant (p<0.05) reduction at the 100 μg /kg [spiked pollen] group (mortality in 2/8 colonies as 

compared to 0/10 colonies in the control (two colonies in the 100 μg/kg group were terminated in early 

September by the study authors after the colonies underwent natural supersedure (queen replacement) 

and the researcher determined that the colonies were no longer healthy enough to survive 

overwintering). In the 2010 trial, there were no treatment-related effects on colony overwintering but it 

is noted the control group for this trial had 57% survival. 

Effects on presence of various life stages:

The same 3 studies discussed above for mortality also assessed the impact of imidacloprid exposure to 

the presence of various life stages within the hive. Additionally, Dively, 2009 (pollen exposure, 5 and 20 

ppb treatment groups, 12-week exposure phase, no overwintering component) examined the presence 

of various life stages within the hive. While not every study examined the same life stages and route of 

exposure, there was generally no impact on the presence of different life stages up to and including the 

highest treatment concentrations assessed (20 μg a.i/L in sucrose, 100 μg/kg in pollen). By contrast, the 

registrant-submitted colony feeding study determined significant reductions in the numbers (as 

percentage of frame coverage within the hive) of adults, eggs, and capped brood (pupae) at the three 

highest treatment groups (50, 100 and 200 μg a.i/L). These effects were usually observed following 
138

exposure and sustained through the duration of the study including after overwintering (100 and 200 μg 

a.i/L groups) but other effects were determined during exposure, showed recovery to the level of the 

control group following exposure, but were again reduced after the overwintering period (50 μg a.i/L 

group). 

Faucon, 2005 did not determine significant effects (p>0.05) to percent frame coverage of adults and eggs 

up to and including the highest treatment group (5 ppb in sucrose). The capped brood (pupae) coverage 

response changed depending on the interval considered. When considering the exposure duration 

interval as well as exposure phase plus 3 weeks post-exposure phase, there were no significant effects 

determined (p>0.05). However, after the overwintering period, there were significant effects on the 

number of frames with capped brood area (p<0.05) although these effects were not dose-responsive (14% 

increase at 0.5 ppb group, 34% decrease in the 5 ppb group). While the route of exposure was distinct 

from Schmuck, 2001 and Faucon, 2005, there were no significant effects (p>0.05) determined in both 

Dively, 2009 and Dively, 2015 on percent frame coverage of adults, eggs, and capped brood (pupae). 

While there was a dose-responsive increase of 27, 35, and 51% (for the 5, 20, and 100 μg/kg treatment 

groups, respectively, the different treatments were not statistically significant, p>0.05) in the coverage of 

capped brood cells in the 2010 trial of Dively, 2015 for observations made at a mid-August CCA. In the 

subsequent early October CCA, these observations did not indicate a treatment-related effect (↑8%, 

↓15%, no change, for the 5, 20, and 100 ppb groups, respectively).

Foraging behavior/foraging success observations:

As noted previously, the majority of the available Tier II open literature studies with Apis include some 

measure of foraging behavior (numbers of foraging trips, time spent on trips, durations of trips) and 

success (amount of pollen and nectar collected). While some studies include foraging measurements with 

other colony health parameters in an attempt to link these effects to short- or long-term colony success, 

other studies assess foraging endpoints only. With the latter case, there is uncertainty in the impact of 

these effects on the success of the colony. As mentioned previously, the registrant-submitted colony 

feeding study did not include foraging endpoints.

It was previously mentioned that a subset of the studies examining foraging endpoints involved a single 

oral exposure for individual bees as opposed to a sustained exposure to colonies with spiked sucrose or 

spiked pollen provisions. While Bortolotti, 2003 does not specify the duration of exposure, as the study 

was 24 hours in duration with endpoints made in pre-defined intervals after exposure, it was assumed to 

be a single oral dose.

In Bortolotti, 2003 (formulated imidacloprid, 100, 500, and 1000 ppb treatment groups, duration of 

exposure not reported), there were decreases ranging from 25-31% of bees returning to the hive from 

directly after exposure to 5 hours post exposure in the 100 ppb group. After 24 hours, this effect was 

reduced to 5.1%, suggesting a recovery of orientation after imidacloprid exposure ceases. Reductions to 

the number of bees returning to a sucrose feeder were observed to be 90% as compared to the control 

group at both the 0-2 and 4-5 hours after exposure intervals (no statistical analysis conducted on the 

results). There was no 24-hour post-exposure observation for this endpoint. Bees in the 500 and 1000 
139

ppb groups were not seen returning to the hive or feeder and therefore no further data were collected. 

It is noted that 500 and 1000 ppb are markedly high concentrations that for a study investigating sublethal 

effects on foraging behavior and success. Tan, 2014 (spiked sucrose, 10, 20, and 40 ppb treatment groups, 

single oral exposure) similarly investigated the numbers of bees retuning to a feeder provided in the field. 

There was a 23% reduction (p<0.05) in the number of bees returning to the feeder at the 40 ppb group. 

Additionally, the mean amount of nectar collected was significantly decreased (p<0.05) from the level of 

the control in the 20 and 40 ppb treatment groups (↓46% and ↓63%, respectively). Schneider, 2012 

(technical imidacloprid, 0.15, 1.5, 3, and 6 ng a.i/bee, RFID tagging) determined significant reductions 

(p<0.05) in number of feeder visits (↓47 - 98%), and significant increases (p<0.05) in length of foraging 

trips (↑50 – 130%), time to feeder (↑65 – 241%), time at feeder (↑25 – 46%), time to hive (↑20 – 210%), 

and interval between trips (↑33 – 993%). It is noted that these effects were observed immediately after 

treatment and were not observed 24 and 48 hours after treatment which corroborate to some extent the 

results of Bortolotti, 2003. Finally, in Eiri and Nieh, 2012, individual honey bees that were exposed to a 

single oral dose of either 24 or 241 ppb in sucrose and subsequently assessed in a sucrose response 

threshold, which is defined as the lowest concentration of sucrose which will elicit a proboscis extension 

response (PER) in honey bees. The results indicated that there was a dose-responsive increase in the 

sucrose response threshold with increasing dose, indicating higher concentrations were needed to elicit 

a PER. No other response variables were assessed in this study.

Several studies also examined the amount of nectar and pollen collected. In the Dively studies (2009 and 

2015) that had a pollen feeding study design, a variety of foraging endpoints were assessed. In Dively 

2009, while there were no significant effects (p>0.05) on the amount of pollen collected and percentage 

of foragers with pollen pellets, there was a significant (p<0.05) reduction from control in the number of 

nectar station visits at the 5 ppb group, although this response was a non-significant increase of 13.1% at 

the 20 ppb group. In Dively 2015, there were no effects on the amount of pollen collected (increases from 

control ranged from 14 – 32% but were not significant and not dose-responsive). 

Summary:

This discussion illustrates that for the available set of higher-tier studies from the open literature, effects 

on colony health parameters such as overwintering survival, worker/forager mortality, and presence of 

various life stages (as percentage of frame coverage within the hive), were not determined at levels in 

spiked sucrose up to and including 40 ppb, and in levels of pollen up to and including 20 ppb. The latter 

statement is based only on two available pollen exposure studies (Dively, 2009 and Dively 2015), but there 

was a significant reduction in overwintering survival at the 100 ppb group in one of the two trial years. 

For foraging measurements in which a single oral dose was administered to individual bees (Bortolotti 

2003, Eiri and Nieh 2012, Schneider 2012, and Tan 2014), significant effects on orientation (bees returning 

to feeder, time to and at feeder, length of foraging trips) were observed at concentrations in sucrose as 

low as 1.5 ng a.i/bee. These effects were noted immediately following oral exposure and were not 

observed 24 hours after exposure. For the colony-level feeding studies, there were no effects up to an 

including 5 ppb in sucrose and 20 ppb in pollen. For effects on pollen and nectar collection, there were 
140

no effects in spiked sucrose feeding studies up to and including 20 ppb and up to and including 100 ppb 

in spiked pollen. 

This summary is based on the results of a small number of studies and therefore there is uncertainty as 

to whether levels of imidacloprid in nectar and pollen above or below the concentrations described above 

could potentially impact the overall health of a colony following continuous exposure. There are no data 

at this time to link impaired foraging behavior of individual bees as a result of acute exposure to relatively 

low doses of imidacloprid to impaired colony condition. 

Bombus 

Summary of Tier II Bombus Studies from the Open Literature

There were a total of 2 Tier II studies (tunnel design, one of which also had a full-field Tier III component) 

and 8 feeding design studies to characterize the colony-level effects on bumble bees (i.e. various species 

of Bombus). Two of the Tier II feeding design studies originate from the same dataset (i.e. Gill 2012 and 

Gill and Raine 2014), with Gill and Raine 2014 re-analyzing a subset of the 2012 data (foraging 

measurements). As with the higher-tier Apis open literature studies, exposure duration, concentrations 

tested, and endpoints assessed varied across the 10 studies. 

There are a few points worth noting:

1. While workers and the queen bee undergo overwintering in honey bee colonies, and 

subsequently build up again the following spring, only the Bombus queen overwinters. Therefore, 

there was no overwintering component included in any of the open literature Bombus studies as 

distinguished from Apis.

2. Colonies of Bombus are much smaller than those of Apis and typically range from several dozen 

to several hundred bees at most. In contrast, Apis colonies consist of thousands of bees and can 

reach sizes up to several tens of thousands. It is therefore expected to some extent that Apis 

colonies are able to compensate for greater losses of their adult population before colony failure 

as compared to Bombus.

3. Some Bombus studies are conducted with microcolonies. Microcolonies are queen-less units of a 

few worker bumble bees where one individual eventually becomes dominant and starts laying 

unfertilized eggs that will eventually become drones (males). 

Table 5-13 below summarizes key elements and findings from the Bombus higher tier studies that were 

evaluated from the open literature.
141

Table 5-13. Summary of semi-field (Tunnel) studies available from the open literature (Bombus)

Test 

Substance 

Purity 

(Test 

species)

Crop

(App. 

Rate)

Exp.

Dur.

(Observ. 

Dura)

Design 

Elements

Endpoints Assessed

(Statistics analysis 

conducted? –

Yes/No)

Effects2 (all comparisons 

made relative to the 

study’s control)

Limitations3

Classification

Citation 

(MRID 

Number)

Gaucho –

not 

reported

(Bombus 

terrestris)

Sunflower

(0.7 mg 

a.i/seed)

4 days

(4 days)

- One tunnel 

per treatment 

group

- 75 total 

workers 

observed for 

foraging 

measurements

Number of workers 

visiting blooming 

heads, number of 

workers with ‘short’ 

foraging trips, 

number of bees with 

‘long’ foraging trips1

--

(Yes)

Numbers of visits: ↑36%, 

Workers with ‘long’ 

foraging trips: ↑60%

- Insufficient demonstration of 

exposure as study authors cite 

unpublished data from nectar 

samples of sunflowers in 

greenhouse used for this 

study.

- Small sample sizes that were 

subject to statistical analysis 

for foraging data.

- Minimal information on the 

husbandry the bees used.

Qualitative

Tasei 2001 

(47800503)

Merit 0.5 

G – not 

reported 

(Bombus 

impatiens)

Turf with 

25-50% 

flowering 

white 

clover

(0.4 lbs 

a.i/A)

28 days

(28 

days)

- 5 plots (3m x 

5m per group

- 1.5 cm of 

irrigation 

following 

exposure

- 1 colony per 

plot

Colony weight, 

worker weight, queen 

weight, number of 

workers, number of 

brood chambers, 

number of honey 

pots, time to initial 

defense response, 

duration of defense 

response, number of 

bees responding

--

(Yes)

Number of workers: 

↓26%, 

Time to initial defense 

response: ↓76%, Number 

of brood chambers: 

↑75%

- Method used for defense 

response has not been 

formally standardized to 

determine its sensitivity

- It was noted that variability 

for some endpoints was high, 

especially in the data for the 

0.5 G experiment, which could 

explain the statistical methods 

failing to detected high (over 

70% reductions and increases 

from the level of control).

- 28 exposure period in a flight 

cage is longer than the 10-14 

days recommended by OECD 

semi field tests for honey bees. 

It is unknown how adaptable 

bumble bees are to this length 

of confinement.

Qualitative

Gels, 2002

Merit 75 47796308

WP – not 

reported

(Bombus 

impatiens)

Turf with 

25-50% 

flowering 

white 

clover

(0.3 lbs 

a.i/A)

- 5 plots (3m x 

5m per group 

(control, spray 

w/ irrigation, 

spray w/ no 

irrigation)

- 1 colony per 

plot

Colony weight (↓54%)†, 

worker weight (↓56%)†, 

number of workers 

(↓60%)†, number of 

brood chambers 

(↓87%)†, number of 

honey pit (↓72%)†, time 

to initial defense response 

(↓67%)†, duration of 

defense response 

(↓73%)†, number of bees 

responding (↓77%)† †Indicates effect was statistically significant (p<0.05). 
142

1’Short’ trips defined by study author to be 1-50 seconds and ‘long’ trips defined to be >50 seconds 2Not all studies were associated with NOAEC/LOAEC values. Reported is the most sensitive statistically derived or otherwise observed difference relative to the control. 3

Limitations considered to be major listed here. Others associated with this study can be found with the study summaries in Appendix D.
143

Table 5-14. Summary of semi-field (feeding) studies available from the open literature (Bombus)

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

Confidor –

20% 

(Bombus 

terrestris)

Sucrose –

component 

1 and 2 (0, 

10, 20, 200, 

20000, and 

200000 ppb; 

component 

3 (0, 2, 10, 

and 20 ppb)

Component 1 

and 2: – 11 

weeks; 

Component 3:

– 2 weeks (11 

weeks and 2 

weeks, 

respectively)

Components 1 

and 2: 

- microcolony (4 

colonies with 5 

workers each 

housed in cages 

for 11 week 

exposure

Component 3:

- queen right 

colonies (25 

bees per colony, 

1 per group) in 2 

week 

greenhouse 

exposure

Component 1: mortality, 

reproduction (total 

amount of brood 

produced)

Components 2 and 3: 

mortality, reproduction, 

and foraging behavior

--

(Yes)

Component 1:

Mortality: 100% in 200, 

2000, 20000, and 20000 

ppb†; Reproduction: no 

reproduction at these 

doses†

Component 2: 

Mortality: 50% in 20 

ppb, 100% in 200, 2000, 

20000, and 200000 

ppb†

Reproduction: ↓62% 

(10 ppb)†, ↓75% (20 

ppb)†, ↓83% (200 

ppb)†

Component 3:

Mortality: 62% (10 

ppb)†, 92% (20 ppb)†

Reproduction: no 

reproduction at 10 and 

20 ppb groups

- There was no 

analytical 

confirmation of 

imidacloprid in the 

treatment 

solutions

- Control 

performance was 

not reported in 

component 1 and 

component 2 of 

the study

Qualitative

Mommaerts, 

2010

48151502

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (0

and 10 μg 

a.i/L)

4 weeks 

(4 weeks)

- 10 colonies per 

group of control, 

IMI, λcyhalothrin 

(spray 

treatment), and 

mix of two

- Colonies 

housed in 

laboratory but 

allowed to freely 

Mean worker mortality, 

% workers getting lost, % 

worker loss + mortality, 

% sucrose consumption, 

pollen foraging size, 

number of 

workers/colony, number 

of foragers/colony, 

brood production, queen 

loss, nest structure mass, 

colony failure, number 

%workers getting lost 

(↑50%)†, %worker 

loss+mortality (↑37%), 

number of 

workers/colony 

(↓27%)†, number of 

foragers/colony 

(↑150%)†, brood 

production/colony 

(↓22%)†, pollen load 

score/foraging worker 

- Vulnerability of 

small size colonies 

may differ from 

that of larger 

colonies; 

- The protozoan 

Crithidia bombi

was found in 55% 

of the colonies 

although there 

was no treatmentQualitative

Gill, 2012

49719618
144

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

forage outside 

through 

connecting tube 

- Foraging 

observations 

recorded with 

RFID tags

of foraging 

bouts/colony, pollen 

load score/foraging 

worker, pollen load 

score/successful 

foraging bout, % 

foraging bouts/forager 

that returned with 

pollen, duration of 

pollen bouts

--

(Yes)

(↓31%)†, %foraging 

bouts/forager that 

returned with pollen 

(↓28%)†, duration of 

pollen bouts (↑18%)†

related effect in 

the incidence of 

the disease among 

the bumble bees.

- No analytical 

confirmation of 

treatment 

concentrations

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (0, 

0.08, 0.20, 

0.51, 1.28, 

3.20, 8.0, 

20.0, 50.0, 

and 125.0 μg 

a.i/L

13 days 

(14 days)

- Micro-colonies 

(4-5 workers, no 

queen) 

established to 

examine effects 

on ovary 

development 

and 

reproduction 

- Varying 

number of 

replicates for 

each 

concentration

Fecundity (number of 

eggs and larvae 

produced), food 

consumption, and 

oocyte development

--

(Yes)

Fecundity: ↓42% at 1 

μg a.i/L. 

- Unknown 

relevance of 

reproductive 

effects on workers 

for effects to 

colony health 

given that worker 

production of 

males is generally 

much less of the 

colony male 

output and 

workers cannot 

produce new 

workers or new 

queens 

- No analytical 

confirmation of 

treatment 

concentrations

Qualitative

Laycock, 

2012

49719622
145

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (of 

0, 0.06, 0.16, 

0.40, 1.0, 

2.5, 6.3, 16, 

39, and 98 

ppb

14 days or 28 

days (14 days 

or 28 days)

- 3 queenright 

colonies (1 

queen, 4 

workers) per 

group for 14-

days on dose 

followed by 14-

days of off dose 

(unspiked syrup)

- Study 

consisted of the 

results of the 

two trials pooled 

- Continuous 

exposure of 1 

group (5 

colonies) to 98 

ppb for 28 days 

(7 colonies for 

control)

Brood production, time 

to first oviposition, 

pollen consumption, 

sucrose consumption

--

(Yes)

Brood production: 14-

day on-dose EC50: 1.44 

ppb, 28-day (on/off 

dose) EC50: >98 ppb

Pollen consumption: 

14-day on-dose EC50: 

4.4 ppb, 28-day (on/off 

dose) EC50: 44 ppb

- Long-term 

impacts of 

decreased brood 

production on 

other colony 

health parameters 

like queen loss and 

worker mortality 

were not 

investigated 

Qualitative

Laycock and 

Cresswell, 

2013

49719621
146

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (0 

and 10 μg 

a.i/L)

6 weeks (6 

weeks)

- 8 queen-right 

colonies in each 

group

- Colonies were 

kept in cages 

within the 

laboratory for 

the entire 

duration of the 

study

- Data informed 

the Sublethal 

Stress Model 

development

(developed by 

the study 

authors)

Mortality, brood 

production

--

(No)

Mortality (day 0-42) 

(↓2.56%), birth rate 

(day 0-42) (↓71.3%)

- Control 

mortality differed 

substantially

between the 8 

colonies, ranging 

from 9-38%, and 

above 25% in 5 of 

8 colonies. As this 

study was 

conducted in the 

laboratory, this is 

suggestive of 

general husbandry 

issues;

- Although there 

appeared to be 

sufficient 

replication among 

the treatment 

groups, the results 

were not 

subjected to 

statistical analysis 

by the study 

authors.

- No analytical 

confirmation of 

treatment 

concentrations

Qualitative

Bryden, 2013

49719607
147

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (0 

and 10 μg 

a.i/L)

4 weeks 

(4 weeks)

- Same 

methodology 

used in Gill 2012

- Temporal 

analysis of the 

foraging data of 

Gill 2012 

assessing weekby-week results.

Number of foragers, 

foraging bouts, foraging 

bout duration, pollen 

load size from all 

foraging bouts, pollen 

load size from successful 

foraging bouts, 

successful pollen 

foraging bout duration

--

(Yes)

Number of foragers (↑ 

for all 

intervals/sampling 

times)†, foraging bouts 

(↓ - week 2 only)†, 

pollen load size from all 

foraging bouts (↓ -

week 4 only)†, 

successful pollen 

foraging bout duration 

(↑ - week 4 only)†

Same limitations 

as those identified 

in Gill 2012

Qualitative

Gill and 

Raine, 2014
148

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

IMI –

technical 

grade

(Bombus 

terrestris)

Sucrose (0, 

0.7, and 1.4 

ppb – ‘low 

group’); 

Pollen (0, 6, 

and 12 ppb –

‘high group’)

14 days – lab 

(6 weeks in 

field)

- 25 colonies per 

group (mean 15 

bees per colony) 

- Colonies 

provided spiked 

sucrose and 

spiked pollen 

simultaneously 

in lab for 2 

weeks

- Moved to field 

(with mixed 

farmland) for 6 

week 

observation 

period 

Colony weight, mean 

numbers of life stages 

(workers, males, pupae, 

and empty pupal cells), 

mean number of queens 

produced 

--

(Yes)

Colony weight: ↓8% 

(‘low group’)†, ↓12% 

(‘high group’)†, Number 

of queens produced: 

↓85% (‘low group’)†, 

↓90% (‘high group’)†

- No analytical 

verification of 

imidacloprid in 

nectar and pollen

- It would have 

been informative 

to have a measure 

of food 

consumption or 

whether the 

pollen and nectar 

stores had to be 

replenished at any 

time during the 

14-day exposure

-Although the 

authors suggest 

that imidacloprid 

may have reduced 

foraging efficiency 

in the treated 

colonies, this 

study did not 

include any 

response variables 

to evaluate 

foraging efficiency

Qualitative

Whitehorn, 

2012

49719634
149

Test 

Substance 

– Purity 

(Test 

species)

Exposure 

Matrix

(Exposure 

Level)

Exposure 

Dur.

(Observ. Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Classification

Citation 

(MRID 

Number)

IMI – not 

reported

(Bombus 

terrestris)

Sucrose: (0, 

0.7 ppb); 

Pollen: ( 0 

and 6 ppb)

14 days – lab 

(4 weeks in 

field)

- 3 colonies per 

group (65 bees 

per colony)

- Colonies 

observed after 

exposure in area 

described as 

urban garden 

area with 

nearest farmed 

area 0.5 miles 

away

Colony survival, weight 

of nectar 

collected/foraging bout, 

nectar foraging 

efficiency, weight of 

pollen collected, pollen 

foraging efficiency 

--

(Yes)

Weight of pollen 

collected: ↓28%†, 

pollen foraging 

efficiency: ↓31%†

- Purity of 

imidacloprid not 

reported

- Analytical 

verification of 

imidacloprid in 

spiked pollen and 

nectar was not 

conducted

Qualitative

Feltham, 

2014

49719617

IMI = imidacloprid; RFID = radiofrequency identification; 

†

Indicates effect was statistically significant (p<0.05). 2Not all studies were associated with NOAEC/LOAEC values. This column will report the most sensitive statistically derived or otherwise observed difference from that of the control. 3

Limitations considered to be major listed here. Others associated with this study can be found in the study summaries in Appendix D.
150

There were 4 feeding design studies that assessed worker bumble bee mortality. Mommaerts, 2010 

(formulated imidacloprid, spiked sucrose) tested both microcolonies (5 bees) and queen-right colonies 

(25 bees) for 11 week and 2-week exposure durations, respectively. In one microcolony trial, there was 

100% mortality in the 200, 2000, 20000, and 200000 ppb treatment groups while mortality was not 

significantly reduced (maximum reduction of 15% from control) at the 10 and 20 ppb groups. In a second 

trial with the same treatment groups but an additional experimental chamber to evaluate foraging, there 

was again 100% mortality in the top 4 groups with 50% mortality in the 20 ppb. For queen-right colonies 

(2, 10, and 20 ppb treatment groups), mortality was significantly increased over the control at the 10 and 

20 ppb groups (↑62 – 92%, respectively). It is not clear why the queen-right colonies were determined 

to be more sensitive to lower concentrations and for shorter durations as compared to the queenless 

microcolonies. In Gill, 2012 (technical imidacloprid, spiked sucrose, 10 μg a.i/L, 4 week exposure), there 

was no significant (p>0.05) impact on worker mortality determined. Bryden, 2013 (technical imidacloprid, 

spiked sucrose, 10 μg a.i/L, 6-week exposure period) observed a 2.6% reduction in survival (no statistical 

analysis conducted) from the control. In Feltham, 2014 (spiked sucrose – 0.7 ppb, and spiked pollen – 6 

ppb, 14-day exposure), 92% of exposed colonies in the treatment group (both food provisions available 

simultaneously) survived until the end of the 4-week post-exposure observation period in the field. 

Effects on numbers of various life stages:

There was one semi-field tunnel design study and 2 semi-field feeding design studies that investigated 

effects on various life stages of bumble bees. In Gels, 2002 (formulated imidacloprid, 28-day exposure 

period in flight cage) bumble bee colonies were placed in flight cages following either granular or spray 

application to turf. For the granular application (0.4 lbs a.i/A of Merit® 0.5 G), there were no significant 

effects (p>0.05) determined although the number of workers decreased by 26% and the number of brood 

chambers increased by 75% relative to the control. There was high variability in the responses for this 

component of the study with statistical methods used failing to identify percent differences of 70% and 

above. In the spray application component (0.3 lbs a.i/A Merit® 75 WP), there were significant (p<0.05) 

decreases in the numbers of workers (↓60%) as well as the number of brood chambers (↓72%). 

Interestingly, these differences were identified when spray application were not followed by irrigation; 

however, when irrigation was administered, there were no significant differences from the control group. 

In Gill, 2012, there was a significant reduction (p<0.05) in the number of workers per colony (↓27%) 

following a week exposure to 10 μg a.i/L. Finally, Whitehorn, 2012 (technical imidacloprid, spiked sucrose 

– 0.7 and 1.4 and spiked pollen – 6 and 12 ppb co-exposure, 14-day exposure period) did not identify 

significant effects (p>0.05) on the numbers of workers, males, and pupae.

Effects on reproduction (brood and queen production)

There were 6 feeding design studies (4 spiked sucrose studies, 2 with co-exposure of spiked sucrose and 

spiked pollen) and one full-field study that assessed endpoints related to reproduction. In Mommearts, 

2010 in addition to no reproduction at the concentrations which elicited 100% mortality in the 

microcolony trials, there was no reproduction in the 10 and 20 ppb groups for the queen-right colony trial. 

Reproduction was not significantly impacted (p>0.05) at 2 ppb. Gill, 2010 found a significant (p<0.05) 22% 

reduction in brood production as compared to the control group. Laycock, 2012 (technical imidacloprid, 
151

spiked sucrose, concentrations ranging from 0.08 – 125 μg a.i/L, 13-day exposure period) tested 

microcolonies (4-5 workers each) and determined a 42% reduction in fecundity at a concentration of 1 μg 

a.i/L. Whitehorn. 2012 exposed queen-right colonies (25 per treatment) for 14 days in the laboratory 

followed by a 6 week observation period in the field. The number of queens produced was significantly 

reduced by 85 and 90%, for the “low” (6 ppb/0.7 ppb pollen/sucrose) and “high” (12ppb/1.2 ppb 

pollen/sucrose) fed groups, respectively. As shown in Figure 5-3, it is evident that queen production is 

highly variable in the control B. terrestris colonies, with the bulk of new queens coming from 5 colonies. 

These same 5 control colonies were also the largest in terms of overall weight (data not shown), which 

supports the hypothesis that queen production is dependent on colony size of B. terrestris. It is also 

noteworthy that 48% of the control colonies did not produce queens whereas 88% and 68% of the low 

and high exposure groups, respectively. It is not known whether the failure of 48% of the control hives to 

produce new queens reflects “normal” development and queen production in natural B. terrestris colonies 

in the field. Nevertheless, it is evident that colonies failed to produce large number of queens in the low 

and high exposed treatments, relative to controls. 
152

Figure 5-3. Queen Production Data from Whitehorn et al (2012) in Controls (A), Low (B) and High Exposure 

Treatments (C). Legend numbers indicate the concentration in pollen and nectar, respectively.

In Bryden, 2013, there was a 71% reduction in brood production the sole treatment group of 10 μg a.i/L 

(statistical analysis not conducted). In Laycock and Cresswell, 2013 (technical imidacloprid, spiked 

sucrose, concentrations ranging from 0.06 – 98 μg a.i/L), queen-right colonies (1 queen, 4 workers) were 

exposed to imidacloprid spiked sucrose for 14 days followed by 14 days of observation while colonies 

were fed untreated sucrose. When assessing the exposure only interval, the EC50 for brood production 

was 1.44 μg a.i/L, a similar finding of Laycock 2012. When assessing the entire 28-day duration (including 

the 14-day ‘off dose’ period) the EC50 was determined to be >98 ppb, suggesting a recovery in brood 

production after exposure to imidacloprid ceases. Finally, in Tasei, 2001, there were 17 and 24 queens 

per colony in the control and treatment fields, respectively, and the difference was not statistically 

significant (p>0.05).

Foraging behavior/foraging success observations:

There was one semi-field tunnel design study and 3 semi-field feeding design studies evaluated for 

Bombus that assessed endpoints related to foraging behavior and success. In Tasei, 2001 (0.7 mg 

a.i/sunflower seed), while the effects were not statistically significant (p>0.05) there increases in the 

number of workers visiting blooming heads of seed treated sunflowers (↑36%), and the number of ‘long’ 

foraging trips (characterized by the study authors as being longer than 50 seconds). In Gill, 2012 (sucrose

exposure to 10 μg a.i/L), the study authors used radio frequency identification (RFID) tags to obtain various 

measures of foraging behavior and success. There were significant increases (p<0.05) in the numbers of 

foragers per colony (↑150%) and the duration of pollen foraging trips (↑18%), as well as significant 

decreases (p<0.05) in pollen load score (amount of pollen collected relative to the size of the worker) 

(↓31%), and the number of foraging trips that returned with pollen (↓28%). The results of this study 

were collected over a 4-week exposure duration. In follow up work by Gill and Raine, 2014, the foraging 

data were analyzed more temporally (i.e. week-by-week) as opposed to collapsing the entire 4-week study 

duration. While means of the response variables were not provided, there were significant (p<0.05) 

increases in the number of foragers (all intervals assessed), and the duration of pollen foraging trips (week 
153

4 only) at the sole treatment concentration of 10 μg a.i/L. Additionally, there were significant decreases 

(p<0.05) in the number of foraging trips (week 2 only), and the pollen load size from all foraging trips 

(week 4 only). Finally, in Feltham, 2014, colonies were evaluated for the weight of nectar and pollen 

collected as well as efficiency (weight collected per hour). While there were no significant effects on 

nectar variables, the weight and foraging efficiency for pollen were significantly decreased (p<0.05), 

↓28% and 31%, respectively at the sole treatment group of 0.7 ppb in nectar and 6 ppb in pollen. 

Summary:

The suite of evaluated higher-tier studies with the Bombussuggest that impacts to reproductive endpoints 

and measures of foraging behavior and success occur at lower concentrations in sucrose and pollen than 

those that elicit effects on mortality (as supported by Gill, 2012, Mommaerts, 2010, Laycock, 2012, 

Laycock and Cresswell, 2013, Whitehorn, 2012, and Bryden, 2013). In these studies, effects to 

reproduction (inclusive of worker and queen production) occurred at sucrose levels as low as 0.7 ppb and 

pollen levels as low as 1.4 ppb. Interestingly, queen-right colonies of bees exposed for 2 weeks appeared 

more sensitive to effects on mortality (62 and 92% reductions at concentrations of 10 and 20 ppb) 

compared with 11-week exposure periods with queenless microcolonies (0 and 15% mortality in 10 and 

20 ppb; 0 and 50% mortality in 10 and 20 ppb for two trials, respectively). However, the effects on 

fecundity when microcolonies and queen-right colonies exposed to similar concentrations of imidacloprid 

appear to be similar as suggested by work by Laycock (2012 and 2013) with microcolonies showing a 42% 

reduction in fecundity at 1 ppb while queen-right colonies showed a 50% decrease in brood production 

at 1.44 ppb.

The impact at 1.44 μg a.i/L identified by Laycock and Cresswell (2013) was determined after a 14-day 

exposure period. These colonies were subsequently observed a further 14 days feeding on untreated 

syrup and when the entire 28-day study duration interval is considered, there were no significant effects 

on brood production up to and including the highest dose (98 ppb). This suggestion of a recuperation 

from significant effects when imidacloprid exposure ceases is also suggested in other work by Cresswell 

on individual bumble bees. As discussed previously, bumble bees exposed to 125 μg a.i/L for 3 days were 

observed to consume less food and exhibit less locomotor activity (p<0.05). When switched to untreated 

sucrose, bumble bee workers were significantly more active after 3 days, and feeding rates were at levels 

similar to control after 8 days. While these results suggest that colonies and individuals can recover from 

short-term exposures to imidacloprid, it is noted that even in crops with short blooming periods that are 

highly synchronous (e.g. canola), the fate of colony could still potentially be impacted as foragers would 

be expected to bring nectar back to the hive, process it, and store it for potentially long periods of time.

The results of these studies also indicate that imidacloprid exposure ranging from 2 – 6 weeks, could 

potentially lead to a greater recruitment of foragers that is hypothesized to be due to a more frequent 

and less efficient foraging trips in collecting pollen. These effects were determined in one study (Gill and 

Raine 2014) to occur immediately following exposure (increased numbers of foragers) or appear after 

several weeks of exposure (decreased pollen load size, increased pollen foraging trip duration). It is noted 

here that pollen is a vital food source for the colony (both for Bombus and Apis species), not only 

promoting the development of the queen’s ovaries, but also serving as the primary food for the Bombus
154

larvae. While the studies indicating increased foraging numbers, and decreased foraging efficiency also 

indicated that these colonies did not fail (at 4 weeks exposure, 10 μg a.i/L), the impact to colonies under 

conditions different from those tested in these studies is uncertain. 

5.3. Tier III 

Tier III represents the highest level of refinement for pollinator studies since they are intended to 

characterize the potential effects of a pesticide on bee colonies under actual use conditions. These studies 

are organized by source (i.e registrant submitted vs open literature) and are discussed below.

5.3.1. Registrant Submitted

There are currently two full field studies that are being conducted by Bayer Crop Science to characterize 

the colony level effects of application of imidacloprid on cotton in California and pumpkin in South Dakota. 

The results of these studies will be incorporated into the preliminary risk assessment expected to be 

complete by the end of 2016.

5.3.2. Open Literature

There are two Apis and one Bombus full field studies that were evaluated in the open literature which are 

summarized below with further details on the methods provided in Appendix D.

Apis

In Pohorecka, 2013 (formulated imidacloprid, seed-treated corn full-field design, 21-day exposure period, 

overwintering observation in one trial year). The number of dead honey bees in the 2011 trial was not 

significantly different from control (p>0.05) throughout the exposure period and up until the last colony 

visit in mid-October (141 dead bees/colony in treatment group compared to 132 dead bees/colony in 

control). In 2012, while the observation period for mortality was a month and a half shorter (last 

assessment made in late August) there was no treatment-related effect on mortality (p>0.05, 22 dead 

bees/colony in treatment group compared to 30 dead bees/colony in the control). All colonies (control 

and treatment) were stated to have overwintered successfully for the 2011 trial (no further information 

given, similar information not provided for the 2012 trial). The analysis of the pollen collected by bees 

indicated only 3% was from the treated crop. Stadler, 2003 (formulated imidacloprid, seed-treated 

sunflower full-field design, 10-day exposure period, overwintering component) reported that there were 

no significant (p>0.05) effects on mortality. It is noted however that this determination appeared to only 

be from the exposure phase of the study and there was no indication of mortality results for the rest of 

the 216-day observation period, including after the overwintering period.

In both the Pohorecka and Stadler studies, there were significant (p<0.05) increases in the percent frame 

coverage of brood area for the treatment groups compared to controls in multiple CCAs. In Pohorecka, 

these increases (relative to controls) were 44% and 87% in the mid-September and early October CCAs, 

respectively, although it was noted in the study report that these numbers were typical for the time of 

season. It is noted that despite these increases, the study authors stated that all control and treatment 
155

group colonies overwintered successfully. Similarly, while Stadler identified significant increases in brood 

area coverage, in was not clear from the study article the actual magnitude of these effects as means were 

not presented and indications of statistical significance were not uniform in their use within the article. 
156

Table 5-15. Summary of Tier III (full field) studies available from the open literature for Apis bees

Test 

Substance 

– Purity

(Test 

species)

Crop

(App. 

Rate)

Exposure 

Dur.

(Observ. 

Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted? – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Qualitative

Citation 

(MRID 

Number)

Gaucho®

600 FS -

60%1

(Apis 

mellifera)

Corn (83.3 

mL/50k 

seeds)

21 days 

(approx. 3 

months)

- One control 

plot (unknown 

size), one 

treatment plot 

(89 acres)

- Colonies 

assessed every 

3-4 weeks 

during 

observation 

period)

Mortality, number of 

combs covered by 

bees, brood area 

--

(Yes)

Brood area: ↑44% 

(mid-September 

CCA)†, ↑87% (early 

October CCA)†

- Pollen analysis indicated 3% 

or less of pollen collected 

originating from treated crop;

- Two fungicides (metalaxyl 

and fludioxonil) were seed 

treated along with 

imidacloprid. Metalaxyl is 

known to be systemic in 

plants and would be expected 

to be available in pollen and 

nectar but no information is 

available for residues in 

pollen from this study.

Qualitative

Pohorecka, 

2013 

(49719625)

Courase®

350 FS -

35%1

(Apis 

mellifera)

Corn (150 

mL/50k 

seeds)

- One control 

plot (unknown 

size), one 

treatment plot 

(74 acres) 

- Colonies 

assessed every 

3-4 weeks 

during 

observation 

period)

Brood area: ↑16% in 

1 colony assessments 
157

Test 

Substance 

– Purity

(Test 

species)

Crop

(App. 

Rate)

Exposure 

Dur.

(Observ. 

Dur)

Design 

Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted? – Yes/No)

Effects2 (all 

comparisons made 

relative to the study’s 

control)

Limitations3

Qualitative

Citation 

(MRID 

Number)

Gaucho®

600FS -

60%1

(Apis 

mellifera)

Sunflower 

(0.24 

mg/seed)

10 days

(216 days, 

including 

overwinter)

- One control 

plot, one 

treatment plot 

(each 60 acres)

- Hives 

acclimated for 

35 days then 

moved to fields 

for 10 days

Hive weight, percent 

cells occupied with 

honey and nectar, 

percent cells occupied 

with pollen, percent 

cells occupied with 

worker brood, percent 

empty cells, foraging 

activity, mortality

--

(Yes)

↑ in hive weight†, ↑ 

in percentage of cells 

occupied with 

pollen†, worker 

brood†, and empty 

cells†

- Means for each response 

variable not reported, only 

direction of effect and 

indication of statistical 

significance are reported;

- Although foraging activity 

and mortality were assessed, 

the summary table differs 

from the text in reporting of 

effects in the study article. 

- Lack of pollen, nectar, and 

soil residue analysis to 

confirm exposure.

Qualitative

Stadler, 

2003

47796301

†Indicates effect was statistically significant (p<0.05). 1Purity not reported but assumed to be percentage indicated assuming a product density of 1 g/L 

2

Not all studies were associated with NOAEC/LOAEC values. This column will report the most sensitive statistically derived or otherwise observed difference from that of the control. 3

Limitations considered to be major listed here. Others associated with this study can be found in with the study summaries in Appendix D.
158

Bombus

In a study by Tasei 2001 (which had a semi-field component that was previously discussed) a full-field 

design component was initiated with two sunflower fields (located in France), one each serving as a 

control and the other with seed-treated sunflower (Gaucho® - purity not provided - 0.7 mg a.i/seed). 

Colonies of bumble bees (10 per field in the control and treatment plots) were exposed to untreated and 

seed-treated sunflower, respectively, for a 9-day exposure period. After this exposure period, the 

colonies were brought into the laboratory, where they were fed with untreated syrup and pollen paste. 

After 26 days, the study authors recorded the number of marked (with colored spot on thorax to delineate

exposure) to estimate their homing rate during the field period and the growth rate of each colony. At the 

conclusion of the colony life cycle, emerged queens were captured, recorded, and housed in cages along 

with male bees for mating purposes. 

When considering the exposure phase duration (Day 0 to Day 9), the mean loss of marked workers per 

colony was 33.5% in the treated group as compared to 23.1% in the control group, a difference that was 

not significant (p>0.05). The mean population increase, which was assessed 26 days after the introduction 

of the hives into the fields, was 86.5 and 78.1 workers/colony in control and treated fields, respectively. 

This difference was not significant (p>0.05). New queens were produced by 8 colonies out of 10 in each 

field. There were 17 and 24 queens/colony in hives of the control and treated fields, respectively, a 

difference that was not significant (p>0.05). While this study investigated individual and colony-level 

effects with bumble bees resulting from exposure to seed-treated sunflower, there are uncertainties as 

to what extent the bumble bees were exposed. Despite the finding that nectar foragers and pollen 

foragers had 98 and 25% of their respective loads originating from sunflowers, there were no confirmatory 

measurements in either the semi-field or field components to indicate that imidacloprid was present in 

the nectar or pollen collected from the bumble bees. Further details on this as well as additional 

information on the methods and results are provided in Appendix D.
159

Table 5-16. Summary of Tier III (full field) studies available from the open literature for Bombus bees.

Test 

Substance 

– Purity

(Test 

species)

Crop

(App. 

Rate)

Exposure 

Dur.

(Observ. 

Dur)

Design Elements

Endpoints Assessed 

--

(Statistical analysis 

conducted? – Yes/No)

Effects1 (all 

comparisons 

made relative to 

the study’s 

control)

Limitations2

Qualitative

Citation 

(MRID 

Number)

Gaucho® –

not 

reported

(Bombus 

terrestris)

Sunflower

(0.7 mg 

a.i/seed)

9 days

( days)

- One control 

field (44 acres) 

and one treated 

field (40 acres);

- 10 colonies 

introduced in 

each field when 

sunflowers 

began blooming, 

removed after 9 

days;

- Control and 

treated field 

were 12 miles 

away.

Mean loss of marked 

workers/colony, colony 

population (Day 0-26), 

mean number of new 

queens.

(Yes)

Colony 

population (Day 

0-26) (↓8.65%); 

mean number of 

new queens 

(↑41.2%)

- There were no pollen, nectar, 

bee, or other samples taken to 

demonstrate exposure to 

imidacloprid. Only soil samples 

were taken in which imidacloprid

was not detected (LOD = 5 ppb), 

and study authors cite previous 

work in same field to state that 

bees were exposed.

- Minimal information on the 

husbandry of the bees and overall 

health at study initiation.

Qualitative

Tasei, 2001

(47800503)

†Indicates effect was statistically significant (p<0.05). 2Not all studies were associated with NOAEC/LOAEC values. This column will report the most sensitive statistically derived or otherwise observed difference from that of the control. 3Limitations considered to be major listed here. Others associated with this study can be found in with the study summaries in Appendix D.
160

5.4. Reported Pollinator Incident Information 

The Office of Pesticide Programs (OPP) maintains a database called the Incident Database System (IDS) in 

which wildlife incidents reported to the Agency from a variety of sources are maintained. Additionally, 

the Environmental Fate and Effects Division (EFED) within OPP maintains an incident database called the 

Environmental Information Incident System (EIIS). There is some overlap with the information housed in 

these databases, but generally a more detailed narrative of an incident is contained in an EIIS report such 

as magnitude of the number of organisms impacted, location, date, product used, use pattern, whether 

the use was a registered use, and any confirmatory residue analysis if available. The sources of 

information for incidents include, registrant reports submitted under the Federal Insecticides, Fungicides, 

and Rodenticides Act (FIFRA) §6(a)(2) reporting requirement, as well as reports from local, state, national 

and international level government reports on bee kill incidents, news articles, and correspondence made 

to EFED by phone or via email (through beekill@epa.gov) generally reported by homeowners and 

beekeepers. 

It is noted that not all reported incidents are associated with narrative or analytical information that 

definitively links imidacloprid exposure to the bee kill event. Analytical information can include residue 

analysis of dead bees observed at a site or within hive residues of pollen and nectar that confirm 

imidacloprid was present. Even in those cases, many incident reports are associated with findings of other 

pesticides, of which the interactions with imidacloprid in contributing to potentially enhanced toxicity to 

bees are not fully understood. In other instances, imidacloprid was only suspected to be the cause of bee 

kill events based on observational accounts between beekeepers in a given area. This, as indicated by the 

summaries below, is not always supported by a confirmatory residue analysis or apiary inspector 

examination of colony health. Typically, the reported wildlife incidents serve as a line of evidence in 

determining the potential effects of imidacloprid, as the reports are useful in understanding how its use 

may impact pollinators under the actual use conditions. As evidenced in the incident summaries below, 

much of the incident information made through phone and email correspondence to EFED does not 

usually include a thorough investigation of the incident or provide any confirmatory residue data to link a 

chemical with a particular incident. Rather, much of these reports are anecdotal in nature. 

For two incidents (I023737-005 and I24127), the reports concerned broad studies/investigations that 

detailed bee losses in Italy and Austria, respectively. Full details and methods are described in Appendix

F. The Italian report details colony losses thought to be due to dust dispersion from the sowing of 

imidacloprid-treated seed. The Austrian report is similar in nature but generally indicated less frequency

of detection (3-11% in samples, depending on the matrix) as compared to 25.7% of dead bees showing 

imidacloprid residues in the Italian report. 

In a report from Health Canada’s PMRA entitled “Update on Neonicotinoid Pesticides and Bee Health 

(2014)”, incident reports were compiled in collaboration with the Regions and Programs Bureau of Health 

Canada that included information on residue analysis of samples and planting practices surrounding the 

affected apiaries. These reports were from 2012 – 2014. The PMRA concluded that neonicotinoids 

(imidacloprid, thiamethoxam, and clothianidin) present in dust generated during planting of treated corn 

and soybean seeds contributed to the reported bee mortalities in 2012 and 2013, that were described 
161

either as dead bee (i.e forager loss) or colony losses. Analytical results and evaluation of the 2014 data 

are still pending. The report stated that 70% of dead bees collected during the corn and soybean planting 

period in 2012 and 2013 had neonicotinoid residues present; whereas, the majority of the live bees 

sampled did not have such residues present. Additionally, it was reported that the 2012 incidents 

primarily identified high numbers of dead bees and symptoms of pesticide poisoning, while the 2013 

reports involved lower number of dead bees but increased incidence of colony-level effects such as lack 

of foragers, and loss of honey production especially in the later months of the beekeeping season. The 

latter was also reported in 2014. It is noted that the report states that unclear how widespread the colony 

losses are as in 2014, three beekeepers accounted for over 72% of the reported incidents. 

In 2012, a total of 278 bee yards from all participating provinces (Ontario, Québec, Alberta, Saskatchewan, 

Manitoba, and Nova Scotia) reported bee loss incidents that represented 53 beekeepers. A follow-up 

investigation revealed that 86% of these incidents were associated with corn and soybean planting. The 

number of reported incidents increased in 2013 and 2014 to 420 and 343, respectively, but the causality 

assessment of these incidents is still pending. Of these incidents, the majority (>85%) originated in Ontario 

where corn in particular is intensively cultivated. The analysis also found that the majority of the incidents 

were reported at the time of seed-treated corn and soybean planting, suggesting exposure to abraded 

treated-seed coatings (dust off) during planting. Interestingly, the corn growing areas of Québec and 

Manitoba are not associated with a similar frequency of incident reports, and the Western Canadian 

provinces where the majority of canola seed is treated with neonicotinoids are also not associated with 

reported incident information. This report does not provide the breakdown of the number of incidents 

associated with a particular chemical, and therefore it is unknown what percentage of the reported 

incidents are the results of seed treated imidacloprid. Also, PMRA responded to these incidents by 

requiring in 2014 a dust-reducing seed flow lubricant when planting neonicotinoid treated seeds using 

pneumatic planting equipment. Additionally, PMRA updated the best management practices (BMPs) for 

the responsible use of treated seed as well as enhanced warnings of pesticide labels and seed package 

labels for directions on how to protect bees were published. 

Summary of Reported Pollinator Incidents

Approximately one half (17/36) of the incidents summarized in Table 5-17 below either included a followup investigation that confirmed through residue analysis the presence of imidacloprid in at least one 

matrix (dead bees, floral pollen, nectar) or were submitted by the registrant under FIFRA 6(a)(2). Only 6 

of these incidents originated from an agricultural use while others were mainly from residential and 

commercial use on ornamentals. In some of these instances, other chemicals (including other 

neonicotinoid chemicals) were also detected. For others, the incident was determined to originate from 

a misuse of the imidacloprid.

Several other incident reports were more anecdotal in the narrative they provided without a confirmatory 

residue analysis such as news reports and beekeeper organization newsletters (there incidents are 

tabulated along with those below in Appendix F. Of the incidents that provided a residue analysis, 

imidacloprid concentrations of dead bee samples were quantified as high as 2456 ppb. It is important to 
162

note incident information serve as one line of evidence and that the absence of reports does not indicate 

an absence of pollinator losses due to pesticides.
163

Table 5-17. Summary of reported pollinator incident reports that are either associated with confirmatory residue analysis or registrant submitted

Incident 

Record Date Use Pattern Product Location Legality App. Method Comments

I020700-001 06-2008 Ornamental Merit 2F DE Registered 

use Soil injection

Submitted under FIFRA 6(a)(2). Linden trees (Tilia cordata) on a 

commercial golf course were treated for Japanese beetle control 

using Merit 2F soil injection treatment. It was stated in the report 

that some months after treatment, the trees bloomed, and dead 

bumble bees (Bombus) and carpenter bees (Xylocopa) were found 

at the base of the tree. It was estimated that 2000-4000 

individuals were affected (11 trees treated). A follow up residue 

analysis (August 2008) conducted by Bayer confirmed 

imidacloprid presence in the leaves of parent imidacloprid 

(ranging from 2.6 – 11.7 ppm), IMI-5-OH (1.6 – 2.2 ppm), and IMIolefin (0.59 – 1.8 ppm). Residues of these products in dead bee 

samples were 0.146, 0.016, 0.138 ppm, respectively (composite 

samples).

I021017-001 03-2009 Ornamental Xytect

75 WSP PA Undetermined NR

Submitted under FIFRA 6(a)(2). Product applied to control aphids 

in 6 linden trees that were reported to 8-10 inches. The 

application took place March 30, 2009. During blooming, it was 

discovered that an unspecified number of bees were killed and 

that the bee deaths ceased when blooming ended. It was 

unspecified of what species of bee was affected.

I022340-001 04-2010 NR NR IN Undetermined NR

Summary report from bee kill incidents at the Purdue University 

Department of Entomology. There were reports of hives with 

dead bees out in front with observations of seed treated corn 

being planted in the fields adjacent to the university lab. A residue 

analysis determined that sampled pollen from affected hives 

(composite sample) had imidacloprid at levels of 2.8 ppb. All 

other sampled matrices (live and dead bees) did not return any 

detectable residues (LOD and LOQ not reported). Clothianidin 

was detected in pollen at 21 ppb.

I023702-001 2006 Canola, 

rapeseed Gaucho ND Undetermined 2006

Part of an April 2009 report from the Nebraska Beekeepers 

Association. Seven beekeepers in North Dakota and Minnesota 

initiated legal action against Bayer Crop Science when they 

suspected Gaucho (used as a seed treatment on neighboring 

canola fields) were responsible for their bee losses. Laboratory 

analysis of the wax comb and honey found imidacloprid in all 

samples with residues ranging from 22 – 671 ppb. Carbofuran, 
164

Incident 

Record Date Use Pattern Product Location Legality App. Method Comments

dichlotvos, and coumaphos were also screened for (no results 

provided).

I023702-005 2007 Citrus Admire FL Undetermined NR

Part of an April 2009 report from the Nebraska Beekeepers 

Association. A beekeeper maintaining 7500 hives for honey 

production and crop pollination provided 18 hives to a research 

project organized by Penn State that would monitor the hives to 

investigate causes for mortality. The beekeeper stated that while 

he provided 18 hives for the study, he only received 4 back with 

only 1 hives in a state sufficient to produce honey. The first 

samples taken were from when the bees were pollinating Florida 

citrus where imidacloprid residues ranging from 14-17 ppb were 

detected in the pollen. Follow up with the grove manager 

revealed that Admire Pro had been used as a ground application 

as the trees began to bloom. 

I025512-001 08-2013 Soybean Leverage 

360 MO Undetermined Ground

Submitted by Bayer Crop Science under FIFRA 6(a)(2). A soybean 

farm was being sprayed by Leverage 360 (imidacloprid and betacyfluthrin) which was adjacent to neighbor who had 11 honey bee 

hives. The neighbor had reported that he had “piles of dead 

honey bees,” back on his property. There was no further 

confirmatory residue information provided in the report.

I025610-001 05-2013 Parking lot QualiPro OR Misuse Soil drench

Submitted under FIFRA § 6(a)(2) by Makhteshim Agan of North 

America (MANA) involving soil drench of linden trees at a golf club 

in Portland, Oregon that resulted in an unspecified number of 

dead bumble bees. Oregon Department of Agriculture 

investigated and conducted residue analysis and determined 

presence of imidacloprid but there were no residues presented in 

the report. It was reported that while this use represented one 

that was permitted by the label, the pest control operator (PCO) 

did not have the necessary licenses to make this application.

I025610-002 2013 Urban QualiPro OR Misuse Soil drench

Submitted under FIFRA § 6(a)(2) by Makhteshim Agan of North 

America (MANA) involving soil drench of linden trees at 200 

Market Street in Portland, Oregon that resulted in an unspecified 

number of dead bumble bees. Oregon Department of Agriculture 

investigated and conducted residue analysis and determined 

presence of imidacloprid but there were no residues presented in 

the report. It was reported that while this use represented one 

that was permitted by the label, the pest control operator (PCO) 

did not have the necessary licenses to make this application.
165

Incident 

Record Date Use Pattern Product Location Legality App. Method Comments

I025980-001 12-2013 Citrus 

Orchard

Admire 

Pro FL Undetermined Surface 

Submitted by Bayer Crop Science as part of FIFRA §6(a)(2). 

Admire® Pro application was made on orange tree orchard in 

which bees had been placed. The hive yards ranged from 1550 –

5500 feet away from the treated grove. The application method 

was described as “surface” application. A follow-up investigation 

conducted by Bayer found that the apiaries had small hive beetle 

in some hives, no varroa present, small amount of early stage 

European foulbrood, adequate honey stores, and all stages of 

brood present in the queen yard (1550 yards from grove). In the 

apiary 5500 yards from grove, there were fewer dead bees than 

in the yard used for queen breeding and evidence of hive robbing 

and heavily infested with all stages of small hive beetle larvae. 

Residue analysis of dead bees from the various yards returned 

total residues of imidacloprid (parent + IMI-olefin+ IMI-5-OH) 

yielded results of 2.5 – 2456 ppb. Live bee residue analysis had 

total residues ranging from 1.1 – 5.1 ppb.

I026301-001 08-2013 Residential Merit 2F CA Undetermined Tree 

injection

Submitted as part of FIFRA 6(a)(2) by Bayer Crop Science. A pest 

control operator (PCO) applied Merit® 2F (imidacloprid) as tree 

injection to Arbutus and Laurel trees on residential property. The 

observation of dead bees (number not specified) occurred shortly 

after the trees were treated. No other confirmatory residue 

analysis provided in the report. 

I026563-001 06-2014 Residential NR OR Misuse NR

Sidewalks were reported to be littered with dead and dying 

bumble bees in Eugene, Oregon. The bees were collected the 

following day by the Oregon Department of Agriculture for 

testing. Imidacloprid and acephate were detected at 0.05 and 

0.30 μg a.i/bee, respectively. An investigation prompted a 

suspension of the pest control operator company who sprayed 

linden trees while in bloom, which is a violation of the label 

restrictions.

I026593-001 06-2014 Residential Ima-Jeet OR Registered 

Use

Tree 

injection

Beaverton, OR incident involving bumble bees and honey bees 

being discovered underneath linden trees in a neighborhood. The 

trees were treated to control aphids. An investigation led to the 

discovery that the same pesticides (imidacloprid, dinotefuran) 

were used here as in a related incident involving linden trees in a 

parking lot (I025610-001). Follow on investigation took bee, 

flower, and leaf samples where analysis determined residue levels 

of 0.050 μg a.i/bee, 0.49 ppm, and 2.2 ppm, respectively. 
166

Incident 

Record Date Use Pattern Product Location Legality App. Method Comments

I026789-001 08-2014 Soybean Leverage 

360 IL Registered 

Use Ground

Submitted by Bayer Crop Science under FIFRA 6(a)(2). Four hives 

adjacent to soybean fields were reported to be implicated, with 

at least 300 dead bees in one hive and 100 dead bees from the 

other 3 hives. The bees were within 1⁄2 mile from the field which 

had been reported to have made applications of Leverage®

(imidacloprid, beta-cyfluthrin) and Stratego® (trifloxystrobin, 

propiconazole). There was no residue analysis of the bee or any 

other in hive matrices to confirm exposure to imidacloprid or any 

other pesticide applied. 

I026904-001 08-2014 Oilseed 

rape

NR United 

Kingdom Undetermined Seed 

treatment

From news article from Smallholder (United Kingdom-based news 

service). The incident was reported to have occurred in Havering, 

East London, next to a field of oilseed rape that was thought to 

have been planted with imidacloprid-treated seeds the previous 

fall. Hundreds of dead bees were scattered all over the ground 

with queens from at least 3 species being identified among dead 

bees. Results of residue analysis of the dead bees determined 

imidacloprid at levels of 6 ppb as well as two fungicides (one being 

flusilazole, the other not being reported). 

I027663-001 05-2014 Commercial 

flowers

Criterion 

75 WSP MO Undetermined NR

Report from the curator of the GT Butterfly House and Bug Zoo in 

Michigan. The facility inquired about neonicotinoid use on the 

commercial flowering plant they wanted to purchase in time for 

their butterflies to arrive in early spring. They settled on Beroske 

Farms in Ohio that confirmed that no neonicotinoids were used 

on their flowering plants. After delivery of the plants, subsequent 

planting, and the beginning of the observation of the foraging of 

the butterflies on the flowers, it was discovered that 4 nectar 

feeding butterflies appeared comatose and then later died. No 

deaths were reported among the fruit eating butterflies. A call 

with Beroske farms confirmed that the product Criterion® 75 WSP 

(75% imidacloprid) was used among 6 other pesticides on the 

flowering plants before delivery to the zoo. It was later confirmed 

that application to commercial flowering plants represented an 

off-label use of this product. A follow-up report (I027748) 

summarized the residue analysis results for imidacloprid for

geranium (<LOQ), butterfly bush (1.5 ppb), coneflowers (<LOQ), 

livewire grass inside butterfly area (0.51 ppb), and potting 

medium (0.12 ppb); no analysis conducted of dead butterflies. 
167

Incident 

Record Date Use Pattern Product Location Legality App. Method Comments

I028034-001 06-2015 Urban NR OR NR Soil drench

Reported by Oregon Department of Agriculture (ODA) to have 

occurred near Portland State University in Portland, Oregon. 

According to ODA, the preliminary investigation revealed that the 

linden trees in the reported incident location had been treated 

with imidacloprid via soil drench in 2013 and with clothianidin via 

soil drench in 2014 to control for aphids. Samples of dead bumble 

bees, linden flowers and leaves were collected for residue 

analyses and indicate residues of imidacloprid, its degradates 

(IMI-5-OH, desnitro and IMI-olefin) and chlorothalonil in leaves 

and flowers, while the samples of dead bumble bees contained 

the parent imidacloprid (0.0009 μg a.i/bee) and chlorothalonil 

(0.029 μg a.i/bee) alone.

I028034-002 06-2015 Urban NR OR NR Soil drench

Reported by Oregon Department of Agriculture (ODA) to have 

occurred near linden trees at 200 & 100 Market Street, Portland, 

Oregon. The ODA investigation indicated that linden trees had 

been treated with imidacloprid by drench application in 2013 to 

control for aphids and that plants in close proximity to the trees 

had been recently treated with clothianidin. These were the same 

trees involved in a previous incident (I025610-002). Residues of 

imidacloprid (0.0095 μg a.i/bee), IMI-olefin (0.0010 μg a.i/bee), 

desnitroimidacloprid HCl (0.0037 μg a.i/bee) and clothianidin 

(0.0026 μg a.i/bee) were detected in bumble bee samples and in 

linden leaves, while linden flowers contained both parent 

imidacloprid (0.028 ppm) and clothianidin (0.065 ppm) alone. It 

was confirmed by ODA that these trees were the same as those 

involved in an earlier incident (I025610-002)

NR: Not reported
168

6. Risk Characterization 

6.1. Risk Estimation 

Estimating risks to bees associated with the registered uses of imidacloprid follows OPP’s published 

guidance entitled: “Guidance for Assessing Pesticide Risks to Bees17.” This guidance presents an iterative, 

tiered process for assessing risks that considers multiple lines of evidence related to exposure and effects 

of pesticides to bees.

Potential for Pesticide Exposure of Bees

The first step in this process involves a qualitative assessment of the potential for exposure of bees to 

imidacloprid. This exposure potential is a function of the application method, timing, location (e.g., indoor 

vs. outdoor), the attractiveness of the crop to bees, agronomic practices (e.g., timing of harvest), and the 

availability of alternative forage sources. For informing the potential for exposure of bees to imidacloprid 

on the treated site, information on the attractiveness of crops was considered based on USDA18

compilations. 

Figure 6-1 below summarizes the process for determining whether an on-field or off-field assessment is 

warranted. Consistent with the guidance, for soil and/or seed treatment uses, it is assumed that contact 

exposure on the treated field would negligible, but oral exposure to residues in pollen and nectar may 

occur, provided the crop is attractive and is not harvested prior to bloom. As spray drift would not be 

present from these use patterns, there would be no off-field exposure expected.

Tables 6-1, 6-2, and 6-3 provide a summary of information on the bee attractiveness of the registered 

foliar, soil, and seed treatment uses of imidacloprid, respectively. This table also indicates whether a Tier 

I contact and/or oral assessment would be conducted for on-field and off-field based on crop 

attractiveness and cultural practices for each use (i.e. whether the crop is harvested before the blooming 

period). 

For any use with a foliar spray component a Tier I off-field assessment would be conducted for contact 

and oral exposure routes for the foliar component only regardless of whether the crop is attractive or is 

harvested prior to bloom. This is due to the potential of bees visiting fields adjacent to the treated crop 

field and subjected to spray drift exposure. If the crop is attractive and is harvested after bloom, a Tier I 

on and off-field assessment is conducted for contact and oral exposure routes.

Where uncertainty exists about bee attractiveness or harvest time, it is assumed that the crop will be 

attractive to bees and harvested after the bloom period, thereby necessitating on-field and off-field Tier 

I assessments for contact and oral exposure routes.

 17 http://www2.epa.gov/sites/production/files/2014-06/documents/pollinator_risk_assessment_guidance_06_19_14.pdf 18 USDA. 2015. Attractiveness of Agricultural Crops to Pollinating Bees for the Collection of Nectar and/or Pollen. Draft. January 13.
169

Figure 6-1. Summarization of the potential scenarios warranting a Tier I on and/or off-field risk 

assessment.

For the tables below, the attractiveness and harvesting information presented represents the most 

conservative scenario that would warrant a Tier I on-field and off-field assessment. For example, if a 

certain member of a crop group indicates no attractiveness to bees, yet another crop within the group is 

considered attractive, a Tier I on-field and off-field assessment would be conducted. 
170

Table 6-1. Attractiveness of crops to bees for the registered foliar uses of imidacloprid.

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive?

Bumble Bee 

Attractive?

Solitary 

Bee 

Attractive

?

Notes

Tier I On-Field 

Contact/Oral 

Assessment?

Tier I Off-Field 

Contact/Oral 

Assessment? 

1 (Root and Tuber Vegetables)1 Y (Pollen 

and Nectar)

Y Y Bees important for seed production, 

typically harvested prior to bloom. 

Potatoes noted to be harvested 

after bloom

Y Y

4A (Leafy Green Vegetables) Y (Pollen 

and Nectar)

Y Y Bees important for seed production, 

crop harvested prior to bloom when 

not used for seed production.

N Y

5 (Brassica Leafy Vegetables) Y (Pollen 

and Nectar)

Y Y Harvested prior to bloom N Y

6 (Legume Vegetables) Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

8 (Fruiting Vegetables) Y (pollen 

and 

nectar)4

Y Y May be grown in glasshouses, with 

bumble bees for pollination

Y Y

10 (Citrus Fruit)* Y (Pollen 

and Nectar)

Y Y -- Y Y

11 (Pome Fruit) Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

12 (Stone Fruit)* Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

13 (Berry and Small Fruit)2 Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

14 (Tree Nuts) Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

9 (Herbs) Y (Pollen 

and Nectar)

Y Y Not harvested prior to bloom Y Y

20 (Oilseed)3,* Y (Pollen 

and Nectar)

Y Y -- Y Y

Non-crop group uses (Globe 

artichoke, banana and plantain, 

peanut, pomegranate, tobacco, 

coffee, hops, tropical fruit)

Y (Pollen 

and Nectar)

Y Y Globe artichoke harvested before 

bloom, tobacco deflowered as part 

of the harvest process

N (globe 

artichoke, 

tobacco) Y for 

all others

Y
171

Groups where members have residue data available are indicated with *

When information was not available from USDA 2014 document, cell was indicated with a “--“ 1

Refer to members of subgroups 1C (potato) and 1D (yams, ginger, others) only

2Includes 13A, 13B, 13-07D, 13-07F, 13-07G 3Cotton represents sole member in oilseed group with registered foliar uses. 4Okra nectar and pollen indicated to be attractive to honey bees (USDA, 2014)

Table 6-2. Attractiveness of crops to bees for the registered soil uses of imidacloprid.

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive?

Bumble 

Bee 

Attractive?

Solitary 

Bee 

Attractive?

Notes

Tier I OnField Oral 

Assessment?

Tier I Off 

Field 

Contact/Oral 

Assessment? 

1 (Root and Tuber Vegetables) Y (Pollen and 

Nectar)

Y Y Bees important for seed production, 

typically harvested prior to bloom. 

Potatoes noted to be harvested after 

bloom

Y N

3 (Bulb Vegetables) Y (Pollen and 

Nectar)

Y Y Typically harvest prior to bloom. N N

4 (Leafy Vegetables) Y (Pollen and 

Nectar)

Y Y Crop harvested prior to bloom when 

not used for seed production.

N N 

5 (Brassica Leafy Vegetables) Y (Pollen and 

Nectar)

Y Y Harvested prior to bloom N N 

6 (Legume Vegetables) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

8 (Fruiting Vegetables)* N Y Y May be grown in glasshouses, with 

bumble bees for pollination

Y N

9 (Cucurbit Vegetables)* Y (Pollen and 

Nectar)

Y Y -- Y N

10 (Citrus Fruit)* Y (Pollen and 

Nectar)

Y Y -- Y N

11 (Pome Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

12 (Stone Fruit) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N 

13 (Berry and Small Fruit)1* Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N 

14 (Tree Nuts) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N
172

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive?

Bumble 

Bee 

Attractive?

Solitary 

Bee 

Attractive?

Notes

Tier I OnField Oral 

Assessment?

Tier I Off 

Field 

Contact/Oral 

Assessment? 

19 (Herbs) Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

20 (Oilseed)

2

* Y (Pollen and 

Nectar)

Y Y -- Y N

Non-crop group uses (Globe 

artichoke, banana/plantain, peanut, 

pomegranate, tobacco, coffee, hops, 

tropical fruit)

Y (Pollen and 

Nectar)

Y Y -- Y N

Groups where members have residue data available are indicated with *

When information was not available from USDA 2014 document, cell was indicated with a “--“ 1Includes 13A, 13B, 13-07D, 13-07F, 13-07G, 13-07H 23

Cotton represents sole member in oilseed group with registered soil uses.

Table 6-3. Attractiveness of crops to bees for the registered seed treatment uses of imidacloprid.

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive?

Bumble 

Bee 

Attractive?

Solitary Bee 

Attractive? Notes

Tier I On-Field 

Oral 

Assessment?

Tier I Off Field 

Contact/Oral 

Assessment? 

1 (Root and Tuber Vegetables)1 Y (Pollen and 

Nectar)

Y Y Bees important for seed 

production, typically harvested 

prior to bloom. Potatoes noted to 

be harvested after bloom

Y N

3 (Bulb Vegetables)2 Y (Pollen and 

Nectar)

Y Y Typically harvest prior to bloom. N N

5 (Brassica Leafy Vegetables)3 Y (Pollen and 

Nectar)

Y Y Requires pollination only when 

grown for seed; small % of acreage; 

harvested prior to bloom

N N 

6 (Legume Vegetables)4 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom N N

15 (Cereal grains)

5

* Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N

19 (Herbs)6 Y (Pollen and 

Nectar)

Y Y Not harvested prior to bloom Y N
173

Crop Group Number (Crop Group 

Name)

Honey Bee 

Attractive?

Bumble 

Bee 

Attractive?

Solitary Bee 

Attractive? Notes

Tier I On-Field 

Oral 

Assessment?

Tier I Off Field 

Contact/Oral 

Assessment? 

20 (Oilseed)

7 Y (Pollen and 

Nectar)

Y Y -- Y N

Non-crop group uses (peanut) Y (Pollen and 

Nectar)

Y Y -- Y N

Groups where members have residue data available are indicated with *

When information was not available from USDA 2014 document, cell was indicated with a “--“ 1Labels specify sugarbeet (1A), carrot (1B), and potato (1C)

2Labels specify onions/leeks and scallions (03-07A, 03-07B) 3Labels specify broccoli (5A) 4Labels specify soybean (6A) and beans/peas (6)

5Labels specify buckwheat, triticale, wheat, barley, oats, millet, sorghum, rye, and corn (pop, sweet, field)

6Labels specify borage (19A) and mustard (19B)

7Labels specify flax, sunflower, safflower, cotton, canola, and crambe
174

6.1.1. Tier I - Screening-level RQs (On-field Contact – Foliar Uses Only)

As described in Section 4, the Tier I method is intended to generate “reasonably conservative” estimates 

of honey bee contact and oral exposure to pesticides for determining the need for additional refinement 

of exposure estimates (e.g. measured residues in pollen and nectar). As such, exposure estimates are 

determined using high-end values predicted from the Bee-Rex model (v.1.0). What follows is a 

summarization of RQs for each route of exposure (contact vs oral) separated by application type (foliar, 

soil, and seed treatment) and whether the on-field or off-field risks(foliar applications only) are estimated. 

For crop uses where an exposure potential of bees is identified the next step in the risk assessment process 

is to conduct a Tier I risk assessment. By design, the Tier I assessment relies on conservative (high end) 

estimates of exposure via contact and oral routes. For contact exposure, only the adult (forager) life stage 

is considered since this is the relevant life stage of honey bees for contact exposure. Effects are defined 

by laboratory exposures to groups of individual bees. As described in Section 4, the endpoint selected for 

acute contact toxicity for adult honey bees is a 96-hour LD50 of 0.043 μg a.i/bee.

Table 6-4 summarizes the screening-level acute contact RQ values for adult honey bees that are assumed 

to be foraging on treated crop during pesticide application. As such, Table 6-4 includes only those crops 

that are considered bee attractive or for which no data are available on bee attractiveness (as a 

conservative assumption). As the Tier I screening-level for acute contact exposure utilizes the maximum

single application rate and a standard contact dose rate of 2.7 μg a.i/bee per 1 lbs. a.i/A, registered use 

patterns with the same maximum single application rate are grouped together in Table 6-4. For all foliar 

uses assessed, acute contact RQ values range from 2.5 (legumes, peanut, herbs) to 15.7 (citrus and pome 

fruits) and exceed the Agency’s acute risk LOC of 0.4. The estimate of contact exposure is considered 

conservative (although not impossible) since it is determined using a high end estimate of forager bees 

exposure to spray droplets. 

Table 6-4. Summary of Tier I screening-level RQs for contact exposure resulting from foliar uses of 

imidacloprid (screening-level contact on-field)

Use pattern

Max. 

Single 

Appl. Rate 

(lbs a.i/A)

Dose (μg a.i./bee

per 1 lbs. a.i./A)1

Imidacloprid Contact 

Dose (μg a.i/bee)

Acute Contact 

RQ1,2

Tuberous and corm 

vegetables, Legume 

vegetables (except soybean), 

Peanut, Herbs

0.04 2.7 0.108 2.5

Strawberry 0.047 2.7 0.127 3.0

Potato, Soybean 0.05 2.7 0.135 3.1

Cotton 0.06 2.7 0.162 3.8

Fruiting vegetables 0.08 2.7 0.216 5.0

Stone fruit, Caneberry, 

Bushberry, Grape, Tree 

nuts, Banana and plantain, 

Pomegranate, Tropical fruit, 

Coffee, Hops

0.10 2.7 0.27 6.3

Citrus, Pome fruit 0.25 2.7 0.675 15.7
175

1 Based on a 96-h acute contact LD50 of 0.043 ug a.i/bee for imidacloprid (MRID 49602717) 2 Bolded value exceeds the acute risk LOC of 0.4. 

6.1.2. Tier I - Screening-level RQs (On-field oral)

Oral Exposure (Foliar, Soil, and Seed Treatment Uses) 

For oral (dietary) exposure, the Tier I assessment initially considers just the caste of bees with the greatest 

oral exposure (foraging adults). If risks are identified, then other factors are considered for refining the 

default Tier I risk estimates. These factors include other castes of bees and available information on 

residues in pollen and nectar which are deemed applicable to the crops of interest. Oral exposure through 

the consumption of imidacloprid-contaminated pollen is considered for on-field and off-field scenarios 

resulting from foliar applications. For soil and seed-treatment applications, where no spray drift is 

expected, oral exposure is assessed for the on-field scenario only. 

For foliar applications, the Bee-REX model uses a standard dose of 32 μg a.i/bee per 1 lbs. a.i/A for adults 

and 13.6 μg a.i/bee for larvae that are based off of consumption rates for these life stages. This dose is 

multiplied by the application rate to yield an oral dose, one each for adults and larvae. For imidacloprid, 

this dose is compared against the most sensitive acute oral LD50 value of 0.0039 μg a.i/bee for adult acute 

exposure and 0.00016 μg a.i/bee for adult chronic exposure. For larvae, there are no acute oral toxicity 

studies for imidacloprid and therefore these cells are shaded in Table 6-15 below. For chronic toxicity, 

the NOAEC was determined to be 0.0018 μg a.i/larva. 

For soil applications, the oral exposure estimate for adults and larvae are determined using Briggs model 

estimates (based off application rate, the log KOW, and organic carbon partition coefficient KOC of 

imidacloprid) multiplied by the consumption rates of 0.292 g/day for adults and 0.124 g/day for larvae. 

The exposure estimates are compared against the same endpoints as described above.

Finally, for seed treatment applications, the exposure estimate is assumed to be 1 μg a.i/g for all uses 

regardless of the application rate. This is multiplied by the consumption rates of 0.292 g/day for adults 

and 0.124 g/day for larvae (as with soil applications) to yield the oral dose that is compared to the Tier I 

toxicity endpoints described previously.

Foliar applications

Table 6-5 below summarizes the on-field acute and chronic oral RQs resulting from the foliar applications 

of imidacloprid. The acute and chronic RQs for adult bees exceed the LOCs of 0.4 and 1, respectively, for 

all use patterns assessed (adult acute RQs ranged from 329 – 2059, and adult chronic RQs ranged from 

8031 – 50195). As noted previously, there are no acute oral toxicity studies to honey bee larvae available 

for imidacloprid; therefore,these cells are shaded in Table 6-5 below. For chronic larval toxicity, RQ values 

exceed the LOC of 1 for all use patterns assessed, with RQs ranging from 321 – 2008.
176

Table 6-5. Summary of Tier I screening-level RQs for oral exposure resulting from foliar uses of 

imidacloprid (based on model-generated exposure values on-field).

4

Use pattern

Max. 

Single 

Appl. Rate 

(lbs a.i/A)

Bee Life 

Stage

Dose (μg 

a.i./bee

per 1 lbs. 

a.i./A)1

Imidacloprid 

Oral Dose (μg 

a.i/bee)

Acute 

RQ2

Chronic 

RQ3

Tuberous and corm vegetables, 

Legume vegetables (except 

soybean), Peanut, Herbs

0.04

Adult 32 1.2850 329 8031

Larval 13.6 0.5878 321

Strawberry 0.047 Adult 32 1.5099 387 9437

Larval 13.6 0.6907 377

Potato, Soybean 0.05 Adult 32 1.6062 412 10039

Larval 13.6 0.7348 402

Cotton 0.06 Adult 32 1.9275 494 12047

Larval 13.6 0.8818 482

Fruiting vegetables 0.08 Adult 32 2.5700 659 16062

Larval 13.6 1.1757 642

Stone fruit, Caneberry, 

Bushberry, Grape, Tree nuts, 

Banana and plantain, 

Pomegranate, Tropical fruit, 

Coffee, Hops

0.10

Adult 32 3.2125 824 20078

Larval 13.6 1.4696 803

Larval 13.6 1.8517 1012

Citrus , Pome fruit 0.25 Adult 32 8.0311 2059 50195

Larval 13.6 3.674 2008 1 Source: USEPA et al. 2014. Guidance for Assessing Pesticide Risks to Bees. 2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4

Bolded value exceeds the acute risk LOC of 0.4 or chronic LOC of 1.

Soil applications

Table 6-6 below summarizes the on-field acute and chronic oral RQs resulting from the soil applications 

of imidacloprid. The acute and chronic RQs for adult bees exceeded the LOCs of 0.4 and 1, respectively, 

for all use patterns assessed (adult acute RQs ranged from 0.47 – 5.8, and adult chronic RQs ranged from 

11 – 142). For chronic larval toxicity, the LOC of 1 was exceeded for all use patterns assessed except 

tobacco (0.04 lbs a.i/A), with RQs ranging from 0.45 – 5.7.

Table 6-6. Summary of Tier I screening-level RQs for oral exposure resulting from soil uses of imidacloprid 

(based on model-generated exposure values on-field).

4

Use pattern

Max. Single 

Appl. Rate (lbs 

a.i/A)

Bee Life 

Stage

Imidacloprid Oral 

Dose (μg 

a.i/bee)1

Acute 

RQ2

Chronic 

RQ3

Tobacco 0.04 Adult 0.0018 0.47 11

Larval 0.0008 0.45

Sugar beet 0.18 Adult 0.0082 2.1 52

Larval 0.0037 2.0

Hops 0.3 Adult 0.0136 3.5 85

Larval 0.0062 3.4
177

1Briggs EEC (derived from Bee-REX) * consumption rate for life stages (0.292g/day for adults; 0.124 g/day for brood) 2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic LOC of 1.

Seed treatment applications

As indicated previously, the Bee-REX model assumes all seed treatment applications to have an EEC in 

pollen and nectar of 1 mg a.i/kg regardless of the application rate. This is multiplied by the consumption 

rate factors for adults and brood (0.292 and 0.124 g/day, respectively) and then compared to the Tier I 

toxicity endpoints previously discussed. All RQs (adult acute oral, adult chronic oral, larval chronic oral) 

exceed the acute and chronic LOCs of 0.4 and 1, respectively. 

Table 6-7. Summary of labeled use information for seed treatment applications of imidacloprid 

(screening-level oral on-field) 4

Use pattern Bee Life Stage EEC in pollen and 

nectar

Imidacloprid Oral 

Dose (μg a.i/bee)1 Acute RQ2 Chronic RQ3

All registered 

seed treatment 

use patterns

Adult 1 mg a.i/kg 

(screening-level

value for all seed 

treatment uses)

0.2920 74.9 1825

Larval 0.1336 73

1 Source: USEPA et al. 2014. Guidance for Assessing Pesticide Risks to Bees. 2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic LOC of 1.

6.1.3. Screening Level RQs (Off-Field)

As described in Section 3, imidacloprid products may be applied to crops via foliar spray applications. 

Consistent with the Agency’s risk assessment process for bees and other taxa, exposure beyond the 

treated field is expected to occur as a result of spray drift. This so-called “off-field” exposure is assessed 

Potato 0.31 Adult 0.0141 3.6 88

Larval 0.0064 3.5

Cotton 0.33 Adult 0.0150 3.8 94

Larval 0.0069 3.8

Root vegetables, Tuberous and 

corm vegetables, Legume 

vegetables (except soybean), 

Cucurbit vegetables, Pome fruit, 

Stone fruit, Peanut, Strawberry 

(perennial and post-harvest), 

Herbs

0.38

Adult 0.0173 4.4 108

Larval 0.0079 4.31

Fruiting vegetables, Citrus, 

Caneberry, Bushberry, Cranberry, 

Grape, Tree nuts, Banana and 

plantain, Pomegranate, Strawberry 

(annual and perennial), Tropical 

fruit, Coffee

0.50

Adult 0.0227 5.8 142

Larval 0.0104 5.7
178

here for honey bees that are assumed to be foraging adjacent to treated fields. The AgDRIFT model (v. 

2.1.119) is used here to estimate the fraction of the foliarly-applied application rate at various distances 

beyond the treated field. The AgDRIFT model accounts for multiple factors that affect the distance and 

amount of spray drift (and consequently the associated risk) of a single spray application. These include 

factors such as wind speed, spray nozzle type, release height, application volume and label restrictions 

pertaining to spray drift mitigation. Table 6-8 below summarizes various aspects of label restrictions 

applicable to foliar spray applications of imidacloprid using the Admire® Pro label as an example (EPA Reg 

No. 264-827).

Table 6-8. Imidacloprid Use Patterns for Crops with or without Specific Application Restrictions

Use pattern

Max. Single 

Appl. Rate

(lbs a.i./A)

Restrictions

Tuberous and corm vegetables, Legume 

vegetables (except soybean), Peanut, Herbs 0.04 No restrictions

Leafy green vegetables, Brassica (Cole) 

Leafy vegetables 0.046 No restrictions

Strawberry 0.047 No restrictions

Potato, Soybean, Tobacco 0.05 No restrictions

Cotton 0.06 No restrictions

Fruiting vegetables 0.08 No restrictions

Caneberry, Banana and plantain, 

Pomegranate, Tropical fruit, Coffee, Hops 0.10 No restrictions; For tree crops, a minimum of 5 gal/A is recommended.

Bushberry 0.10 For ground applications use 20 gal/A and for 

aerial applications use 50 gal/A.

Grape 0.10 Only ground applications are allowed.

Stone fruit, Tree nuts 0.10 For ground applications use 25 gal/A and for 

aerial applications use 50 gal/A.

Globe artichoke 0.126 No restrictions

Citrus, Pome fruit 0.25 For tree crops, a minimum of 5 gal/A is 

recommended.

As shown in Table 6-8, certain crops have limits related to the spray volume. It is expected that higher 

spray volumes will result in lower drift. There are no restrictions related to boom height, droplet size or 

wind speed; however, all these factors are also expected to affect drift levels. Spray drift is expected to 

increase with higher boom heights, smaller droplets, and higher wind speeds. Based on the information 

provided in Table 6-8, nine AgDRIFT scenarios were modeled that span the range of foliar spray application

rates and conditions that favor higher and lower drift estimates in order to bracket the potential for offfield risks. In addition, the Tier I acute and chronic toxicity endpoints for the honey bee summarized in 

Table 5.1 were used to determine the distance required to achieve the applicable acute LOC (0.4) and 

chronic LOC (1.0). These distances can be interpreted as the distance from the edge of the treated field 

beyond which the acute and chronic LOC values would not be exceeded. In modeling using AgDRIFT, 

default conditions were used, except for the variations mentioned in the following paragraphs and/or in 

the tables and footnotes.

 19 Available at http://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/models-pesticide-riskassessment#atmospheric (accessed 11/8/15).
179

Citrus and Pome Fruit 

(Airblast Application) 

Citrus and pome fruits have the highest single application rate among all crops for foliar sprays. The only 

label restriction identified is a minimum of 5 gal/A for tree crops. Ground applications are usually through 

airblast methods for citrus and pome fruits. Spray drift was modeled using AgDRIFT (Tier I 

Orchard/Airblast mode of AgDRIFT) with two options: sparse (default) and orchard (Table 6-9). RQ values 

for contact indicate the acute risk LOC for honey bees is exceeded out to 46 and 66 ft from treated field 

edge for the sparse and orchard scenarios, respectively. Using screening-level oral exposure estimates, 

dietary-based RQ values exceed the acute or chronic risk LOC values from 455 to >1000 ft (the limit of 

model estimation).

Table 6-9. Distance from the edge of the field associated with LOC exceedance, for citrus and pome fruits, 

calculated using AgDRIFT v.1.1.1, the Tier I Orchard/Airblast module, and app rate of 0.25 lbs. a.i./A.

Application 

selection

Distance from the field and point estimate 

of application rate (lbs./A)

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

Sparse

(young, 

dormant)1

0.0571 0.0252 0.0093 0.0026 0.0011 62 581 >1000 455

Orchard2 0.027 0.0126 0.0052 0.0018 0.001 66 >1000 >1000 963

Aerial Application

Aerial applications to citrus and pome fruit were modeled using AgDRIFT in Tier II Aerial mode with wind 

speeds of 10 mph and 15 mph (default) and a range of droplet sizes (fine to medium, medium, medium 

to coarse), respectively. In the absence of additional label restrictions, the default droplet size utilized in 

risk assessments is fine to medium. Results indicate that contact RQ values exceed the acute risk LOC 

from 184 to 318 ft beyond the treated field assuming a wind speed of 15 mph (Table 6-10) and from 141 

to 269 ft beyond the treated field assuming a wind speed of 10 mph (Table 6-11). With aerial application, 

which results in greater amounts of spray drift compared to airblast, acute and chronic Tier I dietary-based 

RQ values exceed their respective LOCs for more than 1000 ft from the edge of the treated field.
180

Table 6-10. Citrus and Pome Fruits: Tier II aerial applications, boom height 10 ft, wind speed 15 mph (label 

required), non-volatile rate 0.25 lbs./A, spray volume 5 gal/A

Droplets Dv0.5 

(μm)

Distance from the field and point estimate of 

application rate

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult Acute 

Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.1408 0.0832 0.0519 0.0305 0.0189 318 >1000 >1000 >1000

M 294 0.1207 0.0705 0.0435 0.0249 0.0148 269 >1000 >1000 >1000

M to C 341 0.1014 0.0575 0.0244 0.0187 0.0107 213 >1000 >1000 >1000

C 385 0.0872 0.049 0.0291 0.0152 0.0088 184 >1000 >1000 >1000

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among others).

Table 6-11. Citrus and Pome Fruits: Tier II aerial applications, boom height 10 ft, wind speed 10 mph (label 

required), non-volatile rate 0.25 lbs./A, spray volume 5 gal/A

Droplets Dv0.5 

(μm)

Distance from the field and point estimate of 

application rate

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.0783 0.0506 0.0389 0.021 0.0128 269 >1000 >1000 >1000

M 294 0.0682 0.0429 0.0319 0.0165 0.01 223 >1000 >1000 >1000

M to C 341 0.0573 0.0347 0.025 0.012 0.0071 167 >1000 >1000 >1000

C 385 0.0496 0.0303 0.0205 0.0098 0.0058 141 >1000 >1000 >1000

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among others).

Globe Artichoke 

Globe artichoke represents the second highest foliar spray application rate at 0.126 lbs a.i./A. Only ground 

applications are allowed according to the label. Two options were modeled using AgDRIFT in the Tier I 

mode: a high boom height (50 inches; default) and a low boom height (20 inches); and two droplet sizes: 

very fine to fine and fine to medium/coarse. Results indicate that the contact-based RQ values exceed

the acute risk LOC from 10 to 52 ft beyond the treated field assuming a high boom height (Table 6-12) and 

from 7 to 20 ft beyond the treated field assuming a low boom height (Table 6-13). With the exception of 

medium/coarse very fine to fine, and fine to medium/coarse droplet size using a low boom height for 

larval chronic oral risk, the acute and chronic Tier I dietary-based RQ values exceed their respective LOCs 

for more than 1000 ft from the treated field.
181

Table 6-12. Globe artichoke (only ground apps allowed): Tier I ground applications, high boom height (50 

inches), application rate 0.126 lbs. a.i./A, 90th percentile results

Droplets Dv0.5 

(μm)

Distance from the field and point estimate of 

application rate (lbs./A)

App rate lbs./A at ea. LOC and distance associated to 

app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

VF to F 175 0.0327 0.0131 0.0063 0.0031 0.0021 52 >1000 >1000 >1000

F to M/C 341 0.0058 0.0026 0.0015 0.0009 0.0006 10 >1000 >1000 771

For ground applications, there are two droplet size options: very fine (VF) to fine (F), and medium (M)/course (C).

Table 6-13. Globe artichoke (only ground apps allowed): Tier I ground applications, low boom height (20 

inches), application rate 0.126 lbs. a.i./A, 90th percentile results

Droplets Dv0.5 

(μm)

Distance from the field and point estimate 

of application rate (lbs./A)

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic

(1.32x10-4 )

VF to F 175 0.0116 0.0044 0.0022 0.0012 0.0008 20 >1000 >1000 932

F to M/C 341 0.0035 0.0016 0.0009 0.0006 0.0004 7 >1000 >1000 587

For ground applications, there are two droplet size options: very fine (VF) to fine (F), and medium (M)/course (C).

Stone Fruit and Tree Nuts

Stone fruit and tree nuts are examples of crops with label restrictions regarding application volumes for 

ground and aerial applications. Since the spray volume is not an option in AgDRIFT modeling for ground 

applications, only aerial applications were modeled assuming wind speeds of 15 mph and 10 mph. Results 

indicate that the contact-based RQ values exceed the acute risk LOC from 115 to 161 ft beyond the treated 

field assuming a wind speed of 15 mph (Table 6-14) and from 66 to 115 ft beyond the treated field 

assuming a wind speed of 10 mph (Table 6-15). All of the Tier I dietary-based RQ values exceed the acute 

and chronic risk LOCs more than 1000 ft from the treated field.

Table 6-14. Stone fruit, Tree nuts: Tier II aerial applications, boom height 10 ft, wind speed 15 mph (label 

required), non-volatile rate 0.10 lbs./A, spray volume 25 gal/A (label required for these crops)

Droplets Dv0.5 

(μm)

Distance from the field and point estimate 

of application rate

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.0553 0.0324 0.02 0.0116 0.007 161 >1000 >1000 >1000

M 294 0.0476 0.0276 0.0168 0.0095 0.0055 138 >1000 >1000 >1000

M to C 341 0.0401 0.0226 0.0136 0.0072 0.004 115 >1000 >1000 >1000

C 385 0.0344 0.0192 0.0113 0.0058 0.0033 92 >1000 >1000 974
182

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among 

others).

Table 6-15. Stone fruit, Tree nuts: Tier II aerial applications, boom height 10 ft, wind speed 10 mph, nonvolatile rate 0.10 lbs./A, spray volume 25 gal/A (label required for these crops).

Droplets Dv0.5 

(μm)

Distance from the field and point estimate 

of application rate

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult Acute 

Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.0303 0.0192 0.0146 0.0076 0.0045 115 >1000 >1000 >1000

M 294 0.0265 0.0164 0.0121 0.006 0.0035 95 >1000 >1000 >1000

M to C 341 0.0224 0.0134 0.0095 0.0044 0.0025 75 >1000 >1000 >1000

C 385 0.0194 0.0117 0.0078 0.0036 0.002 66 >1000 >1000 >1000

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among 

others).

Tuberous and Corm Vegetables 

To model a range of application rates, tuberous and corm vegetables, which represent the lowest single 

application rate for foliar sprays with imidacloprid (0.04 lbs. a.i./A), were also modeled. Aerial applications 

were modeled using various droplet sizes, and assumed boom height of 10 ft, a spray volume of 5 gal/A, 

and wind speeds of 15 and 10 mph. Results indicate that the acute contact-based RQ values exceed the 

acute risk LOC from 36-69 ft beyond the treated field assuming a wind speed of 15 mph (Table 6-16) and 

from 16 to 43 ft beyond the treated field assuming a wind speed of 10 mph (Table 6-17). With the 

exception of coarse and medium to coarse droplet sizes for larval chronic oral risk, all of the acute and 

chronic Tier I dietary-based RQ values exceed their respective LOCs more than 1000 ft from the treated 

field.

Table 6-16. Tuberous & Corm Vegetables and Certain Other Crops: Tier II aerial applications, boom height 

10 ft, wind speed 15 mph (label required), non-volatile rate 0.04 lbs./A, spray volume 5 gal/A

Droplets Dv0.5 

(μm)

Distance from the field and point estimate 

of application rate

App rate lbs./A at ea. LOC and distance associated 

to app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic 

Oral 

(4.98x10-6 

)

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.0222 0.013 0.008 0.0047 0.0028 69 >1000 >1000 >1000

M 294 0.019 0.011 0.0067 0.0038 0.0022 56 >1000 >1000 860

M to C 341 0.016 0.009 0.0055 0.0029 0.0016 43 >1000 >1000 591

C 385 0.0138 0.0077 0.0045 0.0023 0.0013 36 >1000 >1000 534

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among others).
183

Table 6-17. Tuberous & Corm Vegetables and Certain Other Crops: Tier II aerial applications, boom height 

10 ft, wind speed 10 mph, non-volatile rate 0.04 lbs./A, spray volume 5 gal/A

Droplets Dv0.5 

(μm)

Distance from the field and point estimate 

of application rate

App rate lbs./A at ea. LOC and distance associated to 

app rate

10 ft 25 ft 50 ft 100 ft 150 ft

Acute 

Contact 

(0.00637)

Adult 

Acute Oral 

(4.86x10-5 )

Adult 

Chronic Oral 

(4.98x10-6 )

Larval 

Chronic 

(1.32x10-4 )

F to M 255 0.0122 0.008 0.006 0.0031 0.0018 43 >1000 >1000 >1000

M 294 0.0107 0.007 0.005 0.0024 0.0014 30 >1000 >1000 >1000

M to C 341 0.009 0.005 0.004 0.0018 0.001 20 >1000 >1000 673

C 385 0.0078 0.005 0.003 0.0015 0.0008 16 >1000 >1000 551

In the Tier II mode, there are additional droplet size options: fine (F) to medium (M; default), medium, and medium to course (C; among 

others).

6.1.4. Refined RQs (On-field Oral)

As distinguished from the default Tier I assessment, in cases where residue information in pollen and 

nectar are available, these data can be used to refine the estimates of oral exposure as well as further 

characterize the level of risk for other castes of bees using their food consumption rates. These refined 

exposure estimates in pollen and nectar are then compared to the Tier I (i.e. individual level) toxicity 

endpoints in a manner similar to that forthe model-generated or default Tier I exposure estimates. Rather 

than reporting the highest exposure estimates for contact and/or dietary exposure routes (as with the 

default Tier assessment), the Bee-REX model also calculates dietary exposure values and associated RQs 

for larvae of different ages, adult workers with different tasks (and associated energy requirements) and 

the queen using the various aforementioned consumption rates. 

What follows is a summarization of RQs for each use pattern where there are residue data available in 

pollen and/or nectar. The discussion is organized by the application method employed for a given study. 

For adult acute oral RQs, the acute EECs (maximum reported concentration among all individual replicates 

following application) will be compared against the most sensitive acute oral LD50 (0.0039 μg a.i/bee) and 

for the chronic adult oral and chronic larval oral RQs, the chronic EECs (maximum average concentration 

among all individual sampling events following application) will be compared against the larval chronic 

oral (0.0018 μg a.i/bee) and adult chronic oral (0.00016 μg a.i/bee) NOAEC values, respectively. As 

discussed previously, the refined Tier I assessment focuses only on the oral route of exposure and not 

contact. Finally, although Bee-REX includes consumption rates for royal jelly, residue information for this 

matrix is not available from any residue study for imidacloprid. As royal jelly constitutes the exclusive diet 

of the larval and the adult queen, refined Tier I oral RQs are not provided for the queen (larval and adult).

To obviate the need to state it for every analysis described below, the refined oral RQs generally reduced 

adult acute oral, adult chronic oral, and larval chronic oral RQs by 1-2 orders of magnitude (depending on 

the caste and life stage), as compared to the screening-level oral Tier I assessment discussed above which 

relies on model-generated and default exposure values. Nurse bees (6-17 day old workers), nectar 

foragers, and adult drones were the castes generally associated with highest adult acute oral RQs and 

adult chronic oral RQs while drones had the highest larval chronic oral RQ.
184

Finally, all available residue studies assessed parent imidacloprid, IMI-olefin, and IMI-5-OH and therefore 

these combined residues will be referred to as the total residues of imidacloprid.

Foliar Applications 

There are three studies available to characterize the total residues of imidacloprid in pollen and/or nectar 

resulting from foliar applications (citrus, cherry, cotton). Please refer to Appendix E) for a more detailed 

discussion of the methods and findings of these studies. Below is a summary of the RQs using the refined 

estimates of exposure provided by each study.

Crop Group 10 – Citrus Fruits (Orange)

In the available foliar-applied citrus study conducted in Florida (MRID 49521301), imidacloprid (as 

Gaucho® 600 FL Admire® Pro SC) was applied twice at 0.25 lbs a.i/A (maximum permitted single 

application rate for foliar-applied citrus fruits) at a reported 8-10 day interval. Applications were made 

according to the label in that there was no application at or within 10 days of bloom. Residues in pollen 

and nectar were assessed in 3 sites across 2 years (although it is noted that the data reflect the residues 

from two sites only as one site was determined to be directly adjacent to another and therefore 

considered as the same site). Pollen and nectar samples were taken 7- and 4-days after the last 

application, respectively. The default exposure estimates within Bee-REX were supplanted with the 

following values summarized below.

Table 6-18. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for foliar applications on citrus fruits (oranges) based on measured residue data

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 4,100 3,300

Nectar 430 324 1 Refers to hand collected pollen and nectar

2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-19 below summaries the refined Tier I oral RQs for various castes of honey bees using the residue 

data in pollen and nectar from the foliar-applied citrus study. Refined adult acute oral RQs exceeded the 

LOC of 0.4 for every caste (RQs ranged from 4.8 – 32). Similarly, larval chronic oral RQs and adult chronic 

oral RQs exceeded the LOC of 1 for every caste (RQs ranged from 14 – 30 and 89 – 592, respectively). 

Nectar residues were observed to generally decline over time where reliable dissipation half-life (DT50)

values could be calculated for the three trial years that ranged from 8.9 – 14.1 days. Although not 

tabulated below, if acute and chronic nectar concentrations were reduced by 50%, adult acute and chronic 

oral RQs would still all remain well above the LOC of 1.
185

Table 6-19. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total

imidacloprid residues in pollen and nectar from foliar-applications to oranges.1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.025 14

5 120 3.6 0.051 28

Drone 6+ 130 3.6 0.054 30

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.053 14 0.041 259

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.100 26 0.077 482

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.033 8.4 0.025 157

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.019 4.8 0.014 89

Worker 

(foraging for 

nectar)

>18 292 0.041 0.126 32 0.095 592

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.021 5.3 0.016 100

Drone >10 235 0.0002 0.101 26 0.076 476 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003)

3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1.

5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al, 2014)

6Based on concentrations reported in Table 6-18.

Crop Group 12 – Stone Fruits (Cherry)

In the available foliar-applied cherry study conducted in New York and Oregon across 2 years (MRID 

49535601), imidacloprid (as Gaucho® 600 FL Admire® Pro SC) was applied 5 times at 0.1 lbs a.i/A 

(maximum permitted single application rate for foliar applications on cherries) at a reported 8-11 day 

interval per year. For the first year of the trial (2013), applications were made in the fall post-harvest. In 

the second year of the trial (2014), applications were made pre-harvest during the summer. Residues in 

pollen and nectar (sampled 208 days after the last application) were assessed at 4 sites across 2 years 

with the sites in New York being observed to have pollen residues approximately 10X higher as compared 

to the Oregon sites. Table 6-20 below summarizes the residue values that replaced the screening-level
186

model-generated and default exposure estimates within Bee-REX for the refined oral assessment for

foliar-applied stone fruits (cherries).

Table 6-20. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for foliar applications on cherries based on measured residue data

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 1,000 545

Nectar 10 5.6 1 Refers to hand collected pollen and nectar

2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-21 below summaries the refined Tier I oral RQs for various castes of honey bees using the 

measured residue data in pollen and nectar from the foliar-applied cherry study. Refinements to Tier I 

screening-level oral RQs resulted in one caste (pollen foragers) being below the adult acute oral LOC of 

0.4. All other castes and life stages exceed the acute risk LOC (RQs ranged from 0.59 – 2.82). Similarly, 

one caste of larval life stages was below the LOC of 1 for larval chronic oral risk (RQ = 0.72). All other risk 

estimates for larval castes exceed the chronic risk LOC (RQs ranged from 1.4 – 1.5) as well as all adult 

castes exceed the chronic risk LOC for adult (RQs ranged from 1.7 – 38). The highest adult acute and 

chronic oral RQs were associated with the nurse bees which are noted from the table below to have the 

highest pollen consumption rate of all adult castes within the hive. Acute and chronic EECs for pollen 

were approximately 100-fold higher than corresponding values for nectar. 

Table 6-21. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total

imidacloprid residues in pollen and nectar from foliar-applications to cherries1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.0013 0.72

5 120 3.6 0.0026 1.4

Drone 6+ 130 3.6 0.0027 1.5

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.0073 1.9 0.0040 25

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.0110 2.8 0.0060 38

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.0023 0.59 0.0013 7.9

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.0005 0.12 0.0003 1.7

Worker 

(foraging for 

nectar)

>18 292 0.041 0.0030 0.76 0.0017 10
187

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0023 0.59 0.0013 7.8

Drone >10 235 0.0002 0.0024 0.60 0.0013 8.2 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4

Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-20.

Crop Group 20 – Oilseed (Cotton)

In the available foliar-applied cotton study conducted in California across 2-3 years (MRID 49103301), 

imidacloprid (as Provado® 1.6F) was applied once at 0.06 lbs a.i/A (represents maximum single application 

rate but 5-fold less than maximum annual rate) during the bloom period as an aerial application in 2010. 

Given the persistence of imidacloprid in plant tissues, the resulting residues in nectar may be 

underestimated. From 2008-2009, it was reported that imidacloprid (as Admire® Pro) was applied via 

chemigation to other crops on the same fields. It is noted that residues represent only 1 sampling event 

made post application (6-days after application), and are available for nectar only. The screening-level

exposure estimates within Bee-REX were replaced with the following values summarized below. As 

indicated previously, cotton is the sole member in the oilseed group with registered foliar uses.

Table 6-22. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for foliar applications on cotton based on measured residue data

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Nectar 66 56 1 Refers to hand collected nectar

2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Although refinements to exposure estimates based on measured residues in nectar reduced adult acute 

oral, adult chronic oral, and larval chronic oral RQs, all of the acute and chronic RQs are still exceed the 

acute and chronic risk LOCs of 0.4 and 1, respectively (adult acute RQs range from 0.49 – 4.94; adult 

chronic oral RQs range from 10 - 102; larval chronic oral RQs range from 1.8 – 4.07). Despite all acute and 

chronic RQs for adults and larvae being above their respective LOCs, the refined RQs would be expected 

to be even higher if additional foliar applications were made at the maximum rates/reapplication intervals 

as permitted on the label.
188

Table 6-23. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in nectar from foliar-applied cotton1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute dose 

(μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.0034 1.8

5 120 3.6 0.0067 3.7

Drone 6+ 130 3.6 0.0073 4.0

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.0040 1.0 0.0034 21

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.0092 2.4 0.0078 49

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.0040 1.0 0.0034 21

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.0029 0.74 0.0024 15

Worker 

(foraging for 

nectar)

>18 292 0.041 0.0193 4.9 0.0164 102

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0019 0.49 0.0016 10

Drone >10 235 0.0002 0.0155 4.0 0.0132 82 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3

Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MIRD 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-22.

Soil Applications 

There are seven studies available to characterize the total residues of imidacloprid in pollen and/or nectar 

resulting from soil applications that include tomato (2 studies), melons, citrus, blueberries, strawberries, 

and cotton. Please refer to Appendix E) for a more detailed discussion of the methods and findings of 

these studies. Below is a summary of the RQs using the refined estimates of exposure provided by the 

residues data from each of the available studies.
189

Crop Group 8 – Fruiting Vegetables (Tomato)

For tomato, 2 studies are available, both conducted in California. In one study (MRID 49090503), 

imidacloprid (as Admire® Pro) was applied via drip chemigation once at 0.18 lbs a.i/A on 3 sites and two 

applications of 0.13 lbs a.i/A via drip chemigation on 6 sites (study conducted from 2009-2010). These 

two scenarios represent 66 and 47% of the maximum permitted seasonal rate on fruiting vegetables 

including tomato, respectively. The fields in this study were characterized as having medium and fine 

(heavy) soil types. As tomatoes do not produce nectar, only pollen data are available.

In another study (MRID 49665201), one application of imidacloprid (as Admire® Pro Systemic Protectant 

SC) was made at 0.38 lbs a.i/A (maximum permitted single application rate on tomatoes) via soil 

drip/drench. As with the study discussed above, there were no nectar residues sampled in this study. 

Total residues of imidacloprid were analyzed from bumble bee-collected pollen where bees were enclosed 

in tents during the pollen collection period. This study was conducted across 2 years (2013 – 2014) with 

Year 2 still ongoing for 5 sites that were characterized as having fine, medium, and coarse soils.

Table 6-24 below summarizes the pollen residue information from the two available residues studies for 

soil-applied tomato. It is noted that for the first study (MRID 49090503), pollen sampling was 100 days 

after application while it occurred 36 – 38 days after sampling for the second study (MRID 49665201). 

Table 6-24. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for soil applications on tomatoes based on measured residue data.

Matrix Acute EEC (ppb)1 Chronic EEC (ppb)2 Reference

Pollen (including 

anthers) 543 463 MRID 49090503

Pollen (bee collected) 242 198 MRID 49665201

Values in bold represent those that will replace model-generated exposure values within Bee-REX. 1 Acute EEC chosen as the maximum reported concentration among all individual replicates following application. 2 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application. 3

Residues are 2 composites from a single sampling time of 66 – 47% of the maximum seasonal rate permitted.

Refined oral RQs indicate that adult acute oral RQs are generally below the acute risk LOC of 0.4 or 

marginally above for cell cleaning and nurse bee castes (RQs range from <0.01 – 0.60). Larval chronic oral 

RQs were all below the chronic risk LOC of 1 (RQs range from 0.19 – 0.39). Adult chronic oral RQs range 

from <0.01 – 12 where the castes above the LOC were generally those with an appreciable percentage of 

their diet as pollen as with the acute oral RQs (workers less than 10 days old, nurse bees). 

Table 6-25. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen from soil-applications to tomatoes1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 3.56E-04 0.19

5 120 3.6 7.13E-04 0.39

Drone 6+ 130 3.6 7.13E-04 0.39
190

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 1.61E-03 0.41 1.32E-03 8.2

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 2.32E-03 0.60 1.90E-03 12

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 4.11E-04 0.11 3.37E-04 2.1

Worker 

(foraging for 

pollen)

>18 43.5 0.041 9.92E-06 <0.01 8.12E-06 0.05

Worker 

(foraging for 

nectar)

>18 292 0.041 9.92E-06 <0.01 8.12E-06 0.05

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 4.84E-04 0.12 3.96E-04 2.5

Drone >10 235 0.0002 4.84E-08 <0.01 3.96E-08 <0.1 1

For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3

Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5

From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-24.

Crop Group 9 – Cucurbit Vegetables (Melon)

For the available soil-applied melon study (reported as cantaloupes and unknown species), imidacloprid 

(as Admire® Pro, Alias®, and an unknown formulation) was applied at applications rates ranging from 0.23 

– 0.38 lbs a.i/A via soil drip or a seed line drench at the time of transplant, representing 61 – 100% of the 

maximum rate for soil application to Crop Group 8). The study was conducted in California across 10 sites 

from 2008 – 2011 on commercial fields characterized as having medium to fine (heavy) soils. Sampling of 

trapped pollen and hive-collected nectar occurred approximately 90 – 120 days after application. The 

bees Table 6-26 below summarizes the residue values in pollen and nectar to supplant the default 

estimates for soil applied melon. 

Table 6-26. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for soil applications on melons based on measured residue data.

Matrix Acute EEC (ppb)1 Chronic EEC (ppb)2

Pollen (trapped) 32 19

Nectar (hive collected)3 8 4.9
191

1 Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

2 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

3It was noted that the majority of the nectar samples were taken 9-11 days after the bees had been placed into tents.

Refined oral RQs for imidacloprid-treated melon via soil applications indicate that adult acute oral RQs are 

generally below the acute risk LOC of 0.4 (RQs range from <0.01 – 0.60) with nectar foraging worker bees 

and drones representing the two castes marginally above the acute risk LOC. None of the larval chronic 

oral RQs exceed the chronic risk LOC of 1. Chronic oral RQs for all castes of adults exceed the chronic risk 

LOC (RQs range from 1.1 – 8.9), with the same castes as mentioned above associated with the highest 

RQs. 

Table 6-27. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from soil-applications to melons.

1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 3.28E-04 0.18

5 120 3.6 6.56E-04 0.36

Drone 6+ 130 3.6 7.05E-04 0.39

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 6.93E-04 0.18 4.20E-04 2.6

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 1.43E-03 0.37 8.68E-04 5.4

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 5.34E-04 0.14 3.26E-04 2.0

Worker 

(foraging for 

pollen)

>18 43.5 0.041 3.49E-04 0.09 2.14E-04 1.3

Worker 

(foraging for 

nectar)

>18 292 0.041 2.34E-03 0.60 1.43E-03 8.9

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 2.96E-04 0.08 1.80E-04 1.1

Drone >10 235 0.0002 1.88E-03 0.48 1.15E-03 7.2 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3

Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-26.
192

Crop Group 10 – Citrus Fruits (Orange/Grapefruit)

For citrus, two studies (MRIDs 49090504 and 49090505) conducted multiple trials with citrus in California 

both in tunnels and in open fields of oranges, tangerines, and grapefruits. All trials involved one 

application of imidacloprid (as Admire® Pro) at 0.5 lbs a.i/A (maximum single application rate permitted 

for soil-applied citrus) as a post-bloom soil drench except one trial which used an unspecified formulation 

as 0.25 lbs a.i/A. The soils types were characterized as loam for some sites, sandy loam for others, and 

was not provided for two sites. Across all trials within the study, maximum individual residues across all 

replicates (acute EEC) in nectar (hand-collected) ranged from 18.3 – 35.5 and the maximum average 

concentration among all individual sampling events (chronic EEC) ranged from 9.4 – 23.8 ppb. 

Two trials also reported residues for bee-collected nectar which were generally similar in levels as those 

reported for hand-collected nectar. These same two trials also reported residues for hive nectar which 

for one trial was just below hand-collected nectar residues and for the other study was roughly 3-fold 

higher than that of hand-collected nectar, indicating an uncertain relationship between the two methods 

of sampling from these two trials. The sampling interval was the same for both trials (230 days after 

application); the study with higher hive nectar values was conducted in a flight tunnel, while the other 

trial was conducted in an open field, suggesting that the open field bees could have had access alternative 

sources of forage to dilute the level of imidacloprid in the hive nectar stores as compared to the tunnelconfined bees. Finally, only one trial provided pollen data and is the sole trial within the study which did

not test the maximum single application rate. Therefore, the trial with the highest acute EEC value in 

nectar (hand-collected) and its corresponding chronic value will be used for refined Tier I assessment 

purposes.

Table 6-28. Summary of the refined acute and chronic estimated environmental concentrations (EECs)

for soil applications to citrus based on measured residue data.

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)2

Component (inclusive of both 

MRID 49090504 and 

49090505)

Nectar 34.6 21.2 Oranges, 1 x 0.5 lbs. a.i/A; 

post bloom, tunnel Nectar (bee collected) 37.1 17.5

Nectar (hive collected) 95.2 72.8

Nectar 18.3 9.4 Oranges/tangerines, 1 x 0.25 

lbs. a.i/A, post bloom, open 

field 

Nectar (bee collected) 16.0 7.6

Nectar (hive collected) 15.5 11.6

Pollen (trapped) 10.2 9.4

Nectar 29.1 19.3 Oranges, 1 x 0.5 lbs. a.i/A, 

post-bloom, open field

Nectar 35.5 23.8 Grapefruit, 1 x 0.5 lbs. a.i/A, 

post bloom, open field 

Values in bold represent those that will replace model-generated exposure values within Bee-REX. 1Unless otherwise indicated, nectar is hand-collected. 2 Acute EEC chosen as the maximum reported concentration among all individual replicates following application. 

3 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application.

Table 6-29 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using nectar residue data from soil-applied citrus. Adult acute oral RQs were above the LOC of 0.4 with 

the exception of worker bees maintaining the hive overwintering, likely due to a decreased nectar 
193

consumption rate relative to all other adult castes (RQs ranged from 0.26 – 2.1). The adult castes with 

the highest nectar consumption rates (nectar foragers, drones) were associated with the highest acute 

oral RQs. Similarly, these castes also had the highest adult chronic oral RQs (43 and 35, respectively) as 

well as all other adult castes having RQs above the LOC of 1 (RQs ranged from 4.3 – 43). Larval chronic 

oral RQs were above the LOC for two castes (RQs ranged from 0.78 – 1.7). While the acute and chronic 

RQs using refined data from soil-applied citrus are noted to be generally one order of magnitude lower 

than those discussed above using residue data from foliar-applied citrus, it is noted that the pollen residue 

data is not available from the soil applied citrus study that yielded the highest nectar residues and was 

therefore used for the refined Tier I analysis presented below. Pollen residues in the foliar-applied citrus 

study were noted to be as high as 4,100 ppb but it is an uncertainty as to the magnitude of these residues 

resulting from soil application. Additionally, it is noted that residue data from the soil applied citrus 

studies originate from post-bloom applications of imidacloprid whereas for the foliar residue study, 

applications were made at 10 days pre-bloom.

Table 6-29. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total

imidacloprid residues in nectar from soil applications to citrus1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.0014 0.78

5 120 3.6 0.0029 1.6

Drone 6+ 130 3.6 0.0031 1.7

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.0021 0.55 0.0014 8.9

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.0050 1.3 0.0033 21

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.0021 0.55 0.0014 8.9

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.0015 0.40 0.0010 6.5

Worker 

(foraging for 

nectar)

>18 292 0.041 0.0104 2.7 0.0069 43

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0010 0.26 0.0007 4.3

Drone >10 235 0.0002 0.0083 2.1 0.0056 35 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).
194

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-28.

Crop Group 13 – Berries and Small Fruits (Blueberry)

In a study conducted across New York, Michigan, and Illinois from 2012 – 2013 (MRID 49535602), 

imidacloprid as Admire® Pro 600 SC, as applied once at 0.5 lbs a.i/A (maximum single application rate 

permitted on blueberries) 3 days post-harvest as a banded soil application. Bee-collected pollen and hive 

nectar were sampled 240 and 233 days later, respectively. During the collection period, the bees were 

housed in tents to ensure the collected residues were not diluted by other sources of forage. It was 

reported that the experimental fields varied with respect to soil composition, being characterized as either 

sandy or silt loam, with the highest residues being reported in the sandy soils. Residues were also 

reported to have either remained relatively consistent or increased during the course of the trial though 

there was no clear indication of year-to-year carryover. Table 6-30 below summarizes the acute and 

chronic EECs for measured pollen and nectar residues after soil-applications to blueberries.

Table 6-30. Summary of the refined acute and chronic EECs in pollen and nectar following soil 

applications to blueberries based on measured residue data.

Matrix Acute EEC (ppb)1 Chronic EEC (ppb)2

Pollen (bee-collected) 42 16.5

Nectar (hive-collected) 16 8.8 1 Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

2 Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-31 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using measured nectar residue data following soil applications to blueberries. Adult acute oral RQs are 

either below or marginally above the acute risk LOC of 0.4 (RQs range 0.14 – 1.2) with the highest RQs 

associated with nurse bee, nectar forager, and drone castes. Chronic adult oral RQs are above the chronic 

risk LOC of 1 for all castes (RQs range from 2.4 – 16); however, larval chronic oral RQs are below the 

chronic risk LOC for all castes (RQs range from 0.30 – 0.66).

Table 6-31. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured

total imidacloprid residues in pollen and nectar following soil applications to blueberries1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 5.58E-04 0.30

5 120 3.6 1.12E-03 0.61

Drone 6+ 130 3.6 1.20E-03 0.66

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 1.24E-03 0.32 6.38E-04 4.0

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 2.64E-03 0.68 1.39E-03 8.7
195

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 1.03E-03 0.26 5.56E-04 3.5

Worker 

(foraging for 

pollen)

>18 43.5 0.041 6.98E-04 0.18 3.83E-04 2.4

Worker 

(foraging for 

nectar)

>18 292 0.041 4.67E-03 1.2 2.57E-03 16

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 5.48E-04 0.14 2.88E-04 1.8

Drone >10 235 0.0002 3.76E-03 0.96 2.07E-03 13 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4

Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-30.

Crop Group 13 – Berries and Small Fruits (Strawberry)

In a study conducted in California from 2010-2011 (MRID 49090502), imidacloprid (as Alias® 4F, Admire®

Pro, and an unknown formulation) was applied once at 0.5 lbs a.i/A (maximum permitted single 

application rate for soil-applied strawberries). Only pollen samples were available but it was not reported 

the interval between application and sampling (field portion of this study was not conducted under GLP). 

The soils of the treatment plots were characterized as coarse (sandy) and medium (loamy) soils with 

measured residues in pollen generally reported as being higher for sandy soils. Table 6-32 below 

summarizes the acute and chronic EECs for pollen and nectar residues from soil applications to

strawberries.

Table 6-32. Summary of the refined acute and chronic EECs for soil applications to strawberries based 

on measured residue data.

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 320 280 1Refers to hand collected pollen

2Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

3Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-33 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using measured pollen residue data from soil applications to strawberries. Adult acute oral RQs are either 

below or marginally above the acute risk LOC of 0.4 with cleaning bees and nurse bees having the highest 

RQs of all adult castes assessed (RQs range from <0.01 – 0.79). These castes are associated with the 
196

highest pollen consumption rates, which is the sole matrix from which residue data are available. 

Similarly, adult chronic oral RQs exceeded the chronic risk LOC of 1 for these castes as well as comb 

building worker and overwintering maintenance worker castes (RQs range from <0.1 – 17). Nectar and 

pollen foraging bees were the only two castes below the chronic risk LOC, and also have pollen 

consumption rates of less than 0.1 mg/day. Larval chronic oral RQs are below the chronic risk LOC of 1 

for all castes assessed (RQs range from 0.28 – 0.55). Honey bees are attracted to both pollen and nectar 

of strawberries and there is uncertainty regarding RQs had nectar residue data been available to further 

refine exposure estimates. 

Table 6-33. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured

imidacloprid residues in pollen from soil applications to strawberries1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 5.04E-04 0.28

5 120 3.6 1.01E-03 0.55

Drone 6+ 130 3.6 1.01E-03 0.55

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 2.13E-03 0.55 1.86E-03 12

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 3.07E-03 0.79 2.69E-03 17

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 5.44E-04 0.14 4.76E-04 3.0

Worker 

(foraging for 

pollen)

>18 43.5 0.041 1.31E-05 <0.01 1.15E-05 0.1

Worker 

(foraging for 

nectar)

>18 292 0.041 1.31E-05 <0.01 1.15E-05 0.1

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 6.40E-04 0.16 5.60E-04 3.5

Drone >10 235 0.0002 6.40E-08 <0.01 5.60E-08 <0.01 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6

Based on concentrations reported in Table 6-30.
197

Crop Group 20 – Oilseed (Cotton)

In a study conducted in California from 2013-2014 (MRID 49665202), imidacloprid (as Admire® Pro SC) 

was applied once at 0.33 lbs a.i/A (maximum permitted single application rate for soil-applied cotton) as 

an in-furrow spray at planting. Pollen, nectar, and extra-floral nectar samples were obtained 78 days after 

application. The study was conducted across 9 sites where 2 soil plots were characterized as fine (heavy), 

1 as medium, and 6 as coarse (light) texture. Table 6-34 below summarizes the acute and chronic EECs 

for measured pollen and nectar residues from soil applications to cotton. As indicated below, floral nectar 

samples were higher in this study and therefore will be used as the nectar values for modeling purposes. 

As indicated previously, cotton represents the sole member of the oilseed group with registered soil uses.

Table 6-34. Summary of the refined acute and chronic EECs for soil applications on cotton based on 

measured residue data.

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 43.4 41.1

Nectar 127 83.1

Extra-floral nectar 35.9 35.9

Values in bold represent those that will replace screening-level exposure values within Bee-REX 1Refers to hand collected pollen and nectar

2Acute EEC chosen as the maximum reported concentration among all individual replicates following application 

3Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-35 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using measured nectar residue data from soil applications to cotton. All castes of adult bee exceed the 

acute risk LOC of 0.4 from oral exposure (RQs range from 0.97 – 9.51) with the highest RQs are associated 

with the nurse bees, nectar foragers, and drones. This is also the case for the adult chronic oral RQs for 

these castes as well as all other adult castes assessed (RQs range from 16 – 152). Larval chronic oral RQs 

are also above the chronic risk LOC of 1 for all castes assessed (RQs range from 2.8 -6.0). The acute and 

chronic RQs are all higher but within a factor of 2 as compared to the RQs for the refined foliar-applied 

cotton analysis. This is likely due to the addition of pollen data for the soil-applied study (foliar study was 

nectar only), as well as a higher application rate.

Table 6-35. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured

total imidacloprid residues in pollen and nectar from soil-applications to cotton.

1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.0051 2.8

5 120 3.6 0.0101 5.5

Drone 6+ 130 3.6 0.0110 6.0

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.0079 2.0 0.0053 33

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.0182 4.7 0.0120 75
198

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.0077 2.0 0.0051 32

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.0055 1.4 0.0036 23

Worker 

(foraging for 

nectar)

>18 292 0.041 0.0371 9.5 0.0243 152

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0038 0.97 0.0025 16

Drone >10 235 0.0002 0.0298 7.7 0.0195 122 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 ug a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4

Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-34.

Seed Treatment Applications 

There is one study available to characterize the total residues of imidacloprid in pollen resulting from seed 

treatment application to corn. Please refer to Appendix E for a more detailed discussion of the methods 

and findings of these studies. Below is a summary of the RQs using the refined estimates of exposure.

Crop Group 15 – Cereal Grains (Corn)

In a study conducted across 3 sites in Kansas and Nebraska from 2012-2013 (MRID 49511701), 

imidacloprid (as Gaucho® 600 ST) was applied at a rate of 1.34 mg a.i/seed (0.12 lbs a.i/A) which is 

equivalent to the maximum labeled rate for field corn as a seed treatment. Pollen samples were taken 84 

days after application. As the corn plant does not produce nectar, only pollen residue data are available 

from this study. The study was conducted on fields that were characterized as loam, silty loam, and clay 

soils. Residues appeared to increase during the sampling time. Table 6-36 below summarizes the acute 

and chronic EECs for pollen residues in seed-treated corn used to supplant default values in Bee-REX for 

refined Tier I assessment purposes. 

Table 6-36. Summary of the refined acute and chronic EECs for seed treatment applications on corn 

based on measured residue data.

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 39.7 22.3 1

Refers to hand collected pollen 

2Acute EEC chosen as the maximum reported concentration among all individual replicates following application 
199

3Chronic EEC chosen as the maximum average concentration among all individual sampling events following application

Table 6-37 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using nectar residue data from seed-treated corn. The RQ values for all castes of adult bee are below the 

acute risk LOC of 0.4 for oral exposure (RQs range from <0.01 – 0.1). The highest RQ is associated with the 

nurse bee caste given their high pollen consumption rate relative to other adult castes. Similarly, there 

are no larval chronic oral RQs that exceed the chronic risk LOC of 1 (RQs range from 0.02 – 0.04). For adult 

chronic oral risk, all castes are below LOC except for the nurse bee caste, which is marginally above the 

LOC (RQs range from <0.01 – 1.34). 

Table 6-37. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from seed-treated corn1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 4.01E-05 0.02

5 120 3.6 8.03E-05 0.04

Drone 6+ 130 3.6 8.03E-05 0.04

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 2.64E-04 0.07 1.48E-04 0.93

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 3.81E-04 0.10 2.14E-04 1.34

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 6.75E-05 0.02 3.79E-05 0.24

Worker 

(foraging for 

pollen)

>18 43.5 0.041 1.63E-06 <0.01 9.14E-07 0.01

Worker 

(foraging for 

nectar)

>18 292 0.041 1.63E-06 <0.01 9.14E-07 0.01

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 7.94E-05 0.02 4.46E-05 0.28

Drone >10 235 0.0002 7.94E-09 <0.01 4.46E-09 <0.01 1For life stages or castes that rely exclusively on royal jelly (i.e., larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 μg a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5

From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014). 6

Based on concentrations reported in Table 6-36.
200

Combined Method Applications 

There are three studies available to characterize the total residues of imidacloprid in pollen and/or nectar 

resulting from multiple methods of application. This flexibility in multiple types of applications within the 

same growing season is permitted for most use patterns of imidacloprid so long as the annual application 

rate does not exceed 0.50 lbs a.i/A. Two studies examine the total residues in tomato and cotton resulting 

from a soil application followed by two and three foliar applications, respectively (MRIDs 49665201 and 

49665201, respectively). Both of these studies are also discussed in the soil section (same respective 

MRID numbers) where the residues were quantified before foliar applications were made. A third study 

examines the residues in pollen and nectar of seed-treated cotton followed by 5 foliar applications. 

Appendix E provides a more detailed discussion of the methods and findings of these studies. Below is a 

summary of the RQs using the refined estimates of exposure provided by each study.

Crop Group 8 – Fruiting Vegetables (Tomato – Soil + Foliar)

In a study conducted across 9 sites in California in 2013-2014 (MRID 49665201), imidacloprid (as Admire®

Pro SC) was applied at a rate of 0.38 lbs a.i/A at transplant (as described in the soil application above) 

which was followed by two foliar applications of Admire® Pro SC at 0.06 lbs a.i/A with a reported 4-5 day 

interval at bloom. Bumble bee-collected pollen were sampled 2-8 days after application (nectar samples 

not collected as the tomato crop does not produce it). The study was conducted on fields that were 

characterized as having fine, medium, or coarse textures. Table 6-38 below summarizes the acute and 

chronic EECs for pollen residues from soil + foliar-applications to tomato. Pollen residue data alone are 

available as the tomato plant does not produce nectar. As compared to the soil treatment alone 

component of the same study, it is noted that the acute and chronic EECs are over 6-fold higher when the 

foliar component is added.

Table 6-38. Summary of the refined acute and chronic EECs for combined (soil + foliar) applications to

tomato based on measured residue data

Matrix Acute EEC (ppb)1 Chronic EEC (ppb)2

Pollen (bee collected) 1521 1268 1Acute EEC chosen as the maximum reported concentration among all individual replicates following application. 

2

Chronic EEC chosen as the maximum average concentration among all individual sampling events following application.

Table 6-39 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using pollen residue data from combined soil + foliar applications to tomatoes. Oral RQs for adult castes 

with an appreciable percentage of their diet as pollen exceed the acute and chronic risk LOC of 0.4 and 1, 

respectively. These castes include cleaning, nurse, and comb building, and hive maintenance workers 

(acute RQs range from 0.02 – 3.7, chronic RQs range from <0.01 - 76). Larval chronic oral RQs exceed the

chronic risk LOC for all castes assessed (RQs range from 1.2 – 2.5). Pollen foraging workers did not exceed 

the acute or chronic risk LOCs given their low pollen consumption rate. The added foliar component 

increased the RQs over 6 fold relative to the respective RQs of the soil component-applied alone 

component.
201

Table 6-39. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured

imidacloprid residues in pollen from combined soil + foliar-applications to tomatoes.

1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 2.28E-03 1.2

5 120 3.6 4.56E-03 2.5

Drone 6+ 130 3.6 4.56E-03 2.5

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 1.01E-02 2.6 8.43E-03 53

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 1.46E-02 3.7 1.22E-02 76

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 2.59E-03 0.66 2.16E-03 14

Worker 

(foraging for 

pollen)

>18 43.5 0.041 6.24E-05 0.02 5.20E-05 0.3

Worker 

(foraging for 

nectar)

>18 292 0.041 6.24E-05 0.02 5.20E-05 0.3

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 3.04E-03 0.78 2.54E-03 16

Drone >10 235 0.0002 3.04E-07 0.00 2.54E-07 <0.01 1For life stages or castes that rely exclusively on royal jelly (i.e., larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 μg a.i/bee for imidacloprid (MRID 42273003). 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014). 6Based on concentrations reported in Table 6-38.

Crop Group 20 – Oilseed (Cotton – Soil + Foliar)

In a study conducted across 9 sites in California in 2013-2014 (MRID 49665202), imidacloprid (as Admire®

Pro SC) was applied at a rate of 0.33 lbs a.i/A as an in-furrow spray (as described in the soil application

above) which was followed by three foliar applications of Admire® Pro SC at 0.06 lbs a.i/A with a reported 

6-7 day interval at bloom. Pollen, floral nectar, and extra-floral nectar samples were taken 4, 4, and, 5 

days after the last application, respectively. Soil types of the experimental fields were characterized as 

fine, medium, and coarse texture. Table 6-40 below summarizes the acute and chronic EECs for pollen 

residues in the combined soil + foliar applications to cotton used in Bee-REX for refined Tier I assessment 

purposes. As the extra-floral nectar residues were higher than those of floral nectar (as distinguished 
202

from the soil-applied alone component of this study described above), these values will be used for 

refinement of Tier I EECs for the nectar component. 

Table 6-40. Summary of the refined acute and chronic EECs for combined (soil + foliar) applications to

cotton based on measured residue data.

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 328 324

Nectar 171 153

Extra-floral nectar 2775 1952

Bolded values refer to those to be used for refinement of EECs. 1Refers to hand-collected pollen and nectar. 2Acute EEC chosen as the maximum reported concentration among all individual replicates following application. 

3Chronic EEC chosen as the maximum average concentration among all individual sampling events following application.

Table 6-41 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using nectar residue data from the combined soil + foliar-applications to cotton. Adult acute and chronic 

oral and larval chronic oral RQs exceed their respective LOCs for every caste assessed. Adult acute oral 

RQs range from 21 – 208 while adult chronic oral RQs range from 358 – 3562. Similarly, larval chronic oral 

RQs ranged from 64 – 139. The added foliar component increased the RQs over 6 fold the level of the 

respective RQs after the soil treatment alone. These RQs are 20- to 25-fold higher (depending on the life 

stage) as compared to their respective RQs from the soil-only component of this study discussed 

previously. 

Table 6-41. Summary of Tier I oral RQs for honey bees using refined exposure estimates with total 

imidacloprid residues in pollen and nectar from soil + foliar-applied cotton1,4

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.1177 64

5 120 3.6 0.2354 129

Drone 6+ 130 3.6 0.2549 139

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.1687 43 0.1193 745

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.3916 100 0.2764 1727

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.1671 43 0.1177 735

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.1207 31 0.0849 531

Worker 

(foraging for 

nectar)

>18 292 0.041 0.8103 208 0.5700 3562
203

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0811 21 0.0573 358

Drone >10 235 0.0002 0.6521 167 0.4587 2867 1For life stages or castes that rely exclusively on royal jelly (i.e., larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 μg a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MRID 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4

Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014). 6Based on concentrations reported in Table 6-40.

Crop Group 20 – Oilseed (Cotton – Seed + Foliar)

In a study conducted across 3 sites in Missouri in 2013-2014 (MRID 49511702), imidacloprid (as Gaucho®

600 Flowable) was applied at a rate of 0.05 lbs a.i/A as a seed treatment at planting followed by 5 foliar 

spray application of 0.06 lbs a.i/A each at 5-8 day intervals. The experimental fields were characterized as 

sandy, sandy loam, and silty loam soil types. Pollen, floral nectar, and extra-floral nectar samples were 

taken nectar samples were taken 26, 21, and 14 days after the last pesticide application. Table 6-42 below 

summarizes the acute and chronic EECs for pollen residues in seed + foliar-treated cotton to refine modelgenerated values in Bee-REX for Tier I assessment purposes. As the floral nectar residues were higher 

than those of extra-floral nectar for this study, these values will be used for refinement of Tier I default 

EECs for the nectar component. 

Table 6-42. Summary of the refined acute and chronic EECs for combined (seed + foliar) applications on 

cotton based on residue data

Matrix1 Acute EEC (ppb)2 Chronic EEC (ppb)3

Pollen 56.7 25.2

Nectar 39.5 29

Extra-floral nectar 30 16.2

Bolded values refer to those to be used for refinement of EECs. 1

Refers to hand-collected pollen and nectar. 2Acute EEC chosen as the maximum reported concentration among all individual replicates following application. 3Chronic EEC chosen as the maximum average concentration among all individual sampling events following application.

Table 6-43 below summarizes the refined Tier I oral RQs for the various castes of larval and adult bees 

using nectar residue data from seed + foliar-treated cotton. Adult acute and chronic oral and larval chronic 

oral RQs exceed their respective LOCs for every caste assessed except acute oral RQ values for adult 

workers maintaining the hive in winter. Adult acute oral RQs range from 0.3 – 3.0 while adult chronic oral 

RQs range from 5.6 – 53. Additionally, larval chronic oral RQs range from 1.0 – 2.1. 

Table 6-43. Summary of Tier I oral RQs for honey bees using refined exposure estimates with measured

imidacloprid residues in pollen and nectar from combined seed + foliar applications to cotton.

1,4
204

Life 

stage

Caste or task in 

hive

Average 

age (in 

days)

Consumption Rates5 Acute 

dose (μg 

a.i./bee)6

Acute RQ2

Chronic 

dose (μg 

a.i./bee)6

Chronic 

RQ Nectar 3

(mg/day)

Pollen 

(mg/day)

Larval

Worker

4 60 1.8 0.0018 1.0

5 120 3.6 0.0036 2.0

Drone 6+ 130 3.6 0.0039 2.1

Adult

Worker (cell 

cleaning and 

capping)

0-10 60 6.65 0.0027 0.7 0.0019 12

Worker (brood 

and queen 

tending, nurse 

bees)

6 to 17 140 9.6 0.0061 1.6 0.0043 27

Worker (comb 

building, 

cleaning and 

food handling)

11 to 18 60 1.7 0.0025 0.6 0.0018 11

Worker 

(foraging for 

pollen)

>18 43.5 0.041 0.0017 0.4 0.0013 7.9

Worker 

(foraging for 

nectar)

>18 292 0.041 0.0115 3.0 0.0085 53

Worker 

(maintenance 

of hive in 

winter)

0-90 29 2 0.0013 0.3 0.0009 5.6

Drone >10 235 0.0002 0.0093 2.4 0.0068 43 1For life stages or castes that rely exclusively on royal jelly (i.e. larval and adult queen, 1-3 day old worker bees), these rows were removed from 

the table as residues of imidacloprid in royal jelly are not available.

2 Based on a 48-h acute oral LD50 of 0.0039 μg a.i/bee for imidacloprid (MRID 42273003) 3Based on adult 10-day chronic NOAEC of 0.00016 μg a.i/bee (Boily 2013, MIRD 49750601) and larval 21-day chronic NOAEC of 0.0018 μg a.i/larva 

(MRID 49090506).

4Bolded value exceeds the acute risk LOC of 0.4 or chronic risk LOC of 1. 5From Guidance for Assessing Pesticide Risks to Bees (US EPA et al., 2014) 6Based on concentrations reported in Table 6-42.

6.1.5. Refined RQs (Off-field Oral)

In theory, the estimates of spray drift provided by AGDRIFT could be used in conjunction with measured 

concentrations of imidacloprid in pollen and nectar from available residue studies as a way to refine the 

AGDRIFT estimates of spray drift. This approach assumes strict proportionality between measured 

residues in pollen and nectar and application rate. It also assumes that the conditions associated with the 

foliar application in the residue studies and that used in the AGDRIFT modeling are comparable.

Residue data for imidacloprid in pollen and nectar resulting from foliar applications are available for citrus, 

stone fruit (cherry) and cotton. The citrus residue study (MRID 49521301) involved ground applications 

of 2 x 0.25 lbs. a.i./A at approximately 10 days prior to bloom and residue measurements beginning 4-7 

days following the last application. Residue data are available for both pollen and nectar. The stone fruit 
205

residue study (MRID 49535601) consisted of post-harvest airblast applications of 5 x 0.1 lbs. a.i./A with 

residues in pollen and nectar measured at 208 days post application. The cotton foliar residue study 

(MRID 49103301) consisted of a single aerial application of 0.06 lbs. a.i./A made during bloom with 

residues measured approximately 6 days following application, however, residues in nectar were 

measured. 

Unfortunately, none of these studies are considered appropriate for refining AGDRIFT estimates of spray 

drift at off-field locations due to their design and timing of residue measurements. Specifically, the citrus 

study involved 2 sequential applications (with an 8-10 day interval) and a targeted a 10-day pre-bloom 

interval. The AGDRIFT model provides drift estimates based on a single application rate which is not 

comparable to the citrus residue study. Furthermore, actual off-field drift would not reflect a 10-day prebloom interval on application days. Therefore, the citrus residue measurements may underestimate the 

day 0 residues expected on flowering plants adjacent to the treated site on the day of application. The 

stone fruit study also involves multiple application rates and a sampling interval of over 200 days, which 

again is not representative of residues expected off-field due to drift occurring on the day of application. 

Finally, the cotton study only measured residues in nectar. Therefore, comparable pollen data are not 

available to calculate refined Tier I RQ values that reflect drift to adjacent flowering plants. As a result of 

these limitations, refinement of the AGDRIFT-predicted off-field drift RQ value using residue data was not 

conducted.

6.1.6. Uncertainties at the Tier I Level

There are several sources if uncertainty at the screening-level and refined Tier I level associated primarily 

with the screening-level exposure estimates and use of residue data, respectively. What follows are the 

uncertainties associated with each point:

Screening-level Exposure Estimate Uncertainties:

• The extent to which the amount of food consumed by bees for the Tier I exposure estimate 

represent pesticide concentration in food sources.

• The extent that residues on leaves and even soil may be available for bee uptake

• For soil applications, there are three notable limitations to the modified Briggs’ model approach

that include:

o This methodology is based on one species of plant (barley)

o The dataset used to derive elements of the model is based on a limited number of 

chemicals that represent only two classes of pesticides that do not include the 

neonicotinoid insecticides

o The model is based upon data on pesticide concentrations in vegetative plant matrix (i.e., 

shoots) as a surrogate for nectar and pollen 
206

• For seed treatment applications, the screening-level assumption of exposure within Bee-REX is 1 

mg/kg (1 ppm). This is based in the Internal Commission for Plant-Bee Relationships’ 1 mg a.i/kg 

to represent an upper bound concentration in pollen and nectar. This assumption of exposure is 

independent of application rate (i.e mass of chemical applied to the seed0 but is considered to be 

protective of the varying rates currently being used for seed treatments.

• Bee-REX assumes exposures through consumption of nectar and pollen are conservative 

representations of potential exposures through consumption of honey and bee bread, 

respectively. This approach is likely to be conservative because it assumes that pesticides do not 

degrade while honey and bee bread are stored in the hive. For bees that consume honey, it is 

assumed that the estimated pesticide exposures can be related back to the original concentration 

in nectar by accounting for the amount of sugar consumed by bees. It is also assumed that pollen 

and nectar consumption rates and resulting exposures are protective of exposures of bees to 

pesticides through consumption of royal jelly and brood food. 

• The screening-level exposure assumption in Bee-REX assumes pesticide concentrations in pollen 

and nectar are equivalent (i.e. effectively one EEC for bee food). As was shown in the suite of 

available residue studies, pollen and nectar residue values can vary markedly depending on the 

use pattern and application method. For example, maximum residues in pollen in the foliar 

applied cherry study were 100 fold higher than maximum residues in nectar. Conversely, extrafloral nectar residues were over 8-fold higher than pollen residues in the soil + foliar-applied 

cotton study.

Use of Residue Data:

• The use of chemical-specific pollen and nectar residue data reduces the uncertainties associated 

with the methods discussed above; however, these data also require certain considerations to 

ensure they are used in the most appropriate manner use in estimating potential risk. 

6.2. Risk Description

The risk description section further characterizes the findings of Risk Estimation as well as integrates 

additional lines of evidence and uncertainties regarding the potential risks to bees beyond the Tier I RQ 

values. Additionally, the risk description characterizes the effects and findings of the higher tier (i.e. Tier 

II semi-field and Tier III full field) colony-level studies. These effects will be put into context with the 

findings of the available residue studies described previously in Section 5 and in the Section 6.1. This risk 

description includes the following elements for characterizing the risk of imidacloprid’s agricultural uses 

to bees:

1. Additional characterization of Tier I risks and evaluating risks at Tier II level

2. Evaluating risks at the Tier III level

3. Examination of colony-level risks from the pollen route of exposure 

4. Consideration of risks to non-Apis bees
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5. Assessing risks associated with additional lines of evidence

6. Articulating major uncertainties at each risk assessment tier 

6.2.1. Characterization of Tier I Risks and Tier II Analysis

Approach for Characterizing Tier 1 Risks

The additional characterization of the refined Tier I risk estimation is organized by application method 

(e.g., foliar, soil, seed treatment) and crop group, using representative crop residue data when available. 

At the Tier I level, additional characterization includes consideration of the magnitude, duration and 

frequency at which the acute and chronic risk LOC values are exceeded. This refined Tier 1 risk 

characterization considers the totality of available residue data for pollen and nectar rather than just the 

acute or chronic EECs described in Risk Estimation (Section 6.1). In this way, the refined Tier I risk 

characterization is able to distinguish RQ values generated in Risk Estimation that reflect short-term, 

infrequent ‘spikes’ of imidacloprid pollen and nectar residues from those that reflect long-term, frequent 

occurrences. This analysis utilizes the raw data from a given residue study and calculates the maximum 

RQs among bee castes/tasks for each pair of total imidacloprid residues (i.e. parent imidacloprid, IMIolefin, and IMI-5-OH) in pollen and nectar sampled on the same day. Then, the highest RQ was selected 

among the various age/task-specific RQs calculated by BeeREX v. 1.0. For larval bees, this was always the 

5-day old worker larvae20 and for adults it was either nectar foragers or nurse bees. 

Approach for Characterizing Tier II Risks

At the Tier II level, a NOAEC and LOAEC of 25 and 50 ppb of total imidacloprid in sucrose was determined 

from the registrant-submitted colony feeding study (MRID 49501001). At this time, the registrantsubmitted colony feeding study is considered the most comprehensive and robust Tier II study available 

from which to characterize the colony-level effects of imidacloprid to honey bees. Specifically, this study:

• Contains a high degree of replication and adequate statistical power, 

• Demonstrates a robust dose-response relationship between sucrose residues and colony-level 

apical endpoints, 

• Examined a 6-week exposure period that is commensurate with relatively long-term exposures 

expected for some crop use scenarios,

• Provides raw data that enabled an independent statistical evaluation of the responses,

• Was conducted according to Good Laboratory Practice specifications, and

• Included an evaluation of over-wintering colony survival. 

 20 According to BeeREX v. 1.0, drone larvae are not typically included in risk estimation due to their limited role in 

the hive population, although their estimated exposure is slightly greater than worker larvae (130 mg 

nectar/larvae/day for drone larvae vs. 120 mg nectar/larvae/day for worker larvae).
208

The NOAEC and LOAEC of 25 and 50 ppb, respectively, are based on reductions in several colony-level 

apical endpoints including numbers of adults, number of pupae, pollen stores, and honey stores that 

persisted across multiple assessments of the colonies throughout the course of the study. 

As the registrant colony feeding study exposed honey bee colonies via spiked sucrose, only residue data 

for nectar from the Tier II residue studies can be compared to the Tier II NOAEC and LOAEC with a sufficient 

level of certainty. This limitation relates to the differential utilization of pollen by the colony relative to 

nectar and the subsequent differences in exposure of bees to dietary imidacloprid via pollen and nectar. 

Furthermore, there is evidence to suggest that colony-level effects on honey bees via contaminated pollen 

occur at higher residue levels than those in nectar. Potential colony-level risks associated with the pollen 

route of exposure are discussed in a later section within this risk description. Where nectar data are 

available, the average residue values from all sampling events were plotted alongside the NOAEC and 

LOAEC of the colony feeding study in order to determine the magnitude, frequency and duration by which 

the NOAEC and LOAEC values were exceeded. Notably, comparing residues measured in nectar to the Tier 

II NOAEC and LOAEC inherently assumes comparable dosimetry of imidacloprid between bees foraging on 

nectar from the treated crop and bees consuming spiked sucrose from the colony feeding study (i.e., a 6-

week continuous exposure). In some situations, such as when an abundance of uncontaminated alternate 

forage is available, the assumption that bees in the field would be exposed to imidacloprid residues in 

crop nectar continuously for 6 weeks will be conservative (i.e., overestimate exposure). In other 

situations, possibly with managed hives used to pollinate multiple crops over the entire growing season, 

this assumption may underestimate exposure. It is therefore important to consider not only the 

magnitude by which the NOAEC or LOAEC values are exceeded, but also the frequency and duration of 

the exceedance. 

As indicated in Section 5, there are two studies in the open literature that assess the long term effects to 

honey bee colonies resulting from the pollen route of exposure (Dively 2009 and Dively 2015). These 

studies were similar in design to that of the registrant-submitted Tier II colony feeding study in that they 

replicated hives across treatment groups, monitored for disease and parasites, and included an 

overwintering component. Additionally, these studies examined similar response variables as with the 

Tier II colony feeding study in nectar including percentage of comb area covered by various life stages and 

food stores (pollen and nectar). The studies tested up to 100 ppb in pollen (Dively 2015) and employed a 

12 week exposure duration. The findings were restricted to a significant (p<0.05) reduction in 

overwintering survival at the 100 ppb treatment group in one trial within the Dively 2015 paper.

These studies, as indicated in Section 5, are associated with a number of limitations and uncertainties that 

limit their utility for assessing the impact of the spiked pollen route of exposure to honey bee colonies. 

Specifically, the Dively 2015 study (which included two trials conducted in back-to-back years with only 

the number of replicate colonies per treatment group differing) was noted to have high levels of Varroa

mite infestation in all treatment groups as well as the control. In one trial, these levels were 2-4X the level 

which the certain resources indicate as sufficient for treatment.21 Additionally, despite the two trials 

being conducted in a similar manner in consecutive years, the control overwintering survival between the 

 21 http://honeybeehealthcoalition.org/wpcontent/uploads/2015/08/HBHC%20Guide_Varroa_Interactive_23Sep.pdf
209

two years was markedly different (100% vs. 57%, respectively). Furthermore, raw data was not available 

to independently verify the results. The duration of exposure was exceptionally long (12 weeks) which is 

likely not realized in many real world exposure scenarios. Finally, there is a wide dose spacing (i.e. 4-5X) 

between the 3 treatment group (5, 20, and 100 ppb). This wide dose spacing introduces uncertainty in 

the purported overwintering NOAEC of 20 ppb and the LOAEC of 100 ppb. Therefore, this study is 

considered to have limited utility for characterizing colony-level risks to honey bees at the Tier II level. 

Where data for imidacloprid residues in pollen are available, exceedances of the purported 100 ppb 

overwintering LOAEC are noted for informational purposes, but are not considered of similar weight as 

exceedances of the nectar NOAEC and LOAEC described earlier. 

Consideration of Crop Usage and Label Information

Another important aspect that is considered in the Tier 2 risk evaluation is information on the usage of

imidacloprid for the crop of interest (e.g., poundage of active ingredient applied per acre per year as well 

as the average percent of crop acreage that is treated). For example, lettuce was identified as having an 

average percent crop treated of 35% for imidacloprid, which means that out of all the lettuce grown in 

the United States, on average 35% is treated with imidacloprid each year. This information is available 

from the memo entitled “Usage Report in Support of Registration Review Draft Risk Assessment Purposes 

for Imidacloprid (US EPA, 2015),” that includes the Screening Level Usage Analysis (SLUA) that includes 

usage data from 2004 – 2013. The sources for the SLUA include the USDA’s National Agricultural Statistics 

Service (NASS, reporting data from 2004 – 2013), private pesticide market research (reporting data from 

2004 – 2013), and the CDPR Pesticide Use Reporting (PUR) data (reporting from 2004 – 2012). 

To obviate the need to state it for every crop in the analysis that follows, the following are important 

points/caveats to the SLUA data:

• Pesticide usage data are considered for a single active ingredient. That is to say that imidacloprid 

co-formulated with one or more pesticides are not included in the SLUA data.

• The average annual pounds of pesticide applied for each crop originates from the states that were 

surveyed and not the entirety of the United States. It is also noted that usage information for a 

given crop is available from states that produce 80% or more of that crop in most cases.

• Lack of reported usage for a given crop does not necessarily indicate zero usage.

• Usage data on a particular site may be noted in data sources, but not quantified. In these cases, 

the site would not be reported in the SLUA.

• Although some uses for seed treatment applications are delineated, the SLUA does not distinguish 

between foliar and soil applications if a given crop is registered for both application methods.

Finally, label information pertaining to the use of imidacloprid on the crop associated with each residue 

study is also considered in the risk characterization. As the residue studies were generally stated to have 

been conducted with Admire® Pro (EPA Reg. No. 264-827), language from this label is considered for 

determining the extent to which the resulting residues in pollen and nectar reflect current labeled use 

and reasonably “worse case,” exposure conditions. 
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What follows is a discussion of the risks associated with each crop group where residue data are available 

organized by application method.

Foliar Applications

Crop Group 10 – Citrus Fruits (Orange)

The citrus crop group includes, among other members, oranges, grapefruit, lemons, limes, mandarins, 

tangelo, tangerines, and various hybrids thereof. Specific to the uses of imidacloprid on citrus, oranges 

appear to be the dominant crop with an estimated usage of estimated 60,000 lbs/year (Table 6-44). This 

is followed by grapefruit, tangerines and lemons with much lower usage. As noted previously, members

of the citrus crop group are considered highly attractive to honey bees as a source of pollen and nectar 

(e.g., oranges, grapefruit, lemons, mandarins, clementines, tangerines; USDA 2014). According to the 

USDA (2014), oranges, clementine, mandarin and tangerine utilize managed pollination services, although 

this appears dependent on the cultivar. Further, citrus is widely recognized as a source of nectar for 

commercial honey production. Members of the citrus fruits crop group ae associated with a bloom 

duration that is at least 6 weeks in length where nectar and pollen would be potentially be available to 

visiting honey bees. 

Table 6-44. SLUA data for imidacloprid and citrus fruits (2004-2013)

Crop Lbs. Applied/yr. % Acreage Treated (average) % Acreage Treated (maximum)

Grapefruit 8,000 30 60

Lemons 3,000 10 25

Oranges 60,000 25 40

Tangelos <500 15 20

Tangerines 6,000 25 40

Refined Tier I Oral Risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, refined Tier I oral RQ values for honey bees resulting from the foliar use on 

oranges, range from 4.8 – 32 (adult acute), 14 – 30 (larval chronic) and 89 – 592 (adult chronic) depending 

on their caste and function within the hive. These RQ values reflect “high end” estimates of pollen and 

nectar residues obtained from foliar applications at the maximum label rate (2 x 0.25 lbs. a.i/A) for citrus 

with the last application made 10-days prior to bloom (MRID 49521301). Figure 6-2 below shows the 

adult acute oral, adult chronic oral and larval chronic oral RQs in relation to the LOCs with the totality of 

the nectar and pollen data available and how it translates into the magnitude, duration and frequency of 

Tier I LOC exceedances. For this study, only data from trials NT005 and NT006 were included since trial 

NT004 was contaminated by inadvertent applications of imidacloprid. 
211

Figure 6-2. Summary of acute and chronic RQ values using totality of pollen and nectar residue data 

from foliar-applied citrus residue study (MRID 49521301).

A total of 14 oral RQ values were calculated for each life stage/duration. As indicated by Figure 6-2, 100% 

of the refined Tier I acute and chronic RQ values exceed their LOC values (0.4 and 1.0, respectively) up to 

38 days following the last foliar application. As indicated previously, this reflects the majority of the 

blooming period of citrus although there is the potential for regional variation. The magnitude of these 

RQ values range from 1.5 to 32 (adult, acute), 27-592 (adult, chronic), and 1.3-28 (larval chronic). Due to 

the dissipation of residues over time (primarily in citrus nectar), RQ values also decline with time by 

approximately an order of magnitude. 

Tier II Risks

An acceptable Tier II long-term feeding study is available for imidacloprid which quantified effects at the 

colony level following a 6-week exposure to imidacloprid via spiked sucrose solution. This study generated 

a NOAEC of 25 ppb and a LOAEC of 50 ppb (see Section 5.2). To evaluate the risk of foliar application of 

imidacloprid to citrus at the colony level for honey bees, reported residues of total imidacloprid residues

in nectar were compared to the aforementioned NOAEC and LOAEC values (Figure 6-3).
212

Figure 6-3. Imidacloprid nectar residues in nectar from foliar-applied citrus study (MRID 49521301, trials 

NT005 and NT006 only) as compared to effect levels in registrant submitted colony feeding study (MRID 

49510001).

There were 13 of 14 (93%) of daily average residue values that exceeded the NOAEC during the sampling 

intervals of the study. Mean residues of total imidacloprid in orange nectar exceed the LOAEC (50 ppb) 

for approximately 15 days after the last foliar application. The mean residues exceed the NOAEC for at 

least 25 days and appear to drop below the NOAEC by 38 days after application (DAA). As a number of 

the citrus fruits crop group, including oranges are noted to have a blooming period that extends for 6 

weeks or more, the colony feeding study exposure duration of 6 weeks is particularly relevant when 

characterizing the Tier II risk of honey bee colonies to foliar-treated citrus fruits.

Figure 6-4 below shows the average residues in pollen from the foliar-applied citrus fruit study. All 

residues are noted to be above the 100 ppb threshold which, as noted previously was indicated to be a 

level where honey bee colonies exposed to spiked pollen patties for 12 weeks experienced significantly 

reduced (p<0.05) survival after the overwintering period relative to the control. 
213

Figure 6-4. Imidacloprid average residues in pollen from foliar-applied citrus study (MRID 49521301, 

trials NT005 and NT006 only).

Additional Considerations

Due to an oversight by the study author, the plots in trial NT004 were sprayed with PROVADO® (1.6 lb 

imidacloprid a.i/gal, corresponding to 0.1 lb a.i/acre application), in both 2010 and 2011, which was not 

known at the time of the experiment. Additionally, PREY®, another insecticide containing imidacloprid, 

was used as a maintenance pesticide on both the treated and control plots of this trial in September 2012 

and September 2013 (single application of 0.15 lbs/A each year). Because of the additional imidacloprid 

added to the plots prior to the study, the residue values are presumably higher in the NT004 than what 

would be expected from the study applications alone and therefore these data were not included in the 

above analysis. Additionally, all three sites for this study were in relatively close proximity to one another. 

In fact, two sites (NT005 and NT006, which represent the data used in the analysis above) were 

immediately adjacent. Soil types reflect sandy compositions (96-98%) and low organic carbon content 

(0.35-1.9%). Weather conditions (temperature and precipitation) were similar across the three trials. As 

a result of the close proximity of trial sites, this study provides very limited information on how differences 

in environmental conditions across different areas of the US may affect accumulation of total imidacloprid 

residues in pollen and nectar. 
214

Additionally, the extent to which prior year applications of imidacloprid contributed to year-to-year 

carryover in nectar and pollen concentrations could not be reliably assessed due to several study 

limitations including the aforementioned inadvertent pesticide applications in trial NT004 and the 

different sampling times (and plant growth stages) employed during the two trial years (NT005 and 

NT006). Since the application method was foliar spray, concentrations in soil may not be reliable 

indicators of pesticide year-to-year carryover.

Current label language for foliar imidacloprid applications on citrus specify a 0.25 lbs a.i/A maximum single 

application rate (0.5 lbs a.i/A per year) as well as prohibiting applications made 10 days prior to bloom, 

during bloom, or when bees are foraging. This study was conducted with a 10-day pre bloom interval and 

2 applications of 0.25 lbs a.i/A, and was therefore conducted in accordance with the label.

Conclusions

Average residues in nectar resulting from the foliar application (with the labelled restriction of a 10-day 

pre-bloom interval) to citrus fruits (oranges) indicate a colony level risk based on residues being above 

the Tier II NOAEC in nectar for an extended period of time. Similarly, average residues in pollen were also 

above 100 ppb which has been associated with colony level effects following a 12-week exposure via 

pollen (Dively 2015, Qualitative).

Crop Group 12 – Stone Fruits (Cherry)

The stone fruit crop group includes, among other members, apricots, cherries, nectarines, peaches, plums 

and various hybrids. Specific to the uses of imidacloprid, cherries appear to be the dominant crop with 

an estimated usage of estimated 4,000 lbs/year (Table 6-45), with peaches and plums to a lesser extent. 

As noted previously, members of the stone fruit crop group are considered highly attractive to honey bees 

as a source of pollen and nectar (e.g., cherries, nectarines, peaches, plums/prunes; USDA 2014). 

According to the USDA (2014), all of the aforementioned members require bee pollination and utilize 

managed pollination services. Stone fruits, including cherries are noted to bloom for around 2-3 weeks.

Table 6-45. SLUA data for imidacloprid and stone fruits (2004-2013)

Crop Lbs. Applied/yr. % Acreage Treated (average) % Acreage Treated (maximum)

Cherries 4,000 25 50

Peaches 1,000 5 15

Plums/Prunes <500 <2.5 10

Refined Tier I Oral Risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from the foliar use 

on cherries, range from 0.59 – 2.82 (adult acute), 1.4 – 1.5 (larval chronic) and 1.7 - 38 (adult chronic) 

depending on their caste and function within the hive. These RQ values reflect “high-end” estimates of 

pollen and nectar residues obtained from foliar applications at the maximum label rate (5 X 0.1 lbs a.i/A) 

for cherries (MRID 49535601). All applications were post-bloom with applications in the first year being 
215

post-harvest (fall) and applications in the second year were pre-harvest (summer). Residues samples were 

initiated 208 days after the last application for year 1 and approximately 275 days after the last application 

for year 2. Figure 6-5 below shows refined Tier I RQ values in relation the LOC for all matched pollen and 

nectar data from the foliar cherry residue study.

Figure 6-5. Summary of acute and chronic RQ values using totality of pollen and nectar residue data from 

foliar-applied cherry residue study (MRID 49535601).

A total of 13 RQ oral values were calculated for each life stage/duration up to 303 days following the last 

foliar application. The magnitude of these RQ values range from 0.09 - 2.98 (adult, acute), 1.7 - 37 (adult, 

chronic), and 0.09 - 1.4 (larval chronic). There were 38, 100, and 7.6% of all estimated RQs for adult acute 

oral, adult chronic oral, and larval chronic oral that exceeded their respective LOCs. Residues in pollen, 

on average, were typically 10 – 100-fold higher than in nectar. Therefore, the highest acute and chronic 

adult RQs were associated with the nurse bee caste that have the highest consumption of pollen as part 

of their diet than any other adult caste within the hive. In general, adult acute oral RQs and the larval 

chronic oral RQs were below or marginally above the acute risk LOC and chronic risk LOC of 0.4 and 1, 

respectively. In contrast, all adult chronic oral RQs were above the acute risk LOC and as high as 37. While 

LOC exceedances for adult acute oral RQs and larval chronic RQs are not indicated after 274 days after 
216

the last application, chronic adult oral RQs still exceed the chronic risk LOC up to and including the residue 

samples measured at 303 days after the last application.

Tier II Risks

To evaluate the risk of foliar application of imidacloprid to cherries at the colony level for honey bees, 

reported residues of total imidacloprid concentrations in nectar were compared to the aforementioned 

NOAEC and LOAEC values from the available registrant submitted colony feeding study (Figure 6-5). The 

daily average residues were plotted by the year they were sampled.

Figure 6-6. Imidacloprid residues in nectar from foliar-applied cherry study (MRID 49535601) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Based on this comparison, there were no average total residues (0%) of imidacloprid in cherry nectar that 

exceeded the Tier II NOAEC (25 ppb) at sampling intervals up to and including 303 days after the last foliar 

application. Although residues in nectar appeared to be relatively consistent over time, the highest daily 

average value determined was 5.6 ppb, approximately 5-fold lower than the NOAEC. As noted previously, 

cherries typically bloom for 2-3 weeks, which is roughly half the duration of the exposure period of the 

Tier II colony feeding study.
217

Figure 6-7 below shows the daily average residues in pollen from the foliar-applied cherry study. A subset 

of these residues are noted to be above 100 ppb which has been associated with colony level effects 

following a 12-week exposure via pollen (Dively 2015, Qualitative). It is noted that cherries and other 

stone fruits have a bloom duration of 2-3 weeks, which is 4-6 fold less than the exposure period from the 

feeding study conducted with spiked pollen.

Figure 6-7. Imidacloprid average residues in pollen from foliar-applied cherry study (MRID 49535601).

Additional Considerations

Within each residue trial, the data indicate that applications during late summer/early fall resulted in 

greater residues measured in nectar during year 1 (2013) compared to applications made in late spring / 

early summer and subsequent measurement in year 2 (2014). The lower residue values measured during 

year 2 may reflect the greater amount of time available for dissipation of imidacloprid residues between 

application and residue measurement (275-300 days) compared to that from year 1 (about 205-215 days). 

Lower residues in year 2 may also reflect temporal differences in imidacloprid translocation to nectar 

when applied in late summer/early fall vs. spring/early summer. It should be noted that except for one 

value from trial NT016, residues of total imidacloprid in nectar from year 2 (2014) largely reflect levels 

below the limit of analytical detection or quantification where 1⁄2 the LOD or LOQ (0.7 ppb and 1.0 ppb in 

nectar and pollen, respectively) was assumed.
218

The two trials within the NY and OR locations were each within close proximity, such that they shared the 

same weather data. Soil types reflect sandy loam compositions (53-67% sand) and moderate to low 

organic carbon content (0.9-3.4%). Although within NY and OR the sites were in close proximity, this study 

encompasses some regional differences in cherry cultivation which may affect accumulation of total 

imidacloprid residues in pollen and nectar. The available data suggest that year-to-year accumulation of 

total imidacloprid is not evident (as indicated by a decline in pollen and nectar residues from year-toyear); however, the impact of differences in pesticide application timing between year 1 to year 2 on the 

resulting residues in pollen and nectar is not clear. In other words, the lower residues measured in year 

2 may reflect a longer time between application and sampling. It is also noted that the mean residues in 

pollen in the NY sites were an order of magnitude higher (average residues of 153 and 422 ppb) as 

compared to the OR sites (13 and 21 ppb). This observation is less pronounced for nectar (average of 4.8 

and 3.9 ppb in NY compared to 3.4 and 1.6 ppb).

Current label language mandates that imidacloprid not be applied pre-bloom, during bloom, or when bees 

are foraging. This study with 5 foliar applications of 0.1 lbs a.i/A (the maximum single application rate for 

foliar treatment to stone fruits) made post-bloom, was conducted in accordance with current label 

language. 

Conclusions

Average residues in nectar resulting from the foliar application (post bloom) to stone fruits (cherries) do 

not exceed the Tier II NOAEC (25 ppb) in nectar when residues were sampled the following season (200 –

300 days after the last application, depending on the trial year). Average residues in pollen were roughly 

100 fold higher than those in nectar and did exceed 100 ppb for several days in two of the four trials. 

Although colony level effects have been associated with imidacloprid exposure following 12 weeks to 100 

ppb in pollen, these residues exceeded 100 ppb for a short period and the entire bloom duration of cherry 

(2-3 weeks) is much shorter than the 12-week exposure from the spiked pollen feeding study. Therefore, 

despite the fact that nectar is considered the dominant route of exposure for forager honey bees, there 

is uncertainty whether residues in cherry pollen pose a significant colony level risk to honey bees since it 

is not known whether a shorter exposure period would have led to similar effects observed in the open 

literature studies.

Crop Group 20 – Oilseed (Cotton)

The oilseed crop group includes, among other members, cotton, flax, sunflower, and rapeseed (canola). 

Specific to the uses of imidacloprid, cotton is the only crop represented, with 50,000 lbs a.i/A for each of 

foliar/soil uses and seed treatment uses and is the only member of the oilseed group with registered foliar 

and soil uses. Cotton is noted to have nectar that is attractive to bees while the pollen is not considered 

to be attractive to honey bees. Additionally, cotton is associated with a blooming duration of at least 6 

weeks.
219

Table 6-46. SLUA data for imidacloprid and oilseed crops (2004-2013)1

Crop Lbs. 

Applied/yr.

% Acreage Treated 

(average)

% Acreage Treated 

(maximum)

Cotton 50,000 5 10

Cotton (Seed 

Treatment)1 50,000 10 20

1The surveying period for seed treatment uses does not always cover the entire period of the SLUA

There are three residue studies available to characterize the residues of imidacloprid in pollen, nectar, 

and extra-floral nectar in cotton assessing various application methods. For foliar applications, a study 

assessing one aerial application of imidacloprid at the maximum single application rate, 0.06 lbs a.i./A (at 

bloom) yielded floral nectar data only across 1 sampling event. This represented a treatment regimen 

that was less than the maximum annual rate of 0.31 lbs a.i./A (MRID 49103301). Additionally, studies are 

available that assess the combined soil + foliar method (inclusive of a soil only component) and seed + 

foliar regimen that will be discussed in the combined application method section.

Refined Tier I Oral Risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from the foliar use 

on cotton, range from 0.49 – 4.94 (adult acute), 1.8 – 4.1 (larval chronic) and 10 - 102 (adult chronic) 

depending on their caste and function within the hive. These RQ values reflect “high-end” estimates of 

pollen and nectar residues obtained from a foliar application at the maximum single rate (1 X 0.06 lbs 

a.i/A) for cotton (MRID 49103301). An aerial application was made during bloom and it was noted that in 

the two years prior to the study, applications of imidacloprid (ranging from 0.18 – 0.38 lbs a.i/A) had been 

made to other crops via chemigation. Additionally, only nectar was sampled (only one sampling event 

available) at 6 days post-application (noted to be a fraction of the 6 week or longer bloom duration of 

cotton). Figure 6-8 below shows refined Tier I RQ values in relation the LOC for all nectar data available 

from the foliar cotton residue study.
220

Figure 6-8. Summary of acute and chronic RQ values using totality of residue data from foliar-applied 

cotton residue study (MRID 49103301).

A total of 5 RQs were estimated from the totality of nectar data for each life stage and duration. All (100%) 

of the estimated RQs for all life stages were above their respective LOCs at the single sampling period 

assessed (6 days post-application). These results are considered an underestimation of the potential risk 

given that only one application was made while the maximum annual rate for foliar application on cotton 

would have allowed for 4 more applications. Additionally, since there was only one sampling period, it is 

not possible to determine how RQs may have changed over time as a result of the potential changes in 

residue levels. Finally, although the pollen produced by the cotton plant is not considered attractive to 

honey bees, the overall RQs would be higher with the added pollen component of the diet to several 

castes within the hive such as worker bee larvae and nurse bees.

Tier II Risks

To evaluate the risk of a single foliar application of imidacloprid to cotton at the colony level for honey 

bees, daily average residues of total imidacloprid concentrations in nectar were compared to the 

aforementioned NOAEC and LOAEC values from the available registrant-submitted colony feeding study 

(Figure 6-9). As indicated previously, these residues values present only a single sampling period 6 days 

after application. 
221

While the refined Tier I analysis indicated all RQs exceeded the LOC at the honey bee individual level, 2 of 

the 5 (40%) of the available nectar data indicate an average total residue level below the NOAEC of 25 

ppb. One sample was marginally above the LOAEC of 50 ppb (56 ppb) while another sample was between 

the NOAEC and the LOAEC. It is reiterated that the number of samples available from this study is small 

and originated from one sampling interval (6 days post-application). Additionally, it is expected that had 

five applications been made (as permitted on the label), all residues would likely exceed the NOAEC and

possibly the LOAEC. As noted previously, the cotton bloom period can extend for at least 6 weeks, 

consistent with the exposure duration for the sucrose colony feeding study. As noted previously, there 

were no pollen residue data available from this study.

Figure 6-9. Imidacloprid residues in foliar-applied cotton residue study (MRID 49103301) as compared to 

effect levels in registrant submitted colony feeding study (MRID 49510001).

Additional Considerations

In the cotton foliar application residue study, four of the five sites had chemigation applications in 2008 

and 2009 (Sites NT001, NT002, NT003, NT005). One of the five sites had two chemigation applications in 

2009 but no application in 2008 (Site NT004). The formulated products used were Provado® 1.6F (for 

aerial applications) to cotton and Admire® Pro (for prior chemigation uses). All five sites were in one 
222

county in California and therefore the spatial variability was limited and similar climatic conditions would 

be expected. Although prior applications of Admire® Pro had been made to 4 of the 5 treatment sites, 

there was no quantification of the residues in soil prior to aerial application of imidacloprid to determine 

the potential carryover that could affect the resultant residues following foliar application. 

All residue values are noted to originate from fine (heavy) soil types. As indicated previously and will be 

suggested by several soil-applied residue studies discussed below, imidacloprid concentrations in pollen 

and nectar are often higher with increasing soil coarseness (i.e., increasing sand composition). There is 

uncertainty with respect to the potential increased magnitude of residues in nectar if a portion or all 

samples originated from coarser textured soils, but this uncertainty may be minor given that this 

application was as an air blast where soil exposure would be more limited.

As indicated previously, this study assessed one foliar application at the maximum single use rate of 0.06 

lbs a.i/A but did not assess the effect of subsequent applications (an additional 4 would be permitted on 

the label) on the residues in pollen and nectar. Therefore, despite this study not assessing a worst case 

scenario, it is noted that potential colony-level effects are indicated. It is also worth noting that according 

to SLUA data, which do not make the distinction between foliar- and soil-applied application methods, 

that imidacloprid is used to treat an average of 5% of the cotton acreage per year and a maximum of 10% 

of the cotton acreage per year.

Conclusions

Average residues in nectar collected 6 days after resulting a single at bloom foliar application to cotton 

indicate a colony level risk based on residues being above the Tier II NOAEC in nectar. The residues from 

this study would be expected to be even higher if the study had made an additional 4 foliar applications 

as is permitted on current labels. There was no residue information in pollen available from this study.

Additional Foliar Application Use Patterns

Imidacloprid is associated with several other use patterns that are registered for foliar treatment 

applications including members from crop groups 1 (root and tuberous vegetables), 4 (leafy vegetables), 

5 (brassica vegetables), 6 (legumes), 8 (fruiting vegetables), 11 (pome fruits), 13 (berries and small fruits), 

14 (tree nuts), 19 (herbs and spices), and several other use patterns not associated with a crop group 

including banana, plantain, globe artichoke, coffee, hops, peanut, pomegranate, and tobacco. 

Specific to the data provided by the SLUA, Table 6-47 below summarizes the use patterns registered for 

foliar applications of imidacloprid. It is noted that the table below presents crops that are registered with 

both foliar and soil applied uses, although the SLUA does not distinguish between these application 

methods in the usage data. The use patterns are organized by crop group with usage data information on 

crop attractiveness and whether the crop is harvested before bloom. Although residue data are not 

available for at least one member of the majority of crop groups, a large portion of these use patterns 

involve harvesting prior to bloom or are not honey bee attractive, thus minimizing exposure risk to 

foraging honey bees. 
223

Generally speaking, members of Crop Groups 1, 4, and 5 (root and tuberous, leafy, and brassica 

vegetables, respectively) are harvested before bloom while also producing pollen and nectar that are 

noted to be attractive to honey bees. Therefore, with the exception of when used for seed production, 

the exposure to honey bees is expected to be minimal as the crop would be harvested prior to when bees 

would likely be foraging. 

Members of the legume group (Crop Group 6) are also noted to produce pollen and nectar that are 

attractive to bees and some legumes (e.g., cow peas and snap beans) and generally indicated by USDA 

2014 to not be harvested prior to bloom. Soybean represents a total of 430,000 pounds of imidacloprid 

applied a year when considering both foliar/soil uses and seed treatment uses. There is uncertainty as to 

the levels of the total imidacloprid residues in pollen and nectar resulting from applications on soybean, 

which the largest usage of imidacloprid in terms of applied active ingredient per year. 

Although no data are currently available for pome fruits, a combination soil + foliar application study with 

apples is expected in 2016 and will be incorporated into the subsequent preliminary risk assessment for 

imidacloprid. While there are no foliar application studies available for members of the fruiting vegetables 

and berries/small fruits groups (Groups 8 and 13, respectively), data are available for soil applications and 

in the case of tomato, soil + foliar applications. Grapes represent a large use pattern (60,000 pounds per 

year), and while no data are available to characterize the residues of this crop, grapes do not produce 

nectar and are wind pollinated. 

Members of the tree nut group (Crop Group 14), of which several members are also wind pollinated, are 

noted to produce pollen and nectar (except in the case of hazelnuts and walnuts) that are attractive to 

bees. There is uncertainty as to the potential exposure of honey bees during bloom as well as the residues 

in pollen and nectar to members of this group which account for almost 30,000 pounds of imidacloprid 

applied per year according to the SLUA data. 

Finally, there are several uses of imidacloprid that are not associated with a crop group. In two of these 

cases, artichokes and tobacco, the crops are harvested before bloom so exposure to bees is expected to 

be minimal. Pomegranate is not represented in the USDA document and therefore its attractiveness is 

uncertain.

This discussion serves to demonstrate that while foliar application residue studies are only available for 

oranges, cherries, and cotton, several other use patterns are expected to have minimal exposure to bees 

either by being unattractive or harvested prior to bloom, that there are several use patterns including 

potatoes, legumes (soybean), and tree nuts that have uncertainty associated with them with respect to 

serving as sources of forage as well as the residue levels resulting from foliar applications. Table 6-47

shows the SLUA data for additional foliar use patterns with no available residue data. As the SLUA does 

not distinguish between foliar and soil use patterns, the uses below apply to foliar/soil-applied uses. It is 

noted that foliar data are available for oranges, tomatoes, cherries, and cotton and therefore these crops 

are not presented below as their SLUA data are presented above and elsewhere in the risk description 

section).
224

Table 6-47. SLUA data imidacloprid and use patterns registered for additional foliar and soil use patterns 

(2004-2013) with no available residue data.

Crop Group Name 

(Number) Use pattern Lbs. Applied/yr.

% 

Acreage 

Treated 

(average)

% Acreage 

Treated 

(max)

Honey Bee

Attractive? 

(Pollen or 

nectar) (Y/N)

Harvested 

Before Bloom? 

(Y/N)

Root & Tubers (1)

Potatoes 70,000 35 50 N --

Carrots 4,000 15 45 Y Y

Leafy Vegetables 

(4)

Chicory* <500 10 20 Y Y

Lettuce 40,000 65 85 Y Y

Spinach 2,000 25 40 N Y

Celery 1,000 10 20 Y Y

Brassica (Cole) (5)

Brussels 

Sprouts* <500 50 85 Y Y

Broccoli 10,000 65 90 Y Y

Cauliflower 5,000 60 90 Y Y

Cabbage 4,000 30 45 Y Y

Legumes (6)

Dry 

Beans/Peas <500 <1 <2.5 Y --

Peas, green <500 <2.5 <2.5 Y --

Soybeans 30,000 <2.5 <2.5 Y --

Beans, Green 3,000 5 10 Y --

Pome Fruit (11) Apples 10,000 30 45 Y N

Pome Fruit (11) Pears 1,000 5 15 Y --

Berry& Small Fruit 

(13) 

Caneberries

(blackberry 

and 

raspberry)

<500 15 25 Y --

Berry& Small Fruit 

(13) Grapes 60,000 30 50 Y (pollen) N

Berry& Small Fruit 

(13) Strawberries 2,000 5 15 Y --

Berry& Small Fruit 

(13) Blueberries 1,000 10 20 Y N

Tree Nuts (14) Hazelnuts <500 5 20 Y (pollen) --

Tree Nuts (14) Pecans 20,000 15 20 N --

Tree Nuts (14) Pistachios 3,000 5 15 N --

Tree Nuts (14) Walnuts 3,000 10 20 Y (pollen) --

Tree Nuts (14) Almonds 1,000 <2.5 <2.5 Y N

No Group Artichokes <500 15 60 Y Y

No Group Pluots* <500 20 65 -- --

No Group Sugarcane <500 <2.5 <2.5 N --

No Group Tobacco 10,000 25 40 Y (pollen) Y

No Group Pomegranat

es* 4,000 45 65 -- --

*Based on CDPR PUR data only (80% or more of total acreage is in California)
225

Summary of Crop Group/Use Patterns for which Foliar Residues Data are Available

Table 6-48 below summarizes the available residues studies for the foliar-applied method as well as a 

providing a range of the refined Tier I RQs, the percentage of nectar residues above the Tier II NOAEC 

threshold in nectar (25 ppb) and where available, the duration those residues exceed the NOAEC. 

Table 6-48. Summary of risk findings for the foliar applied use patterns of imidacloprid with available 

residue data.

Crop 

Group

(Crop)

Application 

Scenario1

Worst 

Case 

Scenario?

(Y/N)

Refined Tier I RQ Ranges3 (%age 

of Refined Tier I RQs above LOC 

using all residue data)4

Tier II5

Adult 

Acute 

Oral

Adult 

Chronic 

Oral

Larval 

Chronic 

Oral

%age of nectar 

residues above 

NOAEC

Duration above 

NOAEC

Citrus 

fruits

(Oranges)

2 X 0.25 lbs 

a.i/A, 10-day 

pre bloom

Y 4.8 - 32 

(100%)

89 - 592 

(100%)

14 - 30 

(100%) 93% 25 days after last application

Stone 

Fruits

(Cherry)

5 X 0.1 lbs 

a.i/A, post 

harvest

Y 2.8

(38%)

38 

(100%)

1.5

(7.6%) 0% N/A

Oilseed

(Cotton)

1 X 0.06 lbs 

a.i/A, at 

bloom

N 4.9

(100%)

102 

(100%)

4

(100%) 40%

6 days (one 

sampling 

interval only)

Bolded value represent RQ in exceedance of acute or chronic LOC (0.4 and 1.0, respectively). 1

Application rate, number of applications, timing

2

Based on whether rate represents maximum annual rate for a given use pattern 3

Based on highest reported residue concentration of all individual replicates (acute) or highest average concentration among all individual 

sampling events (chronic).

4

Based on all pollen and/or nectar data from all sampling intervals 5

Compared to colony feeding study NOAEC of 25 ppb.

Soil Applications

Crop Group 8 – Fruiting Vegetables (Tomato)

The fruiting vegetables crop group includes, among other members, eggplant, pepper (bell peppers, chili 

peppers, and sweet peppers), and tomatoes and various hybrids thereof. Specific to the uses of 

imidacloprid, tomato appears to be the dominant crop with an estimated usage of estimated 30,000 

lbs/year (Table 6-49). This is followed by peppers with roughly a third of the total usage as reported for 

tomato. According to USDA (2014), members of the fruiting vegetable crop group are largely unattractive 

to honey bees with the exception of okra and chilies, which are noted to have pollen and nectar that is 

attractive (approximately 2400 total acres of okra). The entirety of this group, however, is important for 

bumble bees. Bumble bees extract the pollen granules from the tomato anthers by a technique known 

as “buzz pollination,” in which the bumble bee worker grasps the anthers and rapidly move their wings to 

dislodge and shake out pollen. It is noted here that the tomato plant does not produce nectar. Eggplant, 

peppers, and tomatoes all are noted to require bee pollination but only tomato is listed to use managed 

pollinator resources, specifically bumble bees when grown indoors (e.g., glasshouses). 
226

As discussed previously, there are two studies that examine the residues of pollen in soil treated tomato. 

In one study (MRID 49090503), lower application rates were investigated while in a subsequent study 

(MRID 49665201), the maximum single rate of 0.38 lbs a.i/A to soil-treated fruiting vegetables was 

assessed. Additionally, the field sampling component of the first study was not conducted under GLP. 

Finally, the latter study reported higher residue levels in pollen, as expected from the higher application 

rate, and therefore the residues of this study were used for refining the Tier I risk estimation as well as for 

evaluating the frequency and duration of LOC exceedances using each sampled pollen measurement. 

Table 6-49. SLUA data for imidacloprid and fruiting vegetables (2004-2013)

Crop Lbs. Applied/yr. % Acreage Treated (average) % Acreage Treated (maximum)

Tomatoes 30,000 30 60

Peppers 9,000 35 50

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from the soil 

application on tomatoes range from <0.01 – 0.60 (adult acute), <0.01 - 12 (adult chronic), 0.19 – 0.39 

(larval chronic). These RQ values reflect “high-end” estimates of bumble bee-collected pollen residues 

obtained from soil applications at the maximum label rate (1 X 0.38 lbs a.i/A) for tomatoes (MRID 

49665201). The highest RQs for adults were determined for nurse bees that have a significant portion of 

their diet as pollen. Figure 6-10 below shows refined Tier I RQ values in relation the LOC for all available 

pollen data.
227

Figure 6-10. Summary of acute and chronic RQ values using totality of bee-collected pollen residue data 

from soil-applied tomato residue study (MRID 49665201).

A total of 22 RQs were estimated from pollen residue data for each life stage/duration. Figure 6-10 above 

shows adult acute RQs generally below the acute risk LOC line of 0.4 with the exception of a few residue 

samples that yielded RQs marginally above. Adult chronic oral RQs in contrast are generally above the 

chronic risk LOC of 1 with the exception of roughly 20% of the estimated values. There were 2/22 (9%), 

17/22 (77%), and 0/22 (0%) of estimated adult acute oral, adult chronic oral, and larval chronic oral RQs 

that exceeded their respective LOCs, respectively, using the totality of the residue data. As indicated 

previously, since there were no nectar data available, comparisons cannot be made to the colony feeding 

study effects level. Pollen residue levels were relatively consistent (acute EEC: 242 ppb, chronic EEC: 198 

ppb) during the course of the study which is also indicated by the pattern of the estimated RQs, specifically 

with adult chronic RQs estimated to be above the chronic risk LOC from 32 days to 76 days after 

application.
228

Tier II Risk 

As no nectar data are available (the tomato plant does not produce nectar), comparisons of residues to 

the NOAEC and LOAEC of the colony feeding study cannot be made.

Figure 6-11 shows the average residues in pollen from the soil-applied tomato study. While there is some 

exceptions, the higher residues are generally associated with the coarse soils and lowest with the finer 

soils. A subset of the residues (originating from coarse, medium, and fine soil types) are to be above 100 

ppb which is associated with colony level effects following a 12-week exposure via pollen (Dively 2015, 

Qualitative). 

Figure 6-11. Imidacloprid average residues in pollen from soil-applied tomato study (MRID 49665201).

Additional Considerations

At each sampling interval, bumble bee colonies were placed in bee tunnels (100 to 210 feet long and 20 

to 40 feet wide, and enclosed four to eight rows of tomato plants) at each site. The bees were allowed to 

forage from the tomato flowers for several days after which the bees were collected and pollen was 

removed from pollen baskets (full description of methods provided in Appendix E).
229

There were 9 field sites used for this study all of which were located in California; sites located on the 

central coast and in the Central Valley with 5 fields characterized as coarse soil type, 1 with medium, and 

3 as fine. The maximum distance between the sites was approximately 180 miles. Average residue values 

were variable within and across all sites and soil types and precluded the ability to state whether a given 

magnitude of residue values was associated with a particular soil type. For some sites, there were two 

sampling events within one trial that generally indicated that pollen residues were declining between the 

roughly two week period that residues were sampled although two sites (one coarse soil site and one fine 

soil site) had residues that increased between the sampling periods.

As noted previously, tomatoes do not produce nectar, but their pollen is considered attractive, particularly 

to bumble bees. As discussed previously, the pollen route of exposure will be discussed later in the risk 

description. There are several soil application methods that are allowed on the label and the submitted 

study tests the highest single maximum application rate (0.38 lbs a.i/A) as a 7-day post-transplant soil 

drench which represents a permitted labeled scenario.

Conclusions

Daily average residues in pollen (no nectar produced by tomato) resulting from a soil application (posttransplant) to tomatoes exceeded 100 ppb which has been associated with colony level effects following 

a 12-week exposure to spiked pollen (Dively 2015, Qualitative).Although tomatoes are considered 

indeterminate bloomers, the findings of a potential colony level risk from the pollen route of exposure 

has limited relevance to honey bees since the fruiting vegetable group, including tomatoes, are not 

classified as honey bee attractive with the exception of okra, which produces both nectar and pollen and 

chilies which produce pollen that is attractive to honey bees. Therefore, from an Apis perspective, there 

is limited on-field exposure anticipated for tomato and the rest of the fruiting vegetable group except 

okra. However, member of this group, including tomato, are frequented by bumble bees, where managed

pollination services are utilized for some members including tomatoes and sweet peppers within 

greenhouses. 

Crop Group 9 – Cucurbit Vegetables (Melon)

The cucurbit vegetables crop group includes, among other members, cucumbers, muskmelon (inclusive 

of cantaloupe, honeydew and others) pumpkin, squash, and watermelons. Specific to the uses of 

imidacloprid, cantaloupes and watermelons are the dominant crops with an estimated usage of estimated 

9,000 lbs/year each and an average of 40% of all cantaloupe nationwide treated with imidacloprid (Table 

6-50). This is followed by cucumbers, honeydew, pumpkin, and squash with total poundage of roughly 

one third of that for cantaloupe and watermelon. According to USDA (2014) all members represented in 

the SLUA produce pollen and nectar noted to be honey bee attractive as well as requiring bee pollination 

using managed pollination services. Cucurbit vegetable crops are also associated with blooming duration 

of at least 6 weeks.
230

Table 6-50. SLUA data for imidacloprid and cucurbit vegetables (2004-2013)

Crop Lbs. Applied/yr. % Acreage Treated (average) % Acreage Treated (maximum)

Cantaloupes 9,000 40 60

Cucumbers 3,000 10 20

Honeydews 2,000 30 50

Pumpkins 2,000 10 20

Squash 2,000 15 30

Watermelons 9,000 25 45

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from soil applications 

to melons, range from <0.01 – 0.60 (adult acute), 1.1 – 8.9 (adult chronic), 0.18 – 0.39 (larval chronic). 

These RQ values reflect “high-end” estimates of pollen and nectar residues obtained from a single soil 

application that ranged from 0.23 – 0.38 lbs a.i/A (representing 61 – 100% of the maximum single 

application rate) for melons at transplant (MRID 49090501). The highest RQs for adults were for nectar 

foragers. Figure 6-12 below shows refined Tier I RQ values in relation the LOC for all the totality of 

matched pollen and nectar data.

A total of 10 RQs were estimated from all available pollen and nectar data for each life stage and duration 

(all from the same sampling interval of 100 days after application). Six of 10 (60%) of adult acute oral RQ 

values are below the acute risk LOC of 0.4. All (100%) of the adult chronic oral RQs are above the chronic 

risk LOC of 1 while 100% of the larval chronic oral RQs were below the chronic risk LOC. 
231

Figure 6-12. Summary of acute and chronic RQ values using totality of pollen residue data from soil melon 

residue study (MRID 49090501).

Tier II Risks

To evaluate the risk of soil application of imidacloprid to melons at the colony level for honey bees, daily 

average residues of total imidacloprid concentrations in nectar were compared to the aforementioned 

NOAEC and LOAEC values from the available registrant-submitted colony feeding study (Figure 6-10). As 

indicated previously, these residues values present only a single sampling period 100 days after 

application. The residue samples originate from soils characterized as medium and heavy (Figure 6-10). 

While the refined Tier I analysis indicated several adult acute oral RQs and all adult chronic oral RQs 

exceeded their respective LOCs at the honey bee individual level, all (100%) of the average residue values 

of available nectar data are below the colony feeding study NOAEC value of 25 ppb. As noted previously, 

cucurbit vegetable crops are associated with at least 6-week blooming periods, which matches the 

exposure duration of the colony feeding study. Although the highest average nectar residue value 

determined for this study would have to be approximately 5-fold higher to reach the NOAEC and 

approximately 10-fold higher to reach the LOAEC, the magnitude of these residues in coarser soils is 
232

uncertain. Based on the residue studies of soil applications to other crops (e.g. blueberry), residues in 

pollen and nectar associated with coarser soils may exceed those in medium or fine soils by up to an order 

of magnitude. 

Figure 6-13. Imidacloprid residues in nectar in soil-applied melon residue study (MRID 49090501) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Figure 6-14 below shows the daily average residues in pollen from the foliar-applied melon study. A 

subset of these residues are noted to be above 100 ppb which is associated with colony level effects 

following a 12- week exposure via pollen (Dively 2015, Qualitative). As noted above with nectar residues, 

it is an uncertainty the magnitude of residues in pollen had soil applications been made to melon in 

coarser soils.
233

Figure 6-14. Imidacloprid average residues in pollen from soil-applied melon study (MRID 49090501).

Additional Considerations

All ten sites employed for soil-applied melon study were located in one county in Southern California and 

were noted to have similar climatic conditions. As mentioned previously, application rates for the ten 

sites ranged from 0.23 – 0.38 lbs a.i/A on medium and heavy soils. The study report contained information 

gaps for 5 trials in the association of application rates with specific sites and soils. For the 5 trials with soil 

and application rate information, the data indicate a similar magnitude of residues in nectar and pollen 

from medium and fine soils. For the other 5 trials this information is not provided and is therefore an 

uncertainty. Therefore, the geographic/climatic representation of this study is limited. The extent to 

which prior year applications of imidacloprid contributed to year-to-year carryover was not assessed in 

this study. Finally, it is unclear the timing of the application relative to the bloom period and whether the 

scenario employed in this study represents one where the maximum residues in pollen and nectar would 

be realized.

Additionally, there are two studies available from the open literature that investigate the residues of 

imidacloprid in pollen and nectar from soil-applications to cucurbit vegetables. In a study by Stoner and 

Eitzer (2012, MRID 49719616), imidacloprid (as Admire® Pro) was applied as a soil spray pre-planting at 
234

0.32 lbs a.i/A 1 day before squash was planted. In a subsequent trial, the same application rate was used 

at a 5 day post-transplant via drip irrigation in a greenhouse. The residue values in pollen and nectar were 

pooled together from the various trials and no interval between application and sampling was available. 

Maximum and average values for pollen were 28 and 14 ppb, respectively, and were 14 and 10 ppb, 

respectively, for nectar. It is noted that the application rate is slightly below the maximum labeled single 

application rate 0.38 lbs a.i/A

In a study by Dively and Kamel (2012, MRID 49719612), several trials were conducted with various 

treatment regimen to soil treated pumpkins. Applications rates of Admire® Pro included bedding drench 

applications (0.027 lbs a.i/A, transplant water treatment (0.25 lbs a.i/A and 0.38 lbs a.i/A), and split 

application of 0.19 lbs a.i/A first as transplant water, then as drip irrigation (See Appendix B for further 

details). Average pollen residue levels ranged 4.9– 80.2 ppb across all application methods and rates. 

Average nectar values ranged from 0.4 – 11.2 ppb, with the maximum nectar residue value was 13.7 ppb). 

All treatment regimens utilized are permitted on the label for soil-applied cucurbit vegetables. While 

average pollen residues were noted to be as high as 80 ppb in this study, this level is still under the 

threshold of where overwintering survival effects were noted from the available 12-week spiked pollen 

colony feeding study (Dively 2015, Qualitative). Soil type information was not available from this study. 

Finally, the registrant-submitted study discussed above (MRID 49090501) assessed a range of imidacloprid 

application rates representing 61 – 100% of the maximum application rate cited on the label. Applications 

across all sites were either made by soil drip or seed-line drench at transplant which are methods 

consistent with labeled applications.

Conclusions

While the average residues in nectar and pollen were both below the levels associated with colony level 

effects (25 ppb in nectar, 100 ppb in pollen), the residue data are associated with several uncertainties. 

These include a subset of the residues originating from less than the maximum application rate, unknown 

timing of application relative to bloom, and no residue information on coarse soils, which are indicated to 

potentially lead to residues in pollen and nectar up to an order of magnitude higher than those in medium 

and fine soils. The latter lack of residue data from coarse soils is significant because other residue studies 

suggest residues may be an order of magnitude greater medium and fine soils relative to those in coarse 

soils. Therefore, the on-field colony level risk to honey bees resulting from soil applications to cucurbits 

is considered uncertain. A full field Tier III study for pumpkins is expected in 2016 to further refine this 

determination as well as the potential to bridge to forthcoming soil-applied cucurbit vegetable studies for 

other neonicotinoid pesticides.

Crop Group 10 – Citrus Fruits (Orange/Grapefruit)

The usage and attractiveness of the citrus crop group was previously discussed in the foliar application 

uses section of risk description. The soil applied citrus study investigated the total residues of imidacloprid 

in pollen and nectar of multiple members of the group including orange, tangerine, and grapefruit. All 

trials utilized a post-bloom soil drench at the maximum single application rate for soil applications on 
235

citrus (0.5 lbs a.i/A) with the exception of trials on oranges and tangerines which tested 0.25 lbs a.i./A. 

Because these trials tested only half the maximum labelled application rate for soil treatment to citrus,

they were not included in the refined Tier I and Tier II analysis presented below. Additionally, pollen was 

not sampled in the remaining trials, which, as an important component of the diet for several castes of

individuals such as nurse bees. Therefore, the refined Tier I RQs are considered an underestimation of 

the potential level of risk. 

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis 

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from the soil 

application on citrus, range from 0.26 – 2.1 (adult acute), 4.3 -43 (adult chronic), 0.78 – 1.7 (larval chronic). 

The highest RQs for adults were nectar foragers. 

Figure 6-15 below shows refined Tier I RQ values in relation the LOC for all the totality of and nectar data 

(excluding the trial conducted with 0.25 lbs a.i/A). The 3 trials (3 tunnel sites and 6 field sites) assessed 

the residues in oranges and grapefruit at a maximum single application on citrus fruits at 0.5 lbs a.i/A. 

These trials are separated by lines inserted on the figure below with one trial comprised of three tunnel 

sites (3 mean residue samples), one trial with 2 fields (2 mean residue samples), and a third trial with 4 

fields (4 mean residue samples). It is noted that in the tunnel trials, there were 3 trees (3 samples per 

tree) within each tunnel with one average sample from those three trees to yield an average from that 

tunnel. The soil type for this trial was characterized as loam (medium). In the second trial (field sites in 

LREC and Bakersfield), there were ten trees per field (3 samples per tree) with average samples from each 

field obtained. The soil type in this trial was also characterized as loamy (medium). In the third field trial, 

there were 10 trees in the Hemet site (1 sample per tree), 5 trees at the LREC site (2 samples per tree), 1 

orange tree at the Temecula site (2 samples), and 6 grapefruit trees in Temecula site (2 samples per tree). 

The Hemet and Temecula sites were characterized as a sandy loam and the LREC site as loamy. It is also 

noted that while applications in this study were made post-bloom, that certain labels do not restrict prebloom and during bloom soil applications to citrus fruits, and therefore the residues in nectar from this 

study are likely underestimated.

A total of 9 RQs were estimated for honey bee life stage and duration based on the three included trials 

from the available nectar. There were 8 of 9 (89%) of adult acute oral RQs above the acute risk LOC of 0.4 

across all sites except one sample from the Temecula site with grapefruit. All (100%) of adult chronic oral 

RQs exceeded the chronic risk LOC for all sites. Finally, 4 of 9 (44%) of larval chronic oral RQs exceeded 

the chronic risk LOC across all trials.
236

Figure 6-15. Summary of acute and chronic RQ values using totality of nectar residue data from the soilapplied citrus residue study (MRID 49090505).

Tier II Risks

To evaluate the risk of soil application of imidacloprid to citrus at the colony level for honey bees, reported 

residues of total imidacloprid concentrations in nectar were compared to the aforementioned NOAEC and 

LOAEC values from the available registrant-submitted colony feeding study (Figure 6-16). There were no 

average residue values (0%) in nectar across all trials that exceeded the NOAEC level of 25 ppb; however, 

there is wide variability for several sites likely the result of small numbers of samples informing the 

average residue values for most sites. Despite this variability, the average residue values were relatively

similar for sites with the same application rate of 0.5 lbs a.i/A across all trials (noting some lower values 

in the LREC and Temecula sites) that represent tunnel and field locations on two different members of the 

citrus fruits group. Although the data are not shown below, the field trial conducted with the lower rate 

of 0.25 lbs a.i/A had a maximum nectar residue value of 18.3 ppb that is roughly 50% of the maximum 

nectar residue values noted in tunnel, LREC/Bakersfield field site, and the Hemet/LREC/Temecula field 

sites which were 34.6, 29.1, and 35.5 ppb, respectively. As mentioned previously it was not known for all 

trials the sampling interval and how far after the application nectar samples were obtained.
237

As previously indicated, no pollen residue data were available from the trials that assessed the highest 

application rates for the soil-applied citrus study. The Hemet site was characterized as having a sandy 

loam, while for the Tunnel, LREC, and Bakersfield sites, the soils were characterized as loamy (no soil type 

information provided for the Temecula site) and from Figure 6-16 below, no inference can be made about 

soil type having an effect on the magnitude of residues in nectar, with the exception of one sample from 

the Temecula site.

Figure 6-16. Imidacloprid residues in nectar in soil-applied citrus residue study (MRID 49090505) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Additional Considerations

In the citrus soil residue study, nectar was either sampled directly from the flower or from stores collected 

within the hive, depending on the trial. Nectar samples were obtained from two locations (citrus blocks 

in the Temecula region and at LREC) where the 1X soil application rate of imidacloprid had been made in 

two successive years (2008, 2009) prior to sampling in April 2010. Residue levels at these 11 sites averaged 

8 ppb and ranged from 1 to 18 ppb. 
238

The six locations for the citrus trials were in relatively close proximity. Soil types reflect sandy loam, loam 

or clay compositions (20-40% clay) and low organic carbon content (0.35-1.9%). Weather conditions 

(temperature and precipitation) were similar across the three trials. As a result of the close proximity of 

trial sites, this study provides very limited information on how differences in environmental conditions 

across different areas of the U.S. may affect accumulation of total imidacloprid in pollen and nectar.

The authors speculated that imidacloprid residues at the Hemet site appear to be a function of the rate 

applied at the most recent application only, with no evidence of carryover from previous years. However, 

following the third year of application at the Hemet site, residues were higher at the two sites receiving 

no treatment in 2010 than at the site treated all three years with 1X. This indicates some degree of 

carryover from previous application years, at least for sites treated with the 2X rate during one of the two 

years prior to the no treatment year. This was the only site where samples were collected following a year 

without treatment. 

Conclusions

While the average residues in citrus nectar are just below the threshold indicated to present colony level 

risk in nectar (25 ppb) the residue data are associated with some uncertainties. The primary uncertainty 

relates to the lack of residue information originating from coarse soils which are indicated to potentially 

lead to residues in pollen and nectar up to an order of magnitude higher than those in medium and fine 

soils. Additionally, while the available citrus study employed a post-bloom application, current labels for 

soil applications to citrus fruits do not restrict applications being made pre-bloom or during the bloom 

period. The magnitude of residues in nectar and pollen may have been higher had applications been made 

closer to the bloom period. Therefore, the on-field colony level risk to honey bees resulting from soil 

applications to citrus fruits is considered uncertain. The potential to bridge to forthcoming soil-applied 

cucurbit vegetable studies for other neonicotinoid pesticides will be evaluated for the final risk 

assessment.

Crop Group 13 – Berries/Small fruits (Blueberry and Strawberry)

The berries crop group includes, among other members, blackberry, blueberry, and raspberry. This crop 

group also includes group 13-07 (small fruit and berries group), which itself encompasses 8 subgroups 

that contain other crops such as strawberry, cranberry, and grape. Specific to the uses of imidacloprid, 

grapes are the dominant crop with an estimated usage of estimated 60,000 lbs/year (Table 6-51). This is 

followed by strawberries and blueberries to a far lesser extent. According to USDA (2014), blueberries, 

blackberries, and raspberries require bee pollination blueberries uses managed sources of pollination. 

Although, bee pollination of strawberry is not considered essential, it may be used to compliment wind 

pollination. Similarly, grapes are wind pollinated and therefore do not require honey bee pollination. 

Additionally, grape do not produce nectar, although their pollen is noted to be attractive according to 

USDA (2014). 
239

Table 6-51. SLUA data imidacloprid and berries/small fruit (2004-2013)

Crop Lbs. 

Applied/yr.

% Acreage Treated 

(average)

% Acreage Treated 

(maximum)

Caneberries (includes blackberry 

and raspberry) <500 15 25

Grapes 60,000 30 50

Strawberries 2,000 5 15

Blueberries 1,000 10 20

Residue studies are available for soil-treated blueberry and strawberry and will be discussed separately 

below.

Blueberry

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from soil application 

to blueberries, range from 0.14 – 1.2 (adult acute), 2.4 - 16 (adult chronic), 0.30 – 0.66 (larval chronic). 

These RQ values reflect “high-end” estimates of bee-collected pollen and hive nectar residues obtained 

from a soil application at the maximum label rate (1 X 0.50 lbs a.i/A) post-bloom for blueberries (MRID 

49665201). The highest RQs for adults were for nurse bees that have a significant portion of pollen as 

their diet. Figure 6-17 below shows refined Tier I RQ values in relation to the LOC for all matched pollen 

and nectar data.
240

Figure 6-17. Summary of acute and chronic RQ values using totality of the bee collected-pollen and hive 

nectar residue data from soil-applied blueberry residue study (MRID 49535602).

A total of 17 matched data for pollen and nectar residues were available to estimate acute and chronic 

RQs for honey bee adults and larvae. There were 2 /17 (12%) of the adult acute oral RQs that exceeded

acute risk LOC of 0.4 while 0/17 (0%) of the larval chronic oral RQs exceeded the chronic risk LOC of 1. 

This is distinguished from adult chronic oral RQ values where all (100%) RQs exceeded the chronic risk 

LOC. The highest acute and chronic RQs for adults were associated with nectar foragers.

Tier II Risks

To evaluate the risk of soil application of imidacloprid to blueberries at the colony level for honey bees, 

reported residues of total imidacloprid concentrations in nectar were compared to the aforementioned 

NOAEC and LOAEC values from the available registrant-submitted colony feeding study (Figure 6-14). As 

there was no obvious carry over from year-to-year evident in the data, the average nectar residue values 

were plotted by soil type. While the data are limited, the figure below shows overall higher residue values 

for sandy soils as compared to loam and silty loam soil types, with sandy soils having mean residues almost 

10-fold higher than silty loam soils. None of the daily average residues values in nectar (0%) exceeded the 

NOAEC value of the colony feeding study of 25 ppb.
241

Figure 6-18. Average total residues in nectar from soil-applied blueberry study (MRID 49535602) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Figure 6-19 below shows the average residues in pollen from the foliar-applied melon study. All residues 

were noted to be below 100 ppb which is associated with decreased overwintering survival following a 

12-week exposure to imidacloprid spiked pollen (Dively 2015, Qualitative). 
242

Figure 6-19. Imidacloprid average residues in pollen from soil-applied blueberry study (MRID 49535602).

Additional Considerations

In the blueberry residue study, within each trial and year, the duration of sampling times were relatively 

short, ranging from approximately 3 days to 20 days which apparently reflects the blooming period for 

the blueberry varieties tested. With hive-collected nectar, there appears to be a slight increase in mean 

residues of total imidacloprid residues over the sampling time. With pollen, the temporal trend in mean 

residues appears to be variable or stable over the sampling period, depending on the trial. Furthermore, 

the overall range in reported residues of total imidacloprid residues in pollen is substantially greater than 

that observed with nectar. This may reflect differences in collection methods between the two matrices 

(hive-collected for nectar, bee-collected for pollen).

The geographic coverage among the three trials was relatively broad (northwestern NY, northern IL, and 

southwestern MI) although they were in the northern portion of the US. A range of soil types are 

represented (loam, silty loam and sand) with sandy soils generally associated with the highest average 

residue as indicated by Figures 6-18 and 6-19 above.
243

A qualitative evaluation of the potential for year-to-year accumulation of total imidacloprid residues in 

nectar and pollen could only be conducted for 1 of the 3 trials (NT003); since a majority of the mean 

residue data for trial NT001 and NT002 were below levels of detection or quantification. The data indicate 

the mean (based on those residues >LOQ) of total imidacloprid residues in nectar from Trial NT003 

decreased in year 2 relative to year 1 (7.25 ppb vs 1.8 ppb) while those in pollen remained essentially the 

same (13.7 vs 14.0 ppb). Thus, there is little evidence of year-to-year carryover of residues in bee-relevant 

matrices, despite evidence of increases in soil concentrations from the end of the first to the second 

growing season from two of the trials (soil: 151 to 233 ppb in NT001; 151 to 339 ppb in trial NT002).

Finally, ccurrent label language for soil applications to blueberry specify that application may be made 

post-bloom up to 7 days prior to harvest, or post-harvest until October 1st. Additionally, applications are 

not permitted pre-bloom, during bloom, or when bees are foraging. Therefore, the residue study was 

conducted in accordance with label restrictions. 

Conclusions

Average residues in nectar were below threshold that is associated with colony level risk (25 ppb in 

nectar). Furthermore, residues in pollen were below levels associated with reduced overwintering success 

following a 12-week exposure (Dively 2015, Qualitative). The available blueberry study was conducted 

according to current labels, and while the data suggest average residues in coarser soils are higher than 

those in medium and fine soils, all residues were below colony level risk thresholds in their respective 

matrices. Therefore, post-bloom soil applications to blueberries are determined to present a low on-field 

risk to honey bees at the colony level. 

Strawberry

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, refined Tier I oral RQ values for honey bees range from <0.01 – 0.79 (adult 

acute), <0.1 - 17 (adult chronic), 0.28 – 0.55 (larval chronic). These RQ values reflect “high-end” estimates 

of pollen residues obtained from a single soil application at the maximum label rate (1 X 0.50 lbs a.i/A) for 

strawberries (MRID 49090502). The highest acute and chronic RQs for adults were determined for nurse 

bees that have the highest portion of pollen as their diet among adult. Figure 6-20 below shows refined 

Tier I RQ values in relation to the LOC for all available pollen residue data. It is noted that no nectar data 

are available from this study despite the fact that strawberries produce nectar that is attractive to honey 

bees. Consequently, the RQ values may underestimate risk for the refined Tier I analysis as no nectar 

component is included in the exposure estimates as well as precludes a Tier II analysis to characterize the 

risks of soil applications to strawberries at the colony level. 
244

Figure 6-20. Summary of acute and chronic RQ values using totality of pollen residue data from soil –

applied strawberry residue study (MRID 49090502). (Note: Sampling interval not provided).

A total of 7 RQs were estimated using the totality of pollen residue data for each life stage and duration. 

It is noted that the sampling interval was not available from this study and therefore it is unknown how 

long after application the sampling was conducted, although the samples were stated to have been 

collected during bloom. Additionally, the timing of the application in relation the bloom period is not 

known for this study and therefore there is no x-axis label to indicate the days after application. There 

were 2 of 7 (29%) and 3 of 7 (43%) of adult acute oral and adult chronic oral RQs, respectively, that 

exceeded their respective LOCs. No larval chronic oral RQs (0%) exceeded the chronic LOC. Again, the 

results of this refined Tier I analysis are uncertain given that nectar data would be expected to raise EECs 

and the RQs, and that no sampling interval (other than “during bloom”) was available to ascertain the 

potential behavior of residues over time.

Tier II Risks

As stated previously, as no nectar data are available (despite the strawberry crop producing nectar that is 

attractive to honey bees), comparisons of residues to the NOAEC and LOAEC of the colony feeding study 

were not made. 
245

The available pollen residue data shown below in Figure 6-21 indicate that a subset of the average values 

are above 100 ppb which is associated with reduced overwintering survival following a 12 week exposure 

to spiked pollen (Dively 2015, Qualitative). Additionally, Figure 6-21 indicatesthat the residues associated 

with lighter (sandy) soils have higher average residues as compared to medium (loamy) soil, where 

average residues were approximately an order of magnitude lower. Strawberries are associated with a 

long (i.e. several month) blooming period. 

Figure 6-21. Imidacloprid average residues in pollen from soil-applied strawberry study (MRID 49090502).

Additional Considerations

As indicated previously, there was one sampling event for this study, of an unknown interval in relation 

to time of application, and therefore it is not possible to assess residues over time. All seven sites 

employed for this study were located in a single location in California with the same weather conditions 

(temperature and precipitation) across all sites. As a result of the close proximity of the trial sites, this 

study provides no information on how differences in environmental conditions across different 

production areas of strawberry of the US may affect accumulation of total imidacloprid residues in beerelevant matrices. As all samples were taken at one sampling interval during one year, the potential 

carryover of imidacloprid cannot be assessed.

0

1

10

100

1000

Concentration in Pollen (ppb)

Sand

Loam
246

It is also noted that this study tested 3 coarse (i.e. sandy) soils and 4 medium (i.e. loam) textured soils. 

The 3 sets of highest RQs are from trials conducted on sandy soils. Average residues in pollen ranged from 

87 – 279 ppb in the sandy soil sites and 4.8 – 8.6 ppb in the loamy sites, indicting residues in pollen in the 

sandy sites were one to two orders of magnitude higher than those in loamy sites. It is also noted that 

the LOD and LOQ in pollen for this study were 2.6 and 10 ppb, respectively, meaning that all average 

residues values from the loamy sites were between the LOD and LOQ, rendering these values less certain 

in their estimates.

This study was conducted at the maximum single application rate for soil-applied imidacloprid to 

strawberries (0.50 lbs a.i/A). Labels allow for applications to strawberry at this rate (0.50 lbs a.i/A) when 

applied to annual and perennial varieties where pest pressure may occur later in crop development. 

Alternatively, a maximum single rate of 0.38 lbs a.i/A is permitted for perennial varieties as a post-harvest 

application. Either one method or the other is permitted. As the available study tested the higher rate, 

it is assumed that the application was made post-bloom as the label states to not apply immediately prior 

to bud opening, during bloom, or when bees are foraging. 

Conclusions

The daily average residues in nectar were not available from this study, although honey bees are attracted 

to honey bee nectar. Daily average residues in pollen that originated from coarse soils were above 100 

ppb which is associated with reduced overwintering success following a 12 week exposure to spiked pollen 

(Dively 2015, Qualitative). Additionally, it is an uncertainty of the timing of this application relative to the 

bloom period. Therefore, soil application to strawberries present an uncertain colony level for honey 

bees. While no further soil application studies are expected for strawberries for other neonicotinoid 

chemicals, there is the potential to bridge to data for foliar applications to strawberriesthat is forthcoming 

for other chemicals. 

Crop Group 20 – Oilseed (Cotton)

The usage and attractiveness of cotton was previously discussed in the foliar application uses section of 

risk description. The soil-applied cotton study is one component of a combined soil + foliar application 

study to be later described in the combined application method section (MRID 49665202). The soil 

component of the study made an “at plant” application at the maximum single rate for soil-applied 

imidacloprid on cotton at 0.33 lbs a.i/A. Residues from pollen, nectar and extra-floral nectar were 

sampled but as the floral nectar residues were approximately 3-fold higher than extra-floral data, these 

values were used for the analysis below.

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1,the refined Tier I oral RQ values for honey bees resulting from a soil application 

to cotton, range from 0.97 – 9.5 (adult acute), 16 - 152 (adult chronic), 2.8 – 6.0 (larval chronic). These 

RQ values reflect “high-end” estimates of pollen and nectar residues obtained from a soil application at 

the maximum label rate (1 X 0.33 lbs a.i/A) for cotton at planting (MRID 49665202). 
247

Figure 6-22 Summary of acute and chronic RQ values using totality of pollen and nectar residue data 

from soil-applied cotton residue study (MRID 49665202).

A total of 15 RQs were estimated from matched pollen and floral nectar data for each life stage and 

duration. Figure 6-22 above indicates that 10 of 15 (67%) of the adult acute oral RQs values exceed the 

acute risk LOC of 0.4. All (100%) of the estimated adult chronic oral RQs and 7/15 (47%) of the larval 

chronic oral RQs exceed the chronic risk LOC of 1, respectively. 

Tier II Risks

To evaluate the risk of soil application of imidacloprid to cotton at the colony level for honey bees, daily 

average residues of total imidacloprid concentrations in nectar were compared to the aforementioned 

NOAEC and LOAEC values from the available registrant-submitted colony feeding study (Figure 6-23). As 

data were available for one year, the average nectar residue values were plotted by soil type. While the 

data are limited, the figure below shows overall higher residue values for sandy soils (loamy sand) as 

compared to loam, sandy loam, and clay soil types. 
248

There were 5 of 15 (33%) average residue samples (all soils combined) that exceeded the NOAEC of 25 

ppb and 3 of 15 (20%) samples that exceeded the LOAEC of 50. Ten of fifteen (67%) of the average residue 

samples were below the NOAEC of 25 ppb (range 1.65 – 19.35 ppb). It is noted that the NOAEC and LOAEC 

exceedances are associated with sandy loam and loamy sand soil types, which are the coarsest of those 

included in this study.

Figure 6-23. Imidacloprid residues in nectar in the soil-applied cotton study (MRID 49665202) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

The available pollen residue data shown below in Figure 6-24 indicate that all residues were below 100 

ppb, which is associated with reduced overwintering success following a 12-week exposure in pollen 

(Dively 2015, Qualitative). 
249

Figure 6-24. Imidacloprid average residues in pollen in the soil-applied cotton study (MRID 49665202).

Additional Considerations

The soil-applied cotton study also assessed 3 “at bloom” foliar application at the maximum single 

application rate of 0.06 lbs a.i/A. The combined magnitude of the soil residues with the foliar component 

will be discussed in the combined application method section. As the soil component was associated with 

one sampling event, the behavior of the residues in nectar over time could not be determined. Samples 

were collected four to five days before the first foliar application. There were nine sites employed for this 

study that were all located in California’s Central Valley. It was not reported whether the treatment fields 

had prior applications of imidacloprid. 

Current label language for soil applications to cotton specify that application may at planting at a 

maximum single application rate of 0.33 lbs a.i/A with no specific restrictions to honey bees or other 

pollinators. There are no other registered use patterns of the oilseed crop group for soil applications.

Conclusions

Average residues in nectar and pollen resulting from a single soil application indicate a colony level risk 

based on residues in nectar being above the Tier II NOAEC in nectar (25 ppb) and residues in pollen were 
250

below 100 ppb which is associated with reduced overwintering survival following a 12-week exposure 

(Dively 2015, Qualitative). As indicated with the conclusions for cotton associated with foliar applications 

discussed above, a full field Tier III study is expected in 2016 to potentially further refine this 

determination.

Additional Soil Application Use Patterns

In addition to the use patterns presented above in the foliar use pattern section (which are also registered 

as soil applications), there are several other use patterns registered for imidacloprid for soil only. These 

uses include other members of root and tuber vegetables (crop group 1) as well as crop groups 3 and 9 

(bulb and cucurbit vegetables, respectively). Specific to the data provided by the SLUA, Table 6-52 below 

summarizes these uses in a manner similar to that presented above for the foliar (and soil) uses.

The root vegetables carrot and sugar beet (the latter registered only in California according to the Admire®

Pro label) produce pollen and nectar that are attractive to honey bees but are also harvested before bloom 

unless used for seed production. This is also the case for members of the bulb vegetables group and 

therefore honey bee exposure to these crops is expected to be minimal. 

The cucurbit vegetable group (crop group 9) produces pollen and nectar that are considered to be 

attractive to honey bees, and accounts for approximately 25,000 pounds per year of imidacloprid usage 

according to the SLUA. Residue data are available for melons for this group as presented above although 

it is an uncertainty as to what extent the residues in this crop are representative for other members of 

the group. 

It is noted that the use patterns associated with uncertainty including potatoes, legumes (soybean), and 

tree nuts that were noted in the foliar use section in terms of magnitude of residues in pollen and nectar 

are noted here as well with respect to soil applications.

Table 6-52. SLUA data for imidacloprid on soil only registered use patterns (2004-2013). (Note: Data are 

available for soil treated cucurbit vegetables and therefore is not presented below since its SLUA data is 

shown in the soil treated melon discussion

Crop Group Name 

(Number) Use pattern

Lbs. 

Applied/yr

.

% Acreage 

Treated 

(average)

% Acreage 

Treated 

(max)

Honey Bee 

Attractive? 

(Pollen or 

nectar) (Y/N)

Harvested Before 

Bloom? (Y/N)

Root & Tubers (1)

Carrots 4,000 15 45 Y Y 

Sugar Beets* 2,000 <2.5 5 Y (nectar) Y

Bulb Vegetables 

(3) Garlic <500 5 5 Y Y

Bulb Vegetables 

(3) Onions 1,000 <2.5 5 Y Y

*For use in California only according to Admire® Pro label (EPA Reg. No. 264-827)
251

Summary of Crop Group/Use Patterns for which Soil Residue Data are Available

Table 6-53 below summarizes the available residues studies for the soil-applied method as well as a 

providing a range of the refined Tier I RQs, the percentage of nectar residues above the Tier II NOAEC 

threshold in nectar (25 ppb) and where available, the duration those residues exceed the NOAEC. 

Table 6-53. Summary of risk findings for the soil applied use patterns of imidacloprid with available 

residue data.

Crop Group

(Crop)

Application 

Scenario1

Worst 

Case 

Scenario?

(Y/N)

Refined Tier I RQ Ranges3 (percentage of 

Refined Tier I RQs above LOC using all 

residue data)4

Tier II5

Adult Acute 

Oral

Adult 

Chronic 

Oral

Larval 

Chronic Oral

%age of 

nectar 

residues 

above 

NOAEC

Duration 

above 

NOAEC8

Fruiting 

Vegetable

(Tomato)

1 x 0.38 lbs 

a.i/A, 7-days 

post -

transplant

Y6

<0.01 - 0.60

(9.0%)

<0.01 - 12

(77%)

0.19 - 0.39 

(0%)

- No analysis 

conducted, tomato 

does not produce 

nectar

- Crop group generally 

not attractive to honey 

bees

Cucurbit 

Vegetables

(Melons)

1 x 0.23 –

0.38 lbs 

a.i/A, at 

transplant

N7 <0.01 - 0.60

(60%)

1.1 - 8.9 

(100%)

018 - 0.39 

(0%) 0% N/A

Citrus

(Orange / 

Grapefruit)

1 x 0.50 lbs 

a.i/A, post 

bloom

Y 0.26 – 2.1

(89%)

4.3 - 43 

(100%)

0.78 - 1.7 

(44%) 0% N/A

Berries 

(Blueberry)

1 x 0.5 lbs 

a.i/A, 3 days 

post-harvest

Y 0.14 - 1.2 

(12%)

2.4 - 16 

(100%)

0.3 - 0.66 

(0%) 0% N/A

Berries 

(Strawberry)

1 x 0.5 lbs 

a.i/A, app. 

timing 

unknown 

relative to 

bloom

Unknown <0.01 - 0.79 

(29%)

<0.01 - 17 

(43%)

0.28 - 0.55 

(0%)

- No analysis 

conducted, nectar data 

not available

Oilseed

(Cotton)

1 x 0.33 lbs 

a.i/A, at 

planting

Y 0.97 - 9.5 

(67%)

16 - 152 

(100%)

2.8 – 6.0

(47%) 67% 17 days

Bolded value represent RQ in exceedance of acute or chronic LOC (0.4 and 1.0, respectively); N/A: Not applicable 1Application rate, number of applications, timing

2Based on whether rate represents maximum annual rate for a given use pattern 3Based on highest reported residue concentration of all individual replicates (acute) or highest average concentration among all individual 

sampling events (chronic).

4Based on all pollen and/or nectar data from all sampling intervals

5Compared to colony feeding study NOAEC of 25 ppb 6Maximum soil-applied rate allowed on all fruiting vegetables except okra and pepper, which is 0.50 lbs a.i/A 7Multiple sites for this study using range of application rates depending on site. It is unclear the application rate for some of the sites to which 

to tie back residues to. Maximum annual rate for cucurbit vegetables is 0.38 lbs a.i/A

8Refers to at least one average residue value
252

Seed Treatment Applications

Crop Group 15 – Cereal Grains (Corn)

The cereal grains crop group includes, among other members, barley, corn, oats, rice, rye, and wheat. 

Specific to the uses of imidacloprid, wheat is the dominant crop with an estimated usage of estimated 

100,000 lbs/year (Table 6-54). This is followed by corn and sorghum with approximately 30% and 10%, 

respectively of the total poundage applied to wheat. It is noted that seed treatment is the only registered 

application method to cereal grains. Usage statistics were unavailable from the SLUA for any other cereal 

grain crop. According to USDA (2014), the cereal grain group is generally unattractive to bees or does not 

produce nectar as is the case with corn and wheat, which produce only pollen. An exception to this is 

sorghum, which produces pollen and nectar that is considered to be attractive to honey bees. Although 

not reported in the SLUA, buckwheat is considered highly attractive for honey bees and may be used for 

honey production. Additionally, members of this group generally do not require bee pollination or use 

managed pollination services. 

Table 6-54. SLUA data imidacloprid and cereal grains (2004-2013)1

Crop Lbs. Applied/yr. % Acreage Treated 

(average)

% Acreage Treated 

(maximum)

Corn (Seed Treatment)1 30,000 <2.5 <2.5

Sorghum (seed treatment)1 10,000 15 20

Wheat (seed treatment)1 100,000 15 20 1

The surveying period for seed treatment uses does not always cover the entire period of the SLUA

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from a seed 

treatment application on corn range from <0.01 – 0.1 (adult acute), <0.01 – 1.34 (adult chronic), 0.02 –

0.04 (larval chronic). These RQ values reflect “high-end” estimates of pollen and nectar residues obtained 

from a seed treatment application at the maximum label rate (1.34 mg a.i/seed) for corn at planting (MRID 

49511701). As the corn plant does not produce nectar, Figure 6-25 below shows the totality of all pollen 

data used to estimate refined Tier I RQs for each life stage and duration.
253

Figure 6-25. Summary of acute and chronic RQ values using totality of pollen residue data from seed 

treatment corn residue study (MRID 49511701).

A total of 36 RQs values were estimated for each life stage and duration. Only one chronic adult (3% of 

the total RQs) exceeded the chronic LOC of 1. All other adult acute oral RQs and larval chronic oral RQs 

were below their respective LOCs. Comparisons were not made to the sucrose-based colony feeding study 

effect levels. For corn, comparisons cannot be made to the colony feeding study effect levels. However, 

with the exception of one adult chronic oral RQ value estimated from the maximum sampled residue (40 

ppb), the results of the refined Tier I indicate pollen residues are below Tier I acute and chronic LOCs for 

honey bees when applied as seed treatment to corn.

Tier II Risks

As stated previously, as no nectar data are available (corn does not produce nectar), comparisons of 

residues to the NOAEC and LOAEC of the colony feeding study were not made. The daily average pollen 

residue data shown in Figure 6-26 below indicate that all residues are below 100 ppb which is associated 

with reduced overwintering success following a 12-week exposure to spiked pollen (Dively 2015,

Qualitative). 
254

Figure 6-26. Imidacloprid residues in pollen in the seed treated corn study (MRID 49511701).

Additional Considerations

Among all trials in the corn residue study, the maximum duration of the sampling period during bloom is 

approximately 8 days. Temporal trends in daily average total imidacloprid residues in pollen were variable 

or declining with time in 2012 while interestingly in 2013, concentrations increased with time in each of 

the three trials. The maximum daily average concentration of total imidacloprid (22.3 ppb) from DAA 84 

in trial NT012 is much greater than the previous sampling event for that trial (4.0 ppb) and is also greater 

than daily means from all other trials. Inspection of the raw data indicates this high value is not a result 

of a single outlier among the 5 replicates. The reason for the increasing concentrations of total 

imidacloprid during the second year of sampling is not clear but may reflect greater desiccation of pollen 

over time. This explanation, however, does not explain why such trends were not consistently observed 

during year 1 (2012). With corn tassels, the daily average of total IMI residues generally followed a similar 

trend as pollen, which is expected since tassels are the pollen bearing portion of the corn plant. This also 

suggests that the increase in concentration over time consistently observed in year 2 (and in year 1 of trial 

NT012) is not limited to the pollen matrix alone, but rather reflects the pattern in the entire corn tassel.

The three corn residue trials were located in the midwestern U.S. with a maximum distance of 

approximately 270 miles (further details provided in Appendix E). Examination of the monthly 

precipitation records suggests a similar magnitude and pattern over time. Thus from a climate 

perspective, these trialsreflect similar climatic conditions. A range of soil types isrepresented in the trials 

(silty clay, silt loam, loam) although none were predominately composed of sand. The total imidacloprid 
255

residues in corn pollen and tassels are comparable from years 1 and 2 of one site and suggest no obvious 

year-to-year carry over in total imidacloprid residues in corn pollen and tassels. It is clear that imidacloprid 

residues in soil measured prior to planting in year 2 (9-80 ppb) are elevated compared to those measured 

prior to planting in year 1 (2-4 ppb). Within each trial/plot, concentrations of imidacloprid increased in 

soil by 2-13X prior to planting in year 1 vs. year 2. Comparison of soil concentrations measured at the end 

of each growing season in one site also suggest year-to-year carry over in soil, with year 2 imidacloprid 

concentrations increasing by 4X to 5X relative to year 1. It is important to recognize that these trends 

reflect parent imidacloprid only and do not include possible contribution of toxic degradates (IMI-olefin, 

5-OH IMI). Additionally, while the residues in soil are indicated to be increasing from year 1 to year 2, 

this does not manifest in higher residues in pollen in year 2 relative to year 1.

Additionally, a second series of trials was conducted to measure the same residues in/on bee-relevant 

white clover pollen and nectar samples and in blossoms, leaves, and soil from white clover plants grown 

at locations where seed-treated corn plants were grown the previous year (full description of methods 

provided in Appendix E). Samples of white clover leaves, blossoms, nectar (hive-collected), and pollen 

(hive collected) were collected during four sampling periods in study year 2. The concentrations of total 

IMI measured in white clover nectar and pollen planted following planting and harvesting of seed-treated 

corn the previous year were near or below the combined limits of detection for total imidacloprid (1.24 

ppb for pollen and 1.33 ppb for nectar. In the majority of samples analyzed (detection frequency = 28% 

for clover pollen and 0% for clover nectar). The maximum concentrations of total IMI measured in clover 

pollen in three trials was 3.8 ppb.

In addition to the seed-treated corn study discussed above, there are several registrant and open 

literature studies available that assessed the residues in pollen of seed-treated corn. In the case of the 

registrant-submitted studies (MRIDs 47699416, 47699414, 47699422, 47699423, and 47699425), all were 

semi-field tunnel studies that allowed the bees to forage on seed treated corn or provided pollen that 

originated from seed-treated corn to bees within a tunnel. While these studies are not of utility from an 

effects standpoint due to major deficiencies that are presented in Appendix A, the residue component of 

these studies are considered fit for qualitative use. All studies found that pollen residues were below the 

LOQ of 5 ppb. It is noted that 55% of the 36 pollen samples taken during the seed-treated corn study 

discussed above were at or below 5 ppb. The sampling interval for these studies ranged from 63 to 77 

days after application and was not reported for some studies. Additionally, a study evaluated as part of 

the open literature effort assessed pollen residues of seed-treated corn 130 days after planting and 

residues were below the LOQ of 1 ppb (Donnarumma, 2011, MRID 497196140). 

As the available seed-treated corn study (MRID 49511701) tested the maximum labeled rate of 1.34 mg 

a.i/seed and there are no other restrictions on the label related to bees, this study was conducted in 

accordance with the label. There is uncertainty as to what the residue levels in pollen would be for other 

seed-treated crops, as members of the cereal grain group have varying application rates when expressed 

in terms of mg a.i/seed. Additionally, while this crop group is largely unattractive to bees or does not 

require bee pollination, sorghum is noted to be an exception, as well as being a use pattern identified in 

the SLUA as having 10,000 pounds of a.i/year associated with it. Buckwheat is also an exception, but 

currently not identified in the SLUA.
256

It is also noted here that this analysis does not take into consideration the potential risk of abraded seed 

coating from corn or other seed treatment applications of imidacloprid. This exposure pathway is one 

that is recognized in the Guidance for Assessing Pesticide Risks to Bees (US EPA 2014). At this time, the 

Agency has not addressed this potential route in a quantitative manner. Rather it has worked with 

different sectors of the seed treatment industry to identify means to reduce potential exposure through 

management practices. Efforts have been made to identify best management practices (e.g., Seed 

Treatment Stewardship Guide by the American Seed Trade Association), technology (e.g., new seed 

lubricants used during planting operations with pneumatic planter), and design (e.g., new design 

standards for certain pneumatic planting equipment). The agency will continue to evaluate new 

information on the effectiveness of these best management practices in addition to data that quantify 

off-field drift of imidacloprid via abraded seed coat dust. Pending review of this information, the agency 

will consider additional characterization of the dust-off exposure in its complete Preliminary Risk 

Assessment at the end of 2016.

Conclusions

The available data on seed treated corn indicates that daily average residues in pollen were below 100 

ppb which is associated with decreased overwintering survival following a 12-week exposure to spiked 

pollen (Dively 2015, Qualitative). As indicated previously, data are not available for nectar as corn does 

not produce it. Therefore, seed treatment applications to corn are determined to present a low on-field 

colony level risk to honey bees. While other members of the cereal grain group are not attractive to 

honey bees including wheat, oats, rye, and barley, there are two members, sorghum and buckwheat that 

produce nectar that is attractive to honey bees. No residue data are currently available for these two 

members and registrant-submitted data are not expected for the other neonicotinoids being assessed in 

2016. It is noted, however, that based on the residue studies of seed-treated crops in other crop groups 

(Tables 4-14 and 4-15), levels of imidacloprid in nectar are low (<10 ppb) relative to other application 

methods. Should nectar residue data continue to be unavailable for nectar producing cereal grains 

(sorghum, buckwheat) the agency will consider bridging information from seed-treated crops in other 

crop groups in its 2016 assessment.

Additional Seed Treatment Use Patterns

Imidacloprid is registered for seed treatment for other members of crop groups 1 (root and tuberous 

vegetables), crop group 3 (bulb vegetables), crop group 5 (brassica vegetables), crop group 6 (legumes), 

crop group 19 (herbs and spices), crop group 20 (oilseed) and peanuts (no crop group). 

Screening level use information provided by the SLUA (which is noted to not include all potential uses of 

seed treated application for imidacloprid, Table 6-55 below) indicates that soybean dominates the seed 

treatment application usage, with 400,000 lbs applied annually. This represents the largest use of 

imidacloprid (in terms of poundage applied) from all use patterns delineated in the SLUA. This is followed 

to a lesser extent by the aforementioned seed treated cotton (50,000 lbs) and potatoes at 30,000 lbs per 

year. Soybeans represent an uncertainty in the risk profile because it is a major use pattern is attractive 

to bees via pollen and nectar, yet pollen and nectar data are unavailable from both registrant and open 
257

literature sources. The agency will consider bridging residue information for seed-treated soybean that it 

expects to receive in 2016 for other neonicotinoids. Conversely, potatoes are noted to require bee 

pollination but only for breeding programs and are considered attractive to bumble bees but not honey 

bees. While the USDA 2014 document indicates other members of this crop group are harvested before 

bloom, including sugar beets, this information is not present for potatoes, which is indicated to only 

require pollination when used for breeding purposes, noted to be a small percentage of the acreage. 

Table 6-55. SLUA data imidacloprid and other non-cereal grain seed treatment uses (2004-2013)1

Crop Lbs. Applied/yr. % Acreage Treated 

(average)

% Acreage Treated 

(maximum)

Cotton (Seed Treatment) 50,000 10 20

Potatoes (seed treatment) 30,000 15 20

Soybeans (seed treatment) 400,000 10 25

Sugar Beets (seed treatment) <500 <2.5 <2.5 1The surveying period for seed treatment uses does not always cover the entire period of the SLUA

Additionally, there are several registrant-submitted studies that were either semi-field tunnel or full-field 

designs that included a residue component. Some of these were previously discussed as they related to 

corn, but also tested seed-treated canola and sunflower. These studies share the same uncertainties as 

those identified for corn that limit their utility in the assessment from an effects standpoint but are 

considered qualitatively as a line of evidence from the residue information provided. 

For canola/rapeseed, semi-field studies (MRIDs 47699417, 47699422, 47699423, 47699425, 48699418, 

and 47699419) were conducted across a variety of locations (France, Sweden, Germany), all with 

imidacloprid applied with beta-cyfluthrin as a seed treatment. All studies examined either nectar alone 

or pollen and nectar from hand-collected, bee collected, and hive collected sources (depending on the 

study). All samples were reported to be either below LOD (either not reported or 1.5 ppb, depending on 

the study) or <LOQ (either 5 or 10 ppb, depending on the study). Additionally, in a full-field study (MRID 

49073605), imidacloprid (co-formulated with beta-cyfluthrin) was applied as a seed treatment to canola 

with resulting residues in hand-collected and bee-collected nectar samples below the LOQ (10 ppb). It is 

noted for the above studies that the sampling interval for these studies was reported with varying 

frequency with some studies noting a 55 days after application interval, others with a 59- to 69-day 

interval, and others not reporting this information.

For sunflower, semi-field studies (MRIDs 47699417, 47699422, 47699423, and 47699425) were all 

conducted in Germany with seed treated imidacloprid. All studies examined both hand and bee collected 

pollen and nectar data with some studies noting a 2-8 day interval and other studies not reporting this 

information. All hand collected and bee collected pollen and nectar data (sources of collection varied 

depending on study) were found to be below the LOD (1.5 ppb in all studies). Additionally, in a full field 

study conducted with seed treated sunflower (Schmidt 1998, MRID 49766206), bee collected nectar after 

a 14-day duration in the treatment fields was below the LOQ (10 ppb) Hand collected nectar residues 

were not available.
258

Summary of Crop Group/Use Patterns for which Seed Treatment Residue Data are 

Available

Table 6-56 below summarizes the available residues studies for the soil-applied method as well as a 

providing a range of the refined Tier I RQs, the percentage of nectar residues above the Tier II NOAEC 

threshold in nectar (25 ppb) and where available, the duration those residues exceed the NOAEC. 

Table 6-56. Summary of risk findings for the seed treatment use patterns of imidacloprid.

Crop 

Group

(Crop)

Application 

Scenario1

Worst 

Case 

Scenario

? (Y/N)

Refined Tier I RQ Ranges3 (%age of 

Refined Tier I RQs above LOC using all 

residue data)4

Tier II5

Adult Acute 

Oral

Adult 

Chronic Oral

Larval 

Chronic 

Oral

%age of 

nectar 

residues 

above 

NOAEC

Duration 

above NOAEC

Cereal 

Grains

(Corn)

1.34 mg 

a.i/seed 

(0.12 lbs 

a.i/A)

Y <0.01 - 0.1 

(0%)

<0.01 - 1.34 

(3%)

0.02 - 0.04 

(0%)

- No analysis conducted, 

corn does not produce 

nectar

Bolded value represent RQ in exceedance of acute or chronic LOC (0.4 and 1.0, respectively). 1

Application rate, number of applications, timing

2

Based on whether rate represents maximum annual rate for a given use pattern

3Based on highest reported residue concentration of all individual replicates (acute) or highest average concentration among all individual 

sampling events (chronic).

4

Based on all pollen and/or nectar data from all sampling intervals 5Compared to colony feeding study NOAEC of 25 ppb.

Combined application methods

Soil + Foliar

Crop Group 8 – Fruiting Vegetable (Tomato)

The usage and attractiveness of tomatoes and other members of the fruiting vegetables group were 

previously discussed in the soil application uses section of risk description. The tomato residue study was 

previously discussed also had a foliar component, in which 2 applications of 0.06 lbs a.i/A were made at 

bloom after the soil application of 0.38 lbs a.i/A (MRID 49665201 – further details provided in the soil 

application method section). This application rate is slightly lower than the maximum single application 

rate for foliar use on fruiting vegetables (0.08 lbs a.i/A), but when considered with the previous soil 

application of 0.38 lbs a.i/A, the total pesticide applied is equivalent to maximum annual rate of 0.5 lbs 

a.i/A. As indicated previously, the tomato plant does not produce nectar and therefore only the pollen 

data are used for the refined Tier I analysis examining the totality of all residue information from this 

study. The general pattern of average residues rising nearly tenfold at the sampling event following the 

first foliar application was observed at all sites
259

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from the combined 

soil + foliar application to tomato, range from 0.02 – 3.7 (adult acute), <0.01 - 76 (adult chronic), 1.2 – 2.5 

(larval chronic). These RQ values reflect “high-end” estimates of pollen residues obtained from a soil 

application at the maximum label rate (1 X 0.38 lbs a.i/A), followed by 2 foliar applications of 0.06 lbs a.i/A 

to achieve the maximum annual rate of 0.50 lbs a.i/A for tomato (MRID 49665201). 

Figure 6-27. Summary of acute and chronic RQ values using totality of pollen residue data from the 

combined soil + foliar treatment tomato residue study (MRID 49665201).

A total of 23 RQs were estimated from all available pollen residue data for each life stage and exposure

duration. Figure 6-27 depicts two groups of residue values that were sampled from the same fields with 

the same trial where the first sampling was generally done from days 5-8 after the last application (10 

samples) while the second sampling was done from days 17-20 (13 samples) after the last foliar 

application (across both trial years of the study). The RQs reflect a general decline in pollen residues up 

to an order of magnitude during each sampling interval. However at the first sampling interval, there 

were acute and chronic risk LOC exceedances for adult and larval bees. These RQs generally fell below 

the respective LOC by the time of the second sampling period. That is to say that for the first sampling 
260

interval (5-8 days after the last foliar application), 10/10 (100%) of the acute oral RQs exceeded the acute 

risk LOC. By the time of the final sampling interval (17 – 20 days after the last application), only 1/13 

(7.7%) RQ values exceeded the acute risk LOC. Similarly, for larval chronic oral RQs, 2/10 (20%) RQs 

exceeded the chronic risk LOC at the time of the first sampling interval and these were both below LOC 

by the time of the second interval. For adult chronic oral RQs, 100% of the RQs exceeded the chronic risk 

LOC of 1 (inclusive of all sampling intervals), indicating the residues in pollen were persistent enough to 

exceed chronic risk LOC up to 20 days after the last foliar application. 

As indicated by the discussion of the soil-applied component, the majority of adult acute oral RQ values 

are below the acute risk LOC and all of the larval chronic oral RQs were below the chronic risk LOC. 

However, the majority of adult chronic oral RQs exceeded the chronic risk LOC. Addition of the foliar 

spray component yields higher residue values compared to soil application alone which then results in a 

greater number of RQs exceedances across all bee life stages and durations of exposure.

Tier II Risks

As stated previously, sine no nectar data are available for tomato (tomato does not produce nectar), 

comparisons of residues to the NOAEC and LOAEC of the colony feeding study were not made. However, 

it is noted that residues exceed 100 ppb, which is associated with reduced overwintering survival following 

a 12-week exposure to spiked pollen (Dively 2015, Qualitative).

Figure. 6-28. Imidacloprid residues in pollen in the soil + foliar applied tomato study (MRID 49665202).
261

Additional Considerations

The general pattern of average residues rising nearly tenfold at the sampling event following the first foliar 

application was observed at all sites. A sharp decline in concentration between the first and second foliar 

applications was also observed except for the San Luis Obispo site where there was only a slight decrease 

present by the time of final sampling event. Residue data following the first foliar application indicate 

that residues in pollen increased at all sites irrespective of soil type.

The label permits multiple application methods to be applied to certain crops, including tomatoes, so long 

as the maximum annual application rate does not exceed 0.5 lbs a.i/A. Therefore, with an initial soil 

application of 0.38 lbs a.i/A followed by 2 foliar applications of 0.06 lbs a.i/A, this study represents a 

scenario that is permitted by the label and assesses the highest annual application on tomatoes.

Conclusions

Average residues in tomato pollen resulting from the combined soil + foliar application (post-transplant

and at bloom, respectively) were above 100 ppb indicated by the open literature to cause colony level 

effects through the pollen route of exposure (Dively 2015, Qualitative). As indicated previously in the 

discussion of soil applications to tomatoes, the fruiting vegetables crop group is largely unattractive to 

honey bees. With the exception of okra, which is attractive to honey bees, the on-field colony level risk 

to honey bees resulting from soil + foliar applications to tomatoes is determined to be low.

Crop Group 20 – Oilseed (Cotton)

The usage and attractiveness of cotton to bees was previously discussed in the foliar application uses 

section of risk description. 

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from soil + foliar 

applications on cotton, range from 21-208 (adult acute), 358 - 3562 (adult chronic), 64 - 139 (larval 

chronic). These RQ values reflect “high-end” estimates of pollen and extra-floral nectar residues obtained 

from a soil application at the maximum label rate (1 X 0.33 lbs a.i/A) for cotton at planting followed by 3 

foliar applications of 0.06 lbs a.i/A (MRID 49665202). As noted previously, these RQs are 20- to 25-fold 

higher (depending on the life stage) as compared to their respective RQs from the soil-alone component 

of this study. Figure 6-29 shows RQs estimated using the totality of matched pollen and extra-floral nectar 

samples at the same sampling event. 
262

Figure 6-29. Summary of acute and chronic RQ values using totality of pollen and extra-floral nectar data 

from the combined soil + foliar-applied cotton residue study (MRID 49665202).

A total of 30 RQs were estimated using all available pollen and extra-floral nectar data for each honey bee 

life stage and duration. Two sampling events occurred after the last application at 4-5 days after last 

application and 13-14 days after last foliar application, with 15 data points associated with each interval. 

At the sampling event 4-5 days after the last foliar application, all (100%) of adult acute oral, adult chronic 

oral, and larval chronic oral RQs exceed their respective LOCs. By the time of 14 – 15 days after the last 

foliar application, all (100%) of adult acute oral and adult chronic oral RQs still exceed their respective 

LOCs while 13/15 (87%) of larval chronic oral RQs exceed the LOC of 1. During the time of the two 

sampling intervals, the residues in extra-floral nectar generally decreased by an order of magnitude, with 

residues in pollen generally decreasing as well, although not to the extent observed in extra-floral nectar.

Tier II Risks

To evaluate the risk of soil application of imidacloprid to cotton at the honey bee colony-level, reported 

daily average residues of total imidacloprid concentrations in extra-floral nectar were compared to the 

aforementioned NOAEC and LOAEC values from the available registrant-submitted colony feeding study 

(Figure 6-22). As there were only data available for one year, the daily average extra-floral nectar residue 

values were plotted by soil type. The 30 average residue samples were split between the 4-5 day after 
263

last application and 14-days after last application sampling events. All (100%) of the average extra-floral 

nectar values exceeded the NOAEC (25 ppb) and LOAEC (50 ppb) of the colony feeding study. 

Approximately 10 days later, there were 8/15 (53%) that exceeded the NOAEC. 

It is not known the extent the average extra-floral nectar residue concentrations would decrease had a 

subsequent sampling event occurred. It is also reiterated the blooming duration of cotton is at least 6 

weeks in duration, which matches the exposure duration of the colony feeding study. Additionally, extrafloral nectar sources are available even after the last petal fall, which lengthens the overall potential 

duration of exposure by foraging bees.

Figure 6-30. Imidacloprid residues in nectar from the soil + foliar cotton study (MRID 49665202) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Figure 6-31 below shows the average residues in pollen from the soil + foliar applied cotton study. As 

with the average nectar data above, there is no clear trend in the magnitude of residues associated with 

a given soil type which is 5X the result of foliar applications. Figure 6-31 indicates that a subset of the

daily average residues exceed 100 ppb which is associated with reduced overwintering success following 

a 12-week exposure via spiked pollen (Dively 2015, Qualitative)
264

Figure 6-31. Imidacloprid average residues in pollen from the soil + foliar cotton study (MRID 49665202).

Additional Considerations

The 9 sites employed for this study were in relatively close proximity to each other in California’s Central 

Valley and were shown to have similar climatic condition. This limits the ability to evaluate how regional 

differences in climate may affect residues in cotton pollen and nectar.

Temporal trends in total imidacloprid residues measured in extra-floral nectar following 3 at-bloom foliar 

spray applications at 0.06 lbs a.i/A reflect a 1-2 orders of magnitude increase in concentrations relative to 

residues measured 75 – 95 days following soil application of 0.33 lbs a.i/A. This clearly demonstrates the 

strong impact of at bloom foliar spray applications on pollen and nectar residues, and suggests that 

application timing and method are major factors governing potential exposure of honey bees to 

imidacloprid. 

This study assessed a scenario with a single “at-plant” soil application of 0.33 lbs a.i/A followed by 3 foliar 

applications of 0.06 lbs a.i/A to yield a maximum annual rate of 0.51 lbs a.i/A, which is approximately the 

maximum rate specified on the label (0.50 lbs a.i/A)
265

Conclusions

As documented with foliar and soil applications alone discussed previously, average residues in nectar and 

pollen resulting from the combined soil + foliar application to cotton study indicate a colony level risk to 

honey bees based on residues being well above the Tier II NOAEC in nectar (25 ppb) and furthermore 

above 100 ppb in pollen which is associated with reduced overwintering survival following a 12 week 

exposure (Dively 2015, Qualitative). It is further noted that a full field Tier III study is expected in 2016 to 

potentially further refine this risk determination.

Seed Treatment + Foliar

Crop Group 20 – Oilseed (Cotton)

The usage and attractiveness of cotton was previously discussed in the foliar application uses section of 

risk description. This study assessed the residues in pollen, nectar, and extra-floral nectar of cotton 

following a 0.05 lbs a.i/A seed treatment application and 5 foliar applications of 0.06 lbs a.i/A at bloom (5 

– 8 day intervals). Unlike the soil + foliar treatment described previously, the average floral nectar 

residues from this study were higher than those in extra-floral nectar; therefore, floral nectar data were

used in further refinements to the Tier I analysis and Tier II analysis.

Refined Tier I oral risks and magnitude, duration, and frequency of LOC exceedance analysis

As described in Section 6.1, the refined Tier I oral RQ values for honey bees resulting from seed treatment 

+ foliar applications on cotton, range from 0.3 – 3.0 (adult acute), 5.6 - 53 (adult chronic), and 1.0 – 2.1 

(larval chronic). These RQ values reflect “high-end” estimates of pollen and floral nectar residues from 

the treatment regimen parameters described above (MRID 495117020). Figure 6-32 shows RQs 

estimated using the totality of matched pollen and nectar samples at the same sampling event. 

A total of 43 RQ values (inclusive of 2 years of sampling) were estimated from the available pollen and 

nectar residue data for each bee life stage and exposure duration. These residues spanned 14 – 50 days 

after the last foliar application. A total of 29 of 43 (67%), 5 of 43 (12%), and 43 of 43 (100%) of the RQ 

values exceeded the LOCs for adult acute oral, larval chronic oral and adult chronic oral which exceeded 

their respectively. Residues in pollen and nectar generally declined across the 4-5 sampling events per 

site. Despite this general trend, samples collected as far as 41 days (almost 6 weeks) after the last foliar 

application yielded Tier I RQ values that exceeded the adult acute oral and adult chronic oral LOCs. 

Samples collected 50 days after treatment produced RQ values that exceed the chronic adult oral LOC of 

1. 
266

Figure 6-32. Summary of acute and chronic RQ values using totality of pollen and floral nectar data from 

the combined seed treatment + foliar-applied cotton residue study (MRID 49511702).

Tier II Risks

To evaluate the risk of seed treatment + foliar application of imidacloprid to cotton at the colony level for 

honey bees, reported daily average residues of total imidacloprid concentrations in nectar were compared 

to the aforementioned NOAEC and LOAEC values from the available registrant-submitted colony feeding 

study (Figure 6-32). As there were two years of sampling data, the daily average residues are depicted of 

each year in the figure below. 

There were 3 of 43 (7%) of the daily average extra-floral nectar residue values exceeded the NOAEC (25 

ppb) of the colony feeding study. The remainder (93%) of residues were below the NOAEC. Exceedances 

were associated with the first two sampling periods (approximately 14 and 21 days after the last foliar 

application, respectively). Average floral nectar residues continued to decline after that point. The 

exceedances were also associated with data from 2013, and an examination of the data below from 2013 

compared to 2012 suggest a carryover effect from year to year. This increase could also relate to differing 

climatic conditions in 2013 relative to 2012. As indicated previously and from the sampling duration for 

this study, cotton is associated with a bloom period of at least 6 weeks (matching the exposure duration 

of the colony feeding study) and while there is uncertainty with respect to the magnitude of residues 

beyond the last sampling date 50 days after the last application, the available data suggest a decline in 
267

residues over time with no residues on floral nectar measured greater than or equal to 25 days after the 

last application exceeding the NOAEC.

Figure 6-33. Imidacloprid floral nectar residues in seed + foliar cotton study (MRID 49511702) as 

compared to effect levels in registrant submitted colony feeding study (MRID 49510001).

Figure 6-34 below shows the average residues in pollen from the seed + foliar applied cotton study and 

indicates that all average residue values are below 100 ppb which is associated with reduced 

overwintering survival following a 12-week exposure via spiked pollen (Dively 2015, Qualitative). 
268

Figure 6-34. Imidacloprid averages residues in pollen from the seed + foliar cotton study (MRID 

49665202).

Additional Considerations

Following seed + foliar spray applications, DT50 for residues of total IMI in cotton floral nectar vary from 

19-68 days while those for extra-floral nectar vary from 27 – 52 days (except for one trial where residues 

remained stable over the 40-day sampling period). With floral and extra-floral nectar, 14/16 (88%) of the 

DT50 values of total IMI are between 19 and 51 days. With pollen, most of the trials contained insufficient 

data for reliable determination of DT50 values (<4 sampling points with detectable residues of total IMI). 

Three DT50 values for total IMI in pollen varied from 14 to 58 days.

The three trials were located in the midwestern U.S. within the same general vicinity. Examination of the 

monthly precipitation records suggests a similar magnitude and temporal pattern over time. Thus from a 

climate perspective, these trials are relatively similar. A range of soil types are represented (sandy loam, 

silt loam, sand). 

Based on yearly mean values of total IMI in floral and extra-floral nectar, year 2 means increase by 1.2X 

to 2.7X over year 1 means. With cotton pollen, yearly averages of total IMI increase by 1.5X to 2.9X from 

year 1 to year 2. The majority of increase in year 2 residues of total IMI occurred sooner after application 
269

(13-20 days) across the trials. The two trials with the greatest percent sand in soils (NT013 and NT015) 

show the greatest relative increase in yearly average total IMI from year 1 to year 2 in nectar and pollen 

(1.7X to 2.9X) compared to NT014 which contained mostly silt (1.2-1.5X). It is uncertain whether this 

differential increase is related to differences in soil composition, but all three trials had similar amounts 

of IMI (24-42 ppb) in soil prior to the 2nd year planting. 

Residues of total IMI in soil measured prior to planting in year 2 (24-45 ppb) are elevated compared to 

those (0.3-12 ppb) measured prior to planting in year 1. This could explain some of the higher residues of 

total IMI in pollen and nectar observed in year 2. Unfortunately, soil samples at the end of the 1st growing 

season were not taken; therefore, it is not known whether post-application residues in soil increase from 

year to year or remain similar from year to year. Other factors (weather) may also contribute to these 

observed differences. 

Additionally, white clover was planted as a rotational crop to investigate the residues in pollen and nectar 

in the same cotton fields that were harvested the prior year (for more details on the methods, please 

refer to Appendix E). The concentrations of total IMI measured in white clover nectar and pollen planted 

following foliar application to seed-treated cotton harvested the previous year (trials NT014 and NT015) 

were near or below the level of detection (0.7 ppb) in the majority of samples analyzed (detection 

frequency = 38% for clover nectar and 53% for clover pollen). The maximum concentrations of total IMI 

measured in clover nectar in trials NT014 and NT015 are 1.6 and 2.7 ppb, respectively. The maximum 

concentrations of total IMI measured in clover pollen in trials NT014 and NT015 are 8 and 8.6 ppb, 

respectively.

It is noted the difference in the magnitude of residues between the soil alone, soil + foliar study, and seed 

+ foliar study. Specifically, the soil-applied floral nectar residues are roughly within a factor of 2 of the soil 

+ foliar applied study while roughly 3 fold the level of the seed + foliar study. This is distinguished from 

the extra-floral nectar, which was roughly 1/3 the value of floral nectar residues in the soil-alone study 

yet over 15-fold higher than floral nectar residues in the soil + foliar study. 

Conclusions

As with foliar alone, soil alone, and combined soil + foliar applications discussed above (no pollen data 

from the foliar application alone study), average residues in nectar and pollen resulting from the 

combined seed treatment + foliar application to cotton indicate a colony level risk to honey bees based 

on residues being above the Tier II NOAEC in nectar (25 ppb). There were no average residues in pollen 

above 100 ppb, which is associated with reduced overwintering survival following a 12-week exposure via 

spiked pollen. As indicated previously, a full field Tier III study is expected in 2016 to potentially further 

refine this determination.
270

Summary of Crop Group/Use Patterns for which Combined Method Residue Data are 

Available

Table 6-57 below summarizes the available residues studies for the soil-applied method as well as a 

providing a range of the refined Tier I RQs, the percentage of nectar residues above the Tier II NOAEC 

threshold in nectar (25 ppb) and where available, the duration those residues exceed the NOAEC. 

Table 6-57. Summary of risk findings for the combined method use patterns of imidacloprid.

Crop 

Group

(Crop)

Application 

Scenario1

Worst 

Case 

Scenario

?

(Y/N)

Refined Tier I RQ Ranges3 (percentage 

of Refined Tier I RQs above LOC using 

all residue data)4

Tier II5

Adult 

Acute 

Oral

Adult 

Chronic Oral

Larval 

Chronic 

Oral

Percentage 

of nectar 

residues 

above 

NOAEC

Duration 

above NOAEC6

Fruiting 

Vegetables

(Tomato)7

1 x 0.38 lbs 

a.i/A, at 

plant + 3 x 

0.06 lbs 

a.i/A, at 

bloom

Y

0.02 - 3.7

(100;

7.7%)

<0.01 - 76 

(100%;

100%)

1.2 - 2.5 

(20%; 0%)

- No analysis conducted, 

tomato does not produce 

nectar

- Crop group generally not 

attractive to honey bees

Oilseed

(Cotton)8

1 x 0.33 lbs 

a.i/A, at 

plant + 3 x 

0.06 lbs 

a.i/A at 

bloom

Y

21 - 208

(100%, 

100%)

358 - 3562 

(100%, 

100%)

64 - 139 

(100%, 

87%)

1st interval:

100%

2nd interval: 

53%

10 days

Oilseed

(Cotton)

0.05 lbs 

a.i/A at 

planting + 5 

x 0.06 lbs 

a.i/A at 

bloom

Y 0.3 - 3.0 

(29%)

5.6 - 53 

(100%)

1.0 - 2.1 

(12%) 7% 10 days

Bolded value represent RQ in exceedance of acute or chronic LOC (0.4 and 1.0, respectively). 1

Application rate, number of applications, timing

2

Based on whether rate represents maximum annual rate for a given use pattern 3Based on highest reported residue concentration of all individual replicates (acute) or highest average concentration among all individual 

sampling events (chronic).

4Based on all pollen and/or nectar data from all sampling intervals, for tomato and cotton studies, 1st interval and 2 interval separated 

parenthetically by semicolon)

5Compared to colony feeding study NOAEC of 25 ppb.

6Refers to at least one average residue value.

7Two sampling events occurred after last foliar application, one at 5 – 8 days after last application, the other at 17 – 20 days after last 

application

8

Two sampling events occurred after last foliar application, one at 4 – 5 days after last application, the other at 12 - 14 days after last 

application
271

6.2.2. Tier III analysis 

As indicated in the White Paper (USEPA et al. 2012), full-field studies represent the highest level of 

refinement for pollinator studies since they are intended to reflect the potential effects of a pesticide on 

bee colonies under actual chemical use conditions. Tier III studies may be considered to address specific 

uncertainties, i.e., risk hypotheses, which have been identified through lower-tier studies and/or through 

the open literature under reasonable worst case exposure scenarios in the field. Tier III full field studies 

are generally specific for a given crop and application method. Importantly, interpretation of the results 

from Tier III studies requires careful evaluation of exposure to bees to the treated crop in the context of 

exposures that may be reasonably expected to occur across the landscape. 

There are currently no registrant-submitted Tier III full-field studies available for imidacloprid that are 

considered acceptable for use in risk assessment (i.e. quantitative or qualitative). As discussed in Section 

5, there are currently two full-field studies that are being conducted by Bayer CropScience to characterize 

the colony-level effects of application of imidacloprid on cotton in California and pumpkin in South 

Dakota. The results of these studies will be incorporated in the preliminary risk assessment expected to 

by the end of 2016.

Additionally, there are two full-field studies that were evaluated in the open literature that investigate 

the colony-level effects to honey bees exposed to seed-treated corn and sunflower. 

In Pohorecka 2013 (MRID 49769625, Qualitative), Gaucho® 600 FS; 83.3 mL/50,000 seeds and Courase®

350 FS; 150 mL/50,000 seeds in the 2011 and 2012 trials, respectively) were applied to corn as a seed 

treatment in a full-field design with a 21-day exposure period. The number of dead bees per colony was 

not significantly different from the control fields in both the 2011 and 2012 trials. Additionally, there was 

a significant increase (p<0.05) in percent frame coverage in two colony condition assessments ahead of 

the overwintering period. Despite these increases, the study authors noted that all colonies (control and 

treatment) overwintered successfully in 2011 but similar information was not provided for the 2012 trial. 

It is noted that the analysis of the bee-collected and trapped pollen from corn did not exceed 3%, 

indicating minimal exposure to imidacloprid seed-treated corn.

Stadler 2003 (Qualitative) investigated the colony-level effects of honey bees foraging on seed-treated 

sunflower (0.24 mg a.i/seed) in full-field design imidacloprid for a 10-day exposure period. Similar to 

Pohorecka 2013, there were no effects on mortality but an increase in the brood area coverage. It is noted 

however, the magnitude of this effect is uncertain as means were not presented in the article and 

indications of statistical significance were unclear. 

It is noted that the available Tier III studies from the open literature represent two uses that are only 

approved for seed treatment. In the case of seed-treated corn, the refined Tier I analysis using all available 

data from the seed-treated corn residue study indicated all RQs estimated with exception of one adult 

chronic oral RQ were below their respective LOCs. A Tier II analysis was not conducted as the corn plant 

does not produce nectar to compare to the NOAEC and LOAEC of the colony-feeding study. 
272

For sunflower, there are two registrant-submitted full-field studies that were previously indicated to be 

qualitative from an exposure standpoint but not suitable for risk assessment purposes with regards to 

effects due to major deficiencies in these studies. In Schmidt et al. (MRID 49766206), honey bee colonies 

were exposed for 14 days to seed-treated sunflower applied at 0.7 mg a.i/seed. The bee-collected nectar 

after the 14-day exposure period was determined to be below the LOQ, which was notably less sensitive 

at 10 ppb as compared to other studies. In the full-field component of Schmuck 2001, seed-treated 

sunflower sampled 62 – 66 days after exposure yielded pollen and nectar residues below LOD (1.5 ppb). 

It is noted that the seeds were also treated with carbendazim, metalaxyl and copper oxyquinolate. Finally, 

in a study evaluated in the open literature (Laurent and Rathahao, 2003; MRID 48077902; full details of 

methods provided in Appendix B), pollen residues from radiolabeled seed-treated sunflower (1 mg 

a.i/seed) averaged 13 ppb with maximum residues in pollen of 36 ppb. It was also noted from the analysis 

of radioactive residues that a maximum of 10% of the residues from the treated seed were taken up by 

the various plant parts.

6.2.3. Examination of the pollen route of exposure

Pollen is the chief protein source of honey bees and is therefore an important component of the diet along 

with nectar that provides carbohydrates. Although the pollen route of exposure is considered as part of 

the Tier I risk assessment, it is not explicitly considered in the Tier II assessment since the only acceptable 

(quantitative) data involved feeding hives spiked sucrose22. In this section, information regarding the 

relative importance of the pollen route of exposure to honey bees is described in effort to evaluate the 

importance of not having quantitative data on colony-level effects resulting directly from the pollen route 

of exposure. It is noted here that honey bees do not directly consume raw pollen grains. Rather, nectar 

that is brought back to the hive is processed and mixed with raw pollen to create beebread, which serves 

as the hive’s protein source. (US EPA et al. 2014) 

Consideration of Honey Bee Pollen vs. Nectar Consumption 

As described in the 2014 Guidance for Assessing Pesticide Risks to Bee (USEPA et al. 2014) document, 

pollen consumption rates by honey bees depend on their age and their task. Across all ages and tasks, 

average pollen consumption is estimated to represent 10% or less of the total consumption, with the vast 

majority consisting of nectar (honey). After hatching, worker larvae consume royal jelly until 

approximately 4 days of age after which they start with a diet of processed nectar and pollen (bee bread) 

(US EPA et al.; 2014). At day 4 and 5, worker larvae are estimated to consume an average of 1.8 and 3.6 

mg pollen/day, respectively, while consuming much greater amounts of nectar (60 and 120 mg/day, 

respectively). Thus, pollen consumption by larval bees approximates < 3% of their total daily food 

consumption. 

After emergence, the honey bee worker is engaged with in-hive activities including cell cleaning and 

capping of developing pupae for roughly its first week as an adult and is estimated to consume on average 

 22 Although colonies were fed spiked sucrose solution only, larval and adult honey bees consume bee bread which 

is a mixture of processed nectar and pollen. 
273

6.7 mg/day of pollen and 60 mg/day of nectar. Pollen consumption increases when the worker transitions 

to brood and queen tending, (i.e. nurse bees), with an estimated average consumption rate of 9.6 mg/day 

of pollen. However, nectar consumption also increases to approximately 140 mg/day. When the worker 

is recruited for nectar or pollen foraging, the pollen consumption rate falls sharply and for the purposes 

of Tier I risk assessment, is estimated to be less than 0.1 mg/day as compared to 292 mg/day of nectar for 

nectar foragers. Indeed, even with nurse bees, who have the highest pollen consumption rate of any adult 

caste within the hive, the nectar consumption rate is at least 10-fold higher than the pollen consumption 

rate. 

Consideration of Differential Life Stage Sensitivity

As summarized above, pollen larval and adult hive bees are the primary consumers of pollen (in the form 

of bee bread). Based on toxicity data described in Table 5-1, larval bees are much less sensitive to 

imidacloprid on a chronic exposure basis compared to adult bees (no acute oral toxicity data are available 

for larvae). Specifically, the chronic oral NOAEC for adult bees is 0.00016 μg a.i/bee while that for larvae 

is approximately 10X greater (0.0018 μg a.i./bee). In the larval toxicity study, no statistically significant 

effects were seen at the highest treatment level and therefore the NOAEC could actually be greater than 

0.0018 μg a.i./bee. Given that larval bees consume a maximum of 3% of their total diet as pollen, these 

data suggest that at the organism level, larval bees are expected to be much less sensitive to a given 

concentration of imidacloprid in pollen compared to adult bees. 

Consideration of Higher Tier Studies Involving Pollen

As indicated previously, a Tier II analysis was not conducted for use patterns for which nectar residue data 

were unavailable since the available colony feeding study assessed exposure via spiked sucrose. Although 

no higher-tier studies involving the pollen route of exposure were identified for quantitative use in this 

risk assessment, four higher-tier studies (2 registrant studies and 2 open literature studies) are available 

that are considered appropriate for qualitative use.

Registrant-Submitted Studies. Prior to the initiation of the available colony feeding study, two pilot 

feeding studies were conducted by the registrant in Montana and North Carolina to support protocol 

development and inform the treatment regimens that would later be employed by the current colony 

feeding study. These studies are discussed earlier in Section 5.2.1 and are relevant here because they 

included both a sucrose and pollen route of exposure. Only interim reports are available for these studies 

and raw data were not available to confirm statistical results.

In the North Carolina pilot study (Lawrence 2013; MRID 48962002 small honey bee colonies were fed 

separate treatments of spiked sucrose (50 and 200 ppb) and spiked pollen patties (50 and 200 ppb) along 

with an untreated control for 6 weeks. Based on interim report for the North Carolina Study, colonies fed 

200 ppb imidacloprid in sucrose experienced significant reductions (p<0.05) food consumption, pollen 

stores, colony strength, and the total number of brood while only pollen and nectar stores were 

significantly reduced (p<0.05) at the 50 ppb sucrose group. Unfortunately, the results of the pollen route 

of exposure are considered invalid because control hives appeared to be unable to forage adequately for 
274

nectar (the study was conducted during a nectar dearth). This likely resulted in reduced nectar storage, 

pollen storage and hive strength (adults and brood) over time in the controls and confounded the ability 

to detect treatment effects from pollen exposure.

In the Montana study (Bromenshenk et al., 2012; MRID 48962001) technical imidacloprid was spiked at 

varying nominal concentrations in artificial pollen patties and sucrose at concentrations of 50 ppb pollen 

+ 50 ppb sucrose (50/50), 50 ppb pollen + 200 ppb sucrose (50/200), 200 ppb pollen + 50 ppb sucrose 

(200/50) and 200 ppb pollen + 200 ppb sucrose (200/200) for 6 weeks. Results reported in the interim 

report (without statistical analysis) suggest a combination of 50/50 ppb spiked pollen and sugar (lowest 

combination) reduced the hive strength, pollen stores, pollen consumption, and hive weight relative to 

controls. This combination of 50/50 ppb spiked pollen and sugar appeared to result in a greater number 

of detectable effects compared to 50 ppb spiked sugar alone from the North Carolina pilot study, which 

only found significant reductions in pollen and nectar storage during exposure. At the same 

concentration, spiked sucrose appeared to result in greater effects compared to spiked pollen.

Since these results originated from an interim report, lacked raw data and a formal statistical analysis, the 

findings do not enable a conclusive evaluation of the relative importance of imidacloprid exposure 

through pollen vs. nectar regarding honey bee colony health. 

Open Literature Studies. Two additional studies that were evaluated in the open literature that assessed 

the pollen route of exposure of honey bees colonies to imidacloprid. In the study by Dively (2009; MRID 

47775502) which was previously described in Section 5 and in Appendix D, honey bee colonies were 

exposed to imidacloprid at concentrations of 5 and 20 ppb spiked into pollen cakes provided in the hive, 

in addition to a control group. Multiple measures of colony health were assessed over the course of the 

nearly 12-week exposure period (15 May, 2008 – 06 August 2008). As discussed previously, the hives in 

this study became congested with increased queen failure that was not considered treatment related. 

Results from the 12 week exposure to spiked pollen up to and including 20 ppb did not result in any 

significant effect relative to the control group with one exception. At 5 ppb, the number of marked

foragers visiting nectar stations that was reduced at the 5 ppb group, but increased at the 20 ppb group 

relative to control and thus was not deemed treatment related. 

In further work by Dively (2015), a similar study design was employed for that of the 2009 study (full 

methods and discussion provided in Appendix D) with the addition of a 100 ppb treatment group and 

addition of supers to avoid the crowding reported in the previous study. Replicate trials of the studies 

were conducted consecutively in 2009 and 2010 with the 2009 trial having 10 replicate colonies per group 

and the 2010 trial having 7 replicate colonies per group. Out of several colony health parameters assessed 

over the 12-week exposure period including mean colony size, mean amount of pollen collected, and 

percentage of total frame covered for bees, capped brood cells, capped honey, and beebread, only 

overwintering survival in the 100 ppb group (75%) was significantly reduced relative to the control (100%). 

In the 2010 trial, the study authors suggested that the mild winter in the test area may have led to bees 

consuming their stores too quickly which may explain the low overwintering survival of control colonies 

(57%). Overwintering survival was 50% in each of the 5, 20, and 100 ppb treatments which was not 

significantly different from controls. No other treatment-related effects were indicated compared to 
275

controls. It is worthy to note that while the Dively (2015) study observed that overwintering survival was 

reduced from 100% in controls to 75% after 12 weeks exposure to 100 ppb in spiked pollen, this same 

concentration fed to colonies in sucrose caused catastrophic effects during the course of the 6 week 

colony feeding study such that colonies were removed from the trial due to their weakened condition to 

avoid possible cross contamination via hive robbing. Nearly all of these hives exposed to 100 ppb in 

sucrose were lost following overwintering.

Summary Discussion and Implications for this Risk Assessment

The aforementioned lines of evidence suggest the following:

• Across all honey bee life stages and tasks, consumption of pollen (in the form of bee bread) is far 

less than that of nectar, ranging up to 10% of the total diet. 

• Larval bees, which rely on pollen as a protein source during their rapid development, are at least 

10X less sensitive compared to adult bees on a chronic exposure basis.

• Combined 6-week exposure to imidacloprid spiked pollen and sucrose (50 ppb each) results in a 

greater number of adverse effects (based on mean responses) compared spiked sucrose alone 

(50 ppb) when considering interim results from two separate studies.

• One study indicates that a 12-week exposure to 100 ppb in pollen resulted in a significant (25%) 

reduction in overwintering survival, but a repeat of this study the following year by the same 

author was inconclusive regarding effects on overwintering survival at 20 and 100 ppb.

• At the colony level, exposure to 100 ppb imidacloprid in spiked sucrose results in more rapid and 

severe effects compared to the same concentration in spiked pollen.

Implications for this Risk Assessment

The Tier I risk assessment considers both the pollen and nectar exposure routes to honey bees and 

assumes equal potency at the organism level for these two food sources. No information was 

identified that enabled this assumption to be evaluated at the organism level. At the colony level, the 

Tier II risk assessment is based on colonies feed imidacloprid-spiked sucrose only. While colonies 

were not fed spiked pollen, bees were nonetheless exposed to imidacloprid in pollen in the form of 

bee bread (i.e., a mixture of pollen and honey). As shown in Figure 6-35, mean concentrations of total 

imidacloprid in bee bread were approximately 20% of those measured in uncapped nectar (MRID 

49510001. Therefore, from an in-hive exposure perspective, the effects observed from the Tier II 

sucrose colony feeding study actually reflect a combination of exposure to contaminated nectar and 

pollen in the form of bee bread. Had spiked pollen also been provided, the exposure of bees to 

imidacloprid would undoubtedly have been greater; however it is not clear as to extent to which 

colony-level effects would have increased relative to spiked sucrose alone. The aforementioned 

studies do not provide consistent evidence of the degree to which sucrose only exposures might have 

underestimated colony-level effects. Furthermore, since imidacloprid is known to affect honey bee 

pollen collection in this (and other) studies, providing a pollen source might have masked some of the 

effects resulting from reduced pollen collection. 
276

Figure 6-35. Box and Whisker Plots of Total Imidacloprid Measured in Hive-Collected Uncapped Nectar

(A) and Bee Bread (B) from the Sucrose Colony Feeding Study (MRID 49510001)

The biggest impact for this assessment relating to the pollen route of exposure is the inability to assess 

risks at the colony level for crops where no nectar is produced. This is the case with tomatoes, where RQ 

values exceeded the LOC at the refined Tier I level for both soil and soil + foliar application methods, and 

for corn, where the refined Tier I analysis indicated only a marginal level of risk for adult chronic oral 

component (maximum RQ of 1.34). In the case of tomato, honey bees are not considered attracted to 

the pollen and nectar of members of the fruiting vegetable group, of which tomato is a member. It was 

previously noted, that this crop group is of importance to non-Apis bees, particularly bumble bees, where 

managed pollination services are used for tomatoes. In the case of corn, given the marginal exceedance 

of the adult chronic risk LOC, and the that only 3% of the refined Tier I RQs exceeded this threshold, while 

there were no adult acute oral or larval chronic oral LOC exceedances, the potential impact of the lack of 

Tier II analysis is determined to be low for the seed-treated corn use pattern. Finally, as distinguished 
277

from corn and tomato, the lack of nectar data in the strawberry is considered a limitation as strawberries 

are noted to produce nectar that is attractive to honey bees. In this case, not only were the refined Tier I 

RQs likely underestimated by the absence of a nectar component, but a Tier II analysis could not be 

conducted. Therefore the lack of nectar data in this case presents a higher level of uncertainty as 

compared to the corn and tomato cases.

The second biggest potential impact is for use patterns where the pollen residues are markedly higher 

than nectar residues. This was the case for the cherry study in which residues in pollen were noted to be 

100-fold higher as compared to nectar residues. In this case, while risk was identified all life stages/ 

exposure durations in the refined Tier I analysis, there were no daily average residues in nectar above the 

Tier II NOAEC. To the extent that the Tier II sucrose colony-level study underestimates effects resulting 

from combined nectar/pollen route of exposure, there is potential to underestimate risk for imidacloprid 

in these cases. 

6.2.4. Risk Characterization of Non-Apis Bees

Consistent with the Agency’s 2014 risk assessment guidance for bees, the preliminary risk assessment of 

registered agricultural uses of imidacloprid focuses on the honey bee, A. mellifera. This Apis-centric focus 

reflects two important considerations: 1) honey bees are widely recognized as the most important 

managed pollinator in most regions of the world from both a commercial and ecological perspective;23 

and 2) standardized test methods for evaluating exposure and effects of chemicals in a regulatory context 

are much more developed with the honey bee compared to non-Apis bees (USEPA et al. 2014; USEPA 

201224), although recent progress has been made on test method development for bumble bees25. 

Nonetheless, within North America alone, there are an estimated 4,000 species of bees (Michener 2007) 

and this number rises to more than 20,000 worldwide (Fischer and Moriarty 2014). Several species of nonApis bees are commercially managed for their pollination services, including bumble bees (Bombus spp.), 

leaf cutting bees (Megachile rotundata), alkali bees (Nomia melanderi), and blue orchard bees (Osmia 

lignaria), and the Japanese horn-faced bee (O. cornifrons). Importantly, a growing body of information 

indicates native bees (in addition to other insect pollinators such as flies, moths, butterflies, beetles, 

wasps, and ants) play an important role in crop and native plant pollination, besides their overall 

ecological importance via maintaining biological diversity. Although the current risk assessment process 

for bees does not include a formal process that is specific to non-Apis bees, available data related to the 

 23 According to Tautz, J. (2008), approximately 80% of the world’s flowing plants are pollinated by insects and 85% of these by 

honey bees. In all, the list of flowering plants pollinated by honey bees includes 170,000 species.

24 USEPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees. Submitted to the FIFRA Scientific 

Advisory Panel for Review and Comment September 11 – 14, 2012. Office of Chemical Safety and Pollution Prevention Office of 

Pesticide Programs Environmental Fate and Effects Division, Environmental Protection Agency, Washington DC; Environmental 

Assessment Directorate, Pest Management Regulatory Agency, Health Canada, Ottawa, CN; California Department of Pesticide 

Regulation

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-0543-0004

25 Compilation of results of the ICPPR non-Apis working group with a special focus on the bumble bee acute oral and 

contact toxicity ring test 2014 ICPPR Non-Apis Working Group. Available at: 

http://pub.jki.bund.de/index.php/JKA/article/view/5352
278

potential exposure of non-Apis bees to imidacloprid and subsequent effects are summarized here in 

relation to the previously described risk assessment for the honey bee. 

Exposure Considerations 

Several aspects of the biology and ecology of non-Apis bees lead to important differences in the route and 

extent to which they may be exposed to pesticides compared to honey bees. These aspects have been 

reviewed previously (EFSA 2012, Fisher and Moriarty 2014) and are summarized here briefly. Specifically, 

many non-Apis bees are smaller in size and thus, would receive a higher dose on a contact exposure basis 

(i.e., greater surface area to volume ratio) via intercepting droplets of sprayed pesticide. Most non-Apis

bees are solitary nesting species26 and therefore, loss of a single nesting adult would have a much greater 

consequence on reproduction (at least for that nest) compared to the loss of a single adult foraging honey 

bee. Furthermore, the foraging range of non-Apis bees tends to be much smaller than that of honey bees. 

As a consequence, non-Apis bees that occupy areas adjacent to treated fields may be exposed to 

pesticides at a higher proportion of their foraging area compared to honey bees, which can forage over 

long distances (~7 km) in which they are more likely to encounter untreated forage areas. For ground 

nesting bees, exposure via direct contact with soil may be a major route of exposure unlike that for the 

honey bee. Soil and leaf material are known to be used extensively by some non-Apis bees for nest 

construction, which may lead to different types of exposures (e.g., prolonged contact exposure with 

contaminated residues on treated foliage).

To investigate the extent to which exposure estimates for honey bees may serve as a surrogate for nonApis bees, comparisons were made in the daily consumptions rates of pollen and nectar available from 

the literature as compiled by EFSA (2012). Although there are a number of uncertainties associated with 

these consumption estimates, the data in Tables 6-58 and 6-59 suggest that proposed food consumption 

rate for adult honey bee workers (292 mg/bee/day) is similar to that for adult bumble bee (210-402 

mg/bee/day) and is greater than that of adult female European mason bee and alfalfa leaf cutting bees 

(45-193 and 110-165 mg/bee/day, respectively). Food consumption rates estimated for 5-day old honey 

bee larvae (120 mg/bee/day) are greater than rates for larvae of the other non-Apis bees (7.8-83 

mg/bee/day). These data suggest that the Tier I exposure assessment conducted for oral ingestion of 

imidacloprid by adult honey bees would be representative (and generally protective) for adults these 

particular non-Apis bees. However, it is noted that unlike honey bee larvae which are fed processed pollen 

and nectar in the form of bee bread, larvae of bumble bees and other non-Apis bees consume pollen and 

nectar directly which may lead to differential exposure relative to Apis larvae. 

 26 Colonies of the social non-Apis bees (e.g., bumble bees and stingless bees) tend to be smaller than honey bees. 
279

Table 6-58. Comparison of oral exposure to pollen and nectar for adult Apis and Non-Apis bees1

Species Nectar consumption rate 

(mg/bee/day)*

Pollen consumption 

rate (mg/bee/day)

Total food consumption 

rate (mg/bee/day)

Honey bee worker

(A. mellifera) 292 0.04 292

Bumble bee 

(Bombus spp.) 183-372 27-30 210-402

European mason bee 

(Osmia cornuta) 45-193 N/A 45-193

Alfalfa leaf-cutting bee

(Megachile rotundata) 110-165 N/A 110-165

1From EFSA (2012); N/A = not applicable

Table 6-59. Comparison of oral exposure to pollen and nectar for larval Apis and Non-Apis bees1

Species Male/ 

female

Nectar consumption 

rate (mg/bee/day) *

Pollen consumption 

rate (mg/bee/day) *

Total food consumption 

rate (mg/bee/day)

Honey bee

(A. mellifera) Female 117 2.7 120

Bumble bee

(Bombus spp.) unknown 60 22-23 82-83

European mason bee

(Osmia cornuta)

Female 1.8 16.3 18

Male 1.1 9.5 11

Alfalfa leaf-cutting bee

(Megachile rotundata)

Female 6.2 3.1 9.3

Male 5.2 2.6 7.8 1 From EFSA (2012); * = from stored provisions

As discussed previously, non-Apis bees are expected to be exposed to pesticides via soil and plant material 

used for nest construction. For the European mason bee, contact exposure to mud by adult females has 

been estimated at 200 – 400 mg/bee/day. Similarly, contact exposure of alfalfa leaf cutting bees has been 

estimated at 173 mg/bee/day. Due to the limitations in available data, the current risk assessment process 

for honey bee does not address exposure via soil and foliar contact exposure which are likely more 

important for some non-Apis bees. 

Another important aspect to consider regarding the potential exposure of non-Apis bees to imidacloprid 

is the extent to which they are attracted to agricultural crops to which it is registered for use. Based on a 

recent compilation of crop attractiveness ratings for Apis and non-Apis bees (USDA 2014), bumble bees 

are classified as being as (or more) attracted to the crops registered for imidacloprid use as honey bees. 

For certain crops (e.g., tomatoes, blueberries), bumble bees are commercially managed to provide 

pollination services (although tomato pollination primarily occurs in greenhouses). 
280

Toxicity Considerations

Tier I (Organism) Level

In this section, Tier I (organism level) toxicity data for Apis and non-Apis bees are compared in order to 

evaluate the relative sensitivity of Apis and non-Apis bees to imidacloprid. Details of the studies from 

which these data were obtained are described earlier in Section 5.1. Based on these data, the overall 

range of acute contact toxicity is summarized below in Table 6-60 for Apis and non-Apis bees. While data 

for non-Apis bees are far less abundant compared to Apis bees and uncertainties have been noted 

previously related to the conduct of these studies, the acute contact toxicity of imidacloprid to non-Apis

bees 0.02 – 0.66 μg a.i./bee) appears to be within the lower bound of that observed with Apis bees (0.013 

– 0.24 μg a.i./bee), when considering all formulation types and data sources.

Table 6-60. Comparison of imidacloprid acute contact toxicity to Apis and non-Apis bees

Species Source Formulation LD50 Range

(μg a.i./bee) n

Honey bee

(A. mellifera) Registrant submitted TGAI 0.043 – 0.10 5

Honey bee1

(A. mellifera Open literature TGAI 0.013 – 0.23 11

Honey bee1

(A. mellifera

Registrant and open 

literature TEP 0.03 – 0.24 4

Bumble bee

(Bombus terrestris) Open literature TEP 0.02 1

Japanese orchard bee

(Osmia cornifrons) Open literature TEP 0.66 1

Stingless bee

(Melipona quadrifasciata) Open literature TEP 0.023 1

1 includes subspecies carnica and caucasica. 

Value in bold indicates the LD50 used in to assess risks to the honey bee. Data sources are described in Section 5.1. Non-definitive (>) values are 

excluded from this table.

The overall range in acute oral toxicity of imidacloprid to Apis and non-Apis bees is summarized below in 

Table 6-61. For non-Apis bees, only two definitive LD50 values were available for B. terrestris. While again 

the availability of data for non-Apis bees is extremely limited, these data also suggest that at an organism 

level, the acute oral toxicity of imidacloprid to B. terrestris is well within the ranges observed for A. 

mellifera. Therefore, at least for the few non-Apis bee species for which comparative toxicity data are 

available, the Tier I assessment conducted for the honey bee appears to be reasonably representative of 

these currently tested non-Apis species. 

Table 6-61. Comparison of imidacloprid acute oral toxicity to Apis and non-Apis bees

Species Source Formulation LD50 Range

(μg a.i./bee) n

Honey bee

(A. Mellifera) Registrant submitted TGAI 0.0039 – 0.15 3
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Species Source Formulation LD50 Range

(μg a.i./bee) n

Honey bee1

(A. Mellifera Open literature TGAI 0.0037 – 0.54 7

Honey bee1

(A. Mellifera

Registrant and open 

literature TEP 0.011 – 0.19 8

Bumble bee

(Bombus terrestris)

Registrant and open 

literature TEP 0.02-0.17 2

1 includes subspecies carnica and caucasica. 

Value in bold indicates the LD50 used in to assess risks to the honey bee. Data sources are described in Section 5.1. Non-definitive (>) values are 

excluded from this table.

Tier II (Colony Level)

Data concerning the effects of imidacloprid on non-Apis social bees are available for only the bumble bee 

(B. terrestris or B. impatiens); however, these data are relatively plentiful (2 tunnel studies and 8 feeding 

studies; Table 5.2). For various reasons described in Section 5.2 including lack of raw data to verify 

statistical endpoints, these data are considered only for qualitative use in this risk assessment. When 

evaluating the effects of pesticides on bumble bees at the colony level, it is important to consider the 

differences in biology with respect to honey bees. Specifically, bumble bee colonies do not survive over 

wintering, rather only queens overwinter and are available for propagation in the following spring. 

Although the science behind pesticide risk assessment with bumble bees is still evolving,27 a clearly 

important consideration with respect to maintaining the stability of bumble bee populations is the 

production and propagation of queens; however, this is true for honey bee colonies as well even though 

the queen is not alone.

With respect to the tunnel studies in which application of a formulated product to a surrogate crop is 

evaluated, only data from Gels (2000; MRID 47796308) are considered informative for risk 

characterization. In this study, Gels (ibid) reported statistically significant reductions (60%) in number of 

workers and brood chambers (72%) of tunneled B. impatiens colonies 28-days after spray applications of 

0.3 lbs. a.i./A of Merit® 75 WP to turf containing flowering white clover. Interestingly, statisticallysignificant effects were not observed following application of granular Merit® 0.5 G (0.4 lbs. a.i./A) to turf 

nor when irrigation immediately followed the aforementioned spray application. However, the statistical 

power of this study appears low due to the small sample size such that reductions of up to 70% were not 

statistically significant for some endpoints. It is also noted that there is uncertainty in the suitability of 

maintaining bumble bee colonies in tunnels for 28-days. Give these uncertainties, this study suggests that 

spray applications of 0.3 lbs. a.i./A to turf containing bumble bee-attractive flowering plants may cause 

deleterious effects on bumble bee worker production.

Much more data are available on the prolonged oral exposure of bumble bee colonies to imidacloprid and 

these data suggest a relatively congruent profile of imidacloprid effects at the colony level (Mommaerts 

2010, MRID 48151502; Gill 2012, MRID 49719618; Laycock 2012, MRID 49719622; Laycock and Cresswell 

2013, MRID 49719621; Bryden 2013, MRID 49719607; Gill and Raine 2014; Whitehorn 2012, MRID 

 27 http://pub.jki.bund.de/index.php/JKA/issue/view/1087
282

49719634; Feltham 2014, MRID 49719617). Details of these studies are summarized in Section 5.2 and 

Appendix D. Rather, the levels at which imidacloprid resulted in colony-level effects to bumble bees is 

summarized relative to the oral (sucrose) NOAEC of 25 ppb observed with the honey bee (MRID 

49510001). Specifically, 6 of the 8 aforementioned studies tested sucrose concentrations fed to B. 

terrestris colonies that included (or spanned) 10 ppb in sucrose, as indicated in Figure 6-36,. Despite 

differences in the duration of exposure (14 days to 11 weeks), colony sizes and methods used to assess 

effects on the colonies, 4 of these studies documented major (and statistically significant) effects on B.

terrestris colonies fed 10 ppb imidacloprid in sucrose, including (but not limited to) increased worker 

mortality, decreased numbers of worker bees produced, reductions in foraging efficiency, increased time 

required to collect pollen, and decreases in the quantity pollen collected. In some cases, worker bee 

production increased which was presumably as a compensatory response to reduced food provisions. 

Notably, 2 of these 6 studies (Laycock 2012; Laycock and Cresswell 2013) report 42% reduction in 

fecundity for microcolonies fed 1 ppb imidacloprid (TGAI) for 14 days and an EC50 of 1.4 ppb for brood 

production after the same duration of exposure. These levels approach the limit of detection of 

imidacloprid in nectar (0.7 ppb). Interestingly, when B. terrestris colonies were fed uncontaminated 

sucrose following the 14-day exposure, brood production recovered to the level of the control group. This 

suggests that the duration of exposure of B. terrestris colonies to imidacloprid is critical with respect to 

expression of colony-level effects and that recovery of colonies is possible given sufficient time off dose.

Two other studies (Whitehorn 2012 and Feltham 2014) fed B. terrestris colonies a mixture of low levels of 

imidacloprid in sucrose and pollen for 14 days in the laboratory followed by 4-6 weeks off dose in the 

field. At 0.7 ppb (sucrose) and 6 ppb (pollen), Whitehorn (2012) report an 85% reduction in queen 

production and significantly reduced colony weight relative to controls. Feltham (2014) reported 

reductions of 28% and 31% in collected pollen and foraging efficiency, respectively, following 14 days at 

the same dose levels in sucrose and pollen. These findings are significant given the widespread occurrence 

of imidacloprid in nectar above these levels following application to crops, including seed treatment. As 

noted in the Section 5, however, these studies were not considered suitable for quantitative use in the 

risk assessment, and therefore additional data (i.e. Tier II and Tier III studies with Bombus) would benefit 

the risk characterization for non-Apis bees.
283

Figure 6-36. Comparison of effect levels from qualitative Tier II feeding studies on B. terrestris obtained 

from the open literature (numbers in parentheses refer to the magnitude of effects and/or additional 

exposure to pollen)

6.2.5. Additional Lines of Evidence

Monitoring of residues in agricultural and hives

As described in Section 4, there are several studies available in the open literature that investigated the 

residues of imidacloprid both in agricultural fields with known imidacloprid use, as well as the various hive 

matrices of colonies in areas across the United States and Europe. The agricultural field studies (Bonmatin 

2005 and 2007), which sampled pollen residues from seed-treated corn and sunflower. They determined 

that while the frequency of quantifiable residues ranged from between 36 – 58% of the total samples 

analyzed, the mean concentrations of detectable residues ranged from 0.6 – 3.0 ppb, which was just above 

the LOQ of these studies. 

In the hive monitoring studies, surveys conducted across the United States and Europe measured residues 

in various hive matrices for the presence of imidacloprid. In some studies, known areas of diseased 

colonies were sampled in addition to healthy colonies while for other studies this information was not 

always present. These studies generally indicated (inclusive of all study areas) was generally detected in 

10% or less of pollen, honey, wax or honey bee samples (highest concentration was reported in trapped 

pollen at 149 ppb). In studies where there was a higher frequency of imidacloprid detections (e.g. Chauzat 

et al. 2006 and 2009), the mean residues ranged from below the LOD – 5.7 ppb, a level 5-fold below the 
284

Tier II colony feeding study. In other studies (Bee Research Institutes 2008, and Mullin et al. 2010) several 

hundred samples were tested in each study from various matrices with imidacloprid being detected a 

maximum of 2.9% across both studies. Stoner and Eitzer (2013) screened over 300 pollen samples and 

while imidacloprid was detected in 12% of the samples, the mean residues of 5.2 ppb were similar to 

those found in the work by Chauzat et al. In recent work by Lu (2015), monthly pollen and honey samples 

were collected across hives in Massachusetts and screened exclusively for neonicotinoid pesticides. While 

imidacloprid was detected in 57% of the pollen samples and 53% of the honey samples, the mean residues 

were 0.1 ppb (equivalent to the LOQ) and 0.58 ppb, respectively. 

An additional point to be made from these studies is that for all studies except Lu (2015) (which screened 

only for neonicotinoid pesticides), multiple pesticides were found in the same samples, with some 

samples containing up to 12 pesticides. In the majority of these cases, the Varroa mite treatment 

miticides fluvalinate, coumaphos, and amitraz were detected, in some cases in up to 98% of the assessed 

samples, depending on the matrix (Mullin et al., 2010). Additionally, fungicides, particularly those of the 

sterol biosynthesis inhibitor class that include the triazole fungicides were detected with high frequency. 

This discussion illustrates, that while imidacloprid has an estimated usage of over 1 million pounds of 

applied active ingredient on an annual basis in the U.S., monitoring surveys in agricultural fields and hive 

matrices generally do not detect the chemical with great frequency. In cases where the frequency of 

detections was 50% or more, the mean residues typically did not exceed 5 ppb (inclusive of all assessed 

matrices). 

While the suite of reported pollinator incidents originating from agricultural uses with analytical 

confirmation of residues is small (i.e. 6 incidents), a lack of reported incidents does not equate to the 

absence of honey bees and other pollinators losses due to the registered use patterns of imidacloprid.

6.2.6. Higher Tier and other General Uncertainties

While the uncertainties associated with the screening-level exposure assessment were previously 

discussed, what follows are additional general uncertainties and those related to Tier II:

Tier II Uncertainties

Uncertainties at the Tier II level originate primarily from those identified for the registrant-submitted Tier 

II colony feeding study. These uncertainties and limitations are provided in Section 5.2. 

The primary uncertainty at the Tier II level relates to the interpretation of Tier 2 risks based on the 6-week, 

sucrose colony feeding study. It assumes that bees forage on the treated crop nearly 100% of the time to 

represent the nectar needs of the colony. In the field, bees may forage for significantly shorter periods of 

time particularly for crops such as cherries and blueberries that have a 2-3 weeks blooming duration. Bees 

may also forage on alternative (untreated) plants. Conversely, bees associated with migratory colonies 

used for pollination services may feed on treated crops for similar or possibly longer periods of time over 

the course of a growing season.
285

Additionally, and as was indicated previously, the 6-week duration of the exposure phase in the available 

colony feeding study may be an under or overestimation of the duration of exposure that is potential for 

bees in a given area. For example, citrus fruit trees and cotton are noted to have a bloom duration of at 

least 6 weeks, consistent with the colony feeding study exposure phase, while stone fruit trees have a 

shorter bloom duration of 2-3 weeks.

Finally, as identified previously, there is uncertainty in the lack of a quantitative assessment of effects at 

the colony level resulting from the pollen route of exposure. This stems primarily from the fact that the 

available colony feeding study assessed spiked sucrose as well as an inconsistent picture of effects from 

the available colony-level studies assessing this route of exposure. However, as noted in Section 6.2.3., 

multiple lines of evidence suggest that honey bees are less exposed to pollen compared to sucrose and 

may be less sensitive to imidacloprid residues in pollen compared to nectar at the colony level. 

General Uncertainties

Extrapolation of Effects to Higher Tiers

There is uncertainty with respect to the Tier I suite of studies in the extrapolation of these effects at the 

colony level (i.e. Tier II and Tier III). This uncertainty is reduced with more thorough consideration of 

agronomic practices, pollinator biology, differences in bee life history, and differences in 

pest/pathogen/nutrition/management.

Agronomic Practices

One of the most important considerations within the agronomic practices is the use of managed 

pollinators for crop production. For some crops, growers will bring in managed bees to augment the 

pollination services of local bees if the crop requires pollination and wild bee populations are insufficient 

for adequate pollination. These commercially managed bees may include honey bees, bumble bees, blue 

orchard bees, alfalfa leafcutting bees, etc. When commercially managed bees are used to pollinate a crop, 

the potential for exposure and the magnitude of that exposure to the pollinating bees may be greatly 

increased. Depending on the physical-chemical property of a pesticide and use methods, agronomic 

practices, such as irrigation, may affect the translocation of systemic pesticides and have effects on the 

residues in bee food sources. The agronomic practices may also be related to the extent to which a 

particular registered use may be applied across the landscape of the use. Different use patterns may 

occupy varying spatial extents of coverage.

Pollination Biology

Uncertainties on the exposure of pesticides to pollinators in the field are also associated with plant/crop 

pollination biology. Risk assessment usually encompasses a wide variety of crops or plants that have niche 

pollination characteristics. These plants may include ornamental annuals or perennials, trees or bushes 

covered under forestry uses, or annual or perennial crops. The pollination biology of each of these plants 

is important to consider when developing a description of the potential risk to bees. In the problem 
286

formulation stage of the assessment, information on the attractiveness of plants to pollinators covered 

by the proposed/existing uses should be considered to determine whether exposure may occur and the 

scope of a risk assessment. At the risk description phase of the assessment, information on the pollinator 

attractiveness of the plant as well as acreages treated and application methods will help the risk assessor 

to determine the spatial and temporal aspect of risk to the bee pollinators identified in the problem 

formulation as well as potential mitigation solutions. The pollination biology of plants relates to the 

intrinsic characteristics of the plant itself. These characteristics include the following considerations:

Bee visitation to the flowers of the tree or crop: Not all plants produce flowersthat are attractive sources 

of forage for honey bees, bumble bees, or solitary bees. Conversely, flowers of some plant species last 

only short period. The short blooming duration reduces the potential exposure of flower visitors.

Bloom period of the crop: Different plant species will bloom at different times of the year. In addition,

the length of the bloom period can differ between plants. Some plants bloom within a specific, relatively

narrow window of time called a determinate bloom period. Other crops may produce blossoms

continuously over the course of the growing season (e.g. cotton, cucurbits) or for an extended period of 

time, which is called indeterminate bloom. Indeterminate blooming crops provide a much longer window 

of potential exposure to pollinating bees.

Bee diversity: In terms of the types of bees, honey bees and bumble bees are colonial while there are a

variety of bees, both managed and wild, that are solitary and, depending on the plant, their foraging

strategies may differ substantially; therefore, potential exposure may differ.

Differences in Bee Life History

As noted in the White Paper (USEPA et al. 2012) and as discussed in the FIFRA SAP’s response (SAP 2012), 

there is uncertainty regarding the extent to which any risk assessment process that relies on data on a 

specific species (e.g., A. mellifera) can be considered representative of an entire taxon or multiple taxa. 

This is especially true for honey bees, which are a highly social (eusocial) species, where the colony/hive 

is dependent on the collective tasks of multiple castes and function as a “superorganism”; whereas, the 

majority of other bee species, particularly those species native to North America, are solitary.

Differences in Pests/Pathogens/Nutrition/Management

Multiple factors can influence the strength and survival of bees whether they are solitary or social. These 

factors, including disease, pests (e.g., mites), nutrition, bee management practices, can confound the 

interpretation of studies intended to examine the relationship of the test chemical to a receptor (i.e., 

larval or adult bee). Therefore, most studies attempt to minimize the extent to which these other factors 

impact the study; however, higher-tier studies afford less control over these other factors, and their role 

may become increasingly prominent as the duration of the study is extended. Although studies attempt 

to minimize the confounding effects of other environmental factors, there is uncertainty regarding the 

extent to which the effects of a chemical may be substantially different had these other factors been in 

place.
287

7. Conclusions 

As previously discussed, the risk assessment approach to honey bees proceeds in a stepwise, tiered 

process. The agency conducted a screening level assessment (Tier I) for the various uses of imidacloprid 

utilizing toxicity endpoints and either conservative (modeled) exposures or, as a more refined assessment, 

actual residue values from pollen and/or nectar (where data were available) to determine if there are 

risks to individual bees. If these analysis indicated that the level of concern is not exceeded, the agency 

concluded that there is not a risk and that there is not a concern at the colony level. In these instances, 

no further analysis was necessary. However, if the analysis demonstrated a risk to individual bees, the 

agency did, when data were available, conduct a risk assessment to determine whether there were risks 

posed to the colony. As mentioned above and further described in Section 2 (Problem Formulation), the 

risk assessment approach to honey bees proceeds in a stepwise, tiered process evaluating risks to 

individual bees first and, if needed, risks to the colony. After the initial step in determining the potential 

for exposure of bees to agricultural uses of imidacloprid, risk quotients (or levels of concern) are estimated 

to evaluate the risk to individual bees using modeled/screening-level exposure estimates and the acute 

and chronic laboratory toxicity endpoints (i.e. adult acute contact LD50, adult acute oral LD50, adult chronic 

oral NOAEL, and larval chronic oral LOAEL). For all crops and application methods where on-field 

exposure, is expected, values exceeded risk levels of concern. Even in cases where on-field exposure was 

not expected, an off-field spray drift assessment was conducted and indicated that there could be risk for 

all foliar uses (depending on what crop is adjacent to the field, whether the crop is in bloom, whether the 

crop is pollinator attractive, etc). Additionally, a refined analysis was conducted using available measured 

residue data to supplant the modeled/screening-level estimates of exposure that were mentioned above. 

These refined values were compared to the hazard endpointstabulated above. For all use patterns where 

residue data were available, LOCs were exceeded based on refined estimates of exposure. 

Table 7-1 summarizes the agency’s preliminary risk findings on a crop group-based approach. The table 

presents the findings for groups of crops that have similar use patterns and application methods and are 

further split out into three categories of risk findings. When residue data are available, the crop is 

identified parenthetically within the table along with the respective crop group. 

Crop groups/use patterns where either on-field exposure is not anticipated due to attractiveness or the 

crop is harvested before bloom, or the tiered process indicates a low potential for on-field risk, are listed 

in the green group in Table 7-1. These include all application methods of root/tuberous, bulb, leafy 

greens, and brassica vegetables, globe artichoke, and tobacco (harvested before bloom) as well as soil 

applications to blueberries (berries and small fruits) and seed treatment applications to corn (cereal 

grains). Additional members of the cereal grain group (which is registered for seed treatment uses only) 

including wheat, barley, oats, rye, and millet are either not attractive to honey bees or primarily wind 

pollinated. Finally, members of the fruiting vegetable group (of which soil and soil + foliar residues data 

for tomato are available) are largely unattractive to honey bees with the exception of okra. Therefore, a 

low potential for on-field risk is determined for all members of this group, except okra, for all application 

methods based on a lack of exposure.
288

The yellow/gold group represents crop groups/use patterns for which the assessment for individual bees 

indicates that the LOCs have been exceeded; however, uncertainty exists in the assessment of risk to the 

colony. These include uses where either no data are available (with indications of the potential to bridge 

to other neonicotinoid chemicals where data are expected for that same use pattern and application 

method) or where there is uncertainty in the nectar and pollen residue data originating from uncertainties 

in the available studies. For several crop groups including legumes, tree nuts, and certain application 

methods of stone fruits, berries/small fruits, and oilseed, residue data are unavailable but there is the 

potential to bridge from data for other neonicotinoid chemicals with forthcoming data for certain 

application methods. In other cases, data are not available and there are no data expected for the other 

neonicotinoid chemicals such as certain application methods for legumes, tree nuts, berries/small fruits, 

nectar producing cereal grain members, and herbs and spices. In the case of cucurbit vegetables (soil 

applications to melons data available), citrus fruits (soil applications to oranges and grapefruits data 

available), and berries/small fruits (soil applications to strawberries data available), there are limitations 

with the residue studies that create uncertainty in the risk determinations with these use 

patterns/application methods. This uncertainty is generally associated with these studies having an 

unknown timing of application relative to bloom (strawberry), no nectar data available (strawberry), no 

pollen data available (citrus fruits), and no available residue data from coarse soils, which are shown 

through several studies to yield residues in nectar and pollen up to an order of magnitude higher as 

compared to medium and fine soil types. Furthermore, the soil-applied citrus study was conducted with 

a post-bloom application while the label does not restrict pre-bloom or during bloom applications and 

therefore the residues from this study are likely underestimated. For soil applications to cucurbits and 

citrus fruits, there is a potential to bridge with forthcoming data for other neonicotinoid chemicals. In the 

case of cucurbit vegetables, a full field study (Tier III) on pumpkins is expected in 2016 to further refine 

the risk picture. Additionally, although foliar applications to stone fruits resulted in pollen residues 

exceeding a threshold that is indicated in the open literature to cause colony level effects, the bloom 

duration of stone fruits is markedly shorter than the exposure duration employed in from those studies 

that determined these effects and therefore there is uncertainty with this determination. Finally, while 

data are unavailable for pome fruits, residue data for imidacloprid are expected in 2016.

Lastly, the red grouping within the table indicates use patterns with associated application methods that 

present a risk to individual bees as well as a risk in nectar or both nectar or pollen. These include foliar 

applications (with a 10-day pre-bloom interval) to citrus fruits and foliar, soil, soil + foliar, and seed 

treatment + foliar applications. (with no bloom restrictions) to cotton. A full field study with cotton is 

expected in 2016 to further refine this risk determination. 
289

Table 7-1. Summary of risk findings for honey bees (Apis mellifera) for the registered use patterns of imidacloprid

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

Crop Groups/Use Patterns that Present Low On-Field Risk

Root/Tuber 

Vegetables4

Foliar N Y

No further analysis conducted

Low On-Field Risk (all uses, lack of exposure)1

; Off-Field

Risk (Tier I, foliar uses only) Soil N

Seed N

Bulb 

Vegetables

Soil N Low On-Field Risk (all uses, lack of exposure)1

; Off-Field 

Seed N Risk (Tier I, foliar uses only)

Leafy Greens 

Vegetables

Foliar N Y Low On-Field Risk (all uses, lack of exposure)1

;

Soil N Off-Field Risk (Tier I, foliar uses only)

Brassica 

Vegetables

Foliar N Y

Low On-Field Risk (all uses, lack of exposure)1; Off-Field

Risk (Tier I, foliar uses only) Soil N

Seed N 

Fruiting 

Vegetables 

(Tomatoes)

Foliar Y Y

Y No data2 N

Low On-Field Risk (Tier II, pollen; nectar not produced, 

lack of exposure)

Off-Field Risk (Tier I, foliar uses only)

(Determinations apply to all members except okra due 

to unattractiveness of group to honey bees, Bombus 

used for pollination services in greenhouse)

Soil Y

Berries/Small 

Fruits 

(Blueberry)

Soil Y Y N N Low On and Off-Field Risk (Tier II, nectar and pollen)

Cereal Grains

(Corn) Seed Y Y No data2 N

Low On and Off-Field Risk (pollen; nectar not produced)

(Other members such as wheat, barley, oats, millet and 

rye are either not attractive to bees)

Tobacco, globe 

artichoke

Foliar N Y

No further analysis conducted Low On-Field Risk (all uses, lack of exposure)1

;

Soil N Off-Field Risk (Tier I, foliar uses only)

Crop Groups/Use Patterns with Uncertainty in Colony (Tier II) Assessment

Legumes

Foliar Y Y No data No data No data On Field Risk (Tier I, all uses); Tier II Risk unknown

Off Field Risk (Tier I, foliar uses only)

(Honey bee attractive; no bloom restrictions; seed 

treatment of soybean = highest usage of all registered 

crops (400,000 lbs a.i/year). 

Soil Y No data No data No data

Seed Y No data No data No data
290

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

(Potential 

bridging)

(Potential 

bridging)

(Potential 

bridging)

Cucurbit 

Vegetables 

(Melons)

Soil Y Y

Uncertain 

(Potential 

bridging)

Uncertain 

(Potential 

bridging)

On-Field Risk (Tier I); Tier II Risk uncertain 

(Long [6 weeks +] bloom duration; uncertainty of lower 

than maximum annual rate used and one sampling 

interval, no residues in coarse soils, unknown as to 

whether application closer to bloom would yield higher 

residues; Tier III full field study [pumpkins] expected for 

2016 assessment)

Citrus Fruits 

(Oranges/ 

grapefruits)

Soil Y Y

Uncertain 

(Potential 

bridging)

No data

(Potential 

bridging)

On-Field Risk (Tier I); Tier II Risk uncertain

(6 week + bloom duration; uncertainty of no residues in 

coarse soils and residues do not reflect worst case 

scenario as current labels permit pre and during bloom 

applications where these applications were made postbloom)

Pome Fruits

Foliar Y Y Y No data No data On-Field Risk (Tier I); Off-Field Risk (Tier I, foliar uses 

only)

(Residue data expected in 2016) Soil Y Y No data No data

Stone Fruits 

(Cherries) Foliar Y Y Y N Possible

Low On-Field Risk (Tier II, Nectar;), Tier II Risk possible 

(Pollen); Off-Field Risk (Tier I) 

(Stone fruits associated with short bloom duration [2-3 

weeks] relative to exposure duration in open literature 

pollen feeding study [12 weeks] which likely mitigates 

the potential for colony level from pollen route of 

exposure)

Stone Fruits Soil Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I); Tier II Risk unknown

Berries/small 

fruits Foliar Y Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I); Tier II Risk unknown

Off-Field Risk (Tier I)

Berries and 

small fruits Soil Y Y No data Possible On-Field Risk (Tier I); Tier II Risk possible (pollen)
291

Crop Group

(Available 

Residue Data)

Appl. 

Method

Individual Bee (Tier I) Risk? Colony (Tier II) Risk?

Risk Conclusions

(Basis and Other Considerations )

On Field 

(Screening 

Level)

Off Field 

(Screening 

Level)

On Field

(Refined) Nectar Pollen3

(Strawberries) (Long [6 weeks +] bloom duration; uncertainty of one 

sampling interval, no residues in coarse soils, unknown 

timing of application relative to bloom

Tree nuts

Foliar Y Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-Field Risk (Tier I, all uses); Tier II Risk unknown

(Variable bee attractiveness within group); 

Off-Field Risk (Tier I, foliar uses only) Soil Y Y No data No data

Cereal grains Seed Y Y No data No data

On-Field Risk (Tier I); Tier II Risk unknown 

(Nectar producers within the group (i.e. sorghum, 

buckwheat).

Herbs/Spices

Foliar Y Y Y No data No data On-Field Risk (Tier I); Tier II Risk unknown

Off-field Risk (Tier I, foliar uses only)

(Variable attractiveness within group)

Soil Y Y No data No data

Seed Y Y No data No data

Oilseed5 Seed Y Y

No data

(potential 

bridging)

No data 

(potential 

bridging)

On-field Risk (Tier I), Tier II Risk unknown

Crop Groups/Use Patterns with Colony (Tier II) Risk Indicated

Citrus Fruits

(Oranges) Foliar Y Y Y Y Possible

On-field Risk (Tier I), Tier II Risk (nectar), Tier II Risk 

possible (pollen)

Off-field Risk (Tier I) 

(10-d pre-bloom restriction for foliar uses; 6 week + 

bloom duration; used for honey production)

Oilseed5

(Cotton)

Foliar Y Y Y Y Possible On-field Risk (Tier I), Tier II Risk (nectar), Tier II Risk 

possible (pollen), Off-field Risk (Tier I, foliar uses only)

(Tier III full field study [cotton] expected for 2016 

assessment.

Soil Y Y Y Possible

Hash marks represent no off-field exposure expected for soil and seed treatment uses. 1 Crop is harvested before bloom (except for small acreage for seed production; nectar and pollen residue data were not required as minimal on-field exposure is expected. 2Nectar is not produced by representative crop where residue data are available

3Possible Tier II Risk for pollen indicated when residues in pollen from a residue study exceed 100 ppb, which is indicated in the literature to be a level where colony 

overwintering survival is potentially impacted.

4Two members of this group, potatoes and sweet potato, are noted to be harvested after bloom, although potatoes are not honey bee attractive and in the case of sweet 

potato, require pollination only for breeding, which is a small percentage of the total acreage. 
292

5Cotton is registered for all application methods. All other members of the oilseed group including canola and sunflower are registered only for seed treatment use
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