U. S. ENVIRONMENTAL PROTECTION AGENCY 

Washington, D.C. 20460 

OFFICE OF *\TED S74? % PREVENTION, PESTICIDES ,.$a $ AND TOXIC SUBSTANCES a=.: 3. 

9 4' PRflc~' 8 

Date: April 5,2005 

Chemical: Thidiazwon 

PC Code: 120301 

DP Barcodes: D289980 

D29 1684 

D294536 

D298740 

D294537 

MEMORANDUM 

SUBJECT: Environmental Fate and Effects Division's Phase 2 Risk Assessment for the 

Reregistration Eligibility Decision of Thidiazuron for Use on Cotton 

TO: John Pates, Jr.; Chemical Review Manager; Reregistration Branch I1 

Special Review and Reregistration ~iviiion (7508~) 

FROM: William Evans, Biologist 

lex Clem, Environmental Scientist 

3- & cp" )il'ho"5 

es Hetrick, Ph D., Senior Physical Scientist 

ward Odenkirchen, Ph.D., Senior Biologist /F~~ 6' V77d I Environmental Risk Branch I 

Environmental Fate and Effects Division (7507C) I 

I APPROVED 

BY: Sid Abel, Branch Chief, Environmental Risk Branch I 

Environmental Fate and Effects Division (7507C) ,/ 

-\ 

This memorandum summarizes the attached Environmental Fate and Effects Division's (EFED) 

Phase 2 Thidiazwon Environmental Risk Assessment for a proposed Reregistration Eligibility 

Decision (RED) for use on cotton. The memorandum and attached assessment differ slightly 

from their previous analogs in that we have made appropriate corrections and modifications that 

were prompted the registrant's (Bayer Cropscience) comments in their Phase 1 Error Correction 

document dated February 4,2005. We are pleased to report that the errors Bayer detected 

essentially did not affect the risk characterization and conclusions we reached previously. 

Text Searchable File
Use of Thidiazuron. Thidiazuron is currently registered for use as a defoliant (herbicide) to aid 

in the harvest of cotton. The maximum single application rate is 0.2 Ib ai/A. A second 

application is recommended under low temperature conditions when defoliation does not reach 

the expected level, but the total of the two applications must not exceed 0.3 Ib ai/A. The 

intervals between applications range from 5-7 days in some of the labels, while the other labels 

make no recommendations in this regard. 

Environmental Fate Summary 

In soil, thidiazuron is persistent, as evidenced 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). Such persistence and intermediate mobility 

would allow some year to year accumulation over time and, therefore, more opportunity for 

favorable conditions for runoff from application sites to occur. For example, with a one year soil 

half-life and without redistribution or dispersal from the site of application, theoretical build-up 

in soil would asymptotically approach twice that of annually applied amounts. Of course, given 

time and variable climatic conditions, redistribution or partitioning of thidiazuron into other 

environmental compartments does occur (via runoff, leaching, plant uptake, etc.); therefore, 

maximum build-up in surface soil averages less than the theoretical factor of two. 

When thidiazwon 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 is a structural isomer of parent, whereas the other 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 idon leaves may be an important fate 

pathway. The effects of this uncertainty and alternative assumptions are considered in the risk 

assessment. 

Ecological Risk Summary 

This assessment is based on mainly on toxicity testing with typical end use products 

(formulations) containing thidiazuron as the only active ingredient. In addition to parent 

thidiazuron, we have also factored into our assessment limited data for three of its transformation 

products (degradateslmetabolites). On the basis of testing only with thidiazuron and its 

degradates, apparent risks may be underestimated because: 1) in five of twelve registered 

products thidiazwon is co-formulated with either of two other registered herbicides, and there are 
no test data for any of these co-formulated products; 2) there are no data for mixtures with 

adjuvants (tank-mix data), which product labels state improve performance; and 3) there are no 

test data with mixtures that include organophosphate pesticides, which labels state may increase 

phytotoxicity. A fourth reason that risk may be underestimated is that product label statements 

about citrus and cantaloupe, for which we have no test data, clearly imply that these or other 

plant species may be more sensitive than species for which we have test data. Thus, the absence 

of phytotoxicity data for the cited mixtures and the absence of data for plants that are apparently 

more sensitive than those tested are substantial sources of unquantified uncertainties that would 

reasonably and most likely lead to risk greater than ow estimations. 

Highlights of our ecological risk concerns, based on toxicity testing with typical end-use products 

containing thidiazuron as the only active ingredient and the modicum of data on three 

transformation products, are the following: 

b Non-target Plants: Non-target terrestrial and semi-aquatic dicots, but not monocots, are at 

potential direct and indirect ecological risks. Aquatic plants do not appear to be at risk. 

b Mammals: Mammals are at potential chronic risk, but not acute. 

b Birds: Potential chronic risk to birds cannot be determined because of the absence of 

chronic data. Potential acute risks to birds are below all levels of concern. 

A principal ecological risk concern for the proposed thidiazuron use on cotton is the potential 

risks to non-target plants. If non-target plants come into contact with thidiazuron, these plants 

may be killed or damaged enough to prevent the plant from reproducing or successfully 

competing with other plants for resources. The use of thidiazuron could apply selective pressure 

to dicots along field edges, resulting in changes in species composition. 

Even though thidiazuron is only applied at cotton harvest, application timing is a consequential 

factor to consider. Reproduction abnormalities are some of the plant injuries that can occur due 

to thidiazuron exposure during particular developmental stages. Although the plant may survive, 

sterile florets or non-viable seed production can occur. This will have effects on the non-target 

plant populations in future years. Also, plant material serves as a primary food source for many 

species of animals. If the available plant material (including seeds) is reduced due to the effects 

of thidiazuron, this may have negative effects throughout the food chain. 

The risk assessment for terrestrial plants was based on risk quotients (RQs) calculated from 

toxicity studies using a typically formulated end-use product (TEP). However, it appears that no. 

surfactants or other adjuvants were added to the product during the studies, and all labels clearly 

state the addition of adjuvants has been shown to improve the performance. Therefore, the risk 

to terrestrial non-target plants may be greater than thidiazuron RQs indicate. Hence, the 

production of actual tank-mix data would reduce some of the uncertainties associated with risk to 

plants. 
Additionally, in four products thidiazuron is co-formulated either with the herbicide diuron or the 

herbicide dimethipin (see table in Use section of this document). In these products, diuron 

comprises 33% and thidiazuron 67% of the total active ingredients. In the other formulation, 

dimethipin comprises 80% of the total active ingredients, while thidiazuron comprises only 20%. 

Potential effects of the addition of these other two chemicals are not factored into the risk 

assessment. Combining (adding) known effects of these other two chemicals with those for 

thidiazuron, or, alternatively, producing specific test data for these mixed products would reduce 

the risk uncertainties and more accurately portray the risk associated with their co-application. 

It is also important to note that all labels list precautions when applying the products adjacent to 

lettuce, citrus, or cantaloupe. The labels further state that mixing with organophosphates may 

increase non-target crop phytotoxicity. Special restrictions to reduce drift to lettuce and citrus 

appear on all the labels. In addition, an incident was reported to have caused an estimated 

$350,000 of crop loss to 50 acres of lettuce when thidiazuron allegedly drifted onto 150 acres. 

Thidiazuron products can not be applied by air within '/z mile of lettuce or 5 miles downwind of 

the point of application when citrus is in flush (burst of new growth, as in springtime) in the Rio 

Grande valley of Texas. Ground applications are restricted to 100 feet away from lettuce and 

% mile downwind from the point of application for citrus. Labels do not specify a distance for 

cantaloupe. EFED currently has no toxicity data for citrus or cantaloupe, and the data on lettuce 

indicate that it is the most sensitive of all the crops tested thus far. Therefore, toxicity data on 

cantaloupe and citrus (which has a much larger spraydrift buffer distance than lettuce), may 

indicate that risk is even higher than indicated by the lettuce RQs. 

Acute risk quotients for mammals did not exceed levels of concern (LOCs). However, chronic 

risk quotients for mammals did exceed LOCs, with RQs ranging from 6.3 (15-g mammal feeding 

on short grass) to 0.01 (1000-g mammal feeding on seeds). To further investigate the chronic 

risk to mammals, in addition to the standard default foliar dissipation half-life of 35-days, EFED 

evaluated the impact of a hypothetical half-life as low as one day on thidiazuron residue 

concentrations on reducing risk. We also evaluated potential effects at predicted maximum and 

mean residues for single applications. In these scenarios, chronic RQs would be reduced to the 

range of 2.4 (15-g mammal feeding on short grass) to 0.002 (1000-g mammal feeding on seeds), 

but still would exceed chronic LOCs. 

The 3-generation reproduction study with rats indicates a possible concern for endocrine 

disruption, based on significant effects on mean litter size in the F3b generation. When 

appropriate screening and/or testing protocols have been developed, thidiazuron and its 

degradates may be subject to additional screening andlor testing to better characterize effects 

related to endocrine disruption in mammals or other organisms. 

Acute risk quotients for birds did not exceed LOCs. Chronic risk to birds cannot be evaluated 

because chronic data were not submitted. 
Data Gaps 

Environmental Fate: Status of Envivonmental Fate Data. Although some of the submitted fate 

studies had deficiencies, taken as a whole and within the constraints stipulated in the fate 

assessment, the data enable the Agency to adequately assess the environmental fate of 

thidiazuron. Although no foliar dissipation studies are currently available to the Agency (these 

are not generally required), having such data would make any fate assessment more precise, 

especially in the case of a defoliant such as thidiazuron. However, as discussed in the fate 

assessment, if dislodgement from or transformation idon treated plants were to be an important 

fate pathway, then EFED's inferences from existing fate studies and assumptions for exposure 

modeling for parent and potential degradates should reasonably and conservatively cover the 

possibility. If the registrant wishes to demonstrate that definitive data on foliar dissipation would 

substantially affect any of our inferences, assumptions, or environmental risk conclusions, then 

the registrant should submit appropriate data. Otherwise, except for spray-drift requirements, the 

Agency needs no additional environmental fate studies at this time. 

Ecological Effects: The ecological toxicity data base is fairly complete, however, there are some 

additional key studies that could assist in reducing the uncertainties in this risk assessment. The 

details are summarized below. 

Non-target Terrestrial Plant - Definitive tier 11 EC25s and NOAECs are lacking for the 

vegetative vigor of all the monocots tested and for the seedling emergence of two 

monocot species tested; this is because test concentrations were not low enough to 

establish these endpoints. However, risk quotients from the tests for which definitive 

endpoints were established were below levels of concern. Furthermore, concentrations in 

those tests for which endpoints were not determined were sufficiently low to establish 

that, when compared to estimated field exposure concentrations that correspond to the 

currently-labeled application rates, risk quotients would not exceed any of the terrestrial 

plant LOCs. Therefore, since projected risk quotients for monocots are below levels of 

concern, additional tier I1 testing with a typical end-use product that contains thidiazwon 

as the only active ingredient is not required at this time for these specific crops at 

currently-labeled application rates. 

However, there are other considerations. Even though all labels clearly state that the 

addition of adjuvants has been shown to improve performance, it appears that none were 

added the during the tests. Therefore, apparent risk to terrestrial non-target plants may be 

greater than present RQs indicate. The production of actual tank-mix data would reduce 

this uncertainty. 

Additionally, as previously mentioned, five of twelve registered thidiazuron products are 

mixtures with other herbicides. Four of the twelve are mixtures of thidiazuron and 

diuron, with thidiazuron comprising 67% and diwon 33% of the total active ingredients 

The fifth product is a formulation of thidiazuron (20% of total active ingredients) and 

preponderantly dimethipin (80% of total active ingredients). Potential effects of the 
mixtures with these other two chemicals are not factored into the risk assessment. 

Combining (adding) the known effects for these other two chemicals with those for 

thidiazuron, or, alternatively, producing specific test data for these mixed products would 

reduce the uncertainties associated with their co-application and thereby more accurately 

portray risk. 

Concerning the potential for additional, undetermined risk, the current labels contain 

warnings and specific instructions about the use of thidiazuron in the vicinity of lettuce, 

cantaloupe, and citrus. The language from one of the labels is the following: 

"Particular care should be taken when applying DroppB UltraTM adjacent to 

lettuce, citrus, or cantaloupe. " 

"In addition, for citrus crops, do not apply DroppB UltraTM by air when citrus in 

flush is within Jive (5) miles downwind of the point of application. Do not apply 

DroppB UltraTM by ground when citrus in flush is within one-half(l/2) mile." 

Labels also state that "do not app1y"distances hom lettuce are one-half (112) mile by air 

and 100 feet by ground. 

Despite these warnings, no seedling emergence or vegetative vigor data have been 

submitted for cantaloupe or citrus. Data on citrus and cantaloupe would reduce 

uncertainty about potential increased risk by providing a better species-sensitivity 

distribution for non-target plants. Therefore, EFED recommends definitive tier 11 studies 

for thidiazuron on citrus and cantaloupe, preferably with adjuvants and any other coformulated active ingredient added as part of the test materials. 

Birds - Chronic risks to birds can not be quantitatively evaluated at this time because 

chronic data were not submitted. In order to assess this potential risk, we recommend 

chronic testing with an upland game-bird species (bobwhite quail) and a waterfowl 

(mallard duck). 

Recommended Label Language 

If thidiazuron is reregistered, the following statements should be included in the 

"ENVIRONMENTAL HAZARDS" labeling: 

For Manufacturing-Use Products 

"Do not discharge effluent containing this active ingredient into lakes, streams, ponds, estuaries, 

oceans, or other public waters unless this product is specifically identified and addressed in an 

NPDES permit. Do not discharge effluent containing this product into sewer systems without 

previously notifying the sewage treatment plant authority. For guidance, contact your State 

Water Boardor Regional Office of EPA." 
! 

For End-Use Products 

"Do not apply directly to water or to areas where surface water is present or to intertidal areas 

below the mean high water mark. Do not contaminate water when disposing of equipment 

washwater or rinsate." 

Surface Water Advisory 

"This product may contaminate water through drift of spray in wind. This product has a high 

potential for runoff for several months or more after application. Poorly draining soils and soils 

with shallow water tables are more prone to produce runoff that contains this product. A level, 

well-maintained vegetative buffer strip between areas to which this product is applied and 

surface water features such as ponds, streams, and springs will reduce the potential for 

contamination of water from runoff. Runoff of this product will be reduced by avoiding 

applications when rainfall is forecasted to occur within 48 hours. Sound erosion control 

practices will reduce this product's contribution to surface water contamination." 

Groundwater Advisory 

"This chemical has properties and characteristics associated with chemicals detected in ground 

water. Use of this chemical in areas where soils are permeable, particularly where the water table 

is shallow, may result in groundwater contamination." 

Spray Drift Management 

Drift is transport of pesticides through air away from the target site, and includes, for example, . 

spray drift and volatilization. Spray drift, the movement of pesticide droplets off-target during or 

shortly after application, has been well studied, and is not dependent on the properties of the 

active ingredient. Short-range spray-drift and resulting exposures to non-target organisms is 

quantified in our risk assessments. Currently, when specific label instructions are not available, 

we account for spray drift in both drinking water and ecological exposure assessments by the use 

of default assumptions for the percentage of deposition based on application method. We have 

established default spray drift percentages for pesticides for aerial application, ground 

application, and orchard airblast. 

Default values for drinking water exposure scenarios are 16%, 6.4%, and 6.3% of the application 

rate for aerial, ground boom and airblast applications, respectively. The default values for the 

drinking water exposure were generated using AgDRlFT version 1.03 which uses a semimechan(stic model (AGDISP) for aerial applications and is based an empirical data set (Spray 

Drift Task Force data) for ground boom and airblast applications. The derivation of the spray 

drift loading values is described in Appendix B of the Guidance for Use of the Index Reservoir in 

Drinking Water Exposure Assessments and is based on the following application and wind speed 

assumptions: 
For ground boom applications 

nozzle height no more than 4 feet above the ground or crop canopy 

wind speed is 10 mph or less at the application site and, 

medium or coarser spray according to ASAE 572 definition for standard nozzles. 

For aerial applications 

boom width less than 75% of the wingspan or 90% of the rotary blade, 

use of upwind swath displacement, 

wind speed 3 - 10 mph, and 

use medium or coarser spray according to ASAE 572 definition for standard nozzles. 

For orchardhineyard airblast applications 

spray is not directed above treeslvines, 

outward pointing nozzles are shut off at row ends and outer rows, and 

wind speed is 3 -10 mph at the application site as measured by an anemometer outside of 

the orchard/vineyard on the upwind side. 

Default values for ecological exposure scenarios are 5% for aerial applications and 1% for 

ground applications, with no set value for orchard airblast. For the ecological assessment, the 

default values of 5% for aerial applications and 1% for ground applications were established 

based on open literature and research prior to establishment of the Spray Drift Task Force, and 

were therefore not generated using AgDRIFT. Testing of the default spray drift values for 

ecological assessments using AgDRIFT provides context for the default assumptions. Analysis 

of the 1 % ground application drift percentage using AgDRlFT (with the same assumptions as for 

drinking water outlined above) indicates that the default value likely underestimates drift from 

single events under relatively high drift conditions. A similar underestimation likely occurs for 

the 5% aerial drift value. 

EECs predicted for drinking water and ecological assessments are based on the assumptions of 

drift outlined above for ground and aerial applications. Variance from these assumptions will 

result in differences in predicted concentrations. For the ecological assessment, as with the 

drinking water assessment, variance from the assumptions behind these default values could 

result in increased risk. In general, a decrease in droplet size or increase in wind speed at the 

time of application will result in higher predicted EECs and risk to non-target organisms. 

Alternatively, if droplet size is coarser or wind speeds are lower, exposures due to drift would be 

lower. 

Transport resulting from volatilization is highly dependent on the properties of the active 

ingredient (e.g., Henry's Law constant) as well as a number of environmental parameters. Based 

on its physicochemical properties, thidiazuron is not expected to volatilize. 
Phase 2 

Environmental Fate and Effects Division's Risk Assessment 

for the 

Reregistration Eligibility Document For the Use of 

Thidiazuron on Cotton 
Table of Contents 

I . Environmental Risk Conclusions .................................. Page 4 of 134 

Environmental Fate Abstract .............................. Page 4 of 134 

Terrestrial Plant Risks of Concern .......................... Page 4 of 134 

Terrestrial Vertebrate Risks of Concern ..................... Page 5 of 134 

11 . Introduction .................................................... Page6of 134 

Pesticide Mode of Action ....................................... Page 6 of 134 

Uses and Use Characterization .................................. Page 7 of 134 

Risk Assessment Approach ..................................... Page 9 of 1.34 

Assessment Endpoints ................................... Page 9 of 134 

Measures of Effects ..................................... Page 9 of 134 

Measures of Exposure .................................. Pare 1 1 of 134 

Aquatic Organisms ............................... Page 1 1 of 134 

Active Ingredient and Degradate Aquatic Organism Exposure Approach 

........................................ Page 11 of 134 

Terrestrial Plants .............. -;................. Page 1 2 of 1 34 

Terrestrial Animals .............................. Page 13 of 134 

Risk Characterization Approach ................................ Page 13 of 134 

Problem Formulation Analysis Plan ............................. Page 1 3 of 1 34 

111 . Integrated Environmental Risk Characterization ....................... Page 17 of 134 

Risks to Aquatic Organisms .................................... Page 17 of 134 

Risk to Birds . ; ............................. ;......... Page 1.7 of 134 

Acute Risks - Birds .............................. Page 17 of 134 

Chronic Risk . Birds ............................. Page 1 8 of 1 34 

Risks to Mammals ........................................... Page 18 of 134 

Acute Risks . Mammals ................................. Page I . 8 of 134 

Chronic Risk . Mammals .......................... Page 18 of 134 

Risks to Non-Target Insects .................................... Page 20 of 1.34 

Risks to Terrestrial Plants ..................................... Page 21 of 134 

Endocrine Diskption Assessment ............................... Page 43 of 134 

Endangered Species Assessment ................................ Pace 44 of 134 

IV . Environmental Fate Assessment ................................... Page 45 of 134 

Basis ................................................ Page45of 134 

Fate Assessment ....................................... Page 46 of 134 

Abstract .............................................. Page 46 of 134 

Physical/Chemical Properties ....................... Page 47 of 134 

Transformation Products (degradateslmetabolites) ...... Page 5 1 of 134 

V . Drinking Water Assessment Summary ............................... Page 54 of 134 

VI . Aquatic Hazard. Exposure. and Risk Quotient Calculation ............... Page 56 of 134 

Toxicity to Fish ............................................. Page 56 of 134 

Page 2 of 134 
I 

Toxicity to Aquatic Invertebrates ................................ Page 56 of 134 

Toxicity to Aquatic Plants ..................................... Page 57 of 134 

Aquatic Exposure ............................................ Page 57 of 134 

RiskQuotients ........................................ Page58of 134 

VII . Terrestrial Hazard. Exposure. and Risk Quotient Calculation ............ Pawe 58 of 134 

Toxicity to Birds ............................................ Page 58 of 134 

Toxicity to Mammals ......................................... Page 58 of 134 

Toxicity to Non-target Insects .................................. Page 59 of 134 

Toxicity to Earthworms ....................................... Page 60 of 134 

Toxicity to Non-Target Plants ............................ Page 60 of 134 

Terrestrial Plant Exposure and Risk Quotient Calculations ..... Page 61 of 134 

Terrestrial Exposure for Birds and Mammals ................ Page 63 of 134 

Terrestrial Risk Quotients ............................... Page 65 of 134 

APPENDIX C: Model Input Parameters Derived fiom Environmental Fate Studies 

for Thidiazuron ............................................. Page 8 1 of 134 

APPENDIX LIST 

APPENDIX A: Detailed Drinking Water Assessment Memo ................ Pape 66 of 134 

APPENDIX B: Chemical Structural Formulas. Names .................... Pawe 78 of 134 

APPENDIX D: GENEEC Input & Output Files for Ecological Assessment .... Page 84 of 134 

APPENDIX E: Ecological Hazard Data ................................ Page 89 of 134 

APPENDIX F: The Risk Quotient Method ............................. Page 106 of 134 

APPENDIX G: Detailed Risk Quotients .............................. Pave 109 of 134 

APPENDIX H: Data Requirements for Thidiazuron ...................... Page 129 of 134 

Page 3 of 134 
I. Environmental Risk Conclusions 

Environmental Fate Abstract 

In soil, thidiazuron is persistent, as evidenced 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). Such persistence and intermediate mobility 

would allow some year-to-year accumulation over time and, therefore, more opportunity for 

favorable conditions for runoff from application sites to occur. 

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 is a structural isomer of parent, whereas the other has a substantially altered 

chemical structure. The photolysis process requires special exposure and modeling 

considerations. 

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

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

thidiazuron should not bioconcentrate. 

No data on foliar dissipation are currently available to EFED. Since thidiazuron is used to 

defoliate cotton prior to harvest, dislodgement from or transformation idon leaves may be an 

important fate pathway. The effects of this uncertainty and alternative assumptions on the risk 

assessment are considered. 

Terrestrial Plant Risks of Concern 

Thidiazuron poses potential risks to terrestrial plants based on the results of a screening-level 

assessment. The risk quotients exceed the levels of concern for non-endangered and endangered 

plants adjacent to treated sites as well as plants inhabiting semi-aquatic sites. This is especially 

true for dicots for both single and multiple applications. The RQs resulting from single 

applications at the maximum label rates range from 0.39 to 9.09 for non-endangered plants and 

up to as high as 52.63 for endangered plants. The RQs for multiple applications range from 0.59 

to 9.09 for non-endangered species and up to 52.63 for endangered species. 

In addition to the RQ calculations from the maximum label rates, EFED completed a 

characterization of potential risks by assessing the effect of RQ calculations using EECs 

predicted from modeling with average application rates (0.05 - 0.07 lb ai/A) as reported in the 

BEAD QUA report. When the average application rates are compared to the maximum 

allowable application rates, RQs are reduced by at least one-half, but levels of concern are still 

exceeded, except for the single ground application for non-endangered species scenario. 

It should be noted that RQs for this risk assessment were based on toxicity studies using the 

typical end-use product "Thidiazuron SC42" (presumably DROPP SC (Reg. #: 264-700)). All 

labels clearly state the addition of adjuvants has been shown to improve the performance, and yet 

Page 4 of 134 
no surfactants or adjuvants were added to the product during the studies. Therefore, the risk to I terrestrial non-target plants may be greater than RQs indicate. The production of core data using 

actual tank-mixes may assist in reducing some of the uncertainties associated with risk. 

I Additionally, as previously noted, four thidiazuron products are mixtures (co-formulations) of 

thidiazuron (67% of total active ingredients) and diuron (33% of total active ingredients), and 

still another is a combination of thidiazuron (20% of total active ingredients) and preponderantly 

dimethipin (80% of total active ingredients) (see table in section on Use and Use 

Characterization). Adding the known effects of these other two chemicals to those for 

thidiazuron, or, alternatively, having test data specifically for these products would reduce the 

uncertainties associated with their co-application and more realistically portray risk. 

Since the sole use of thidiazuron is as a cotton defoliant, the primary concern is for non-target 

plants which inhabit areas near cotton fields. If such plants are sensitive to leaf drop and other 

symptoms which may result from thidiazuron exposure, death, severe damage, or reproductive 

impairment could occur. This damage could apply selective pressure to dicot plants along field 

edges and changes in species composition could occur. 

There is another reason to believe that our assessment may underestimate risk to terrestrial 

plants. All labels list precautions when applying the products adjacent to lettuce, citrus, or 

cantaloupe. The labels further state that mixing with organophosphates may increase non-target 

crop phytotoxicity. Special restrictions to reduce spraydrift potential to lettuce and citrus appear 

on all the labels. Generation of toxicity data for these crops and other plant species could assist in 

better characterizing the risks to non-target plants. 

To explore the apparent risk posed by spraydrift, we used EFED's current AgDRlFT computer 

model and thidiazuron phytotoxicity data to estimate plant exposure as a function of downwind 

distances from application areas. Since the current labels lack specific information on droplet 

size or boom height, we also looked at the effects of varying these. We then compared the 

various exposure distributions with toxicity level percentiles from the EClO to EC90 (in 

increments of ten). I 

As can be seen from the sequence of the following bar graphs, there is considerable variation 

with droplet size or boom height for the EC30 (the closest toxicity level to the EC25). The nospray zones for aerial applications for the EC30 effect levels range from slightly over 60 feet to 

greater than 1000 feet for lettuce, the most sensitive crop tested so far. Ground application nospray zones for the EC30 vary from about 3 to 200 feet. Similar trends are demonstrated for the 

other crops for which slope data could be determined. In addition, according to the labels, 

cantaloupe and citrus many be more sensitive than lettuce, and no data have been submitted on 

these crops. All the factors influencing the downwind drift distance, including droplet size, wind 

speed, boom height, method of application, and targeted toxidty effect should be considered 

when placing restrictions on labels. 

Terrestrial Vertebrate Risks of Concern 

I 

Page 5 of 134 
Thidiazwon may pose chronic risk to mammals, based on a screening-level assessment. 

Conclusions are derived from a three-generation rat study, the only chronic study currently 

available to EFED. From the study, the reproductive NOAEC and LOAEC values are 200 and 

600 mglkg-diet, respectively, based on reduction in litter size in the F3b generation. When 

standard conversion factors are employed, the respective NOAEL and LOAEL values are 10 and I 30 mglkg-bwlday. To estimate risk, we used two screening-level methodologies, and found that 

they give different results. 

Preferably, EFED uses the NOAEL of 10 mglkg-bwlday as the evaluation endpoint for RQ 

calculations based on the Nagy small mammal assumptions of percent body weight consumed for 

the 15,35, and 1000 g mammals. We then compare the NOAEL (expressed as mglkg-bwlday) to 

a dietary EEC adjusted for food consumption. When this method is used, the LOCs are exceeded 

I for herbivores/insectivores foraging in short grass, broadleaf forage, and small insects. The 

highest RQ is 8.5 (15-g mammal, short grass). EFED believes this analysis is reasonable because 

the percent reproductive impairment that is significant to a given species would be based on the 

reproductive strategy of that species. For long-lived species (K-selected), episodic reductions in 

reproduction might not be of the same significance as short-lived species (r-selected). A shortlived species reproductive impairment could be enough to igpact any number of species with 

low generational survival rates and a dependance on high reproductive output for population 

I continuance (r-selected species). 

Another, less preferable, method of calculating risk quotients is to directly compare the NOAEC 

(expressed as mglkg-diet) to the dietary EEC for all the forage groups. Using the NOAEC of 200 

mg/kg-diet, chronic LOCs are not exceeded for any of the food categories. 

11. Introduction 

Thidiazuron is undergoing review for reregistration under Section 4 of FIFRA as amended. It is 

currently registered for use as a defoliant to aid in the harvest of cotton. Defoliation, or leaf 

abscission, is a natural process in all plants, but can be manipulated in cotton plants to cause 

them to drop their leaves in a relatively short period of time to aid in the efficiency of cotton 

harvesting. 

Pesticide Mode of Action 

Thidiazuron is a phenylurea herbicide that is said to inhibit regrowth through photosynthetic 

electron transport inhibition at the photosystem 11 receptor. This mode of action increases the 

ethylene concentration in the leaves; this inhibits the transport of the hormone auxin, and results 

in leaf drop. 

While the defoliation process is quite complex, it can be simplified as being governed by two 

major compounds in the plant, auxin and ethylene. Auxin is a growth-promoting hormone that 

stimulates leaf growth and development. Ethylene can be classified as a ripening agent that 

causes leaf drop. Leaves fall from the plant once ethylene moves fi-om the leaf blade to the base 

of the petiole and stimulates the formation of an abscission (separation) layer. The amount of 

auxin or ethylene present in the leaves of the cotton plant is related to leaf age. Younger leaves 

Page 6 of 134 
I I 

have a higher level of auxin, whereas older leaves have lower levels of auxin and higher levels of 

I ethylene. This is why older leaves are more conditioned for defoliation than younger leaves. 

Furthermore, because of the hormone balance of younger leaves, low rates of harvest aids often 

have no effect, and higher rates may actually kill the leaf, leading to desiccation and leaf sticking. 

I 

Eventually, all the leaves on a cotton plant age so they will defoliate naturally. However, cotton 

producers can manipulate these hormone levels with the use of defoliants so all the leaves 

abscise (separate and fall) at the same time. When these defoliants are applied, ethylene levels 

artificially increase so the abscission process begins. 

Uses and Use Characterization 

Thidiazuron is used exclusively as a cotton defoliant to aid in the uniform removal of both 

mature and juvenile leaves. This facilitates machine harvesting and helps eliminate sources of 

gin trash and stains. Because of its purpose, thidiazuron is applied during the cotton harvesting 

period, which is typically in late August and early September. All labels indicate that citrus trees 

are particularly sensitive to thidiazuron. 

All current products containing thidiazuron and their corresponding EPA registration numbers 

are listed in the following table. Product label statements about use indicate that the areas 

potentially at direct and/or indirect risk from the application of thidiazuron in the manner 

prescribed by the labels are the cotton agronomic environments. 

Page 7 of 134 

- - - - - - -- - - - - ---- 
' 12% thidiazuron and 6% diuron mixture 

50% thidiazuron and 25% diuron mixture 

30.3% thidiazuron and 15.1 % diuron mixture 

32.7% dimethipin and 8.4% thidiazuron mixture 

For cotton defoliqnt formulation use only 

EPA Registration Number 

264-622 

264-634 

264-66 1 

264-700 

264-82 1 

400-505 

51036-401 

5 1036-402 

5 1036-429 

51036-430 

72167-28 

72167-30 

As indicated in the table above, thidiazuron is used exclusively as a cotton defoliant or growth 

regulant. ~h;ee of the twelve registered thidiazuron products are mixed with diuron at half the 

concentration of thidiazuron. In another product (Harvade 41 98SC), dimethipin is present at 

approximately four times the concentration of thidiazuron. 

Product Name 

Dropp 50WP Cotton Defoliant 

GINSTAR EC Cotton ~efoliant' 

Dropp Ultra Cotton Defoliant2 

Dropp SC Cotton Defoliant 

GINSTARB 4.5 SC Cotton ~efoliant~ 

Harvade 4 198SC Harvest Growth Regulant 

for Cotton4 

Thidiazuron 50W Cotton Defoliant 

Thidiazuron Technical5 

Thidiazuron-Diuron EC Cotton Defoliant' 

Thidiazuron 4SC Cotton Defoliant 

Nation's AG 11 Thidiazuron Technical5 

Thidiazuron 5OWSB Cotton Defoliant 

In the case of the mixed products, care must be taken when calculating the use rates for 

thidiazuron. This is because the "active ingredient" application rates given on the labels are for 

the sums both active ingredients. Since each compound is different in physicochemical 

properties and biological activity, it is more meaningful, useful, and transparent to separate the 

rates for each. This risk assessment was conducted for that fraction of the application rate 

contributed by thidiazuron only. 

The maximum single application rate for all labels on which thidiazuron is the sole active 

, ingredient is 0.2 lb ai/A (EPA Registration Numbers 264-622, 264-700, 51036-401', 51036-430, 

and 72167-30). A second application is recommended under low temperature conditions when 

defoliation does not reach the expected level, but the total of the two applications must not 

exceed 0.3 lb ai/A. The intervals between applications range from 5-7 days in some of the labels, 

while the other labels make no recommendations with regard to intervals. 

Page 8 of 134 
The maximum single application rate when thidiazuron is mixed with diuron is 0.125 lb of 

thidiazuron per acre and 0.0625 lb of diuron per acre (EPA Registration Numbers 264-634,264- 

661, and 5 1036-429). The combined rate for both active ingredients is 0.1875 lb ai/A, and since 

diuron also has the same mode of action as thidiazuron, the combined total use rate is 

approximately the same as in the products which contain only thidiazwon as the sole active 

ingredient. The total number of applications is not specified on any of these labels, however, the 

maximum combined active ingredient dosage is not to exceed 0.1 875 lb ai/A per season. 

When thidiazuron is mixed with dimethipin the maximum single application rate is 0.00625 Ib 

thidiazuron and 0.1 Ib dimethipin/A (EPA Registration Number 400-505). A second application 

may be required at a maximum rate per acre of 0.003 lb thidiazuron and 0.05 lb dimethipin 5 - 7 

days after the initial treatment. The maximum yearly application should not exceed 0.01 lb 

thidiazuron and 0.16 lb dimethipinlAlyr. 

Risk Assessment Approach 

For terrestrial risk, we consider the active ingredient thidiazuron only. For aquatic risk, in 

addition to parent thidiazwon, we include the compounds AE F132147 and AE C421200 

(photodegradates of thidiazwon produced in water) and the thidiazuron metabolite AE F132 145. 

We do not consider any additional risk posed by the co-formulations of thidiazwon with either of 

the active ingredients diuron or dimethipin, (see previous section for specific products and 

concentrations of each) or by the addition of adjuvants (surfactants). 

Assessment Endpoints 

The typical assessment endpoints for screening-level pesticide ecological risks are reduced 

survival and reproductive impairment for both aquatic and terrestrial animal species. These 

effects extend to a consideration of direct acute and direct chronic exposures. The assessment 

endpoints for aquatic plants are impairments due to a decrease in alga cell density or macrophyte 

growth. Terrestrial plant assessment endpoints are impairments due to shoot weight, shoot 

length, or dry weight. There are currently no chronic assessment endpoints for reproductive 

impairment for plants. While these endpoints are measured at the individual level, they do 

provide insight about risks at higher levels of biological organization (e.g., populations). 

Measures of Effects 

The screening level risk assessment process relies on a suite of toxicity studies performed on a 

limited number of organisms in the following broad groupings: 

Birds (mallard duck and bobwhite quail) (see Toxicity to Birds section) 

Acute data are complete for this risk assessment 

Chronic data are not available. 

Mammals (laboratory rat) (see Toxicity to Mammals section) 

Acute data are complete for this risk assessment 

Chronic data are available from a 3-generation rat study 

No rabbit developmental study is available at this time. 

Page 9 of 134 
Freshwater fish (bluegill sunfish, rainbow trout, and fathead minnow) (see Toxicity to 

Fish section) 

Acute data are available for thidiazuron. 

Acute data are available for the AE F132145 metabolite and the AE F132147 and 

AE C421200 photodegradates, and these data are incorporated into the risk 

assessment. 

Chronic freshwater fish data are available for thidiazuron 

I Freshwater invertebrates (Daphnia magna) (see Toxicity to Aquatic Invertebrates section) 

Acute data are available for thidiazuron • Acute data are available for the AE F132145 metabolite and the AE F132147 and 

AE C421200 photodegradates, and these data are incorporated into risk I assessment 

Chronic data are available for thidiazuron 

Estuarinelmarine fish (sheepshead minnow) (see Toxicity to Fish section) I Acute data are available for thidiazuron. 

No chronic data are available 

Estuarinelmarine invertebrates (Crassostrea virginica and Mysidopsis bahia) (see 

Toxicity to Aquatic Invertebrates section) , Acute data are available for thidiazuron 

Chronic data are not available 

Algae and aquatic plants (see Toxicity to Aquatic Plants section) 

Data are available for green algae, blue-green algae, marine diatoms, and aquatic 

vascular macrophytes for thidiazuron. 

Data are not available for freshwater diatoms. 

I Data are available for green algae for the AE F132145 metabolite and the 

AE F132 147 and AE C421200 photodegradates, and these data are incorporated 

into risk assessment. 

Terrestrial plants (corn, soybean, turnip, oats, wheat, tomato, onion, cabbage, lettuce, , cucumber) (see Toxicity to Terrestrial Plant section) 

Data are available for thidiazuron 

Data are available for thidiazuron SC42 formulated product 

Within each of these very broad taxonomic groups, an acute and chronic endpoint is selected 

from the available test data, as the data sets allow. The selection is made from the most sensitive 

species tested within that organism group. 

Additional effects data were available for other taxa and for aquatic community effects measures. 

These have been incorporated into the risk characterization as other lines of evidence. These data 

include: 

Acute laboratory toxicity study of technical and formulated product with oligochaete 

worms 

Toxicity data on beneficial insects 

laboratory tests for honeybee contact toxicity 

laboratory tests for predatory mites and parasoid wasps 

Page 10 of 134 
In addition, a search of EPA's publically accessible Ecotoxicity Database 

(htt~://www.er>a.~ov/ecotox) for relevant toxicological studies revealed no additional data for 

effects on mortality, growth, behavior, reproduction, population, or ecosystem function for 

thidiazuron. A complete discussion of -all toxicity data available for this risk assessment and the 

resulting measurement endpoints selected for each taxonomic group are included in Sections VI 

and VII of this document. 

Measures of Exposure 

Exposures estimated in the screening level risk assessment for non-target organisms are not 

specific to a given species. Rather, general taxonomic exposure assumptions are made that 

provide separate exposure measures for aquatic and terrestrial plants and animals. The 

approaches for each are discussed separately below. 

Aquatic Organisms 

In addition to parent thidiazuron, effects data are available for the aqueous photodegradates 

AE F132147 and AE C421200 and the metabolite AE F132145 in fish, aquatic invertebrates, and 

aquatic plants. These data suggest that exposure modeling for these degradates is warranted. 

The following sections describe the general analysis approach used for active ingredient and the 

three transformation products. 

Active Ingredient and Degradate Aquatic Organism Exposure Approach 

This risk assessment considers thidiazuron exposure in aquatic organisms (animals and plants) 

via the fraction of pesticide dissolved in the water column. Thidiazuron is assumed to be 

introduced to surface waters via runoff and spray drift. Currently, no monitoring data are 

available that would provide information on thidiazuron levels in surface water bodies receiving 

runoff or spray drift from agricultural fields treated with the pesticide. Consequently, aquatic 

organism exposure is estimated through the use of runoff and spray drift models. Aquatic 

organism exposures are based on a set of standardized receiving surface water body assumptions 

(water body size, watershed size, proximity to field, etc.) that result in reasonable upper-level 

estimates of exposure. For this risk assessment the GENEEC model was used for making these 

exposure estimates. GENEEC is the most conservative exposure model available in OPP. It 

estimates exposure concentrations in aquatic environments representative of geographic locations 

where cotton is grown in large quantities. If exposure concentrations resulting from the use of 

GENEEC yield risk quotients below all aquatic levels of concern, it is concluded that direct and 

indirect aquatic risks would not occur. In this regard, EFED used the most sensitive 

toxicological endpoint available from data on thidiazuron and the photoproducts for each of the 

aquatic organism categories (i.e., fish, aquatic invertebrates, plants). 

For acute aquatic risk, no averaging time $or exposure was assumed for this assessment. The use 

of the instantaneous peak assumes that instantaneous exposure is of sufficient duration to elicit 

acute effects on par with those observed over more protracted exposure periods tested in the 

laboratory, typically 48 to 96 hours. For assessment of chronic aquatic invertebrate risks with 

thidiazuron, the peak water concentration was again used. This is a departure from the normal 

Page 11 of 134 
assessment where the 2 1 -day average, 1 -in- 10 year return frequency values are compared to the 

measurement endpoint for aquatic invertebrate chronic risks. The decision to use a peak 

concentration exposure measure was predicated on the belief that the LOCs would not be 

exceeded even if the higher peak concentrations were utilized. 

As discussed in the Environmental Fate section of this document and in Appendix D, thidiazuron 

undergoes relatively rapid branching photolysis in water into only two photoproducts. Formation 

of these products is in constant ratio and in complementary stoichiometric yield with parent. The 

key features of these two photoproducts are as follows: 

Photoproduct I (AE F132347, see chemical structure in Appendix B) is an isomer of parent 

differing only in the relative positions of the sulfur atom and the two nitrogen atoms in the 5- 

membered ring. Like parent, photoproduct I is moderately ecotoxic (slightly more toxic than 

parent, based on the limited data discussed in the main text of this document). It has a reviewercalculated asymptotic experimental limit of production of 77% of parent at pH 5, 28% at pH 7, 

and 17% at pH 9. This product was stable to further photolysis under the experimental study 

conditions and durations (original MRID 41 188201, related follow-up MRIDs 4 13649 10 and 

43075202, and fate overview document with MRID 44436901). SoiVsediment sorption data and 

non-photolytic transformation data are available only for parent. However, based on its isomeric 

chemical structure, photoproduct I, except for photolysis, would be expected to have fate 

properties similar to parent. 

Photoproduct II (AE C421200, chemical structure in Appendix B) is a substantially degraded, 

practically non-ecotoxic product (based on the limited data discussed in the main text of this 

document), and has a complementary asymptotic limit of production to 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. SoiVsediment sorption data and non-photolytic transformation data are 

available only for parent. However, because of its degraded nature and a modicum of direct 

evidence indicating that photoproduct I1 is practically non-toxic for ecological risk assessment 

purposes, EFED assumed that photoproduct I1 does not contribute to ecological risk, and ignored 

its potential presence. 

Aquatic exposure concentrations are derived from a combination of GENEEC model runs for the 

summation of thidiazuron and its photoisomer as explicitly derived in Appendix D 

Terrestrial Plants 

The focus of the terrestrial plant exposure assessment is on terrestrial plants inhabiting dry and 

semi-aquatic areas. Potential routes of exposure to pesticides considered are runoff, spray drift 

and volatilization. Since thidiazuron is non-volatile, volatilization is not a concern. 

Semi-aquatic areas are those low-lying wet areas that may be dry at certain times of the year. 

EFEDfs runoff exposure estimate in these areas is: (1) based on a pesticide's water solubility and 

the amount of pesticide present in the top one inch of soil, (2) characterized as "sheet runoff' 

(one treated acre to an adjacent acre) for dry areas, (3) characterized as "channelized runofff (10 

treated acres to a distant low-lying acre) for semi-aquatic areas, and (4) based on percent runoff 

Page 12 of 134 
values of 0.01,0.02, and 0.05 for water solubilities of <10 ppm, 10-1 00 ppm, and >I00 ppm, 

respectively. 

I 

I Spray drift exposure from ground and overhead chemigation applications is assumed to be 1% of 

the application rate. Spray drift from aerial, airblast, and forced-air applications is assumed to be 

5% of the application rate with an application efficiency of 60%. The effects of multiple 

applications are addressed by summing the application rates from individual applications. 

Terrestrial Animals 

The focus of the terrestrial wildlife exposure assessment is on birds and mammals with the 

exposure route emphasis on uptake through the diet. For exposure to terrestrial organisms, such 

I as birds and small mammals, OPP looks at the residues of pesticides on food items, and assumes 

that organisms are exposed to a single pesticide residue in a given exposure scenario. For 

thidiazuron spray applications, estimation of pesticide concentrations in wildlife food items 

I focuses on quantifying possible dietary ingestion of residues on vegetative matter and insects. 

The residue estimates are based on a nomogram that relates food item residues to a pesticide's 

application rate. Residues may be compared directly with dietary toxicity data or converted to an 

oral dose, as is the case for small mammals. The screening-level risk assessment for thidiazuron 

uses an upper bound predicted residue as the measure of exposure. For mammals, the residue 

concentration is converted to daily oral dose based on fractions of body weight consumed daily, 

as estimated through mammalian allometric relationships. 

Risk Characterization Approach 

For this assessment of thidiazuron risks, the risk quotient (RQ) method is used to compare 

exposure and measured toxicity. Estimated environmental concentrations (EECs) are divided by 

acute and chronic toxicity values. The RQs are compared to the Agency's levels of concern 

(LOCs). These LOCs are the Agency's interpretative policy, and are used to analyze potential 

risk to non-target organisms and the need to consider regulatory action. These criteria are used to 

indicate when a pesticide used as directed on the label has the potential to cause adverse effects 

on non-target organisms. Appendix G of this document summarizes the LOCs employed in this 

risk assessment. 

Problem Formulation Analysis Plan 

In accordance with US EPA7s "Guidelines for Ecological Risk Assessment," April 1998, and 

"Overview of the Ecological Risk Assessment Process in the Office of Pesticide Program," 

January 2004, an analysis plan and rationale were developed to determine if current label uses of 

thidiazuron indicate a likelihood that unreasonable adverse effects to nontarget and listed animals 

and plants could potentially impact the reregistration eligibility decision under the Federal 

Insecticide, Fungicide, and Rodenticide Act, the Food Quality Protection Act, and the 

Endangered Species Act. Over the course of its assessment, OPP has attempted to refine its 

problem formulation by integrating registrant-submitted guideline data and various levels of 

exposure refinements and information on usage to characterize the environmental fate and 

ecological effects of thidiazuron and its degradates. 

Page 13 of 134 
Thidiazuron is exclusively used as a cotton defoliant to aid in the machine harvesting of cotton 

by removing both mature and juvenile leaves. EFED made several assumptions concerning some 

ambiguities on the labels. Three of the product labels list diuron as an additional active 

ingredient, and another label lists dimethipin, but none of these labels report the application rates 

of each active ingredient separately. EFED quantitatively considered the ribk resulting from 

thidiazuron only, rather than the combination of the two active ingredients. Thidiazuron rates 

were calculated from total product application rates and product chemical composition. The 

possible additive or synergistic effects resulting from the consideration of the other active 

ingredients were not evaluated. Additional information provided by BEAD and other sources 

identified more typical use rates and frequencies in addition to the maximum rates which are 

routinely used in screening level assessments. These typical rates were considered in the 

terrestrial plant risk characterization. 

The risk assessment considers aquatic and terrestrial risks for parent thidiazuron. In addition, it 

considers risks posed to aquatic organisms from AE F132347 and AE C421200 (photodegradates 

formed in water) and the metabolite AE F132345 by using aquatic exposure concentrations 

derived from a combination of GENEEC model runs for the summation of thidiazuron and its 

photoisomer as explicitly derived in Appendix D. 

Risk to terrestrial plants from the formulated product, Thidiazuron SC42 (DROPP) is considered 

in the risk assessment. Exposure from foliar routes of exposure are primarily explored for 

terrestrial and aquatic plants and animals. However, this risk assessment does not take into 

account atmospheric transport, other than spray drift, in estimation of environmental 

concentrations. Further, it does not account for ingestion of residues by animals in drinking 

water or contaminated grit, ingestion through preening activities, or uptake through inhalation or 

dermal absorption by terrestrial animals. Exposure to terrestrial animals is based primarily on 

dietary consumption of foliar residues, whereas aquatic exposures assume that all potential routes 

of direct exposure are accounted for and are based on standard surface water modeling used in 

EFED. 

As previously discussed, exposure to terrestrial plants Inhabiting dry and semi-aquatic areas 

involves runoff assumptions based on the water solubility and the amount of pesticide present in 

the top one inch of soil. This runoff exposure is characterized as "sheet runoff' (one treated acre 

to an adjacent acre) for dry areas, and "channelized runoff' (10 treated acres to a distant lowlying acre) for semi-aquatic areas. Terrestrial plant spray drift exposure assumptions are based 

on 1 % of the application rate for ground applications and 5% for aerial applications with an 

application efficiency of 60%. 

Maximum application rates on vulnerable soils for representative crops are selected for modeling 

environmental concentrations for this screening-level deterministic (risk quotient based) risk 

assessment. This assessment is not intended to represent a site or time-specific analysis; i.e., 

assessments are intended to represent a national level exposure based on vulnerable soils as 

opposed to being regionally specific. However, if exposures exceed LOCs, the assessment will 

be refined to include higher-tiered models with spatially specific use scenarios. Likewise, the 

most sensitive toxicity endpoints are used from surrogate test species to estimate treatmentrelated direct effects on acute mortality and chronic reproductive, growth, and survival 

Page 14 of 134 
assessment endpoints. Toxicity tests are intended to determine pesticidal effects on birds, 

mammals, fish, terrestrial and aquatic invertebrates and plants. These tests include short-term 

acute, subacute and reproduction studies and are typically arranged in a hierarchical or tiered 

system that progresses from basic laboratory tests to applied field studies. These studies are used 

to evaluate the potential of a pesticide to cause adverse effects, determine whether further testing 

is required, and to determine the need for precautionary label statements to minimize the 

potential adverse effects to nontarget animal and plants (CFR 40 5 158.202,2002). 

The conceptual model used to depict the ecological risk associated with thidiazuron was initially 

generic and assumed that thidiazuron was capable of affecting terrestrial and aquatic animals 

Conceptual Model for Thicliazuron Ask Assessment 

Thidiaaurm applied 

to agricultural field 

Attribute 

CfTF 

mO= ,Aquatic plants Pquatic invertetn-ates 

Mudtic vertwes 

Individual fish 

Rdm sulvival 

Muted repoducticn 

Rparianhe 

Terrestrial plants 

Terrestrial insects 

- 

l-lab~tat integrity 

Decreased &er quality 

wucedccwer 

Stream destabilizatrcn 

"4 Terrestrial Birds lvhmmls 

provided environmental concentrations were sufficiently elevated as a result of labeled uses. 

This initial conceptual model is presented below. 

Page 15 of 134 

- - - - - - - -- ----- 
However, through an iterative process of examining fate and effects data, the conceptual model 

has been refined to reflect the exposure pathways and the organisms for which risk is most likely. 

Based on OPPs screening level assessment, the refined conceptual model for depicting the risks 

of thidiazuron primarily focuses on risk to terrestrial plants. Additionally, there are concerns 

about possible chronic effects to small mammals, depending on the methodology and toxic 

endpoints used to calculate the risk quotients. Further, since there are no chronic data currently 

available for birds, chronic risk to birds is undetermined. The refined conceptual model for 

depicting risks to nontarget terrestrial plants is presented below. 

Refined Conceptual Model for a Screening level 

Assessment of Thdiazuronum on Cotton 

Page 16 of 134 

- - - --- -- ---- 

Source: Aerial and ground Application to Cotton 

Thidiazuron/Degradates/Products 

Terrestrial Plant Exposure from 

Spray Drift, Runoff, and Sheet Runoff 

Receptor: Plants Adjacent 

to treated sties 

v v 7 

Direct and Indirect Acute Effects to Acute Effects to Chronic Effects to Non-endangered Non-endangered Non-endangered and and Endangered Plants and Endangered Plants 

Receptor: Plants in 

Semi-Aquatic Areas 

Receptor: Mammals in and 

Adjacent to treated sites 
111. Integrated Environmental Risk Characterization 

Risks to Aquatic Organisms 

The results of the risk assessment suggest that acute and chronic risk quotients for all freshwater 

and marine fish and invertebrates are below all levels of concern established'by the Agency's 

screening-level assessment. 

The acute risk quotients ranged from <0.0003 (marine fish) to 0.002 (freshwater fish and 

estuarine invertebrates). Chronic risk quotients ranged from 0.002 (freshwater fish) to >0.1 

(freshwater invertebrates). In addition, the acute risk quotients for the non-endangered and 

endangered species are well below all established levels of concern for aquatic plants. The 

definitive acute risk quotients ranged from 0.0005 to 0.013 and the endangered species risk 

quotients ranged fiom 0.006 to 0.2. 

Although there may be some uncertainty with respect to chronic risk to freshwater and marine 

invertebrates because a definitive NOAEC value could not be obtained, it is possible to estimate 

chronic risk based on a 21-day EC,,value which was obtained in the chronic freshwater 

invertebrate study. The 21-day EC,, value, was 720 yg/L based on the length of the test 

organisms. Using this generated EC,, value, the definitive 21-day RQ value of 0.015 is well 

below the chronic level of concern. In order for the chronic aquatic invertebrate LOC to be 

exceeded, the test would have to be repeated to test levels lower than 11 pg/L (nearly 10-fold the 

current test level of 100 yg1L). It is doubtful that definitive NOAECILOAECs would exceed the 

chronic levels of concern. Even if the persistence and intermediate mobility were taken into 

account where some year-to-year build-up in soil would occur (asymptotic limit twice that of 

annually applied amounts), the resulting RQs would still not exceed any aquatic LOCs. 

Risk to Birds 

Acute Risks - Birds 

Acute risks to birds for this use are below all levels of concern. 

To determine potential acute risk to birds, we use exposure data from Hoeger and Kenaga 

(1972), as modified by Fletcher et al. (1994). These data provide maximum and mean residue 

concentrations of a pesticide on various terrestrial food items on the basis of a one pound per 

acre application in the field. Predicted residue concentrations are available for short grass, tall 

grass, broad-leaved plantslsmall insects, and seedsllarge insects. 

Toxicant concentrations on food items following multiple applications are predicted using a firstorder residue decline method, EFED's "ELLFATE" model, which allows determination of 

residue dissipation over time incorporating degradation half-life. Predicted maximum and mean 

EECs resulting from multiple applications estimates the highest one-day residue, based on the 

maximum or mean initial EEC hom the first application, the total number of applications, 

interval between applications, and a first-order degradation rate, consistent with EFED policy. 

The input parameters for the cotton scenario were based on a maximum single application of 0.2 

Page 17 of 134 
lbs a.i.1A. For multiple applications, a follow-up application of 0.1 lb ai/A at an interval between 

5 and 7 days is allowed on all labels, and for this scenario EFED used the ELLFATE program to 

estimate the maximum residue concentrations occurring over a 99-day period based on a 35-day 

foliar dissipation half-life and a 7-day interval between applications. The first application was 

run at 0.2 lb ai1A. To factor in the second application of 0.1 lb ai/A (on day 6), the maximum 

residue concentrations from it were added to those remaining from the first application (on 

day 0). 

Acute data submitted for thidiazuron indicate that the dietary LC5Os were greater than 5000 

mg/kg-diet for bobwhite quail and mallard duck. The LD5Os were greater than 3 160 mglkg-bw 

for both species. Since the results from both the dietary LC50 and the oral gavage LD50 classify 

thidiazuron as practically non-toxic to birds and the formulations are non-granular, the dietary 

LC50 for bobwhite quail was selected for risk quotient calculations. 

The resulting acute risk quotients (Appendix G) at all sites were below all levels of concern 

using the most sensitive LC50 of >5000 mglkg-diet. The highest RQ observed for two 

applications of 0.2 Ib ai/A for birds foraging in short grass was c0.013. 

Chronic Risk - Birds 

Chronic risks to birds cannot be quantitatively evaluated at this time because chronic data were 

not submitted. However, available data showing that chronic levels of concern for mammals are 

exceeded and that thidiazuron is persistent in the field suggest that there is also potential for 

chronic risk to birds should they be of equal of greater sensitivity than mammals. 

Risks to Mammals 

Acute Risks - Mammals 

Acute risks to mammals were evaluated using the rat LD50 of > 2000 mglkg-bw. There were no 

acute LOCs exceeded (Appendix H). The highest risk quotient observed for the 15 g mammal 

foraging on short grass is <0.03, well below the endangered species LOC of 0.1. 

Chronic Risk - Mammals 

The results from the 3-generation rat study was used to assess chronic risk to mammals. This 

study was classified as core-supplemental in 1995 due to several deficiencies in the study design. 

Most notable of these was that there was no measure of food consumption and the amount of 

food consumed was estimated by using default food consumption values developed by the FDA 

many years ago. Since this study is the only chronic study currently available, EFED will use 

this data for this assessment. 

The reproductive NOAEC and LOAEC values based on reduction of litter size are 200 and 600 

mglkg-diet, respectively. By standard conversion factors the NOAEL and LOAEL are 10 and 30 

mglkg-bwlda, respectively. The mean litter size data from the study is summarized in the 

Page 18 of 134 
following table and indicate that significant reductions are occurring in the F3b generation at the 

highest dose, and possibly the lowest as well. 

** statistically significant at p<0.01 

* statistically significant at p<0.05 

Reduction in Mean Litter Size for the Three-Generation Rat Study 

For calculating the chronic RQ values, the NOAEL of 10 mg/kg/day and the LOAEL of 30 

mg/kg/day based on reduced litter size in a 3-generation reproduction study in the rat were 

selected. In the second group of litters of the third generation, there was a statistically significant 

decrease in litter size at the lowest dose tested (3 mgikgiday). This apparent reduction in litter 

size is not considered to be related to exposure to thidiazuron for the following reasons: 

1) The effect was only observed in the second group of litters (F3b) in the third. generation. 

There was no indication of an effect at this dose level in any of the other generations or in the 

first group of litters in the same generation (F3a). If this were a genuine effect, it would likely 

have appeared in other generations at this dose level. 

High Dose 

(30 rnglkg-bw/day) 

11.2 

10.3 

7.9 

9.9 

9.3 

9.6" 

2) There was no dose-response. No decreases in litter size were observed at the mid-dose level 

of 10 mg/kg/day. 

Mid Dose 

(10 mg/kg-bwlday) 

12.1 

12.1 

9.0 

9.8 

9.5 

11.4 

3) In the F2a generation, the litter size of the control group was less than the litter size for the 

low-dose group in the F3b generation. The variability of the litter sizes in the control group 

indicate that the apparent reduction in litter size at the low dose is not likely to be toxicologically 

significant. 

Low Dose 

(3 mg/Kg-bw/day) 

11.6 

13.2 

9.0 

11.0 

9.0 

9.1"" 

Generation 

Fla 

Flb 

F2a 

F2b 

F3a 

F3b 

Using the NOAEL of 10 mg/kg-bwiday is reasonable because the percent of reproductive 

impairment that is significant for a given species would be based on their reproductive strategy. I 

For long-lived (K-selected species), episodic reductions in reproduction might not be of the same 

significance as short-lived (r-selected) species. Reproductive impairment for a short-lived 

species could be enough to impact any number of species with low generation survival rates and 

a dependance on high reproductive output for population continuance (r-selected species). 

Control 

10.4 

12.8 

8.7 

10.3 

10.3 

12.2 

Page 19 of 134 

___ _- -. - _- - _ __ ---___ _ _ . - 
To calculate mammalian risk quotients, we preferentially use the Nagy small mammal 

assumptions of percent body weight consumed for the 15,35, and 1000 g mammals and divide 

the dietary EEC adjusted for food consumption by the NOAEL (expressed as mglkg-bwlday). 

The resulting risk quotients are presented in Tables 7 and 8 of Appendix H and indicate that 

LOCs are exceeded for mammals (herbivores/insectivores) foraging in short grass and broadleaf 

forage and small insects. The risk quotients range fiom 6.33 (short grass) to 0.01 (large insects). 

The chronic LOCs are not exceeded for mammalian granivores. 

A less-preferred method of calculating risk quotients is to divide the dietary EEC for each of the 

forage groups by the NOAEC (expressed as mgkg-diet). Using the NOAEC of 200 mglkg-diet, 

the chronic LOCs are not exceeded for any of the food categories. These risk quotients are 

presented in Table 9 of Appendix H. 

There are uncertainties associated with the risk quotient exceedances. As mentioned above, there 

was no measure of food consumption in the study or the amount of pesticide consumed. Further, 

there are no measured foliar dissipation half-life data to evaluate residue concentrations; 

therefore, the EFED standard default foliar half-life of 35 days was used. However, even if a 

I foliar dissipation half-life of one day were used, LOCs are still exceeded, as indicated by risk 

quotients that range from 4.6 to 0.02 for herbivores/insectivores. 

Chronic risk can also be evaluated for single applications for maximum Fletcher values for 

predicted maximum and mean residues. Using the default 35-day foliar half-life and a maximum 

single application rate of 0.2 lb ai/A resulted in risk quotients exceeding the chronic LOCs for 

the same food categories as multiple applications, except that the RQs ranged fiom 4.6 to 0.02 

for herbivores/insectivores (coincidentally the same as range as multiple applications using a one 

day foliar dissipation half-life. 

Additionally, EFED looked at the impact of a reduced application rate on mammalian risk. In I 

this regard, LOCs would be exceeded unless the use rate for each of two applications separated 

by 7 days is reduced by a factor of approximately twenty to 0.014 lb ai/A. 

Risks to Non-Target Insects 

EFED currently does not estimate risk quotients for terrestrial non-target insects. Whenever an 

LD,, is < 11 pghee, an appropriate label statement is required to protect foraging honeybees. 

The acute contact toxicity study to honeybees revealed a contact LD,, > 100 pghee for the 

technical thidiazuron and a contact LD,, >98.1 &bee for the SC42 formulated product. This 

classifies thidiazuron technical and.the formulated product as practically non-toxic to honeybees. 

The two other non-target insect OECD studies, which investigated the lethal and sub-lethal effect 

of the SC42 water miscible suspension concentrate 500 g/L on the parasitoid wasp (Aphidius 

rhopalosiphi) and the predatory mite (Typhlodromuspyri), were found not to show statistical 

differences from the controls for mortalities or reproduction effects. The wasp study showed the 

percent mortality was 12.5% in the highest exposed concentration of 0.2 lb ai/A compared to 

10% in the controls. The percent reduction when compared to the control was 27.3 and 38.1% in 

the 100 and 200 g ailha treatment groups, respectively. In the predatory mite study the 

Page 20 of 134 
mortalities were 8% in the treated group compared to 13% in the untreated group, and the mean 

number of eggs per female was 9.32 in the control group compared to 8.99 in the 200 g ailha 

treatment. Since these studies showed no statistically significant effects at the highest maximum 

single rate (0.2 lb ai/A), there is little concern for non-target insects. 

Risks to Terrestrial Plants 

RQs for terrestrial plants in dry and semi-aquatic areas are calculated for multiple and single 

spray applications for endangered and non-endangered species. As mentioned in the exposure 

section, the runoff scenarios are based on solubility, and since the solubility of thidiazuron is 3 1 

mg/L (ppm), the percent runoff value used in the risk quotient calculations was 0.02. A 60% 

efficiency factor is also included for aerial applications. The detailed procedures for terrestrial 

exposure and plant risk quotient calculations are discussed in the terrestrial exposure section of 

this assessment. The detailed exposure and risk quotient calculations are tabulated in detail in 

Appendix H, and a summary of the resulting RQs for single applications is presented below. 

* Exceeds terrestrial plant Levels of Concern (LOC) 

Multiple spray applications - All thidiazuron labels allow two applications. The sum of these 

is a maximum of 0.3 lb ai/A/yr (0.2 lb ai/A for the first application and up to 0.1 1b ai1A for the 

second application). Only one label specifies that intervals between applications should be 

between 5-7 days. Since there are no residue data over time for either one or two applications, 

we used the same procedure here to estimate exposure concentrations that we stated previously 

for birds. That is, we used exposure data from Hoeger and Kenaga (1972), as modified by 

Fletcher et al. (1 994), and the ELLFATE program for calculation. From these we estimated the 

maximum residue concentrations occurring on short grass over a 99-day period based on the 

default 35-day foliar dissipation half-life and a 7-day interval between applications. Again, the 

Page 21 of 134 
first application was run at 0.2 lb aiiA, and to factor in the second application of 0.1 1b aiiA (on 

day 6), the maximum residue concentrations from it were added to those remaining from the first 

application (on day 0). The detailed exposure and risk quotient calculations are tabulated in 

detail in Appendix H, and a summary of the resulting RQs is presented below. 

Nonendangered 

Endangered 

Seedling Monocot ~0.1783 >0.05 - B0.07 i >0.22 - > 0.35 Emergence ! ............................................................. a ............................................ <. .................................... 

i Dicot 0.0152 0.59 - 0.86 i 2.63 - 4.14" ........................ + ............................................................. + ............................................. : ...................................... 

Vegetative Monocot >0.1783 <0.02 - >0.06 i <0.02 - >0.06 Vigor ;.. .... .................................................. .'. ........................................ ....... .................................... 

i Dicot 0.0011 ; 2.73 - 9.09" i 2.73 - 9.09" 

........................+............................................................. A ............................................ < ...................................... 

Seedling Monocot 0.1783 i 0.05 - 0.07 0.22 - 0.35 

Emergence ! .............................................................+............................................ < ...................................... 

i Dicot 0.0111 ; 0.81 - 7.18' 3.6 - 5.68" 

................... 

i Vegetative i Monocot 0.1783 0.02 - 0.06 0.02 - 0.06 

! Vigor ~................. ......................................................................................... A ...................................... 

i Dicot 0.00019 i 15.79 - 52.63" ' 15.79 - 52.63" 

* Exceeds terrestrial plant Levels of Concern (LOC) 

Potential risks to terrestrial plants are possible because RQs exceed the LOCs. This is especially 

true for dicots for both single and multiple applications to plants inhabiting dry and semi-aquatic 

areas. The RQs resulting from single applications to dicots range from 0.39 to 9.09 for nonendangered plants and up to 53 for endangered plants. The multiple applications RQs to dicots 

range from 0.59 to 9.09 for non-endangered species and up to 53 for endangered species. 

Risk to monocot species appear to be substantially lower. However, definitive EC25s and 

NOAECs have not been determined because tier IT tests were not conducted for some of the more 

sensitive species such as onion and oats. These EC25s range from >0.05 to > 0.35 and are 

presented in more detail in the Appendix C. 

Definitive tier I1 EC25s and NOAECs are lacking for the vegetative vigor of all the monocots 

tested and for the seedling emergence of two monocot species tested because test concentrations 

were not low enough to establish these endpoints. However, risk quotients from the tests for 

which definitive endpoints were established were below levels of concern. Furthermore, 

compared to the labeled application rates for field use and corresponding estimated field 

exposure concentrations, the lowest concentrations for the tests for which endpoints were not 

Page 22 of 134 
determined were adequate to establish that risk quotients would not exceed any of the terrestrial 

plant LOCs at current application rates for thidiazuron. 

Thidiazuron inhibits regrowth by uptake through the roots and foliage. When thidiazuron is 

applied, ethylene levels artificially increase so the defoliation (leaf drop) process begins. If nontarget plants which are more sensitive than cotton come into contact with thidiazuron, the leaves 

of these plants may be killed or severely damaged. As a result, the plant may die, or the damage 

may be sufficient to prevent the plant from reproducing or successfully competing with other 

plants for resources. The use of thidiazuron could apply selective pressure to dicots along field 

edges resulting in changes in species composition. 

Even though thidiazuron is only applied at cotton harvest, application timing is a consequential 

factor to consider. Reproduction abnormalities are some of the plant injuries that can occur due 

to thidiazuron exposwe during particular developmental stages. Late season annuals complete 

I their life-cycle in one year beginning with seed germination, plant growth, and reproduction. 

Late season spraying of thidiazuron could cause defoliation during the plant growth phase which 

I can directly affect populations and may reduce their capability to reproduce. Biennials are plants 

that complete their life cycles in two years, producing foliage the first year and flowers the 

following year. Defoliation resulting fiom applications of thiadiazuron in the late season may 

inhibit these plants from reproducing the following year. Perennials are plants that grow 

throughout the year and produce flowers andlor seeds at some point in their growing season. 

Late blooming perennials may be impacted by spraying of thidiazuron during their blooming 

phase and thidiazuron may inhibit the plants' ability to adequately reproduce.Although the plant I 

may survive, sterile florets or non-viable seed production can occur. This could have effects on I non-target plant populations' ability to sustain a viable population in future years. 

Plant material serves as a primary food source for many species of animals. If the available plant 

material (includng seeds) is reduced due to the effects of thidiazuron, this may have negative 

effects throughout the food chain. 

The exceedances discussed above were calculated using maximum applications rates derived 

fiom the thidiazuron labels. EFED completed a characterization of potential risks by assessing 

the effect of calculating RQ using EECs predicted from modeling with average application rates 

(0.05 - 0.07 lb ai/A) as reported in the BEAD QUA report. In the table below the average 

application rates are compared to the maximum allowable application rates. Although the RQs 

at the average rates are reduced by at least one-half, only the risk to non-endangered species 

caused by the single ground application would be eliminated in all categories expect dicot plants 

(vegetative vigor). 

Page 23 of 134 
Seedling Monocot ....; ; + ; + ................................. ................................. .............................. .............................. ................................. emergence i 2.63 : Dicot ............ ................................................................. .................................+................... ; ................................. * ................................. .. 

Vegetative i . Monocot i , ............................................ ; ................................. ; -! ..................... ................................. ................................. vigor i 1.82 1.82 i Dicot .................................. ;... ...........................A. .................................%......................... ................................. .........; , ................. ............... 

Single Aerial Application - Non-Endangered Species 

(max. 0.2 lb aiIA; average 0.07 lb ailA) ............................. ................................7..............................T.................................T.... .._ ............................................................ ....... 

Seedling Monocot i .; ................................... .......... 6 % ................................. , ..................... .................................. Emergence 1.97 : Dicot ................. ................................. ................................. ............ ; + -! ; -! 

.................... .............................. ................................. ................ 

Vegetative ! Monocot i .............................. .................................. ............................... A ;... $ .................................. .................................. Vigor i 3.18 9.09 9.09 i Dicot 3.18 .......................................... ;... ............................%.......................... .................................. 

; % ................................. ................................ 

Single Ground Application - Endangered Species 

(max. 0.2 lb aiIA, average 0.07 lb ailA) ........................................................................ ................................................................................................................... ............ 

Page 24 of 134 

,' 

Seedling ; Monocot i .............................. ; ................................. 6 ................................. + ................................. * ................................. Emergence 3.6 

: Dicot ! 1.32 ................................. .................................. ; + ; ; .................................. ................................ .............................. ................................. 

Vegetative ! Monocot ........ + 4 ................................. ; ..............................+................................. + ................................. ......................... Vigor i 3.68 

i Dicot 10.53 10.53 [ 3.68 ......... ;.. ............................A......................... ;..... ................................. ................................ 

Single Aerial Application - Endangered Species 

(max. 0.2 lb ailAlyr; average 0.07 lb ailNyr) .................................. -.- ...........................T................................. T ...................................................................................... ..... .......... 

Seedling .................................................................. Monocot i ; ; -! ................................. ................................. ................................. Emergence ! 1.08 2.7 : Dicot 1.07 ......................................................... .................................. ;.................................A ................................. * .. ................................. 

Vegetative Monocot i ................................. ;..............................A .................................. ; $ ................................. .! ................................. 

Vigor i 18.42 i 52.63 : i Dicot 52.63 : 18.42 
Page 25 of 134 

. 

Multiple Ground Applications - Non-Endangered Species 

(Maximum Application: 0.2 lb ailA 1" App.; 0.1 lb ailA Zndapp; 7 day intervals; ) 

(Average Application: 0.07 lb ailA 1' App.; 0.035 lb .... aiiA Zndapp; 7 day intervals; ) ... ..................................................................................................................... ...........-.. .............................................................. 

Seedling Monocot ! .. , ................................ ................ .............. * ................................. * ; .................................................................... Emergence i 4.14 ! Dicot 1.38 .. ..s..................................,................................. ................................ ; ..............................*.................................+.............................. . 

Vegetative i Monocot i ........................ 1 * .. .................................. : ! .............................. ......... .................................. ................................. Vigor i Dicot 2.73 2.73 ........................... ..... ; ...............................%.................................&............... ..................A..................................%................................. 

Multiple Aerial -Applications - Non-Endangered Species 

(Maximum Application: 0.2 lb ai1A 1" App.; 0.1 lb ailA tndapp; 7 day intervals; ) 

(Average Application: 0.07 lb ailA 1" App.; 0.035 lb aiiA fndapp; 7 day intervals; ) .. ............................................................................ .......................................................................................................................... 

Seedling Monocot i ......... . .................................................................+........................ ; ................................. .. ................................. Emergence i 2.63 ; Dicot 1.32 ................................ ; ..............................+.................................+......................... ........ .................................... ................................. 

Vegetative i Monocot A i................................. .. * I ............................... .................................................................... vigor i ................................. 

9.09 9.09 4.55 i Dicot 4.55 ;........ ........A...........;............\...........A ................................ ................................. 

Multiple Ground Applications - Endangered Species 

(Maximum Application: 0.2 lb aiiA 1' App.; 0.1 lb ailA tndapp; 7 day intervals; ) 

(Average Application: 0.07 lb aiIA 1' App.; 0.035 lb ai1A 2ndapp; 7 day intervals; ) ...................................................................................................................................................................... : 

................................. 

Seedling j Monocot ! ;.................................+......................... ........ ; ................................. ., ................................. ............................... Emergence i 5.68 : Dicot 1.89 ........... ................................. ............................... .; ..............................&.................................+...................... ; ., ................................. 

Vegetative ! Monocot 1 ........ , ..............................*......................... ;.................................A ................................. + ................................. 

Vigor i 5.26 i Dicot 15.79 1 15.79 1 5.26 .................... : ....... ............................................................................................................................................................... ............ 
The risk assessment for terrestrial plants was based on RQs calculated from toxicity studies using 

a TEP (typical end-use product). The product used in this case was "Thidiazuron SC42", 

presumably DROPP SC (EPA Registration Number 264-700)., However, it appears that no 

surfactants or adjuvants were added to the product during the study, and all labels clearly state 

the addition of adjuvants has been shown to improve the performance. Therefore, the risk to 

terrestrial non-target plants may be greater than RQs indicate. The production of actual tank-mix 

data would reduce some of the uncertainties associated with risk. - 

Additionally, as previously stated, five of twelve thidiazwon products are formulated as mixtures 

with other herbicides (see table in section on Use and Use Characterization). In four of the 

mixed products, thidiazwon comprises 67% of total active ingredients and diuron the remaining 

33%. In the fifth co-formulated product, thidiazuron comprises only 20% of the total active 

ingredients, while dimethipin comprises the remaining 80%. Plant data on these products with 

typical adjuvants would reduce the undetermined risk associated with these mixtures and more 

precisely portray risk. An alternative to the submission of such data, assuming no synergism or 

antagonism among compounds, would be to simply add the known effects for these other two 

chemicals to those for thidiazuron. 

Finally, all labels list precautions when applying the products adjacent to lettuce, citrus, or 

cantaloupe. The labels further state that mixing with organophosphates may increase non-target 

crop phytotoxicity. Special restrictions to reduce drift potential to lettuce and citrus appear on all 

the labels. Thidiazuron products cannot be applied by air within % mile of lettuce or 5 miles 

downwind of the point of application when citrus is in flush in the Rio Grande valley of Texas. 

Grpund applications are restricted to 100 ft downwind from the point of application for lettuce or 

?4 mile downwind from the point of application when citrus is in flush. No restrictions are 

prescribed for cantaloupe. EFED currently has no toxicity data for citrus or cantaloupe, and the 

Page 26 of 134 
data on lettuce indicate that it is the most sensitive of all the crops tested so far. Toxicity data on 

these additional crops may indicate that risk may be even higher than indicated by the RQs. 

Phytotoxicity and Downwind Distance -Spray drift from aerial and ground applications may 

also impact non-target plants or other crops grown adjacent to cotton, although the density of the 

spray droplets is expected to be considerably lower in these areas. Thidiazuron labels do not have 

many restrictions on how and under what conditions the product may be applied. For example, 

there are no restrictions on droplet size or boom height. Using EFED's current AgDRIFT 

computer model and registrant phytotoxicity data, it is possible to estimate distances downwind 

from application areas at which a particular toxic effect level would be experienced by a particular 

tested plant species. These estimates can be presented for combinations of aerial and ground 

applications for varying spray droplet size in the form of three-dimensional bar charts. To 

calculate the EC,, .,, values in increments of 10, the EC2, values from the seedling emergence and 

vegetative vigor studies are used in conjunction with the toxicity slopes from each of these 

studies1. Unfortunately slopes for only two of the ten crops for seedling emergence tests and four 

of the ten crops tested for vegetative vigor are currently available. These EC,, - ,, values and the 

maximum application rate for cotton were entered into an Excel spreadsheet with Tier 1 

Ag DRIFT (version 2.01). Calculated downwind deposition levels were then compared to the 

EC,, - 90 values to identify the downward distance at which the EC,, - ,, values would be reached. 

The few crops for which downwind distances could be calculated include lettuce and turnip for 

the seedling emergence tests, and lettuce, cucumber, tomato, and soybean for the vegetative vigor 

tests. Although these crops were the most sensitive in the available two tests, the current labels 

contain warnings and specific instructions concerning the use of thidiazuron on lettuce, 

cantaloupe, and citrus. The language from one of the labels is the following: 

"Particular care should be taken when applying DvoppB UltraTM adjacent to lettuce, citrus, or 

cantaloupe. " 

"In addition, for citrus crops, do not apply DroppB UltraTM by air when citrus in flush is within 

five (5) miles downwind of the point of application. Do not apply DroppB ultraTM by ground 

I when citrus in flush is within one-half(l/2) mile." 

The long protective distances on labels for citrus imply that it, or other plants, may be 

substantially more sensitive than other tested crops, including lettuce, for which "do not apply" 

distances are much shorter. Despite the warnings, no seedling emergence or vegetative vigor data 

have been submitted for cantaloupe or citrus. Submission of such data would allow a better 

characterization of the distribution of plant toxicity, and a better comparative measure of the 

necessary downwind distances and droplet sizes necessary to protect plants. 

The following equation is used to calculate the various EC, levels: [~~~~/10~~~~~

x 

10-"/~'~*" = EC, where a = 1.28, 0.84. 0.54, 0.25, 0, -0.25, -0.54, -0.84, -1.28 for EC,,, EC,,, EC,,, 

I EC,,, EC,,, EC,,, EC,,, EC,,, EC,,, respectively. A normal toxicity distribution is assumed. 

Page 27 of 134 
The bar graphs for the data for which we have slopes are presented on the following pages for 

aerial and ground application under varying droplet sizes. Following the graphs is a discussion of 

their interpretation and EFED policy for terrestrial plants. 

Page 28 of 134 

, 
Aerial: Coarse Spray 

(Seedling emergence) 

Page 29 of 134 
Aerial: Medium Spray 

(Seedling emergence) 

I Percent Effect- " 97 

Page 30 of 134 
Aerial: Fine Spray 

(Seedling emergence) 

.turnip 

lettuce 

I U Percent Effect 80 90 

Page 31 of 134 
Aerial: Coarse Spray 

(Vegetative vigor) 

Page 32 of 134 
Aerial: Medium Spray 

(Vegetative vigor) 

Percent Effect 

Page 33 of 134 
NO-Spray 

Zone (ft) 

/ I cucumber / 

Aerial: Fine Spray 

(Vegetative vigor) 

Uu YU Percent Effect 

Bsoybean 

tomato 

Page 34 of 134 
No-Spray Zone 1 (ftI 

Ground: Low Boom, Medium-Coarse Spray 

(Seedling emergence) 

Page 35 of 134 
Ground: Hlgh Boom, Medium-Coarse Spray 

(Seedling Emergence) 

Page 36 of 134 
Ground: Low Boom, Medium Spray 

(Seedling emergence) 

No-Spray 

Zone (ft) 

Page 37 of 134 
Ground: Hlgh Boom, Medium Spray 

(Seedling emergence) 

NO-Spray 50- 

Zone (ft) 40- 

30 - 

20 

I0 

Page 38 of 134 
No-Spray 

(ft) 

Ground: Low Boom, Medium-Coarse Spray 

(Vegetative vigor) 

Zone 

Page 39 of 134 
No-Spray 

Zone (ft) 

Ground: Hlgh Boom, Medium-Coarse Spray 

(Vegetative vigor) 

-T. Percent Effect ' " 80 

Page 40 of 134 
Ground: Low Boom, Medium Spray 

(Vegetative vigor) 

Page 4 1 of 134 
Ground: Hlgh Boom, Medium Spray 

(Vegetative vigor) 

I Zone /ft) 

Page 42 of 134 

m - 
EFED policy with regard to non-endangered terrestrial plants is that the LOC is exceeded when 

the environmental exposure concentration is greater than the EC2.5. In the case of thidiazuron, 

aerial application is the most common mode of application, and, as demonstrated in the charts 

above, no-spray zones for the EC,, effect levels range from > 60 ft for course sprays to over 1000 

ft. for fine sprays for the most sensitive crop tested so far (lettuce). Ground applications indicate . that no-spray zones for the EC,, effect level range from < 3 ft for low boom, medium-course spray 

to < 200 ft for high boom medium sprays. The charts also show similar trends for the other crops. 

The endangered species terrestrial plant LOC is exceeded when the environmental concentration 

is greater than the EC,,. Aerial applications of thidiazuron indicate that the no-spray zone the 

EC,, effect level ranges fi-om > 95 ft. for course sprays to > 1000 ft for fine sprays for lettuce. 

Ground applications ranges from > 95 ft for low boom, medium course sprays to < 1000 ft for 

high boom, medium sprays. 

The results of this analysis indicate that downwind distance for a specific EC, can be determined 

I for four of the crops which were tested. However, two crops, cantaloupe and citrus, lack the 

toxicity data with which to determine a downwind drift distance. This is important since these 

two crops are listed as being extremely sensitive to thidiazuron. All the factors influencing the 

downwind drift distance including droplet size, wind speed, boom height, method of application, 

and targeted toxicity effect should be considered when placing restrictions on labels. 

Endocrine Disruption Assessment 

The 3-generation reproduction study with rats indicates the potential for endocrine disruption, 

based on significant effects on mean litter size in the F3b generation. The reproductive NOAEC 

and LOAEC values based on reduction of litter size are 200 and 600 mgikg-diet, respectively. By 

standard conversion factors, the NOAEL and LOAEL, are 10 and 30 mgkg-bwlda, respectively. 

This reproductive effect could be an indicator of potential endocrine disruption in mammals or 

other organisms. 

EPA is required under the Federal Food, Drug, and Cosmetic Act (FFDCA), as amended by the 

Food Quality Protection Act (FQPA), to develop a screening program to determine whether 

certain substances (including all pesticide active and other ingredients) "may have an effect in 

humans that is similar to an effect produced by a naturally occurring estrogen, or other such 

endocrine effects as the Administrator may designate. " Following the recommendations of its 

Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC), EPA determined 

that there was scientific bases for including, as part of the program, the androgen and thyroid 

hormone systems, in addition to the estrogen hormone system. EPA also adopted EDSTACYs 

recommendation that the Program include evaluations of potential effects in wildlife. For 

pesticide chemicals, EPA will use The Federal Insecticide, Fungicide, and Rodenticide Act 

(FIFRA) and, to the extent that effects in wildlife may help determine whether a substance may 

have an effect in humans, FFDCA authority to require the wildlife evaluations. As the science 

develops and resources allow, screening of additional hormone systems may be added to the 

Endocrine Disruptor Screening Program (EDSP). When the appropriate screening and/or testing 

protocols being considered under the Agency's EDSP have been developed, thidiazuron and its 

Page 43 of 134 

-- - - -- - - -- - - - - -- - - 
degradates may be subjected to additional screening and/or testing to better characterize effects 

related to endocrine disruption. 

Endangered Species Assessment 

The Agency's level of concern for endangered and threatened non-target terrestrial plants 

and mammals is exceeded for the use of thidiazuron on cotton. 

Action Area 

For listed species assessment purposes, the action area is considered to be the area affected 

directly or indirectly by the Federal action and not merely the immediate area involved in the 

action. At the initial screening-level, the risk assessment considers broadly described taxonomic 

groups and so conservatively assumes that listed species within those broad groups are collocated 

with the pesticide treatment area. This means that terrestrial plants and wildlife are assumed to be 

located on or adjacent to the treated site and aquatic organisms are assumed to be located in a 

surface water body adjacent to the treated site. The assessment also assumes that the listed 

species are located within an assumed area which has the relatively highest potential exposure to 

the pesticide, and that exposures are likely to decrease with distance from the treatment area. 

Section II of this risk assessment presents the pesticide use sites that are used to establish initial 

collocation of species with treatment areas, 

If the assumptions associated with the screening-level action area result in RQs that are below the 

listed species LOCs, a "no effect" determination conclusion is made with respect to listed species 

in that taxa, and no further refinement of the action area is necessary. Furthermore, RQs below 

the listed species LOCs for a given taxonomic group indicate no concern for indirect effects upon 

listed species that depend upon the taxonomic group covered by the RQ as a resource. However, 

in situations where the screening assumptions lead to RQs in excess of the listed species LOCs for 

a given taxonomic group, a potential for a "may affect" conclusion exists and may be associated 

with direct effects on listed species belonging to that taxonomic group or may extend to indirect 

effects upon listed species that depend upon that taxonomic group as a resource. In such cases, 

additional information on the biology of listed species, the locations of these species, and the 

locations of use sites could be considered to determine the extent to which screening assumptions 

regarding an action area apply to a particular listed organism. These subsequent refinement steps 

could consider how this information would impact the action area for a particular listed organism 

and may potentially include areas of exposure that are downwind and downstream of the pesticide 

use site. 

Species Affected 

The preliminary risk assessment for listed species indicates that thidiazuron exceeds the 

endangered species LOCs for the following combinations of analyzed uses and species: 

• Use of thidiazuron on cotton indicate that chronic LOCs are exceeded for mammals 

foraging on short grass and broadleaf forage and small insects. These risks could be 

mitigated by reducing the use rates approximately 20-fold to 0.014 lb ai/acre. 

Page 44 of 134 
Use of thidiazuron on cotton results in potential risks to listed species of plants. 

County-level location data for listed species (terrestrial plants and mammals) were compared with 

county-level information on crop production to identify coarsely overlaps of listed species with 

the proposed labeled uses of thidaizuron. This analysis was limited to those counties with 10 or 

more acres of land in production for the labeled crops. The results are presented below: 

11 Oklahoma I - I 

Numbers of Listed Plant and Mammal Species in States with at Least 10 Acres of Cotton 

I[ South Carolina 10 - II 

State 

Arkansas 

California 

Florida 

Georgia 

Kansas 

Louisiana 

Mississippi 

Missouri 

New Mexico 

North Carolina 

IV. Environmental Fate Assessment 

Number of Listed Species 

Tennessee 

Texas 

Virginia 

Basis 

Plants 

2 

49 

5 

14 

1 

- 

- 

5 

5 

17 

This assessment is based on an integration of 1) environmental fate study data from previously 

reviewed studies, in some cases with subsequent amendments, addenda, or other written 

explanatory responses or updated information; 2) environmental fate study data from the current 

review of new fate studies submitted to support the re-registration of thidiazuron; 3) intrinsic 

Mammals 

- 

9 

4 

3 

- 

2 

1 

1 

5 

3 

4 

19 

2 

Page 45 of 134 

2 

3 

1 
physical/chemical properties; and 4) information from product labels. As with all EFED fate 

assessments, it does not take into consideration any alteration of the environmental fate properties 

of thidiazuron that may be associated with the addition of co-formulated pesticides (currently 

diuron or dimethipin); so-called "inert" ingredients (formulants); tank mixtures with other 

pesticides, for example, organophosphates, for which label statements indicate increased 

phytotoxicity; or tank mixes with adjuvants, such as surfactants or oils, for which product label 

statements indicate "improved performance." 

Status of Environmental Fate Data 

Although some of the submitted fate studies had deficiencies, taken as a whole and within the 

constraints stipulated in the fate assessment, the data enable the Agency to adequately assess the 

environmental fate of thidiazuron. Although no foliar dissipation studies are available to the 

Agency at this time (these are not generally required), having such data would make any fate 

assessment more precise, especially in the case of a defoliant such as thidiazuron. However, as 

discussed in the fate assessment, if dislodgement from or transformation inlon treated plants were 

to be an important fate pathway, then EFED's inferences from existing fate studies and 

assumptions for exposure modeling for parent and potential degradates should reasonably and 

conservatively cover the possibility. If the registrant wishes to demonstrate that definitive data on 

foliar dissipation would substantially affect any of EFED's inferences, assumptions, or 

environmental risk conclusions, the registrant should submit appropriate data. Otherwise, except 

for spray-drift requirements, the Agency needs no additional environmental fate studies at this 

time. 

Required field spray-drift and spray droplet-size spectral data have not been submitted. However, 

since the applicant is a member of the Spray Drift Task Force (SDTF), the registrant may be able 

to satisfy the spray-related requirements with data the SDTF has submitted to the Agency. , 

Fate Assessment 

----- 

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

intervals on the order of one year. It has intermediate soil sorption coefficients. Such persistence and intermediate 

mobility would allow some year-to-year accumulation over time and, therefore, more opportunity for favorable 

conditions for runoff from application sites to occur. With a one-year half-life, build-up in soil could asymptotically 

approach twice that of annually applied amounts. 

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 is a structural isomer of parent, whereas the 

other has a substantially altered chemical structure. The photolysis process requires special modeling considerations. 

Based on its solubility, vapor pressure, and other laboratory evidence, tbdiazuron is non-volatile. Based on its 

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

No data on foliar dissipation are available to EFED. Since tbdiazuron is used to defoliate cotton prior to harvest, 

dislodgement from or transformation idon leaves may be an important fate pathway. The effect of th~s uncertainty 

and alternative assumptions on the risk assessment are considered. 

Page 46 of 134 
Physical/Chemical Properties 

Thidiazuron's chemical structural formula, various names and table of physical/chemical 

properties are as follows: 

Common Name: 

Other Designations: 

IUPAC name: 

CAS name: 

CAS No.: 

PC Code No.: 

Empirical formula: 

Molecular Weight: 

SMILES string: 

Thidiazuron 

AE B049537, SN 49537, NC 1921 1, Hoe 080279 

I -Phenyl-3-(I ,2,3-thiadiazol-5-yl)urea 

N-Phenyl-N'-I ,2,3,-thiadiazol-5-yI urea 

51 707-55-2 (330-54-1 also found) 

120301 

C, H8 N4 0s 

220.25 

O=C(Ncl cccccl)Nclcnnsl 

Page 47 of 134 

7 - - - ----- 
Physical and 

GLN 

830.6302 

830.6303 

830.6304 

830.6313 

830.6314 

830.631 5 

830.631 6 

830.6317 

830.6319 

830.6320 

830.7000 

830.7050 

830.71 00 

830.7200 

830.7220 

830.7300 

830.7370 

830.7550 

830.7840 

As the table shows, thidiazuron is a weak organic acid with a pKa of approximately 8.9. 

Therefore, at pHs found in most aquatic and terrestrial environments, it exists mainly in neutral 

form (299% neutral at pH 7 or below, 88% at pH 8, and 42% at pH 9). Thidiazuron is nonvolatile, as indicated by its reviewer-calculated Henry's Law constant (H = 3 x atm m3/mol 

and dimensionless K, = 1 x 10'" at 20 OC) and by collateral data showing the absence of 

thidiazuron in collection traps for volatile products in various laboratory degradation and 

830.7950 

Page 48 of 134 

--- 

Chemical Properties of 

Requirement 

C o l o r 

Physical state 

Odor 

Stability to normal and 

elevated temperatures, 

metals, and metal ions 

Oxidation/reduction: 

chemical 

incompatlb~litv 

Flammability 

Explodability 

Storage stability 

Miscibility 

Corrosion 

characteristics 

pH 

UVNisible absorption 

Viscosity 

Melting point 

Boilinq D0int 

Relative Density 

Dissociation constants 

in water 

Partition coefficient. 

(n-octanollwater), 

shake flask method 

Water solubility: 

column elution 

method; shake flask 

method 

(ritprl) 

A = Acceptable; N = Unacceptable (see Deficiency); N/A = Not Applicable.; G = Data gap; I = In progress or need upgrade 

Vapor pressure 

Thidiazuron (from product 

MRlD 

459340-01 

I< ,, ,, 

,, ,, ,, 

'I I, I' 

,, ,, ‘A 

459340-01 

459340-01 

459340-01 

459340-01 

459340-01 

N A 

459340-01 

459340-01 

413649-01 

(cited) 

41 3649-06 

(cited) 

41 3649-06 

41 7862-02 

(cited) 

942460-01 

chemistry 

Status 

A 

A 

A 

A 

A 

N A 

A 

I 

N A 

I 

A 

A 

A 

N A 

A 

A 

A 

A 

review) 

Result or Deficiency 

TGAI: yellow Brown; PAI: Brown 

TGAI: amorphous solid; PAI: crystalline solid 

TGAl & PAI. characteristics Sulfur l~ke odor 

TGAI: None 

PAI: None 

TGAI: No reaction with-H20, KMnO,, Zn or CO, 

PAI: No reaction with-H,O, KMnO,. Zn or CO, 

TGAI: exotherm onset observed at 116.3"C 

PAI: NA 

1 year under progress 

1 year under progress 

TGAI: 5.21 @ 20% 

PAI: 6.50 

A max = 238 nm; e = 985 M-' cm-' 

PAI: 223% (decomposition) 

TGAI: 0.324 gmlcc at 20 "C 

PAI: pKa = 8.86 25 "C 

PAI: K olw = 50.0 (25 OC, pH = 7.3) 

PAI: water = 31.0 mglL (pH = 7.0; 25 'C) 

organic solvents (gll):acetone, 6.67; CH2C1,,0.003; 

ethyl acetate, 1.10; hexane, 0.002; MeOH, 4.20; 

Toluene, 0.40 (all measured at 20 "C) 

A PAI: 3.5 x 10‘~ Pa @ 20 "C 
metabolism studies. Its relatively low n-octanol to water partitioning ratio of approximately 50 

indicates little likelihood of bioconcentration. 

Persistence--Degradation/Metabolism Half-lives 

In standard laboratory studies, thidiazuron did not hydrolyze (MRID 42069203), and was stable 

against anaerobic metabolism in water (MRID 42666101) and soil (MRD 41945201). 

Thidiazuron was relatively long-lived in aerobic soil. As a measure of its persistence, laboratory 

aerobic soil metabolism half-lives2 in four soils were approximately 206 (173-254), 253 (1 87- 

393), 436 (310-737), and 140 (1 19-170) days (95% confidence intervals in parentheses as a 

measure of precision). The first three listed half-lives are derived fiom MRID 461 196013, the 

fourth from MRlD 419501014. The average of these four values is 259 days, and the one-tailed 

upper 90% confidence bound for the mean, which is used for EFED standard environmental 

modeling purposes, is approximately 363 days (259-day average plus a confidence level of 

104 days). Soil metabolites are identified in a Transformation Products (degradateslmetabolites) 

subsection below. 

Crop rotational intervals also provide a measure of thidiazuron's persistence in soil. Product 

labels list rotational crop intervals as long as one year. 

Another index of thidiazuron's persistence in soil is dissipation in terrestrial field studies. It 

should be recognized, however, that field dissipation is punctuated by variable climatic events, 

and generally involves a combination of physical dispersal/transport (leaching, runoff, 

volatilization) and chemical and/or biochemical transformation. Furthermore, transformation 

does not necessarily imply detoxification, since byproducts may also be toxic. Results from 

terrestrial field studies are as follows: 

'~alf-lives are based on first-order linear regression of time (x-axis) versus the logarithm of the relative 

concentration (percent of radioactivity) of tbdiazuron (y-axis). These experimental half-lives were then adjusted by 

the reviewer to the EFED standard temperature of 25 "C and to the standard soil moisture content of 75% of 113-bar 

water potential (suction) by the methods described in the next footnote. 

3~RID 461 19601, Allan, J.G., 2003; EFED review 24 May 2004. Experimental half lives were adjusted by 

the reviewer to the EFED standard temperature of 25 "C by use of the Arrhenius equation and to the standard soil 

moisture content of 75% of 113-bar water potential (suction) by use of the Walker equation, as described in the Data 

Evaluation Record (DER). Actual study conditions were 20 "C and 40% maximum water holding capacity 

(MWHC); under these conditions half-lives were 163, 355, and 322 days, respectively, as extrapolated from study 

periods lasting 160-1 68 days. 

4 MRTD 41950101, Feyerabend, M., 1991; EFED review 28 Apr 1993. Previous reviewer incorrectly ' 

reported the half-life as 11 1 days at 21 "C and 78% of 1/3-bar. The 11 1-day value was the registrant-reported DT50, 

not the half-life. Based on this reviewer's calculation using standard semi-logarithmic transformed data, the 

experimental half-life is 182 days (95% confidence range of 155-221 days). After adjustment to 25 "C and 75% of 

113-bar soil water potential by the method described by the reviewer in the DER for MRID 461 19601 (see previous 

footnote), the standard half-life becomes 140 (1 19-170) days. (The registrant's calculated half-life using 

untransformed, simple first-order data was 147 days (no confidence range reported.) 

Page 49 of 134 
In three supplemental, bare-ground field studies (MRID 44454001), each conducted for 18 

months, linear first-order dissipation half-lives were 237 days (California), 173 days (Florida), and 

161 days (North Carolina). Reviewer-calculated respective times required for 90% loss, based on 

simple first-order kinetics were approximately 800, 600, and 500 days. Corresponding registrantcalculated DT90 values, based on a two-compartment model, were 1 190,655, and 475 days. 

Although these measures of field persistence are conventionally reported, because of the inherent 

variability and randomness in dissipation processes, they should be interpreted only in a crude, 

qualitative sense. Three transformation products were sporadically identified, but were most 

frequently below the limit of quantitation; therefore, a kinetic pattern could not be established. In 

another supplemental study in Florida (MRID 41 76 1 105), there was no significant dissipation of 

thidiazuron during a nine-month study period in which only small amounts of two transformation 

products were detected. Identified soil transformation products and observations concerning 

mobility are discussed separate subsections below. 

Thus, laboratory aerobic soil metabolism studies, product label statements, and field dissipation 

studies show a general consistency and a similar degree of persistence for thidiazuron. The 

demonstrated persistence would allow some year to year accumulation. With the hypothetical 

assumption that there is no significant physical movement of thidiazuron from a site of 

application for several years, then, with the upper 90% confidence bound half-life of 

approximately one year and with uniform annual applications, the asymptotic limit of build-up in 

. soil would result in soil concentrations that would approach twice that of the yearly application 

amount. 

Photolysis of thidiazuron requires special consideration. In water (original MRID 41 188201, 

related follow-up MRIDs 41364910 ,43075202, and fate overview document MRID 44436901), 

it undergoes relatively rapid (see below), pH dependent, branching photolysis into two 

photoproducts. Formation of these products is in constant ratio, and in complementary 

stoichiometric yield with parent. A product originally identified as "photoproduct I" and 

subsequently identified as AE F132347 (see chemical structure in Appendix B) is an isomer of 

parent differing only in the relative positions of the sulfur atom and the two nitrogen atoms in the 

5-membered ring. Like parent, photoproduct I is moderately ecotoxic (slightly more ecotoxic than 

parent, based on limited data). It has a reviewer-calculated asymptotic experimental limit of 

production of approximately 77% of parent at pH 5,28% at pH 7, and 17% at pH 9. This product 

was stable to further photolysis under the experimental study conditions and durations. 

Photoproduct I1 (AE C421200) is a significantly degraded, practically non-ecotoxic product, and 

has a compleme*tary reviewer-calculated asymptotic limit of production to 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. 

Under standard, ideal laboratory exposure conditions, the pH dependent photolysis half-lives were 

approximately one to two hours (adjusted to twelve hours of sun per day). [Note: When these 

half-lives are put directly into current EFED aquatic modeling scenarios, because of water depth, 

clarity, and other factors, the effective or practical photolysis half-lives become equivalent to 

approximately five to ten days.] For the purpose of aquatic ecological exposure assessment, we 

protectively assumed the pH 5 results for which 77% of parent reaching water is converted to the 

Page 50 of 134 
stable isomer photoproduct I (AE F132347). Under these conditions, the thidiazuron photolytic 

half-life was 1.44 hours (0.0602 days). Half-lives at higher pHs were somewhat shorter 

(approximately 6% shorter at pH 7 and approximately 40% shorter at pH 9. 

To further simplify exposure estimates for aquatic ecological assessment, rather than separately 

apportioning parent and photoproduct I exposures and toxicities, we chose to add exposure 

concentrations of parent and photoproduct I (ignoring the practically non-toxic ecotoxic 

photoproduct II), and use the toxicity of the slightly more ecotoxic photoproduct I for parent also. 

Whether we use this simplified procedure or add the separate contribhtions from parent and 

photoproduct I makes no meaninghl difference in the aquatic ecological risk assessment in this 

particular case. Appendix D describes in detail the special quantitative procedure used for 

estimating aquatic exposure concentrations. For surface drinking water sources for humans, we 

provide two other altematives, depending on potential human health concerns for parent and 

photoproducts; these alternatives are discussed in the Drinking Water Assessment Appendix A. 

Because of thidiazuron's demonstrated persistence in soil, modeled concentrations in water could 

be underestimated by as much as a factor of two. 

A unacceptable laboratory soil photolysis study conducted on thin-plated soil samples had 

numerous flaws and inconsistencies (MRID 001 56241 and MRID 41 364902, the latter being 

essentially a re-submission of a published version of the unpublished former; and fate overview 

document MIUD 44436901, which summarized study issues and responses). The previously cited 

unacceptability of the study is confirmed by the current reviewer (who has noticed even additional 

study irregularities). Ostensibly, the study authors maintained that soil photolysis was occurring 

at a relatively rapid rate, with a half-life on the order of one hour or less, and that the only 

significant product was the isomer photoproduct I (AE F132347). The reviewer notes that 

photoproduct I is the same photoisomer found in the aqueous photolysis study discussed above. 

In view of the study unacceptability and the inherent marked physical differences between the 

effectiveness of photolysis on soil that is plated thin (0.5 rnm) on glass an that applied on soil in 

the field, the registrant and the Agency are in agreement that terrestrial field studies would serve 

to determine the relative importance of soil photodegradation. In all field studies (the four studies 

discussed above), thidiazuron was sprayed on bare soil without incorporation. In these studies 

thidiazuron was long-lived, and there was only minimal evidence of photolysis. Hence, to 

whatever extent photolysis may occur inlon field soil, it is definitely a slow process. 

Transformation Products (degradateslmetabolites) , 

Chemical structures, chemical names, and other forms of past and present identifiers are in 

Appendix B. Naming conventions, different company codes for the same compounds, and 

incorrectly named compounds have been and can easily be tedious sources of confusion and 

potential error. Major photodegradates in water and soil have already been discussed. 

In the two submitted laboratory aerobic metabolism studies identified above, no major (defined as 

- > 10% of applied radioactivity) organic transformation products were isolated or identified in any 

of a total of four soils tested. In the most recent study (MRID 461 19601), the three soils tested 

were treated with thidiazuron radiolabeled with 14C in the thiadiazol-ring position; in the other 

I 

Page 5 1 of 134 
study (MRID 41950101), the one soil was treated with thidiazuron uniformly radiolabeled with 

14C in the phenyl ring. Such labeling insures that major molecular changes can be monitored. 

In the three soils treated with the thidiazol-ring label (MRID 461 19601), the only organic 

transformation product identified was AE F 132345 (thidiazolylurea or 1,2,3-thiadiazol-5-ylurea). 

In the soil in which metabolism was fastest, AE F132345 appeared to reach an approximately flat 

maximum concentration of roughly 6-9% of applied radioactivity during the period fi-om 

approximately 88 days to study end at 168 days. The broad plateau is evidence that AE F132345 

would not form in greater yield, and that it may be a persistent metabolite. Maximum 

concentrations of AE F132345 in the other two soils were roughly 1% to 3%. Extracted 

substances that were not identified in any soil at any sampling interval comprised no more than 

roughly 1-6% of applied radioactivity. Non-extracted 14C material (so-called "bound" residues) in 

these soils increased variably from approximately 16-32% of applied at study end. In one soil, 

14C0, reached a broad total plateau of 5-12% of applied radioactivity during later study periods. 

In the two other soils, 14C0, totaled around 1-2% and 6-8%, respectively, during later study 

periods. For all three soils, volatile organics totaled 50.1% of applied radioactivity. The data 

evaluation record (DER) for the three-soil study has additional details for individual soils, and a 

brief comment on a postulated transformation pathway. 

In the one soil treated with the uniformly labeled phenyl ring (MRID 41950101), three very minor 

metabolites were identified, and at least one these may have been an initial impurity. All 

concentrations of each were < 0.8% of applied radioactivity. The three minor metabolites were: 

AE F132347 (0.8%), the compound that may have been an initial impurity and which is the same 

compound identified above as photoproduct I; phenylurea (0.3%); and AE F147706 (0.8%). 

(Appendix B shows chemical structures and has various past and present forms of identification.) 

Of extracted substances, approximately 3% of applied radioactivity was not identified. By the end 

of the 361-day study, unextracted ("bound") residues increased to approximately 45% of applied, 

and 14C0, accounted for approximately 2 1 %. 

In the supplemental terrestrial field studies cited above (MRID 44454001 and MRID 41761 105), 

three transformation products were detected, but rarely above the limit of quantitation of 

0.01 parts per million (ppm). The three compounds were AE F132345, AE F132347 

(photoproduct I), and phenylurea. Since they were almost never detected above the limit of 

quantitation of 0.01 ppm for any sampling interval, no kinetic analysis is possible for them. The 

maximum concentrations for the sporadic detections of these products ranged, without correlation, 

from roughly <4% to perhaps as high as 19% of the maximum concentration ever detected for 

parent. 

Thus, based on laboratory and field studies, thidiazuron undergoes very slow transformation in 

soil by aerobic metabolism, with an indefinite contribution from photolysis. Laboratory studies 

further suggest that the identified transformation products will slowly be converted in soil into the 

carbon pool of bound residues, carbon dioxide, and other products of mineralization. 

Page 52 of 134 
Mobility in Soil 

Based on batch equilibrium sorption coefficients in four soils (MRID 41364909, three German 

standard soils and one United Kingdom soil), thidiazuron has intermediate mobility, as indicated 

by Freundlich adsorption coefficients of approximately 4.4,7.3, 16, and 19 Freundlich units. 

Slopes (1111) of the isotherms ranged from approximately 0.7-0.8. Sorption correlated reasonably 

well with organic carbon, and respective Freundlich coefficients normalized for organic carbon 

(Kfoc) were approximately 900, 800, 800, and 500. Soil textural identification and detailed 

sorption values are in Appendix C. 

In the four supplemental terrestrial field studies in bare soil discussed above, no significant 

leaching was apparent for parent or transformation products. However, because of imposed 

experimental limitations that were common to each study (for example, limits of quantitation and 

the absence of a water balance or use of a tracer) potential for leaching was not clearly 

established. However, based on the intermediate range of its sorption coefficients and the 

supplemental field evidence, leaching should not be major route of dissipation. Model-estimated 

potential exposure to thidiazuron from surface and ground drinking water source are presented in 

the separate Drinking Water section of this document. 

The one major metabolite found in laboratory aerobic soil studies, 1,2,3-thiadiazol-5ylurea (also 

identified as thidiazolylurea, AE 132345, and ZK 85290 ), had somewhat lower sorption 

coefficients than parent. In four United States soils, its Freundlich adsorption coefficients were 

approximately 1.5, 8.3,2.0, and 1.7 Freundlich units. Values for lln ranged from approximately 

0.8-1. Sorption also correlated reasonably well with organic carbon, and respective Freundlich 

coefficients normalized for organic carbon (Kfoc) were approximately 200,280, 170, and 340. 

Soil textural identification and detailed sorption values are in the Data Evaluation Record (DER) 

for MRID 44436902. As was the case for parent in the same supplemental field studies, no 

significant leaching pattern was evident for this metabolite, which was detected sporadically and 

without correlation at c0.05 ppm. However, as previously stated, potential for leaching was not 

clearly established in the field. 

No data on foliar dissipation are available to EFED. Since thidiazuron is used to defoliate cotton 

prior to harvest, dislodgement from or transformation idon leaves may be an important fate 

pathway. Although photolysis was the rapid, major route of transformation in water, in field soil 

parent was long-lived with only minimal evidence of photolysis or leaching. However, in 

laboratory plates of thin soil, the photoisomer AE F132347 (photoproduct I) was a quickly-fonned 

major degradate. Thus, whether photolysis in or on cotton plants is a major fate process is 

unclear, and can be decided only by experiment. However, based on available data, if photolysis 

were to be substantial on foliage, it is a reasonable inference that the same major structural isomer 

formed in water and soil also be produced on foliage. In this case, EFED's modeling assumptions 

for parent and the photoisomer should cover the possibility, and, within standard model 

limitations, our estimated water exposure concentrations remain essentially the same. 

Page 53 of 134 
It bears repeating that thidiazuron is persistent in soil, as evidenced by laboratory and field halflives of the order of one year, and that it has intermediate soil sorption coefficients. Such 

persistence and intermediate mobility allows some year-to-year build-up in soil over time and: 

therefore, greater opportunity for favorable conditions for runoff from application sites to occur. 

With the hypothetical assumption that there is no significant physical movement of thidiazuron 

from a site of application for several years, then, with a one-year half-life and uniform annual 

applications, the asymptotic limit of build-up would result in soil concentrations that would 

approach twice that of the yearly application amount. Runoff concentrations would be affected 

similarly. 

V. Drinking Water Assessment Summary 

There are no useful water monitoring data for thdiazuron in EFED's standard monitoring 

sources5. The exposure estimates in this assessment come from the two EFED Tier 1 screeninglevel models: 1) FIRST (Ver. 1.0; Aug. 1,2001) for surface water concentrations resulting from 

runoff and spray drift into an Index Reservoir and 2) SCI-GROW (Ver. 2.3; Nov. 4,2003) for 

ground water concentrations resulting from leaching. These models and their descriptions are 

available at the EPA internet site: 

As described in the Fate Assessment, 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 the two photoproducts identified as AE F132347 and AE C421200. The first is a 

structural isomer of parent, while the latter has a substantially altered chemical structure. The 

photolysis process, chemical identification, and special modeling and computational 

considerations are in Appendix A. Ground water is not expected to be impacted by these 

degradates because all field studies show that parent is long-lived in soil (half-lives of the order of 

one year) with only minimal evidence of photolysis. 

Modeling Results. The following table shows the FIRST-derived potential surface water 

concentrations for the three respective combinations of 1) parent plus both aqueous 

photodegradation products (most conservative), 2) parent plus photodegradate 1, and 3) parent 

only (least conservative). Degradate modeling is based on mass balance using simultaneous 

decline and formation over time. All reported concentration combinations are in terms of parent - 

mass chemical equivalents. However, concentrations for each photodegradate can easily be 

separated. 

The tabulated exposure concentrations are for a single aerial application at a maximum rate of 

0.3 lb a.i.lacre. However, as specifically illustrated in Appendix A, whether the total annual 

maximum is applied all at once or is split in two (as specified on product labels), differences are 

5~~~~'s Pesticides in Ground Water Database has no thidiazuron data. The United States Geological 

Survey (USGS), as part of a large water monitoring program, does not analyze for thidiazuron. The EPA STORET 

water quality databases have no useful information. 

Page 54 of 134 
negligible for the particular case of thidiazuron because of its long life in soil. Again, complete 

model inputloutput tables with detailed explanations of the computational procedures used to 

handle the photodegradates are in Appendix A. 

Surface Water Concentrations (derived from FIRST model, Ver. 1.0) 

Parent Only 3.4 I 0.068 

PEAK DAY (ACUTE) 

DEGREES OF PROTECTION CONCENTRATION 

b or pg/L) 

Parent + Both Photoproducts 3.5 

Parent + AE F132347 (photoproduct 1) 

The next table shows the SCI-GROW-estimated potential groundwater exposure concentration. 

Groundwater is not expected to be impacted by photodegradates because all field studies show 

that parent is long-lived in soil (half-lives on the order of one year) with only minimal evidence of 

photolysis or leaching. However, as previously discussed, dislodgeability, photolysis (or other 

chemical changes) in or on plant surfaces are uncertain. Therefore, it is possible that 

photodegradates identified in water and soil or other compounds may form in or on cotton plants, 

reach the soil surface at unknown rates, and be subject to unknown degrees of leaching or runoff. 

EFED's assessment cannot specifically account for this absent information. However, based on 

all available fate evidence, it appears that photodegradation would be the most likely candidate for 

any rapid degradation of thidiazwon that may occur on plants. If this is the case, then, to a first 

approximation, the physical and chemical fate properties of the photodegradates should be similar 

to parent, and any resulting loss in parent would be offset by production of photodegradates. 

Therefore, combined concentrations of all these compounds should approximate those given in 

the table for parent only. 

ANNUAL AVERAGE (CHRONIC) 

CONCENTRATION 

(ppb or pg/L) 

1 .O 

Groundwater Concentration 

(derived from SCI-GROW regression model, Ver. 2.3 ) 

3.5 

I Acute and Chronic Concentration*: 0.066 ppb (pg/L) 

0.82 

*As also noted in Appendix A, the SCI-GROW-estimated groundwater concentration given here is an extrapolation 

beyond the regression model limits for Koc. SCI-GROW was developed using Koc values ranging from 

approximately 32 to 180 mL/g of organic carbon, whereas the median Koc value for thidiazuron is 783 mL/g. 

Extrapolation increases uncertainty, but without ability to estimate confidence. 

Page 55 of 134 
VI. Aquatic Hazard, Exposure, and Risk Quotient Calculation 

Appendix F summarizes the results for toxicity studies material to this risk assessment. 

Discussions of the effects of thidiazuron, formulated product, and degradates on aquatic 

taxonomic groups are presented in the following sections. 

Toxicity to Fish 

An acute freshwater fish toxicity classification (slightly toxic, moderately toxic, and practically 

non-toxic) for thidiazuron cannot be definitively established because available acute toxicity data 

(MRID 420692-01 and 420692-02) do not establish an LC50 (the endpoint critical to 

classification). However, the available data for parent thidiazuron indicate that no mortality nor 

signs of intoxication were observed at the highest concentration tested 19 and 32 mg/L. 

Additional acute freshwater toxic testing was performed on three metabolites of thidiazuron. The 

LC50 for the photodegradate AE F132347 was established at 6,700 pg/L (MRID 462035-08), 

indicating that this degradate is more toxic than the parent thidiazuron. This compound is 

classified as moderately toxic to freshwater fish. The metabolite (AE F132345) indicated that the 

LC50 was >101,000 yg/L (MRID 462035-15) and suggest that this compound as practically nontoxic to freshwater fish. In addition the metabolite (AE C421200) LC50 could not be determined 

definitively, but it was tested up to 103,000 pg/L, and the results also suggest classification of this 

metabolite as practically non-toxic to freshwater fish. 

The acute thidiazuron LC50 for marine fish was not determined by available toxicity data (MRID 

41 8461-01). Lack of an LC50, precluded a toxicity classification. However, the data do suggest 

that the NOAEC for thidiazuron is at least as high as the highest dose tested (36,000 yg/L), which 

is just above the established solubility limit of the compound (31,000 pg/L). No sub-lethal effects 

were observed in this study. Because of the lack of a reported LOAEC, the NOAEC result from 

this study is uncertain. However, it is unlikely that a repeat of the study will generate an acute 

lethality endpoint consistent with screening risk assessment methods that would be at 

environmentally relevant concentrations. There are no data for either the formulated product or the 

degradates. 

A chronic toxicity study was submitted for freshwater fish early life-stage (fathead minnow) for 

parent thidiazuron. The NOAEC of the parent was 5,700 pg/L based on hatchability, fky survival, 

and total survival (MRID 421320-02). A chronic full life cycle test was not submitted for 

freshwater fish or marinelestuarine fish. 

Toxicity to Aquatic Invertebrates 

Acute freshwater invertebrate data were submitted for the parent thidiazuron and the LC50 of 

5,700 pg/L (MRID 464053-03) classifies the parent as moderately toxic to freshwater 

invertebrates. A previous study conducted in 1979 determined the LC50 to be 10,000 pg/L 

(MRID 0998 19). 

Page 56 of 134 
Acute freshwater invertebrate studies on the major transformation products were also submitted. 

The LC50 for the photodegradate AC F132347 was >12,000 pgL (MRID 462035-09). The 

LC5Os for the AC F132345 metabolite (MRJD 462035-16) and the AE C421200 photodegradate 

(MRID 462035-12) were >98,000 pg/L. 

Acute marine invertebrate studies show that the mysid shrimp (MRID 41 8416-02) and the eastern 

oyster embryollarvae (MRID 421320-01) LC5Os are 3,240 and 5,384 pg/L, respectively. This 

classifies thidizuron as moderately toxic to marine acute invertebrates. No studies were submitted 

for any of the metabolites. 

A chronic daphnid toxicity study was also available for thidiazuron. The LOAEC was determined 

to be 400 pg/L based on length (MRID 421320-021436650-01). Even though a definitive 

NOAEC could not be obtained, the 21-day EClO was found to be 720 pg/L. No chronic data 

were submitted for marine invertebrates. 

Toxicity to Aquatic Plants 

Aquatic plant testing was conducted on the parent thiadiazwon. The E50 was determined to be 

>24,000 pg/L for the vascular plant (Lemna gibba) based on frond count as the most sensitive 

parameter. The NOAEC and EC05 were determined to be 8,600 and 570 pg/L, respectively 

(MFUD 462035-06). Non-vascular plant data were submitted for the green alga (Selenastrum 

copricornatum), blue-green algae (AnabaenaJlos-aquae), and the marine diatom (Skeletonema 

costatum) Skeletonema costatum). The most sensitive of these non-vascular plants was the 

marine diatom with a E50 of 850 pg/L, a NOAEC of 110 pg/L, and EC05 of 56 pg/L. No data 

are currently available for the freshwater diatom (Navicula pelliculosa). 

/ 

Additional non-vascular plant data (green alga) was submitted for the photodegradate 

AE F132347, the metabolite AC F132345, and the photodegradate AE C421200. The most 

sensitive of these metabolites was determined to be the AE F132347 photodegradate, with EC50, 

NOAEC, and EC05 values of 980,220, and 3 10 pg/L, respectively. All these studies were 

considered as Tier 1 studies and classified as supplemental because they were conducted for 3 

days instead of 5 days as required by the USEPA. 

Aquatic Exposure 

Aquatic exposure and the special emphasis on photolysis in water is discussed in the 

Environmental Fate Assessment section of this document and in greater detail in.Appendix D. 

Aqueous photolysis is rapid, and occurs by branching in quantitative yield into the two 

photoproducts identified as photoproduct I (AE F132347) and photoproduct 11 (AE C421200) (see 

chemical structures in Appendix B). The first is a structural isomer of parent, while the latter has 

a substantially altered chemical structure. 

For reasons previously stated, including the toxicity data available for degradates, the exposure 

concentrations used for the aquatic risk assessment are the time dependent summations of the 

concentrations of parent thidiazuron and its photoisomer (photoproduct I). Exposure 

Page 57 of 134 
concentrations were derived from a combination of GENEEC model runs, as given explicitly in 

Appendix D. Resulting concentrations for parent plus the photoisomer are in the following table: 

Risk Quotients 

Estimated Environmental Concentrations (ppb or pg/L) 

The methodology for calculating RQs is presented in Appendix G. The resulting RQs for the 

parent toxicity values are presented in detail in Appendix H. 

Peak 

11 

VII. Terrestrial Hazard, Exposure, and Risk Quotient Calculation 

Appendix F provides tabular summaries of the toxicity studies material to the risk assessment for 

thidiazuron. The toxicities of thidiazuron, formulated products, and degradates are discussed 

below for each terrestrial taxonomic group studied. 

Toxicity to Birds 

4-Day Avg. 

11 

Acute testing through oral gavage of technical thidiazuron to bobwhite quail indicates that the 

LD,, is >3 160 mglkg-body weight (MRID 099819). A similar study was not submitted for the 

mallard duck. However, a Japanese quail study with the formulated product CP 503 WP indicated 

a LD50 >9989 mglkg-bw (4995 mg ailkg-bw). These LD5O values classify thidiazuron as 

practically non-toxic to birds. 

21-Day Avg. 

9.7 

60-Day Avg. 

8.6 

The dietary LC,, for both species is > 5000 mg/kg-diet for the technical thidiazuron. No sublethal effects or other treatment related effects were observed in any of these studies. This 

classifies technical thidiazuron as practically non-toxic to birds. Chronic testing of thidiazuron 

was not submitted. Details of the results are tabulated in Appendix F. 

90-Day Avg. 

8.2 

Toxicity to Mammals 

Acute testing through oral gavage of technical Thidiazuron (AE Bo49537 00 ID99 0003) to 

laboratory rats indicates that the LD,, is > 2000 mg/kg-body weight. This classifies the technical 

thidiazuron as a category III compound. 

For chronic mammalian risks a two generation rat or mouse study, and/or the developmental 

rabbit study are more relevant than the longer term mammalian carcinogenicity1oncogenicity 

studies. No rabbit developmental study is available at this time. The review of the two generation 

rat study is currently in progress within HED, so EFED is using the results fiom a previous three 

generation rat study on the thidiazuron technical. This study was classified as CoreISupplemental 

because several deficiencies were noted including the following: 

Page 58 of 134 
Pup body weights were only submitted for lactation day 21. The guidelines require body 

weight data on days 0,4 (pre- and post-cull), 7, 14, and 21. 

There was no measure of food consumption. The amount of pesticide consumed was 

estimated by using default food consumption values for the laboratory rat developed by 

FDA years ago. 

A dietary analysis was conducted but was not provided in the report. 

Little detail was provided on the mating design. For example, EPA guidelines specify that 

sibling from the same litters should not be mated. It was unclear if this guideline was 

followed. 

The reproductive/offspring NOEL was 200 ppm (10 mg ailkghwlda) based on a decrease in litter 

size. 

Toxicity to Non-target Insects 

Acute contact toxicity studies to honeybees revealed contact LD,,s of > 100 pg ai /bee for 

thidiazuron technical and > 98.1 pg ai /bee for the SC42 water miscible suspension concentrate 

500 g/L (MRID 462035-01,462035-18). These results classify thidiazuron as practically nontoxic to non-target insects. In addition, a 72-hour acute oral toxicity test was conducted for the 

SC42 water miscible suspension concentrate 500 g/L in accordance with OECD guidelines 

(MRID 462035-19). This study resulted in an oral contact LD,, of >197.8 pg ai/bee. 

Two other non-target insect studies investigated the lethal and sub-lethal effect of the SC42 water 

miscible suspension concentrate 500 glL on the parasitoid wasp (Aphidius rhopalosiphi) and the 

predatory mite, Typhlodromuspyri, when exposed on a glass surface. These studies were 

performed to satisfy the OECD guideline requirements, and although scientifically sound, do not 

fulfill the OPP guideline requirement. 

The parasitoid wasp study (MRID 46203520) exposed 4 replicates (10 waspslreplicate) for 48 

hours, at concentrations of 100 and 200 g ailha (0.1 and 0.2 lb ai/A). The percent mortality was 5 

and 12.5%, respectively, as compared to 10% in the controls. Therefore, it was concluded that the 

LC,, was > 200 g a.i.lha (0.2 lb ai1A). To determine reproductive success through parasitation, 15 

wasps were placed on aphid-infested oat plants. One day later the wasps were removed and the 

plants kept for 12 additional days. The parasitation rate was determined by counting the number 

of mummies for each individual wasp. The percent reductions when compared to the control were 

27.3 and 38.1 % in the 100 and 200 g ailha treatment groups, respectively. 

In the predatory mite study (MRID 46203521) 5 replicates (20 miteslreplicate) were exposed to 

200 g ai/ha (0.2 lb ai/A) via a floating glass plate. Dead mites and escapees (mites that left the 

glass plate and were found stuck in the water barrier) were counted as mortalities. Mites were 

observed over a 14-day period; the mortalities were 8% in the treated group and 13% in the 

untreated group. To measure reproductive performance the number of eggs per female was 

Page 59 of 134 
determined by counting the numbers of females and eggs from day 7 to day 14. The mean number 

of eggs per female was 9.32 in the control group and 8.99 in the 200 g ai/ha treated group. 

Toxicity to Earthworms 

Acute toxicity studies to earthworms (Eisenia foetida) were performed in accordance with OECD 

guidelines for the thidiazuron SC 500 G/L (42.6% ai), the AE F132347 photodegradate, and the 

AE F132345 metabolite (MRID #s 46203522,46203507, and 46203514). All studies exposed 

earthworms at nominal test concentrations of 62.5, 125,250,500, and 1000 mglkg for 14 days. 

Endpoints observed included mortality and worm body weights. A definitive LC,, based on 

I mortality was not determined for any of the tested compounds, however, sub-lethal NOAECs and 

LOAECs were determined based on worm body weight loss. The results are presented in the table 

I below. 

Test Substance 

AE F132347 

photodegradate 

AE F132345 

metabolite 

XC,, > 1000 mg produc 

Toxicity to Non-Target Plants 

LC50 (mg am9 

>426" 

31000 

Terrestrial plant testing (seedling emergence and vegetative vigor) is required for herbicides that 

have terrestrial non-residential outdoor use patterns and that may move off the application site 

through volatilization (vapor pressure >1.0 x 10'5mm Hg at 25OC) or spraydrift (aerial or 

irrigation) andlor that may have endangered or threatened plant species associated with the 

application site. 

Currently, terrestrial plant testing is not required for pesticides other than herbicides except on a 

case-by-case basis (e.g., labeling bears phytotoxicity warnings, or incident datafliterature indicaate 

phytotoxicity). Since thidiazuron is used as a defoliant on cotton to inhibit regrowth and removal 

of both mature and juvenile leaves in cotton, non-target terrestrial plant data are required. 

NOAEC (mg ailkg) 

125 

250 

For seedling emergence and vegetative vigor testing the following plant species and groups should 

be tested: (1) six species of at least four dicotyledonous families, one species of which is soybean 

(Glycine max) and the second is a root crop, and (2) four species of at least two 

monocotyledonous families, one of which is corn (Zea mays). 

LOAEC (mg ai/kg) 

250 

5 00 

Tier I tests measure the response of plants, relative to a control, at a test level that is equal to the 

highest use rate (expressed as Ibs ai/A). If effects are observed in this test, the registrant is 

required to proceed to the Tier I1 level. 

Page 60 of 134 
Tier I terrestrial plant testing for the seedling emergence test conducted in 1991 showed that the 

response of the tested plants was less than 25% when compared to the controls. However, the 

vegetative vigor test conducted in 1990 indicated negative growth response (>25%) in one of the 

four monocots (onion) and five of the six species of dicots tested. 

Tier I1 tests measure the response of plants, relative to a control, and five or more test 

concentrations. Terrestrial Tier I1 studies are required for all low dose herbicides (those with the 

maximum use rate of 0.5 lbs ai/A or less) and any pesticide showing a negative response equal to 

or greater than 25% in Tier I tests. The registrant may opt to proceed directly to Tier I1 testing. 

Recent data submitted (2003) combined tier I and I1 testing for seedling emergence and vegetative 

vigor (MRID #s 459085-01 and 459215-01) for the SC42 formulated product. The seedling 

emergence tests indicated negative growth responses to plant emergence for onion and oat by 

more than 25% at the maximum single application label rate of 0.2 lb ai/A. This would indicate 

that a need exists for tier 11 testing for these two monocot species. All other tested species showed 

definitive results or an EC2, above the maximum single application rate. 

Similar results were indicated for the vegetative vigor tests. Negative growth responses below the 

EC2, level were indicated for all the monocots at the maximum single application rate. This 

indicates that tier II testing will need to be performed for these monocot species. All of the dicot 

plants were tested at the tier I1 level with the exception of turnip and cabbage and since these 

plants are not among the most sensitive dicots tested, further testing will not be required. The 

results from these tests are detailed in Appendix F and summarized in the table below. 

0.0152 / 0.003 1 Tomato 

Terrestrial Plant Exposure and Risk Quotient Calculations 

To calculate the RQs for non-endangered plants the EC,, value of the most sensitive species in the 

seedling emergence study is compared to runoff and drift exposure to determine the RQ 

(EECitoxicity value). The EC2, value of the most sensitive species in the vegetative vigor study is 

Page 61 of 134 
compared to the drift exposure to determine the acute RQ. RQs are calculated for the most 

sensitive monocot and dicot species. 

To calculate the RQs for endangered plants, the NOAEC or EC,, value of the most sensitive 

species in the seedling emergence study is compared to runoff and drift exposure (EECItoxicity 

value). The NOAEC or EC,, value of the most sensitive species in the vegetative vigor study is 

compared to the drift exposure to determine the acute RQ. 

Exposure to terrestrial plants inhabiting dry and semi-aquatic areas may be exposed to pesticides 

from runoff, spray drift or volatilization. Volatilization is not included in this assessment. Semiaquatic areas are those low-lying wet areas that may be dry at certain times of the year. EFED's 

runoff exposure estimate is: (1) based on a pesticide's water solubility and the amount of pesticide 

present on the soil surface and its top one inch, (2) characterized as "sheet runoff' (one treated 

acre to an adjacent acre) for dry areas, (3) characterized as "channelized runoff" (10 treated acres 

to a distant low-lying acre) for semi-aquatic areas, and (4) based on percent runoff values of 0.01, 

0.02, and 0.05 for water solubility of <lo ppm, 10-100 ppm, and >I00 ppm, respectively. 

Spray drift exposure from ground and overhead chemigation applications is assumed to be 1% of 

the application rate. Spray drift from aerial, airblast, and forced-air applications is assumed to be 

5% of the application rate with an application efficiency of 60%. The effects of multiple 

applications are addressed by summing the application rates from individual applications. 

EECs are calculated for the following application methods: (1) unincorporated ground 

applications, (2) incorporated ground application, and (3) aerial, airblast, forced-air, and 

chemigation applications. Formulas for calculating EECs for dry areas adjacent to treatment sites 

and EECs for semi-aquatic areas follow. 

EEC Formulas: 

Calculating EECs for terrestrial plants inhabit in^ drv areas adiacent to treatment sites 

Unincorporated ground application: 

Runoff = maximum application rate (lbs aiIA) x runoff value x number of applications 

Drift = maximum application rate x 0.01 

Total Loading = runoff (Ibs ailacre) + drift (lbs ai/A) 

Incorporated ground application: 

Runoff = [maximum application rate (Ibs ai/A) + minimum incorporation depth (cm.)] x 

runoff value x number of applications 

Drift = maximum application rate x 0.01 

(Note: drift is not calculated if the product is incorporated at the time of application.) 

Total Loading = runoff (lbs ai/A) + drift (lbs ai/A) 

Page 62 of 134 
Aerial, airblast, forced-air, and chemigation applications: 

Runoff = maximum application rate (lbs ai/A) x 0.6 (60% application efficiency assumed) 

x runoff value x number of applications 

Drift = maximum application rate (lbs ai1A) x 0.05 

Total Loading = runoff (lbs ai/A) + drift (lbs ai1A) 

Calculatinp EECs for terrestrial plants inhabiting semi-aauatic low-lvin~ areas 

Unincorporated ground application: 

Runoff = maximum application rate (lbs ai/A) x runoff value x 10 acres x number of 

applications 

Drift = maximum application rate x 0.0 1 

Total Loading = runoff (lbs ai1A) + drift (lbs ai/A) 

Incorporated ground application: 

Runoff = [maximum application rate (lbs ai1A)lminimum incorporation depth (cm)] x 

runoff value x 10 acres x number of applications 

Drift = maximum application rate x 0.01 

(Note: drift is not calculated if the product is incorporated at the time of application.) 

Total Loading = runoff (lbs ailA) + drift (Ibs ai1A) 

Aerial, airblast, and forced-air applications: 

Runoff = maximum application rate (lbs ailacre) x 0.6 (60% application efficiency 

assumed) x runoff value x 10 acres x number of applications \ 

Drift = maximum application rate (lbs ai1A) x 0.05 

Total Loading = runoff (lbs ailA) + drift (lbs ai/A) 

runoff values = 0.01, 0.02, and 0.05 for water solubility of <10 ppm, 10-100 ppm, and 

>I00 ppm, respectively 

Incorporation depth: Use the minimum incorporation depth prescribed in the label. 

Terrestrial Exposure for Birds and Mammals 

Terrestrial exposure estimations differ for the groups of terrestrial organisms. One major 

difference in the way in which exposure scenarios are evaluated for terrestrial species is the 

methodology used for non-granular and granular applications. Since the sole use of thidiazuron 

is limited to spray applications to cotton, exposure scenarios were only considered for nongranular applications. 

Toxicant concentrations on terrestrial food items are based on data from by Hoerger and Kenaga 

(1 972) as modified by Fletcher et al. (1994) that determined residue levels on various terrestrial 

Page 63 of 134 
items immediately following toxicant application in the field. These values are summarized in the 

table below. 

Estimated Environmental Concentrations on Avian and Mammalian Food Items (ppm) 

Following a Single Application at 1 lb ai/A) 

EEC (PP~) EEC (PP~) 

Food Items Predicted Maximum Residue' Predicted Mean Residue' 

Short grass 240 85 

Tall grass 110 3 6 

Broadleafjforage plants and 135 

small insects 

Fruits, pods, seeds, and large 15 7 

: insects 

modified by Fletcher et al. (1994). 

Toxicant concentrations on food items following multiple applications are predicted using a firstorder residue decline method, EFED's "FATES" model, which allows determination of residue 

dissipation over time incorporating a degradation half-life. Predicted maximum and mean EECs 

resulting fiom multiple applications estimates the highest one-day residue, based on the maximum I 

or mean initial EEC from the first application, the total number of applications, interval between 

applications, and a first-order degradation rate, consistent with EFED policy. 

The cotton scenario was based on two applications spaced 7 days apart. The first application was 

at the maximum single application rate of 0.2 lbs a.i./acre. The second at 0.1 lbs a.i./acre, since 

the maximum allowable total yearly application rate is 0.3 1bs ailA. All the above scenasios used 

the default 35-day foliar dissipation half-life since no suitable data were available. For 

comparative purposes, residues were also calculated based on a 1 -day foliar dissipation half-life to 

observe the effects on the risk quotients. Results are discussed in the risk characterization section. 

Dietary exposure to mammals from liquid sprays is based upon EFED's draft 1995 SOP for 

mammalian risk assessments and methods used by Hoerger and Kenaga (1 972) as modified by 

Fletcher et al. (1994). The concentration of thidiazuron in the diet that is expected to be acutely 

lethal to 50% of the test population (LC,,) is determined by dividing the LD,, value (usually a rat 

LD,,) by the amount of food, as percent (decimal of) body weight consumed. A risk quotient is 

then determined by dividing the EEC by the derived LC,, value. Acute RQs are calculated for 

three separate weight classes of mammals (15,35, and 1000 g), each presumed to consume four 

different kinds of food (grass, forage, insects, and seeds). Chronic mammalian RQs are calculated 

using the most sensitive NOAEC fiom the 2-generation rat study and the residue concentration 

expected on food items from Hoerger and Kenaga (1972) as modified by Fletcher et al. (1994). 

Page 64 of 134 
Terrestrial Risk Quotients 

The methodology for calculating RQs is presented in Appendix G. Resulting RQs for the parent 

toxicity values are presented in detail in Appendix H. 

Page 65 of 134 

-- - - -- ---. 

MEMORANDUM_ 24 August 2004 

Subject: Thidiazuron (PC 120301): Drinking Water Assessment (D244574) 

From: Alex Clem, Environmental Scientist, EFEDIERB 1 (7507C) 

Peer 

Review: James Hetrick, Senior Physical Scientist EFEDIERB 1 (7507C) 

Through: Sid Abel, Chief, EFEDIERB 1 (7507C) 

To: Felicia Fort, Team Leader, HEDIRRB 1 (7509C) 

This memorandum provides you with the potential drinking water exposure concentrations for 

thidiazuron in surface water and ground water resulting from its existing use as a cotton 

defoliantharvest aid. Since the HED MARC has not yet convened, at this time EFED is also 

informing you of two major transformation products formed in surface water (not ground water) 

that may be of potential toxicological interest to you. We also describe three reasonable exposure 

alternatives for surface water involving thidiazuron and the two by-products. 

There are no useful water monitoring data for thidiazuron in EFEDYs standard monitoring 

sources6. Our exposure estimates come from the two EFED Tier I screening-level models: 

1) FIRST (Ver. 1 .O; Aug. 1,2001) for surface water concentrations resulting from runoff and 

spray drift into an Index Reservoir and 2) SCI-GROW (Ver. 2.3; Nov. 4,2003) for ground water 

concentrations resulting from leaching. These models and their descriptions are available at the 

EPA internet site: 

htt~://www.epa. govloppefed 1 /models/water. 

Before proceeding further, we now provide an environmental fate capsule for thidiazuron7. This 

provides basic background for placing the drinking water assessment in perspective. Included 

within are key assumptions, limitations or uncertainties. 

Environmental Fate Capsule: In soil, thidiazuron is persistent, as evidenced by laboratory and 

field half-lives of the order of one year. It has intermediate soil sorption coefficients. Such 

persistence and intermediate mobility would allow some year to year accumulation as well as time 

and, therefore, opportunity for favorable conditions for runoff from application sites to occur. 

With the hypothetical assumption that there is no significant physical movement of thidiazuron 

6~~~~'s Pesticides in Ground'Water Database has no thidiazuron data. The United States Geological 

Survey (USGS), as part of a large water monitoring program, does not analyze for thidiazuron. The EPA STORET 

water quality databases have no useful information. 

7~ more complete environmental fate assessment is included in the EFED science chapter of the RED, 

which also includes sources of all model input parameters. 

Page 67 of 134 
fi-om a site of application for several years, then, with a one year half-life and uniform annual 

applications, the asymptotic limit of build-up would result in soil concentrations that would 

approach twice that of the yearly application amount. Runoff concentrations would be affected 

similarly. 

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 is a structural 'isomer of parent, while the other has a substantially altered 

chemical structure. These degradates are identified below and in Appendix 1 by chemical names, 

structural formulas, and other designations. The photolysis process and special modeling 

considerations are discussed briefly below and in detail in Appendix 2. Ground water is not 

impacted by these degradates because all field studies show that parent is long-lived in soil (halflives of the order of one year) with only minimal evidence of photolysis. 

The structural isomer is AE F132347 or photoproduct I (see Appendix 1 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 1 for chemical identification). Photoproduct II is substantially different from 

parent thidiazuron, and has a complementary asymptotic limit of production to 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 is always 

quantitatively converted in complementary proportions into the two photodegradates. 

Assumptions/Limitations/Uncertainties: Since EFED does not know yet whether the two 

photodegradates in surface water will be of toxicological concern to HED, we assume that they 

may be, and present three alternative exposure combinations that cover the range of potential 

surface water exposures that include parent and the two aqueous photoproducts. One alternative 

is the most conservative; a second, least conservative; and a third is intermediate in nature. These 

are discussed below and in more detail in Appendix 2. Among the three exposure assessments, 

there is no meaningful difference in peak (acute) surface water exposure concentrations. The only 

significant change for any of the three is in the annual average (chronic) concentrations. 

We have no data on foliar dissipation. Since thidiazuron is used to defoliate cotton prior to 

harvest, dislodgement from or transformation in or on leaves may be an important fate pathway. 

Although photolysis was the rapid, major route of transformation in water, in field soil parent was 

long-lived with only minimal evidence of photolysis or leaching. However, in laboratory plates of 

thin soil, the photoisomer AE F132347 (photoproduct I) was a quickly formed major degradate. 

Thus, whether photolysis in or on cotton plants is a major fate process is unclear, and can be 

decided only by experiment. However, based on available data, if photolysis were to be 

substantial on foliage, it is a reasonable inference that the same structural isomer would form. In 

this case, ow surface water assumptions and three options still cover possibilities, and, within 

standard model limitations, our estimated surface water exposure concentrations remain 

essentially the same. 

Page 68 of 134 
Modeling Results. Table 1 shows the FIRST-derived potential surface water concentrations for 

the three respective combinations of 1) parent plus both aqueous photodegradation products (most 

conservative), 2) parent plus photodegradate 1, and 3) parent only (least conservative). Degradate 

modeling is based on mass balance using simultaneous decline and formation over time. All 

reported concentration combinations are in terms of parent mass chemical equivalents. However, 

if desired, concentrations for each photodegradate can easily be separated. 

Inspection of thidiazuron product labels shows that no more than two annual applications of 

thidiazuron by aerial or ground spray are allowed; their summation is not to exceed 0.3 lb of the 

active ingredient (a.i.) per acre. The maximum single application rate is no more than 

0.2 lb a.i./acre, with split applications as close as five to seven days apart. The exposure 

concentrations we give below are for a single aerial application at a maximum rate of 

0.3 lb ailacre. However, as specifically discussed and illustrated in Appendix 2, whether the 

total annual maximum is applied all at once or is split in two (as specified on product labels), 

differences are negligible for the particular case of thidiazuron because of its long life in soil. 

Complete model input/output tables with detailed explanations of the computational procedures 

used to handle the photodegradates are in Appendix 2. 

Table 1. Surface Water Concentrations (derived from FIRST model, Ver. 1.0) 

PEAK DAY (ACUTE) 

DEGREES OF PROTECTION CONCENTRATION 

b or pgL) 

Parent + Both Photoproducts 

Table 2 shows the SCI-GROW-estimated potential groundwater exposure concentration. 

Groundwater is not expected to be impacted by photodegradates because all field studies show 

that parent is long-lived in soil (half-lives of the order of one year) with only minimal evidence of 

photolysis or leaching. However, as previously discussed, dislodgeability, photolysis (or other 

chemical changes) in or on plant surfaces is uncertain. Therefore, it is possible that 

photodegradates identified in water and soil or other compounds may form in or on cotton plants, 

reach the soil surface at inknown rates, and be subject to unknown degrees of leaching or runoff. 

Our assessment, of course, cannot specifically account for this absent information. However, 

based on all available fate evidence, it appears that photodegradation would be the most likely 

candidate for any rapid degradation of thidiazuron that may occur on plants. If this is the case, 

then, to a first approximation, the physical and chemical fate properties of the photodegradates 

should be similar to parent, and any resulting loss in parent would be offset by production of 

ANNUAL AVERAGE (CHRONIC) 

CONCENTRATION 

(ppb or pg/L) 

Parent + AE F132347 (photoproduct 1) 

Parent Only 

Page 69 of 134 

3.5 1.0 

3.5 

3.4 

0.82 

0.068 
photodegradates. Therefore, combined concentrations of all these compounds should approximate 

those given for Table 2 for parent only. 

Table 2. Groundwater Concentration 

(derived from SCI-GROW regression model, Ver. 2.3 ) 

Acute and Chronic Concentration*: 0.066 ppb (pg/L) 

*As noted in Appendix 3, the SCI-GROW-estimated groundwater concentration given here is an extrapolation 

beyond the regression model limits for Koc. SCI-GROW was developed using Koc values ranging from 

approximately 32 to 180 nL/g of organic carbon, whereas the median Koc value for thidiazuron is 783 mug. 

Extrapolation increases uncertainty, but without ability to estimate confidence. 

Page 70 of 134 
APPENDIX A-1 

Chemical Structures, Names, and Other Designations 

for 

Thidiazuron and Two Photoproducts 

Thidiazuron (AE B049537; SN 49537; NC 19211; Hoe 080279; DROPP) 

IUPAC name: 1 -phenyl-3-(1,2,3-thiadiazol-5-y1)urea. 

CAS name: N-phenyl-W- 1,2,3-thiadiazol-5-ylurea. 

CAS No: 5 1707-55-2 (330-54-1 also found). 

SMILES string: O=C(Nc 1 ccccc 1)Nc 1 cnns 1 

AE F132347; ZK 79173; ZK 80178; photoproduct I; photothidiazuron (peak 3 in 

MRID 41188201) 

IUPAC name: 1 -phenyl-3-(l,2,5-thiadiazol-3yl)urea 

CAS name: N-phenyl-N'- l,2,5-thiadiazol-3ylurea 

CAS No: Not reported. 

N-C-N 

AE C421200; photoproduct I1 (peak 1 in MRID 41188201) 

Page 71 of 134 

- - -- - -- 
APPENDIX A-2 

INPUT/OUTPUT TABLES FOR FIRST (EQPA EX RESERVOIR SCREENING TOOL) 

(A Tier I Simulation Model for Drinking Water Exposure Assessment) 

(Version 1.0; August 1, 2001) 

WITH 

i 

EXPLANATION OF THE BASIS FOR APPLICATION RATES AND FOR HANDLING THE 

SPECIAL CASE OF AQUEOUS PHOTOLYSIS 

Product labels specify no more than two annual applications of thidiazuron by 

aerial or ground spray, their summation not to exceed 0.3 lb of the active 

ingredient (a.i.) thidiazuron per acre. The maximum single application rate 

is no more than 0.2 lb a.i./acre, with split applications as close as five to 

seven days apart. However, as indicated in the FIRST input/output tables 

below for model runs numbered as 201 and 202, which are for a hypothetical 

single, aerial application at the maximum total annual rate of thidiazuron of 

0.3 lb/acre, whether the total annual maximum is applied all at once or is 

split variously, [for example, (0.1 + 0.2) lb/acre or, as illustrated in 

runs 203 and 204 below, (0.15 + 0.15) lb/acrel, differences are negligible for 

our purposes for the particular case of thidiazuron. 

In water, photolysis is the major route of transformation for thidiazuron; 

other processes are essentially negligible by comparison. Aqueous photolysis 

is rapid (see below), pH dependent, and occurs by branching into two 

photoproducts. The process merits special modeling consideration, as will be 

discussed. , 

Formation of the two photoproducts is in constant ratio and in complementary 

stoichiometric yield with parent. Photoproduct I (AE F132347, see chemical 

structure in Appendix 1) is an isomer of parent differing only in the relative 

positions of the sulfur atom and the two nitrogen atoms in the 5-membered 

ring. The Health Effects Division (HED)should evaluate the relevance of this 

compound to human health. Photoproduct I has a reviewer-calculated asymptotic 

experimental limit of production of 77% of parent at pH 5, 28% at pH 7, and 

17% at pH 9. This product was stable to further photolysis under experimental 

study conditions and durations (original MRID 41188201, related follow-up 

MRIDs 41364910 and 43075202, and fate overview document MRID 44436901). 

Photoproduct I1 (AE C421200, chemical structure in Appendix 1) is a 

substantially degraded product, and has a complementary asymptotic limit of 

production to 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. HED should consider any relevance of this compound to human 

health. 

Of the three compounds, soil/sediment sorption data and non-photolytic 

transformation data are available only for parent. In the absence of these 

data for the photoproducts, we assume they share the same properties as 

parent. For photoproduct I, because of its particular isomeric chemical 

structure, we have a relatively high de,gree of confidence in this assumption. 

For photoproduct 11, because of its altered structure, we have a lower degree 

of confidence. However, photoproduct I1 is formed in lower yield at more 

typical pHs, and any error introduced in the modeling analysis described 

below by assuming that its fate properties are equivalent to those of parent 

and photoproduct I would have virtually no effect on peak exposure 

concentrations, and only a minor effect on chronic exposure concentrations. 

Page 72 of 134 
For the purposes of this assessment, of the seven possible combinations of 

parent plus two photoproducts, two cover the high and low ends of potential 

exposure. A third combination, is intermediate between the extremes, and 

could be considered as a reasonable alternative. In any of the possible 

cases, the only significant differences in exposure occur in the annual 

average (chronic) concentrations. The two extreme combinations and the 

intermediate alternative are the following: 

1) The high end of exposure (most conservative for risk assessment) 

corresponds to including parent plus both photoproducts 

(parent + photoproduct I + photoproduct II). This is equivalent to saying 

that, in effect, parent did not photolyze or that photolysis is "turned off". 

There is also the inherent assumption that all three compounds have equivalent 

toxicities. Results for this high-end exposure analysis, are given in the 

table below for model run number 201. 

2) The low end of exposure (least conservative for risk assessment) 

corresponds to including only parent thidiazuron concentrations as a function 

of time. Photolysis is "turned on," but any potential exposure risk from 

photoproducts I and I1 is ignored. Results for this low-end exposure 

analysis, are given in the table below for model run number 202. 

3) The third combination is parent + photoproduct I. This is an intermediate 

combination between the extremes, and, based on chemical structural 

similarity, could be considered as a reasonable exposure alternative. 

For the purposes of the third alternative, we assume the pH 5 results for 

which parent was slightly more stable and for which 77% of parent reaching 

water is converted to the isomer photoproduct I (AE F132347). Under these 

conditions, the thidiazuron photolytic half-life, after adjustment of 

continuous experimental radiation intensity to 12 hours of solar intensity at 

40 deg. North latitude at end of March or beginning of September, was 1.44 

hours or 0.0602 days. These are reviewer-calculated values from 

MRID 41188201, page 40, Table 5a; and page 36, Table 2, and are in close 

agreement with those of the study author. Alteration of latitude or date of 

application have but minor influence on results for our purposes. Half-lives 

at higher pHs were somewhat shorter (approximately 6% shorter at pH 7 and 

approximately 40% shorter at pH 9, based on same study Table 21, and would 

result in lower chronic exposures. 

With the restrictions on chemical kinetics imposed by current standard Agency 

simulation models, including FIRST, in order to determine the exposure 

contributions from parent and photoproduct I, it is necessary to do some 

external calculations using the output from two simulations or "runs." As 

described and derived below, one model run is with aqueous photolysis "turned 

off," (model run number 201), the other with photolysis "turned on" (model run 

number 202). This is an accurate, simple accounting procedure, and allows the 

full time-dependent capabilities of the FIRST model to be utilized. 

The following reviewer-derived formulas provide the combined exposure 

concentrations for parent thidiazuron and the isomer photoproduct I for the 

described photolysis process: 

Page 73 of 134 
ci (t) = 0.77 [Cp (201) - Cp (202) 1 , where 

C!i(t) = concentration of photoisomer as a function of time 

0.77 = the branching fraction of parent that is photolyzed to 

photoisomer 

Cp(201) = concentrations of parent given in Run 201 below, which are 

concentrations that would result if parent were stable to 

photolysis (photolysis "turned off") 

Cp(202) = concentrations of parent given in Run 202 below with 

observed photolysis "turned on." 

Therefore, the summation of parent and photoisomer concentrations is given by: 

Conc. of Parent + Conc. of Photoisomer = Cp(202) + 0.77 [Cp(201) - Cp(202)I 

Rounding to two digits, Table 1 shows the resulting drinking water sums of 

parent and the photoproduct I isomer derived from the FIRST runs 201 and 202 

below: 

Table 1 

PEAK DAY (ACUTE) I ANNUAL AVERAGE (CHRONIC) 

Table 2. RUN No. 201 for Thidiazuron on Cotton 

(Most Conservative with Photolysis "Turned Off") .................................................................... 

RATE (#/Ac) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP 

ONE (MULT) INTERVAL Koc (PPM ) (%DRIFT) AREA (IN) .................................................................... 

.300( .300) 1 1 494.0 31.0 AERIAL(~~.~) 20.0 .O 

CONCENTRATION 

(PP~ or pg/L) 

FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RuNOFF (RESERVOIR) (RES . -EFF) (RESER. ) (RESER. ) .................................................................... 

363.00 2 N/A .OO- .OO 726.00 726.00 

CONCENTRATION 

(PP~ or pg/L) 

UNTREATED WATER CONC (MICROGRAMS/LITER (PPB) Ver 1.0 AUG 1, 2001 .................................................................... 

PEAK DAY (ACUTE ) ANNUAL AVERAGE (CHRONIC) 

CONCENTRATION CONCENTRATION 

Page 74 of 134 
Table 3. RUN No. 202 for Thidiazuron on Cotton 

(Least Conservative with Photolysis "Turned On") .................................................................... 

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP 

ONE (MULT) INTERVAL Koc (PPM ) (%DRIFT) AREA (IN) .................................................................... 

.300 ( .300) 1 1 494.0 31.0 AERIAL (16.0) 20.0 .O 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (RESERVOIR) (RES . -EFF) (RESER. ) (RESER. ) 

UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 AUG 1, 2001 .................................................................... 

PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

CONCENTRATION CONCENTRATION .................................................................... 

3.442 -068 

As can be seen from the three tables above, the only significant change for 

any combination is in the annual average (chronic) concentrations. 

Tables 4 and 5 below for runs 203 and 204 are for split applications, but 

otherwise correspond, respectively, to the above runs 201 (photolysis "turned 

offn)and 202 (photolysis "turned on"). As can be seen, there is no meaningful 

difference between corresponding runs 201 and 203 or between 202 and 204. 

Therefore, the effect of splitting thidiazuron applications can be neglected. 

Table 4. RUN No. 203 for Thidiazuron on Cotton with Split Applications 

(Most Conservative with Photolysis "Turned Off") .................................................................... 

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP 

ONE (MULT) INTERVAL KOC (PPM ) (%DRIFT) AREA (IN) .................................................................... 

.I50 ( .298) 2 7 494.0 31.0 AERIAL(16.0) 20.0 .O 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (RESERVOIR) (RES . -EFF) (RESER. ) (RESER. ) 

UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 AUG 1, 2001 .................................................................... 

PEAK DAY (ACUTE) ANNUAL AVERAGE ( CHRONIC) 

CONCENTRATION CONCENTRATION 

Page 75 of 134 
Table 5. RUN No. 204 for Thidiazuron on Cotton with Split ~pplications 

(Least Conservative with Photolysis "Turned On") .................................................................... 

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP 

ONE (MULT) INTERVAL Koc (PPM ) (%DRIFT) AREA (IN) .................................................................... 

.I50 ( .298) 2 7 494.0 31.0 AERIAL (16.0) 20.0 .O 

FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (RESERVOIR) (RES . -EFF) (RESER. ) (RESER. ) 

.................................................................... 

363.00 2 N/ A 0.0602 7.46 726.00 7.39 

UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 AUG 1, 2001 .................................................................... 

PEAK DAY (ACUTE) ANNUAL AVERAGE ( CHRONI c ) 

CONCENTRATION CONCENTRATION .................................................................... 

3.355 .066 

Page 76 of 134 

---- -- - - -- 
APPENDIX A-3 ., 

INPUT/OUTPUT TABLE FOR SCI-GROW 

A SCREENING MODEL FOR EXPOSURE TO PESTICIDES THROUGH GROUNDWATER 

(Version 2.3; Nov. 4, 2003) 

Chemical: Thidiazuron ........................................................................ 

Application Number of Total Use Koc Soil Aerobic 

rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days) ........................................................................ 

0.300 1.0 0.300 7.83~+02' 230.0 ........................................................................ 

Groundwater Screening Concentration (pg/L or ppb) = 6.60~-02' 

'The groundwater value estimated from the SCI-GROW regression model is an extrapolation beyond tested model 

limits for Koc. SCIGROW was developed using Koc values ranging from approximately 32 to 180 mL/g of organic 

carbon, whereas the median Koc value for thidiazuron is 783. Using extrapolated values increases uncertainty 

without ability to estimate confidence beyond subjective, qualitative statements. 

Page 77 of 134 

---- -- -- - - - ---- 
APPENDIX B: Chemical Structural Formulas, Names, and Other Designations 

for 

Thidiazuron and Transformation Products 

Page 78 of 134 
Thidiazuron (AE B049537; SN 49537; NC 19211; Hoe 080279; DROPP) 

IUPAC name: 1 -phenyl-3-(l,2,3-thiadiazol-5-y1)wea. 

CAS name: N-phenyl-M-1,2,3-thiadiazol-5-ylwea. 

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

SMILES string: O=C(Nc 1 ccccc 1)Nc 1 cnnsl 

Thidiazolurea (AE F132345; ZK 85290) 

IUPAC name: 1,2,3-thiadiazol-5-ylurea. 

CAS name: Not reported. 

CAS No: Not reported. 

AE P132347; ZK 79173; ZK 80178; photoproduct I; photothidiazuron (peak 3 in 

MRID 41188201) 

IUPAC name: 1 -phenyl-3-(1,2,5-thiadiazol-3yl)urea 

CAS name: &-phenyl-&'-I ,2,5-thiadiazol-3ylurea 

CAS No: Not reported. 

Page 79 of 134 
AE C421200; photoproduct I1 (peak 1 in MRID 41188201) 

ZK 44483; BTS 13225 

IUPAC name: phenylurea 

CAS name: phenylurea 

CAS No: not reported 

IUPAC name: 1 -(3-hydroxypheny1)-3-(1,2,3-thiadiazol-5-y1)urea 

CAS name: N-(3-hydroxypheny1)-g- l,2,3-thiadiazol-5-ylurea 

CAS No: not reported 

Other names: 1 -m-hydroxyphenyl-3-(1,2,3-thiadiazol-5-y1)-urea 

N-m-hydroxyphenyl-N y -(1,2,3-thiadiazol-5-y1)urea 

Page 80 of 134 

----- -- - - - - 
APPENDIX C: Model Input Parameters Derived from Environmental Fate Studies 

for Thidiazuron 

Page 81 of 134 
Model In~ut Parameters 

Study 

Hydrolysis (161-1) 

Photolysis in water (161-2) 

Photolysis on soil (161-3) 

Not a model input. 

Derived from Environmental Fate 

Model Input Value(s) 

Stable 

(at pH 5,799) 

-- - - 

Assume 77% of parent reaching water is 

converted to the stable photoisomer 

AE F132347 (photoproduct I). Parent 

TI12 = 0.0602 days (1.44 hours) at pH 5 

after adjustment of continuous 

experimental radiation intensity to 12 

hours solar intensity at 40 deg. North 

latitude at end of March or beginning of 

September. (Somewhat shorter TI12 

values at pH 7 (approx.6% shorter) and 

pH 9 (approx. 40% shorter). 

Degradates: Branching transformation 

into only two photoproducts in 

stoichiometric yield. Photoproduct I 

(AE F132347) is a photoisomer of 

parent, that, like parent, is moderately 

ecotoxic (but slightly more toxic than 

parent). It has an asymptotic 

experimental limit of production of 77% 

of parent at pH 5,28% at pH 7, and 17% 

at pH 9. Stable to further photolysis 

under study conditions. 

Photoproduct I1 (AE C421200) is a 

significantly degraded, practically nonecotoxic product, and has an asymptotic 

limit of production of 23% of parent at 

pH 5,72% at pH 7, and 83% at pH 9. 

Stable to further photolysis under study 

conditions. 

In spite of numerous study flaws, 

inconsistencies, and irregular kinetics, 

TI12 in laboratory soil plates is 

apparently relatively short. However, 

the major identified transformation 

product, AE F132347 (photoproduct I) is 

the same photoisomer found in aqueous 

photolysis. In field studies on bare soil, 

parent was long-lived with only minimal 

evidence of photolysis. 

Studies for Thidiazuron 

Comments 

MRID 42069203 

Tschampel, M., 1991 

EFED review 28 Apr 1993 

MRID 41188201; MRID 41364910 (a 

supplement to MRID 4 1 188201 

responding to deficiencies cited in 

DER; addresses characterization of 

previously unidentified 

photodegradates); MRID 43075202 

(an addendum in response to DER for 

MRID 41 188201 that addresses light 

sources and their comparability); 

MRID 44436901 (Environmental Fate 

Overview summarizing DER issues 

and responses). 

Supplemental. Essentially the same 

study authored by Klehr, Iwan, and 

Riernann was submitted with 

MRID 00156241(1981) and 

MRID 41364902 (1983). The latter 

MRID is a essentially a published 

version of the former. EFED review 

28 Apr 1993. MRID 44436901 

(Environmental Fate Overview 

summarizing DER issues and 

responses). 

Page 82 of 134 

- ---- - - --- 
Page 83 of 134 

Aerobic Soil Met. (162-1). 

All half-lives adjusted by 

this reviewer to 25 "C and 

75% of 113 bar soil suction. 

MRID 41950101: 

140 days--phenyl label, 

German standard sandy 

loam soil 2.3. 

MRID 46119601: 

206,436, and 253 days-- 

thiadiazol label in sandy 

loam (UK), loamy sand 

(NC), and silt loam (Il), 

respectively. 

Anaerobic Soil (162-2) 

Anaerobic Aquatic Met. 

(1 62-3) 

Aerobic Aquatic Met. 

(1 62-4) 

Sorption 

(163-1) 

Upper 90% one-tail C.1: 

TI, = 363 days 

N=4 (140,206,436, and 253 days) 

Mean TI12 = 259 days 

Std. Dev. = 2 127 days 

90% C.I. = 2 104 days 

(Upper 90% T,, = Mean + 90% C.I.) 

Essentially Stable [43 1-day extrapolated 

regression half-life from anaerobic 

period lasting 90 days (from day 30 to 

day 120) at 21C. Large 95% C.I. in halflife of 230-3400 days (0.6-9.0 years); rsq = 0.39.1 

Stable 

No data submitted. 

Default = 2 x aerobic soil TI12 

T1/2 = 2 x 364 days = 728 days 

Sand: German Std. Soil # 2.1 

Loamy sand: German Std. Soil #2.2 

Sandy loam: German Std. Soil #2.3 

Sandy clay loam: UK, Schering #I71 

Respectively (Freundlich units): 

Kflads Kfloclads l/n r2 

4.36 908 0.721 0.997 

16.2 786 0.771 0.999 

7.33 780 0.719 0.999 

18.8 494 0.817 1.000 

Koc model is appropriate (12 = 0.88) 

N = 4 values 

Min Kf, Kfoc: 4.36,494 

Max Kf, Kfoc: 18.8, 908 

Median Kfd, Kfoc: 1 1.8, 783 

Average Kfd, Kfoc: 11.726.9, 7422176 

MRID 41950101, Feyerabend, M., 

1991; EFED review 28 Apr 1993. 

Reviewer incorrectly reported half-life 

as 11 1 days at 21 "C and 78% of 113- 

bar. Value should be 182 days (based 

on serni-log transformed data). Halflife becomes 140 days after adjustment 

to 25 "C and 75% of 113-bar soil water 

potential. 

MRID 461 19601, Allan, J.G., 2003; 

EFED review May 2004. Half-lives at 

20 "C and 40% MWHC were 163, 

355, and 322 days, respectively, 

extrapolated from study periods of 

160-168 days. 

MRID 4 1945201, Feyerabend, M., 

1991. EFED review 28 Apr 1993. 

MRID 4266601, Leake and Allan, 

1993; EFED review May 2004. 

No data submitted. 

MRID 41364909, Bruhl, R., 1988. 

EFED review 28 Apr 2004. 

Foreign soils, 3 German standard soils 

and 1 United Kingdom soil; not able to 

compare these 4 soils with USDA 

classification. 
\ 

APPENDIX D: GENEEC Input & Output Files for Ecological Assessment 

Page 84 of 134 
INPUT/OUTPUT TABLES FOR GENEEC2 

(A Tier I Simulation Model for Aquatic Ecological Exposure Assessment) 

(Version 2.0; August 1, 2001) 

WITH 

EXPLANATION OF THE BASIS FOR APPLICATION RATES AND FOR HANDLING THE 

SPECIAL CASE OF AQUEOUS PHOTOLYSIS 

Product labels specify no more than two annual applications of thidiazuron by 

aerial or ground spray, their summation not to exceed 0.3 lb of the active 

ingredient (a.i.) thidiazuron per acre. The maximum single application rate 

is no more than 0.2 lb a.i./acre, with split applications as close as five to 

seven days apart. However, as indicated in the GENEEC input/output tables 

below for model runs numbered as 101 and 102, which are for a hypothetical 

single, aerial application at the maximum total annual rate of thidiazuron of 

0.3 lb/acre, whether the total annual maximum is applied all at once or is 

split variously, [for example, (0.1 + 0.2) lb/acre or, as illustrated in 

runs 103 and 104 below, (0.15 + 0.15) lb/acre], differences are negligible for 

our purposes. 

Photolysis is a special modeling consideration for thidiazuron. Thidiazuron 

undergoes relatively rapid branching photolysis in water into only two 

photoproducts. Formation of these products is in constant ratio and in 

complementary stoichiometric yield with parent. 

Photoproduct I (AE F132347, see chemical structure in Appendix 11) is an 

isomer of parent differing only in the relative positions of the sulfur atom 

and the two nitrogen atoms in the 5-membered ring. Like parent, 

photoproduct I is moderately ecotoxic (slightly more toxic than parent, based 

on the limited data discussed in the main text of this document). It has a 

reviewer-calculated asymptotic experimental limit of production of 77% of 

parent at pH 5, 28% at pH 7, and 17% at pH 9. This product was stable to 

further photolysis under the experimental study conditions and durations 

(original MRID 41188201, related follow-up MRIDs 41364910 and 43075202, and 

fate overview document with MRID 44436901). Soil/sediment sorption data and 

non-photolytic transformation data are available only for parent. However, 

based on its isomeric chemical structure, photoproduct I, except for 

photolysis, would be expected to have fate properties similar to parent. 

Photoproduct I1 (AE C421200, chemical structure in Appendix 11) is a 

substantially degraded, practically non-ecotoxic product (based on the limited 

data discussed in the main text of this document), and has a complementary 

asymptotic limit of production to 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. Soiljsediment sorption data and non-photolytic 

transformation data are available only for parent. However, because of its 

degraded nature and a modicum of direct evidence indicating that 

photoproduct I1 is practically non-toxic for ecological risk assessment 

purposes, we assume that photoproduct I1 does not contribute to ecological 

risk, and ignore its potential presence. 

For the purposes of this assessment, we shall protectively assume the pH 5 

results for which parent was slightly more stable and for which 77% of parent 

reaching water is converted to the stable isomer photoproduct I (AE F132347). 

Under these conditions, the thidiazuron photolytic half-life, after adjustment 

of continuous experimental radiation intensity to 12 hours of solar intensity 

at 40 deg. North latitude at end of March or beginning of September, was 

0.0602 days or 1.44 hours. These are reviewer-calculated values from 

MRID 41188201, page 40, Table 5a; and page 36, Table 2, and are in close 

Page 85 of 134 
agreement with those of the study author. Alteration of latitude or date of 

application have but a minor influence on results for our purposes. Halflives at higher pHs were somewhat shorter (approximately 6% shorter at pH 7 

and approximately 40% shorter at pH 9, based on same study Table 2). 

To further simplify exposure estimates, rather than separately apportioning 

parent and photoproduct I exposures and toxicities, we chose to add exposure 

concentrations of parent and photoproduct I (ignoring the practically nontoxic photoproduct 11), and use the toxicity of the slightly more toxic 

photoproduct I for parent also. Whether we use this simplified procedure or 

add the separate contributions from parent and photoproduct I makes no 

meaningful difference in the risk assessment in this particular case. 

With the restrictions on chemical kinetics imposed by current standard Agency 

simulation models, including GENEEC, in order to determine the exposure 

contributions from parent and photoproduct I, it is necessary to do some 

external calculations using the output from two simulations or "runs." One 

model run is with aqueous photolysis "turned off," the other with photolysis 

'turned on." 

The following, reviewer-derived formulas account for the photolysis process 

and risk assessment approach described above: 

Ci (t) = 0.77 [Cp (101) - Cp (102) 1 , where 

Ci(t) = concentration of photoisomer as a function of time 

0.77 = the branching fraction of parent that is photolyzed to the 

photoisomer 

Cp(lO1) = concentrations of parent given in Run 101 below, which are 

concentrations that would result if parent were stable to 

photolysis (photolysis "turned off") 

Cp(102) = concentrations of parent given in Run 102 below with 

observed photolysis "turned on." 

Therefore, the summation of parent and photoisomer concentrations is: 

Conc. of Parent + Conc. of Photoisomer = Cp (102) + 0.77 [Cp (101) - Cp (102) 1 

Rounding to two digits, the resulting aquatic exposure concentration 

summations derived from the GENEEC runs and used for the aquatic risk 

assessment are: 

Applying the same methodology to the results of runs 103 and 104 for split 

applications gives the analogous equation: 

Concentrations (ppb or pg/L) 

Conc. of Parent + Conc. of Photoisomer = Cp(104) + 0.77[Cp(103) - Cp(104)l 

Peak 

11 

As is apparent from the tables, corresponding pairs of concentrations are 

virtually the same as for the single application at the maximum annual rate 

In all cases, peak concentrations are virtually the same. Minor decreases in 

the summation of parent and the photoisomer concentrations over time reflect 

primarily the 77% conversion to photoproduct I. 

4-Day Avg. 

11 

21-Day Avg. 

9.7 

60-Day Avg. 

8.6 

90-Day Avg. 

8.2 
RUN No. 101 FOR Thidiazuron ON Cotton * INPUT VALUES * .................................................................... 

RATE (#/Ac) NO-APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

ONE (MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) .................................................................... 

.300( -300) 1 1 494.0 31.0 AERL-B( 13.0) .O .O 

FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) ( POND) .................................................................... 

364.00 2 N/A -00- .OO 728.00 728.00 

GENERIC EECs (IN MICROGRAMS/LITER (PPB)) Version 2.0 Aug 1, 2001 

.................................................................... 

PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC .................................................................... 

10.87 10.84 10.65 10.23 9.93 

RUN No. 102 FOR Thidiazuron ON Cotton * INPUT VALUES * .................................................................... 

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

ONE (MULT) INTERVAL KOC (PPM ) (%DRIFT) (FT) (IN) .................................................................... 

.300( -300) 1 1 494.0 31.0 AERL-B( 13.0) .O .O 

FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) IIAIN/RUNOFF (POND) (POND-EFF) ( POND) (POND) .................................................................... 

364.00 2 N/A -0602- 7.46 728.00 7.39 

GENERIC EECS (IN MICROGRAMS/LITER (PPB)) Version 2.0 ~ug 1, 2001 

PEAK MAX 4 DAY MAX 2 1 DAY MAX 60 DAY MAX 90 DAY 

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

Page 87 of 134 

- --------- - --- -- 
RUN No. 103 FOR Thidiazuron ON Cotton * INPUT VALUES * .................................................................... 

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

ONE (MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) 

FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (POND) (POND-EFF) ( POND) (POND 1 

GENERIC EECs (IN MICROGRAMS/LITER (PPB)) Version 2.0 Aug 1, 2001 

PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

RUN No. 104 FOR Thidiazuron run1 ON Cotton * INPUT VALUES * .................................................................... 

RATE (#/Ac) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

ONE (MULT) INTERVAL KOC (PPM) (%DRIFT) (FT) (IN) .................................................................... 

.150( .298) 2 7 494.0 31.0 AERL-B( 13.0) .O .O 

FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) .................................................................... 

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

(FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) .................................................................... 

364.00 2 N/ A .0602- 7.46 728.00 7.39 

GENERIC EECs (IN MICROGIIAMS/LITER (PPB)) Version 2.0 Aug 1, 2001 .................................................................... 

PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC .................................................................... 

10.23 9.52 6.37 3.18 2.19 

Page 88 of 134 
APPENDIX E: Ecological Hazard Data 

Page 89 of 134 
EPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) 

Rainbow 42069202, 

trout 99.3 >19,000 >19,000 M, S slightly toxic Schupner, Coreb 

et. al. 1991. 

Bluegill 42069201, 

sunfish 99.3 >32,000 >32,000 M, S slightly toxic Schupner, coreb 

et. al. 1991. 

EPA PC Code: 120301 - Thidiazuron Photodegradate (AE F132347) 

Rainbow 6700 (5700 - 2300 97.4 w:w M, S moderately toxic Blankinship core 

trout 7900) , AS.., et. 

al., 2003. 

EPA PC Code: 120301 - Thidiazurqn metabolite (AE F132345) 

Rainbow 97.4 w:w >101,000 2300 M, S moderately toxic Palmer, 

trout core S.J., et.al, 

2003. 

EPA PC Code: 120301 - Thidiazuron Photodegradate (AE ~421200) 

Rainbow 98.4~:~ >103000 103,000 M, S Practically non- Palmer, trout core toxic S.J., et.al., 

2003. 

M=mean-measured chemical concentrations, N=nominal chemical concentrations; F-T=flowhrough; S=static. 

 he compound was tested to the limits of solubility. No mortalities were observed. 

Page 90 of 134 
~PA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) II 

462035-03, 

Daphnia 99.5 5,700 (4700 - 1200 

M, S moderately Blankinship, 

9500) core 

magna wlw toxic A.S..,et. al., 

2003. 

EPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) 

(Accession # Daphnia (9000 5,6001 N, S mOderatel~ 0998 19), 

magna 98.4 -II,IOO) core toxic Brown. A., 

1979. I~PA PC Code: 120301 - Thidiazuron PhotometaBOlite (RE F132347) 

462035-09, 

Daphnia 97.4 >12,000 * 5100 M, S Blankinship, core * Slightly toxic magna wlw A.S.., et. al., 

2003. 

EPA PC Code: 120301 - Thidiazuron Urea meta~~lite (AE ~132345) 

Daphnia 462035-16, 

magna 

91 wlw >98,000 12000 M, S Slightly toxic Palmer, S.J., core 

2003. 

~~EPA PC Code: 120301 - Thidiazuron PhotometaBOlite (AE C421200) II 

Daphnia 462035-12, 

magna 

98.4 >99,000 24000 M, S Slightly toxic Palmer, S.J., core 

2003. 

" M=rnean-measured chemical concentrations, N=nominal chemical concentrations: F-T=flow-through: S=static. " z 

A definitive LC50 could not be determined because the compound was tested to the limit of solubility. 

A definitive LC50 could not be determined due to the lack of a 50% mortality at the highest test level. 

Page 91 of 134 

-- - - - - - -- 
EPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) 

Sheephead 99.3 418461-01, 

>36,000 <36,000 M,S slightly toxic Schupner, J.K. minnow Coreb 

et a1 1991 

I' --. --.) A// *. 11 

a M=mean-measured chemical concentrations, N=nominal chemical concentfations; F-T=flow-through; S=static. 

A definitive LC50 could not be determined because the compound was tested to the limit of solubiky. 

Page 92 of 134 

-*--- - --A -- - -- ---. 
IIEPA PC Code: 120301 - Thidiazueon Technical (AE B049537 00 ID99 0003) 

418461-02, 

Mysid shrimp 99.3 3,240 (2,890 1400 M, S moderately toxic Schupner, core - 3,770) J.K., 1991. 

Eastern oyster- 421320-01, 5,384 (4,768 1.06 

99'3 - 6,157) M, F-T moderately toxic Ward, S.G., core larvae/embryo 1991. 

" M=rnean-measured chemical concentrations, N-ominal chemical concentrations; F-T=flow-through; S=static. 

Page 93 of 134 

99.5 6000 

99.5 860 1 101 Growth rate & 

Plant biomass 

97.4 980 220 I 3 10 Cell density 

Page 95 of 134 
Marine diatom 

(Skeletonema 

costatum) 

EPA PC Code: 120301 - Thidiazuron Urea metabolite (AE F132345) 

11 Vascular Plant .._................ ................................................................................................................................................................. - ...................... 

(1 Duck weed )I (Lemna gibba) ............................................................................................................................................................................................................. 11 Nonvascular Plants ..__ ................................................... " .............................................................................................................................. ....................... 

Greeen Algae 462035-17, 

(Scenedesmus w'w 3400 1300 Cell density mean Supplementa Desjardins, 

subspicatus) ai 1200 D. et.al., 

2003. .._ ..................... .............................................................................................................................................................. - ...................... 

Freshwater diatom 

(Navicula 

pelliculosa) 

.. - ............................................................................................................................. - ...................................................... - ...................... 

Blue-green algae 

(Anabaenaflosaquae) ............................................................................................................................................................... " ........................................... 

Marine diatom 

(Skeletonema ,: 

costaturn) ~IEPA PC Code: 120301 - Thidiazuron Photodegradate (AE C421200) ..............a ........................................................................................................................................................................... 

Vascular Plants 

.................... .............................................................................................................................................................. - ..................... 

Duck weed 

(Lemna ~ibba) 

............................................................................................................................................................................................................. Nonvascular plants 

Green Algae 462035-13, 

(Scenedesmus 98.4 70000 49'000 Cell density mean Desjardins, 

53,000 Supplementa subspicatus) D. et.al., 

2003. 

Freshwater diatom 

(Navicula 

pelliculosa) 

Blue-green algae 

(Anabaena flosPage 96 of 134 
II Marine diatom 

(Skeletonema 1) costaturn) I 

" Since the study duration was for 3 days instead of 5, the study can only be classified as a Tier 1 study. 

EPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) 

No sub-lethal effects 00066168 

60 or other treatment practically non- (Accession 

Bobwhite quail 98.43 >3 160 related effects were # 0998 19), core toxic 

observed. Fletcher, D. 1979. 

Table 8: Acute Toxicity to Thidiazuron to Birds (oral administration) 

EPA PC Code: 120301 - Thidiazuron formulated product (CP 503 WP 50) 

00038144, 

9989a NO sub-lethal effects practically non- (Accession Japanese Quail 50 >9989" . observed toxic # 095969), Supplemental 

Ross, D., 

Species 

I 1976. 

" The LD50 is 4995 mg aikg-bw when adjusted to 100% ai. \ 

Page 97 of 134 

Yo 

a.i. 

LD509 

mglkg-bw 

(conf. 

interval) 

NOABC, 

mglkgbw 

Effects 

Toxicity 

Classification 

(based on a.i.) 

MRID, 

Author, 

Year 

Status 
No sub-lethal effects 

or other treatment 

5000 related effects were Fletcher, Core 

observed. 

No sub-lethal effects 

or other treatment 

52 related effects were Bowers, Core 

observed. 

No sub-lethal effects 

or other treatment 

5000 related effects were 

observed. 

No sub-lethal effects 

or other treatment 

related effects were 

Page 98 of 134 
IIEPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) II 

laboratory rat 11 (Rattus 98.7 

46121501, 

>2000 Category I11 Coleman, Acceptable 11 

nowegicus) D.G., 2001 

" Status (acceptability) based on HEDs guidelines. 

Page 99 of 134 
'I~PA PC Code: 120301 - Thidiazuron Technical SN 49537 II 

200 ppm 600 (30 429589-01, 3 -generation Core- 100 (10 mgkg mgkg - Decreased F, litter size at 200 ppm Rees, S.J., reproductive (rats) 1992. Supplementary bwida) b~ida)~ 

" Status (acceptability) based on HEDs guidelines. . - -, - 

Based on decrease epidermal sperm counts. 

~PPA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) II 

Honey bee (Apis 

mellifera) >loo 462035-01, Kling 11 Practically non-toxic A., 2003. 

II EPA PC Code: 120301 -Thidiazuron AE B049573 00 SC42 (formulated product) II 

462035-18, Honey bee (Apis 41.9 >98.1 Practically non-toxic Waltersdorfer, A., coreb mellifera) 2002. 

" Sup~lemental for Oral LD50 of >105.25 rrnibee. 

~ui~lemental forOral LD50 of >197.8 igTbee. 

I@PA PC Code: 120301 - Thidiazuron Technical (AE B049537 00 ID99 0003) I1 

Predatory mite 

(Typlodromus pyri) 41.9 

462035-21, 

NIA Supplemen Waltersdorfer, A., 

2002.. tal" 

~~EPA PCCode: 120301 -Thidiazuron AE B049573 00 SC42 (formulated product) II 

Parasoid wasp 462035-20, 

(Aphidius 4 1.9 >0.1782 NIA Waltersdorfer, A,, Coreb 

3007 

a Non-gu~dehne study 

Page 100 of 134 
~~EPA PC Code: 120301 -Thidiazuron, SC 500 GL I1 II Earthworm (Eiseenia 

foetide) , 42.6 53.3" 462035-22Niestedt K.M., 2003. Supplementar 

~~EPA PC Code: 120301 - Thidiazuron photometaboite (AE F132347) II 

462035-07, 

250 Sindermann, et. al., ~upplemental~ 

2003. I~PA PC Code: 120301 - Thidiazuron photometaboite (AE F1323457) II 

462035-14, Earthworm (Eiseenia 9 1 1000 62.5 Sinderrnann, et. al., supplementalb foetide) 2003. 

" LC,, and NOEC > 1000 and 125 mg productkg 

Performed under OECD guidelines. Not required by EPA. 

Terrestrial Plants 

Terrestrial plant testing (seedling emergence and vegetative vigor) is required for herbicides 

that have terrestrial non-residential outdoor use patterns and that may move off the application site 

through volatilization (vapor pressure >1.0 x 10"mm Hg at 25OC) or drift (aerial or irrigation) andlor 

that may have endangered or threatened plant species associated with the application site. 

Currently, terrestrial plant testing is not required for pesticides other than herbicides except on 

a case-by-case basis (e.g., labeling bears phytotoxicity warnings incident data or literature that 

demonstrate phytotoxicity). 

For seedling emergence and vegetative vigor testing the following plant species and groups 

should be tested: (1) six species of at least four dicotyledonous families, one species of which is 

soybean (Glycine max) and the second is a root crop, and (2) four species of at least two 

monocotyledonous families, one of which is corn (Zea mays). 

Tier 1 tests measure the response of plants, relative to a control, at a test level that is equal to 

the highest use rate (expressed as lbs ai/A). If effects are observed in this test, the registrant is 

required to proceed to the Tier 2 level. 

Terrestrial Tier 2 studies are required for all low dose herbicides (those with the maximum use 

rate of 0.5 lbs ai/A or less) and any pesticide showing a negative response equal to or greater than 25% 

in Tier 1 tests. The registrant may opt to proceed directly to Tier 2 testing. 

Page 101 of 134 
Tier 2 tests measure the response of plants, relative to a control, and five or more test 

concentrations. Results of Tier 1 and 2 toxicity testing on the technical1TEP material are tabulated 

below. 

<25 none 

<25 none 

<25 none 

<25 none 

<25 none 

Page 102 of 134 
<25 none 

increased suckering 

>25 necrosis, weight 

inhibition 

necrosis, leaf darkening, 

deformity, height 

inhibition 

deformity, height >25 inhibition 

Page 103 of 134 

- - -- "- ----- - - - 
onion' 

wheat 

Dicot 

turnip (root 

0.17831 emergence, shoot 

'0'1783 >0.1783 length, dry weight, 

emergence 

Supplemental 

Core 

Supplemental 

0.17831 emergence, shoot 459085-01, 

>0.1783 length, dry weight, Teineira, D., 2003. Core 

0.17831 emergence, shoot 

0.1783 length, dry weight, 

<0.0223/ shoot length, dry 

0'0410 0.003 1 weight 

l1 emergence, dry weight 0.0039 

0.17831 emergence, shoot 

'0'1783 ~0.1783 length, dry weight 

0.17831 emergence, shoot 

50'1783 s0.1783 length, dry weight 

Supplemental 

Core 

Core 

Supplemental 

Supplemental 

cabbage 0.17831 emergence, shoot 

>'.I7" <O. 1783 length, dry weight Supplemental 

'Tier 2 tests will need to be performed because oat and onion sensitivity indicated a 25% reduction in growth. 

'Conducted as Tier 2 tests. 

Page 104 of 134 
Ip PC Code: 120301 -Thidiazuron AE 8049573 00 SC42 (formulated product) 

onion 0.17831 shoot length, 

<O. 1783 dry weight Core 

I1 corn 0.17831 shoot length, 

<0.1783 dry weight, Core 

I1 oat 0.17831 shoot length, 

<0.1783 dry weight, Core 

wheat 0.17831 shoot length, 459215-01, 

0.1783 Teixeira, Core dry weight, D., 2003. 

II Dicot 

2 soybean 0.01 111 shoot length, 

0.0024 dry weight, Core 

II turnip (root crop) >0.1783 0.17831 shoot length, 

<0.1783 dry weight Core 

0.00019/ 

0.00005 dry weight Core 

cucumber 2 0.00561 

0.0075 dry weight Core 

0'02221 shoot length 0.0098 Core 

cabbage >0.1783 0.17831 shoot length, 

<0.1783 Core dry weight 

'Tier 2 tests will need to be performed to determine the most sensitive monocot species. 

*Conducted as Tier 2 tests. 

Page 105 of 134 
APPENDIX F: The Risk Quotient Method 

Page 106 of 134 
The Risk Quotient Method is the means used by EFED to integrate the results of exposure and 

ecotoxicity data. For this method, RQs (RQs) are calculated by dividing exposure estimates by 

ecotoxicity values (i.e., RQ = EXPOSURE/TOXICITY), both acute and chronic. These RQs are then 

compared to OPP's levels of concern (LOCs). These LOCs are criteria used by OPP to indicate 

potential risk to non-target organisms and the need to consider regulatory action. EFED has defined 

LOCs for acute risk, potential restricted use classification, and for endangered species. 

The criteria indicate that a pesticide used as directed has the potential to cause adverse effects on 

nontarget organisms. LOCs currently address the following risk presumption categories: 

(1) acute - there is a potential for acute risk; regulatory action may be warranted in addition to 

restricted use classification; 

(2) acute restricted use - the potential for acute risk is high, but this may be mitigated through 

restricted use classification L 

(3) acute endangered species - the potential for acute risk to endangered species is high, 

regulatory action may be warranted, and 

(4) chronic risk - the potential for chronic risk is high, regulatory action may be warranted. 

Currently, EFED does not perform assessments for chronic risk to plants, acute or chronic risks to 

non-target insects, or chronic risk from granularhait formulations to mammalian or avian species. 4 

The ecotoxicity test values (i.e., measurement endpoints) used in the acute and chronic RQs are 

derived from required studies. Examples of ecotoxicity values derived from short-term laboratory 

studies that assess acute effects are: (1) LC,, (fish and birds), (2) LD,, (birds and mammals), (3) EC,, 

(aquatic plants and aquatic invertebrates), and (4) EC,, (terrestrial plants). Examples of toxicity test 

effect levels derived from the results of long-term laboratory studies that assess chronic effects are: (1) 

LOAEL (birds, fish, and aquatic invertebrates), and (2) NOAEL (birds, fish and aquatic invertebrates). 

The NOAEL is generally used as the ecotoxicity test value in assessing chronic effects. 

Risk presumptions, along with the corresponding RQs and LOCs are summarized in Table G-1. 

Page 107 of 134 

-- --- - -- 
Acute Risk EECILC,, or LD, Jsqft or LD,,/day 0.5 

Acute Restricted Use EECILC,, or LD, Jsqft or LD,,/day (or LD,, < 50 mgkg) 0.2 

Acute Endangered Species EECLC,, or LD,,/sqft or LD,,/day 0.1 

Chronic Risk EEChTOAEC 1 

Wild Mammals1 

Acute Risk EECILC,, or LD, Jsqft or LD,,/day 0.5 

Acute Restricted Use EECLC, or LD,olsqfi or LDdday (or LD,, < 50 mgkg) 0.2 

Acute Endangered Species EECLC,, or LD,,Isqft or LD,,Iday 0.1 

Chronic Risk EEC/NOAEC 1 

Aquatic Animals2 I 

Acute Risk EECILC,, or EC,, 0.5 

Acute Restricted Use EECLC,, or EC,, 0.1 

Acute Endangered Species EECILC,, or EC,, 0.05 

Chronic Risk EECLNOAEC 1 

Terrestrial and Semi-Aquatic Plants 

Acute Risk EECIEC,, 1 

Acute Endangered Species EECIEC,, or NOAEC 1 

Aquatic Plants2 

Acute Risk EECIEC,, 1 

Acute Endangered Species EECIEC,, or NOAEC 1 

LD5,1sqft = (mglsqft) 1 (LD, * wt. of animal) 

LD,,Iday = (mg of toxicant consumedday) 1 (LD,, * wt of animal) 

Table 1: Risk Presumptions and LOCs 

EEC = (ppm or ppb) in water 

Risk Presumption RO 

Page 108 of 134 

- F - - - - - 

LOC 

Birds' 
APPENDIX G: Detailed Risk Quotients 

Page 109 of 134 
Pome Fruit (0.32 Ib ailA1App.; 3 applyr; 10 day intervals; ground and air blast equipment) 

Freshwater Fish 6700' 5700 11.00 9.70 8.60 0.002 0.002 

Estuarine fish >36,000 No data 11.00 9.70 8.60 < 0.0003 

Freshwater 

Invert. 5,700 <I00 11.00 9.70 8.60 0.002 > 0.10 

Estuarine Invert. 

(shrimp) 3,240 Nodata 11.00 9.70 8.60 0.003 

Estuarine Invert. 

(oyster) 5,384 No data 11.00 9.70 8.60 0.002 

a * indicates an exceedance of endangered Species Level of Concern (LOC). 

** indicates an exceedance of Acute Restricted Use LOC. 

*** indicates an exceedance of Acute Risk LOC. 

+ indicates an exceedance of Chronic LOC. 

Thidiazuron photodegradate AE F132347 

*Estimated on the assumption that acute to chronic ratio for estuarine invertebrates applies to freshwater invertebrates. 

Page 1 10 of 134 
II Cotton (0.2 lb ai/A; single application aerial equipment 5% drift) 

lpular Plant 

Aquatic Vascular Plant 

(Lemna gibba) >24,000 

Nonvascular Plants 

Green algae 

(Selenastrum capuicornutum) ...................................................... 

algae (Anabaena 

0s-aquae) 6000 

II Marine diatom (Skeletonema 

costaturn) 8 60 

I' 

a * indicates an exceedence of Acute Risk LOC b ** indicates an exceedence of listed Endangered Species LOC. e There are currently no endangered nonvascular plant species 

Page Ill of 134 

--- -- ---- 
Short grass >SO00 No data 62.62 < 0.013 

Tall grass 15000 No data 30.54 < 0.006 

Broadleaf forage, 

small insects 15000 No data 37.48 < 0.008 

Fruit, pods, seeds, 

laru~ inc~ote >So00 Nn - . - data --.- 7.lU < U.UU1 

ELLFATE program. (Assumed peak residue values (day 6) from first a~olicatinn at n 7 lh ~ila n~~-~ :-lr:-j 11. .I. r A - -, lo ail^ Ior second application.) a * indicates an exceedence of Endangered Species Level of Concern (LOC), ** indicates an exceedence of Acute Restricted Use LOC. 

*** indicates an exceedence of Acute Risk LOC. 

+ indicates an exceedence of Chronic LOC. 

Page 112 of 134 
Page 113 of 134 

P - --- - - - - 

a 

Predicted 

Acute Toxicity Maximum Chronic Chronic Chronic Toxicity 

~ood'1tems Residue Acute RQ Threshold, NOEC RQb RQb 

LC,, (Max. (Mean 

(mg ailkg-diet) (mg ai/kg-diet) (EEC)(mg Residue) Residue 

ai/kg-diet) 

Cotton (0.2 lb ai/A/App.) 

Short grass >5000 No Data 48.0 < 0.010 - 

Tall grass 15000 No Data 22 < 0.004 - 

Broadleaf forage, , 15000 No Data 27 < 0.005 - small insects 

Fruit, pods, seeds, >5000 No Data 3.0 < 0.001 - - large insects 

* indicates an exceedence of Endangered Species Level of Concern (LOC). 

** indicates an exceedence of Acute Restricted Use LOC. 

*** indicates an exceedence of Acute Risk LOC. 

+ indicates an exceedence of Chronic LOC. 
Cotton (0.2 Ib ai/A lSt A 

Short grass 

Broadleaf 

15 95 forage, small 

insects 

Large insects 

Short grass 

Broadleaf 

35 66 forage, small 

insects 

Large insects 

Short grass 

1000 15 Broadleaf 

forage, small 

insects 

Page 1 14 of 134 

15 21 Seeds 12000 4.16 < 0.0004 1.94 < 0.0002 

3 5 15 Seeds 12000 4.16 < 0.0003 1.94 < 0.0001 

1000 3 Seeds 12000 4.16 < 0.0001 1.94 < 0.00003 I ELLFATE program. (Assumed peak residue values (day 6) &om first application at 0.2 lb aifA and added initial residue value at 0.1 lb a1fA for second application.) 

b~~ = EEC 

I LD,, 1 % Body wt consumed 

! 

1 

I 

i 

i 

* indicates an exceedence of Endangered Species Level of Concern (LOC). 

** indicates an exceedence of Acute Restricted Use LOC. 

*** indicates an exceedence of Acute Risk LOC. 

Page 1 16 of 134 

Table 6: Mammalian (Granivore) Acute Risk Quotient 

Calculations for Spray Applications 

Animal Body - 

Weight (g) 

% Body 

Weight 

Consumed 

(melkg-bw) 

Scenario 

Cotton (0.2 lb ail* I" App.; 0.1 lb ri/A zndapp; 7 day intervals; ground and air blast equipment) 

(mglkg-diet) 

Acute ~~~i~i~ 

Threshold, 

LD50 

(mglkg-diet) Acute RQ 

Predicted Mean 

Residue Levels 

EEC 

Predicted Maximum Residue 

Levels 

EEC " Acute RQ 


21 Seeds 10 4.16 0.09 

t 

I 

i 

1 

3 5 15 Seeds 10 4.16 0.06 1.94 0.03 

1000 3 Seeds 10 4.16 0.01 1.94 0.01 

' ELLFATE program. (Assumed peak residue values (day 6) &om first application at 0.2 lb ai/A and added initial residue value at 0.1 lb ai/A for second application.) 

RQ = EEC x % body weight consumed - 

NOEL 

* indicates .an exceedence of Chronic Level of Concern (LOC). 

i 

Table 8 : Mammalian (Granivore) Chronic Risk Quotient 

Calculations for Spray Applications (NOAEL = 10 me/kg-bwlda) 

Animal 

Body 

Weight (g) 

ll~hort grass 

Table 9 : Mammalian Chronic Risk Quotient C~lculations for Spray Applications (NOAEC = 200 mglkg-diet) 

Page 119 of 134 

% Body 

Weight 

Consumed 

Scenario 

(mg/kg-diet) 

Scenario 

Chronic Toxicity 

Threshold, 

NOAEC (mglkgCotton (0.2 Ih aiiA I" App.; 0.1 Ib aiiA 2""app; 7 day intervals; ground and air blast equipment) 

(mg/kg-diet) 

dict) 

Chronic Toxicity 

Threshold, 

NOAEL (mg/kgbw/da) Chronic RQ " 

Predicted Maximum Residue Levels 

EEC 

Cotton (0.2 Ib aiiA I" App.; 0.1 lb aiIA fn*app; 7 day intervals; ground and air blast equipment) 

Predicted Mean 

Residue Levels 

(mglkg-diet) 

Predicted Mean 

Residue Levels 

EEC 

Predicted Maxim urn Residue Levels 

Chronic RQ " (mgkg-d ie t) 

EEC 

EEC" 

Chronic KQ " 

Chronic RQ " 

Cotton - ground Apulications (0.2 Ibs ailaclapp.) 

0.2 0 0.02 0.004 0.040 0.002 0.006 0.04 

Cotton - aerial avplications (0.2 Ibs ailaclapp.) 

0.2 0 0.02 0.002 0.020 0.010 0.012 0.030 

Assumed drift at I % for ground applications and 5% for aerial applications. 

Non-endangered Plant Risk Quotients - Single Spray Applications of Thidiazuron 

The EC2, value of the most sensitive species in the seedling emergence study is compared to runoff and drift exposure to determine 

the RQ (EECItoxicity value). The EC,, value of the most sensitive species in the vegetative vigor study is compared to the drift exposure to 

determine the acute RQ. RQs are calculated for the most sensitive monocot and dicot species for ground and aerial applications. 

Page 121 of 134 
Cotton - Ground Applications (0.2 Ibs ailaclapp.) 

Monocot <O. 1783 NIA 0.006 > 0.03 

Seed Emerg. 

Dicot 0.0152 N/A 0.006 0.39 

Monocot >O. 1783 0.002 NIA < 0.01 0.002 NIA < 0.01 

Veg Vigor 

Dicot . 0.001 1 0.002 N/A 1.82 * 0.002 N/A 1.82 * 

Cotton -Aerial Applications (0.2 Ibs ai/ac/app) 

Monocot <O. 1783 NIA 0.012 > 0.07 N/A 0.03 0.17 

Seed Emerg. 

Dicot 0.0152 N/A 0.012 0.79 NIA 0.03 1.97 * 

Monocot 20.1783 0.01 N/A < 0.06 

Veg - Vigor - 

Dicot 0.001 1 0.01 N/A 9.09 * 0.01 NIA 9.09 * 

indicates an exceedence of Acute Risk LOC. 

Page 122 of 134 
Endangered Plant Risk Quotients - Single Spray Applications of Thidiazuron 

The endangered plant RQs are calculated by comparing the NOEC or EC, value of the most sensitive species in the seedling 

emergence study to runoff and drift exposure (EECItoxicity value). The NOEC or EC, value of the most sensitive species in the vegetative 

vigor study is compared to the drift exposure to determine the acute RQ. RQs are calculated for the most sensitive monocot and dicot 

species. 

Monocot 0.1783 NIA 

Seed Emerg. 

Dicot 0.01 11 NIA 

Table 12: Acute Endangered Terrestrial Plant Risk Quotient Calculations For Single Spray Applications of thidiazuron 

Monocot 0.1783 0.002 

Veg Vigor 

Dicot 0.00019 0.002 

Scenario Toxicity Threshold, NOEC 

or EC,, 

(Ib ailac) 

I1 Cotton -Aerial A~ulications (0.2 Ibs aiiaclapp) 

Monocot 0.1783 NIA 

Seed Emerg. 

Dicot 0.01 11 NIA 

Cotton - Crormd Applications (0.2 Ibs ailaclapp) 

Page 123 of 134 

Plants Adjacent to Treated Sites 

Total Drift 

(Ib ailac) 

Plants in Semi-aquatic Areas 

Total Drift 

(Ib ailac) 

Total 

Loading(Sheet 

runoff + Drift) 

(I b aihac) 

RQ" 

Total Loading 

(Channel 

runoff + Drift) 

(Ib ailac) 

1U.y 
Table 12: Acute Endangered Terrestrial Plant Risk Quotient Calculations For Single Spray Applications of thidiazuron 

Page 124 of 134 

Scenario Toxicity Threshold, NOEC 

or EC,, 

(Ib ailac) 

i Monocot 0.1783 0.01 N/A 0.06 0.01 N/A 0.06 

Veg Vigor 

Dicot 0.000 19 0.0 1 N/A 52.63 * 0.01 N/A 52.63 * 

"Indicates acute risk to endangered plants 

I 

I Most of the thidiazuron labels recommend a second application not to exceed a seasonal total of 0.3 lb ai/A. The number of days between 

1 the intervals is not stated on any of the labels so EFED has assumed a somewhat conservative scenario of 7 days between applications. For 

1 multiple spray applications the environmental concentrations and the RQs would increase according to a linear relationship at the most , conservative level. The estimated environmental concentrations for dry and semi-aquatic areas with an assumed runoff of 2% are tabulated 

I 

I below. 

1 

Plants Adjacent to Treated Sites 

Total Drift 

(Ib ailac) 

Plants in Semi-aquatic Areas 

Total Drift 

(Ib ailac) 

Total 

Loading(Sheet 

runoff + Drift) 

(Ib ailac) 

RQ" 

Total Loading 

(Channel 

runoff + Drift) 

(Ib ailac) 

w" 
Cotton (0.2 Ib ai/A 1" App.; 0.1 Ib ai/A 2"*app; 7 day intervals; ground applications) 

0.276 0 0.02 0.006 0.06 0.003 0.009 0.063 

Cotton (0.2 lb ai/A lSt App.; 0.1 lb ai/A 2"*app; 7 day intervals; aerial applications) 

n 776 n n n:, n nnq n n? n ni n ni? n 04 

' Assumed short grass peak residue value (day 6) from first application at 0.2 Ib ai/A and added initial residue value at 0.1 lb ai/A for second application. 

Assumed drift at 1% for ground applications and 5% for aerial applications. 

Non-endangered Plant Risk Quotients - Multiple Spray Applications of Thidiazuron 

The RQs for multiple applications are determined in the same manner as single applications. The EC,, value of the most sensitive 

species in the seedling emergence and vegetative vigor studies are compared to runoff and drift exposure.) The RQs are calculated for the 

most sensitive monocot and dicot species for aerial and ground applications.. 

Page 125 of 134 
lotton (0.2 Ib ai/A 1" App.; 0.1 Ib ailA Indapp; 7 day intervals; ground applications) 

Monocot <o. 1783 N/A 0.009 

Seed Emerg. 

Dicot 0.0152 N/A 0.009 

Monocot >O. 1783 0.003 N/A 

Veg Vigor 

Dicot 0.001 1 0.003 N/A 

Jotton (0.2 Ib ai/A 1" App.; 0.1 Ib ai/A 2ndapp; 7 day intervals; aerial applications) 

Monocot <O. 1783 NIA 0.013 

Seed Emerg. 

Dicot 0.0152 N/A 0.013 

Monocot >O. 1783 0.01 NIA < 0.06 0.01 N/A < 0.06 

Veg Vigor 

Dicot 0.001 1 0.01 N/A 9.09 * 0.01 NIA 9.09 * 

indicates an exceedence of Acute Risk LOC. 

Page 126 of 134 
Endangered Plant Risk Quotients - Multiple Spray Applications of Thidiazuron 

The RQs for multiple applications of thidizuron are determined in the same manner as single applications. The NOEC or EC,, value 

of the most sensitive species in the seedling emergence and vegetative vigor studies are compared to runoff and drift exposure. RQs are 

calculated for the most sensitive monocot and dicot species for ground and aerial applications. 

otton (0.2 Ib ai/A 1'' App.; 0.1 Ib ai/A 2"*app; 7 day intervals; aerial applications) 

Veg Vigor Monocot 

Page 127 of 134 

APPENDIX H: Data Requirements for Thidiazuron 

Page 129 of 134 
Photodegradation in Water 

summarizing previous DER issues and 

Photodegradation on Soil 

Page 130 of 134 
reported half-life as 1 1 1 days at 2 1 OC and 

78% of 113-bar. Value should be 182 days 

(based on semi-log transformed data). 

Half-life becomes 140 days after 

adjustment to 25 "C and 75% of 113-bar 

Aerobic Soil Metabolism soil water potential. (German standard 

sandy loam soil 2.3) 

respectively. (Half-lives at 20 OC and 40% 

MWHC were 163,355, and 322 days, 

Terrestrial Field Dissipation 

Page 131 of 134 
Page 132 of 134 
Avian Dietary LC,, 

Freshwater Fish LC,, Metabolite (AE F132345) 

Freshwater Invertebrate 

Page 133 of 134 
72-5 i 850.15 Freshwater Fish Full LifeCycle N A 

122- i 850.41 

l(a) ; 

122- ; - 850.415 

l(b) ; 

122-2 i 850.44 

123- : 850.4225 

l(a> i 

123- i 850.425 l(b) i 

123-2 ; 850.44 

Seed Germ./Seedling 

Emergence 

Vegetative Vigor 

Aquatic Plant Growth 

Seed Germ./Seedling 

Emergence (Tier 2) 

141-1 i 850.302 

I I I I 

This test may be recommend if chronic mammalian issues can not be resolved 

Vegetative Vigor (Tier 2)' 

Aquatic Plant Growth (Tier 

212 

1 141-2 / 850.303 

Core 

Core 

TGAI 

TGAI 

42.6 a.i. 

Honey Bee Acute Contact 

no 

Honey Bee Residue on 

Foliage I 

Core 

418213-01 

418191-01 

459085-01, 

42.6 a.i. 

TGAI 

These tests are,highly recommended for cantaloup and citrus due to the high sensitivity of these crops as evidenced in 

label warnings. 

459215-01 

459085-01 

TG A1 

41.9 

Page 134 of 134 

462035-01 

462035-18