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OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

August 18, 2015

PC Codes: 032201

MEMORANDUM DP Barcode: 427095

SUBJECT: Preliminary Ecological Risk Assessment for the Registration Review of Diquat 

Dibromide

FROM: Elyssa Arnold, Biologist, Risk Assessment Process Leader

James Lin, Environmental Engineer

Environmental Risk Branch 2

Environmental Fate and Effects Division (7507P)

THROUGH: Brian Anderson, Branch Chief

Environmental Risk Branch 2

Environmental Fate and Effects Division (7507P)

TO: Bonnie Adler, Chemical Review Manager

Jill Bloom, Team Leader

Linda Arrington, Branch Chief

Risk Management and Implementation Branch V

Pesticide Re-evaluation Division (7508P)

This memorandum transmits the preliminary ecological risk assessment for the Registration 

Review of diquat dibromide. Based on currently registered uses and labeled rates, this 

preliminary assessment identifies potential risks to non-target organisms. This assessment is 

focused on species that are not federally listed as threatened or endangered (referred to as 

“listed”); however, some general discussion of taxa for which “no effect” determinations are 

expected is included. For taxa to which potential risk is identified, specific determinations for 

listed species are considered uncertain at this time. Currently labeled rates of diquat dibromide

pose a potential risk of adverse effects, i.e., risk, to non-target terrestrial and aquatic animals, 

including birds, reptiles, amphibians, mammals, fish, and aquatic invertebrates. In addition, 

diquat dibromide uses result in risks above levels of concern for terrestrial and aquatic plants. 

These risks may be reduced through label changes such as decreases in maximum single 

application rates, decreases in the number of applications allowed per year, removal of aerial 

applications, use of spray drift buffers and specification of coarse droplet spectrum to reduce 

spray drift. 
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Ecological Risk Assessment for the Registration Review of 

Diquat Dibromide

U.S. EPA PC CODE 032201

CAS No. 85-00-7

Prepared by:

Elyssa Arnold, Biologist

Jim Lin, Environmental Engineer

Reviewed by:

Christine Hartless, Wildlife Biologist

R. David Jones, Senior Agronomist

Approved by:

Brian Anderson, Branch Chief, Environmental Risk Branch 2

Environmental Fate and Effects Division

Office of Pesticide Programs

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, NW

Washington, D.C. 20460

August 18, 2015
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Table of Contents

1. Executive Summary............................................................................................................ 4

1.1. Use Summary.......................................................................................................... 4

1.2. Predominant Risk Concerns.................................................................................... 4

1.3. Major uncertainties and Critical Data Gaps............................................................ 6

1.4. Risk Reduction Options.......................................................................................... 7

2. Introduction......................................................................................................................... 7

3. Problem Formulation Update.............................................................................................. 8

3.1. Chemical Identification and Mode of Action ......................................................... 8

3.2. Use Characterization............................................................................................... 8

3.3. Label Uncertainties............................................................................................... 12

4. Exposure Assessment........................................................................................................ 13

4.1. Environmental Fate Summary .............................................................................. 13

4.2. Residues of Concern ............................................................................................. 14

4.3. Aquatic Exposures................................................................................................ 15

4.4. Bioaccumulation ................................................................................................... 20

4.5. Monitoring Data.................................................................................................... 20

4.6. Terrestrial Exposures............................................................................................ 21

4.7. Spray Drift Transport............................................................................................ 24

5. Effects Analysis................................................................................................................ 25

5.1. Aquatic Toxicity Data........................................................................................... 25

5.2. Terrestrial Toxicity Data....................................................................................... 28

5.3. Incidents................................................................................................................ 29

5.4. Endocrine Disruptor Screening Program.............................................................. 30

6. Risk Characterization........................................................................................................ 31

6.1. Aquatic Animals ................................................................................................... 31

6.2. Aquatic Plants....................................................................................................... 36

6.3. Terrestrial Animals ............................................................................................... 38

6.4. Terrestrial Plants................................................................................................... 45

6.5. Risk Conclusions .................................................................................................. 47

7. Listed Species of Concern ................................................................................................ 48

7.1. Taxonomic Groups Potentially at Risk................................................................. 49

8. References......................................................................................................................... 50

8.1. Cited Environmental Fate and Chemistry Studies................................................ 52

8.2. Cited Ecotoxicity Studies...................................................................................... 53

Appendix A. Risk Quadrant Figures and Tables.......................................................................... 56

Appendix B. SIP Output ............................................................................................................... 58

Appendix C. STIR Output ............................................................................................................ 59

Appendix D. SWCC Example Output .......................................................................................... 60

Appendix E. T-REX Model Example Inputs................................................................................ 62

Appendix F. TERRPLANT Model Example Inputs..................................................................... 64

Appendix G. MCnest Output........................................................................................................ 65

Appendix H. Ecotoxicity Data...................................................................................................... 69
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1. Executive Summary

Diquat dibromide is a non-selective contact herbicide and desiccant used to control broadleaf and 

grassy weeds in terrestrial and aquatic areas, and as a desiccant in seed crops and potatoes. It is 

in the bipyridylium class. Diquat formulations contain diquat in the dibromide salt form, which is 

the registered active ingredient (PC 032201). This assessment, consistent with previous 

assessments, focuses on the cation form of diquat, referred to in this assessment as diquat (PC 

032202). Diquat is persistent in the environment, binds strongly to soil and sediment, and is toxic 

to nearly all tested taxa.

1.1. Use Summary

Agricultural use sites for diquat dibromide include alfalfa, canola, clover, and a large variety of 

fruit and vegetable crops. Diquat dibromide is used in agriculture for weed control as well as preharvest defoliation for seed crops and post-harvest desiccation for some fruit and vegetable 

crops. The most significant agricultural use by amount applied and percent crop treated is on 

potatoes (U.S. EPA, 2015a). Diquat dibromide is used as a desiccant/defoliant for commercial 

greenhouses and nurseries, ornamental seed crops, and commercial and residential landscaping. 

It is also used in aquatic environments to control algae and aquatic weeds, and to control tree 

root intrusions in drainage systems. When used as a desiccant, diquat dibromide can be applied 

by aircraft or ground equipment. In aquatic sites, diquat dibromide may be injected below the 

water surface for submerged weeds, or sprayed for weed control along the edges of aquatic sites. 

While diquat dibromide is the registered active ingredient (a.i.), the active component of the 

compound is diquat cation. Therefore, all application rates, exposure estimates, and toxicity 

endpoints discussed in this assessment are in units of diquat cation, referred to as diquat. Diquat 

cation (184.24 g/mol) is 53.55% of diquat dibromide (344.05 g/mol) by molecular mass. The 

maximum treatment concentration for aquatic sites is 1.47 ppm diquat. The maximum single 

application rate for terrestrial sites is 0.5 lb diquat /acre (A). The maximum annual application 

rate is 3 lb diquat/A/yr for food uses and 6 lb diquat/A/yr for ornamentals. 

1.2. Predominant Risk Concerns

Diquat dibromide is expected to be transported by spray drift and runoff to non-targeted areas. 

Since it binds strongly to soil and mainly to clay matter, diquat dibromide may also be 

transported to non-target areas by eroded soil and transported dust. In addition, because available 

studies suggest that diquat dibromide degrades slowly, if at all, the potential for accumulation 

exists in areas of continual use. There were no major degradates identified in any of the 

environmental fate studies on diquat dibromide.

Diquat dibromide exhibits acute and chronic toxicity to nearly all taxa tested, including fish, 

aquatic invertebrates, birds, mammals, and aquatic and terrestrial plants. Diquat dibromide is 

practically non-toxic to adult honey bees (Apis mellifera) on an acute contact and oral exposure 

basis, however no data are available on chronic toxicity to adult honey bees or on acute or 

chronic toxicity to larval bees. Incidents have been reported involving aquatic invertebrates, fish, 

reptiles, and terrestrial plants.
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Figure 1.1 and Table 1.1 give a broad overview of the potential risk concerns and supporting 

evidence for registered uses of diquat dibromide. An explanation of these tools can be found in 

Appendix A.

Figure 1.1. Graphic Summary of Potential Risks of Current Uses of Diquat Dibromide Certainty

Risk

Diquat Dibromide

Terr. Plants

Mammals Fish

Birds Aq. Inverts

Aq. Plants

Terr. Inverts
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Table 1.1. Evidence of Risk to Taxonomic Groups from Current Uses of Diquat Dibromide

Predominant Evidence of Negligible Risk Predominant Evidence of Risk

N/A

Diquat is expected to persist for extended periods in both 

terrestrial and aquatic environments. 

Terrestrial Plants: RQs>LOC (up to 66) via spray drift

for listed and non-listed dicots for all terrestrial uses and 

for listed and non-listed monocots for some use 

scenarios. Multiple incidents reported.

Birds, Reptiles, and Terrestrial-phase Amphibians:

Acute and chronic RQs>LOC for all terrestrial uses. One 

reptile incident reported.

Mammals: Acute and chronic RQs>LOC for all 

terrestrial uses.

Aquatic Plants: RQs>LOC for listed and non-listed 

vascular and non-vascular aquatic plants for nearly all 

use patterns. Consistent with use as an aquatic herbicide.

Aquatic Invertebrates: Acute and chronic RQs>LOC for 

multiple uses for water column and benthic aquatic 

invertebrates. One mollusk incident reported.

Fish and Aquatic-phase Amphibians: Acute RQ>LOC 

for non-listed freshwater fish following direct aquatic 

application. Acute RQs>LOC for listed freshwater fish 

for one terrestrial use. No exceedances for 

estuarine/marine fish. Multiple incidents reported.

Limited Evidence of Negligible Risk Limited Evidence of Risk

Terrestrial Invertebrates: No risks exceeding levels of 

concern for adult honey bee acute contact or oral 

exposures. No chronic toxicity data or larval toxicity 

data available.

N/A

1.3. Major uncertainties and Critical Data Gaps

Uncertainties specific to this assessment include:

• There are no chronic toxicity data available for estuarine/marine benthic 

invertebrates, only a 10-day (subchronic) study on Leptocheirus plumulosus which 

evaluated mortality. As reproductive toxicity but not lethality was observed for the

freshwater amphipod, Hyalella azteca, there is uncertainty as to whether similar 

effects would be seen in a chronic study on L. plumulosus. Diquat is expected to 

partition to sediment in aquatic systems.

• While no risks of concern to terrestrial invertebrates are identified based on acute 

contact and oral exposure routes, no toxicity data are available with which to assess 

chronic exposure to adult honey bees or acute or chronic exposure to larval bees. 

These exposure routes remain significant uncertainties for the assessment of the 

impact of diquat dibromide on pollinators. The problem formulation for diquat 
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dibromide was completed in 2010, prior to the introduction of the requirements for 

chronic testing on honey bees (USEPA et al., 2014). The Agency expects to publish 

additional guidance on addressing gaps in bee toxicity data for pesticides that have 

undergone Registration Review in the near future.

Data gaps include:

• Non-guideline: Chronic sediment toxicity for estuarine/marine invertebrates

• Terrestrial invertebrate studies

o Non-guideline (Tier 1): Honey bee larvae acute toxicity

o Non-guideline (Tier 1): Honey bee adult chronic oral toxicity

o Non-guideline (Tier 1): Honey bee larvae chronic toxicity

o Non-guideline (Tier 2): Semi-field testing for pollinators (tunnel or colony 

feeding studies)

o Non-guideline (Tier 2): Field trial of residues in pollen and nectar

o OCSPP 850.3040 (Tier 3): Field testing for pollinators

1.4. Risk Reduction Options

EFED has identified potential options for reducing the risks identified in this assessment or the 

spatial extent of potential impacts; however, risk management decisions will be based on an 

assessment of both risks and benefits:

• Use of spray drift buffers, specification of medium to coarse droplet size, and/or further 

restriction of aerial applications to reduce exposure via spray drift.

• Restricting timing of applications to fall and winter to reduce reproductive risks to avian 

species.

• Reducing the maximum single application rate, decreasing the number of applications per 

year, and/or increasing application intervals to reduce overall exposure.

2. Introduction

This preliminary risk assessment (PRA) examines the potential ecological risks associated with 

labeled uses of the herbicide diquat dibromide based on the best available scientific and 

commercial information on the use, environmental fate and transport, and effects of diquat 

dibromide on non-target organisms. This assessment is focused on species that are not federally 

listed as threatened or endangered (referred to as “listed”); however, some general discussion of 

taxa for which “no effect” determinations are expected is included. For taxa to which potential 

risk is identified, specific determinations for listed species are considered uncertain at this time. 

Methodology consistent with the recommendations of the April 2013 National Academy of

Sciences (NAS) report1 for evaluating risk to listed species is currently under development.

 1 http://www.nap.edu/catalog.php?record_id=18344
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3. Problem Formulation Update

A detailed problem formulation for the registration review of diquat dibromide was completed in 

2010 (USEPA, 2010a, 2010c). Since that time, an ecological risk assessment was conducted for a 

Section 3 new use on canola at 0.5 lb diquat/A (USEPA, 2010e) and an endangered species 

assessment was conducted for the federally threatened delta smelt (USEPA, 2010f). A drinking 

water memo in support of the new use on canola referred to the previous assessment conducted 

for a higher use rate (USEPA, 2008, 2010d). The endangered species assessment concluded that 

diquat was “likely to adversely affect” the delta smelt (USEPA, 2010f). A number of ecotoxicity 

studies have been submitted to the Agency since 2010 and additional studies have been identified 

from the open literature. The results of these studies were incorporated into this assessment.

Guidance documents and models implemented by EFED since 2010 were also applied in this 

assessment. These include ecological risk assessment guidance for benthic invertebrates

(USEPA, 2014c), guidance for evaluating ecological toxicity data in the open literature (USEPA, 

2011b), the Surface Water Concentration Calculator (SWCC) (USEPA, 2014b), and the Markov 

Chain Nest Productivity Model (MCnest) (USEPA, 2014a).

Additionally, drinking water and inhalation exposure pathways were screened using the SIP 

(Screening Imbibition Program) and STIR (Screening Tool for Inhalation Risk) screening 

methods, which were not yet implemented at the time the problem formulation was written. SIP 

results indicated that drinking water exposure alone is a potential acute and chronic exposure

pathway of concern for birds and mammals (see Appendix B). STIR results showed that droplet 

inhalation may be a potential exposure pathway of concern for birds, but exposure via vapor 

inhalation is not likely to be significant (see Appendix C). This is based on the highest single 

application rate (0.5 lbs diquat/A). SIP and STIR are described in detail at:

http://www.epa.gov/oppefed1/models/terrestrial/index.htm.

3.1. Chemical Identification and Mode of Action

Diquat is a contact herbicide in the bipyridylium class.2 The bipyridylium herbicides are 

quaternary ammonium salts. Compounds in this group, e.g., diquat and paraquat, cause rapid 

disruption of cell membranes and very rapid death of plants. Diquat penetrates into the 

cytoplasm and causes the formation of peroxides and free electrons (light is required) which 

destroy the cell membranes almost immediately. Rapid destruction of cell membranes prevents 

translocation to other regions of the plant. Severe injury is evident hours after application, first as 

water-soaked areas which later turn yellow or brown. Maximum kill is attained in a week or less. 

Partial coverage of a plant with spray results in spotting and/or partial shoot kill. New growth on 

surviving plants will be normal in appearance. Some translocation of diquat can occur within a 

plant if a period of darkness follows application (Ashton and Crafts, 1973).

3.2. Use Characterization

Diquat dibromide is used as a nonselective contact herbicide for agricultural and non-crop weed 

control, aquatic weed control, and as a plant regulator and desiccant for agricultural seed crops. 

 2 https://www.extension.purdue.edu/extmedia/WS/WS-23-W.html
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The most significant agricultural use is on potatoes, with an annual average of 200,000 lbs a.i. 

applied per year from 2004-2013, and an average of 35% and a maximum of 50% of the crop 

treated (U.S. EPA, 2015a). The quantity of diquat dibromide applied to potatoes during this 

period is approximately 100 times greater than that applied to any other agricultural crop, and the 

percent crop treated is approximately 10 times greater. 

Tables 3.1, 3.2, and 3.3 summarize the registered uses of diquat for agricultural, landscaping, 

and aquatic uses, respectively. The diquat dibromide containing end-use products include 

Reglone® Dessicant (EPA Reg. No. 100-1061), Reward® Landscape and Aquatic Herbicide 

(EPA Reg. No. 100-1091), and Tribune® Herbicide (EPA Reg. No. 100-1390). These products

and their associated Section 24(c) labels are used as surrogates in this assessment to represent 

over 100 active Section 3 and Section 24(c) labels. All three of these end-use products are 

formulated as SL (soluble liquid concentrate) with a concentration of 37.3% a.i. (formulated as

3.73 lbs diquat dibromide/gallon equivalent to 2 lb of the active ingredient diquat/gallon). 

Registered application methods include foliar sprays via ground (broadcast and hand-held 

equipment), aerial, and aquatic (boat) methods. Diquat dibromide products include coformulations with other herbicides including glyphosate, prodiamine, dicamba, fluazifop-P-butyl, 

oxyfluorfen, imazapic, and indaziflam. However, only risk from diquat dibromide exposure

alone is assessed here.

Table 3.1. Maximum Use Patterns for Reglone® Dessicant (EPA Reg. No. 100-1061 and Associated 24(c) 

labels)

Site

Max 

Single 

Applicati

on Rate 

(lb 

diquat/A)

Max # 

Applications 

/Year

Max 

Application 

Rate per 

year 

(lb diquat 

/A/yr)

Minimum 

Retreatment 

Interval 

(days)

Application Type

Application Equipment

Application Timing

Geo Restrictions

Bearing Crops

Artichoke, 

Asparagus, Ginseng 0.5 3 1.5 5

Directed spray, foliage

Ground boom Sprayer

When emerged weeds are small

Canola 0.5 1 0.5 NA Broadcast, pre-harvest foliage

Ground boom Sprayer, Aerial 

boom Potato 0.5 2 1 5

Inter-row Weed Control

Eggplant, Green 

Pepper, Tomato 0.5 6 3 5

Directed spray, row middle 

weeds

Ground boom Sprayer

Post-directed prior to 

transplanting or as a postdirected hooded spray 

application when transplants are 

well established.

FL only – Eggplant

FL, PR only – Green Pepper, 

Tomato

Non-bearing Crops (Small Fruits – non-bearing)

Blackberry, 

Blueberry, 

Boysenberry, 

0.5 1 0.5 NA

Directed spray, foliage

Ground boom Sprayer

When emerged weeds are small
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Cranberry, 

Dewberry, 

Gooseberry, Grapes, 

Huckleberry, Jojoba, 

Loganberry, 

Raspberry

Non-bearing Crops (Trees – non-bearing)

Acerola, Almonds, 

Apples, Apricots, 

Banana, Cherries, 

Coffee, Conifers, 

Crabapple, Dates, 

Figs, Filberts, 

Grapefruits, Guava, 

Kiwi, Lemon, Lime, 

Macadamia, Mango, 

Nectarines, Olive, 

Orange, Papaya, 

Passion Fruit, 

Peaches, Pears, 

Pecan, Persimmons, 

Pistachios, Plantains, 

Plums, 

Pomegranates, 

Prunes, Tangelos, 

Tangerines, Walnuts

0.5 1 0.5 NA

Directed spray, foliage

Ground boom Sprayer

When emerged weeds are small

Non-bearing Crops (Vines)

Cantaloupe Vines 0.25 6 1.5 5

Post-harvest

Ground boom Sprayer

FL only, except Tomato Vines 

for FL and SC only

Cucumber Vines, 

Squash, except 

Acorn,

Tomato Vines,

Watermelon Vines

0.38 6 2.28 5

Pepper Vines,

Squash, Acorn 0.5 6 3 5

Non-cropped or Non-planted Areas on Farms

Fence Lines, 

Farmyards, Farm 

Buildings, Fuel 

Storage Areas, 

Barrier Strips, 

Equipment Areas, 

Dry (non-flooded 

areas) around ponds, 

lakes and drainage 

ditches on farms

0.5 NA 1.5 5

Broadcast, Spot treatment 

foliage

Backpack Sprayer

When emerged weeds are small

Seed Crops

Alfalfa seed crop,

Clover seed crop 0.5 4 2 5

Broadcast, Pre-harvest foliage

Ground boom sprayer, Aerial 

boom

Calendula, Carrots, 

Coriander, Radishes, 

except Daikon 

variety, Spinach, 

0.5 1 0.5 NA

Broadcast, Pre-harvest foliage

Ground boom sprayer, Aerial 

boom

3 to 10 days before harvest prior 

to swathing
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Sugar beets, Table 

Beets, Turnips

MN only – Calendula

WA only – Spinach, Table beets

WA, OR only - Sugar beets

WA, ID only – Carrots, 

Coriander, Radishes, except 

Daikon variety, Turnips

Table 3.2. Maximum Non-Aquatic Use Patterns for Reward® Landscape and Aquatic Herbicide (EPA Reg. 

No. 100-1091) and Tribune® Herbicide (EPA Reg. No. 100-1390)

Site

Max 

Single 

Application Rate

Max # 

Applications 

/Year

Max 

Application Rate 

per year 

(lb diquat 

/A/yr)

Minimum 

Retreatment 

Interval 

(days)

Application Type/

Application Equipment/

Application Timing/

Geo Restrictions

Commercial 

Greenhouse (beneath 

benches), ornamental 

field grown and 

container stock, and 

other similar areas1

0.5 lb 

diquat/A 12 6 28

Broadcast, foliage of target weed/

Ground boom sprayer, Backpack 

Sprayer/

Pre-plant, Pre-emergence or 

Post-Emergence Directed Spray/

No pre-emergence application on 

ornamental crops in CA

0.01 lb 

diquat/gal 12 NA 5

Spot-Treatment, foliage target 

weed/

Backpack sprayer/

Pre-plant, Pre-emergence or 

Post-Emergence Directed Spray/

No preemergence application on 

ornamental crops in CA

Ornamental Seed 

Crops

0.5 lb 

diquat/A 3 1.5 5

Broadcast, foliage of target weed/

Ground boom sprayer, backpack 

sprayer/

Pre-harvest Desiccation/

Except CA.

Landscape, Industrial, 

Commercial, 

Residential and 

Public Areas

0.5 lb 

diquat/A 3 1.5 28

Broadcast, foliage of target weed/

Ground boom sprayer/

As needed for control of 

undesired foliage

0.01 lb 

diquat/gal 3 NA 5

Spot-Treatment, undesired target 

weed/

Backpack sprayer/

As needed for control of 

undesired foliage

Turf Renovation 0.5 lb 

diquat/A 2 1 28

Broadcast, undesired turf/

Ground boom sprayer/

As needed

Dormant Established 

Turf Grass

0.5 lb 

diquat/A 2 1 28

Broadcast, to desired turf/

Ground boom sprayer/

Apply when turf grass is dormant 1 Use inside greenhouses is considered an indoor use and will not be assessed. Ornamental field grown and container 

stock and “other similar areas” is considered an outdoor use.
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Table 3.3. Maximum Direct Aquatic Use Patterns for Reward® Landscape and Aquatic Herbicide (EPA Reg. 

No. 100-1091) and Tribune® Herbicide (EPA Reg. No. 100-1390 and Associated 24(c) Labels)

Site Max Single 

Application Rate

Max # 

Applications 

/Year

Max 

Application 

Rate per 

year (lb 

diquat/

A/yr)

Minimum 

Retreatment 

Interval 

(days)

Application Type/

Application Equipment/

Application Timing/

Geo Restrictions

Floating and 

Marginal 

Weeds

4 lb diquat/

Surface Acre/1 ft

(1.47 ppm diquat)

12 NA 14

Broadcast, foliage of the target 

weed species/

Backpack, airboat, spray 

handgun, helicopter, airplane, or 

similar application equipment 

results in thorough spray 

coverage/

As needed/

Not for use in NY without 

supplemental labeling

0.04 lb diquat/gal 12 NA 14

Spot-Treatment, foliage of target 

weed species/

Backpack, spray handgun, or 

similar application equipment 

results in thorough spray 

coverage/

As needed/

Not used on Frog's Bit 

(Limnobium spongia) in CA

1. Submersed 

Weeds.

2. Standing and 

Flowing Water 

- Submerged 

Weeds

4 lb diquat/

Surface Acre/4 ft

(0.37 ppm)

12 NA 14

Broadcast to water surface or 

below water surface to contact 

foliage of weed species./

Backpack, airboat, spray 

handgun, helicopter, airplane, or 

similar application equipment 

results in thorough spray 

coverage/

As needed.

2. FL, ID, AL, GA only

3.NY only

3. Still Water -

Submerged 

Weeds

4 lb diquat/

Surface Acre/1 ft

(1.47 ppm)

Still Water -

Floating Weeds

1.5 lb diquat/

Surface Acre/1 ft

(0.55 ppm)

12 NA 14

Broadcast, foliage of the target 

weed species/

Backpack, airboat, spray 

handgun, or similar application 

equipment results in thorough 

spray coverage/

As needed/

NY only

Still Water -

Duckweed, 

Cattails

2 lb diquat/

Surface Acre/1 ft

(0.74 ppm)

12 NA 14

Note: Treat only 1/3 to 1/2 of water body area at one time and wait 14 days between treatments to prevent oxygen 

loss from decomposition dead weeds which may cause fish suffocation.

3.3. Label Uncertainties

Three diquat dibromide end-use products and their associated Section 24(c) labels are used as 

surrogates in this assessment to represent over 100 active Section 3 and Section 24(c) labels. 

These labels are representative of most diquat dibromide uses (U.S. EPA, 2015b). For the 
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assessed labels, missing information such as maximum number of applications per year, 

maximum annual application rate, and minimum retreatment interval was reconciled through 

communication with the registrant. For other diquat dibromide labels, ambiguities in this type of 

information may remain. The degree to which this risk assessment is protective of aquatic and 

terrestrial organisms depends on the extent to which the assessed labels represent the maximum 

use patterns allowed across all the labels.

4. Exposure Assessment

EFED used standard available models to evaluate potential exposures to aquatic and terrestrial 

organisms as described at http://www.epa.gov/pesticides/science/models_db.htm. Aquatic 

exposure was estimated using the SWCC. Terrestrial exposure was estimated using the 

Terrestrial Residue Exposure Model (T-REX) v.1.5.2 for animals and TERRPLANT v.1.2.2 for 

plants. Spray drift exposure was estimated using AgDRIFT v2.1.1. Estimates of chronic risk to 

birds were refined using the MCnest model.

4.1. Environmental Fate Summary

Diquat dibromide has a high water solubility (700,000 mg/L at pH 7) and a low vapor pressure (1 

x 10-7 mmHg at 20oC). The calculated Henry's Law Constant of 1.4 x 10-13 atm-m3/mol)

indicates that diquat dibromide is not expected to be volatile under field conditions or from 

water. In laboratory batch equilibrium studies, diquat dibromide was shown to be immobile with 

Freundlich Kads of 15–42 in sand sediment and two sand soils and Freundlich Kads values of 

1882–10740 in sandy loam, sandy clay loam, and loam soils. Diquat dibromide binds strongly to 

clay particles. Due to its low BCF values (0.7X for edible, 2.5x for nonedible, and 1.03x for 

whole fish), fish bioaccumulation is not likely to occur. 

The primary route of environmental dissipation of diquat dibromide is strong adsorption to soil 

particles. Diquat dibromide does not hydrolyze or photodegrade and is resistant to microbial 

degradation under aerobic and anaerobic conditions. Reinert and Rodgers (1987) studied various 

sediment types and found that unbound, biologically available diquat dibromide can be 

biodegraded by bacteria in the laboratory. However, because of the rapid adsorption of diquat

dibromide to sediments in the environment which may render it less available to biodegradation, 

Reinert and Rodgers (1987) suggested that the opportunity for microbial decomposition of 

sorbed diquat dibromide is reduced.

When used as an aquatic herbicide, diquat dibromide is removed from the water column by 

adsorption to sediments, aquatic vegetation, and organic matter. Adsorbed diquat dibromide is 

persistent and immobile, and is not expected to be a groundwater contaminant. The available 

environmental fate data for diquat dibromide are summarized in Table 4.1. First order 

degradation calculations were used to determine the half-lives in this table.
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Table 4.1. Chemical Properties and Environmental Fate Parameters of Diquat Dibromide

Parameter Value Source

Selected Physical/Chemical Parameters

Molecular mass (molecular formula) 344.05 g/mol (C12H12Br2N2 ) (Calculated)

Vapor pressure (20°C) <1 x 10-7 torr Product Chemistry

Aqueous solubility (20°C, pH 7) 700,000 mg/L

Henry’s Law Constant (20°C) 1.4x10-13 atm-m3/mol (Calculated)

Log octanol-to-water partition coefficient (log Kow) 

(20°C, pH 7)

-4.6 MRID 00138928

Persistence in Water

Hydrolysis half-life (20°C) (pH 5, 7, 9) stable MRID 40418801

Aqueous photolysis half-life (20°C) (pH 7) 74 days MRID 40418801

Persistence in Soil

Photodegradation half-life on soil stable MRID 40246101

Aerobic soil metabolism half-life (20°C) stable MRID 40972301

Aerobic aquatic metabolism half-life (20oC) stable MRID 40927601

Anaerobic aquatic metabolism half-life (20°C) stable MRID 40972302

Mobility

Freundlich Adsorption coefficient (Kads) (mL/g) 15-42 (sand sediment and soil)

1882-10740 (sandy loam, sandy clay 

loam and loam soils)

MRID 40348601

Field Dissipation

Aquatic Field Dissipation half-life 1 – 2 days (removed by adsorption) MRID 40917403

Terrestrial Field Dissipation half-life stable MRID 42060301

MRID 42060302

Fish Bioconcentration

Bioconcentration Factor (BCF) 0.7X (edible), 2.5x (nonedible), 1.03x 

(whole fish). 

Rapid depuration (50% in 3 days) 

MRID 40326901

4.2. Residues of Concern

There were no major degradates isolated from any of the standard environmental fate studies, so 

no degradates of concern for diquat dibromide have been identified. The aqueous photolysis 

study included a test system without sediment, which resulted in a half-life of 74 days and 

breakdown to the major degradate 1,2,3,4-tetrahydro-1-oxopyrido (1,2-a) pyrazin-5-ium ion. 

However, in an aquatic environment with sediment (i.e., natural aquatic environment) photolysis 

is not expected to be a significant route of diquat degradation.
Page 15 of 76

4.3. Aquatic Exposures

The available physical/chemical and environmental fate properties of diquat dibromide were 

used to calculate exposure model input parameters to estimate potential environmental 

concentrations. The exposure modeling for aquatic environments was done separately for 

terrestrial and aquatic uses. 

4.3.1. Aquatic Modeling for Terrestrial Uses

The expected environmental concentrations (EECs) for diquat dibromide in surface water and 

sediment were generated based on maximum application rates (Table 3.1 and 3.2). Simulations 

were conducted using the SWCC model (v 1.106, May, 2014). Sample model inputs and output 

are in Appendix D. The SWCC simulates pesticide runoff from a treated area to an adjacent 

water body following applications, in addition to loadings that occur directly to the water as a 

consequence of spray drift. The SWCC scenarios assume a standard 1-hectare, 2-meter depth 

pond (static water body) into which a 10-hectare field drains (see

http://www.epa.gov/oppefed1/models/water/). 

The SWCC modeling scenarios are presented in Table 4.2. For each modeling scenario, the 

maximum single and annual application rates, the maximum number of applications per year, 

and the minimum retreatment interval are listed. The application timing is determined according 

to the label directions and the crop stage timing of the modeling scenarios.

Table 4.2. SWCC Scenarios and Input Parameters Describing Maximum Patterns of Diquat Use on 

Representative Terrestrial Use Sites 

Use Sites

Max Single Application Rate (lb 

diquat/A)

Max # Applications /yr

Max Application Rate per year (lb 

diquat/A/yr)

Minimum Retreatment Interval (days)

SWCC Model Scenarios / 1st application 

date / Aerial (12.5% drift) or Ground 

(6.2% drift) Applications

(Inter-row Weed 

Control) Eggplant, 

Green Pepper, Tomato

0.5

6

3

5

CA Tomato / May 1st / Ground

FL Tomato / April 22nd /Ground

PA Tomato / July 10th / Ground

Non-bearing Crops 

(Vines) – Cucumber 

Vines, Squash, except 

Acorn,

Tomato Vines,

Watermelon Vines

0.38

6

2.28

5

FL Cucumber / December 20th / Ground

Non-bearing Crops 

(Vines) – Pepper 

Vines,

Squash, Acorn

0.5

6

3

5

FL Pepper / December 10th / Ground

(Seed Crops) Alfalfa 

seed crop, Clover seed 

crop

0.5

4

2

5

CA Alfalfa (OP) / December 30th /Aerial

MN Alfalfa (OP) / August 25th / Aerial

NC Alfalfa (OP) / June 1st / Aerial

PA Alfalfa (OP) / October 31st / Aerial

Ornamental field

grown and container 

0.5

12

6

CA Nursery / February 25th / Ground

FL Nursery / January 15th / Ground

MI Nursery / January 15th / Ground
Page 16 of 76

stock and other similar 

areas

28 NJ Nursery / January 15th / Ground

OR Nursery / January 15th / Ground

TN Nursery / April 1st / Ground

(Bearing Crops) 

Artichoke, Asparagus, 

Ginseng

0.5

3

1.5

5

CA Row-Crop / March 1st / Ground

MI Asparagus / July 1st / Ground

(Bearing Crops) 

Canola

0.5

1

0.5

NA

ND Canola / June 1st / Aerial

(Bearing Crops) Potato

0.5

2

1

5

ID Potato / June 15th / Aerial

ME Potato / August 1st / Aerial

Non-bearing Crops 

(Small Fruits, Trees)*

0.5

1

0.5

NA

MI Cherries / June 1st / Ground

GA Peaches / April 15th / Ground

GA Pecans / June 1st / Ground

(Seed Crops) 

Calendula, Carrots, 

Coriander, Radishes, 

except Daikon variety, 

Spinach, Sugar Beets, 

Table Beets, Turnips

0.5

1

0.5

NA

FL Carrots / November 1st / Ground

MN Sugarbeets / July 1st / Ground

Ornamental Seed 

Crops, Landscape, 

Industrial, 

Commercial, 

Residential and Public 

Areas

0.5

3

1.5

5

CA Residential / March 1st / Ground

Residential BSS / March 1st / Ground

Turf Renovation,

Dormant Established 

Turf Grass

0.5

2

1

28

PA Turf / April 10th / Ground

FL Turf / February 10th / Ground

CA Turf / January 15th / Ground

*Refers to maximum use table (Table 3.1)

For aquatic exposure, modeling was conducted for parent diquat dibromide only. Half-lives used 

for modeling are presented in Table 4.3. Application parameters were chosen based on the likely 

method of application at the maximum application rate as indicated in the label instructions. 

Chemical properties and model input values were chosen according to the current Input 

Parameter Guidance (USEPA, 2009).

Table 4.3. Chemical Input Parameters for Diquat Aquatic Exposure Modeling 

Parameter Value Source

Molecular Weight (g/mol) 344.05 Calculated

Water Solubility (mg/L) 700,000 Product Chemistry Vapor Pressure (torr) 1 x 10-7

Hydrolysis half-life (days) @pH 7 0 (stable) MRID 40418801

Aqueous photolysis half-life (days) 74 days MRID 40418801

Aerobic soil metabolism half-life (days) 0 (stable) MRID 40972301
Page 17 of 76

Parameter Value Source

Aerobic aquatic metabolism half-life (days) 0 (stable) MRID 40927601

Anaerobic aquatic metabolism half-life (days) 0 (stable) MRID 40972302

Adsorption coefficients (Kd) 5839 (mean) MRID 40348601

The EECs listed in Table 4.4 and 4.5 reflect daily 1-in-10 year return frequency surface water 

concentrations in the water column and in benthic environments, respectively, based on 

maximum application rates for labeled uses of diquat dibromide. The nursery modeling scenarios 

generally produce the highest aquatic EECs due to the high annual application rates for this use. 

Table 4.4. Aquatic Water Column Exposure Estimates for Diquat Terrestrial Uses

Use Sites SWCC Scenario 1-in-10-year Peak

Exposure (μg/L)

1-in-10-year

21-day Average 

Exposure (μg/L)

1-in-10-year

60-day Average 

Exposure (μg/L)

(Inter-row Weed 

Control) Eggplant, 

Green Pepper, Tomato

CA Tomato 5.05 2.94 2.88

FL Tomato 19.1 12.3 12.2

PA Tomato 22.4 12.8 12.7

Non-bearing Crops 

(Vines) – Cucumber 

Vines, Squash, except 

Acorn,

Tomato Vines,

Watermelon Vines

FL Cucumber 13.6 9.24 9.18

Non-bearing Crops 

(Vines) – Pepper Vines,

Squash, Acorn

FL Pepper 17.7 11.8 11.8

(Seed Crops) Alfalfa 

seed crop, Clover seed 

crop

CA Alfalfa 6.69 4.18 4.12

MN Alfalfa 4.37 1.96 1.86

NC Alfalfa 6.93 3.48 3.38

PA Alfalfa 11.3 6.42 6.30

Ornamental field grown 

and container stock and 

other similar areas

CA Nursery 52.6 23.3 23.1

FL Nursery 36.6 22.2 22.1

MI Nursery 34.5 20.0 19.9

NJ Nursery 41.7 23.1 22.4

OR Nursery 13.0 9.75 9.70

TN Nursery 34.4 22.1 21.9

(Bearing Crops) 

Artichoke, Asparagus, 

Ginseng

CA Row-Crop 8.52 4.14 4.09

MI Asparagus 16.4 4.30 3.83

(Bearing Crops) Canola ND Canola 4.37 1.69 1.66

(Bearing Crops) Potato

ID Potato 4.63 2.16 2.13

ME Potato 8.19 4.66 4.60

Non-bearing Crops 

(Small Fruits, Trees)

MI Cherries 5.03 2.17 2.17

GA Peaches 6.17 1.89 1.88
Page 18 of 76

Use Sites SWCC Scenario 1-in-10-year Peak

Exposure (μg/L)

1-in-10-year

21-day Average 

Exposure (μg/L)

1-in-10-year

60-day Average 

Exposure (μg/L)

GA Pecans 4.01 2.07 2.06

(Seed Crops) Calendula, 

Carrots, Coriander, 

Radishes, except 

Daikon variety, 

Spinach, Sugar Beets, 

Table Beets, Turnips

FL Carrots 3.37 1.99 1.98

MN Sugarbeets 3.47 1.49 1.48

Ornamental Seed Crops, 

Landscape, Industrial, 

Commercial, 

Residential and Public 

Areas

CA Residential 2.51 1.26 1.22

Residential BSS 4.08 1.91 1.87

Turf Renovation,

Dormant Established 

Turf Grass

PA Turf 4.04 1.57 1.55

FL Turf 2.22 0.870 0.859

CA Turf 2.05 0.771 0.760

Bolded values indicate the highest EECs for the use pattern

Table 4.5. Aquatic Benthic Exposure Estimates for Diquat Terrestrial Uses

Use Sites SWCC Scenario

1-in-10-year 

Peak

Pore Water 

Exposure 

(μg/L)

1-in-10-year

21-day Average 

Pore Water 

Exposure 

(μg/L)

1-in-10-year 

Peak

Total/ Dry 

Sediment 

(μg/kg)

1-in-10-year 21-

day Average

Total/ Dry 

Sediment 

(μg/kg)

(Inter-row Weed 

Control) Eggplant, 

Green Pepper, 

Tomato

CA Tomato 2.86 2.86 16702 16702

FL Tomato 12.2 12.2 71248 71248

PA Tomato 12.7 12.7 74168 74168

Non-bearing Crops 

(Vines) – Cucumber 

Vines, Squash, 

except Acorn,

Tomato Vines,

Watermelon Vines

FL Cucumber 9.17 9.15 53553 53436

Non-bearing Crops 

(Vines) – Pepper 

Vines,

Squash, Acorn

FL Pepper 11.7 11.7 68328 68328

(Seed Crops) 

Alfalfa seed crop, 

Clover seed crop

CA Alfalfa 4.12 4.10 24060 23944

MN Alfalfa 1.80 1.80 10512 10512

NC Alfalfa 3.34 3.35 19506 19564

PA Alfalfa 6.24 6.24 36442 36442

Ornamental field 

grown and container 

stock, and other 

similar areas

CA Nursery 23.1 23.1 134904 134904

FL Nursery 22.0 22.0 128480 128480

MI Nursery 19.9 19.9 116216 116216

NJ Nursery 22.4 23.0 130816 134320

OR Nursery 9.69 9.69 56590 56590
Page 19 of 76

Use Sites SWCC Scenario

1-in-10-year 

Peak

Pore Water 

Exposure 

(μg/L)

1-in-10-year

21-day Average 

Pore Water 

Exposure 

(μg/L)

1-in-10-year 

Peak

Total/ Dry 

Sediment 

(μg/kg)

1-in-10-year 21-

day Average

Total/ Dry 

Sediment 

(μg/kg)

TN Nursery 21.9 21.9 127896 127896

(Bearing Crops) 

Artichoke, 

Asparagus, Ginseng

CA Row-Crop 4.10 4.09 23944 23886

MI Asparagus 3.80 3.80 22192 22192

(Bearing Crops) 

Canola ND Canola 1.64 1.64 9578 9578

(Bearing Crops) 

Potato

ID Potato 2.12 2.12 12380 12380

ME Potato 4.60 4.50 26864 26806

Non-bearing Crops 

(Small Fruits, 

Trees)

MI Cherries 2.04 2.17 11914 12673

GA Peaches 1.87 1.87 10921 10921

GA Pecans 2.06 2.06 12030 12030

(Seed Crops) 

Calendula, Carrots, 

Coriander, 

Radishes, except 

Daikon variety, 

Spinach, Sugar 

Beets, Table Beets, 

Turnips

FL Carrots 1.97 1.97 11504 11504

MN Sugarbeets 1.47 1.47 8585 8585

Ornamental Seed 

Crops, Landscape, 

Industrial, 

Commercial, 

Residential and 

Public Areas

CA Residential 1.20 1.20 7008 7008

Residential BSS 1.85 1.85 10804 10804

Turf Renovation,

Dormant 

Established Turf 

Grass

PA Turf 1.55 1.55 9052 9052

FL Turf 0.851 0.851 4970 4970

CA Turf 0.751 0.750 4385 4380

Bolded values indicate the highest EECs for the use pattern

4.3.2. Aquatic Modeling for Aquatic Uses

The maximum single application rate for the direct aquatic use is 4 lb diquat/surface acre/ft (see 

Table 3.3). The labels also specify to treat only 1⁄3 to 1⁄2 of water body area at one time with a 

minimum retreatment interval of 14 days. Considering the receiving water body as the standard 

1-hectare (i.e., 2.47 acre), 2-meter (i.e., 6.56 ft) depth pond, the initial loading of diquat is shown 

below: 

4 lb diquat/surface acre/ft x 1⁄2 x 6.56 ft x 2.47 acre = 32.4 lb diquat

Factoring in the volume of the pond, the initial concentration in the pond after the first treatment 

will be 0.735 mg/L (ppm). Considering the minimum retreatment interval of 14 days, the diquat 

concentrations in the pond following the repeated treatments are shown in Figure 4.1, based on
Page 20 of 76

the aquatic field dissipation half-life of 2 days. The maximum 21-day average concentration is 

0.133 mg/L, and the maximum 60-day average concentration is 0.116 mg/L. For reference 

purposes, all these expected values are higher than the drinking water MCL of 0.02 mg/L.

Figure 4.1. Diquat Concentration in the Standard Pond Following Aquatic Applications

The results of SWCC runs were used to estimate the sediment and pore water concentrations. In

a total of 30 SWCC runs, the range for the ratios between the peak water concentrations to the 

peak pore water concentrations is from 23.1% to 74.54% with the average of 50.36%. Similarly, 

the range for the ratios between 21-day average water concentrations to the 21-day average pore 

water concentrations is from 88.37% to 100% with the average of 97.8%. Using the average 

values to derive the pore water concentrations, the peak water concentration of 0.735 mg/L 

would yield a value of 0.370 mg/L for peak pore water concentration. The 21-day average pore 

water concentration will be 0.130 mg/L (based on the water concentration of 0.133 mg/L). The 

results for sediment concentrations of all the 30 SWCC runs show a conversion factor of 

0.0171%; this factor was used to derive the sediment concentrations in terms of μg/kg. Based on 

this conversion factor, the peak sediment concentration is 2161733 μg/kg (0.370 mg/L * 1000 

μg/mg ÷ 0.0171%) and the 21-day average value is 760124 μg/kg. The 21-day average sediment 

EEC is compared to the chronic sediment toxicity endpoints.

4.4. Bioaccumulation

Based on the low BCF values (see Table 4.1), bioaccumulation is not a concern for diquat 

dibromide.

4.5. Monitoring Data

No surface water or groundwater monitoring data for diquat were found from searches of the

EPA Pesticides in groundwater database (1994) or the USGS National Water-Quality 

Assessment Program (NAWQA).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70

Diquat Conc. (mg/L) in the Pond with Time
Page 21 of 76

Surface and groundwater monitoring data are available for diquat from the South Florida Water 

Management District (SFWMD) and the EPA Office of Water. An MCL of 0.02 mg/L (20 ppb)

was established for diquat in 1999, and the Florida Department of Environmental Protection 

(FDEP) has accepted this standard. 

The SFWMD database contained a total of 42 samples taken from April 1992 to November 2000 

at approximately a 1-3 month interval. For diquat, the only detection observed in surface water 

was 0.0045 ppm in 1994 (2.4% detection frequency). Further monitoring beyond 1994 has 

shown no detections in surface water. Also in the SFWMD, diquat was detected in 9 sediment 

samples out of 17 sediment samples total taken from canals with a maximum concentration of 

3.1 ppm (LOD of 0.25 ppm; Miles and Pfeuffer, 1997). 

The EPA Office of Water has also collected monitoring data for diquat at intake pumps of

drinking water utilities that use surface water and groundwater, provided by community water 

systems. Monitoring was conducted in 14 states over many years. However, data from only eight 

states in the years 1993-1997 were included in the report. In these eight states, the percent of 

combined surface water and groundwater systems reporting exceedances of the 20 ppb MCL was 

0.06 %, resulting in an exceedance for 0.27 % of the population (USEPA, 2000).

4.6. Terrestrial Wildlife Exposures

4.6.1. Birds and Mammals

Dietary EECs for birds and mammals were calculated using T-REX v.1.5.2. EECs are based on 

the application rates, number of applications, and retreatment intervals presented in Tables 3.1

and 3.2. A foliar dissipation rate of 35 days (default value) was used for this analysis to estimate 

dissipation after each application. Model inputs are detailed in Appendix E. Resulting upper 

bound exposure estimates to birds and mammals are in Table 4.6. Use patterns with the same 

application rate, number of applications, and application intervals are combined in the table 

below because they result in identical EECs in T-REX.

Terrestrial exposure via drinking water may result from the aquatic uses outlined in Table 3.3, 

however those uses are not addressed in this section. See Section 3 and Appendix B for 

discussion of the drinking water exposure pathway for birds and mammals. See Section 3 and 

Appendix C for discussion of the inhalation exposure pathway for birds and mammals.

Table 4.6. T-REX Calculated EECs as Food Residues for Terrestrial Animals from Diquat Terrestrial Uses

Food Type

Dietary-Based EEC

(ppm)

(Mammals and 

Birds)

Dose-Based EEC for Birds

(mg/kg-bw)

Dose-Based EEC for Mammals

(mg/kg-bw)

All Size Classes Small

(20 g)

Medium

(100 g)

Large

(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

1. (Inter-row Weed Control) Eggplant, Green Pepper, Tomato

2. Non-bearing Crops (Vines) – Pepper Vines, Squash, Acorn

Use pattern: 0.5 lb diquat/A, 6 applications, 5 day retreatment interval

Short grass 570 649 370 166 544 376 87

Tall grass 261 298 170 76 249 172 40
Page 22 of 76

Food Type

Dietary-Based EEC

(ppm)

(Mammals and 

Birds)

Dose-Based EEC for Birds

(mg/kg-bw)

Dose-Based EEC for Mammals

(mg/kg-bw)

All Size Classes Small

(20 g)

Medium

(100 g)

Large

(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

Broadleaf plants 321 365 208 93 306 211 49

Fruits/pods 36 41 23 10 34 23 5.4

Arthropods 223 254 145 65 213 147 34

Seeds 36 9.0 5.1 2.3 7.6 5.2 1.2

Non-bearing Crops (Vines) – Cucumber Vines, Squash, except Acorn, Tomato Vines, Watermelon Vines

Use pattern: 0.38 lb diquat/A, 6 applications, 5 day retreatment interval

Short grass 433 494 281 126 413 286 66

Tall grass 199 226 129 58 189 131 30

Broadleaf plants 244 278 158 71 232 161 37

Fruits/pods 27 31 18 7.9 26 18 4.1

Arthropods 170 193 110 49 162 112 26

Seeds 27 6.9 3.9 1.8 5.7 3.97 0.92

(Seed Crops) Alfalfa seed crop, Clover seed crop

Use pattern: 0.5 lb diquat/A, 4 applications, 5 day retreatment interval

Short grass 416 474 270 121 397 274 64

Tall grass 191 217 124 55 182 126 29

Broadleaf plants 234 267 152 68 223 154 36

Fruits/pods 26 30 17 7.6 25 17 4.0

Arthropods 163 186 106 47 155 107 25

Seeds 26 6.6 3.8 1.7 5.5 3.8 0.88

Ornamental field grown and container stock, and other similar areas

Use pattern: 0.5 lb diquat/A, 12 applications, 28 day retreatment interval

Short grass 281.56 321 183 82 268 186 43

Tall grass 129.05 147 84 38 123 85 20

Broadleaf plants 158.38 180 103 46 151 104 24

Fruits/pods 17.60 20 11 5.1 17 12 2.7

Arthropods 110.28 126 72 32 105 73 17

Seeds 17.60 4.5 2.5 1.1 3.7 2.6 0.60

1. (Bearing Crops) Artichoke, Asparagus, Ginseng

2. Ornamental Seed Crops, Landscape, Industrial, Commercial, Residential and Public Areas

Use pattern: 0.5 lb diquat/A, 3 applications, 5 day retreatment interval

Short grass 327 373 212 95 312 216 50

Tall grass 150 171 97 44 143 99 23

Broadleaf plants 184 210 120 54 175 121 28

Fruits/pods 20 23 13 5.9 19 13 3.1

Arthropods 128 146 83 37 122 84 20

Seeds 20 5.2 2.9 1.3 4.33 3.0 0.69

1. (Bearing Crops) Canola

2. Non-bearing Crops (Small Fruits, Trees)

3. (Seed Crops) Calendula, Carrots, Coriander, Radishes, except Daikon variety, Spinach, Sugar Beets, Table 

Beets, Turnips

Use pattern: 0.5 lb diquat/A, 1 application

Short grass 120 137 78 35 114 79 18

Tall grass 55 63 36 16 52 36 8.4

Broadleaf plants 68 77 44 20 64 44 10

Fruits/pods 7.5 8.5 4.9 2.2 7.2 4.9 1.2

Arthropods 47 54 31 14 45 31 7.2

Seeds 7.5 1.9 1.1 0.48 1.6 1.1 0.25
Page 23 of 76

Food Type

Dietary-Based EEC

(ppm)

(Mammals and 

Birds)

Dose-Based EEC for Birds

(mg/kg-bw)

Dose-Based EEC for Mammals

(mg/kg-bw)

All Size Classes Small

(20 g)

Medium

(100 g)

Large

(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

(Bearing Crops) Potato

Use pattern: 0.5 lb diquat/A, 2 applications, 5 day retreatment interval

Short grass 229 260 149 66 218 151 35

Tall grass 105 119 68 30 100 69 16

Broadleaf plants 129 147 84 370 123 85 20

Fruits/pods 14 16. 9.3 4.2 14 9.4 2.2

Arthropods 90 102 58 26 85 59 14

Seeds 14 3.6 2.1 0.92 3.0 2.1 0.49

Turf Renovation, Dormant Established Turf Grass

Use pattern: 0.5 lb diquat/A, 2 applications, 28 day retreatment interval

Short grass 189 215 123 55 180 124 29

Tall grass 87 99 56 25 83 57 13

Broadleaf plants 106 121 69 31 101 70 16

Fruits/pods 12 13 7.7 3.4 11 7.8 1.8

Arthropods 74 84 48 22 71 49 11

Seeds 12 3.0 1.7 0.76 2.5 1.7 0.40

4.6.2. Terrestrial Invertebrates

The Guidance for Assessing Pesticide Risks to Bees (USEPA et al., 2014) provides equations for 

the calculation of EECs for 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. Bees foraging in a field treated with a 

pesticide through foliar spray, such as diquat, could potentially be exposed to the pesticide 

through direct spray as well through consuming contaminated food. Foraging honey bees may 

also be exposed to pesticides via consumption of water from surface water, puddles, dew droplet 

formation on leaves, and guttation fluid; however, the Tier I exposure method does not include 

quantification of exposures via these routes.

The EECs for a single foliar application of diquat at the maximum application rate are calculated 

below.3

Adult Contact EEC = 0.50 lbs diquat/A * 2.7 μg diquat/bee = 1.35 μg diquat cation/bee

Adult Dietary EEC = 0.50 lbs diquat/A * 110 μg diquat/g * 0.292 g/day = 16.1 μg diquat 

cation/bee/day

Larval Dietary EEC = 0.50 lbs diquat/A * 110 μg diquat/g * 0.124 g/day = 6.82 μg diquat/bee/day

 3Based on food consumption rates for larvae (0.124 g/day) and adult (0.292 g/day) worker bees and concentration in 

pollen and nectar.
Page 24 of 76

4.6.3. Terrestrial Plants

EECs for terrestrial plants were calculated using TERRPLANT v.1.2.2. TERRPLANT inputs are 

detailed in Appendix F. Resulting upper bound exposure estimates to terrestrial and semiaquatic (wetland) plants are in Table 4.7. EECs are based on the maximum single application 

rate for terrestrial uses, solubility, and spray drift fraction. The default spray drift assumptions in 

the model were updated to reflect the most recent spray drift modeling guidance with 6% drift

for ground applications and 12% drift for aerial applications, consistent with the SWCC 

modeling for aquatic exposure via spray drift (USEPA, 2013).

Table 4.7. Calculated EECs for Terrestrial and Semi-Aquatic Plants near Diquat Terrestrial Use Areas

Crop

Single Max. 

Application 

Rate

(lbs

diquat/A)

EECs (lbs diquat/A)

Ground Aerial

Dry 

Areas 

(Total)

SemiAquatic 

Areas 

(Total)

Spray 

Drift 

Dry 

Areas 

(Total)

SemiAquatic 

Areas 

(Total)

Spray 

Drift 

Non-bearing Crops 

(Vines) – Cucumber 

Vines, Squash, except 

Acorn, Tomato Vines, 

Watermelon Vines

0.38 0.042 0.21 0.023 0.065 0.24 0.046

All other terrestrial use

sites 0.50 0.055 0.28 0.030 0.085 0.31 0.060

4.7. Spray Drift Transport

Exposure via spray drift was assessed using AgDRIFT v2.1.1. Spray drift deposition as a fraction 

of the on-field application rate varies by distance from the field and by droplet size (Figure 4.2), 

and is not dependent on the active ingredient applied. The AgDRIFT Tier 1 model estimates 

deposition up to 1,000 ft from a treated field. Figure 4.2 depicts spray drift deposition for the first 

100 ft. Spray drift exposure can impact both terrestrial and aquatic taxa.
Page 25 of 76

DSD = Droplet Size Distribution

Figure 4.2. Spray Drift Deposition Fractions at Varying Distance from a Treated Field

5. Effects Analysis

Summaries of the most sensitive aquatic and terrestrial toxicity data for diquat are provided in 

Tables 5.1 and 5.2, respectively. A full list of toxicity studies considered can be found in 

Appendix H. All toxicity endpoints are presented in units of diquat cation.

5.1. Aquatic Toxicity Data

Diquat is slightly to highly toxic to freshwater and estuarine/marine fish on an acute exposure 

basis. The most sensitive freshwater fish acute endpoint is from an open literature study on 

several age classes of walleye (Stizostedion vitreum) identified in the ECOTOX database (Paul et 

al., 1994; MRID 49611201; E14903). Diquat dibromide (tested as the end-use product Diquat 

HA) had an LC50 of 0.75 mg diquat/L in 8-10 day old walleye. While many acute toxicity studies 

for freshwater fish are available (see Table A-1 in Appendix H), none of them were conducted 

using the technical grade active ingredient (TGAI). Therefore, the endpoint used for risk 

estimation is based on end-use product testing (referred to as TEP, typical end-use product). The 

most sensitive estuarine/marine fish acute endpoint is 51.1 mg diquat/L for the sheepshead 

minnow (Cyprinodon variegatus) (MRID 40316101); however, this endpoint may be an 

underestimate of the true toxicity due to the presence of precipitate in the test system. Chronic 

exposure studies on freshwater and estuarine/marine fish demonstrated effects on fish growth at 

0.316 mg diquat/L (NOAEC = 0.122 mg diquat/L) and 7.7 mg diquat/L (NOAEC = 3.0 mg 

diquat/L), respectively (MRIDs 40380703, 49095901).

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 10 20 30 40 50 60 70 80 90 100

Deposition Fraction

Distance from Edge of Treated Field (ft)

Aerial Spray, Fine to Medium DSD Ground Spray, Very Fine to Fine DSD

Aerial Spray, Medium to Coarse DSD Ground Spray, Fine to Medium/Coarse DSD
Page 26 of 76

A 4-day study on the African clawed frog (Xenopus laevis) resulted in mortality of 20% at 0.50

mg diquat/L, 45% at 0.75 mg diquat/L, 75% at 1.00 mg diquat/L, and 100% at 1.25, 1.50, and 

2.00 mg diquat/L (MRID 05005290). Inhibition in growth and pigmentation was seen in 

survivors after day 3 and distortion in body shape was observed at 1.5 mg diquat/L.

Diquat is slightly to very highly toxic to freshwater and estuarine/marine invertebrates on an 

acute exposure basis. The most sensitive species tested is Hyalella azteca with an LC50 of 0.09 

mg diquat/L (MRID 48826701). The most sensitive chronic aquatic invertebrate endpoint is from 

an open literature study identified in the ECOTOX database evaluating the toxicity of diquat 

dibromide to great pond snails (Lymnaea stagnalis) (Ducrot et al., 2010; MRID 49613201; 

E168652). Over the full snail lifecycle, diquat exposure impacted development and food 

consumption at 0.0032 mg diquat/L (NOAEC = 0.0011 mg diquat/L). Hatching rate and 

mortality were impacted at 0.0067 mg diquat/L. The study simulated exposure from four aquatic 

applications of diquat dibromide (tested as the end-use product Reglone 2). Recovery was 

observed following transfer of surviving snails to clean water on day 168 of the study. The most 

sensitive chronic aquatic invertebrate endpoint from a study using TGAI is on Daphnia magna, 

with a NOAEC of 0.036 and a LOAEC of 0.057 mg diquat/L based on survival (MRID 

40380702).

Because diquat binds strongly to sediment, tests on benthic invertebrates were also required. A 

42-day lifecycle test on the freshwater amphipod Hyalella azteca showed that diquat exposure 

significantly impacted reproduction at 11 mg diquat/kg sediment or 0.043 mg diquat/L pore 

water (MRID 49262901). A 63-day lifecycle test on the freshwater midge Chironomus dilutus

showed no effects on survival, growth, or reproduction up to 37 mg diquat/kg sediment or 0.54 

mg diquat/L pore water (MRID 49261902). A 10-day test on the marine amphipod Leptocheirus 

plumulosus measured only impacts on survival, and found no effect at 110 mg diquat/kg 

sediment (MRID 49584101). Diquat was not detected above the LOQ in pore water (0.0048 mg

diquat/L on Day 0 and 0.0043 mg diquat/L on Day 10) in that study.

In tests with vascular and non-vascular aquatic plants, significant toxic effects including 

decreases in biomass, growth rate, and frond number (for Lemna gibba) were observed. EC50

values for aquatic plants range from 0.00070 to 102 mg diquat/L (see Table A-11 in Appendix 

H).

Table 5.1. Most Sensitive Toxicity Endpoints for Aquatic Organisms Exposed to Diquat 

Taxonomic 

Group

Study 

Type

Test 

Substance

(% diquat)

Test Species

Toxicity Value

measured mg diquat/L

(Acute Toxicity Category or 

Most Sensitive Endpoints)

MRID and 

Study 

Classification

Aquatic Vertebrates

Freshwater 

fish 1

Acute

TEP

(240 g 

diquat/L)

Walleye 

(Stizostedion vitreum) 

96-hr LC50 = 0.75

(Highly toxic)

49611201

Quantitative

Chronic 

(Early-life 

Stage)

TGAI

(21.1%)

Fathead minnow 

(Pimephales 

promelas)

34-d NOAEC = 0.122

34-d LOAEC = 0.316

(Growth)

40380703

Acceptable
Page 27 of 76

Taxonomic 

Group

Study 

Type

Test 

Substance

(% diquat)

Test Species

Toxicity Value

measured mg diquat/L

(Acute Toxicity Category or 

Most Sensitive Endpoints)

MRID and 

Study 

Classification

Estuarine/

marine fish

Acute TGAI

(21.1%)

Sheepshead Minnow 

(Cyprinodon 

variegatus)

96-hr LC50 = 51.1

(Slightly toxic)

40316101

Acceptable

Chronic TGAI

(21.5%) Sheepshead Minnow

34-d NOAEC = 3.0

34-d LOAEC = 7.7

(Growth)

49095901

Acceptable

Aquatic Invertebrates in Water Column

Freshwater 

invertebrates

Acute TGAI

(23.6%) Hyalella azteca 96-hr EC50 = 0.09

(Very highly toxic)

48826701

Acceptable

Chronic

TEP

(16.7%)

Great Pond Snail 

(Lymnaea stagnalis)

168-d NOAEC = 0.0011

168-d LOAEC = 0.0032

(Development, Food 

consumption)

49613201 

Quantitative

TGAI

(21.1%) Daphnia magna

21-d NOAEC = 0.036

21-d LOAEC = 0.057

(Survival)

40380702

Acceptable

Estuarine/

marine 

invertebrates

Acute TGAI

(21.1%)

Mysid Shrimp 

(Americamysis bahia)

96-hr LC50 = 0.42

(Highly toxic)

40315701

Acceptable

Chronic TGAI

(21.5%) Mysid Shrimp

31-d NOAEC = 0.052

31-d LOAEC = 0.104

(Female dry weight)

49095902

Acceptable

Benthic Invertebrates

Freshwater 

invertebrates Chronic TGAI(21.5%) Hyalella azteca

42-d NOAEC = 11 mg/kg sed.

= 0.043 mg/L pore water 2

42-d LOAEC = 23 mg/kg sed.

= 0.037 mg/L pore water 2

(Reproduction)

49261901

Acceptable

Estuarine/ 

marine 

invertebrates

Subchronic TGAI 

(22.2%)

Leptocheirus 

plumulosus

10-d NOAEC = 110 mg/kg 

sed.

10-d LOAEC >110 mg/kg sed.

(No effects) 3

49584101

Acceptable

Aquatic Plants

Aquatic 

plants and 

algae

Vascular 4 TGAI

(22.4%)

Duckweed

(Lemna gibba)

14-d NOAEC = 0.0012

14-d EC50 = 0.0047

(Frond number)

48784007

Acceptable

NonvascularTGAI

(20.5%)

Freshwater Diatom 

(Navicula 

pelliculosa)

120-hr NOAEC = 0.00033 

120-hr EC50 = 0.00070

(Yield)

48784008

Supplemental

1 Fish are used as surrogate species for aquatic-phase amphibians. 2 Due to the relatively high 0-hour recovery of pore water concentration at the 11 mg diquat/kg level, the TWA 

calculated concentration at this level exhibited a shift in order, i.e., was higher than the TWA-calculated pore water 

concentration for the higher nominal level, 23 mg diquat/kg. 3 Pore water concentrations were below the LOQ (0.0048 mg diquat/L on Day 0 and 0.0043 mg diquat/L on Day 

10). Survival was the only endpoint measured. 4 A non-guideline Tier III study cited in previous assessments (MRID 41883002) provided an EC50 of 0.00075 mg 

diquat/L for giant duckweed (Spirodela punctata) based on decrease in dry weight. This endpoint will be used for 

additional risk characterization.
Page 28 of 76

5.2. Terrestrial Toxicity Data

Diquat is moderately to highly toxic to birds on an acute exposure basis. When the LD50 values 

for the mallard duck (Anas platyrhynchus) (60.6 mg diquat/kg-bw; MRID 00106559) and zebra 

finch (Taeniopygia guttata) (28.9 mg diquat/kg-bw; MRID 49095903) are scaled by body weight 

using the Mineau scaling factor of 1.15, they are equivalent. Diquat is moderately toxic to birds 

on a sub-acute dietary exposure basis. In chronic studies on the mallard duck, effects on egg 

laying have been observed as low as 24 mg diquat/kg-diet (NOAEC=4.2 mg diquat/kg-diet)

(MRID 00116565). Additional studies have demonstrated that varying the window of exposure 

during the egg laying period can result in higher LOAECs (MRIDs 48784012, 48784013). One 

study demonstrated full recovery of egg production during a post-treatment recovery period 

(MRID 48784014). 

Diquat is moderately toxic to mammals on an acute exposure basis (MRID 00081506). In a 2-

generation reproduction study on laboratory rats (Rattus norvegicus), the number of pups/litter 

and pup body weight were affected by exposure to diquat at 12 mg diquat/kg-bw, equivalent to 

240 mg diquat/kg-diet (NOAEC=4 mg diquat/kg-bw, equivalent to 80 mg diquat/kg-diet) (MRID 

41531301).

Diquat is practically non-toxic to adult honey bees (Apis mellifera) on an acute contact and oral 

exposure basis. No data are available on chronic toxicity to adult honey bees or on acute or 

chronic toxicity to larval bees.

No effects on seedling emergence were observed following diquat application to terrestrial plant

species (MRID 40165101). Toxicity was observed in studies of diquat’s effects on vegetative 

vigor (MRID 49141801), with greater impacts to dicots than to monocots. This comports with 

diquat’s mode of action as a contact herbicide.

Table 5.2. Most Sensitive Toxicity Endpoints for Terrestrial Animals Exposed to Diquat 

Taxonomic 

Group Study Type

Test 

Substance

(% diquat)

Test Species

Toxicity Value

measured diquat 

(Acute Toxicity Category or 

Most Sensitive Endpoints)

MRID and 

Study 

Classification

Birds 1

Acute Oral TGAI

(21.5%)

Zebra finch 

(Taeniopygia 

guttata)

LD50 = 28.9 mg/kg-bw

(Highly toxic)

49095903

Supplemental

Sub-acute 

dietary

TGAI

(18.9%)

Bobwhite quail

(Colinus virginianus)

LC50 = 540 mg/kg-diet

(Moderately toxic)

00116565

Supplemental

Chronic TGAI

(19.6%) Mallard duck

NOAEC = 4.2 mg/kg-diet

LOAEC = 24 mg/kg-diet

(Eggs laid)

00114230

Acceptable

Mammals 2

Acute Oral TEP

(19.3%) Laboratory rat

(Rattus norvegicus)

LD50 = 120 mg/kg-bw

(Moderately toxic)

00081506

Acceptable

Acute 

Inhalation

TEP

(19.5%)

LC50 = 0.19 mg/L

(Toxicity Category III)

00162046

Acceptable
Page 29 of 76

Taxonomic 

Group Study Type

Test 

Substance

(% diquat)

Test Species

Toxicity Value

measured diquat 

(Acute Toxicity Category or 

Most Sensitive Endpoints)

MRID and 

Study 

Classification

Chronic (2-

generation 

reproduction)

TGAI

(21.1%)

NOAEC = 80 mg/kg-diet

= 4 mg/kg-bw/day

LOAEC = 240 mg/kg-diet

= 12 mg/kg-bw/day

(F1 pups/litter; F1 body 

weight gain during lactation)

41531301

Acceptable

Insects

Acute 

contact TGAI

Honey bee

(Apis mellifera)

48-hr LD50 >100 μg/bee

(Practically non-toxic)

00072012

Acceptable

Acute oral TEP

(16.7%)

48-hr LD50 = 47 μg/bee

(Practically non-toxic)

40208001

Acceptable

Terrestrial 

Plants

Seedling 

Emergence TEP Monocots & Dicots

NOAEC = 4.65 lb/A

IC25 >4.65 lb/A

(No effects)

40165101

Acceptable

Vegetative 

Vigor

TGAI

(21.5)

Monocot: Corn 

(Zea mays)

NOAEC = 0.027 lb/A

IC25 = 0.059 lb/A

(Dry weight) 49141801

Supplemental Dicot: Cabbage 

(Brassica oleracea)

NOAEC = 0.00091 lb/A

IC25 = 0.0022 lb/A

(Dry weight) 1Birds are used as surrogate species for terrestrial-phase amphibians and reptiles. 2 Mammalian toxicity endpoints provided and reviewed by the U.S. EPA Office of Pesticide Programs Health Effects 

Division (HED).

5.3. Incidents

A review of the Ecological Incident Information System (EIIS, version 2.1.1), the ‘Aggregate 

Incident Reports’ database, and the Avian Monitoring Information System (AIMS) for ecological 

incidents involving diquat dibromide was completed on April 21, 2015 using PC codes 032201 

(diquat dibromide) and 032202 (diquat).

Eight aquatic incidents classified as ‘possible,’ ‘probable,’ or ‘highly probable’ were reported in 

EIIS. All eight incidents were as a result of direct application of diquat dibromide to a water 

body. Reported effects for all incidents were mortality, with species including clams, fish, frogs, 

turtles, and alligators. Of these eight reported incidents, two were misuses, four were registered 

uses, and two were of unknown legality of use. For three of the incidents that were associated 

with registered uses, the cause of death was reported as likely to be due to low dissolved oxygen 

following a large die-off of aquatic plants. No further information regarding cause of mortality 

was provided for the fourth registered use incident or the two unknown use incidents. 

Thirteen terrestrial plant incidents classified as ‘possible,’ ‘probable,’ or ‘highly probable’ are

reported in EIIS. Three of the 13 incidents were recorded in EIIS as misuses, four were 

recorded as registered uses, and the remaining six were recorded as unknown legality. 

Exposures were recorded as direct spray or spray drift. Reported effects were plant damage 

and plant mortality to species including alfalfa, strawberry, ornamental plants, grapes, spinach, 

and wildflowers. 
Page 30 of 76

In the Aggregate Incident Reports database, 29 fish and wildlife incidents, 47 plant incidents, 

and three other non-target incidents were reported. As no additional information on these 

aggregated incidents is available, they are assumed to be representative of registered uses of 

diquat dibromide.

No avian incident reports for diquat or diquat dibromide were identified in the AIMS database. 

A lack of reported incidents does not necessarily mean that such incidents have not occurred. In 

addition, incident reports for non-target plants and animals typically provide information on 

acute mortality events only. Reports for other adverse effects, such as reduced growth or 

impaired reproduction, are rarely received.

5.4. Endocrine Disruptor Screening Program

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse 

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and 

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental, 

reproductive, and general or systemic toxicity. These studies include endpoints which may be 

susceptible to endocrine influence, including effects on endocrine target organ histopathology, 

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss, 

and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and 

chronic studies that assess growth, developmental and reproductive effects in different 

taxonomic groups. As part of the Preliminary Problem Formulation for Registration Review (DP 

Barcode 365771), EPA reviewed these data and selected the most sensitive endpoints for 

relevant risk assessment scenarios from the existing hazard database. However, as required by 

FFDCA section 408(p), diquat dibromide is subject to the endocrine screening part of the 

Endocrine Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the Administrator 

may designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine-related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between October 

2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 chemicals, 

which contains 58 pesticide active ingredients and 9 inert ingredients. A second list of chemicals 

identified for EDSP screening was published on June 14, 20134 and includes some pesticides 

 4 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals.
Page 31 of 76

scheduled for registration review and chemicals found in water. Neither of these lists should be 

construed as a list of known or likely endocrine disruptors. 

For further information on the status of the EDSP, the policies and procedures, the lists of 

chemicals, future lists, the test guidelines and the Tier 1 screening battery, please visit our 

website.5

6. Risk Characterization

For this risk assessment of diquat dibromide, the deterministic RQ method was used to provide a 

metric of potential risks. The RQ is a comparison of exposure estimates to toxicity endpoints

(i.e., RQ = EEC/toxicity endpoint). The resulting RQs are compared to the Agency’s risk levels 

of concern (LOC), which are the Agency’s interpretive policy such that when acute or chronic 

risk LOCs are exceeded, the need for regulatory action may be considered. These criteria are 

used to indicate when the use of a pesticide, as directed on the label, has the potential to cause 

adverse effects to non-target organisms. For acute and chronic risks to animals, the LOCs are 0.5 

and 1.0, respectively, and for plants, the LOC is 1.0. The LOC for terrestrial invertebrates is 0.4. 

For listed aquatic animals, the acute LOC is 0.05 and for listed terrestrial animals the acute LOC 

is 0.1. For each taxa, RQs are generated and risks are discussed. Risk characterization includes 

consideration of calculated RQs as well as additional data and lines of evidence.

6.1. Aquatic Animals

6.1.1. Fish

The acute risk to non-listed species LOC (0.5) is exceeded for freshwater fish following direct 

aquatic use of diquat at an application rate of 4 lb diquat/surface acre/ft. The chronic RQ for 

freshwater fish (0.95) is very close to the chronic risk LOC (1.0) for direct aquatic use of diquat. 

No terrestrial uses result in LOC exceedances for non-listed freshwater fish, and no aquatic or 

terrestrial uses result in LOC exceedances for estuarine/marine fish (Table 6.1). The acute risk 

LOC (0.05) for listed freshwater fish is exceeded for one terrestrial use patterns in addition to the 

direct aquatic use, field grown and container stock of ornamentals. 

Freshwater fish are used as surrogate species for aquatic-phase amphibians. An acute toxicity 

study with diquat on the African clawed frog (Xenopus laevis) (MRID 05005290) demonstrates 

similar toxicity levels as studies on freshwater fish. In this study, 50% mortality was observed at 

1.0 mg diquat/L (most sensitive freshwater fish LC50=0.75 mg diquat/L, MRID 49611201) and 

inhibition of growth was seen at the lowest concentration tested, 0.5 mg diquat/L (most sensitive 

freshwater fish NOAEC=0.122 mg diquat/L, MRID 40380703). This study provides evidence 

that the use of freshwater fish as surrogate species for aquatic-phase amphibians is appropriate 

for assessing risk from diquat exposure.

There are five fish kill incidents reported in EIIS that are attributed to diquat exposure. One 

incident was a misuse, involving dumping of a diquat product into a storm drain. Another 

 5 http://www.epa.gov/endo/
Page 32 of 76

incident in a creek was attributed to the use of diquat and malathion in or near the creek. The 

remaining three incidents were fish kills in golf course ponds that were directly treated with 

diquat. In these incidents, it was not clear whether the fish kills were the result of direct toxicity 

of diquat or a decrease in dissolved oxygen following the application from the decomposition of 

the targeted aquatic weeds. Affected species included bream, carp, bass, and suckermouth 

catfish. There was also one frog incident reported in EIIS attributed to diquat exposure. Few 

details are available for this incident, which was reported by a concerned citizen who observed 

animal and plant deaths in a creek following municipal spraying of diquat dibromide into the 

water body. Additionally, 29 fish and wildlife incidents were reported in aggregate incident 

reports. 

The water monitoring data available for diquat are very limited, and unlikely to represent highend exposures. These data do not represent aquatic applications. The single surface water 

detection value of 0.0045 mg diquat/L in South Florida is approximately 10 times lower than the 

concentrations predicted to result in acute or chronic risk concerns for fish. The MCL of 0.02 mg 

diquat/L is approximately half the concentration predicted to result in acute or chronic risk 

concerns for fish.

Based on the lines of evidence above, direct aquatic use of diquat dibromide poses acute risks of 

concern to freshwater fish and aquatic-phase amphibians. Terrestrial uses of diquat dibromide do 

not pose risks of concern to non-listed fish but may pose risks of concern to listed freshwater 

fish.

Table 6.1. Acute and Chronic Fish Risk Quotients for Aquatic and Terrestrial Uses of Diquat Dibromide

Use Sites

Freshwater Estuarine/Marine 

Acute Chronic Acute Chronic

LC50 = 0.75 

mg diquat/L

NOAEC = 

0.122 mg 

diquat/L

LC50 = 51.1 

mg diquat/L

NOAEC = 

3.0 mg 

diquat/L

(Inter-row Weed Control) Eggplant, Green 

Pepper, Tomato 0.030 0.10 0.0004 0.004

Non-bearing Crops (Vines) -Cucumber Vines, 

Squash, except Acorn, Tomato Vines, 

Watermelon Vines

0.018 0.075 0.0003 0.003

Non-bearing Crops (Vines) -Pepper Vines, 

Squash, Acorn 0.024 0.10 0.0003 0.004

(Seed Crops) Alfalfa seed crop, Clover seed 

crop

0.015 0.052 0.0002 0.002

Ornamental field grown and container stock, 

and other similar areas 0.070 0.19 0.001 0.008

(Bearing Crops) Artichoke, Asparagus, 

Ginseng 0.022 0.034 0.0003 0.001

(Bearing Crops) Canola 0.006 0.014 0.0001 0.001

(Bearing Crops) Potato 0.011 0.038 0.0002 0.002

Non-bearing Crops (Small Fruits, Trees) 0.007 0.018 0.0001 0.001
Page 33 of 76

(Seed Crops) Calendula, Carrots, Coriander, 

Radishes, except Daikon variety, Spinach, 

Sugar beets, Table Beets, Turnips

0.005 0.016 0.0001 0.001

Ornamental Seed Crops, Landscape, 

Industrial, Commercial, Residential and Public 

Areas

0.005 0.015 0.0001 0.001

Turf Renovation, Dormant Established Turf 

Grass 0.005 0.013 0.0001 0.001

Aquatic Use 0.98 0.95 0.014 0.039

Bolded values exceed the LOC for non-listed species of 0.5 for acute risk or 1.0 for chronic risk.

Shaded values exceed the LOC for listed species of 0.05 for acute risk. The LOC of 1.0 for chronic risk to listed 

species is not exceeded in any of the scenarios in this table.

6.1.2. Aquatic Invertebrates – Water Column

Acute and chronic risk LOCs are exceeded for non-listed and listed freshwater and 

estuarine/marine invertebrates following direct aquatic use of diquat. The acute risk LOC is also

exceeded for non-listed freshwater invertebrates for one of the modeled terrestrial use patterns, 

field grown and container stock of ornamentals. For listed species, the acute risk LOC is 

exceeded for eight of the 12 terrestrial use patterns for freshwater invertebrates and for two of the

terrestrial use patterns for estuarine/marine invertebrates. The chronic risk LOC is exceeded for 

all use patterns for non-listed and listed freshwater invertebrates when the most sensitive TEP 

endpoint is used, but only for direct aquatic use when the most sensitive TGAI endpoint is used. 

Chronic risk for estuarine/marine invertebrates exceeds the LOC only for direct aquatic use

(Table 6.2).

Chronic risk to freshwater invertebrates was assessed using a NOAEC of 0.0011 mg diquat/L (as 

TEP) from an open literature study on great pond snails (Lymnaea stagnalis) (Ducrot et al., 

2010; MRID 49613201; E168652). This species was affected by diquat exposure at 

concentrations an order of magnitude lower than the NOAEC for the standard test species, 

Daphnia magna. If the NOAEC from the Daphnia study (NOAEC=0.036 mg diquat/L; MRID 

40380702) is used to calculate RQs, the chronic risk LOC is exceeded for direct aquatic use only. 

While the endpoint for the great pond snail was derived from a study using TEP while the 

endpoint for D. magna was derived from a study using TGAI, it is likely that the difference in 

sensitivity is due primarily to the differences between mollusks and daphnids rather than the 

different test substances. However, exposure to a formulated product is generally expected only

from spray drift, not from runoff. Any toxicity due to the formulation rather than the active 

ingredient may be alleviated when exposure is via runoff only.

There is one mollusk incident reported in EIIS that is attributed to diquat exposure. For two 

consecutive years, following treatment of a large lake with diquat dibromide and copper sulfate, 

thousands of dead clams were observed in the lake. It is not known whether the clam mortality 

was the result of direct toxicity of diquat or copper sulfate or due to a decrease in dissolved 

oxygen from the decomposition of the targeted aquatic weeds.

The monitoring data available for diquat are very limited, and unlikely to represent high-end 

exposures. The data available also do not represent aquatic applications. The single surface water 

detection value of 0.0045 mg diquat/L in South Florida is approximately 10 times lower than the 
Page 34 of 76

concentrations predicted to result in acute or chronic risk concerns for aquatic invertebrates. The 

MCL of 0.02 mg diquat/L is approximately half the concentration predicted to result in acute or 

chronic risk concerns for aquatic invertebrates.

Based on the lines of evidence above, direct aquatic use of diquat dibromide poses acute and 

chronic risks of concern to freshwater and estuarine/marine invertebrates. Terrestrial uses of 

diquat dibromide pose acute and chronic risks of concern to non-listed and listed freshwater 

invertebrates. Terrestrial uses also pose acute risks of concern to listed estuarine/marine 

invertebrates.

Table 6.2. Acute and Chronic Aquatic Water Column Invertebrate Risk Quotients for Aquatic and 

Terrestrial Uses of Diquat Dibromide

Use Sites

Freshwater Estuarine/Marine

Acute Chronic Acute Chronic

LC50 = 0.09 

mg diquat/L

NOAEC = 

0.0011 mg 

diquat/L 

(TEP, snail)

NOAEC = 

0.036 mg 

diquat/L 

(TGAI, 

daphnid)

LC50 = 0.42 

mg 

diquat/L

NOAEC = 

0.052 mg 

diquat/L

(Inter-row Weed Control) 

Eggplant, Green Pepper, Tomato 0.25 12 0.36 0.053 0.25

Non-bearing Crops (Vines) -

Cucumber Vines, Squash, except 

Acorn, Tomato Vines, 

Watermelon Vines

0.15 8.4 0.26 0.032 0.18

Non-bearing Crops (Vines) -

Pepper Vines, Squash, Acorn 0.20 11 0.33 0.042 0.23

(Seed Crops) Alfalfa seed crop, 

Clover seed crop 0.13 5.8 0.18 0.027 0.12

Ornamental field grown and 

container stock, and other similar 

areas

0.58 21 0.65 0.13 0.45

(Bearing Crops) Artichoke, 

Asparagus, Ginseng 0.18 3.9 0.12 0.039 0.083

(Bearing Crops) Canola 0.049 1.5 0.047 0.010 0.033

(Bearing Crops) Potato 0.091 4.2 0.13 0.020 0.090

Non-bearing Crops (Small Fruits,

Trees) 0.056 2.0 0.060 0.012 0.042

(Seed Crops) Calendula, Carrots, 

Coriander, Radishes, except 

Daikon variety, Spinach, Sugar 

beets, Table Beets, Turnips

0.039 1.8 0.055 0.008 0.038

Ornamental Seed Crops, 

Landscape, Industrial, 

Commercial, Residential and 

Public Areas

0.045 1.7 0.053 0.010 0.037

Turf Renovation, Dormant 

Established Turf Grass 0.045 1.4 0.044 0.010 0.030

Aquatic Use 8.2 121 3.7 1.75 2.6

Bolded values exceed the LOC for non-listed species of 0.5 for acute risk or 1.0 for chronic risk.

Shaded values exceed the LOC for listed species of 0.05 for acute risk or 1.0 for chronic risk.
Page 35 of 76

6.1.3. Aquatic Invertebrates – Benthic

The acute risk LOC is exceeded for non-listed freshwater benthic invertebrates for direct aquatic 

use only. The chronic risk LOC is exceeded for listed and non-listed freshwater benthic 

invertebrates for nine of the 12 terrestrial uses (Table 6.3). The chronic risk LOC may be 

exceeded for estuarine/marine benthic invertebrates for one or more uses, however this is an 

uncertainty due to the lack of effects in the 10-day study and the lack of available growth and 

reproductive data. Diquat is expected to bind strongly to sediment in aquatic systems.

LOC exceedances are seen for freshwater benthic invertebrates based on acute water column 

exposure as well as chronic exposure via sediment or pore water. The most sensitive freshwater 

organism tested was the amphipod Hyalella azteca, which experienced decreases in reproductive 

output with chronic exposure to diquat. In a chronic test with the freshwater midge Chironomus 

dilutus, no effects on growth or reproduction were observed up to 37 mg diquat/kg sediment, 

0.54 mg diquat/L pore water (MRID 49261902). These results likely indicate differential 

sensitivity to diquat among benthic invertebrate families. 

While there are two LOC exceedances for estuarine/marine invertebrates based on the NOAEC 

from a 10-day sub-chronic study of effects of diquat on survival of the amphipod Leptocheirus 

plumulosus, no effects were observed in this study. The NOAEC for this study is the highest 

concentration tested. Because a chronic study measuring growth and reproduction was not 

conducted, whether diquat impacts reproduction in L. plumulosus as it does in the freshwater 

amphipod H. azteca is unknown. Pore water RQs are not reported for L. plumulosus because 

pore water measurements of diquat in that study were <LOQ (0.0048 mg diquat/L on Day 0 and 

0.0043 mg diquat/L on Day 10). 

As noted above, aquatic monitoring data for diquat are very limited, are unlikely to represent 

high-end exposures, and do not represent aquatic applications. The highest sediment detection 

value of 3.1 mg diquat/kg in South Florida is one-third of the lowest modeled sediment EEC and 

approximately one-third of the lowest concentration predicted to result in a chronic risk concern 

for benthic aquatic invertebrates.

Based on the lines of evidence above, terrestrial and aquatic uses of diquat dibromide may pose 

acute and chronic risks of concern to both non-listed and listed freshwater and estuarine/marine 

benthic invertebrates.

Table 6.3. Chronic Aquatic Benthic Invertebrate Risk Quotients for Terrestrial Uses of Diquat Dibromide

Use Sites

Freshwater Estuarine/Marine

Pore Water 

Acute

Pore Water 

Chronic

Sediment 

Chronic

Sediment 

Sub-chronic

LC50 = 0.09 

mg 

diquat/L

NOAEC = 

0.043 mg 

diquat/L

NOAEC = 

11 mg 

diquat/kg 

sediment

NOAEC = 110 mg 

diquat/kg 

sediment

(Inter-row Weed Control) Eggplant, Green 

Pepper, Tomato 0.14 0.30 6.7 0.67
Page 36 of 76

Non-bearing Crops (Vines) -Cucumber 

Vines, Squash, except Acorn, Tomato Vines, 

Watermelon Vines

0.10 0.21 4.9 0.49

Non-bearing Crops (Vines) -Pepper Vines, 

Squash, Acorn 0.13 0.27 6.2 0.62

(Seed Crops) Alfalfa seed crop, Clover seed 

crop

0.069 0.15 3.3 0.33

Ornamental field grown and container stock, 

and other similar areas 0.26 0.54 12 1.2

(Bearing Crops) Artichoke, Asparagus, 

Ginseng 0.046 0.10 2.2 0.22

(Bearing Crops) Canola 0.018 0.038 0.87 0.087

(Bearing Crops) Potato 0.051 0.10 2.4 0.24

Non-bearing Crops (Small Fruits, Trees) 0.023 0.048 1.1 0.11

(Seed Crops) Calendula, Carrots, Coriander, 

Radishes, except Daikon variety, Spinach, 

Sugar beets, Table Beets, Turnips

0.022 0.046 1.0 0.10

Ornamental Seed Crops, Landscape, 

Industrial, Commercial, Residential and 

Public Areas

0.021 0.043 0.98 0.098

Turf Renovation, Dormant Established Turf 

Grass 0.017 0.036 0.82 0.082

Aquatic Use 4.1 3.0 69 6.9

Bolded values exceed the LOC for non-listed species of 0.5 for acute risk or 1.0 for chronic risk.

Shaded values exceed the LOC for listed species of 0.05 for acute risk or 1.0 for chronic risk.

6.2. Aquatic Plants

LOCs are exceeded for risk to non-listed and listed vascular and non-vascular aquatic plants 

following aquatic applications and nearly all terrestrial uses of diquat (Table 6.4). Diquat 

appears to be more toxic to non-vascular than to vascular aquatic plants. However, a nonguideline Tier III study cited in previous assessments (MRID 41883002) provided an EC50 of 

0.00075 mg diquat/L for giant duckweed (Spirodela punctata) based on decrease in dry weight. 

If this endpoint is used for characterization of risk to non-listed vascular aquatic plants, the

toxicity and risk estimation would be equivalent to that for non-vascular aquatic plants. The Tier 

III study was an outdoor, microcosm study conducted in sediment-water systems. Due to the 

multitude of variables in this study, a more recent acceptable guideline study was considered 

more appropriate for the calculation of RQs in this assessment. However, the Tier III study 

provides useful supplemental information, demonstrating that some vascular aquatic plants may 

be as sensitive as non-vascular plants to diquat exposure. This study also demonstrates that the 

toxicity to aquatic plants from diquat exposure observed in guideline laboratory studies is also 

seen in more complex systems.

As an aquatic herbicide, risks of concern to aquatic plants as a result of exposure to diquat are 

expected. Consistent with its mode of action, diquat’s toxicity appears to be non-selective across 

a variety of species of vascular and non-vascular aquatic plants (see Appendix H, Table A-11). 

One study on green algae, Pseudokirchneriella subcapitata, demonstrated decreased toxicity in 

the presence of sediment (MRID 48784011), suggesting that binding of diquat to sediment in 
Page 37 of 76

aquatic systems may decrease the toxicity to plants in the water column. 

Spray drift buffer widths that would be needed to reduce risks below the LOCs can be calculated 

to inform potential risk mitigation options. Tables 6.5 and 6.6 below show the calculated buffer 

widths for reducing risks below levels of concern for aquatic plants from ground and aerial 

(terrestrial) applications of diquat dibromide at a rate of 0.50 lb diquat/A, which is the maximum 

single application rate for 11 of the 12 diquat use patterns modeled in this assessment. For diquat 

dibromide, aquatic plants are the most sensitive aquatic taxa, so the calculated buffers would also 

be protective of other aquatic species. 

As shown, for aerial applications with droplet size distribution of fine to medium, buffers of 275 

or 900 ft are estimated to reduce risks to below levels of concern for non-listed and listed aquatic 

plant species, respectively. For ground applications with droplet size distribution in the very fine 

to fine range and high boom height (50 in or 4.2 ft), buffer widths are calculated as 30 or 120 ft 

for non-listed and listed aquatic plant species, respectively. These droplet size distributions are 

the defaults modeled when labels do not specify required droplet size distributions. Buffer widths 

for alternative droplet size spectra are provided in Tables 6.5 and 6.6 for comparison. The buffer

widths in Tables 6.5 and 6.6 are calculated for single applications only. If a water body receives 

exposure from multiple applications in one treatment area, risks may be underestimated. 

Table 6.4. Aquatic Plant Risk Quotients for Aquatic and Terrestrial Uses of Diquat Dibromide

Use Sites

Vascular Non-vascular 

Non-listed Listed Non-listed Listed

IC50 = 0.0047 

mg diquat/L

NOAEC = 

0.0012 mg 

diquat/L

IC50 = 0.00070 

mg diquat/L

NOAEC = 

0.00033 mg 

diquat/L

(Inter-row Weed Control) Eggplant, 

Green Pepper, Tomato 4.8 19 32 68

Non-bearing Crops (Vines) -

Cucumber Vines, Squash, except 

Acorn, Tomato Vines, Watermelon 

Vines

2.9 11 19 41

Non-bearing Crops (Vines) -Pepper 

Vines, Squash, Acorn 3.8 15 25 54

(Seed Crops) Alfalfa seed crop, 

Clover seed crop 2.4 9.4 16 34

Ornamental field grown and 

container stock, and other similar 

areas

11 44 75 159

(Bearing Crops) Artichoke, 

Asparagus, Ginseng 3.5 14 23 50

(Bearing Crops) Canola 0.93 3.6 6.2 13

(Bearing Crops) Potato 1.7 6.8 12 25

Non-bearing Crops (Small Fruits, 

Trees) 1.1 4.2 7.2 15

(Seed Crops) Calendula, Carrots, 

Coriander, Radishes, except Daikon 

variety, Spinach, Sugar beets, Table 

Beets, Turnips

0.74 2.9 5.0 11
Page 38 of 76

Ornamental Seed Crops, Landscape, 

Industrial, Commercial, Residential 

and Public Areas

0.87 3.4 5.8 12

Turf Renovation, Dormant 

Established Turf Grass 0.86 3.4 5.8 12

Aquatic Use 156 613 1050 2227

Bolded values exceed the LOC of 1.0.

Table 6.5. Spray Drift Buffer Distances to Reduce Risk to Aquatic Plants below Levels of Concern, Derived 

using AgDRIFT v2.1.1 for Aerial Spray at 0.5 lb diquat/A

Droplet Size

Buffer Distance for no LOC Exceedance (ft)

Vascular Aquatic Plants Non-vascular Aquatic Plants

Non-listed Listed Non-listed Listed

Coarse to Very Coarse 0 20 60 155

Medium to Coarse 0 55 115 275

Fine to Medium 0 130 275 900

Very Fine to Fine 70 840 >1,000 >1,000

Bolded rows are the recommended default assumptions for AgDRIFT modeling if droplet size is not otherwise 

specified on the label.

Table 6.6. Spray Drift Buffer Distances to Reduce Risk to Aquatic Plants below Levels of Concern, Derived 

using AgDRIFT v2.1.1 for Ground Spray at 0.5 lb diquat/A

Droplet Size Boom Height

Buffer Distance for no LOC Exceedance (ft)

Vascular Aquatic Plants Non-vascular Aquatic 

Plants

Non-listed Listed Non-listed Listed

Fine to Medium/Coarse Low 0 0 0 0

High 0 0 0 4

Very Fine to Fine Low 0 0 4 17

High 0 7 30 120

Bolded rows are the recommended default assumptions for AgDRIFT modeling if droplet size or boom height are 

not otherwise specified on the label.

6.3. Terrestrial Animals

6.3.1. Birds

Acute and chronic LOCs are exceeded across all terrestrial use patterns for both non-listed and 

listed bird species. In this assessment, birds are used as surrogate species for terrestrial-phase 

amphibians and reptiles. RQs calculated using T-REX based on various dietary items are 

presented in Table 6.7. 

The chronic RQs in Table 6.7 are calculated using the NOAEC of 4.2 mg/kg-diet from a 

guideline avian reproduction study with the mallard duck (MRID 00114230, test concentrations 

4.2, 24, and 98 mg/kg-diet). The only endpoint affected in the study was number of eggs laid. 

Subsequent studies demonstrated that beginning exposure to diquat later in the reproductive 

period than is done in the guideline study may result in higher NOAECs for this endpoint, 

although reductions in egg laying were seen consistently across all reproduction studies with the 
Page 39 of 76

mallard. A study that began exposure at photostimulation (when the period of light and dark is 

adjusted to stimulate egg production) and continued for 9 weeks resulted in a NOAEC of 19.2 

mg/kg-diet based on reductions in egg laying (MRID 48784012). A study that began exposure 

after egg laying was already underway and continued for 6 weeks resulted in a NOAEC of 33.8 

based on the same endpoint (MRID 48784013). If either of these NOAECs are used to calculate 

RQs, the LOC is still exceeded for every use pattern and food item, with the exception of some 

seed-eating birds (granivores) for some use patterns. Another study demonstrated that recovery 

of normal egg production is possible following cessation of a 6 week exposure (MRID 

48784014). Overall, these studies demonstrate that a real decrease in egg production is likely to 

occur following exposure of birds to diquat through food items. The severity of the reduction and 

its impact on the population may vary by the timing and duration of the exposure. The impacts of 

these factors may also vary between different species of birds. While full recovery is possible 

after exposure to diquat dibromide ends, there is likely to be considerable variability based on the 

timing of the laying period relative to the cessation of exposure. Prolonged exposure to diquat is 

likely if a bird population remains in the same location (e.g., during nesting season), due to 

diquat’s persistence in the environment.

The MCnest model was used to further evaluate the impact of diquat exposure on avian 

reproduction. The MCnest model estimates the number of broods per female expected in one 

year for 60 bird species following a pesticide application based on the application rate and avian 

toxicity values (USEPA, 2014a). The model was run a total of 12 times. Single applications of 

0.5 lb diquat/A on March 1, June 1, September 1, and December 1 were simulated. At each of 

these time points, the model was run using the three toxicity thresholds (NOAECs) for egg 

production discussed in the previous paragraph (i.e., 4.2, 19.2, and 33.8 mg/kg-diet). The results 

show substantial decreases in broods per female across nearly all 60 species for applications on 

March 1 and June 1 for all three toxicity thresholds. Fall and winter applications had much less 

impact on number of broods per female. There were only four species impacted by a diquat 

application in September, with equal impact across the three toxicity levels, and none impacted 

by an application in December. This reflects the fact that the majority of bird species breed in 

spring and summer months. As shown in Table 4.2, diquat applications may occur throughout 

the year for various uses. The higher toxicity thresholds impacted results for some species 

following a spring or summer application. This demonstrates that the window of exposure during 

the reproductive cycle does make a difference in the impact of diquat on avian reproduction in 

these cases. However, the overall picture shows that substantial impacts on avian reproduction 

would be experienced by the majority of species modeled even with a shorter window of 

exposure during egg laying.

The use of only one application in these model simulations underestimates the potential impact 

for use patterns that allow for multiple applications of diquat. Some use patterns allow for up to 6 

consecutive applications with only 5 day intervals, considerably prolonging exposure to nearby 

nesting bird populations. An example of graphical output from MCnest depicting a June 1 

application is in Figure 6.1; full graphical output is in Appendix G.

An additional acute and chronic exposure pathway of concern for birds may include 

consumption of diquat-contaminated drinking water (see Appendix B). However, the potential

risk from that exposure pathway is not quantified here. Exposure via drinking water may be 
Page 40 of 76

mitigated by the strong binding of diquat to sediment in aquatic systems, removing it from the 

water, if the birds do not consume that sediment while drinking. Similarly, exposure via 

inhalation of droplets following aerial or ground applications of diquat may contribute to 

potential risk for birds.

There is one reptile incident reported in EIIS attributed to diquat exposure with probable 

certainty and considered to be an accidental misuse. After application of diquat dibromide to a 

pond at three times the labeled rate, 400 turtles and 3 alligators died. No incidents were reported 

with birds.

Based on these lines of evidence, terrestrial uses of diquat dibromide pose acute and chronic 

risks of concern to non-listed and listed birds, reptiles, and terrestrial-phase amphibians.

Table 6.7. Upper Bound Kenaga, Acute and Chronic Avian Dose- and Dietary-Based Risk Quotients for 

Terrestrial Uses of Diquat Dibromide

Dietary item

Avian RQs

Acute Dose-based Size Classes Acute 

Dietarybased

Chronic 

Dietarybased 20 g 100 g 1000 g

Adjusted LD50 = 

31 mg/kg-bw

Adjusted LD50 = 

39 mg/kg-bw

Adjusted LD50 = 

55 mg/kg-bw

LC50 =

540 ppm

NOAEC =

4.2 ppm

1. (Inter-row Weed Control) Eggplant, Green Pepper, Tomato

2. Non-bearing Crops (Vines) – Pepper Vines, Squash, Acorn

Short grass 21 9.5 3.0 1.1 136

Tall grass 9.7 4.4 1.4 0.48 62

Broadleaf plants 12 5.3 1.7 0.59 76

Fruits/pods 1.3 0.59 0.19 0.066 8.5

Arthropods 8.3 3.7 1.2 0.41 53

Seeds 0.29 0.13 0.042 0.066 8.5

Non-bearing Crops (Vines) – Cucumber Vines, Squash, except Acorn, Tomato Vines, Watermelon Vines

Short grass 16 7.2 2.3 0.80 103

Tall grass 7.4 3.3 1.0 0.37 47

Broadleaf plants 9.1 4.1 1.3 0.45 58

Fruits/pods 1.0 0.45 0.14 0.050 6.4

Arthropods 6.3 2.8 0.90 0.31 40

Seeds 0.22 0.10 0.032 0.050 6.4

(Seed Crops) Alfalfa seed crop, Clover seed crop

Short grass 15 6.9 2.2 0.77 99

Tall grass 7.1 3.2 1.0 0.35 45

Broadleaf plants 8.7 3.9 1.2 0.43 56

Fruits/pods 0.97 0.43 0.14 0.048 6.2

Arthropods 6.1 2.7 0.86 0.30 39

Seeds 0.22 0.1 0.031 0.048 6.2

Ornamental field grown and container stock, and other similar areas

Short grass 10 4.7 1.5 0.52 67

Tall grass 4.8 2.1 0.68 0.24 31

Broadleaf plants 5.9 2.6 0.84 0.29 38

Fruits/pods 0.65 0.29 0.093 0.033 4.2
Page 41 of 76

Arthropods 4.1 1.8 0.58 0.20 26

Seeds 0.15 0.065 0.021 0.033 4.2

1. (Bearing Crops) Artichoke, Asparagus, Ginseng

2. Ornamental Seed Crops, Landscape, Industrial, Commercial, Residential and Public Areas

Short grass 12 5.4 1.7 0.61 78

Tall grass 5.6 2.5 0.79 0.28 36

Broadleaf plants 6.8 3.1 0.97 0.34 44

Fruits/pods 0.76 0.34 0.11 0.038 4.9

Arthropods 4.8 2.1 0.68 0.24 31

Seeds 0.17 0.076 0.024 0.038 4.9

1. (Bearing Crops) Canola

2. Non-bearing Crops (Small Fruits, Trees)

3. (Seed Crops) Calendula, Carrots, Coriander, Radishes, except Daikon variety, Spinach, Sugar beets, Table 

Beets, Turnips

Short grass 4.5 2.0 0.63 0.22 29

Tall grass 2.0 0.92 0.29 0.10 13

Broadleaf plants 2.5 1.1 0.36 0.13 16

Fruits/pods 0.28 0.12 0.04 0.014 1.8

Arthropods 1.7 0.8 0.25 0.087 11

Seeds 0.06 0.028 0.009 0.014 1.8

(Bearing Crops) Potato

Short grass 8.5 3.8 1.21 0.42 54

Tall grass 3.9 1.75 0.55 0.19 25

Broadleaf plants 4.8 2.1 0.68 0.24 31

Fruits/pods 0.53 0.24 0.075 0.026 3.4

Arthropods 3.3 1.5 0.47 0.17 21

Seeds 0.12 0.053 0.017 0.026 3.4

Turf Renovation, Dormant Established Turf Grass

Short grass 7.0 3.1 1.0 0.35 45

Tall grass 3.2 1.44 0.46 0.16 21

Broadleaf plants 4.0 1.8 0.56 0.20 25

Fruits/pods 0.44 0.2 0.062 0.022 2.8

Arthropods 2.8 1.2 0.39 0.14 18

Seeds 0.10 0.044 0.014 0.022 2.8

Bolded values exceed the LOC for non-listed species of 0.5 for acute risk or 1.0 for chronic risk.

Shaded values exceed the LOC for listed species of 0.1 for acute risk or 1.0 for chronic risk.
Page 42 of 76

Figure 6.1. Modeled Number of Broods by Species for Varying NOAECs using the MCnest Model 

for a June 1 Diquat Application (30 of 60 modeled species presented)

6.3.2. Mammals

Acute and chronic LOCs are exceeded for most terrestrial use patterns for both non-listed and 

listed mammalian species (Table 6.8). The only exception is for acute risk to non-listed 

mammals for the use patterns with a single application of diquat at 0.5 lb diquat/A (these include 

(1) canola, (2) non-bearing small fruits and trees, and (3) seed crops of calendula, carrots, 

coriander, radishes, spinach, sugar beets, table beets, and turnips). Additionally, acute and 

chronic risks do not exceed levels of concern for granivorous mammals for any use pattern.

An additional acute and chronic exposure pathway of concern for mammals may be realized 

through consumption of diquat-contaminated drinking water (see Appendix B). However, the 

potential risk from this exposure pathway is not quantified here. Exposure via drinking water 

may be mitigated by the strong binding of diquat to sediment in aquatic systems, removing it 

from the water, if the mammals do not consume that sediment while drinking. Inhalation is not 

an exposure pathway of concern for mammals.

No incidents have been reported involving diquat and mammals, however this does not 

necessarily mean that no incidents have occurred.

Based on these lines of evidence, terrestrial uses of diquat dibromide pose acute and chronic 

risks of concern to non-listed and listed mammals.

0

0.5

1

1.5

2

2.5

3

House sparrow

Mourning dove

Horned Lark

American robin

House finch

Chipping sparrow

American goldfinch

Cedar waxwing

Northern mockingbird

House wren

Barn swallow

Dark-eyed junco

Wood thrush (contiguous)

White-winged dove

Lark sparrow

Grasshopper sparrow

Boat-tailed grackle

Black-capped chickadee

Northern cardinal

American kestrel

Wood thrush...

Carolina chickadee

Yellow warbler

Common grackle

Red-winged blackbird

Canada goose (giant)

Blue Jay

Dickcissel

Northern bobwhite

Mallard

Number of Broods Expected Following a June 1 Diquat 

Application of 0.5 lb diquat cation/A 

Baseline (No diquat exposure) NOAEC = 33.8 mg/kg-diet (6 mg/kg-bw/d)

NOAEC = 19.2 mg/kg-diet (3.2 mg/kg-bw/d) NOAEC = 4.2 mg/kg-diet (0.412 mg/kg-bw/d)
Page 43 of 76

Table 6.8. Upper Bound Kenaga, Acute and Chronic Mammalian Dose- and Dietary-Based Risk Quotients

for Terrestrial Uses of Diquat Dibromide

Dietary item

Mammalian RQs

Acute Dose-based Size Classes Chronic Dose-based Size Classes Chronic 

Dietarybased 15 g 35 g 1000 g 15 g 35 g 1000 g

Adjusted 

LD50 = 264 

mg/kg-bw

Adjusted 

LD50 = 213 

mg/kg-bw

Adjusted 

LD50 = 92 

mg/kg-bw

Adjusted 

NOAEL 

= 8.8

Adjusted 

NOAEL 

= 7.1

Adjusted 

NOAEL 

= 3.1

NOAEC = 

80 ppm

1. (Inter-row Weed Control) Eggplant, Green Pepper, Tomato

2. Non-bearing Crops (Vines) - Pepper Vines, Squash, Acorn

Short grass 2.1 1.8 0.94 62 53 28 7.1

Tall grass 0.94 0.81 0.43 28 24 13 3.3

Broadleaf plants 1.2 0.99 0.53 35 30 16 4.0

Fruits/pods 0.13 0.11 0.059 3.9 3.3 1.8 0.45

Arthropods 0.81 0.69 0.37 24 21 11 2.8

Seeds 0.029 0.024 0.013 0.86 0.73 0.39 0.45

Non-bearing Crops (Vines) - Cucumber Vines, Squash, except Acorn, Tomato Vines, Watermelon Vines

Short grass 1.6 1.3 0.72 47 40 22 5.4

Tall grass 0.72 0.61 0.33 22 18 10 2.5

Broadleaf plants 0.88 0.75 0.40 26 23 12 3.0

Fruits/pods 0.098 0.084 0.045 2.9 2.5 1.3 0.34

Arthropods 0.61 0.52 0.28 18 16 8.4 2.1

Seeds 0.022 0.019 0.010 0.65 0.56 0.30 0.34

(Seed Crops) Alfalfa seed crop, Clover seed crop

Short grass 1.5 1.3 0.69 45 39 21 5.2

Tall grass 0.69 0.59 0.32 21 18 9.5 2.4

Broadleaf plants 0.85 0.72 0.39 25 22 12 2.9

Fruits/pods 0.09 0.08 0.043 2.8 2.4 1.3 0.33

Arthropods 0.59 0.5 0.27 18 15 8.1 2.0

Seeds 0.021 0.018 0.01 0.63 0.54 0.29 0.33

Ornamental field grown and container stock, and other similar areas

Short grass 1.0 0.87 0.47 31 26 14 3.5

Tall grass 0.47 0.40 0.21 14 12 6.4 1.6

Broadleaf plants 0.57 0.49 0.26 17 15 7.9 2.0

Fruits/pods 0.064 0.054 0.029 1.9 1.6 0.87 0.22

Arthropods 0.40 0.34 0.18 12 10 5.5 1.4

Seeds 0.014 0.012 0.006 0.42 0.36 0.19 0.22

1. (Bearing Crops) Artichoke, Asparagus, Ginseng

2. Ornamental Seed Crops, Landscape, Industrial, Commercial, Residential and Public Areas

Short grass 1.2 1.0 0.54 35 30 16 4.1
Page 44 of 76

Tall grass 0.54 0.46 0.25 16 14 7.4 1.9

Broadleaf plants 0.67 0.57 0.30 20 17 9.1 2.3

Fruits/pods 0.074 0.063 0.034 2.2 1.9 1.0 0.26

Arthropods 0.46 0.40 0.21 14 12 6.4 1.6

Seeds 0.016 0.014 0.008 0.49 0.42 0.23 0.26

1. (Bearing Crops) Canola

2. Non-bearing Crops (Small Fruits, Trees)

3. (Seed Crops) Calendula, Carrots, Coriander, Radishes, except Daikon variety, Spinach, Sugar beets, Table 

Beets, Turnips

Short grass 0.43 0.37 0.20 13 11 6.0 1.5

Tall grass 0.20 0.17 0.091 6.0 5.1 2.7 0.69

Broadleaf plants 0.24 0.21 0.11 7.3 6.3 3.4 0.84

Fruits/pods 0.027 0.023 0.012 0.81 0.69 0.37 0.094

Arthropods 0.17 0.15 0.078 5.1 4.4 2.3 0.59

Seeds 0.006 0.005 0.003 0.18 0.15 0.083 0.094

(Bearing Crops) Potato

Short grass 0.83 0.71 0.38 25 21 11 2.9

Tall grass 0.38 0.32 0.17 11 9.7 5.2 1.3

Broadleaf plants 0.47 0.40 0.21 14 12 6.4 1.6

Fruits/pods 0.052 0.044 0.024 1.6 1.3 0.71 0.18

Arthropods 0.32 0.28 0.15 9.7 8.3 4.4 1.1

Seeds 0.011 0.010 0.005 0.34 0.29 0.16 0.18

Turf Renovation, Dormant Established Turf Grass

Short grass 0.68 0.58 0.31 20 18 9.4 2.4

Tall grass 0.31 0.27 0.14 9.4 8.0 4.3 1.1

Broadleaf plants 0.38 0.33 0.18 12 10 5.3 1.3

Fruits/pods 0.043 0.036 0.020 1.3 1.1 0.59 0.15

Arthropods 0.27 0.23 0.12 8.0 6.9 3.7 0.9

Seeds 0.009 0.008 0.004 0.28 0.24 0.13 0.15

Bolded values exceed the LOC for non-listed species of 0.5 for acute risk or 1.0 for chronic risk.

Shaded values exceed the LOC for listed species of 0.1 for acute risk or 1.0 for chronic risk.

6.3.3. Terrestrial Invertebrates

The acute oral toxicity endpoint for honey bees is 47 μg diquat/bee and the EEC for adult dietary 

exposure is 16.1 μg diquat/bee/day. The acute contact RQ for honey bees is 0.34, slightly below 

the LOC of 0.4 (USEPA et al., 2014).

The acute contact toxicity endpoint for honey bees is non-definitive. Therefore, an RQ value is 

not calculated for acute risk to terrestrial invertebrates via contact exposure. However, even if the 

EEC of 1.35 μg diquat/bee is compared to the highest concentration tested, 100 μg diquat/bee,

for the honey bee acute contact study, the RQs would be 0.014, over 10 times lower than the 
Page 45 of 76

acute risk LOC of 0.4 for bees (USEPA et al., 2014). 

No toxicity data are available for calculation of chronic RQs for adult bees or for acute or 

chronic RQs for bee larvae. Therefore, there is uncertainty regarding the potential acute and 

chronic effects of diquat on honey bee larvae as well as potential for chronic effects on adult

bees. While diquat is not a systemic compound, residues from sub-lethal concentrations of spray 

drift on plants may be present in pollen and/or nectar and represent a route of exposure for bees 

through both contact and ingestion. Therefore, additional data on pollinators are needed to fully 

characterize the risk associated with diquat use to all developmental stages of honey bees, as 

sensitivity may vary according to life-stage and length of exposure (adult vs. larval and acute vs. 

chronic, respectively). These data are recommended under the Guidance for Assessing Pesticide 

Risks to Bees (USEPA et al., 2014). The Agency expects to publish additional guidance on 

addressing gaps in bee toxicity data for pesticides that have undergone Registration Review in 

the near future.

No incidents associated with diquat involving terrestrial invertebrates, such as bee kills, have been 

reported to the Agency.

6.4. Terrestrial Plants

LOCs are exceeded for risk to non-listed and listed dicots following a single ground or aerial 

application of diquat at 0.38 or 0.50 lb diquat/A as a result of spray drift (Table 6.9). At 0.38 lb 

diquat/A, LOCs are exceeded for risk to listed monocots following aerial application of diquat as 

a result of spray drift. At 0.50 lb diquat/A, LOCs are additionally exceeded for risk to listed 

monocots following ground application and for risk to non-listed monocots following aerial 

application of diquat as a result of spray drift.

As a means of understanding the potential impact of spray drift buffers on risks to terrestrial 

plants, EFED has evaluated buffer widths for various droplet size spectra for ground and aerial 

applications at 0.50 lb diquat/A, the maximum single application rate for 11 of the 12 diquat use 

patterns modeled in this assessment (Tables 6.10 and 6.11). Based on these evaluations, for 

aerial applications, buffer widths of 610 to >1,000 ft would reduce risks below levels of concern 

for non-listed and listed dicot species, depending on droplet size. For ground applications, buffer 

distances of 100 to 850 ft would reduce risks below levels of concern for non-listed dicot 

species, depending on droplet size and boom height. Monocots are relatively less sensitive than 

dicots to diquat exposure and buffer widths for monocots are therefore smaller.

These modeled buffer widths are for a single application only. Risks associated with multiple 

applications may be greater. 

Exposures from runoff alone do not result in LOC exceedances for any of the application 

scenarios modeled. This is due to the lack of toxicity in the seedling emergence study using 

diquat (MRID 40165101), which simulates exposure in the soil from runoff. Diquat binds 

strongly to soil particles and is expected to be present in runoff primarily bound to eroded soil 

rather than freely dissolved in runoff water. As a non-systemic contact herbicide, this exposure 

pathway is not expected to result in toxicity exceeding risk levels of concern to terrestrial plants.
Page 46 of 76

There are 13 terrestrial plant incidents reported in EIIS that were attributed to diquat exposure.

Three of the 13 incidents were misuses. The remaining 9 incidents all resulted from exposure 

via direct spray or spray drift. Reported effects were plant damage and mortality; affected 

species included strawberries, spinach, alfalfa, grapes, and ornamentals. Additionally, 47 plant 

incidents were reported in aggregate incident reports. As no additional information on these 

aggregated incidents is available, they are assumed to be representative of registered uses of 

diquat dibromide.

Based on these lines of evidence, uses of diquat dibromide on terrestrial sites poses risks of 

concern to non-listed and listed terrestrial plants. Aquatic uses are assumed to result in minimal 

exposure to terrestrial plants, although any overspray or spray drift from aquatic applications 

onto terrestrial sites could pose the same risks as spray drift resulting from terrestrial uses.

Table 6.9. Terrestrial Plant Risk Quotients for Terrestrial Uses of Diquat Dibromide

Type of Plant

Ground spray Aerial Spray

Dry Areas

SemiAquatic 

Areas

Spray Drift 

Only Dry Areas

SemiAquatic 

Areas

Spray Drift 

Only

Non-bearing Crops (Vines) – Cucumber Vines, Squash, except Acorn, Tomato Vines, Watermelon Vines

0.38 lb diquat/A

Monocot

Nonlisted <0.1 <0.1 0.39 <0.1 <0.1 0.77

Listed <0.1 <0.1 0.84 <0.1 <0.1 1.7

Dicot

Nonlisted <0.1 <0.1 10 <0.1 <0.1 21

Listed <0.1 <0.1 25 <0.1 <0.1 50

All other terrestrial uses 0.50 lb diquat/A

Monocot

Nonlisted <0.1 <0.1 0.51 <0.1 <0.1 1.0

Listed <0.1 <0.1 1.1 <0.1 <0.1 2.2

Dicot

Nonlisted <0.1 <0.1 14 <0.1 <0.1 27

Listed <0.1 <0.1 33 <0.1 <0.1 66

Bolded RQ values exceed the LOC of 1.0.

Table 6.10. Spray Drift Buffer Distances to Reduce Risk to Terrestrial Plants below Levels of Concern, 

Derived using AgDRIFT v2.1.1 for Aerial Spray at 0.5 lb diquat/A

Plant Type Droplet Size

Buffer Distance for no LOC Exceedance (ft)

Non-listed Species Listed Species

Dicot

Coarse to Very Coarse 610 >1,000

Medium to Coarse >1,000 >1,000

Fine to Medium >1,000 >1,000

Very Fine to Fine >1,000 >1,000

Monocot

Coarse to Very Coarse 35 75

Medium to Coarse 50 110

Fine to Medium 80 170

Very Fine to Fine 250 670
Page 47 of 76

Bolded rows are the recommended default assumptions for AgDRIFT modeling if droplet size is not otherwise 

specified on the label.

Table 6.11. Spray Drift Buffer Distances to Reduce Risk to Terrestrial Plants below Levels of Concern, 

Derived using AgDRIFT v2.1.1 for Ground Spray at 0.5 lb diquat/A

Plant Type Droplet Size Boom Height1

Buffer Distance for no LOC 

Exceedance (ft)

Non-listed Species Listed Species

Dicot

Fine to Medium/Coarse Low 100 320

High 180 470

Very Fine to Fine Low 240 570

High 460 850

Monocot

Fine to Medium/Coarse Low 4 7

High 7 10

Very Fine to Fine Low 10 20

High 25 50

Bolded rows are the recommended default assumptions for AgDRIFT modeling if droplet size or boom height are 

not otherwise specified on the label.

1 Low boom height = 20 in, high boom height = 50 in.

6.5. Risk Conclusions

Risks exceeding levels of concern as a result of labeled diquat dibromide uses are identified for 

all assessed taxa with the exception of terrestrial invertebrates (for which critical toxicity data are 

lacking). The specific risks identified for aquatic and terrestrial uses are listed below. Incidents 

attributed to diquat have been reported involving aquatic invertebrates, fish, reptiles, and 

terrestrial plants.

Risks exceeding levels of concern for aquatic uses:

• Acute risk to freshwater fish and aquatic-phase amphibians. 

• Acute and chronic risk to freshwater and estuarine/marine invertebrates.

• Risk to vascular and non-vascular aquatic plants.

Aquatic uses are assumed to result in minimal exposure to terrestrial plants and animals, 

although any overspray or spray drift from aquatic applications onto terrestrial sites could pose 

the same risks as spray drift resulting from terrestrial uses.

Risks exceeding levels of concern for terrestrial uses:

• Acute risk to freshwater fish (listed species only).

• Acute and chronic risk to water column and benthic freshwater invertebrates.

• Acute risk to water column estuarine/marine invertebrates (listed species only) and 

acute and chronic risk to benthic estuarine/marine invertebrates.

• Risk to vascular and non-vascular aquatic plants.

• Acute and chronic risk to birds, reptiles, and terrestrial-phase amphibians.

• Acute and chronic risk to mammals.

• Risk to terrestrial plants.
Page 48 of 76

While no risks to terrestrial invertebrates are identified above concern levels based on acute 

contact and oral exposure routes, no toxicity data are available with which to assess chronic 

exposure to adult honey bees or acute or chronic exposure to larval bees. 

7. Listed Species of Concern

Consistent with EPA’s responsibility under the Endangered Species Act (ESA), the Agency will 

evaluate risks to federally listed threatened and endangered (listed) species from registered uses 

of pesticides in accordance with the Joint Interim Approaches developed to implement the 

recommendations of the April 2013 National Academy of Sciences (NAS) report, Assessing 

Risks to Endangered and Threatened Species from Pesticides. The NAS report6 outlines 

recommendations on specific scientific and technical issues related to the development of 

pesticide risk assessments that EPA and the Services must conduct in connection with their 

obligations under the ESA and FIFRA. EPA will address concerns specific to diquat dibromide

in connection with the development of its final registration review decision for diquat dibromide. 

In November 2013, EPA, the U.S. Fish and Wildlife Service, National Marine Fisheries (the 

Services), and USDA released a white paper containing a summary of their joint Interim 

Approaches for assessing risks to listed species from pesticides. These Interim Approaches were 

developed jointly by the agencies in response to the NAS recommendations, and reflect a 

common approach to risk assessment shared by the agencies as a way of addressing scientific

differences between the EPA and the Services. Details of the joint Interim Approaches are 

contained in the November 1, 2013 white paper

7

, Interim Approaches for National-Level 

Pesticide Endangered Species Act Assessments Based on the Recommendations of the National 

Academy of Sciences April 2013 Report.

Given that the agencies are continuing to develop and work toward implementation of the 

Interim Approaches to assess the potential risks of pesticides to listed species and their 

designated critical habitat, this ecological risk assessment supporting the registration review of

diquat dibromide does not describe the specific ESA analysis, including effects determinations 

for specific listed species or designated critical habitat, to be conducted. While the agencies 

continue to develop a common method for ESA analysis, this risk assessment for the registration 

review of diquat dibromide describes only the level of ESA analysis completed at this time. This 

assessment allows EPA to focus its future evaluations on the types of species where the potential 

for effects exists, once the scientific methods being developed by the agencies have been fully 

vetted. Once the agencies have fully developed and implemented the scientific methods 

necessary to complete risk assessments for listed species and their designated critical habitats, 

these methods will be applied to subsequent analyses of diquat dibromide as part of completing 

this registration review.

 6 http://www.nap.edu/catalog.php?record_id=18344

7 http://www.epa.gov/espp/2013/nas.html
Page 49 of 76

7.1. Taxonomic Groups Potentially at Risk 

Based on this screening-level assessment, there are potential risks of direct effects to listed and 

non-listed terrestrial plants from the use of diquat dibromide on all of its registered use sites. 

Listed species of all taxa may also be affected through indirect effects because of the potential 

direct effects on listed and non-listed species. Potential direct effects on listed and non-listed 

terrestrial plants, aquatic plants, birds, mammals, fish, and aquatic invertebrates from the use of 

diquat dibromide may be associated with modification of primary constituent elements of 

designated critical habitats, where such designations have been made. However, at this current 

stage of the Registration Review process, it is premature to make effects determinations for listed 

species until further refinements are conducted, based on recommendations received from the 

National Academy of Sciences’ (NAS) National Research Council (NRC) April 2013 report8

. 

The NAS report outlines recommendations on specific scientific and technical issues related to 

the development of pesticide risk assessments that are compliant with the ESA and FIFRA.

To make effects determinations for individual listed species, useful refinements may include, but 

are not limited to, analyses of: 1) more detailed, species-specific ecological and biological data; 

2) more detailed and accurate information on diquat dibromide use patterns; and 3) sub-county 

level spatial proximity data for the co-occurrence of potential effects areas and listed species and 

any designated critical habitat. Examples of such refinements are described below.

The refinements based on individual species data; additional, detailed usage information, when 

available; and recommendations from the NRC report are expected to help to more accurately 

identify potential areas of effect and to better inform effects and habitat determinations for listed 

species and any designated critical habitats.

Table 7.1. Potential Effects to Federally-Listed Taxa Associated with Direct Effects from the Labeled Uses of 

Diquat

Listed Taxon Direct Effects1Uses of Concern

Yes/No Acute/Chronic

Terrestrial and semi-aquatic 

plants - monocots Yes NA All terrestrial uses

Terrestrial and semi-aquatic 

plants - dicots Yes NA All terrestrial uses

Terrestrial invertebrates No2 NA None

Birds Yes Acute and Chronic All terrestrial uses

Terrestrial-phase amphibians Yes Acute and Chronic All terrestrial uses

Reptiles Yes Acute and Chronic All terrestrial uses

Mammals Yes Acute and Chronic All terrestrial uses

Aquatic plants Yes NA All terrestrial uses 

Direct aquatic use

Freshwater fish Yes Acute Ornamental field grown and container stock

Direct aquatic use

Aquatic-phase amphibians Yes Acute Ornamental field grown and container stock

Direct aquatic use

Freshwater invertebrates Yes Acute and Chronic All terrestrial uses 

Direct aquatic use

Marine/estuarine fish No NA None

 8 http://www.nap.edu/catalog.php?record_id=18344
Page 50 of 76

Listed Taxon Direct Effects1Uses of Concern

Yes/No Acute/Chronic

Marine/estuarine 

invertebrates3 Yes Acute and Chronic

(Inter-row Weed Control) Eggplant, Green 

Pepper, Tomato

Ornamental field grown and container stock

Direct aquatic use 1 If direct effects are identified to any taxon, indirect effects or effects to designated critical habitat may occur for all 

other taxa.

2 Based on acute toxicity data only. Presumed direct effects for larval and chronic toxicity. 3 Based on acute and chronic water column toxicity data and on 10-day benthic toxicity data. Chronic benthic 

toxicity data may reveal additional terrestrial uses of concern.

8. References

Ashton, Floyd M. and Alden S. Craft. 1973. Mode of Action of Herbicides. John Wiley & Sons, 

New York, NY.

Miles and Pfeuffer. 1997. Pesticides in Canals of South Florida, Arch. Environ. Contam. Toxicol. 

32:337-345.

Reinert, K.H. and J.H. Rodgers. 1987. Fate and persistence of aquatic herbicides. Rev. Envtl.

Contamin. Toxicol. 98:61-98.

USEPA. 2000. Occurrence of Regulated Contaminants in Drinking Water: First Stage 

Occurrence and Exposure Report for Six-Year Regulatory Review. U.S. Environmental 

Protection Agency, Office of Water. Working Draft dated May 12, 2000.

USEPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. U.S. Environmental Protection Agency, Office of Pesticide Programs, 

Environmental Fate and Effects Division. Available at 

http://www.epa.gov/espp/consultation/ecorisk-overview.pdf

USEPA. 2008. Abdel-Saheb, I. Drinking Water Assessment for diquat dibromide (Reward) 

use as landscape and aquatic herbicide. DP Barcode 355147. U.S. Environmental Protection 

Agency, Office of Pesticide Programs, Environmental Fate and Effects Division. 

Memorandum to the Registration Division. Sep. 3, 2008.

USEPA. 2009. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and 

Transport of Pesticides. Version 2.1. U.S. Environmental Protection Agency, Office of 

Pesticide Programs, Environmental Fate and Effects Division. Oct. 22, 2009. Available

at: http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm

USEPA. 2010a. Abdel-Saheb, I. and M. Davy. Registration Review – Preliminary Problem 

Formulation for the Ecological Risk Assessment of Diquat Dibromide (Revised). DP 

Barcode 365771. U.S. Environmental Protection Agency, Office of Pesticide Programs, 

Environmental Fate and Effects Division. Memorandum to the Special Review and 

Reregistration Division. Apr. 20, 2010.
Page 51 of 76

USEPA. 2010b. Abdel-Saheb, I. and M. Davy. EFED Response to Public Comments for Diquat 

Dibromide PF: Docket ID# EPA-HQ-OPP-2009-0846; Case No. 0288. DP Barcode 374529. 

U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate 

and Effects Division. Memorandum to the Special Review and Reregistration Division. Apr. 

20, 2010.

USEPA. 2010c. Davy, M. Addendum to Diquat Dibromide Problem Formulation for 

Registration Review. DP Barcode 365771. U.S. Environmental Protection Agency, Office of 

Pesticide Programs, Environmental Fate and Effects Division. Memorandum to the Product 

Reregistration Division. Jun. 7, 2010.

USEPA. 2010d. Abdel-Saheb, I. Drinking Water Assessment for Diquat Dibromide (Reglone®) 

Use on Canola Desiccant. DP Barcode 372018. U.S. Environmental Protection Agency, 

Office of Pesticide Programs, Environmental Fate and Effects Division. Memorandum to the 

Registration Division. Apr. 13, 2010.

USEPA. 2010e. Davy, M. And I. Abdel-Saheb. Ecological Risk Assessment for Diquat 

Dibromide for use on Canola. DP Barcode 372017. U.S. Environmental Protection Agency, 

Office of Pesticide Programs, Environmental Fate and Effects Division. Memorandum to the 

Registration Division. May 11, 2010.

USEPA. 2010f. Hartless, C. and J. Lin. Risks of Diquat Dibromide use to the Federally 

Threatened Delta Smelt (Hypomesus transpacificus). U.S. Environmental Protection Agency, 

Office of Pesticide Programs, Environmental Fate and Effects Division. Dec. 17, 2010.

USEPA. 2011a. Hartless, C. and J. Lin. Justification for Changes to DCI for Diquat Dibromide. 

DP Barcode 384111. U.S. Environmental Protection Agency, Office of Pesticide Programs, 

Environmental Fate and Effects Division. Memorandum to the Pesticide Re-evaluation 

Division. Feb. 9, 2011.

USEPA. 2011b. Evaluation Guidelines for Ecological Toxicity Data in the Open Literature. U.S. 

Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and 

Effects Division. Available at

http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/endangered_speci

es_reregistration_workgroup/esa_evaluation_open_literature.htm. 

USEPA. 2013. Guidance on Modeling Offsite Deposition of Pesticides via Spray Drift for 

Ecological and Drinking Water Assessments. U.S. Environmental Protection Agency, Office 

of Pesticide Programs, Environmental Fate and Effects Division, December 20, 2013.

USEPA. 2014a. Implementation of Markov Chain Nest Productivity Model (MCnest). U.S. 

Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and 

Effects Division. Available at http://www.epa.gov/med/Prods_Pubs/mcnest.htm. 
Page 52 of 76

USEPA. 2014b. Surface Water Concentration Calculator. U.S. Environmental Protection 

Agency, Office of Pesticide Programs, Environmental Fate and Effects Division. Available at 

http://www.epa.gov/oppefed1/models/water/index.htm#swcc

USEPA. 2014c. Toxicity Testing and Ecological Risk Assessment Guidance for Benthic 

Invertebrates. U.S. Environmental Protection Agency, Office of Pesticide Programs, 

Environmental Fate and Effects Division. Available at 

http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/aquatic_biology_t

ech_team/ToxTesting_EcoRiskAssessmentForBenthicInvertebrates.pdf

USEPA. 2015a. Alsadek, J. Updated Screening Level Usage Analysis (SLUA) Report for Diquat 

dibromide PC #032201. U.S. Environmental Protection Agency, Office of Pesticide 

Programs, Biological and Economic Analysis Division. Memorandum to the Pesticide Reevaluation Division. Aug. 13, 2015.

USEPA. 2015b. Doucoure, C. BEAD Review of Diquat Dibromide (032201) Use Summary 

Tables Submitted by Syngenta Crop Protection, DP #427305. U.S. Environmental Protection 

Agency, Office of Pesticide Programs, Biological and Economic Analysis Division.

Memorandum to the Pesticide Re-evaluation Division. Jul. 2, 2015.

USEPA, PMRA, and CDPR. 2014. Guidance for Assessing Pesticide Risks to Bees. Office of 

Pesticide Programs, USEPA, Washington D.C. Health Canada Pest Management Regulatory 

Agency, Ottawa, ON, Canada. California Department of Pesticide Regulation, Sacramento, 

CA. June 23, 2014.

8.1. Cited Environmental Fate and Chemistry Studies

MRID Citation

00138928 McAllister, W.; Franklin, L.; Cohle, P.; et al. (1984) Uptake, Depuration and Bioconcentration of 

14C-R-40244 by Bluegill Sunfish (Lepomis macrochirus): ABC Final Report #30973; T-11414. 

(Un- published study received Feb 1, 1984 under 476-EX-104; prepared by Analytical BioChemistry Laboratories, Inc., submitted by Stauffer Chemical Co., Richmond, CA; CDL:252340-

A)

40246101 Joseph, R.; Skidmore, M. (1987) Diquat: Photolytic Stability on Soil Surfaces: Lab Project ID: 

RJ0573B. Unpublished study prepared by ICI Agricultural Experiment Station. 50 p.

40326901 Hamer, M.; Farrelly, E.; Hill, I. (1987) Diquat: Investigation of Accumulation in Bluegill Sunfish 

in a Flow-through System: Lab. Proj. ID RJ0608B. Unpublished study prepared by Imperial 

Chemical Industries, PLC. 33 p.

40348601 Pack, D. (1987) Freundlich Soil Adsorption Coefficients of Diquat: Laboratory Project I.D: MEF0069/8716930. Unpublished study prepared by Chevron Chemical Co. 41 p.

40418801 Tegala, B.; Skidmore, M. (1987) Diquat: An Aqueous Photolysis Study: Laboratory Project ID: RJ 

0613B. Unpublished study prepared by Imperial Chemical Industries, PLC. 54 p.

40917403 Fujie, G. (1988) Aquatic Field Dissipation Studies for Aquatic Uses and Aquatic Impact Uses: 

Laboratory Project ID: R10/1642AQDISS. Unpublished study prepared by Chevron Chemical Co. 

183 p.

40927601 Johnston, J. (1988) Aerobic Aquatic Metabolism of Diquat: Laboratory Project ID MEF-0073. 

Unpublished study prepared by Chevron Chemical Co. 116 p.

40972301 Johnston, J. (1988) Aerobic Soil Metabolism of Diquat: Project ID: MEF-0071. Unpublished study 

prepared by Chevron Chemical Co. 116 p.
Page 53 of 76

40972302 Johnston, J. (1988) Anaerobic Aquatic Metabolism of Diquat: Project ID: MEF-0072. Unpublished 

study prepared by Chevron Chemical Co. 129 p.

42060301 Fujie, G. (1991) Diquat Cation Soil Dissipation Study on Potatoes in New York: Supplement to: 

Lab Project Number: R010/7059SOILF. Data submitted as alternate to Craven Laboratories 

generated data. Unpublished study prepared by A&S Environmental Testing, Inc. 465 p.

42060302 Fujie, G. (1991) Diquat Cation Soil Dissipation Study on Potatoes in Idaho: Supplement to: Lab 

Project Number: R010/7060SOILF. Data submitted as alternate to Craven Laboratories generated 

data|. Unpublished study prepared by A&S Environmental Testing, Inc. 320 p.

8.2. Cited Ecotoxicity Studies 

MRID Citation

00072012 Kimura, E.T.; Young, P.R.; Staniszewski, K. (1959) Gibberellic acid: Toxicologic 

and pharmacologic studies. American Pharmaceutical Association Journal XLVIII(2):127-129. 

(Also in unpublished submission received Jan 31, 1977 under 275-32; submitted by Abbott 

Laboratories, North Chicago, Ill.; CDL: 095795-D)

00081506 Rittenhouse, J.R. (1979) The Acute Oral Toxicity of Diquat Water Weed Killer: SOCAL 

1396/37:67 (S-1431). (Unpublished study received Sep 14, 1979 under 239-2247; submitted by 

Chevron Chemical Co., Richmond, Calif.; CDL:098973-B)

00106559 Fink, R.; Beavers, J.; Joiner, G.; et al. (1982) Final Report: Acute Oral LD50--Mallard Duck: 

Diquat Technical (SX-1260): Project No. 162-131. Rev. (Unpublished study received May 13, 

1982 under 239-1663; prepared by Wildlife International Ltd. and Washington College, submitted 

by Chevron Chemical Co., Richmond, CA; CDL:247503-A)

00114230 Fink, R.; Beavers, J.; Joiner, G.; et al. (1982) One-generation Reproduction--Mallard Duck: Diquat 

Technical (SX-1306): Project No. 162-143; S-2069. Final rept. (Unpublished study received Sep 3, 

1982 under 239-1663; prepared by Wildlife International Ltd., submitted by Chevron Chemical 

Co., Richmond, CA; CDL: 248290-D)

00116565 Fink, R.; Beavers, J.; Joiner, G.; et al. (1981) Subacute Feeding-- Reproduction Screening 

Bioassay--Bobwhite Quail: Diquat Technical (SX-1306): Project No. 162-137; S-1984. Final rept. 

(Unpublished study received Sep 3, 1982 under 239-1663; prepared by Wildlife International Ltd., 

submitted by Chevron Chemical Co., Richmond, CA; CDL:248290-B)

00162046 Bruce, E. (1985) The Acute Inhalation Toxicity of Diquat Water Weed Killer (SX-1574) in Rats: 

SOCAL 2353. Unpublished study prepared by Chevron Environmental Health Center. 67 p.

05005290 Anderson, R.J.; Prahlad, K.V. (1976) The deleterious effects of fungicides and herbicides on 

Xenopus laevis embryos. Archives of Environmental Contamination and Toxicology 4(3):312-323.

40165101 Shilling, D. (1987) The Influence of Preemergence Applied Diquat in the Germination and 

Emergence of Selected Plant Species: Laboratory Project ID: 8704721. Unpublished study 

prepared by Univ. of Florida, Dept. of Agronomy. 11 p.

40208001 Gough, H.; Collins, I.; Jackson, D.; et al. (1987) Diquat: Acute 5-day Contact and Oral Toxicity to 

Honey Bees (Apis mellifera): Laboratory Project ID: RJ055B. Unpublished study prepared by 

Imperial Chemicals Industries. 39 p.

40315701 Hoberg, J. (1987) Acute Toxicity of Diquat Concentrate to Mysid Shrimp (Mysidopsis bahia):

Laboratory Project Identification: S-2910: 981.0287.6111.510. Unpublished study prepared by 

Springborn Life Sciences, Inc. 52 p.

40316101 Nicholson, R. (1987) Acute Toxicity of Diquat Concentrate to Sheepshead Minnow (Cyprinodon 

variegatus): Laboratory Project ID: S-2909: 981.0287.6110.500. Unpublished study prepared by 

Springborn Life Sciences, Inc. 55 p.

40380702 Surprenant, D. (1987) The Chronic Toxicity of Diquat Conc. to Daphnia magna under Flowthrough Conditions: Lab. Proj. ID. 87-8- 2473. Unpublished study prepared by Springborn Life 

Sciences, Inc. 67 p.

40380703 Surprenant, D. (1987) The Toxicity of Diquat Conc. to Fathead Minnow (Pimephales promelas) 

Embryo and Larvae. Unpublished study prepared by Springborn Life Sciences, Inc. 64 p.

41531301 Hodge, M. (1990) Diquat: Multigeneration Study in the Rat: Lab Project Number: CTL/P/2462: 

RRO393. Unpublished study prepared by ICI Central Toxicology Laboratory. 2174 p.
Page 54 of 76

41883002 Bellet, E. (1990) Effect of Diquat on Aquatic Plants-A Tier 3 Study: Lab Project Number: DQ 

124-2. Unpublished study prepared by Chevron Chemical Co. 272 p.

43532703 Smyth, D.; Tapp, J. (1988) Diquat: Determination of Toxicity to the Green Alga Selenastrum 

capricornutum: Lab Project Numbers: FT51/88: BL/B/3271. Unpublished study prepared by ICI 

PLC. 18 p.

48784007 Magor, S.; Shillabeer, N. (2001) Diquat Dibromide - Toxicity to Duckweed (Lemna gibba): Final 

Report. Project Number: BL6966/B, TK0025805. Unpublished study prepared by AstraZeneca 

UK, Ltd. 25p.

48784008 Kent, S. (1998) Diquat - Toxicity to the Freshwater Diatom Navicula pelliculosa: Final Report. 

Project Number: BL6150/B, TK0117407. Unpublished study prepared by AstraZeneca UK, Ltd. 

25p.

48784009 Kent, S. (1998) Diquat - Toxicity to the Marine Alga Skeletonema costatum: Final Report. Project 

Number: BL6149/B, TK0117408. Unpublished study prepared by AstraZeneca UK, Ltd. 24p.

48784010 Kent, S. (1998) Diquat - Toxicity to the Blue-Green Alga Anabaena flos-aquae: Final Report. 

Project Number: BL6148/B, TK0117409. Unpublished study prepared by AstraZeneca UK, Ltd. 

26p.

48784011 Shillabeer, N. (1998) Diquat - Toxicity to the Marine Alga Selenastrum capricornutum in the 

Presence of Sediment: Final Report. Project Number: BL6471/B, TK0117406. Unpublished study 

prepared by AstraZeneca UK, Ltd. 23p.

48784012 Temple, D.; Martin, K.; Beavers, J.; et al. (2004) Diquat - An Egg Production Study with the 

Mallard: Final Report. Project Number: 528/158, TK0025806. Unpublished study prepared by 

Wildlife International, Ltd. 82p.

48784013 Temple, D.; Martin, K.; Beavers, J.; et al. (2004) Diquat - An Egg Production Study with the 

Mallard (6 Weeks Exposure during Egglaying): Final Report. Project Number: 528/174, 

TK0025807. Unpublished study prepared by Wildlife International, Ltd. 106p.

48784014 Temple, D.; Martin, K.; Beavers, J.; et al. (2009) Diquat - A Reproduction Study with the Mallard 

to Evaluate Reversibility in Effects following Withdrawal of Dietary Exposure: Final Report. 

Project Number: 528/273, T001762/08. Unpublished study prepared by Wildlife International, Ltd. 

127p.

48826701 Bender, E. (2006) Diquat Technical - Acute Toxicity to Hyalella azteca Under Static Conditions: 

Final Report. Project Number: TK0120969, 1666/CRA/510/05, 1666/CRC/510/05. Unpublished 

study prepared by Bioensaios Analises e Consultoria Ambiental Ltda. 23p.

49095901 Minderhout, T.; Kendall, T.; Gallagher, S.; et al. (2012) Diquat Dibromide - An Early Life-Stage 

Toxicity Test with the Sheepshead Minnow (Cyprinodon variegatus): Final Report. Project 

Number: 528A/232, TK0175769. Unpublished study prepared by Wildlife International, Ltd. 77p.

49095902 Claude, M.; Kendall, T.; Gallagher, S.; et al. (2013) Diquat Dibromide - A Flow-Through LifeCycle Toxicity Test with the Saltwater Mysid (Americamysis bahia): Final Report. Project 

Number: 528A/240A, TK0024700. Unpublished study prepared by Wildlife International, Ltd. 

94p.

49095903 Hubbard, P.; Martin, K.; Beavers, J. (2013) Diquat Dibromide - An Acute Oral Toxicity Study 

with the Zebra Finch Using a Sequential Testing Procedure: Final Report. Project Number: 

528/366, TK0024765. Unpublished study prepared by Wildlife International, Ltd. 98p. Relates to 

L0001544.

49141801 Martin, J. (2013) Diquat Dibromide-Evaluation of the Phytotoxicity to Non-Target Terrestrial 

Plants Vegetative Vigor Test: Final Report. Project Number: 1787.6872. Unpublished study 

prepared by Syngenta Crop Protection, LLC. 347p. Relates to L0001563.

49262901 Daun, C. (2013) Enforcement Analytical Method: Maquat 86-M. Unpublished study prepared by 

Mason Chemical Company. 8p.

49261902 Bradley, M. (2013) Final Report: Diquat Dibromide - Life Cycle Toxicity Test Exposing Midges 

(Chironomus dilutus) to Diquat Dibromide Applied to Sediment Under Static-Renewal Conditions 

Following EPA Test Methods. Project Number: 1781/6869. Unpublished study prepared by 

Smithers Viscient Laboratories. 143p.

49584101 Bradley, M. (2015) Diquat Dibromide: 10-Day Toxicity Test Exposing Estuarine Amphipods 

(Leptocheirus plumulosus) to Diquat Dibromide Applied to Sediment Under Static Conditions: 
Page 55 of 76

Final Report. Project Number: 1781/7059, TK0024762. Unpublished study prepared by Smithers 

Viscient Laboratories. 98p.

49611201 Paul, E.; Simonin, H.; Symula, J.; et al. (1994) The Toxicity of Diquat, Endothall, and Fluridone to 

the Early Life Stages of Fish. Journal of Freshwater Ecology. 9(3).

49613201 Ducrot, V.; Pery, A.; Lagadic, L. (2010) Modelling Effects of Diquat under Realistic Exposure 

Patterns in Genetically Differentiated Populations of the Gastropod Lymnaea stagnalis. 

Philosophical Transactions of the Royal Society (365):3485-3494.
Page 56 of 76

Appendix A. Risk Quadrant Figures and Tables

Risk quadrant figures and tables are a tool for use by EFED to better communicate with risk 

managers, who need to know which taxa have greater or lower risk and the certainty of the risk 

conclusions. The risk quadrant figure is extremely summarized to display the seven assessed 

taxa in one of four quadrants, whichever quadrant represents the highest risk for the taxon. The 

risk quadrant table summarizes the rationales for the placement of the taxa in the figure, as 

succinctly as possible. A taxon is only discussed once in the table: in the quadrant matching its 

location in the figure.

For example, if there are chronic risks of concern to mammals and acute risks to mammals are 

not of concern, then the term “Mammals” appears in the upper-right quadrant of the figure and 

the table. The table summarizes (under the “Mammals” heading) the chronic RQ range and any 

other salient lines of evidence for chronic risks of concern. In this example, the figure and table 

do not directly indicate low acute risks to mammals; it is implied by the absence of discussion. 

Only the rationales for the location of the taxa in the figure are discussed in the table, with one 

exception: No Effect Determinations are summarized in the upper-left quadrant of the table (not 

the figure).

The risk versus certainty quadrants are represented as four color-coded categories, as represented 

in the examples below. The left quadrants represent no risks of concern (i.e., no LOC 

exceedances). The right quadrants represent a risk of concern (i.e., at least one LOC 

exceedance). The upper quadrants represent higher certainty (i.e., a relatively complete data set 

and/or multiple lines of evidence support the risk conclusions). The lower quadrants represent 

lower certainty (i.e., a relatively limited data set and/or lines of evidence are limited or 

conflicting).

While communication of risk is inherently subjective, use of subjective terminology or risk 

management values should be avoided. The intent of the risk quadrant figure is to simply 

display for the risk manager which taxa are of concern and the degree of confidence in the risk 

conclusions. The intent of the risk quadrant table is to simply explain the supporting evidence 

for the placement of taxa in the figure and serves essentially like a key to the figure. The table 

should not summarize all of the risk conclusions, just the conclusions that support the placement 

of taxa in the figure.

The risk quadrant figure and table are placed in an assessment’s Executive Summary, the risk 

transmittal memo, or may be used for formal or informal communications with risk managers. 

A key to the placement of taxa in the figure and graph is placed in an appendix, as follows:
Page 57 of 76

Key to Four Color-Coded Categories in Risk Quadrant Figures and Tables:

Green: Greater Certainty of Negligible Risk Red: Greater Certainty of Risks of Concern

• No LOC exceedance with relatively complete data set 

or risks have been mitigated

• High confidence in data

• Fate and ecotox profiles clearly support lack of a risk 

concern

• Multiple lines of evidence predict effects will occur

• LOC exceedances, fate, and ecotox profile should be 

considered 

• Exposure is expected to occur based on fate profile 

and uses

• Probable or highly probable cause of incident(s)

Yellow: Lower Certainty of Negligible Risk Orange: Lower Certainty of Risks of Concern

• No LOCs exceeded but other non-compelling 

circumstances suggest that there may be issues

• Possible cause of incident(s)

• Missing endpoints, poorly spaced test concentrations, 

low confidence in data showing low toxicity

• An LOC exceedance is judged to not be of concern 

due to use pattern or other circumstances that limit 

exposure potential

• Low risk calls based on extrapolated data

• LOCs exceeded; however, lines of evidence 

predicting risk are either limited or conflicting

• Possible cause of incident(s)

• Chronic risk called for non-persistent chemical

• Risk calls based on lack of data without other lines of 

evidence available

• Risk calls based on uncertain extrapolation methods 

or conflicting data

• Missing endpoints, poorly spaced test concentrations, 

low confidence in data
Page 58 of 76

Appendix B. SIP Output

Table 1. Inputs

Parameter Value

Chemical name Diquat

Solubility (in water at 25oC; mg/L) 700000

Mammalian LD50 (mg/kg-bw) 120

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian species

Mammalian NOAEL (mg/kg-bw) 4

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian species

Avian LD50 (mg/kg-bw) 28.9

Avian test species other

Body weight (g) of "other" avian species 13.6

Mineau scaling factor 1.15

Mallard NOAEC (mg/kg-diet) 4.2

Bobwhite quail NOAEC (mg/kg-diet)

NOAEC (mg/kg-diet) for other bird species

Body weight (g) of other avian species

NOAEC (mg/kg-diet) for 2nd other bird species

Body weight (g) of 2nd other avian species

Table 2. Mammalian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 120400.0000 120400.0000

Adjusted toxicity value (mg/kg-bw) 92.2993 3.0766

Ratio of exposure to toxicity 1304.4524 39133.5714

Conclusion*

Exposure through drinking 

water alone is a potential 

concern for mammals

Exposure through drinking 

water alone is a potential 

concern for mammals

Table 3. Avian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 567000.0000 567000.0000

Adjusted toxicity value (mg/kg-bw) 30.6212 0.2084

Ratio of exposure to acute toxicity 18516.6131 2721084.9932

Conclusion*

Exposure through drinking 

water alone is a potential 

concern for birds

Exposure through drinking 

water alone is a potential 

concern for birds

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, when 

aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through drinking water may contribute 

to a total exposure that has potential for effects to non-target animals.
Page 59 of 76

Appendix C. STIR Output

Input

Application and Chemical Information

Enter Chemical Name Diquat

Enter Chemical Use Herbicide

Is the Application a Spray? (enter y or n) y

If Spray What Type (enter ground or air) air

Enter Chemical Molecular Weight (g/mole) 184.25

Enter Chemical Vapor Pressure (mmHg) 1.00E-07

Enter Application Rate (lb a.i./acre) 0.5

Toxicity Properties

Bird

Enter Lowest Bird Oral LD50 (mg/kg bw) 28.9

Enter Mineau Scaling Factor 1.15

Enter Tested Bird Weight (kg) 0.0136

Mammal

Enter Lowest Rat Oral LD50 (mg/kg bw) 120

Enter Lowest Rat Inhalation LC50 (mg/L) 0.19

Duration of Rat Inhalation Study (hrs) 4

Enter Rat Weight (kg) 0.35

Output

Results Avian (0.020 kg )

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 9.92E-04

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.25E-04

Adjusted Inhalation LD50 3.75E-01

Ratio of Vapor Dose to Adjusted Inhalation LD50 3.32E-04 Exposure not Likely Significant

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 4.80E-02

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 1.28E-01 Proceed to Refinements

Results Mammalian (0.015 kg )

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 9.92E-04

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.57E-04

Adjusted Inhalation LD50 1.13E+01

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.39E-05 Exposure not Likely Significant

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 6.04E-02

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 5.34E-03 Exposure not Likely Significant
Page 60 of 76

Appendix D. SWCC Example Output

Summary of Water Modeling of Diquat and the USEPA Standard Pond

Estimated Environmental Concentrations for Diquat are presented in Table 1 for the USEPA 

standard pond with the CAnurserySTD_V2 field scenario. A graphical presentation of the yearto-year peaks is presented in Figure 1. These values were generated with the Surface Water 

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

This model estimates that about 4.7% of Diquat applied to the field eventually reaches the water 

body. The main mechanism of transport from the field to the water body is by erosion (85.9% of 

the total transport), followed by spray drift (13.2%) and runoff (0.91%).

In the water body, pesticide dissipates with an effective water column half-life of 10888.7 days. 

(This value does not include dissipation by transport to the benthic region; it includes only 

processes that result in removal of pesticide from the complete system.) The main source of 

dissipation in the water column is photolysis (effective average half-life = 10888.8 days) 

followed by volatilization (5.844262E+09 days).

In the benthic region, pesticide is stable. The vast majority of the pesticide in the benthic region 

(99.99%) is sorbed to sediment rather than in the pore water.

Table 1. Estimated Environmental Concentrations (ppb) for Diquat.

Peak (1-in-10 yr) 52.6

4-day Avg (1-in-10 yr) 24.7

21-day Avg (1-in-10 yr) 23.3

60-day Avg (1-in-10 yr) 23.1

365-day Avg (1-in-10 yr) 22.8

Entire Simulation Mean 12.0

Table 2. Summary of Model Inputs for Diquat.

Scenario CAnurserySTD_V2

Cropped Area Fraction 1

Koc (ml/g) 5839

Water Half-Life (days) @ 25 °C 0

Benthic Half-Life (days) @ 25 °C 0

Photolysis Half-Life (days) @ 25 

°Lat

74

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 25 °C 0
Page 61 of 76

Foliar Half-Life (days) 0

Molecular Wt 344.05

Vapor Pressure (torr) 1e-7

Solubility (mg/l) 700000

Table 3. Application Schedule for Diquat.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

2/25 Ground 0.56 0.99 0.062

3/28 Ground 0.56 0.99 0.062

4/25 Ground 0.56 0.99 0.062

5/23 Ground 0.56 0.99 0.062

6/17 Ground 0.56 0.99 0.062

7/15 Ground 0.56 0.99 0.062

8/12 Ground 0.56 0.99 0.062

9/9 Ground 0.56 0.99 0.062

10/7 Ground 0.56 0.99 0.062

11/5 Ground 0.56 0.99 0.062

12/3 Ground 0.56 0.99 0.062

12/31 Ground 0.56 0.99 0.062

Figure 1. Yearly Peak Concentrations
Page 62 of 76

Appendix E. T-REX Model Example Inputs

TREX MODEL INPUTS

These values will be used in the calculation of exposure estimates for foliar, granular, liquid and/or 

seed applications of pesticides.

Chemical Identity and Application Information

Chemical Name: Diquat

Seed Treatment? (Check if yes)

FALSE

 Use: (Seed Crops) Alfalfa seed crop, Clover seed crop

Product name and form:

% A.I. (leading zero must be entered for 

formulations <1% a.i.): 100.00%

Application Rate (lb ai/acre) 0.5

Half-life (days): 35

Application Interval (days): 5

Number of Applications: 4

Are you assessing applications with variable 

rates or intervals? no

Assessed Species Inputs (optional, use defaults for RQs for national level assessments)

What body weight range is assessed (grams)? Birds Mammals

Small 20 15

Medium 100 35

Large 1000 1000
Page 63 of 76

Endpoints

Avian

Endpoint Toxicity value

Indicate test 

species below

Optional 

Test 

Organism 

Body 

weight 

(g)

Optional 

Test 

Species 

Name

Toxicity 

Value 

Referenc

e (MRID)

LD50 (mg/kg-bw)

28.90

3

13.60

zebra 

finch 49095903

LC50 (mg/kgdiet) 540.00 1 116565

NOAEL (mg/kgbw) 1

NOAEC (mg/kgdiet) 4.20 2 114230

Enter the Mineau et al. Scaling Factor 1.15

Mammalian

Acute Study

Chronic 

Study

Size (g) of mammal used in toxicity study

Default rat body weight is 350 grams 350 350

Endpoint Toxicity value

Reference 

(MRID)

LD50 (mg/kgbw) 120.00 81506

LC50 (mg/kgdiet)

Reported 

Chronic 

Endpoint

4.00

415313011

Is dietary 

concentration 

(mg/kg-diet) 

reported from 

the available 

chronic 

mammal 

study? (yes or 

no)

yes

41531301

Enter dietary 

concentration 

(mg/kg-diet)

80.00
Page 64 of 76

Appendix F. TERRPLANT Model Example Inputs

TerrPlant v. 1.2.2

Green values signify user inputs (Tables 1, 2 and 4). 

Table 1. Chemical Identity.

Parameter User Inputs

Chemical Name Diquat 

PC code 32201

Use Many

Application Method Broadcast

Application Form liquid 

Solubility in Water (ppm) 700000

Table 2. Input parameters used to derive EECs.

Input Parameter Symbol Value (user inputs) Units

Application Rate A 0.5 lb a.i./A

Incorporation I 1 none

Runoff Fraction R 0.05 none

Drift Fraction D 0.12 none

Table 3. EECs for Diquat. Units in lb a.i./A.

Description Equation EEC

Runoff to dry areas (A/I)*R 0.025

Runoff to semi-aquatic areas (A/I)*R*10 0.25

Spray drift A*D 0.06

Total for dry areas ((A/I)*R)+(A*D) 0.085

Total for semi-aquatic areas ((A/I)*R*10)+(A*D) 0.31

Table 4. Plant survival and growth data used for RQ derivation. Units are in lb a.i./A

Seedling Emergence Vegetative Vigor

Plant type EC25 NOAEC EC25 NOAEC 

Monocot 4.65 4.65 0.059 0.027

Dicot 4.65 4.65 0.0022 0.00091

Table 5. RQ values for plants in dry and semi-aquatic areas exposed to Diquat through runoff 

and/or spray drift.*

Plant Type Listed Status Dry Semi-Aquatic Spray Drift

Monocot non-listed <0.1 <0.1 1.02

Monocot listed <0.1 <0.1 2.22

Dicot non-listed <0.1 <0.1 27.27

Dicot listed <0.1 <0.1 65.93
Page 65 of 76

Appendix G. MCnest Output

Figure F-1. Modeled Number of Broods by Species for Varying NOAECs using the MCnest Model for a March 1 Diquat Application
Page 66 of 76

Figure F-2. Modeled Number of Broods by Species for Varying NOAECs using the MCnest Model for a June 1 Diquat Application
Page 67 of 76

Figure F-3. Modeled Number of Broods by Species for Varying NOAECs using the MCnest Model for a September 1 Diquat Application
Page 68 of 76

Figure F-4. Modeled Number of Broods by Species for Varying NOAECs using the MCnest Model for a December 1 Diquat Application
Page 69 of 76

Appendix H. Ecotoxicity Data

1. Toxicity to Aquatic Organisms

1.1. Aquatic Vertebrate Toxicity

1.1.1. Freshwater Fish Toxicity

Table A-1. Acute Toxicity of Diquat Cation to Freshwater Fish

Species 1,2 % Diquat 

Cation

96-hour LC50

(ppm diquat 

cation)

Toxicity Category MRID No.

Author, Year

Study 

Classification

Black bullhead

(Ameiurus melas) 18 6 94 Slightly toxic 40098001Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish

(Lepomis macrochirus) 19.8 3 14 Slightly toxic 00138962Thompson et al., 1980 Supplemental

Bluegill sunfish 18 4 33 5

(72-hr) Slightly toxic 00115572McCann, 1967 Supplemental

Bluegill sunfish 18 6 79 Slightly toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 92 Slightly toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 107 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 227 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 243 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 >107 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 >241 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Bluegill sunfish 18 6 >386 Practically non-toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Brown trout

(Salmo trutta) 18 4 17.8 Slightly toxic 00115858Simonin et al., 1977 Supplemental

Goldfish

(Carassius auratus auratus) 18 6 >54 Slightly toxic 40098001Mayer and Ellersick, 1986 Supplemental

Largemouth bass 

(Micropterus salmoides) 240 g/L 4.9 Moderately toxic 49611201 (E014903)

Paul et al., 1994 Quantitative

Rainbow trout

(Oncorhynchus mykiss) 19.8 3 14.8 Slightly toxic 00138961Thompson et al., 1980 Supplemental

Rainbow trout 18 6 >54 Slightly toxic 40098001

Mayer and Ellersick, 1986 Supplemental

Smallmouth bass 

(Micropterus dolomieu) 240 g/L 3.9 Moderately toxic 49611201 (E014903)Paul et al., 1994 Quantitative

Walleye (Stizostedion 

vitreum) 240 g/L 0.75 Highly toxic 49611201 (E014903)Paul et al., 1994 Quantitative

Yellow perch

(Perca flavescens) 18 6 32 Slightly toxic 40098001Mayer and Ellersick, 1986 Supplemental

Bolded values indicate the endpoints used for risk estimation. 1 In previous risk assessments for diquat dibromide, an acute toxicity study for goldfish (LC50 >5000 mg/L, MRID 00059586) 

was listed. This study was conducted with dikegulac sodium (PC code 109601) and will not be included in this or future risk 

assessments for diquat dibromide. 

2 In previous risk assessments for diquat dibromide, an acute study for striped bass (Roccus saxatilis, MRID 00028002, Wellborn, 

1969) was listed. This study is reclassified as Unacceptable (previously classified Supplemental). Reasons for the reclassification 

include no raw data, not recommended test species, formulated product, laboratory method concerns as study author noted issues 

with aquaria containing residue from previous toxicity tests. 
Page 70 of 76

3 Test substance: Diquat 2 Spray. 4 Test substance was an unspecified EUP. 5 This endpoint is based on a 72-hr test. Previous EFED risk assessments indicated the LC50 was 64.6 mg/L; this value is correct 

for mg a.i./L. The LC50 when calculated in terms of cation is 33 mg cation/L. 6 Product tested was described as a ‘Liquid Concentrate’ in summary tables and as Reglone (an EUP) in some raw data sheets. 

Table A-2. Chronic Toxicity of Diquat Cation to Freshwater Fish

Species % Diquat 

Cation

NOAEC / 

LOAEC

(ppm diquat 

cation)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Fathead minnow 

(Pimephales promelas) 21.1 0.122 / 0.316 Wet weight 40380703

Surprenant, 1987 Acceptable

1.1.2. Estuarine/Marine Fish Toxicity

Table A-3. Acute Toxicity of Diquat Cation to Estuarine/Marine Fish

Species % Diquat 

Cation

96-hour LC50

(ppm diquat 

cation)

Toxicity Category MRID No.

Author, Year

Study 

Classification

Longnose killifish (Fundulus 

similis) 20 >20(48-hr) Slightly toxic 40228401

Mayer, 1986 Supplemental

Sheepshead minnow 

(Cyprinodon variegatus) 21.1 51.1 1 Slightly toxic 40316101

Nicholson, 1987 Acceptable

1 Slope = 2.14

Table A-4. Chronic Toxicity of Diquat Cation to Estuarine/Marine Fish

Species % Diquat 

Cation

NOAEC / 

LOAEC

(ppm diquat 

cation)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Sheepshead minnow 21.5 3.0 / 7.7 Length and wet 

weight

49095901

Minderhout et al., 2012 Acceptable

1.1.3. Amphibian Toxicity

Table A-5. Toxicity of Diquat Cation to Amphibians

Species % Diquat 

Cation Toxicity Description MRID No.Author, Year

Study 

Classification

African clawed frog

(Xenopus laevis) TGAI

Mortality at Day 3:

90% at 1.5 and 1.25 ppm diquat; 50% 

at 1.0 ppm; 20% at 0.5 ppm

Inhibition in growth and 

pigmentation for survivors starting at 

Day 3

Distortion in body shape observed at 

1.5 ppm

05005290

 Anderson et al., 1976 Supplemental
Page 71 of 76

1.2. Aquatic Invertebrate Toxicity

1.2.1. Freshwater Invertebrate Toxicity

Table A-6. Acute Toxicity of Diquat Cation to Freshwater Invertebrates

Species % Diquat 

Cation

48-hour LC50

(ppm diquat 

cation)

Toxicity Category MRID No.

Author, Year

Study 

Classification

Apple snail

(Family: Ampullariidae) 18 5 1.1(96-hr) Moderately toxic 40098001Mayer and Ellersick, 1986 Supplemental

Gammarus fasciatus 18 5 >54

(24-hr) Slightly toxic 40098001Mayer and Ellersick, 1986 Supplemental

Freshwater amphipod 

(Hyalella azteca) 23.6 0.09 (96-hr) Very highly toxic 48826701Bender, 2006 Acceptable

Freshwater amphipod 

(Hyalella azteca) Unknown 1 0.14 Highly toxic 00115862Nicholson et al., 1974 Supplemental

Water flea

(Daphnia magna) 17.9 2 0.77 Highly toxic 00115577Wheeler et al., 1978 Supplemental

Water flea 46.6 1.17 3 Moderately toxic 00115576

Wheeler et al., 1978 Acceptable

Water flea TGAI 7.1

(26-hr) 4 Moderately toxic 05001465Crosby et al., 1966 Supplemental

Bolded values indicate endpoints used for risk estimation. 1 Test substance: Ortho EUP

2 Test substance: Diquat Water Weed Killer 3 Slope = 5.7

4 1 ppm of non-ionic surfactant Tween 20 added to test system 5 Product tested was described as a ‘Liquid Concentrate’ in summary tables and as Reglone (an EUP) in some raw data sheets. 

Table A-7. Chronic Toxicity of Diquat Cation to Freshwater Invertebrates 

Species % Diquat 

Cation

NOAEC / 

LOAEC

(ppm diquat 

cation)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Great Pond Snail

(Lymnaea stagnalis) 16.7 0.0011 /0.0032

Embryonic stage 

duration, Age at first 

spawning, Food 

consumption

49613201 (E168652)

Ducrot et al., 2010 Quantitative

Water flea 21.1 0.036 / 0.057 Survival 40380702

Surprenant, 1987 Acceptable

1.2.2. Estuarine/Marine Invertebrate Toxicity

Table A-8. Acute Toxicity of Diquat Cation to Estuarine/Marine Invertebrates

Species % Diquat 

Cation

96-hour LC50

(ppm diquat 

cation)

Toxicity Category MRID No.

Author, Year

Study 

Classification

Eastern Oyster 

(Crassostrea virginica) 21.1 >54.9 1 Slightly toxic 40316001Dionne, 1987 Acceptable

Eastern Oyster 21.1 >0.50

(48-hr) Highly toxic 40228401Mayer, 1986 Supplemental

Mysid Shrimp 

(Americamysis bahia) 21.1 0.42 2 Highly toxic 40315701Hoberg, 1987 Acceptable

Pink Shrimp 

(Penaeus duorarum) 21.1 >20(48-hr) Slightly toxic 40228401Mayer, 1986 Supplemental

1 28% reduction in shell growth at 54.9 ppm diquat cation

2 Slope = 5.4
Page 72 of 76

Table A-9. Chronic Toxicity of Diquat Cation to Estuarine/Marine Invertebrates 

Species % Diquat 

Cation

NOAEC / 

LOAEC

(ppm Cation)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Mysid Shrimp 21.5 0.052 / 0.104 Female dry weight 49095902

Claude et al., 2013 Acceptable

1.2.3. Benthic Invertebrate Toxicity

Table A-10. Toxicity of Diquat Cation to Benthic Invertebrates 

Species % Diquat 

Cation

NOAEC / 

LOAEC

(ppm diquat 

cation)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Freshwater amphipod 

(Hyalella azteca) 21.5

Sediment: 

11 / 23 mg/kg

Pore water:

0.043 / 0.037 

mg/L 1

Reproduction

(42-day test)

49261901

Bradley, 2013 Acceptable

Freshwater amphipod 

(Hyalella azteca) 21.5 Sediment: 

47 / >47 mg/kg

None

(28-day test) 2

48784016

Bender, 2006 Supplemental

Midge

(Chironomus dilutus) 21.5

Sediment: 

37 / >37 mg/kg

Pore water:

0.54 / >0.54 

mg/L

None

(63-day test)

49261902 

Bradley, 2013 Acceptable

Midge

(Chironomus dilutus) 22.4

Sediment: 

100 / >100

mg/kg

None

(20-day test) 3

48784015

Ashwell, 1999 Supplemental

Saltwater amphipod 

(Leptocheirus plumulosus) 22.2

Sediment:

110/>110 

mg/kg

None 

(10-day test) 4

49584101

Bradley, 2015 Acceptable

1 Due to the relatively high 0-hour recovery of pore water concentration at the 11 mg diquat cation/kg level, the TWA calculated 

concentration at this level exhibited a shift in order, i.e., was higher than the TWA-calculated pore water concentration for the 

higher nominal level, 23 mg diquat cation/kg.

2 Reproduction not assessed

3 OECD 218 method, endpoint measured: emergence

4 Endpoint measured: survival only. Pore water concentrations were below the LOQ (0.0048 mg diquat cation/L on Day 0 

and 0.0043 mg diquat cation/L on Day 10).

1.3. Aquatic Plant Toxicity

Table A-11. Toxicity of Diquat Cation to Aquatic Plants 

Species

% 

Diquat 

Cation

EC50

(ppm diquat 

cation)

EC05 or NOAEC

(ppm diquat 

cation)

MRID No.

Author, Year

Study 

Classification

Duckweed 

(Lemna gibba) 22.4 0.0047

(14-d) 0.0012

48784007

Magor and 

Shillabeer, 2001

Acceptable

Green algae 

(Pseudokirchneriella 

subcapitata)

26.8 0.0094

(96-hr) 0.0068

43532703

Smyth and Tapp, 

1988

Acceptable

Green algae

(Pseudokirchneriella 

subcapitata)

20.5 >0.30 0.30 48784011

Shillabeer, 1998 Supplemental 1
Page 73 of 76

Species

% 

Diquat 

Cation

EC50

(ppm diquat 

cation)

EC05 or NOAEC

(ppm diquat 

cation)

MRID No.

Author, Year

Study 

Classification

Blue-green Algae 

(Anabaena flos-aquae) 20.5 0.0087(120-hr) 0.0013 48784010 Kent, 1998 Acceptable

Freshwater Diatom 

(Navicula pelliculosa) 20.5 0.00070(120-hr) 0.00033 48784008Kent, 1998 Supplemental

Marine Diatom 

(Skeletonema costatum) 20.5 0.011(120-hr) 0.00057 48784009 Kent, 1998 Supplemental

Marine diatom

(Isochryris galbana) 20 7.6(240-hr) No data 40228401Mayer, 1986 Supplemental

Marine diatom

(Phaeodactylum 

tricornutum)

20 7.6

(240-hr) No data 40228401

Mayer, 1986 Supplemental

Marine algae

(Dunaliella tertiolecta) 20 16(240-hr) No data 40228401Mayer, 1986 Supplemental

Green algae

(Chlorococcum sp.) 20 102(240-hr) No data 40228401Mayer, 1986 Supplemental

Giant duckweed

(Spirodela punctata) 35.3 0.00075 3 No data 41883002Bellet, 1990 Supplemental 2

Water hyacinth

(Eichhornia crassipes) 35.3 0.014 4 No data 41883002Bellet, 1990 Supplemental 2

Water lettuce

(Pistia stratiotes) 35.3 0.0158 5 No data 41883002Bellet, 1990 Supplemental 2

Watershield

(Brasenia schreberi) 35.3 No data 6 No data 41883002Bellet, 1990 Supplemental 2

Torpedograss

(Panicum repens) 35.3 3.513 7 No data 41883002Bellet, 1990 Supplemental 2

Bahiagrass

(Paspalum notatum) 35.3 No data 8 No data 41883002Bellet, 1990 Supplemental 2

Hydrilla

(Hydrilla verticillata) 35.3 0.0099 No data 41883002Bellet, 1990 Supplemental 2

Azolla

(Azolla caroliniana) 35.3 0.0116 9 No data 41883002Bellet, 1990 Supplemental 2

Bolded values indicate endpoints used for risk estimation. 1 Study conducted with sediment present in the test vessels 2 Tier III outdoor study

3 Giant duckweed EC50 with foliar application: 0.0036 lb cation/A 4 Water hyacinth EC50 with foliar application: 0.0198 lb cation/A 5 Water lettuce EC50 with foliar application: 0.1067 lb cation/A 6 Watershield EC50 with foliar application: 3.5900 lb cation/A 7 Torpedograss EC50 with foliar application: 3.6391 lb cation/A 8 Bahiagrass EC50 with foliar application: 14.409 lb cation/A 9 Azolla EC50 with foliar application: 0.0277 lb cation/A

2. Toxicity to Terrestrial Organisms

2.1. Avian Toxicity 

Table A-12. Acute Oral Toxicity of Diquat Cation to Birds
Page 74 of 76

Species

% 

Diquat 

Cation

LD50

(mg diquat 

cation/kg-bw)

Toxicity 

Category

MRID

Author, Year

Study

Classification

Mallard duck 

(Anas platyrhynchus) 45.6 60.6 1 Moderately toxic 00106559

Fink et al., 1982 Acceptable

Mallard duck 30 90.7 Moderately toxic 00160000

Tucker et al., 1970 Supplemental 2

Zebra finch 

(Taeniopygia guttata) 21.5 28.9 3 Highly toxic

49095903

Hubbard and 

Beavers, 2013

Supplemental

1 Slope = 4.2 2 Test substance: formulated product. Only male birds tested.

3 Slope = 4.82

Table A-13. Sub-acute Dietary Toxicity of Diquat Cation to Birds

Species

% 

Diquat 

Cation

LC50

(mg diquat 

cation/kg-diet)

Toxicity 

Category

MRID

Author, Year

Study

Classification

Japanese quail

(Coturnix coturnix 

japonica)

37 264 1,5 Highly toxic 00022923, 00008595

Heath et al., 1972 Supplemental

Mallard duck 37 >981 5 Moderately toxic 00022923, 00008595

Heath et al., 1972 Supplemental

Northern bobwhite quail

(Colinus virginianus) 18.9 540.4 2 Moderately toxic 00116565

Fink et al., 1982 Supplemental

Northern bobwhite quail 37 575 3,5 Moderately toxic 00022923, 00008595

Heath et al., 1972 Supplemental

Ring-necked Pheasant

(Phasianus colchicus) 37 734 4,5 Moderately toxic 00022923, 00008595

Heath et al., 1972 Supplemental

1 Slope = 4.8 2 Egg production was significantly affected from 215 to 1000 ppm cation.

3 Slope = 7.6

4 Slope = 7.5

5 Calculated in units of diquat cation based studies conducted with a 37% diquat dibromide formulation.

Table A-14. Chronic Toxicity of Diquat Cation to Birds 

Species

% 

Diquat 

Cation

NOAEC / LOAEC

(mg diquat 

cation/kg-diet)

Most Sensitive 

Endpoint

MRID No.

Author, Year

Study 

Classification

Mallard duck 19.6 4.2 / 24 Eggs laid 1 00114230

Fink et al., 1982 Acceptable

Mallard duck 22.7 19.2 / 36.9

Eggs laid and 

female body 

weight gain 2

48784012

Temple et al., 2004 Supplemental

Mallard duck 22.7 33.8 / 74.5 Eggs laid 3 48784013

Temple et al., 2004 Supplemental

Mallard duck 22.0

<71.8 / 71.8 

(treatment phase)

152 / >152 (posttreatment phase)

Eggs laid, 

hatchling weight, 

and mean food 

consumption 4

48784014

Temple et al., 2009 Supplemental

Northern bobwhite quail 19.6 100 / >100 None 00119988

Fink et al., 1982 Supplemental

1 At 24 mg cation/kg-diet, a marked reduction was observed in the numbers of eggs laid (376 total = 31 per pen), normal 

hatchlings (153 total = 13 per pen), and 14-day old survivors (151 total = 13 per pen) when compared to the control group (498

total = 42 per pen, 261 total = 22 per pen, and 256 total = 21 per pen, respectively). These differences, although not statisticallysignificant, were considered treatment-related and biologically meaningful. Statistical power was limited due to the use of only 

12 pairs of birds per treatment group. At 98 mg cation/kg-diet, a statistically-significant reduction was observed in the numbers 
Page 75 of 76

of eggs laid, eggs set, ratio of eggs set to eggs laid, viable embryos, live embryos, number of normal hatchlings, and 14-day old 

survivors when compared to the control group. 

2 Study with 9 weeks of exposure beginning at photostimulation to induce egg production. Egg production and egg weight were 

the only reproductive parameters measured.

3 Study with 6 weeks of exposure beginning 5 weeks into the egg laying period. Egg production and egg weight were the only 

reproductive parameters measured.

4 Study with 6 weeks of exposure beginning at start of egg laying period (treatment phase) and a 6-week recovery period 

immediately following the exposure period (post-treatment phase). 

2.2. Mammalian Toxicity 

Table A-15. Acute Oral Toxicity of Diquat Cation to Mammals

Species

% 

Diquat 

Cation

LD50

(mg diquat 

cation/kg-bw)

Toxicity 

Category

MRID

Author, Year

Study 

Classification

Laboratory rat 

(Rattus norvegicus) 19.3 120 Moderately toxic 00081506Rittenhouse, 1979 Acceptable

Table A-16. Acute Inhalation Toxicity of Diquat Cation to Mammals

Species

% 

Diquat 

Cation

4-hr LC50

(mg diquat 

cation/L)

Toxicity 

Category

MRID

Author, Year

Study 

Classification

Laboratory rat 19.5 0.19 Toxicity category 

III

00162046

Bruce, 1985 Acceptable

Table A-17. Acute Oral Toxicity of Diquat Cation to Mammals

Species

% 

Diquat 

Cation

NOAEC / LOAEC

(mg diquat 

cation/kg-bw)

Most Sensitive 

Endpoint

MRID

Author, Year

Study 

Classification

Laboratory rat 21.1 4 / 12 1

F1 pups/litter; F1 

body weight gain 

during lactation

41531301

Hodge, 1990 Acceptable

1 LOAEC / NOAEC = 80 / 240 mg/kg-diet

2.3. Terrestrial Invertebrate Toxicity

Table A-18. Acute Toxicity of Diquat Cation to Honey Bees

Species

% 

Diquat 

Cation

48-hr LD50

(μg diquat 

cation/bee)

Toxicity 

Category

MRID

Author, Year

Study 

Classification

Honey bee

(Apis mellifera) TGAI >100 (contact) Practically nontoxic

00131562

Atkins, 1983 Acceptable

Honey bee 16.7 47 (oral) Practically nontoxic

40208001

Gough, 1987 Acceptable

2.4. Terrestrial Plant Toxicity

Table A-19. Tier I Seedling Emergence Toxicity of Diquat Cation

Type of 

Study 

Species

Crop

IC25

(lbs diquat 

cation/A)

NOAEC

or IC05

(lbs diquat 

cation/A)

Most Sensitive 

Endpoint 

MRID

Author, Year

Study 

Classification

Monocots Barnyard grass >4.65 4.65 None 40165101

Corn >4.65 4.65 None Shilling, 1987
Page 76 of 76

Onion >4.65 4.65 None Acceptable

Rye >4.65 4.65 None

Dicots

Jimson weed >4.65 4.65 None

Morning glory >4.65 4.65 None

Potato >4.65 4.65 None

Sicklepod >4.65 4.65 None

Soybean >4.65 4.65 None

Sunflower >4.65 4.65 None

Table A-20. Tier II Vegetative Vigor Toxicity of Diquat Cation

Type of 

Study 

Species

Crop

IC25

(lbs diquat 

cation./A)

NOAEC

or IC05

(lbs diquat 

cation/A)

Most Sensitive 

Endpoint 

MRID

Author, Year

Study 

Classification

% diquat cation

Monocots

Corn 0.059 0.027 Dry weight

49141801

Martin, 2013

Supplemental

21.5%

Oat >0.24 0.24 None

Onion 0.12 0.075 Dry weight

Ryegrass 0.15 0.075 Dry weight

Dicots

Cabbage 0.0022 0.00091 Dry weight

Carrot 0.013 0.00091 Dry weight

Oilseed Rape 0.0075 0.0027 Dry weight

Soybean 0.010 0.0027 Dry weight

Sugarbeet 0.0042 0.00091 Dry weight

Sunflower 0.0042 0.0027 Dry weight

Monocots Corn 0.011 0.037 Height

41883001

Bellet, 1990

Supplemental

35.3%

Onion 0.18 0.037 Height

Dicots

Sunflower 0.049 0.020 Height

Cotton 0.0047 0.020 Height

Yellow 

nutsedge 0.14 0.037 Height

Soybean 0.0074 0.020 Dry weight

Monocots Sweet corn 0.023 0.017 Dry weight 44755601

Porch and 

Krueger, 1999

Supplemental

35.9%

Dicots Wheat 0.091 0.070 Dry weight

Bolded values indicate the most sensitive endpoints.