UNITED STA TES ENVIRONM E NT AL PROTECTIO N AGENCY
WAS H INGT O N 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
Nei l Anderson

, Branch Chief

Risk Management and Implementation Branch l

Pesticide Re-Evaluation Division (7508P)**~ ~**  /; 

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Stephen carey , Biologist

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.. William P. Eckel

, Senior Science Advisor 

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Environmental Risk Branch 6**~** Environmental Fate and Effect#J_sivisio (7507P) · · / 

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# ~ G,-l b-\:S 

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# 0 Mark Corbin, Branch Chief (_;..-/

MonicaWait

, RAPL

### ~**\J~ ~/** **h/15** Amy Blankinship

, Sen i or Scientist
Environmental Risk Branch 6
Environmental Fate and Effects Division (7507P)

The Environmenta l 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 12030 1 )

.

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 E li gibility Decision (RED) i n 2005

, thus

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

to past risk assessments

, mainly the RED. This document will use current standard models to update the potential exposures l ast 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 availab l e. 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)

.

Pa ge I of 45Page 2 of 45 

### Risk Conclusions Summary

The risks of thidiazuron to terrestrial and aqua tic animals including plants are summarized below. As a result of the joint PF/DRA review for Regist ration Review, EFED determined that thidiazuron exposures to aquatic organisms a nd vascular/nonvascular plants con tinue to be unlikely as the new aquatic EECs (Estimated Environmental Concentrati ons) are similar to previous calculated EECs. Hence, discussions of risks to aquatic organi sms, with the exception of chronic risk to estuarine/marine species, have no t been updated in this risk asse ssment 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 estuar y environment was previously not assessed and therefore, is assessed in this doc ument. 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 invert ebrates 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 a quatic vascular and nonvasc ular plants. No listed or non-listed plant LOC exceedances were obser ved 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. Howeve r, bird reproduction

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

and the risk assessment indicates no anticipated repr oductive risks in bird s. 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 bi rds via the oral and di etary routes; however, the Screening Imbibition Program (SIP) v.1.0 in dicates 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 w ith 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. EF ED currently uses surrogate data (birds) for terrestrialphase amphibians and reptiles when taxa-speci fic data are not available. There were no

LOC exceedances for birds via the oral and diet ary routes; these findings also apply to terrestrial-phase amphibians and reptiles. Howe ver, SIP indicates a drinking water concernPage 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 rept iles. 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 diet ary-based risks are below the levels of concern (RQ = 0.8, LOC = 1). Also in Registration Review, it wa s 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 invertebrate s were previously not assessed quantitatively. As part of the re gistration 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. Ch ronic 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 harv est and are contaminated by spray drift from cotton fields may be exposed to thidi azuron. See section 6.4 for details.

 Terrestrial Plants: Thidiazuron is expected to adversel y affect the survival and growth of terrestrial plants, mainly dicots. No-e ffect 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 expos ure 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 dr ift EECs for plants inhabiting in terrestrial areas) beyond the edge of field. Along with m onocots, the least susceptible dicots were not affected at the edge of the field.

Analysis of potential effects to federally-l isted endangered and threatened plant species will require a more in-depth evaluation to de termine 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 persiste nt, as shown by laboratory and fiel d 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 esse ntially negligible by compar ison. The two photoproducts are: (photothidiazuron, photoproduct I), a structural isomer of pare nt, and photoproduct II, which has a substantially altered chemical structure.

Based on its solubility and vapor pressure (Henry's law), thidiazu ron is non-volatil e. Based on its relatively low n-octanol/water pa rtitioning coefficient, thidiazu ron should not bioconcentrate.

No foliar dissipation data are currently available, and since thidiazuron is used to defoliate cotton prior to harvest, dislodgement fr om 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 data base is largely completed for thidiazuron except for the following required studies under 40 CFR Part 158:

Environmental Fate

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

 Enforcement Analytical me thod for Water. (850.6100)

 Enforcement Analytical Method for So il. (850.6100) Submitted method (MRID 41987102) not reviewed.

Ecological Effects

 Thidiazuron may cause exposure of pollinators visi ting 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 1 

### . 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 LD 50 

### is >100 μg a.i./bee, well above the 11 μg a.i./bee trigger criterion and thidiazur on 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 studie s, 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 (sp ecial study renamed as honeybee larval chronic oral toxicity study, Tier I)

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

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

o Field trial of residues in pollen a nd 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 LD 50 

### (acute endpoint of interest) and a NOAEC (chronic endpoint of interest) from a single study. If the chronic larval toxicity st udy is conducted in such a way that it derives
both an LD 50 

### and NOAEC, then a separate acute larv al toxicity study is not needed. For pollinator studies without finalized OECD gui delines, protocols should be submitted for EPA review and comment prior to study initiation.

The need for higher Tier semi-field and full fi eld 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 th e available bobwhite quail study indicates thidiazuron has low toxicity to upland game bi rd 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 reprodu ction effects of numb er of survivors, hatchlings, viable embryos, and live embryos observed in birds, EFED cannot preclude the potential risk to passerines wh ich 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

P

repared 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 R eviewed by: 

### Amy Blankinship, Senior Scientist Monica Wait, RAPL Mark Corbin, ERB6 Chief

June 16, 2015 

### Office of Chemical Safety
and Pollution PreventionPage 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 ............................................................................................................. 2 2
6.5. Endangered Species Assessments .................................................................................. 28
6.6. Endocrine Disruptor Screening Program ....................................................................... 29

7. Environmental Fate and Ec ological Effects Data ................................................................. 30
7.1. Environmental Fate ....................................................................................................... ..... 30
7.2. Effects .................................................................................................................. .............. 31
8. References ................................................................................................................ ................ 37Page 8 of 45 

## 1. Introduction

EFED evaluated the most recent ecological risk assessments for th idiazuron in association with the updated toxicity, exposure, and usage informati on to determine if suffici ent data are available and if further updates are needed to support regist ration review. With suff icient 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 -- ‘Environmen tal Fate and Effects Division Revised Risk Assessment for the Reregistration Eligibility Decision of Thidiazuron for Use on Cotton’ document (September 15, 2005; D319339). Those expos ures and taxa previously assessed and
determined not to be of concern will be discus sed briefly here with de tails available in the September 15, 2005 document. EFED will use applic ation 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 regulat or 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) repor ted that during the 2008-2012 time frame, an average of approximately 250,000 pounds of active ingr edient per acre (lb a. i./A) of thidiazuron were applied annually to 3.8 million acres at an overa ll average application rate of 0.07 lb a.i./A. The top states, in terms of the percentage of th e 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 emulsifiab le 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 fo r 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 fo r 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
mammals 1 Terrestrial
invertebrates 2

, 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 w ith adult bees; were not assessed quantitatively.



### As a result of the joint PF/DRA review for Regist ration Review, EFED determined that thidiazuron exposures to aquatic organisms a nd vascular/nonvascular plants con tinue to be minimal as the new aquatic EECs (Estimated Environmental Concentrat ions) are similar to previous calculated EECs. Hence, discussions of risks to aquatic organi sms, with the exception of chronic risk to estuarine/marine species, have no t been updated in this risk asse ssment but rather are summarized from the RED. Details of the aquatic risk conc lusions can be found in th e 2005 RED. The chronic risk of thidiazuron in an estuary environment was previously not assessed a nd therefore, is assessed
in this document. For thidiazur on exposures to terrestrial animal s, the chronic risk conclusions have changed significantly, due to submission of ne w 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-asse ssed taxa of terrestrial-phase amphibians and reptiles and terrestrial in vertebrates are availabl e in this document. Th e 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 a quatic vascular and nonvasc ular plants. No listed or non-listed plant LOC exceedances were obser ved 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. Howeve r, bird reproduction

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

### and the risk assessment indicat es no adverse reproductive effect s 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 bi rds via the oral and di etary routes; however, the Screening Imbibition Program (SIP) v.1.0 in dicates drinking water exposure alone is not a concern on an acute exposure basis; but is a concern for birds fo raging in thidiazurontreated cotton fields on a chronic exposure basis ( Appendix III ). This pathway will be explored further with the deve lopment 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. EF ED currently uses surrogate data (birds) for terrestrialphase amphibians and reptiles when taxa-speci fic data are not available. There were no

LOC exceedances for birds via the oral and diet ary routes; these findings also apply to terrestrial-phase amphibians and reptiles. Howe ver, 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 rept iles. 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 diet ary-based risks are below the levels of concern (RQ = 0.8, LOC = 1). The potential risk to mammals is ex pected to be minimal. Also in Registration Review, it was determined that exposure to drinking water alone a nd via inhalation is expected to not a concern for mamm als. See section 6.4 for details.



###  Terrestrial Invertebrates (Pollinators): Terrestrial invertebrate s were previously not assessed quantitatively. As part of the regi stration 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 di etary risk to adult honeybees and acute and chronic dietary risk to larv al honeybees is uncertain due to a lack of data. Because
thidiazuron is used at cotton ha rvest, 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 adversel y affect the survival and growth of terrestrial plants, mainly dicots. No-e ffect 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 expos ure alone for the most sensitive dicot at aPage 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 dr ift EECs for plants inhabiting in terrestrial areas) beyond the edge of field. Along with m onocots, the least susceptible dicots were not affected at the edge of the field.

Analysis of potential effects to federally-l isted endangered and threatened plant species will require a more in-depth evaluation to de termine 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 Quot ients 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: <0.06 A 

### 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 larv al are unknown.Page 12 of 45 

### 2.3. Clean Water Act Programs

Thidiazuron is not identified as a cause of impairment for an y 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 a quatic benchmarks have been established for
thidiazuron and are not available at http://www.epa.gov/oppefed1/ecoris k_ders/aquatic_life_benchmark.htm . Any data submitted or otherwise located as part of th e registration review process may be used to update aquatic life benchmarks if applicable.



## 3. Environmental Fate and Transport

In soil, thidiazuron is persiste nt, as shown by laboratory and fiel d half-lives and rotational crop intervals on the order of one y ear. It has intermediate soil sorption coefficients (sorption coefficients ranged from 4.4 to 19 Freundlich units).

When thidiazuron reaches surface water, photoly sis is expected to be the major route of transformation; other degradativ e processes are essentially ne gligible by comparison. Aqueous photolysis is rapid, and occurs by branching in quantitative yield into two photoproducts. One of the photodegradates (photothidiazu ron, photoproduct I) is a structural isomer of parent, whereas photoproduct II has a substantially altered chemical structure.

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

thidiazuron is non-volatile. Based on its relativel y 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 fr om 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 degr ade to unidentified mi nor compounds which are incorporated into the organic material and mineralized to CO 2 

### . However, there was insufficient data to support these contentions, and produc tion of additional meta bolites 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. Phot oproduct I was stable to further photolysis under experimental study conditions. The second phot odegradate is AE C421200 or photoproduct II (see Appendix V for chemical identification). Photoproduct II (phenylurea) is substantially different from parent thidiazu ron, and has a complementary asym ptotic limit of production toPage 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 quantit atively converted in compleme ntary 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 C 9 H 8 N 4 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(Nc 1ccccc1)Nc1cn ns1 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 o C 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 o C MRID 41364901 pH TGAI: 5.21 (20 0 C); PAI: 6.50 MRID 45934001 Henry’s Law Constant 2.5 x 10 -l6 atm m 3 /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 (90 t h %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



### MobilityPage 14 of 45 

### Table 2. Physical Chemical and Environmen tal Fate Properties of Thidiazuron and Two photoproducts.

Parameter Value Source Adsorption/desorption Kf/ads Kf/oc/ads 1/n r 2

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 (r 2 = 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 (photothidi azuron), photoproduct II (phenylurea), and thiazoylurea. Structures are given in A ppendix V. Photoproduct I is considered the same as parent thidiazuron for th e 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 tabl es provide a preliminary overview of the potential acute toxicity of thid iazuron by providing the acute toxi city 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 pract ically non-toxic to upland game birds, waterfowl, and mammals. Thidiazuron is acutely not toxic to adult bees via the contact and or al 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 availabl e. Avian reproduction studies have been submitted
since the last risk assessment. Th e toxicity to benthic invertebrate s is not available; however, the chemical properties indicate thidi azuron exposures to benthic inve rtebrates are unlikely and data on sediment toxicity to benthic invertebrates ar e not needed. Additional details are included in the following tables.

Toxicity data on three degradates of thidi azuron are available. Da ta indicate aquatic photodegradate AE C421200 (photopr oduct II) and metabolite AE F 132345 (thiadiazolurea) are generally practically non- toxic to freshwater fish and invert ebrates. Aquatic photodegradate AE F132347 (photoproduct I) is moderately toxic and more toxic than the parent compound toPage 15 of 45 

### freshwater fish and practically nontoxic to freshw ater invertebrates and is more toxic than the other degradates but not more t oxic 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 Mo st 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 fish 1 Acute Rainbow trout LC 50 = 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 EC 50 = 5,700 Moderately
toxic 46203503 (Acceptable)

Chronic NOAEC <100; [EC 10 = 720] 2 Affected Endpoint: length [survival] 42132002 (Upgraded to acceptable under 43665001)

Estuarine/marine
fish Acute

Sheepshead
minnow LC 50 >36,000 Slightly toxic at solubility limit 41846101 (Acceptable)

Chronic Estimated NOAEC = 6,400 (assuming values are definitive) -- ACR 3

Estuarine/marine
invertebrates Acute Mysid shrimp EC 50 = 3,240 Moderately
toxic 41846102 (Acceptable)

Chronic Estimated
NOAEC = 400 -- ACR 4

Aquatic plants and algae Vascular Duckweed EC 50 >24,000 NOAEC = 8,600 -- 46203506 (Acceptable)

Nonvascular Marine diatom ( Skeletonema

costatum ) EC 50 = 860
NOAEC = 110 -- 46203505 (Acceptable)

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

2 An EC 10 is used in lieu of an NOAEC (D216628).

3 The ACR (Acute-to-Chronic Ratio method) using rainbo w trout acute and fathead minnow chronic data is 5.6, assuming the values are definitive. Employ ing the ACR of 5.6 to the sheepshead minnow LC 50 of >36 mg a.i./L calculates a chronic toxicity value (NOA EC) 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
LC 50 of 3.2 mg a.i./L calculates a chronic toxicity va lue (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 Se nsitive 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

Birds 1 Acute oral
Bobwhite quail LD 50 >3,160 mg a.i./kg-bw Practically nontoxic 099819 (Acceptable)

Subacute Dietary LC 50 >5,215 mg a.i./kg-diet Practically nontoxic 46203502 (Acceptable)

Chronic Mallard
duck NOAEC = 298 mg a.i./kgdietLOAEC = 906 mg a.i./kgdiet Affected Endpoint: no. of survivors, hatchlings, viable embryos, and live embryos 48098704 (Acceptable)

Mammals Acute Oral

Laboratory

rat LD 50 >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 LD 50 > 100 μg a.i./bee Practically nontoxic 46203501 (Acceptable) Acute Oral -
Adults LD 50 > 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) EC 25 = 0.015 lb a.i./A [EC 05 ] 2 = 0.004 lb a.i./A -- 45908501 (Supplemental)

Monocot (All) EC 25 >0.18 lb a.i./A
NOAEC = 0.18 lb a.i./A -- 48098706 (Supplemental)

Vegetative Vigor Dicot (Lettuce) EC 25 = 0.001 lb a.i./A
NOAEC = 0.0002 lb a.i./A -- 45921501 (Acceptable) Monocot (All) EC 25 = >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 EC 05 is used in lieu of the NOAEC since the NOA EC 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 repor ted to the Agency. When ava ilable, 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 inci dent, and results of any chemical residue analysis or other analyses conducted during in cident investigation. EIIS incidents are categorized according to the certai nty that the incident resulted from pesticide exposure. The Avian Monitoring System (AIMS) is a database administered by the Amer ican Bird Conservancy that contains publicly available data on reported avian incidents involving pesticides. Many of
the incidents listed in this data base are also in the EIIS.

As of April 2015, there are 4 reported incidents fo r thidiazuron in the E IIS database with a certainty index of ‘possible’ and ‘probable’. Of these four incide nts, 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 dire ctly; however, one incident in volves chickens. The chicken incident is also reported in the AIMS database.

In addition to the incident reports available in EIIS, there were no aggreg ate ‘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 th idiazuron suggests that direct spray, spray drift, leaching and runoff represent the likely exposure pathways for terrestrial and aquatic non-target organisms. Drinking water and i nhalation exposure pathways were also screened using the SIP (Screening Imbibition Program) and STIR (Scr eening 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 th at the only stressor of ecological concern for terrestrial and aquatic organisms (except fi sh) 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 Invertebrate s, and Aquatic Plants

EFED updated the aquatic EECs using the cu rrent surface water model, Surface Water Concentration Calculator (SWCC), to calcula te the aquatic EECs for cotton. EECs for the registered use on cotton are based on 1 st 

### application at 0.21 and 2 nd 

### 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 usi ng the model SWCC indicate that exposures for cotton uses are marginally highe r or similar to those previo usly 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-ba sed 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 1 st 

### application at 0.21 and 2 nd 

### at 0.1 lb a.i./A, applied 5 days apart broadcast. Results of the updated diet ary-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 ol d and new EECs for cotton. ELLFATE dose-based
EECs were not available in the 2005 risk assess ment; thus, a comparison could not be performed.

As of a result, the updated terr estrial EECs using the model T-RE X indicate that dietary-based exposures for cotton uses are sli ghtly higher than or similar to those previously assessed using
ELLFATE.

Table 7. Comparison of Dietary-Base d 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 ra te and method of application for spray drift. STIR incorporates results from several toxicity studies including acute oral and inhalation ratPage 20 of 45 

### toxicity endpoints (rat LD 50 

### >2000 mg/kg-bw; rat inhalation LC 50 

### >2.4 mg/L) as well as the most
sensitive acute oral avian t oxicity endpoint (bobwhite LD 50 

### >3160 mg/kg-bw). Based on the results of the STIR model ( Appendix III ), exposure through i nhalation of spray drift or the vapor phase of thidiazuron is not determ ined 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 usi ng thidizuron’s solubility ( i.e ., 31 mg/L at 20 o 

### C), and compared to the
most sensitive acute and chronic avian and mamma lian toxicity endpoints. Upon results of the screening model SIP, drinking wa ter exposure alone is not a c oncern 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 thidiazur on on a chronic basis. This pathwa y will be explor ed further with the development of SIP v.2.0 in a future Ecologica l Risk Assessment for th idiazuron. For a sample of the output generated by SIP v.1.0, please see Appendix IV

.

Terrestrial Invertebrates

The pollinator risk assessment guidance (U SEPA, 2014c) expresses estimates of contact exposure to honey bees from a folia r spray as the application rate for a single application (in lbs a.i./A) multiplied by 2.7 μg a.i./b ee and estimates of dietary expos ure to honey bees from a foliar spray as the application rate for a single appl ication (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 LD 50 

### data available for the adult honey bee. Ma ximum 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 ma y be exposed via runoff a nd/or spray drift from foliar applications has been updated using TERRP LANT. Only a single a pplication of 0.21 is considered. A 60% application e fficiency factor that previous ly was included in the runoff calculations for aerial app lications is not used any longer an d removed from calculations in this assessment. Results of the updated EECs for terr estrial 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-targ et plant exposure from spray drift alone, Tier 1 modules in AgDRIFT were used to calculate the EECs for off-site spray drift e xposure to plants inhabiting in terrestrial areas ( Table 11) for wettable powder formulations. For aerial applica tions, EFED used the American Society of Agricultural and Biolog ical 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 applicat ion parameters. For ground applications, EFED used the ASABE Very Fine-to-Fi ne 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 fo r 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 Areas 1 

### Total Loading to SemiAquatic Areas Adjacent to
Treated Areas 2



### 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 + Drif t (1% for ground; 5% for aerial



### Table 11. Maximum Drift Loadings at Sp ecific 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 0.0003Page 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 aqua tic and terrestrial organisms not assessed previously.



### 6.4 Risk Assessment

The risks to aquatic and terrestri al organisms not assessed previous ly will be assessed in this
section of the document. The taxa and routes no t quantitatively assessed previously are chronic exposure to thidiazuron in estuary waters; avia n and mammalian reproduct ion; acute and chronic exposures to terrestrial invertebra tes (pollinators); and distance fr om 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, exposur e to aquatic nonvascular and vascular plants, acute exposure to birds and mammals, a nd exposure to terrestrial plants.

Estuarine/marine Fish and Invertebrates

Chronic RQs were not available in past assessmen ts for estuarine/marine fish and invertebrates due to lack of data. To measure the potential ri sk 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 ra inbow 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 LC 50 

### of >36 mg a.i./L calculates a chronic t oxicity value (NOAEC) of >6.4 mg a.i./L for estuarine/marine fish. For invertebrates, the A CR using water flea acute and chronic data is 7.9. Employing the invertebrate ACR of 7.9 to the mysid shrimp LC 50 

### of 3.2 mg a.i./L calculates a chronic toxicity value (NOAEC) of 0.4 mg a.i./L for estuarine/mari ne invertebrates. With a peak EEC of 0.012 mg a.i./L, the chronic RQ for estuar ine/marine fish and invertebrates is <0.01 and 0.03, respectively, which is be low 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 a nd growth to fish and invertebra tes that inhabit in freshwater and estuary waters with thidiazu ron residues as a result of su rface runoff and spray drift from adjacent treated fields is expected to be unlikely.

Birds

Avian reproduction studies were not previously av ailable 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 gr ound 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 fo r birds were below the LOC of 1 for potentialPage 23 of 45 

### risk to non-listed and listed bi rds consuming vegetation, insects, and seeds with thidiazuron residues ( Table 12 ).

Drinking water exposure alone a ccording to the SIP model is a potential pathway of concern for birds on a chronic exposure basis. SIP is a screeni ng (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 solubilit y limit of the chemical, 2) the assessed animals obtain 100% of their daily water n eeds through drinking water, 3) the daily water need is equivalent to the daily water flux rate, 4) th e 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 ra tio 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 exposur e estimates will be refined further when SIP v.2.0 is developed that could increase or decr ease the potential pathway of concern for birds drinking water with thidiazuron residues.

Overall, the potential risk for adverse effects on survival, reproduc tion and growth to bird species that inhabit in treated fields foraging on vege tation and insects with thidiazuron residues is expected to be unlikely. There is a potential con cern for birds exposed to drinking water alone on a chronic basis; this pathway will be explored furt her 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

EEC 2 

### RQ

Cotton 1 

### 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 amphib ians and reptiles when data are not available.
There were no LOC exceedances for birds; these apply to terrestria l-phase amphibian and reptiles.

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

### phase amphibian and reptiles that inhabit in treated fields fora ging 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; th is 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 ex posure 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 chr onic RQs of 0.01-0.8 were belo w the chronic LOC of 1.0.

Overall, the potential risk for adverse effect s on survival, reproduction and growth to mammals
that inhabit in thidiazuron-treated fields is anti cipated 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 Di etary-Based Risk Quotients

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

EEC 2 

### RQ

Cotton 1 

### 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

EEC 2 

### RQ EEC RQ EEC RQ

Cotton 1 

### 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 exceedancesPage 25 of 45 

### Beneficial Invertebrates (Pollinators)

For beneficial terrestrial invert ebrates, this assessment follows the guidance established in the recently issued Pollinator Risk Assessment Gu idance (USEPA, 2014c) to ev aluate acute contact and dietary risk to honey bees usi ng a Tier I screening approach. Ho wever, due to a lack of data, chronic dietary risk to adult hone ybees and acute and chronic diet ary risk to larval honeybees (who may be exposed from pollen and nectar contam inated by either direct spray or spray drift and carried back to the colony by adult foragers) cannot be quan titatively 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 acu te mortality was observed in either the acute contact or acute oral honeybee studies at the hi ghest levels tested. Only acute non-definitive (“greater than”) endpoints are available for adul t honeybees; therefore, the acute RQs presented
in Table 15 are also non-definitive (“less than”) ma ximum RQs. There is no acute dietary LOC exceedance for foliar applications of thidiazuron (o r 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 hone ybees and acute and chronic diet ary risk to larval honeybees cannot be precluded for applications of thidiazuron due to a lack of ecotoxicity data. Because
thidiazuron is used at cotton harv est, long after flowering, it is not likely to cause exposure of pollinators visiting cotton plants. Pollinators that ar e visiting off-site plants that are blooming at
the time of the cotton harvest and are contamin ated 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 maximu m application rate for thidiazuron since the previous risk assessment (no change in exposure concentrations or ch ange 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 RQ s for runoff to semi-aquatic areas result in the highest RQs compared to dry areas. This scenar io represents a 10 acre area running off into a one acre area. The highest RQ for a combination of r unoff 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-dietPage 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 we re calculated using the maximum application rate.

2 Non-listed toxicity thresholds (EC 25 ) were >0.18, 0.015, >0.18, an d 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 exceedance s (Exceeds the LOC of 1 for potential risk to non-listed and listed plant species).



### AgDRIFT was used to assess thidi azuron exposure to non-target terre strial dicots in terrestrial areas ( Table 17 ) adjacent to thidiazuron-treated sites. Th e spray drift exposure analysis using the
default scenario indicates that fo r 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 distan ce greater than 997 feet off-site. However, for non-lis ted dicots using the default inputs, the LOC falls at >997 feet for most scenarios with the exception for groundbased applications where the LOC fa lls at 328 feet. This suggests th at adverse effect s 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 se nsitive plant and 2) the Listed Plant RQs at the edge of the fiel d, at 250 feet offsite, and the dist ance the potential drift ends for the other nine plant species agai nst the most conservative exposur e scenario -to observe whether all or some terrestrial plants woul d be affected when exposed. For th e 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 plan t 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 e xposed to off-site drift.

Overall, there is a potential of risk for adverse e ffects on survival and growth to terrestrial dicots that inhabit areas adjacent to thidiazuron-treate d 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 ex posed 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 expo sed 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 ex posed 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 expo sed 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 EC 25 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 fo r 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 Ed g e 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 0.178 b 

### 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 Servi ces), and the U.S. Department of Agriculture (USDA) released a summary of th eir joint Interim Approaches for assessing risks to listed species from pesticides. The Inte rim 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 prio r to a stakeholder workshop held on November 15, 2013. In addition, the EPA presented the jo int 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 stake holders to comment on the Interim Approaches. As part of a phased, iterative process for developing the Inter im Approaches, the agencies will also consider public comments on the Interim Approaches in connection with the development of upcoming Registration Review decisions. Th e details of the joint Interim A pproaches are contained in the white paper “Interim Approaches fo r National-Level Pesticid e 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 de velop and work toward implementation of the Interim Approaches to assess the potential risk s of pesticides to listed species and their designated critical habitat, this preliminary ri sk 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 complete d effects determinations for specific species or habitats, for this prelim inary assessment EPA conducted a screening-level
assessment for all taxa of non-ta rget wildlife and plants that assumes for the sake of the assessment that listed species and designated criti cal habitats may be present in the vicinity of the application of thidiazuron. This screeni ng level assessment will allow EPA to focus its future evaluations on the types of species where th e potential for effects exists once the scientific methods being developed by the agencies have b een fully vetted. This screening-level risk assessment for thidiazuron indicates potential risks of direct effects to list ed terrestrial plants. Listed species of plant-dependent taxa may also be affected th rough 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 designate d critical habitats, these methods will be applied to subsequent analyses for thidiazuron as part of completing this regi stration review.



### 6.6. Endocrine Disruptor Screening Program

As required by FIFRA and FFDCA, EPA reviews num erous studies to asse ss potential adverse outcomes from exposure to chemicals. Collectiv ely, these studies includ e acute, subchronic and chronic toxicity, including asse ssments of carcinogenicity, ne urotoxicity, developmental, reproductive, and general or systemic toxicity. These studies include e ndpoints which may be susceptible to endocrine influe nce, including effects on endocri ne target organ histopathology, organ weights, estrus cyclicity, sexual maturatio n, fertility, pregnancy rates, reproductive loss, and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and chronic studies that assess gr owth, developmental and reprod uctive effects in different taxonomic groups. As part of (its most recent registration decision fo r thidiazuron or (its reregistration decision for thidiazuron), EPA review ed these data and selected the most sensitive endpoints for relevant risk assessment scenarios fr om the existing hazard database. However, as required by FFDCA section 408(p), th idiazuron is subject to the e ndocrine screening part of the Endocrine Disruptor Screening Program (EDSP).

EPA has developed the EDSP to determine whet her 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 ot her such endocrine effect s as the Administrator may designate.” The EDSP employs a two-tiered approach to making th e 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 estrog en, androgen, or thyroid (E, A, or T) hormonal systems. Chemicals that go th rough Tier 1 screening and are found to have the potential to interact with E, A, or T hormonal systems will proceed to the next stag e of the EDSP where EPA will determine which, if any, of th e Tier 2 tests are necessary base d on the available data. Tier 2Page 30 of 45 

### testing is designed to identify a ny adverse endocrine-related eff ects 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 pest icide active ingredient s and 9 inert ingredients. A second list
of chemicals identified for EDSP sc reening was published on June 14, 2013 3 

### and includes some pesticides scheduled for registration review and chemicals found in water

. Neither of these lists
should be construed as a list of know n or likely endocri ne disruptors.

For further information on the status of the ED SP, the policies and procedures, the lists of chemicals, future lists, the test guidelines and the Tier 1 screening ba ttery, 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 classi fications and whether or not furt her 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 MR ID that offer data for each guideline requirement, as well as study classi fications and whether or not furt her data are needed in order to support risk assessment. Rationale for the additional data requested is presented below.

 Thidiazuron may cause exposure of pollinators visi ting 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 to xicity of residues on foliage (OCSPP 850.3030)

is not needed since results of the honey bee acute contact LD 50 

### is >100 μg a.i./bee, well above the 11 μg a.i./bee trigger criterion and thidiazur on 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 studie s, 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 (sp ecial study renamed as honeybee larval chronic oral toxicity study, Tier I)

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

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

o Field trial of residues in pollen a nd 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 LD 50 

### (acute endpoint of interest) and a NOAEC (chronic endpoint of interest) from aPage 32 of 45 

### single study. If the chronic larval toxicity st udy is conducted in such a way that it derives
both an LD 50 

### and NOAEC, then a separate acute larv al toxicity study is not needed. For pollinator studies without finalized OECD gui delines, protocols should be submitted for EPA review and comment prior to study initiation.

The need for higher Tier semi-field and full fi eld 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 indicate s thidiazuron has low toxicity to upland game bird species, it is uncertain whether passeri nes 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 numb er 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 45Table 21. Submitted Aquatic Ecological Effects Data for ThidiazuronOCSPPGuidelineDataRequirementSubmittedStudies(MRID)StudyClassificationsAre dataneeded forassessment?Justification and Assumptions EPA will Makein Absence of Data850.1010Freshwaterinvertebrate acutetoxicity46203503(TGAI)Acceptable No --099819(AEF132347)6203509(AEF132345)46203516(AEC421200)850.1025850.1035Estuarine/Marineinvertebrate acutetoxicity4213200141846101Acceptable No --850.1075Freshwater fishacute toxicity42069202(TGAI)Acceptable No --42069201(AEF132347)46203515(AEF132345)46203511(AEC421200)850.1075Estuarine/Marinefish acute toxicity41846101 Acceptable No --850.1300Freshwaterinvertebrate lifecycle42132002 Acceptable No --Page 34 of 45Table 21. Submitted Aquatic Ecological Effects Data for ThidiazuronOCSPPGuidelineDataRequirementSubmittedStudies(MRID)StudyClassificationsAre dataneeded forassessment?Justification and Assumptions EPA will Makein Absence of Data850.1350Estuarine/Marineinvertebrates lifecycleNo data N/A NoNo chronic data is available for estuarine/marineorganisms. However, EFED is not asking for thechronic studies and will use the freshwater datato make a conservative assumption of sensitivityfor estuarine/marine organisms.850.1400Estuarine/Marinefish early-life stageNo data N/A No850.1400Freshwater fishearly-life stage42273001 Acceptable No --850.1500 Fish life cycle (None) (N/A) NoNo LOC exceedances were observed withaquatic organisms thus a FLC study is notrequired.850.4400Aquatic plantgrowth: Vascularplants46203506 Acceptable No --850.4500Aquatic PlantGrowth: Algal4620350541761104(TGAI)Acceptable NoNo data is available for freshwater diatom.However, EFED is not asking for the freshwaterdiatom study since the data need to be 10x moretoxic to exceed the LOCfor aquatic non-vascularplants which is unlikely.46203510(AEF132347)Supplemental No46203517(AEF132345)Supplemental No46203513(AEC421200)Supplemental No850.4550Aquatic PlantGrowth:Cyanobacteria46203504 Supplemental No --Page 35 of 45Table 22. Submitted Terrestrial Ecological Effects Data for ThidiazuronOCSPPGuidelineData RequirementSubmittedStudies(MRID)Study ClassificationsAre dataneeded forassessment?Current Additional Data Need850.2100 Avian oral toxicity 099819 AcceptableYesA passerine bird study is necessary tosatisfy standard data requirement.Without data, risk would be presumedfor small-sized birds, including listedspecies.850.2200Avian dietarytoxicity008162946203502099819245834AcceptableAcceptableAcceptableAcceptableNo --850.2300 Avian reproduction4809870448098705AcceptableAcceptableNo --NonguidelineAvian inhalation (None) (N/A) NoThe STIR model indicates exposurethrough inhalation is not a concern forbirds.850.3020Adult honey beeacute contacttoxicity4620350146203518AcceptableAcceptableNo --850.3030Honey bee residueon foliageNo data No data NoNo data is available; however, EFED isnot asking for the toxicity of residue onfoliage since the bee contact LD50was>100 μg a.i./bee, which is well above the11 μg a.i./bee criterion that triggers thestudy.Page 36 of 45Table 22. Submitted Terrestrial Ecological Effects Data for ThidiazuronOCSPPGuidelineData RequirementSubmittedStudies(MRID)Study ClassificationsAre dataneeded forassessment?Current Additional Data Need850.3040Field testing forpollinatorsNo data No dataYesPollinator Tier I, II and III studies withthe honeybees are necessary to satisfystandard data requirement. Withoutdata, risk would be presumed forpollinators, including listed species.SSLarval honey beechronic oral toxicityNo data No dataYesSSAdult honey beechronic oral toxicityNo data No dataYesSSLarval honey beeacute toxicityNo data No dataYesSSAdult honey beeacute oral toxicityNo data No dataYesSSField testing ofresidues in pollenand nectarNo data No dataYesSSSemi-field testingfor pollinatorsNo data No dataYes850.4100Tier I or II seedlingemergence4182130145908501AcceptableAcceptable/SupplementalNo--850.4150Tier I or IIvegetative vigor4181910145921501AcceptableAcceptable/SupplementalNoPage 37 of 45 

## 8. References

USEPA 2005. Environmental Fate and Effects Division Revised Risk Assessment for the Reregistration Eligibility Decision of Thid iazuron for Use on Cotton. D319339. September 15,
2005Page 38 of 45Appendix I. IncidentsEIIS Pesticide Summary Report: General InformationThidiazuron (120301)Incident #DateCountyStateCertaintyLegal.Formul.Appl. MethodTotal MagnitudePLANTSCottonI016036-02410/22/2004RiversideCA2RUSpray53 acresN/RI013645-0029/30/2002FresnoCA3MSpray150 acresRyeI021411-01910/20/2009HalifaxNC2RUSCN/R75.0 acresTERRESTRIALAgricultural AreaI005204-0149/18/19963MAFBroadcast9Wednesday, April 22, 2015Certainty Code: 0=Unrelated,1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable.Page 1 of 1Legality Code: RU=Registered Use,M=Misuse, MA=Misuse (Accidental),MI=Misuse (Intentional), U=Unknown.Page 39 of 45EIIS Pesticide Summary Report: Species InformationThidiazuron (120301)Incident #SpeciesScientific NameMagnitudeResponseRt. ExposurePLANTSCottonI016036-024lettuce53 acresplant damageDrift, sprayN/RI013645-002lettuce50 acresplant damageDrift, sprayRyeI021411-019ryegrass100% of 75 acresplant damageTreated directlyTERRESTRIALAgricultural AreaI005204-014chickenGallus gallus9mortalityUNKNOWNn/rn/rmortalityOVERSPRAYEDWednesday, April 22, 2015Page 1 of 1Page 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 pe a

is presented in Figure 1. These values were generated with the Surface Water Concentration Calcula t (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 t o 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 v a
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 pesti c

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 289Page 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 ConcentrationsPage 42 of 45 

# Appendix III. STIR

Input

Application and Chem ical 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 Molecu lar 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 LD 50 (mg/kg bw) 3160 Enter Mineau Scaling Factor 1.15 Enter Tested Bird Weight (kg) 0.178

Mammal

Enter Lowest Rat Oral LD 50 (mg/kg bw) 2000

Enter Lowest Rat Inhalation LC 50 (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/m 3 ) 3.11E-07 Maximum 1-hour Vapor Inhalation Dose (mg/kg) 3.90E-08

Adjusted Inhalation LD 50 2.11E+01
Ratio of Vapor Dose to Adjusted Inhalation LD 50 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
LD 50 9.09E-04 Exposure not Likely Significant



### Results Mammalian (0.015 kg )
Maximum Vapor Concentration in Air at Saturation (mg/m 3 ) 3.11E-07 Maximum 1-hour Vapor Inhalation Dose (mg/kg) 4.91E-08

Adjusted Inhalation LD 50 1.43E+02
Ratio of Vapor Dose to Adjusted Inhalation LD 50 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
LD 50 1.69E-04 Exposure not Likely SignificantPage 43 of 45 

# Appendix IV. SIP

Table 1. Inputs

Parameter Value
Chemical name Thidiazuron Solubility (in water at 25 o C; mg/L) 31

Mammalian LD 50 (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 LD 50 (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 (photoproduc t I) CAS Reg. No. 71769-74-9

Photoproduct II (phenylurea) CAS Reg. No. 64-10-8Page 45 of 45 

# Appendix V. Degradate Structures

Thidiazolurea (AE 132345)