UNITED STA TES ENVIRONMENT AL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

PC Code: 120301 

DP Barcode: 424977 

MEMORANDUM June 16, 2015 

Subject: 

To: 

From: 

Through: 

Registration Review: A Joint Problem Formulation and Draft Risk Assessment of 

the Environmental Fate and Ecological Risk of Thidiazuron 

Khue Nguyen, Chemical Review Manager 

Neil Anderson, Branch Chief 

Risk Management and Implementation Branch l 

Pesticide Re-Evaluation Division (7508P) ~ ~ /; t 1..1 

Stephen carey, Biologist . ../~~ ~ I' ICU) i .. 

William P. Eckel, Senior Science Advisor 1:0// // 1 f 

Environmental 

Environmental Fate 

Risk 

and 

Branch 

Effect#J_sivisio 

6 

(7507P) 

~ 

· · / J L ( 

I ~ G,-l b-\:S 0 / 0 Mark Corbin, Branch Chief (_;..-/ 

MonicaWait, RAPL ~ \J~ ~/ h/15 

Amy Blankinship, Senior Scientist 

Environmental Risk Branch 6 

Environmental Fate and Effects Division (7507P) 

The Environmental Fate and Effects Division (EFED) has completed the joint problem formulation 

(PF) I draft risk assessment (ORA) for the environmental fate and ecological risk, and endangered 

species, for the registration review of the cotton defoliant thidiazuron (PC Code 120301 ). 

Thidiazuron is registered for use as a defoliant and harvest aid for use on cotton. The use pattern 

for thidiazuron has not changed since the Reregistration Eligibility Decision (RED) in 2005, thus 

a full risk assessment is not warranted. This assessment will mostly be abbreviated with references 

to past risk assessments, mainly the RED. This document will use current standard models to 

update the potential exposures last evaluated in the RED. If any exposure is higher than assessed 

in the RED or the route was not evaluated previously, it will be assessed here. Potential exposures 

to previously un-evaluated aquatic and terrestrial organisms will also be assessed in this document 

because toxicity data has become available. Those exposures and taxa previously assessed and 

determined not to be a concern will be discussed briefly here with the details available in EFED's 

2005 risk assessment (EFED RED Chapter; D3 l 9339; 9/15/2005). 

Page I of 45 
Page 2 of 45 

Risk Conclusions Summary 

The risks of thidiazuron to terrestrial and aquatic animals including plants are summarized below. 

As a result of the joint PF/DRA review for Registration Review, EFED determined that thidiazuron 

exposures to aquatic organisms and vascular/nonvascular plants continue to be unlikely as the new 

aquatic EECs (Estimated Environmental Concentrations) are similar to previous calculated EECs. 

Hence, discussions of risks to aquatic organisms, with the exception of chronic risk to 

estuarine/marine species, have not been updated in this risk assessment but rather are summarized 

from the RED and below. Details of the aquatic risk conclusions can be found in the 2005 RED. 

The chronic risk of thidiazuron in an estuary environment was previously not assessed and 

therefore, is assessed in this document. For thidiazuron exposures to terrestrial animals, the chronic 

risk conclusions have changed significantly, due to submission of new data and/or new EFED 

models, resulting in removal of the reproductive concerns for birds and mammals. Terrestrial 

plants, specifically dicots, remain a concern. Risks to the previously un-assessed taxa of terrestrialphase amphibians and reptiles and terrestrial invertebrates are available in this document. The risks 

to each taxon are summarized below: 

 Fish and Aquatic Invertebrates: There are no acute or chronic risks to freshwater and 

estuarine/marine fish and invertebrates. No LOC exceedances were observed in any of the 

aquatic organisms. See section 6.4 for details.

 Aquatic Plants: There are no risks to aquatic vascular and nonvascular plants. No listed 

or non-listed plant LOC exceedances were observed in any of the plants. See section 6.4 

for details.

 Birds: There are no acute or chronic risks to birds. Avian acute oral and dietary studies 

resulted in endpoints that were “greater than” the highest test doses. Chronic risks were 

presumed in the RED due to lack of avian reproduction data. However, bird reproduction 

data are now available and have been reviewed by EPA (MRIDs 48098704 and 48098705), 

and the risk assessment indicates no anticipated reproductive risks in birds. Highest chronic 

RQ was 0.23, which is below the chronic LOC of 1.0. See section 6.4 for details. 

Thidiazuron does not cause adverse effects to birds via the oral and dietary routes; however, 

the Screening Imbibition Program (SIP) v.1.0 indicates drinking water exposure alone is 

not a concern on an acute exposure basis; but is a potential pathway of concern for birds 

foraging in thidiazuron-treated cotton fields on a chronic exposure basis (Appendix IV). 

This pathway will be explored further with the development of SIP v.2.0 in a future 

Ecological Risk Assessment for thidiazuron. 

 Terrestrial-phase Amphibians and Reptiles: Terrestrial-phase amphibians and reptiles 

were not assessed previously. EFED currently uses surrogate data (birds) for terrestrialphase amphibians and reptiles when taxa-specific data are not available. There were no 

LOC exceedances for birds via the oral and dietary routes; these findings also apply to 

terrestrial-phase amphibians and reptiles. However, SIP indicates a drinking water concern 
Page 3 of 45 

for birds on a chronic exposure basis, and results of the future SIP v.2.0 runs apply to 

terrestrial-phase amphibians and reptiles. See section 6.4 for details.

 Mammals: There are no acute or chronic risks to mammals. Mammalian acute oral studies 

resulted in endpoints that were “greater than” the highest test doses. The RED concluded 

that there was reproductive risk to mammals, but use of the newer T-REX model indicates 

that both dose-based and dietary-based risks are below the levels of concern (RQ = 0.8, 

LOC = 1). Also in Registration Review, it was determined that exposure to drinking water 

alone and via inhalation is expected to not a concern for mammals. See section 6.4 for 

details.

 Terrestrial Invertebrates (Pollinators): Terrestrial invertebrates were previously not 

assessed quantitatively. As part of the registration review risk assessment, EFED has 

assessed this taxa and determined that there was no acute contact or acute dietary risk to 

adult honeybees. Highest acute RQ was <0.06. Chronic dietary risk to adult honeybees and 

acute and chronic dietary risk to larval honeybees is uncertain due to a lack of data. 

Because thidiazuron is used at cotton harvest, long after flowering, it is not likely to cause 

exposure of pollinators visiting cotton plants. Pollinators that are visiting off-site plants 

that are blooming at the time of the cotton harvest and are contaminated by spray drift from 

cotton fields may be exposed to thidiazuron. See section 6.4 for details. 

 Terrestrial Plants: Thidiazuron is expected to adversely affect the survival and growth 

of terrestrial plants, mainly dicots. No-effect toxicity endpoints for all monocots are 

above the application rate, indicating that monocot species on and adjacent to cotton 

fields would not be adversely affected. The highest dicot RQ = 13 for exposure to a 

combination of surface water runoff and spray drift, which significantly exceeds the plant 

LOC of 1.0. 

The LOC is also exceeded by spray drift exposure alone for the most sensitive dicot at a 

distance of more than 1000 feet from the edge of cotton fields. For the other dicots tested 

(n=9), only 1 species (other than the most sensitive species) exceeded the spray drift LOC 

up to a distance of 328 feet (based on spray drift EECs for plants inhabiting in terrestrial 

areas) beyond the edge of field. Along with monocots, the least susceptible dicots were 

not affected at the edge of the field. 

Analysis of potential effects to federally-listed endangered and threatened plant species 

will require a more in-depth evaluation to determine the potential co-occurrence of listed 

species and the areas in which cotton is grown. See section 6.4 for details. 

Environmental Fate Summary 

In soil, thidiazuron is persistent, as shown by laboratory and field half-lives and rotational crop 

intervals on the order of one year. It has intermediate mobility in soil. 
Page 4 of 45 

In surface water, photolysis is expected to be the major route of transformation; other 

degradative processes are essentially negligible by comparison. The two photoproducts are: 

(photothidiazuron, photoproduct I), a structural isomer of parent, and photoproduct II, which has 

a substantially altered chemical structure. 

Based on its solubility and vapor pressure (Henry's law), thidiazuron is non-volatile. Based on its 

relatively low n-octanol/water partitioning coefficient, thidiazuron should not bioconcentrate. 

No foliar dissipation data are currently available, and since thidiazuron is used to defoliate cotton 

prior to harvest, dislodgement from or transformation in/on leaves may be an important fate 

pathway. The default foliar dissipation half-life of 35 days was assumed. 

Data Gaps 

The ecological effects and environmental fate database is largely completed for thidiazuron except 

for the following required studies under 40 CFR Part 158: 

Environmental Fate 

 Terrestrial Field Dissipation study: storage stability portion only (835.6100) 

 Enforcement Analytical method for Water. (850.6100) 

 Enforcement Analytical Method for Soil. (850.6100) Submitted method (MRID 

41987102) not reviewed. 

Ecological Effects 

 Thidiazuron may cause exposure of pollinators visiting off-site plants that are contaminated 

by spray drift. Acceptable toxicity data to honey bee adults and larvae have not been 

submitted for thidiazuron and these data are needed to fulfill EPA’s recent policy on 

pollinator protection1

. A tier I study on the toxicity of residues on foliage (OCSPP 850.3030) 

is not needed since results of the honey bee acute contact LD50 is >100 μg a.i./bee, well 

above the 11 μg a.i./bee trigger criterion and thidiazuron is used at cotton harvest, long after 

flowering, it is not likely to cause exposure of pollinators visiting cotton plants. Thus, the 

following six Tier I, II and III honey bee studies, yet to be submitted for thidiazuron, are 

required: 

o Honeybee larval acute oral toxicity study (following OECD 237, Tier I) 

 

1

 (Guidance for Assessing Pesticide Risks to Bees, 06.23.2014; signed by Donald Brady, Director, EFED; 

http://www2.epa.gov/sites/production/files/2014-

06/documents/pollinator_risk_assessment_guidance_06_19_14.pdf) 
Page 5 of 45 

o Honeybee adult 10-day chronic oral (special study; Tier I) 

o Honeybee larval repeated dose study (special study renamed as honeybee larval 

chronic oral toxicity study, Tier I) 

o Semi-field testing for pollinators (special study, Tier II). 

o Field testing for pollinators (OCSPP 850.3040, Tier III) 

o Field trial of residues in pollen and nectar (special study, Tier III) 

Note that it is possible to conduct the larval chronic oral study in such a way as to derive both 

an LD50 (acute endpoint of interest) and a NOAEC (chronic endpoint of interest) from a 

single study. If the chronic larval toxicity study is conducted in such a way that it derives 

both an LD50 and NOAEC, then a separate acute larval toxicity study is not needed. For 

pollinator studies without finalized OECD guidelines, protocols should be submitted for EPA 

review and comment prior to study initiation. 

The need for higher Tier semi-field and full field pollinator studies for thidiazuron will be 

determined after review of these Tier I studies. 

 Acceptable acute avian acute oral toxicity (Guideline 850.2100) data have not been 

submitted for a passerine species exposed to thidiazuron, which is now required under the 40 

CFR Part 158 data requirements. Although the available bobwhite quail study indicates 

thidiazuron has low toxicity to upland game bird species, it is uncertain whether passerines 

may be more or less sensitive to this chemical as a result of their higher metabolic rate and 

differing metabolic pathways. Given the reproduction effects of number of survivors, 

hatchlings, viable embryos, and live embryos observed in birds, EFED cannot preclude the 

potential risk to passerines which are small-sized birds. An avian oral toxicity study is 

needed for a passerine species. 
Page 6 of 45 

 

A Joint Problem Formulation/Preliminary Risk 

Assessment of the Environmental Fate and Ecological 

Risk in Support of the Registration Review of 

Thidiazuron 

N-phenyl-N’-1,2,3-thiadiazol-5-ylurea

CAS Registry Number: 51707-55-2

PC Code: 120301 

Prepared by:

Stephen Carey, Biologist 

William P. Eckel, Senior Science Advisor 

 U. S. Environmental Protection Agency

 Office of Pesticide Programs

 Environmental Fate and Effects Division

 Environmental Risk Branch VI

 1200 Pennsylvania Ave., NW

 Mail Code 7507P

 Washington, DC 20460

Reviewed by:

Amy Blankinship, Senior Scientist 

Monica Wait, RAPL 

Mark Corbin, ERB6 Chief 

June 16, 2015 

	 	

Office of Chemical Safety 

and Pollution Prevention 
Page 7 of 45 

Contents

1. Introduction ............................................................................................................................. 8

2. Conclusions ............................................................................................................................. 8

2.1. Ecological Risk Assessments .......................................................................................... 8

2.3. Clean Water Act Programs ............................................................................................. 12

3. Environmental Fate and Transport ........................................................................................... 12

3.1. Transformation Products ................................................................................................ 14

4. Receptors............................................................................................................................... 14

4.1. Effects to Aquatic Organisms ........................................................................................ 15

4.2. Effects to Terrestrial Organisms .................................................................................... 16

4.3. Ecological Incidents ....................................................................................................... 17

5. Exposure Pathways of Concern ............................................................................................ 17

6. Analysis Plan ........................................................................................................................... 18

6.1. Stressors of Concern....................................................................................................... 18

6.2. Measures of Exposure ................................................................................................... 18

6.3. Measures of Effect ......................................................................................................... 22

6.4 Risk Assessment ............................................................................................................. 22

6.5. Endangered Species Assessments .................................................................................. 28

6.6. Endocrine Disruptor Screening Program ....................................................................... 29

7. Environmental Fate and Ecological Effects Data ................................................................. 30

7.1. Environmental Fate ............................................................................................................ 30

7.2. Effects ................................................................................................................................ 31

8. References ................................................................................................................................ 37

 
Page 8 of 45 

1. Introduction 

EFED evaluated the most recent ecological risk assessments for thidiazuron in association with 

the updated toxicity, exposure, and usage information to determine if sufficient data are available 

and if further updates are needed to support registration review. With sufficient data available 

and no new actions since the RED issued in 2005, EFED proceeded with an abbreviated/ 

summarized risk assessment using the latest EPA science policies and risk assessment 

methodologies for those exposure routes or/and taxa not previously assessed in the most recent 

risk assessment for thidiazuron -- ‘Environmental Fate and Effects Division Revised Risk 

Assessment for the Reregistration Eligibility Decision of Thidiazuron for Use on Cotton’ 

document (September 15, 2005; D319339). Those exposures and taxa previously assessed and 

determined not to be of concern will be discussed briefly here with details available in the 

September 15, 2005 document. EFED will use application scenarios that result in upper bound 

exposure scenarios for the sole registered use of thidiazuron, cotton, for the risk assessment. 

Thidiazuron is a plant growth regulator used as a defoliant to aid in the harvest of cotton. It is a 

phenylurea herbicide and a member of the morpholine and urea chemical families. Its mode of 

action is to inhibit regrowth of the plant through photosynthetic electron transport inhibition at 

the photosystem II receptor causing the leaf to fall off. It has two functional groups, the phenyl 

and thiadiazole ring moieties, and replacement or modification of either of these groups will 

result in the reduction of activity. 2

Thidiazuron is currently registered for use on only cotton. Thidiazuron is applied during the 

cotton harvesting period, which is typically in late August and early September. The BEAD 

chemical profile (dated January 26, 2015) reported that during the 2008-2012 time frame, an 

average of approximately 250,000 pounds of active ingredient per acre (lb a.i./A) of thidiazuron 

were applied annually to 3.8 million acres at an overall average application rate of 0.07 lb a.i./A. 

The top states, in terms of the percentage of the total pounds a.i. applied, were Georgia, Texas, 

Mississippi, and Arkansas. The average percent crop treated (PCT) for use of thidiazuron on 

cotton during 2004-2012 was 30%. According to 2008-2012 usage data, the highest usage of 

thidiazuron was observed in Georgia and Texas, and these two states account for 46% of the 

cotton acreage treated with thidiazuron. 

Thidiazuron is formulated in 18 active end-use products as a wettable powder in water soluble 

bags, or as suspension, flowable, and emulsifiable concentrates. Applications can be made by 

either aerial or ground equipment. Thidiazuron products are not to be applied using any type of 

irrigation system. 

2. Conclusions 

2.1. Ecological Risk Assessments 

The Agency has conducted multiple ecological risk assessments on thidiazuron for use on cotton 

that serve as a basis for this joint problem formulation and preliminary risk assessment 

 

2

 Murthy, B.N.S., Murch, S.J., Saxena, P.K, In Vitro Cell. Dev. Biol. Plant, 1998, 34, 267. 
Page 9 of 45 

document. The most recent risk assessment for thidiazuron is the ‘Environmental Fate and 

Effects Division Revised Risk Assessment for the Reregistration Eligibility Decision of 

Thidiazuron for Use on Cotton’ document (Dated September 15, 2005; D 319339). The major 

ecological conclusions presented in the previous risk assessment for cotton use were as follows: 

Summary of Risk Concerns Identified for Thidiazuron in Previous Assessments

Uses Taxa with 

Acute LOC 

Exceedances 

Taxa with 

Chronic 

LOC 

Exceedances 

Taxa with 

No LOC 

Exceedances 

BCF Persistence Degradates of 

Concern 

Cotton Dicots Birds and 

mammals1

Terrestrial 

invertebrates2, 

monocots, 

aquatic 

organisms 

No Yes Photothidiazuron 

(photoproduct I) 

treated as parent 

1 Chronic LOC exceeded for mammals based on dietary-based effects; dose-based effects were not assessed 

previously. Chronic risk presumed for birds due to lack of data. 

2 Based on acute contact and oral toxicity with adult bees; were not assessed quantitatively.

As a result of the joint PF/DRA review for Registration Review, EFED determined that thidiazuron 

exposures to aquatic organisms and vascular/nonvascular plants continue to be minimal as the new 

aquatic EECs (Estimated Environmental Concentrations) are similar to previous calculated EECs. 

Hence, discussions of risks to aquatic organisms, with the exception of chronic risk to 

estuarine/marine species, have not been updated in this risk assessment but rather are summarized 

from the RED. Details of the aquatic risk conclusions can be found in the 2005 RED. The chronic 

risk of thidiazuron in an estuary environment was previously not assessed and therefore, is assessed 

in this document. For thidiazuron exposures to terrestrial animals, the chronic risk conclusions 

have changed significantly, due to submission of new data and/or new EFED models, resulting in 

removal of the reproductive concerns for birds and mammals. Terrestrial plants, specifically dicots, 

remain a concern. Risks to the previously un-assessed taxa of terrestrial-phase amphibians and 

reptiles and terrestrial invertebrates are available in this document. The risks to each taxon are 

summarized below: 

 Fish and Aquatic Invertebrates: There are no acute or chronic risks to freshwater and 

estuarine/marine fish and invertebrates. No LOC exceedances were observed in any of the 

aquatic organisms. See section 6.4 for details.

 Aquatic Plants: There are no risks to aquatic vascular and nonvascular plants. No listed 

or non-listed plant LOC exceedances were observed in any of the plants. See section 6.4 

for details.

 Birds: There are no acute or chronic risks to birds. Avian acute oral and dietary studies 

resulted in endpoints that were “greater than” the highest test doses. Chronic risks were 

presumed in the RED due to lack of avian reproduction data. However, bird reproduction 

data are now available and have been reviewed by EPA (MRIDs 48098704 and 48098705), 
Page 10 of 45 

and the risk assessment indicates no adverse reproductive effects in birds. Highest chronic 

RQ was 0.23, which is below the chronic LOC of 1.0. See section 6.4 for details. 

Thidiazuron does not cause adverse effects to birds via the oral and dietary routes; however, 

the Screening Imbibition Program (SIP) v.1.0 indicates drinking water exposure alone is 

not a concern on an acute exposure basis; but is a concern for birds foraging in thidiazurontreated cotton fields on a chronic exposure basis (Appendix III). This pathway will be 

explored further with the development of SIP v.2.0 in a future Ecological Risk Assessment 

for thidiazuron. 

 Terrestrial-phase Amphibians and Reptiles: Terrestrial-phase amphibians and reptiles 

were not assessed previously. EFED currently uses surrogate data (birds) for terrestrialphase amphibians and reptiles when taxa-specific data are not available. There were no 

LOC exceedances for birds via the oral and dietary routes; these findings also apply to 

terrestrial-phase amphibians and reptiles. However, SIP indicates a drinking water concern 

for birds on a chronic exposure basis, and results of the future SIP v.2.0 runs apply to 

terrestrial-phase amphibians and reptiles. See section 6.4 for details.

 Mammals: There are no acute or chronic risks to mammals. Mammalian acute oral studies 

resulted in endpoints that were “greater than” the highest test doses. The RED concluded 

that there was reproductive risk to mammals, but use of the newer T-REX model indicates 

that both dose-based and dietary-based risks are below the levels of concern (RQ = 0.8, 

LOC = 1). The potential risk to mammals is expected to be minimal. Also in Registration 

Review, it was determined that exposure to drinking water alone and via inhalation is 

expected to not a concern for mammals. See section 6.4 for details.

 Terrestrial Invertebrates (Pollinators): Terrestrial invertebrates were previously not 

assessed quantitatively. As part of the registration review risk assessment, EFED have 

assess this taxa and determined that there was no acute contact or acute dietary risk to adult 

honeybees. Highest acute RQ was <0.06. Chronic dietary risk to adult honeybees and acute 

and chronic dietary risk to larval honeybees is uncertain due to a lack of data. Because 

thidiazuron is used at cotton harvest, long after flowering, it is not likely to cause exposure 

of pollinators visiting cotton plants. Pollinators that are visiting off-site plants that are 

blooming at the time of the cotton harvest and are contaminated by spray drift from cotton 

fields may be exposed to thidiazuron. See section 6.4 for details. 

 Terrestrial Plants: Thidiazuron is expected to adversely affect the survival and growth 

of terrestrial plants, mainly dicots. No-effect toxicity endpoints for all monocots are 

above the application rate, indicating that monocot species on and adjacent to cotton 

fields would not be adversely affected. The highest dicot RQ = 13 for exposure to a 

combination of surface water runoff and spray drift, which significantly exceeds the plant 

LOC of 1.0. 

The LOC is also exceeded by spray drift exposure alone for the most sensitive dicot at a 
Page 11 of 45 

distance of more than 1000 feet from the edge of cotton fields. For the other dicots tested 

(n=9), only 1 species (other than the most sensitive species) exceeded the spray drift LOC 

up to a distance of 328 feet (based on spray drift EECs for plants inhabiting in terrestrial 

areas) beyond the edge of field. Along with monocots, the least susceptible dicots were 

not affected at the edge of the field. 

Analysis of potential effects to federally-listed endangered and threatened plant species 

will require a more in-depth evaluation to determine the potential co-occurrence of listed 

species and the areas in which cotton is grown. See section 6.4 for details. 

Table 1 provides an updated summary of the highest acute and chronic RQs for each taxon 

exposed to thidiazuron uses. 

Table 1. Updated Summary of Risk Quotients for Thidiazuron from Use on 

Cotton. 

Species Cotton Use RQs 

Aquatic Organisms and Plants 

Freshwater Fish (and aquatic-phase 

amphibians) 

 Highest Acute RQ: <0.01 

Highest Chronic RQ: <0.1 

Freshwater Invertebrates Highest Acute RQ: <0.01 

Highest Chronic RQ: <0.1 

Marine/estuarine Fish Highest Acute RQ: <0.01 

Highest Chronic RQ: <0.1 

Marine/estuarine Invertebrates Highest Acute RQ: <0.01 

Highest Chronic RQ: <0.1 

Aquatic Vascular Plants Highest Non-Listed RQ: <0.1 

Highest Listed RQ: <0.1 

Aquatic Non-Vascular Plants Highest Non-Listed RQ: <0.1 

 Highest Listed RQ: 0.1 

Terrestrial Animals and Plants 

Birds (and terrestrial-phase 

amphibians and reptiles) 

Highest Acute RQ: <0.01 

Highest Chronic RQ: 0.23 

Mammals Highest Acute RQ: <0.01 

Highest Chronic RQ: 0.8

Terrestrial Plants 

Highest Terrestrial Plant RQ: 13 (Dicots, 

Potential drift exceeds LOC for >1000 feet); 

<0.1 (monocots) 

Terrestrial Invertebrates (and 

pollinators) 

Highest Acute RQ: <0.06A

Highest Chronic RQ: Presumed due to no data

Bold RQs indicate LOC exceedance or concern 

A Data available for adult bees only, acute effects to bee larval are unknown. 
Page 12 of 45 

2.3. Clean Water Act Programs 

Thidiazuron is not identified as a cause of impairment for any water bodies listed as impaired 

under section 303(d) of the Clean Water Act. No Total Maximum Daily Load (TMDL) criteria 

have been developed for thidiazuron. No aquatic benchmarks have been established for 

thidiazuron and are not available at 

http://www.epa.gov/oppefed1/ecorisk_ders/aquatic_life_benchmark.htm. Any data submitted or 

otherwise located as part of the registration review process may be used to update aquatic life 

benchmarks if applicable. 

3. Environmental Fate and Transport 

In soil, thidiazuron is persistent, as shown by laboratory and field half-lives and rotational crop 

intervals on the order of one year. It has intermediate soil sorption coefficients (sorption 

coefficients ranged from 4.4 to 19 Freundlich units). 

When thidiazuron reaches surface water, photolysis is expected to be the major route of 

transformation; other degradative processes are essentially negligible by comparison. Aqueous 

photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of 

the photodegradates (photothidiazuron, photoproduct I) is a structural isomer of parent, whereas 

photoproduct II has a substantially altered chemical structure. 

Based on its solubility and vapor pressure (Henry's law), and other laboratory evidence, 

thidiazuron is non-volatile. Based on its relatively low n-octanol/water partitioning coefficient, 

thidiazuron should not bioconcentrate. 

No foliar dissipation data are currently available, and since thidiazuron is used to defoliate cotton 

prior to harvest, dislodgement from or transformation in/on leaves may be an important fate 

pathway. The default foliar dissipation half-life of 35 days was assumed. 

Thidiazuron may degrade to thiadiazolurea (also known as AE F132345 or 1,2,3-thidiazol-5-

ylurea) and unidentified simple phenyl compounds via hydrolysis of the urea bridge. These 

compounds in turn were then thought to degrade to unidentified minor compounds which are 

incorporated into the organic material and mineralized to CO2. However, there was insufficient 

data to support these contentions, and production of additional metabolites through AE F132345 

appears to be a minor route of transformation. 

The structural isomer is AE F132347 or photoproduct I (see Appendix V for chemical 

identification). It has a maximum experimental limit of production of 77% of parent equivalents 

at pH 5, 28% at pH 7, and 17% at pH 9. Photoproduct I was stable to further photolysis under 

experimental study conditions. The second photodegradate is AE C421200 or photoproduct II 

(see Appendix V for chemical identification). Photoproduct II (phenylurea) is substantially 

different from parent thidiazuron, and has a complementary asymptotic limit of production to 
Page 13 of 45 

photoproduct I of 23% of parent at pH 5, 72% at pH 7, and 83% at pH 9. This product was also 

stable to further photolysis under study conditions. Thus, at different environmental pHs, parent 

thidiazuron is always quantitatively converted in complementary proportions into the two 

photodegradates. 

A selection of thidiazuron environmental fate and transport properties is summarized in Table 2 

below. 

Table 2. Physical Chemical and Environmental Fate Properties of Thidiazuron and 

Two photoproducts. 

Parameter Value Source

Selected Physical/Chemical Parameters

Chemical Structure 

Empirical Formula C9 H8 N4 O S D294544 

IUAPC Name 1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea D294544 

CAS Name N-phenyl-N’-1,2,3-thiadiazol-5-ylurea D294544 

CAS No. 51707-55-2 (330-54-1 also found) D294544 

PC Code No. 120301 D294544 

SMILES string: O=C(Nc1ccccc1)Nc1cnns1 D294544 

Molecular Weight 220.25 g/mole Product Chemistry 

Physical state TGAI: amorphous solid; PAI: crystalline 

solid 

MRID 45934001 

Vapor pressure 3.5 x l0-9 Pa @ 20 oC MRID 94246001 

Aqueous solubility (25C) PAI: water = 31.0 mg/L (pH = 7.0) MRID 41364906 

Dissociation constants (pKa ) PAI: pKa = 8.86 25 oC MRID 41364901 

pH TGAI: 5.21 (20 0

C); PAI: 6.50 MRID 45934001 

Henry’s Law Constant 2.5 x 10-l6 atm m3/mol calculated 

Octanol-to-water partition 

coefficient at 25 o

C 

PAI: K o/w = 50.0 (25 0

C, pH = 7.3) 

shake flask method 

MRID 41364906 

Persistence

Hydrolysis half-life (pH 5, 7, 9) Stable MRID 42069203 

Aqueous photolysis half-life Stable due to formation of stable 

photodegradates 

MRID: 44537108 

 

Aerobic soil metabolism halflife (range in days) 349 days (90th%ile upper bound on mean) MRID 41950101; 46119601 

DP Barcode D289980 

Anaerobic aquatic metabolism 

half-life 

289 days MRID 42666101and 

44505132 

Aerobic aquatic metabolism 

half-life (total system) 

697 days (2 x soil metabolism) MRID: 45196011 

Mobility
Page 14 of 45 

Table 2. Physical Chemical and Environmental Fate Properties of Thidiazuron and 

Two photoproducts. 

Parameter Value Source

Adsorption/desorption Kf/ads Kf/oc/ads 1/n r2

Sand: German Std. Soil # 2.1 

4.36 908 0.721 0.997 

Loamy sand: German Std. Soil #2.2 

16.2 786 0.771 0.999 

Sandy loam: German Std. Soil #2.3 

7.33 780 0.719 0.999 

Sandy clay loam: UK, Schering #171 

18.8 494 0.817 1.000 

Koc model is appropriate (r2

 = 0.88) 

Min Kf, Kfoc (n=4): 4.36, 494 

Max Kf, Kfoc (n=4): 18.8, 908 

Median Kfd, Kfoc: 11.8, 783 

Average Kfd, Kfoc: 11.7+6.9, 742+176 

MRID 41364909 

3.1. Transformation Products 

The major degradates of thidiazuron include photoproduct I (photothidiazuron), photoproduct II 

(phenylurea), and thiazoylurea. Structures are given in Appendix V. Photoproduct I is 

considered the same as parent thidiazuron for the assessment, because of its minimal structural 

difference (rearrangement of S and N atoms in thidiazoyl ring). 

4. Receptors 

Tables 3 and 4 provide an update of the aquatic and terrestrial taxonomic groups, respectively, 

and most sensitive surrogate species tested to characterize the potential acute and chronic 

ecological effects of thidiazuron. In addition, the tables provide a preliminary overview of the 

potential acute toxicity of thidiazuron by providing the acute toxicity classifications. Based on 

the available ecotoxicity information, thidiazuron is moderately toxic to freshwater fish and 

freshwater and estuarine/marine invertebrates; slightly toxic at the solubility limit to 

estuarine/marine fish; and practically non-toxic to upland game birds, waterfowl, and mammals. 

Thidiazuron is acutely not toxic to adult bees via the contact and oral routes; however, the 

toxicity to bees from chronic exposure and to bee larval from exposure to the TGAI is unknown. 

No toxicity data for passerines are available. Avian reproduction studies have been submitted 

since the last risk assessment. The toxicity to benthic invertebrates is not available; however, the 

chemical properties indicate thidiazuron exposures to benthic invertebrates are unlikely and data 

on sediment toxicity to benthic invertebrates are not needed. Additional details are included in 

the following tables. 

Toxicity data on three degradates of thidiazuron are available. Data indicate aquatic 

photodegradate AE C421200 (photoproduct II) and metabolite AE F132345 (thiadiazolurea) are 

generally practically non-toxic to freshwater fish and invertebrates. Aquatic photodegradate AE 

F132347 (photoproduct I) is moderately toxic and more toxic than the parent compound to 
Page 15 of 45 

freshwater fish and practically nontoxic to freshwater invertebrates and is more toxic than the 

other degradates but not more toxic than the parent compound to aquatic nonvascular plants. No 

estuarine/marine toxicity data for the degradates were available. 

4.1. Effects to Aquatic Organisms 

Table 3. An Updated Summary of the Most Sensitive Endpoints from Aquatic 

Toxicity Studies for Thidiazuron and its Degradates 

Taxonomic 

Group 

Study 

Type 

Surrogate 

Species 

Toxicity Value 

(μg a.i./L) 

Acute Toxicity 

Classification 

or Adverse 

Effect 

Observed 

Source 

(Study 

classification) 

Freshwater fish1

Acute Rainbow trout LC50 = 6,700 

(AE F132347) 

Moderately 

toxic 

46203508 

(Acceptable) 

Chronic Fathead minnow NOAEC = 5,700 

Affected 

Endpoint: 

Hatchability, 

fry survival, 

total survival 

42270301 

(Acceptable) 

Freshwater 

invertebrates 

Acute 

Water flea 

EC50 = 5,700 Moderately 

toxic 

46203503 

(Acceptable) 

Chronic NOAEC <100; 

[EC10 = 720] 2

Affected 

Endpoint: 

length 

[survival] 

42132002 

(Upgraded to 

acceptable under 

43665001) 

Estuarine/marine 

fish 

Acute 

Sheepshead 

minnow 

LC50 >36,000 Slightly toxic at 

solubility limit 

41846101 

(Acceptable) 

Chronic 

Estimated 

NOAEC = 6,400 

(assuming values 

are definitive) 

-- ACR3

Estuarine/marine 

invertebrates 

Acute 

Mysid shrimp 

EC50 = 3,240 Moderately 

toxic 

41846102 

(Acceptable) 

Chronic Estimated 

NOAEC = 400 -- ACR4

Aquatic plants 

and algae 

Vascular Duckweed EC50 >24,000 

NOAEC = 8,600 -- 46203506 (Acceptable) 

Nonvascular 

Marine diatom 

(Skeletonema 

costatum) 

EC50 = 860 

NOAEC = 110 -- 46203505 (Acceptable) 

1 Freshwater fish may be surrogates for aquatic-phase amphibians. 

2

 An EC10 is used in lieu of an NOAEC (D216628). 3 The ACR (Acute-to-Chronic Ratio method) using rainbow trout acute and fathead minnow chronic data is 

5.6, assuming the values are definitive. Employing the ACR of 5.6 to the sheepshead minnow LC50 of >36 

mg a.i./L calculates a chronic toxicity value (NOAEC) of 6.4 mg a.i./L for estuarine/marine fish. 4 The ACR using water flea acute and chronic data is 7.9. Employing the ACR of 7.9 to the mysid shrimp 

LC50 of 3.2 mg a.i./L calculates a chronic toxicity value (NOAEC) of 0.4 mg a.i./L for estuarine/marine 

invertebrates. 
Page 16 of 45 

4.2. Effects to Terrestrial Organisms 

Table 4. An Updated Summary of the Most Sensitive Endpoints from Terrestrial Toxicity 

Studies for Thidiazuron 

Taxonomic 

Group 

Study Type Surrogate 

Species 

Toxicity Value Acute Toxicity 

Classification or 

Chronic Effects 

Observed 

Source 

(Study 

classification) 

Passerine Acute oral N/A No data N/A No data 

Birds1

Acute oral 

Bobwhite 

quail 

LD50 >3,160 mg a.i./kg-bw Practically nontoxic 

099819 

(Acceptable) 

Subacute Dietary LC50 >5,215 mg a.i./kg-diet Practically nontoxic 

46203502 

(Acceptable) 

Chronic Mallard 

duck 

NOAEC = 298 mg a.i./kgdiet 

LOAEC = 906 mg a.i./kgdiet 

Affected 

Endpoint: no. of 

survivors, 

hatchlings, 

viable embryos, 

and live embryos 

48098704 

(Acceptable) 

Mammals 

Acute Oral 

Laboratory 

rat 

LD50 >2000 mg a.i./kg-bw Practically nontoxic 

46121501 

(Acceptable) 

Chronic 

NOAEL = 400 mg a.i./kgdiet (35.4/39.8 mg a.i./kgbw [M/F]) 

LOAEL = 1200 mg a.i./kgdiet (108.5/121.1 mg a.i./kg 

bw [M/F]) 

Affected 

Endpoint: delay 

in sexual 

maturation and 

irregular estrous 

cycles 

(acyclicity) 

46209601 

(Acceptable) 

Insects 

Acute Contact - 

Adults 

Honey bee 

LD50 > 100 μg a.i./bee Practically nontoxic 

46203501 

Acute Oral - (Acceptable) 

Adults LD50 > 105.25 μg a.i./bee 

Acute Oral - 

Larval No data N/A No data 

Chronic Oral – 

Adults No data N/A No data 

Chronic Oral - 

Larval No data N/A No data 

Terrestrial 

plants 

Seedling 

Emergence 

Dicot 

(Lettuce) 

EC25 = 0.015 lb a.i./A 

[EC05]

2

 = 0.004 lb a.i./A -- 45908501 

(Supplemental) 

Monocot 

(All) 

EC25 >0.18 lb a.i./A 

NOAEC = 0.18 lb a.i./A -- 48098706 

(Supplemental) 

Vegetative Vigor 

Dicot 

(Lettuce) 

EC25 = 0.001 lb a.i./A 

NOAEC = 0.0002 lb a.i./A -- 45921501 

Monocot (Acceptable) 

(All) 

EC25 = >0.178 lb a.i./A 

NOAEC = 0.178 lb a.i./A -- 

1 Birds represent surrogates for terrestrial-phase amphibians and reptiles. 

2 Per EPA guidelines, the EC05 is used in lieu of the NOAEC since the NOAEC is below the lowest test concentration 

tested.
Page 17 of 45 

4.3. Ecological Incidents 

The ecological incident information system (EIIS) is an OPP database that houses ecological 

incidents that have been reported to the Agency. When available, EIIS includes date and 

location of an incident, type and magnitude of effects observed in various species, use(s) of 

pesticides known or suspected of contributing to the incident, and results of any chemical residue 

analysis or other analyses conducted during incident investigation. EIIS incidents are 

categorized according to the certainty that the incident resulted from pesticide exposure. The 

Avian Monitoring System (AIMS) is a database administered by the American Bird Conservancy 

that contains publicly available data on reported avian incidents involving pesticides. Many of 

the incidents listed in this database are also in the EIIS. 

As of April 2015, there are 4 reported incidents for thidiazuron in the EIIS database with a 

certainty index of ‘possible’ and ‘probable’. Of these four incidents, two are from a registered 

use, one misuse, and one accidental misuse (see Table 5 and Appendix I). The dates of the 

incident reports range from 1996 to 2009. The reports involve damage to lettuce via spray drift 

and to ryegrass by treating directly; however, one incident involves chickens. The chicken 

incident is also reported in the AIMS database. 

In addition to the incident reports available in EIIS, there were no aggregate ‘Other Non-Target 

incidents reported to the Agency. 

Table 5. Incident Reports from EIIS 

INCIDENT 

NUMBER YEAR CERTAINTY INDEX STATE LEGALITY USE SITE 

SPECIES 

AFFECTED 

EFFECT/ 

MAGNITUDE 

I005204-014 1996 Probable n/r Misuse 

(accident) Ag. area Chicken Mortality / 9 died 

I013645-002 2002 Probable CA Misuse n/r Lettuce Damage from drift / 

150 acres 

I016036-024 2004 Possible CA Registered Use Cotton Lettuce Damage from drift / 

53 acres 

I014341-044 2009 Possible WA Undetermined Rye Ryegrass Treated directly / 75 

acres 

n/r – Not reported 

5. Exposure Pathways of Concern 

The environmental fate and toxicity profile for thidiazuron suggests that direct spray, spray drift, 

leaching and runoff represent the likely exposure pathways for terrestrial and aquatic non-target 

organisms. Drinking water and inhalation exposure pathways were also screened using the SIP 

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

methods. 
Page 18 of 45 

6. Analysis Plan 

6.1. Stressors of Concern 

Ecological Risk Assessment 

Based on available toxicity data, it appears that the only stressor of ecological concern for 

terrestrial and aquatic organisms (except fish) is thidiazuron; however, the aquatic 

photodegradate AE F132347 (Photoproduct I) was used as the stressor for freshwater fish as it 

was more toxic than the parent compound. 

6.2. Measures of Exposure 

EFED used standard available models to evaluate potential exposures to not previously assessed 

aquatic and terrestrial organisms and/or exposure routes as described at 

http://www.epa.gov/pesticides/science/models_db.htm. 

Aquatic Exposure Assessment 

Fish, Aquatic Invertebrates, and Aquatic Plants 

EFED updated the aquatic EECs using the current surface water model, Surface Water 

Concentration Calculator (SWCC), to calculate the aquatic EECs for cotton. EECs for the 

registered use on cotton are based on 1st application at 0.21 and 2nd at 0.1 lb a.i./A, applied 5 days 

apart aerially using the CA, MS, and NC scenarios. Results are shown in Table 6 (see model 

input and output in Appendix II). GENEEC EECs in Table 6 were obtained from EFED’s 

September 15, 2005 risk assessment to compare the old and new EECs for cotton. 

As of a result, the updated aquatic EECs using the model SWCC indicate that exposures for 

cotton uses are marginally higher or similar to those previously assessed using GENEEC. 

Table 6. Comparison of Aquatic Estimated Environmental Concentrations (μg/L) for 

Thidiazuron. 

Crop 

(Model) 

Peak 

(μg/L) 

21-day Avg 

(μg/L) 

60-day Avg 

(μg/L) 

Cotton* 

(SWCC) 12.0 11.1 10.1 

Cotton 

(GENEEC) 11.0 9.7 8.6 

*: Mississippi cotton scenario gave the highest EECs compared to other modeled scenarios (i.e. California and 

North Carolina). 
Page 19 of 45 

Terrestrial Exposure Assessment 

Birds and Mammals 

EFED updated the terrestrial dose- and dietary-based EECs using the current model, Terrestrial 

Residue Exposure (T-REX), to calculate the avian/mammalian EECs for cotton. EECs for the 

registered use on cotton are based on 1st application at 0.21 and 2nd at 0.1 lb a.i./A, applied 5 days 

apart broadcast. Results of the updated dietary-based and dose-based EECs are shown in Tables 

7 and 8, respectively. ELLFATE dietary-based EECs in Table 7 were obtained from the EFED’s 

Sept. 2005 risk assessment to compare the old and new EECs for cotton. ELLFATE dose-based 

EECs were not available in the 2005 risk assessment; thus, a comparison could not be performed. 

As of a result, the updated terrestrial EECs using the model T-REX indicate that dietary-based 

exposures for cotton uses are slightly higher than or similar to those previously assessed using 

ELLFATE. 

Table 7. Comparison of Dietary-Based EECs (mg/kg diet) for Terrestrial 

Vertebrates Exposed to Thidiazuron. 

Crop 

(Model) 

Short 

Grass 

Tall 

Grass 

Broadleaf 

Plants 

Fruits/pods

/seeds 

Arthropods 

Cotton 

(TREX V.1.5.2) 69.65 31.92 39.18 4.35 27.28 

Cotton 

(ELLFATE) 62.62 30.54 37.48 4.16 N/A 

Table 8. Dose-Based EECs (mg/kg bw) for Terrestrial Vertebrates Exposed to 

Thidiazuron. 

Bodyweight 

Size 

Short 

Grass 

Tall 

Grass 

Broadleaf 

Plants 

Fruits/

pods 

Arthropods Seeds 

Avian 

20 g 79.32 36.36 44.62 4.96 31.07 1.1 

100 g 45.23 20.73 25.44 2.83 17.72 0.63 

1000 g 20.25 9.28 11.39 1.27 7.93 0.28 

Mammalian 

15 g 66.4 30.44 37.35 4.15 26.01 0.92 

35 g 45.89 21.03 25.82 2.87 17.98 0.64 

1000 g 10.64 4.88 5.99 0.67 4.17 0.15 

The Screening Tool for Inhalation Risk (STIR) was used to calculate an upper-bound estimate of 

wildlife inhalation exposure using thidiazuron’s vapor pressure and molecular weight for vapor 

phase exposure as well as the maximum application rate and method of application for spray drift. 

STIR incorporates results from several toxicity studies including acute oral and inhalation rat 
Page 20 of 45 

toxicity endpoints (rat LD50 >2000 mg/kg-bw; rat inhalation LC50 >2.4 mg/L) as well as the most 

sensitive acute oral avian toxicity endpoint (bobwhite LD50 >3160 mg/kg-bw). Based on the 

results of the STIR model (Appendix III), exposure through inhalation of spray drift or the vapor 

phase of thidiazuron is not determined to be a potential pathway of concern for either avian or 

mammalian species on an acute exposure basis. 

 

The Screening Imbibition Program (SIP) was used to calculate an upper-bound estimate of wildlife 

drinking water exposure using thidizuron’s solubility (i.e., 31 mg/L at 20oC), and compared to the 

most sensitive acute and chronic avian and mammalian toxicity endpoints. Upon results of the 

screening model SIP, drinking water exposure alone is not a concern for mammals on both acute 

and chronic basis. For birds, drinking water is not a concern on an acute exposure basis; however, 

is a concern for birds on a chronic exposure basis (Appendix IV). Thidiazuron has a low solubility, 

which is the only parameter used in estimating potential exposure, and birds and mammals are 

particularly sensitive to thidiazuron on a chronic basis. This pathway will be explored further with 

the development of SIP v.2.0 in a future Ecological Risk Assessment for thidiazuron. For a sample 

of the output generated by SIP v.1.0, please see Appendix IV. 

Terrestrial Invertebrates 

The pollinator risk assessment guidance (USEPA, 2014c) expresses estimates of contact 

exposure to honey bees from a foliar spray as the application rate for a single application (in lbs 

a.i./A) multiplied by 2.7 μg a.i./bee and estimates of dietary exposure to honey bees from a foliar 

spray as the application rate for a single application (lbs a.i./A) multiplied by 110 μg a.i./g 

(upper-bound residue for tall grass used in T-REX) and 0.292 g/d (maximum estimated daily 

dietary consumption by honey bees). These values are then compared to the acute contact and 

oral LD50 data available for the adult honey bee. Maximum EECs for adult honey bees are shown 

in Table 9. 

Table 9. Exposure Estimates to Adult Honey Bees From 

Foliar Applications of Thidiazuron 

Foliar Contact EEC (μg a.i.) Foliar Dietary EEC (μg a.i.) 

0.54 6.424 

Terrestrial Plants 

EECs for plants residing near a use area that may be exposed via runoff and/or spray drift from 

foliar applications has been updated using TERRPLANT. Only a single application of 0.21 is 

considered. A 60% application efficiency factor that previously was included in the runoff 

calculations for aerial applications is not used any longer and removed from calculations in this 

assessment. Results of the updated EECs for terrestrial and semi-aquatic plants for a single 

application of thidiazuron at 0.21 lb a.i./A are presented in Table 10. TERRPLANT EECs in Table 

10 were obtained from the EFED September 2005 risk assessment to compare the old and new 

EECs for cotton. The comparison of the old/new EECs shows identical results. 
Page 21 of 45 

To characterize potential non-target plant exposure from spray drift alone, Tier 1 modules in 

AgDRIFT were used to calculate the EECs for off-site spray drift exposure to plants inhabiting in 

terrestrial areas (Table 11) for wettable powder formulations. For aerial applications, EFED used 

the American Society of Agricultural and Biological Engineers (ASABE, formerly ASAE) Fineto-Medium droplet size category in its modeling. This drop size category is EFED’s default 

distribution when labels do not specify application parameters. For ground applications, EFED 

used the ASABE Very Fine-to-Fine droplet size category with a high boom height. Again, this 

droplet size category is EFED’s default distribution when labels do not specify the application 

parameters for ground applications. 

Table 10. Comparison of TERRPLANT EECs for Terrestrial Plants Located Adjacent 

to Thidiazuron Treated Sites. 

Use site Application 

Rate 

 TerrPlant EECs (lb a.i./A) 

Total Loading to Dry 

Areas Adjacent to 

Treated Areas1

Total Loading to SemiAquatic Areas Adjacent to 

Treated Areas2

Aerial 

Cotton 

(Updated) 0.21 0.014 0.05 

Cotton 

(2005 RA) 0.2 0.012 0.03 

Ground 

Cotton 

(Updated) 0.21 0.006 0.04 

Cotton 

(2005 RA) 0.2 0.006 0.04 

1 EEC = Sheet Runoff + Drift (1% for ground; 5% for aerial) 2

 EEC = Channelized Runoff + Drift (1% for ground; 5% for aerial 

Table 11. Maximum Drift Loadings at Specific Distances for Terrestrial Areas 

Near Thidiazuron Use Areas Using Default Conservative Scenario. 

Broadcast 

Application 

Rate 

(lb a.i./A) 

Application 

Method 

Distance from Edge 

of Field 

(ft) 

Drift EEC 

Terrestrial Areas 

(lb a.i./A) 

0.2 Aerial 

0 0.1 

250 0.0073 

500 0.0037 

997 0.0023 

0.2 Ground 

0 0.2 

250 0.002 

500 0.0008 

997 0.0003 
Page 22 of 45 

6.3. Measures of Effect 

Toxicity data presented in Section 4 of this document will be used to calculate risk quotients for 

those exposure routes to aquatic and terrestrial organisms not assessed previously. 

6.4 Risk Assessment 

The risks to aquatic and terrestrial organisms not assessed previously will be assessed in this 

section of the document. The taxa and routes not quantitatively assessed previously are chronic 

exposure to thidiazuron in estuary waters; avian and mammalian reproduction; acute and chronic 

exposures to terrestrial invertebrates (pollinators); and distance from edge of field the potential 

risk to terrestrial plants ends. 

Exposure routes or/and taxa assessed previously will not be reanalyzed in this document, but 

details are available in the 2005 risk assessment. The taxa and routes assessed previously are 

acute and chronic exposures to freshwater fish and invertebrates, acute exposure to 

estuarine/marine fish and invertebrates, exposure to aquatic nonvascular and vascular plants, 

acute exposure to birds and mammals, and exposure to terrestrial plants. 

Estuarine/marine Fish and Invertebrates 

Chronic RQs were not available in past assessments for estuarine/marine fish and invertebrates 

due to lack of data. To measure the potential risk for chronic exposure to estuarine/marine fish 

and invertebrates, the Acute-to-Chronic Ratio (ACR) method from available data was used to 

estimate the NOAECs. For fish, the ACR using rainbow trout acute and fathead minnow chronic 

data is 5.6, assuming the values are definitive. Employing the fish ACR of 5.6 to the Sheepshead 

minnow LC50 of >36 mg a.i./L calculates a chronic toxicity value (NOAEC) of >6.4 mg a.i./L for 

estuarine/marine fish. For invertebrates, the ACR using water flea acute and chronic data is 7.9. 

Employing the invertebrate ACR of 7.9 to the mysid shrimp LC50 of 3.2 mg a.i./L calculates a 

chronic toxicity value (NOAEC) of 0.4 mg a.i./L for estuarine/marine invertebrates. With a peak 

EEC of 0.012 mg a.i./L, the chronic RQ for estuarine/marine fish and invertebrates is <0.01 and 

0.03, respectively, which is below the chronic LOC of 1.0. 

Thus, thidiazuron exposures to estuarine/marine fish and invertebrates on a chronic basis are 

expected to be minimal. Together with other aquatic organisms, the potential risk for adverse 

effects on survival, reproduction and growth to fish and invertebrates that inhabit in freshwater 

and estuary waters with thidiazuron residues as a result of surface runoff and spray drift from 

adjacent treated fields is expected to be unlikely. 

Birds 

Avian reproduction studies were not previously available for assessment, but have subsequently 

been submitted to EPA. The mallard duck NOAEC of 298 mg a.i./kg-diet and T-REX EECs of 

selected feed items from broadcast sprays via ground or aerially at 0.2 lb a.i./A then again 5 days 

later at 0.1 lb. a.i./A; the chronic RQs of 0.01-0.23 for birds were below the LOC of 1 for potential 
Page 23 of 45 

risk to non-listed and listed birds consuming vegetation, insects, and seeds with thidiazuron 

residues (Table 12). 

Drinking water exposure alone according to the SIP model is a potential pathway of concern for 

birds on a chronic exposure basis. SIP is a screening (tier I) assessment used to determine whether 

there is a potential for a pathway of concern, which is intended to be evaluated in further 

developments of SIP. The following conservative assumptions to derive upper bound exposure 

estimates that triggered the potential pathway of concern for birds drinking water alone with 

thidiazuron residues were based on 1) the solubility limit of the chemical, 2) the assessed animals 

obtain 100% of their daily water needs through drinking water, 3) the daily water need is equivalent 

to the daily water flux rate, 4) the body weight of the assessed bird is equivalent to the smallest 

bird (i.e., 20 g) modeled in T-REX, which results in the highest ratio of exposure to toxicity for 

the 3 assessed avian body weights of 20, 100, and 1000 g in T-REX, and 5) the body weight of the 

assessed mammal is equivalent to the largest mammal (i.e., 1000 g) modeled in T-REX, which 

results in the highest ratio of exposure to toxicity for the 3 assessed mammalian body weights of 

15, 35, and 1000 g in T-REX. The upper bound exposure estimates will be refined further when 

SIP v.2.0 is developed that could increase or decrease the potential pathway of concern for birds 

drinking water with thidiazuron residues. 

Overall, the potential risk for adverse effects on survival, reproduction and growth to bird species 

that inhabit in treated fields foraging on vegetation and insects with thidiazuron residues is 

expected to be unlikely. There is a potential concern for birds exposed to drinking water alone on 

a chronic basis; this pathway will be explored further with the development of SIP v.2.0 in a future 

ecological risk assessment for thidiazuron. Exposed to thidiazuron via inhalation was not a concern 

in birds. 

Table 12. Chronic Avian Dietary-Based Risk Quotients 

Site Selected Feed 

Items 

NOAEC = 298 mg a.i./kg-diet 

EEC2 RQ 

Cotton1

Short grass 67.47 0.23 

Tall grass 30.93 0.10 

Broadleaf plants 37.95 0.13 

Fruits/pods/seeds 4.22 0.01 

Arthropods 26.43 0.09 

Bold values indicate terrestrial LOC exceedances 1 Exposure scenario based on 2 broadcast sprays at 0.2 lb a.i./A and 5 days later at 0.1 lb a.i./A 

2 mg a.i./kg-diet 

Terrestrial-phase Amphibians and Reptiles 

Terrestrial-phase amphibians and reptiles were not assessed previously. EFED currently uses 

surrogate data (birds) for terrestrial-phase amphibians and reptiles when data are not available. 

There were no LOC exceedances for birds; these apply to terrestrial-phase amphibian and reptiles. 

Overall, the potential risk for adverse effects on survival, reproduction and growth to terrestrial-
Page 24 of 45 

phase amphibian and reptiles that inhabit in treated fields foraging on vegetation and insects with 

thidiazuron is expected to be unlikely. There is a potential pathway of concern for birds from 

drinking water alone on a chronic exposure basis; this applies to terrestrial-phase amphibian and 

reptiles as well. 

Mammals 

EFED used the current model, T-REX, to update the dietary- and dose-based EECs to calculate 

chronic RQ values for mammals, and the results are presented in Tables 13 and 14. Based on the 

rat chronic NOAEL of 35.4 mg a.i./kg-bw/day (400 mg/kg-diet) for exposure to thidiazuron, both 

dietary and dose-based RQ values do not exceed the chronic risk LOC (LOC = 1) for all size 

classes of mammals that consume short grass, tall grass, broadleaf plants, fruits, seeds and pods, 

and arthropods. These chronic RQs of 0.01-0.8 were below the chronic LOC of 1.0. 

Overall, the potential risk for adverse effects on survival, reproduction and growth to mammals 

that inhabit in thidiazuron-treated fields is anticipated to be low. There is no concern for mammals 

exposed to drinking water alone contaminated with thidiazuron or via inhalation of the test 

material. 

Table 13. Chronic Mammalian Dietary-Based Risk Quotients 

Site Selected Feed Items NOAEC = 400 mg a.i./kg-diet 

EEC2 RQ 

Cotton1

Short grass 64.33 0.17 

Tall grass 29.49 0.08 

Broadleaf plants 36.19 0.09 

Fruits/pods 4.02 0.01 

Arthropods 25.20 0.07 

Seeds 0.89 0.17 

Bold values indicate terrestrial LOC exceedances 1 Exposure scenario based on 2 broadcast sprays at 0.2 lb a.i./A and 5 days later at 0.1 lb a.i./A 

2 mg a.i./kg-diet

Table 14. Chronic Mammalian Dose-Based Risk Quotients 

Site Selected 

Feed Items 

Adjusted NOAEC of 35.4 mg a.i./kg-bw for Size Classes 

15 g NOAEC = 77.8 35 g NOAEC = 62.95 1000 g NOAEC = 27.23

EEC2 RQ EEC RQ EEC RQ 

Cotton1

Short grass 64.33 0.83 44.46 0.71 10.31 0.38 

Tall grass 29.49 0.38 20.38 0.32 4.72 0.17 

Broadleaf 

plants 36.19 0.47 25.01 0.40 5.80 0.21 

Fruits/pods 4.02 0.05 2.78 0.04 0.64 0.02 

Arthropods 25.20 0.32 17.41 0.28 4.04 0.15 

Seeds 0.89 0.01 0.62 0.01 0.14 0.01 

Bold values indicate terrestrial LOC exceedances 
Page 25 of 45 

Beneficial Invertebrates (Pollinators) 

 

For beneficial terrestrial invertebrates, this assessment follows the guidance established in the 

recently issued Pollinator Risk Assessment Guidance (USEPA, 2014c) to evaluate acute contact 

and dietary risk to honey bees using a Tier I screening approach. However, due to a lack of data, 

chronic dietary risk to adult honeybees and acute and chronic dietary risk to larval honeybees 

(who may be exposed from pollen and nectar contaminated by either direct spray or spray drift 

and carried back to the colony by adult foragers) cannot be quantitatively evaluated in this 

assessment. 

Maximum EECs and the resulting RQs for adult honeybees are shown in Table 15. Based on the 

available toxicity data, no treatment-related acute mortality was observed in either the acute 

contact or acute oral honeybee studies at the highest levels tested. Only acute non-definitive 

(“greater than”) endpoints are available for adult honeybees; therefore, the acute RQs presented 

in Table 15 are also non-definitive (“less than”) maximum RQs. There is no acute dietary LOC 

exceedance for foliar applications of thidiazuron (or exposure to targeted plants from direct spray 

or to non-targeted plants from spray drift) at the maximum proposed rates (0.2 lbs a.i./A) using 

the non-definitive acute dietary endpoint for adult honeybees; the highest RQ = 0.06, which is 

below the acute dietary LOC of 0.4. 

Chronic dietary risk to adult honeybees and acute and chronic dietary risk to larval honeybees 

cannot be precluded for applications of thidiazuron due to a lack of ecotoxicity data. Because 

thidiazuron is used at cotton harvest, long after flowering, it is not likely to cause exposure of 

pollinators visiting cotton plants. Pollinators that are visiting off-site plants that are blooming at 

the time of the cotton harvest and are contaminated by spray drift from cotton fields may be 

exposed to thidiazuron. 

Table 15. EECs and Resulting RQs to Adult Honeybees From Foliar Applications of 

Thidiazuron 

Foliar Contact EEC Foliar Contact RQ Foliar Dietary EEC Foliar Dietary RQ 

0.508 (μg a.i.) <0.01 6.039 (μg a.i.) ≤ 0.06 

Risk to Terrestrial Monocots and Dicots 

There has been no change in the single maximum application rate for thidiazuron since the 

previous risk assessment (no change in exposure concentrations or change in toxicity values); thus, 

RQs are unchanged (Table 16). Based on the available data from past assessments, monocots are 

less sensitive to thidiazuron than dicots. The RQs for runoff to semi-aquatic areas result in the 

highest RQs compared to dry areas. This scenario represents a 10 acre area running off into a one 

acre area. The highest RQ for a combination of runoff and spray drift to semi-aquatic areas is 13, 

which exceeds the Agency’s LOC of 1 for potential risk to plants. 

1 Exposure scenario based on 2 broadcast sprays at 0.2 lb a.i./A and 5 days later at 0.1 lb a.i./A 

2 mg a.i./kg-diet
Page 26 of 45 

Table 16. Terrestrial Plant Risk Quotient 1, 2, 3, 4 

Scenario 

Non-listed RQs Listed RQs 

Terrestrial 

Adjacent area 

Semi-aquatic 

Adjacent Area

Terrestrial 

Adjacent area 

Semi-aquatic 

Adjacent Area 

One Aerial Spray at 0.2 lb a.i./A

Monocot <0.1 0.29 <0.1 0.29 

Dicot 0.98 3.5 3.7 13 

One Ground Spray at 0.2 lb a.i./A 

Monocot <0.1 0.25 <0.1 0.25

Dicot 0.42 2.9 1.58 11 1 RQs for aerial and ground sprays in this table were calculated using the maximum application rate. 2

 Non-listed toxicity thresholds (EC25) were >0.18, 0.015, >0.18, and 0.001 lb a.i./A for seedling 

emergence monocot, seedling emergence dicot, vegetative vigor monocot, and vegetative vigor dicot. 

3

 Corresponding listed toxicity thresholds (NOAEC) were 0.18, 0.004, 0.18, and 0.0002 lb a.i./A for 

seedling emergence monocot, seedling emergence dicot, vegetative vigor monocot, and vegetative vigor 

dicot. 

4 Values in bold are LOC exceedances (Exceeds the LOC of 1 for potential risk to non-listed and listed 

plant species). 

AgDRIFT was used to assess thidiazuron exposure to non-target terrestrial dicots in terrestrial 

areas (Table 17) adjacent to thidiazuron-treated sites. The spray drift exposure analysis using the 

default scenario indicates that for aerial or ground applications, exposure falls below the drift LOC 

for listed dicots inhabiting terrestrial areas adjacent to a treated field exposed to thidiazuron as a 

result of spray drift at a distance greater than 997 feet off-site. However, for non-listed dicots using 

the default inputs, the LOC falls at >997 feet for most scenarios with the exception for groundbased applications where the LOC falls at 328 feet. This suggests that adverse effects to non-target 

terrestrial plants susceptible to thidiazuron could occur outside the site of application. 

In addition to the AgDRIFT assessment using the most conservative scenario, two other scenarios 

were conducted- 1) a coarser droplet size scenario using the most sensitive plant and 2) the Listed 

Plant RQs at the edge of the field, at 250 feet offsite, and the distance the potential drift ends for 

the other nine plant species against the most conservative exposure scenario -to observe whether 

all or some terrestrial plants would be affected when exposed. For the first scenario, drift is reduced 

as much as 200-500 feet in terrestrial areas when coarser droplets are used (Table 17). In the 

second scenario, the eight of the nine tested plant species NOAEC values did not exceed the spray 

drift LOC (Table 18); the distance the RQ falls below the LOC for the only susceptible dicot (other 

than the most sensitive dicot) was 328 feet. While likely for one plant species, possible for one 

species and not likely for the other 8 tested species, it is anticipated that the potential risk to 

terrestrial plants is limited to susceptible species when exposed to off-site drift. 

Overall, there is a potential of risk for adverse effects on survival and growth to terrestrial dicots 

that inhabit areas adjacent to thidiazuron-treated fields through a combination of runoff and spray 

drift or by drift alone. 
Page 27 of 45 

Table 17. Spray Drift (Terrestrial) Assessment for Listed and Non-listed Terrestrial Plant 

Species a

No. of 

Applications and 

Application Rate

(lb a.i/.A)

Distance from 

Edge of Field

% of Application 

Rate

Terrestrial 

Area EEC

(lb a.i./A)

Non-listed 

Plant

RQ

Listed

Plant 

RQ

Spray 

Method

Terrestrial plants inhabiting areas exposed to fine to medium droplet sizes (Default scenario)

1 @ 0.2

0 feet 50% 0.1 100 500 

Aerial

50 feet 17% 0.034 34 170 

500 feet 1.9% 0.0037 3.7 19 

997 feet 1.2% 0.0023 2.3 11.5 

Terrestrial plants inhabiting areas exposed to coarse to very coarse droplet sizes

1 @ 0.2

0 feet 50% 0.1 100 500 

Aerial 50 feet 7.4% 0.015 15 75 

500 feet 0.5% 0.0011 1.1 5.5 

997 feet 0.3% 0.0006 0.6 3 

Terrestrial plants inhabiting areas exposed to very fine to fine droplet sizes (Default scenario)

1 @ 0.2

0 feet 100% 0.2 200 1000 

Ground

50 feet 5% 0.01 10 50 

100 feet 2.5% 0.005 5 25 

300 feet 0.75% 0.0015 1.5 7.5 

500 feet 0.4% 0.0008 0.8 4 

997 feet 0.1% 0.0003 0.3 1.5 

Terrestrial plants inhabiting areas exposed to fine to medium/coarse droplet sizes

1 @ 0.2

0 feet 100% 0.2 200 1000 

Ground

50 feet 1.2% 0.0024 2.4 12 

100 feet 0.7% 0.0014 1.4 7 

500 feet 0.2% 0.0003 0.3 1.5 

997 feet 0.08% 0.0002 0.2 1 a Based on lettuce EC25 of 0.001 lb a.i./A and NOAEC of 0.0002 lb a.i./A. 

*Value in bold indicates the LOC is exceeded. 
Page 28 of 45 

Table 18. Spray Drift (Terrestrial) Assessment for Other Nine Terrestrial Plant Species 

using Listed Toxicity Endpoint a

Species 

Terrestrial Area EEC 

(lb a.i./A) Toxicity 

(lb a.i./A) 

Listed Plant RQ Distance from 

Edge of Field 

RQ Falls Below 

LOC 

At Edge 

of Field 

250 feet from 

Edge of Field

At Edge of 

Field 

250 feet from 

Edge of Field 

Terrestrial plants inhabiting terrestrial areas (Default scenario for aerial application)

Corn

0.01 0.0073 

0.178 b

 0.06 0.04 0 feet 

Onion 0.18 b

 0.06 0.04 0 feet

Oat 0.18 b 0.06 0.04 0 feet 

Wheat 0.178 b 0.06 0.04 0 feet 

Cabbage 0.178 0.06 0.04 0 feet 

Cucumber 0.0056 1.8 1.3 328 feet 

Soybean 0.011 0.9 0.7 0 feet 

Tomato 0.0223 0.4 0.3 0 feet 

Turnip 0.178b 0.06 0.04 0 feet a Based on NOAEC. 

b Highest test concentration tested. No effects to the species. 

*Value in bold indicates the LOC is exceeded. 

6.5. Endangered Species Assessments 

In November 2013, the EPA, along with the U.S. Fish & Wildlife Service (USFWS), the 

National Marine Fisheries Service (NMFS) (collectively, the Services), and the U.S. Department 

of Agriculture (USDA) released a summary of their joint Interim Approaches for assessing risks 

to listed species from pesticides. The Interim Approaches were developed jointly by the 

agencies in response to the National Academy of Sciences’ (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. The NAS report 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 

Endangered Species Act (ESA) and FIFRA. 

The joint Interim Approaches were released prior to a stakeholder workshop held on November 

15, 2013. In addition, the EPA presented the joint Interim Approaches at the December 2013 

Pesticide Program Dialogue Committee (PPDC) and State-FIFRA Issues Research and 

Evaluation Group (SFIREG) meetings, and held a stakeholder workshop in April 2014, allowing 

additional opportunities for stakeholders to comment on the Interim Approaches. As part of a 

phased, iterative process for developing the Interim Approaches, the agencies will also consider 

public comments on the Interim Approaches in connection with the development of upcoming 

Registration Review decisions. The details of the joint Interim Approaches are contained in the 

white paper “Interim Approaches for National-Level Pesticide Endangered Species Act 

Assessments Based on the Recommendations of the National Academy of Sciences April 2013 

Report,” dated November 1, 2013. 
Page 29 of 45 

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 preliminary risk assessment for thidiazuron does not contain a 

complete ESA analysis that includes effects determinations for specific listed species or 

designated critical habitat. Although EPA has not yet completed effects determinations for 

specific species or habitats, for this preliminary assessment EPA conducted a screening-level 

assessment for all taxa of non-target wildlife and plants that assumes for the sake of the 

assessment that listed species and designated critical habitats may be present in the vicinity of 

the application of thidiazuron. This screening level assessment will allow 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. This screening-level risk 

assessment for thidiazuron indicates potential risks of direct effects to listed terrestrial plants. 

Listed species of plant-dependent taxa may also be affected through indirect effects because of 

the potential for direct effects on listed and non-listed species upon which such species may rely. 

Potential direct effects on listed terrestrial plants from the use of thidiazuron may be associated 

with modification of Primary Constituent Elements (PCEs) of designated critical habitats, where 

such designations have been made. Once the agencies have fully developed and implemented 

the scientific methods necessary to complete risk assessments for endangered and threatened 

(listed) species and their designated critical habitats, these methods will be applied to subsequent 

analyses for thidiazuron as part of completing this registration review. 

6.6. 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 (its most recent registration decision for thidiazuron or (its 

reregistration decision for thidiazuron), 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), thidiazuron 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 
Page 30 of 45 

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, 20133

 and includes some 

pesticides 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.4

7. Environmental Fate and Ecological Effects Data 

7.1. Environmental Fate 

Table 20 identifies environmental fate studies by MRID that offer data for each guideline 

requirement, as well as study classifications and whether or not further data are needed in order 

to support risk assessment. 

Table 20. Submitted Environmental Fate Data for Thidiazuron

OCSPP Guideline Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications

Are data needed 

to conduct risk 

assessment? 

Justification and 

Assumptions EPA will 

Make in Absence of Data

835.2120 Hydrolysis 42069203 

41188201 

Acceptable No 

835.2240 Aqueous 

photolysis 

44537108 Acceptable No 

835.2410 Soil photolysis 00156241 

41364902 

Supplemental No 

835.4100 Aerobic soil 

metabolism 

41950101 

46119601 

Supplemental No 

835.4200 Anaerobic soil 

metabolism 

41945201 

46119602 

Acceptable No 

835.4300 Aerobic aquatic 

metabolism 

45196011 Acceptable No 

835.4400 Anaerobic 

aquatic 

metabolism 

42666101 

44505132 

Acceptable No 

 

3

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

chemicals. 4

 http://www.epa.gov/endo/ 
Page 31 of 45 

Table 20. Submitted Environmental Fate Data for Thidiazuron

OCSPP Guideline Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications

Are data needed 

to conduct risk 

assessment? 

Justification and 

Assumptions EPA will 

Make in Absence of Data

835.1230 

835.1240 

Adsorption/ 

desorption and 

leaching 

41364909 

44436902 

(degradate) 

Acceptable 

Acceptable 

No 

835.6100 Terrestrial field 

dissipation 

444540-01 

417611-05 

Supplemental No 

Storage Stability 

Study (TFD) 

 Yes

850.1730 Fish 

bioconcentration 

 No 

Environmental 

Chemistry 

Method: Water 

850.6100 

Analytical 

method in water 

 Yes 

Environmental 

Chemistry 

Method: Soil 

Analytical 

method in soil 

41987102 Not Reviewed Yes 

7.2. Effects 

Tables 21 and 22 identify ecological effects studies by MRID that offer data for each guideline 

requirement, as well as study classifications and whether or not further data are needed in order 

to support risk assessment. Rationale for the additional data requested is presented below. 

 Thidiazuron may cause exposure of pollinators visiting off-site plants that are contaminated 

by spray drift. Acceptable toxicity data to honey bee adults and larvae have not been 

submitted for thidiazuron and these data are needed to fulfill EPA’s recent policy on 

pollinator protection. A tier I study on the toxicity of residues on foliage (OCSPP 850.3030) 

is not needed since results of the honey bee acute contact LD50 is >100 μg a.i./bee, well 

above the 11 μg a.i./bee trigger criterion and thidiazuron is used at cotton harvest, long after 

flowering, it is not likely to cause exposure of pollinators visiting cotton plants. Thus, the 

following six Tier I, II and III honey bee studies, yet to be submitted for thidiazuron, are 

required: 

o Honeybee larval acute oral toxicity study (following OECD 237, Tier I) 

o Honeybee adult 10-day chronic oral (special study; Tier I) 

o Honeybee larval repeated dose study (special study renamed as honeybee larval 

chronic oral toxicity study, Tier I) 

o Semi-field testing for pollinators (special study, Tier II). 

o Field testing for pollinators (OCSPP 850.3040, Tier III) 

o Field trial of residues in pollen and nectar (special study, Tier III) 

Note that it is possible to conduct the larval chronic oral study in such a way as to derive both 

an LD50 (acute endpoint of interest) and a NOAEC (chronic endpoint of interest) from a 
Page 32 of 45 

single study. If the chronic larval toxicity study is conducted in such a way that it derives 

both an LD50 and NOAEC, then a separate acute larval toxicity study is not needed. For 

pollinator studies without finalized OECD guidelines, protocols should be submitted for EPA 

review and comment prior to study initiation. 

The need for higher Tier semi-field and full field pollinator studies for thidiazuron will be 

determined after review of these Tier I studies. 

 Acceptable acute avian acute oral toxicity data were not submitted for a passerine species 

exposed to thidiazuron, which is now required under the 40 CFR Part 158 data requirements. 

Though the available bobwhite quail study indicates thidiazuron has low toxicity to upland 

game bird species, it is uncertain whether passerines may be more or less sensitive to this 

chemical as a result of their higher metabolic rate and differing metabolic pathways. Given 

the reproduction effects of number of survivors, hatchlings, viable embryos, and live 

embryos observed in birds, EFED cannot preclude the potential risk to passerines which are 

small-sized birds. An avian oral toxicity study is needed for a passerine species. 
Page 33 of 45 

Table 21. Submitted Aquatic Ecological Effects Data for Thidiazuron OCSPP Guideline 

Data Requirement 

Submitted Studies (MRID) 

Study Classifications

Are data needed for assessment? 

Justification and Assumpin Absence of Data 

850.1010 

Freshwater invertebrate acute toxicity 

46203503 (TGAI) 

Acceptable No -- 

099819 

(AEF132347) 

6203509 (AEF132345) 46203516 (AEC421200) 

850.1025 850.1035 

Estuarine/Marine invertebrate acute toxicity 

42132001 41846101 Acceptable No -- 

850.1075 Freshwater fish acute toxicity 

42069202 (TGAI) 

Acceptable No -- 

42069201 (AEF132347) 46203515 (AEF132345) 46203511 (AEC421200) 

850.1075 Estuarine/Marine fish acute toxicity 41846101 Acceptable No -- 850.1300 

Freshwater invertebrate life cycle 

42132002 Acceptable No -- 
Page 34 of 45 

Table 21. Submitted Aquatic Ecological Effects Data for Thidiazuron OCSPP Guideline 

Data Requirement 

Submitted Studies (MRID) 

Study Classifications

Are data needed for assessment? 

Justification and Assumpin Absence of Data 

850.1350 

Estuarine/Marine invertebrates life cycle 

No data N/A No 

No chronic data is availablorganisms. However, EFED chronic studies and will use to make a conservative assufor estuarine/marine organi

850.1400 Estuarine/Marine fish early-life stage No data N/A No 850.1400 Freshwater fish early-life stage 42273001 Acceptable No -- 850.1500 Fish life cycle (None) (N/A) No 

No LOC exceedances were aquatic organisms thus a FLrequired. 

850.4400 

Aquatic plant growth: Vascular plants 

46203506 Acceptable No -- 

850.4500 Aquatic Plant Growth: Algal 

46203505 41761104 (TGAI) 

Acceptable No 

No data is available for freHowever, EFED is not askidiatom study since the data toxic to exceed the LOC foplants which is unlikely. 

46203510 (AEF132347) Supplemental No 46203517 (AEF132345) Supplemental No 46203513 (AEC421200) Supplemental No 

850.4550 

Aquatic Plant Growth: Cyanobacteria 

46203504 Supplemental No -- 
Page 35 of 45 

Table 22. Submitted Terrestrial Ecological Effects Data for Thidiazuron

OCSPP 

Guideline Data Requirement 

Submitted 

Studies 

(MRID) 

Study Classifications 

Are data 

needed for 

assessment?

Current Addit

850.2100 Avian oral toxicity 099819 Acceptable Yes 

A passerine bird stsatisfy standard daWithout data, risk for small-sized birdspecies. 

850.2200 Avian dietary toxicity 

0081629 

46203502 

099819 245834 

Acceptable Acceptable Acceptable Acceptable 

No -- 

850.2300 Avian reproduction 48098704 48098705 Acceptable Acceptable No -- 

Nonguideline Avian inhalation (None) (N/A) No The STIR model ind

through inhalation ibirds. 

850.3020 

Adult honey bee 

acute contact 

toxicity 

46203501 46203518 

Acceptable Acceptable No -- 

850.3030 Honey bee residue on foliage No data No data No 

No data is availablenot asking for the tofoliage since the bee 

>100 μg a.i./bee, wh11 μg a.i./bee criteristudy. 
Page 36 of 45 

Table 22. Submitted Terrestrial Ecological Effects Data for Thidiazuron

OCSPP 

Guideline Data Requirement 

Submitted 

Studies 

(MRID) 

Study Classifications 

Are data 

needed for 

assessment?

Current Addit

850.3040 Field testing for pollinators No data No data Yes 

Pollinator Tier I, II the honeybees are nstandard data requdata, risk would be pollinators, includi

SS Larval honey bee chronic oral toxicity No data No data Yes SS Adult honey bee chronic oral toxicity No data No data Yes SS Larval honey bee acute toxicity No data No data Yes SS Adult honey bee acute oral toxicity No data No data Yes SS 

Field testing of 

residues in pollen 

and nectar 

No data No data Yes 

SS Semi-field testing for pollinators No data No data Yes 

850.4100 Tier I or II seedling 

emergence 

41821301 45908501 

Acceptable 

Acceptable/Supplemental No 

-- 

850.4150 Tier I or II vegetative vigor 41819101 45921501 Acceptable Acceptable/Supplemental No 
Page 37 of 45 

8. References 

USEPA 2005. Environmental Fate and Effects Division Revised Risk Assessment for the 

Reregistration Eligibility Decision of Thidiazuron for Use on Cotton. D319339. September 15, 

2005 
Page 38 of 45 

Appendix I. Incidents 

EIIS Pesticide Summary Report: General Information 

Thidiazuron (120301)

Incident # Date County State Certainty Legal. Formul. Appl. Method 

PLANTS

Cotton

I016036-024 10/22/2004 Riverside CA 2 RU Spray 

N/R

I013645-002 9/30/2002 Fresno CA 3 M Spray 

Rye

I021411-019 10/20/2009 Halifax NC 2 RU SC N/R 

TERRESTRIAL

Agricultural Area

I005204-014 9/18/1996 3 MA F Broadcast 

Wednesday, April 22, 2015 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=Unknown.
Page 39 of 45 

EIIS Pesticide Summary Report: Species Information 

Thidiazuron (120301)

Incident # Species Scientific Name Magnitude Response 

PLANTS

Cotton

I016036-024

lettuce 53 acres plant damage 

N/R

I013645-002

lettuce 50 acres plant damage 

Rye

I021411-019

ryegrass 100% of 75 acres plant damage 

TERRESTRIAL

Agricultural Area

I005204-014

chicken Gallus gallus 9 mortality n/r n/r mortality 

Wednesday, April 22, 2015 
Page 40 of 45 

Appendix II. SWCC 

Summary of Water Modeling of Thidiazuron and the USEPA Standar

Pond

Estimated Environmental Concentrations for thidiazuron are presented in Table 1 for the USEPA

standard pond with the MScottonSTD field scenario. A graphical presentation of the year‐to‐year pea

is presented in Figure 1. These values were generated with the Surface Water Concentration Calculat

(SWCC Version 1.106). Critical input values for the model are summarized in Tables 2 and 3.

This model estimates that about 4.1% of thidiazuron applied to the field eventually reaches the wate

body. The main mechanism of transport from the field to the water body is by runoff (66.1% of the to

transport), followed by spray drift (30.6%) and erosion (3.24%).

In the water body, pesticide dissipates with an effective water column half‐life of 725.0 days. (This va

does not include dissipation by transport to the benthic region; it includes only processes that result

removal of pesticide from the complete system.) The main source of dissipation in the water column

metabolism (effective average half‐life = 725 days) followed by volatilization (9.381325E+08 days).

In the benthic region, pesticide dissipates very slowly (300.6 days). The main source of dissipation in t

benthic region is metabolism (effective average half‐life = 300.6 days). The vast majority of the pestic

in the benthic region (98.77%) is sorbed to sediment rather than in the pore water.

Table 1. Estimated Environmental Concentrations (ppb) for thidiazuron.

Peak (1‐in‐10 yr) 12.2

4‐day Avg (1‐in‐10 yr) 11.9

21‐day Avg (1‐in‐10 yr) 11.1

60‐day Avg (1‐in‐10 yr) 10.1

365‐day Avg (1‐in‐10 yr) 8.02

Entire Simulation Mean 6.28

Table 2. Summary of Model Inputs for thidiazuron.

Scenario MScottonSTD

Cropped Area Fraction 1

Koc (ml/g) 742

Water Half‐Life (days) @ 20 °C 697

Benthic Half‐Life (days) @ 20 °C 289
Page 41 of 45 

Photolysis Half‐Life (days) @ 40 °Lat 0

Hydrolysis Half‐Life (days) 0

Soil Half‐Life (days) @ 20 °C 349

Foliar Half‐Life (days) 35

Molecular Wt 220.25

Vapor Pressure (torr) 2.62E‐11

Solubility (mg/l) 31

Table 3. Application Schedule for thidiazuron.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

09/12 Foliar 0.235 0.95 0.125

09/17 Foliar 0.112 0.95 0.125

Figure 1. Yearly Peak Concentrations
Page 42 of 45 

Appendix III. STIR

Input 

Application and Chemical Information 

Enter Chemical Name Thidiazuron 

Enter Chemical Use Cotton 

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) 220.25 

Enter Chemical Vapor Pressure (mmHg) 2.62E-11 

Enter Application Rate (lb a.i./acre) 0.2 

 

Toxicity Properties 

Bird 

Enter Lowest Bird Oral LD50 (mg/kg bw) 3160 

Enter Mineau Scaling Factor 1.15 

Enter Tested Bird Weight (kg) 0.178 

Mammal 

Enter Lowest Rat Oral LD50 (mg/kg bw) 2000 

Enter Lowest Rat Inhalation LC50 (mg/L) 2.4 

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) 3.11E-07 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 3.90E-08 

Adjusted Inhalation LD50 2.11E+01 

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.85E-09 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 1.92E-02 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 9.09E-04 Exposure not Likely Significant 

 

Results Mammalian (0.015 kg ) 

Maximum Vapor Concentration in Air at Saturation (mg/m3) 3.11E-07 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 4.91E-08 

Adjusted Inhalation LD50 1.43E+02 

Ratio of Vapor Dose to Adjusted Inhalation LD50 3.43E-10 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 2.42E-02 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 1.69E-04 Exposure not Likely Significant 
Page 43 of 45 

Appendix IV. SIP

Table 1. Inputs 

Parameter Value 

Chemical name Thidiazuron 

Solubility (in water at 25oC; mg/L) 31 

 

Mammalian LD50 (mg/kg-bw) 2000 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian species 

 

Mammalian NOAEL (mg/kg-bw) 35.4 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian species 

 

Avian LD50 (mg/kg-bw) 3160 

Avian test species northern bobwhite quail 

Body weight (g) of "other" avian species 

Mineau scaling factor 1.15 

 

Mallard NOAEC (mg/kg-diet) 298 

Bobwhite quail NOAEC (mg/kg-diet) 903 

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) 5.3320 5.3320 

Adjusted toxicity value (mg/kg-bw) 1538.3211 27.2283 

Ratio of exposure to toxicity 0.0035 0.1958 

Conclusion* 

Drinking water exposure 

alone is NOT a potential 

concern for mammals 

Drinking water exposure 

alone is NOT a potential 

concern for mammals 

 

Table 3. Avian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 25.1100 25.1100 

Adjusted toxicity value (mg/kg-bw) 2276.5573 14.7845 

Ratio of exposure to acute toxicity 0.0110 1.6984 

Conclusion* 

Drinking water exposure 

alone is NOT 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 44 of 45 

Appendix V. Degradate Structures 

Photothidiazuron (photoproduct I) CAS Reg. No. 71769-74-9 

Photoproduct II (phenylurea) CAS Reg. No. 64-10-8 

 
Page 45 of 45 

Appendix V. Degradate Structures 

Thidiazolurea (AE 132345)