Page 1 of 121 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

 OFFICE OF 

 CHEMICAL SAFETY AND 

POLLUTION PREVENTION

 PC Code: 105501 

DP Barcode: D414217 

MEMORANDUM

DATE: May 20, 2014 

SUBJECT: Transmittal of the Draft Environmental Fate and Ecological Risk Assessment in 

Support of the Registration Review of Tebuthiuron 

TO: Joel Wolf, Chemical Review Manager 

 Risk Management and Implementation Branch IV 

 Pesticide Re-evaluation Division 

 7508P 

FROM: Ibrahim Abdel-Saheb, Ph.D., Environmental Scientist 

Steve Carey, Biologist 

 Environmental Risk Bran ch 6 

 Environmental Fate and Effects Division 

7507P 

THROUGH: Mark Corbin, Branch Chief 

 Environmental Risk Branch 6 

 Environmental Fate and Effects Division 

7507P 

The Environmental Fate and Effects Division (EFED) has completed the preliminary ecological risk, 

environmental fate, endangered species, and drinking water assessments conducted in support of the 

Registration Review of the herbicide Tebuthiuron (PC Code 105501). 
-Page 2 of 121- 

Risk Conclusions 

The results of this screening-level risk assessment indicate that the current uses of tebuthiuron may 

potentially cause direct adverse effects to Federally-listed threatened and endangered (hereafter 

referred to as “listed”) and non-listed mammals, terrestrial-phase amphibians, reptiles, small-sized 

birds on an acute exposure basis. Chronic risk to listed and non-listed birds, terrestrial-phase 

amphibians, reptiles, and mammals may also occur. Risk to listed and non-listed species of 

terrestrial plants and aquatic vascular and nonvascular plants may occur. Whereas for terrestrial 

invertebrates, available lines of evidence indicate that the potential for risk is low. Tebuthiuron is not 

expected to adversely affect aquatic vertebrates and invertebrates. The risks may extend to indirect 

effects upon listed species that depend upon that taxonomic group as a resource. 

At this current stage of the registration review process, it is premature to make direct or indirect 

effects determinations until further refinements are conducted. An in-depth listed species evaluation 

determining the potential co-occurrence of listed species and use patterns of tebuthiuron is needed; 

however, additional information on the biology of listed species, the locations of these species, and 

the locations of use sites are needed to determine the extent to which screening assumptions 

regarding an action area apply to a particular listed organism. 

Tables 1 and 2 summarize the maximum uses assessed and the taxa that may potentially be directly 

at risk from exposure to tebuthiuron as a result of broadcast spray and granular applications, 

respectively. Table 3 summarizes the area percentage of spot treatment uses needed to exceed the 

listed species LOC from spraying or with granules/pellets/tablets using ground-based equipments. 
Page 3 of 121 

Table 1. Risk to Taxon Resulting from Tebuthiuron Use Patterns Sprayed at Maximum Single ApplicatioUse Pattern Category

Aquatic Organisms (surrogate for aquatic-phase amphibians)

Birds, (surrogate for terrestrialphase amphibians and reptiles) Mammals Plants1 

Rangeland and Pastures 

Acute risk: Not expected. Highest RQ = 0.02Chronic risk: Not expectedHighest RQ = 0.2

Acute risk: Expected for small-sized birds consuming tebuthiuron directly. Highest RQ=0.98, up to 10x the listed LOC of 0.1; however, extremely low food consumption of the test material was observed in passerines. No effect for medium- and large-sized birds. Days of exceedance for acute risk to birds using maximum scenario = 115 days. Application rate needed to not exceed the listed species LOC = 0.25 lb a.i./A. Chronic risk: Expected for birds. Highest RQ = 14, exceeds 14x the listed LOC of 1. Days of exceedance for chronic risk to birds using maximum scenario = 140 days. Application rate needed to not exceed the listed species LOC = 0.4 lb a.i./A.Distance of risk to non-target birds downwind (drift only) from edge of field:Up to 139 feet with aerial applications; up to 38 feet with ground applications. 

Acute risk: Expected for all sizes of mammals. Highest RQ=1.6, exceeds 16x the listed LOC of 0.1. Days of exceedance for acute risk to mammals using maximum scenario = 60 days. Application rate needed to not exceed the listed species LOC = 0.15 lb a.i./A.Chronic risk: Expected for mammals. Highest RQ=45, exceeds 45x the listed LOC of 1. Days of exceedance for chronic risk to mammals using maximum scenario = 100 days. Application rate needed to not exceed the listed species LOC = 0.15 lb a.i./A.Distance of risk to non-target mammals downwind (drift only) from edge of field: up to 479 feet (aerial app.); up to 125 feet (ground app.).

Terrestrial 

Risk to plaHighest RQ plants = 18(semi-aqua(drift). Higaquatic pla55 (alga/diApplication exceed listeand/or driftTerrestrial area), 0.005 aquatic are(drift only)a.i./A (vasc(alga/diatomDistance of species dowfrom edge maximum up to and efeet; Aquat10 feet; and None, runoexposure to 

1 Results are from aerial-based applications at which exposure is at its highest. 
-Page 4 of 121- 

Table 2. Risk to Taxon Resulting from Tebuthiuron Use Patterns Applied as Granules at Maximum Singa.i./A Use Pattern Category

Aquatic Organisms (surrogate for aquatic-phase amphibians)

Birds, (surrogate for terrestrial-phase amphibians and reptiles)

Mammals Plants1 

Rangeland and Pastures 

Acute risk: Not expected. Highest RQ = 0.02

Acute risk: Expected for smallsized birds that consume tebuthiuron-treated granules as their only source of food; however, low food consumption and regurgitation of the active ingredient were observed in the passerine bird studies. Granular RQs not available, assumed to be equivalent to spray RQs; however, some uncertainty exists since granular and pelleted formulations do not contain 100% of the active ingredient. No effect for medium- and largesized birds. 

Acute risk: Expected for small- and medium-sized mammals that consumes tebuthiuron-treated granules directly. Highest RQ=1.96, exceeds up to 20x the listed LOC of 0.1. Not expected for large-sized mammals; highest RQ = 0.08 Application rate needed to not exceed the listed species LOC for pelleted (40% a.i.) formulations = 0.3 lb a.i./A.Application rate needed to not exceed the listed species LOC for granular (5% a.i.) formulations = 2.5 lb a.i./A

Terrestrial and Expected for plareas adjacent twith granules ctebuthiuron. Hilisted terrestriaarea) and 938 (area). Highest Raquatic plants = and 26 (alga/diApplication ratexceed listed LTerrestrial = 0.area) and 0.006 aquatic area). Aa.i./A (vasculara.i./A (alga/diat

1 Results are from the maximum application rate of 6 lb a.i./A at which exposure is at its highest
Page 5 of 121 

Table 3. Area Percentage of Spot Treatment Needed to Exceed the Listed Species LOC 

Taxa Application 

rates 

(lb a.i./A) 

RQs 

assuming 

100% area 

treated 

(highest to 

lowest)* 

Listed Species LOC Area Percentage 

of application 

needed to exceed 

LOC1

Birds

(acute effects, liquid 

spray, broadcast)

6 0.98

Acute = 0.1

10% 

4 0.66 15% 

Mammals

(acute effects, liquid 

spray, broadcast)

6 1.6 6% 

4 1.1 9% 

Mammals

(acute effects, pellets, 

broadcast)

6 1.96 5%

4 1.31 8%

Mammals

(acute effects, 

granules, broadcast)

6 0.24 42%

4 0.16 63%

Birds

(chronic effects, 

broadcast, dietarybased)

6 14

Chronic = 1.0

7%

4

10 10%

Mammals

(chronic effects, 

liquid spray, 

broadcast, dietarybased)

6 7 14%

4 5

20%

Mammals

(chronic effects, 

broadcast, liquid 

spray, dose-based)

6 45 2%

4 30

3%

Terrestrial Plants

(listed effects, liquid 

spray, ground, 

broadcast)

6 956

Listed plants = 1.0

0.1%

4 638

0.2%

Terrestrial Plants

(listed effects, 

granules, ground, 

broadcast)

6 938 0.1%

4 625

0.2%

Aquatic vascular 

plants 

(listed effects, liquid 

spray, ground, 

broadcast)

6 20 5%

4 14 7%

6 26 4%
-Page 6 of 121- 

Table 3. Area Percentage of Spot Treatment Needed to Exceed the Listed Species LOC 

Taxa Application 

rates 

(lb a.i./A) 

RQs 

assuming 

100% area 

treated 

(highest to 

lowest)* 

Listed Species LOC Area Percentage 

of application 

needed to exceed 

LOC1

Aquatic nonvascular 

plants

(listed effects, liquid 

spray, ground, 

broadcast)

4 18 6%

Aquatic vascular 

plants 

(listed effects, 

granules, ground, 

broadcast)

6 20 5%

4 13 8%

Aquatic nonvascular 

plants

(listed effects, 

granules, ground, 

broadcast)

6 26 4%

4 17 6%

*Bold values indicate LOC exceedance 

1 The threshold percentage is assessed to determine how much percent of exposure that RQ needs to exceed the LOC. 

For example, the highest acute RQ for birds inhabiting a field treated at 6 lb a.i./A is 0.98, which is up to 10x the 

listed species LOC of 0.1; to assess the % of exposure needed at which the LOC exceedance occurred, results 

indicate 10% is needed for the RQ to exceed the LOC. A “no effect” or “may affect” determination was not made 

since it is unknown what % of an actual area is typical spot treatment. 

Basic Uncertainties 

Application dates were chosen in accordance with the product label: “For optimum results, 

applications should be made prior to the resumption of active seasonal growth in the spring or 

before expected seasonal rainfall.” This can occur through a broad portion of a crop year and 

applications can therefore occur over a broad range of time. This creates uncertainty as to when 

to assign the application date for aquatic exposure modeling. Additional information regarding 

tebuthiuron usage, specifically concerning the timing of applications and geographical 

limitations of use, is of high value to EFED.

 
-Page 7 of 121- 

Environmental Fate and Ecological Risk Assessment for the 

Registration of Tebuthiuron

 

Tebuthiuron 

1-(5-tert-butyl-1,3,4-thiadiazol-2-yl)-1,3-dimethylurea 

CAS Registry Number 34014-18-1

PC Code 105501

 

Prepared by:

Ibrahim Abdel-Saheb, Ph.D., Environmental Scientist

Steve Carey, Biologist 

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch VI

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington DC 20460

Reviewed by:

Amy Blankinship, Senior Scientist, ERB VI

William P. Eckel, Ph.D, Senior Science Advisor, ERB VI

Mark Corbin, Branch Chief, ERB VI

May 20, 2014 
Page 8 of 121 

Table of Contents

1. Executive Summary ................................................................................................................. 9

2. Problem Formulation .............................................................................................................. 13

3. Analysis.................................................................................................................................. 19

4. Risk Characterization ............................................................................................................. 46

4.1. Risk Estimation................................................................................................................46

4.1.1 Risk Quotient Calculations for Aquatic Organisms ................................................ 47

4.1.2 Risk Quotient Calculations for Aquatic Plants ........................................................ 51

4.1.3. Risk Quotient Calculations for Terrestrial Animals ............................................... 54

4.1.4. Risk Quotient Calculations for Terrestrial Plants in Terrestrial and Semi-aquatic 

Environments ........................................................................................................................ 62

4.2. Risk Description ...............................................................................................................63

4.2.1. Risks to Aquatic Animals and Plants ...........................................................................64

4.2.2. Risks to Terrestrial Animals and Plants ........................................................................66

5. Federally Threatened and Endangered (Listed) Species Concerns ........................................ 78

 
Page 9 of 121 

1. Executive Summary 

EFED has completed the Registration Review draft ecological risk assessment for Tebuthiuron (1-

(5-tert-butyl-1, 3,4-thiadiazol-2-yl)-1 ,3-dimethylurea; CAS#: 34014-18-1; PC Code: 105501) and 

its end-use products. Tebuthiuron is a systemic, relatively nonselective herbicide, which is 

absorbed into the plant roots and is then translocated into other plant tissues. Once absorbed by 

the roots and translocated into the plant, it acts by inhibiting photosynthesis. 

Tebuthiuron is used to control broadleaf and woody weeds, grasses and brush on feed crop sites 

(pasture and rangeland) and a variety of non-food crop sites including airports/landing fields, 

outdoor industrial areas, non-agricultural rights-of-way, fencerows, hedgerows, uncultivated 

areas/soils, and under paved roads and sidewalks in areas where no future landscaping is 

planned. Primary uses include rangeland and near railroads and other industrial facilities. Single 

active ingredient formulations include granular, pelleted/tableted, wettable powder, water 

dispersible granules, and technical grade/solid products. Four multiple active ingredient 

formulations (granules) also are registered. All formulations may be applied as broadcast, banded 

or spot treatments using ground equipment. The pelleted/tableted formulations also may be 

applied using aerial equipment. 

The currently registered yearly single application rate of tebuthiuron is 6 lb a.i./A for the use on 

rangeland. 

Tebuthiuron is persistent, mobile and can leach to ground water. It is resistant to biological and 

chemical degradation, and its principal route of dissipation in the environment appears to be 

leaching and/or runoff. Transport to ground water through leaching and to surface water through 

runoff are likely as a result of tebuthiuron's environmental persistence and low sorption to soil. 

Tebuthiuron has been detected in ground water in Texas and California. Considering its low 

Henry's Law constant (7.58 x 10-11 atm-m3/mol; at 25°C), the potential for vapor phase and longrange transport is not important. 

With the exception of the pink shrimp, an estuarine/marine invertebrate, a small number of 

available data indicates tebuthiuron is practically non-toxic to freshwater and estuarine/marine 

fish and invertebrates on an acute exposure basis. On a chronic exposure basis, tebuthiuron 

resulted in decreased survival, reproduction (number of young/female), and growth (length) in 

these aquatic organisms. 

On an acute exposure basis, tebuthiuron is slightly toxic to passerines, practically nontoxic to 

upland game birds and waterfowls and moderately toxic to mammals. Significant treatmentrelated reductions in eggs laid per pen, 14-day hatchling per number hatched, hatchling body 

weight, 14-day survivor weight, female and male weight gains, and eggshell thickness were 

determined from the chronic avian reproduction studies, while significant reductions in body 

weight and body weight gains were determined from the chronic mammalian reproduction study. 

Available studies with beneficial insects (i.e., honey bees) recorded no effects (i.e., the no-effect 

level was greater than the highest concentration tested). As expected for an herbicide, the 
Page 10 of 121 

available plant toxicity data showed tebuthiuron to be toxic to aquatic and terrestrial plants based 

on reduced growth. 

Risks are anticipated for terrestrial vertebrates that directly consume vegetation and granules 

treated with tebuthiuron on a daily basis. Exceedances of the listed species LOC were up to 10x 

for birds and up to 20x for mammals. The highest chronic RQs were approximately 14x and 45x 

the chronic LOC for birds and mammals, respectively. For spot treatment uses using sprayers, 

T-REX modeling indicates a field applied at the maximum application rate and with an 

assumption that 100% of the field is treated; an area percentage (threshold) of less than 10% and 

5% of the field treated is needed to not exceed the listed species LOC for birds and mammals, 

respectively, consuming tebuthiuron-treated vegetation. Of the same scenario above, an area 

percentage of less than 7% and 2% is needed to not exceed the chronic LOC for birds and 

mammals, respectively. For granule uses, applying less than 42% of the field at the maximum 

rate would not exceed the listed species LOC for mammals. 

Tebuthiuron is an herbicide; listed plant LOC exceedances for broadcast spraying and granular 

uses were expected. RQs were as high as 1031 for terrestrial plants and 55 for aquatic plants; and 

as high as 938 and 26 for terrestrial and aquatic plants, respectively, when granules are used. 

No risks are expected for fish and aquatic invertebrates. Risks to terrestrial invertebrates are 

unlikely. 

Identified potential risks to listed species are summarized in Table 1.1 and detailed in Table 5.1.

Table 1.1. Potential Effects to Federally-Listed Taxa Associated with Direct Effects from 

the Registered Uses of Tebuthiuron. 

Listed Plant Taxon Potential Direct Effects 

Terrestrial and semi-aquatic 

plants – monocots and dicots Yes* 

Aquatic nonvascular plants Yes* 

Aquatic vascular plants Yes* 

Listed Animal Taxon Potential Direct Effects 

Acute Chronic 

Terrestrial invertebrates Uncertain1 N/A2

Mammals Yes* Yes*

Birds 

Yes* for small-sized classes; 

No effect for mid- and largesized classesYes* (all size classes)

Reptiles3 Yes* Yes* 

Terrestrial-phase amphibians3 Yes* Yes* 

Freshwater fish No Effect No Effect 

Aquatic-phase amphibians4 No Effect No Effect 

Freshwater invertebrates No Effect No Effect

Estuarine/marine fish No Effect No Effect 

Estuarine/marine invertebrates No Effect No Effect 
Page 11 of 121 

* At this current stage of the Registration Review process, it is premature to make effects determinations until further 

refinements are conducted. 1 Although no effects were seen in the acute contact toxicity test with honey bees at the max dose (100 μg a.i./bee), public data 

suggest individual terrestrial invertebrate taxonomic groups increased or decreased in population when exposed to tebuthiuron. 

Further, given that no endpoints were available, EFED cannot preclude effects to this taxon. 2 No methodology available to measure chronic effects to terrestrial invertebrates. 3 Birds are surrogates for reptiles and terrestrial-phase amphibians 

4 Fish are surrogates for aquatic phase amphibians.

Potential effects to federally-listed endangered and depth threatened species (listed species) 

based on LOC exceedances require an in-depth listed species evaluation determining the 

potential co-occurrence of listed species and use patterns of tebuthiuron. 

 

Based on this screening-level assessment, acute and chronic risk resulting from direct effects is 

not expected for listed freshwater and estuarine/marine fish (surrogates for aquatic-phase 

amphibians), and freshwater and estuarine/marine invertebrates. 

Precautionary labeling for beneficial insects is not required based on acute toxicity data for 

honeybees. Risk resulting from direct effects to listed terrestrial invertebrates is uncertain from the 

use of tebuthiuron on its registered use sites. 

Potential direct effects on listed birds (surrogates for terrestrial-phase amphibians and reptiles), 

mammals, and terrestrial and aquatic plants from the use of tebuthiuron may be associated with 

modification of primary constituent elements (PCEs) of designated critical habitat, where such 

designations have been made. However, at this current stage of the Registration Review process, 

it is premature to make effects determinations for listed species until further scientific analysis 

and refinements are conducted, based on recommendations received from the National Academy 

of Sciences’ (NAS) National Research Council (NRC) April 2013 report, available at

http://www.nap.edu/catalog.php?record_id=18344. The NAS report outlines recommendations 

on specific scientific and technical issues related to the development of pesticide risk 

assessments that are compliant with the Endangered Species Act (ESA) and the Federal 

Insecticide, Fungicide, and Rodenticide Act (FIFRA). 

The EPA along with the U.S. Fish & Wildlife Service (USFWS), the National Marine Fisheries 

Service (NMFS) (collectively, the Services), and the U.S. Department of Agriculture (USDA), 

released a summary of their implementation plan for assessing risks of pesticides to listed species 

ahead of the stakeholder workshop held on November 15, 2013. This plan was developed in 

response to the NAS’ recommendations, including a common approach to risk assessment as a 

way of addressing scientific differences between EPA and the Services. During the workshop, 

the agencies received feedback from the public on the interim scientific approaches that were 

developed as part of the initial implementation of the NAS recommendations. These approaches 

will be jointly implemented and vetted as part of a phased iterative process. Once fully vetted, 

EPA will further refine the listed species effects determination portion of this risk assessment. 

To make effects determinations for individual listed species, useful refinements may include, but 

are not limited to, analyses of: 1) more detailed, species-specific ecological and biological data; 

2) more detailed and accurate information on tebuthiuron use patterns; and 3) sub-county level 
Page 12 of 121 

spatial proximity data for the co-occurrence of potential effects areas and listed species and any 

designated critical habitat. Examples of such refinements are described below. 

EFED is currently developing tools that are expected to further refine the assessment and are 

designed to support effects determinations for individual federally listed species and their 

designated critical habitats (where applicable). Scientific information obtained from the 

Services, and other reliable sources is being collated by EFED to address all currently listed 

species. This information is being stored in an Office of Pesticide Programs (OPP) Pesticide 

Registration Information SysteM (PRISM) listed species knowledgebase. The listed species 

knowledgebase consists of an information repository that houses biological and behavioral 

information relevant to individual species (e.g., habitat, diet, and life history, including specific 

temporal and spatial associations) and a document repository that contains supporting documents 

(e.g., USFWS recovery plans) and electronic information (e.g., GIS data files). For terrestrial 

taxa, biological information relevant to the assessment (e.g., diet and body weight) will be used 

to parameterize exposure estimates using a method consistent with currently used methods in the 

T-REX and T-HERPS models. 

Refinements will also include more detailed analyses of the registered uses and specific use 

patterns that result in either “Likely to Adversely Affect” (LAA) or “Not Likely to Adversely 

Affect” (NLAA) determinations for federally listed species. The analyses may include more 

information on where, when, and how tebuthiuron is used on all use sites. Actual usage data 

(when available) and national land-cover datasets that indicate potential use sites [e.g., National 

Land Cover Dataset (NLCD), Cropland Data Layer (CDL)] may be used to support a more 

refined analysis of where tebuthiuron is reasonably expected to be used. Similarly, refinements 

on the timing of applications and a more in-depth exploration of agronomic practices for 

tebuthiuron may be included as part of the refinement. 

The refinements based on individual species data; additional, detailed usage information, when 

available; and recommendations from the NRC report are expected to help to more accurately 

identify potential areas of effect and to better inform effects and habitat determinations for listed 

species and any designated critical habitats. 

Uncertainties 

The conclusions of this risk assessment are limited by uncertainties concerning both exposure and 

toxicity. 

Degradate 104 (N-[5-(1,14-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea) has the highest 

concentrations detected. Additionally, degradate 104 was detected in a retrospective ground water 

monitoring study and was a major degradate in a terrestrial field dissipation study, accounting for 

up to 23% of the mass applied. The degradate 104 was also found in an aerobic soil metabolism 

and soil photolysis studies comprising close to l7% of the mass applied. Degradate 104 was 

identified by the Health Effects Division (HED) as the only environmental fate degradate of 

tebuthiuron of toxicological concern to human health. The lack of data regarding maximum 

occurrence, mobility, and toxicity of degradate 104 are further uncertainties which are addressed 

through a total toxic residues approach. 
Page 13 of 121 

2. Problem Formulation 

2.1. Nature of Regulatory Action 

Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), all pesticides distributed 

or sold in the United States must be registered by the United States Environmental Protection 

Agency (EPA or the Agency). To determine whether a pesticide can be registered, the EPA 

evaluates the product’s safety to non-target species based on a wide range of environmental and 

health effects studies. In 1996, FIFRA was amended by the Food Quality Protection Act (FQPA), 

and EPA was mandated to implement a new program for the periodic review of pesticides, i.e., 

Registration Review1

. The registration review program is intended to ensure that as the ability to 

assess risk evolves and as policies and practices change, all registered pesticides continue to meet 

the statutory standard of no unreasonable adverse effects to human health and the environment. 

Changes in science, public policy, and pesticide use practices will occur over time. Through the 

registration review program, the Agency periodically reevaluates pesticides to make sure that as 

changes occur, products in the marketplace can be used safely. 

Additionally, potential effects to Federally listed endangered and threatened species (hereafter 

referred to as “listed”) and their designated critical habitat are also considered under the 

Endangered Species Act (ESA) in order to ensure that the continued registration of tebuthiuron is 

not likely to jeopardize the continued existence of listed species or adversely modify their critical 

habitat. In order to meet the requirements of FIFRA and the ESA, this assessment follows EPA 

guidance on conducting ecological risk assessments (USEPA, 1998) and Office of Pesticide 

Program’s Overview Document, which contains guidance for assessing pesticide risks to nonlisted and listed species (USEPA, 2004). 

Recent tebuthiuron ecological risk assessment- related documents performed by EFED include Tier 

II Aquatic Exposure Assessment for Selected Tebuthiuron Uses on Rangeland, Pastureland, and 

Other Non-Crop Lands in the Pacific Northwest: Endangered Species (ES) Consultation Package. 

Dated: July 12, 2004. DP Barcode D304225, and an initial Problem Formulation for Registration 

Review (2009, DP Barcode D363982). 

2.2. Mode of Action 

Tebuthiuron is a systemic, relatively nonselective herbicide which is absorbed into plant roots 

and is then translocated throughout the plant. It acts by inhibiting photosynthesis. 

2.3. Use Characterization 

 

1 http://www.epa.gov/oppsrrd1/registration_review/
Page 14 of 121 

Single active ingredient formulations include granular, pelleted/tableted, wettable powder, water 

dispersible granules, and technical grade/solid products. Four multiple active ingredient 

formulations (granules) also are registered. All formulations may be applied as broadcast, banded 

or spot treatments using ground equipment. The pelleted/tableted formulations also may be 

applied using aerial equipment. 

Labeled use patterns for tebuthiuron are summarized in Table 2.1, below. 

Table 2.1. Tebuthiuron Uses by Label 

Reg.No. Formul

ation 

% a.i. Uses # of apps/yr Method of App. 

013283-

00018 

Granular 

2 

6 (Diuron) 

COMMERCIAL/INSTITU

TIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT 

(OUTDOOR) 

2 Broadcast, No 

aerial app allowed 

013283-

00021 

Granular 1 

3 (Diuron) 

COMMERCIAL/INSTITU

TIONAL/INDUSTRIAL 

PREMISES/EQUIPMENT 

(OUTDOOR) 

2 Broadcast, 

Application via 

shaker can 

034913-

00010 

Granular 5 DRAINAGE SYSTEMS 1/3 yr, 

1 time in 3 yr period up 

to 4 lbs a.i./A, no more 

than 6 lbs a.i./A per 2 

consecutive treatments in 

any 6 year period 

Broadcast, Band 

treatment, Spot 

treatment, , no 

aerial applications 

034913-

00015 

Granular 1 

3 (Diuron) 

INDUSTRIAL AREAS 

(OUTDOOR), 

NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HED

GEROWS, 

NONAGRICULTURAL 

UNCULTIVATED 

AREAS/SOILS, PAVED 

AREAS (PRIVATE 

ROADS/SIDEWALKS) 

1/3 yr, 

1 time in 3 yr period up 

to 4 lbs a.i./A, no more 

than 6 lbs a.i./A per 2 

consecutive treatments in 

any 6 year period 

Soil band 

treatment, Spot 

soil treatment, Soil 

broadcast 

treatment 

034913-

00016 

Granular 2 

6 (Diuron) 

TERRESTRIAL NONFOOD+OUTDOOR 

RESIDENTIAL 

1/3 yr, 1 time in 3 yr 

period up to 4 lbs a.i./A, 

no more than 6 lbs a.i./A 

per 2 consecutive 

treatments in any 6 year 

period 

Soil band 

treatment, Spot 

soil treatment, Soil 

broadcast 

treatment 

062719-

00107 

Wettable 

powder 

80 TERRESTRIAL NONFOOD CROP; 

TERRESTRIAL FEED 

CROP 

Max rate is 1-2 lb a.i./A, 

every 3 years for 

vulnerable sites; 1/3 yr, 1 

time in 3 yr period up to 

4 lbs a.i./A, no more than 

6 lbs ai/A per 2 

consecutive treatments in 

Soil band 

treatment, Spot 

soil treatment, Soil 

broadcast 

treatment 
Page 15 of 121 

any 6 year period; spot 

treatment is allowed up 

to 6 lb a.i./A when 

needed 

062719-

00121 

Pelleted/ 

Tableted 

20 TERRESTRIAL NONFOOD CROP, 

TERRESTRIAL FEED 

CROP 

Max rate is 1-2 lb a.i./A, 

every 3 years for 

vulnerable sites; 1/3 yr, 1 

time in 3 yr period up to 

4 lbs a.i./A, no more than 

6 lbs a.i./A per 2 

consecutive treatments in 

any 6 year period; spot 

treatment is allowed up 

to 6 lb a.i./A when 

needed 

Soil band 

treatment, Spot 

soil treatment, Soil 

broadcast 

treatment 

062719-

00122 

Pelleted/ 

Tableted 

40 AQUATIC NON-FOOD 

INDUSTRIAL, 

TERRESTRIAL NONFOOD CROP, 

TERRESTRIAL FEED 

CROP 

Max rate is 1-2 lb a.i./A, 

every 3 years for 

vulnerable sites; 1/3 yr, 1 

time in 3 yr period up to 

4 lbs a.i./A, no more than 

6 lbs a.i./A per 2 

consecutive treatments in 

any 6 year period; spot 

treatment is allowed up 

to 6 lb a.i./A when 

needed 

Soil band 

treatment, Spot 

soil treatment, Soil 

broadcast 

treatment 

81927-37 Pelleted 80 TOTAL VEGETATION 

CONTROL IN NONCROPLAND AND FOR 

WOODY PLANT 

CONTROL IN NONCROPLAND, 

RANGELAND AND 

PERMANENT 

PASTURES. 

Max rate is 1-2 lb a.i./A, 

every 3 years for 

vulnerable sites; 1/3 yr, 1 

time in 3 yr period up to 

4 lb a.i./A; spot 

treatment is allowed up 

to 6 lb a.i./A when 

needed 

Surface applied, 

broadcast 

treatment, as a 

banded treatment, 

and aerial 

Application on 

rights-of-way is 

limited to 

helicopter only 

 81927-41 Pelleted 20 Max rate is 1-2 lb a.i./A, 

every 3 years for 

vulnerable sites; For all 

other areas, the 

maximum use rate and 

frequency of application 

is up to 4 lb a.i/acre

once every three years, 

and no more than two 

treatments totaling 6 lb 

a.i./acre in any 6 year 

period, spot treatment is 

allowed up to 6 lb a.i./A 

when needed 

A screening level estimate of current usage of tebuthiuron is presented in Table 2.2. This 

screening level usage assessment presents usage data on an annual basis and is derived from 
Page 16 of 121 

USDA’s National Agricultural Statistics Service with data updated to 2013. This analysis amounts 

to 8,000 pounds of active ingredient used nationally on an annual basis with use on Pastureland 

being the dominant use pattern. 

Table 2.2. Tebuthiuron Screening Level Usage Analysis (SLUA) for 2013 

 Average Percent Crop Treated 

 Crop Lbs. a.i. Average Maximum 

1 Pastureland 8,000 <1 <2.5 

2.4. Conceptual Model 

The potential terrestrial and aquatic exposure pathways and effects of the registered uses of 

tebuthiuron are shown in Figures 1 and 2. The conceptual model used to depict the potential 

ecological risk associated with the registered uses of tebuthiuron assumes that, as an herbicide, 

tebuthiuron is capable of adversely affecting terrestrial animals and plants including aquatic plants, 

provided that environmental concentrations are sufficiently elevated as a result of the labeled uses. 

Solid arrows in Figures 1 and 2 depict the most likely routes of exposure and effects; dashed lines 

depict potential routes of exposure that are not considered likely for tebuthiuron. 
Page 17 of 121 

Figure 1. Conceptual Model of the Transport and Effects of Tebuthiuron for Aquatic Environments. 

*Dotted lines indicate that, although this exposure route/media was considered, its contribution to the fate and transport of 

tebuthiuron is expected to be negligible
Page 18 of 121 

Figure 2. Conceptual Model of the Transport and Effects of Tebuthiuron for Terrestrial 

Environments. 

*Dotted lines indicate that, although this exposure route/media was considered, its contribution to the fate and 

transport of tebuthiuron is expected to be negligible
Page 19 of 121 

The hypothesis for the risks of tebuthiuron to non-target animals focuses on both aquatic and 

terrestrial environments via potential exposure to the parent from direct spray, spray drift, and 

runoff. Leaching is considered a significant route of transport considering tebuthiuron is mobile 

in water. Dotted exposure routes are not likely (not anticipated to volatize). For terrestrial 

organisms, the major route of exposure is via diet, such as through consumption of plant leaves or 

insects, which contain tebuthiuron residues as a result of direct application and/or spray drift. For 

aquatic animal species, the major routes of exposure are direct contact via the respiratory surface 

(gills) or the integument. Aquatic vascular and non-vascular plants may also be exposed via direct 

uptake and adsorption. Risk to terrestrial plants is also considered in this screening-level 

assessment. In addition, tebuthiuron has a low potential for sorption to sediment; therefore, 

exposure to organisms that reside primarily in the benthos is not likely. Estimated exposure 

concentrations for all organisms are obtained through the use of several Agency exposure models.

The following risk hypothesis is presumed for this screening level assessment: 

Based on the application methods, mode of action, direct toxicity to non-target aquatic and 

terrestrial species, the registered uses of tebuthiuron have the potential to reduce survival, 

reproduction, and/or growth in listed and non-listed terrestrial and aquatic animals and plants. 

3. Analysis 

 3.1. Exposure Characterization 

 3.1.1. Environmental Fate and Transport Characterization 

Tebuthiuron is persistent, mobile and can leach to ground water, as indicated by a small-scale 

retrospective ground water study (MRID 42390901). It is resistant to biological and chemical 

degradation, and its principle route of dissipation in the environment appears to be leaching and/or 

runoff. Transport to ground water through leaching and to surface water through runoff are likely 

as a result of tebuthiuron's environmental persistence and low sorption to soil. Because tebuthiuron 

has the potential to leach through the ground and because of the detections in Texas and California, 

the Agency is concerned about the potential for ground water contamination. Considering its low 

Henry's Law constant, (7.58 x 10-11 atm-m3/mol; at 25°C), the potential of vapor phase and long 

range transport is considered insignificant. The environmental fate study summaries are presented 

as Appendix A. 

Based on available environmental fate laboratory studies, several minor degradates (< 10%) were 

identified (Table 3.1; Appendix B). Among these degradates, degradate 104 (N-[5-(1,14- 

dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea) has the highest concentration detected. 

Additionally, degradate 104 was detected in a retrospective ground water monitoring study and 

was a major degradate in a terrestrial field dissipation study accounting for up to 23% of the mass 

applied. The degradate 104 was also found in an aerobic soil metabolism (t1/2= 0.8 d) and soil 

photolysis (t1/2= 18 d) studies (MRID # 41050201) comprising close to 7% of the mass applied. 

Degradate 104 was identified by the Health Effects Division (HED) as the only environmental fate 

degradate of tebuthiuron of toxicological concern to human health. A cumulative residue approach 
Page 20 of 121 

to model total tebuthiuron residues was applied for drinking water assessment purposes as well as 

in this ecological risk assessment. 

In regards to the bioconcentration potential of tebuthiuron, a 28-day flow-through study in which 

bluegill sunfish were exposed to a nominal tebuthiuron concentration of 5.0 ppm resulted in 

bioconcentration factors of 1.98, 3.40, and 2.63 for edible tissue, non-edible tissue, and whole 

fish, respectively. Residues in the tissues consisted primarily of tebuthiuron and two metabolites 

(N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methyl-N’ hydroxymethyl-urea [compound 

109] and compound 103(OH) (hydroxylated form of parent tebuthiuron). Accumulated residues 

depurated rapidly from fish tissue with depuration half-lives of 0.33 and 0.51 days reported for 

edible and non-edible tissue, respectively (MRID 40819501). Due to the rapid depuration, 

bioaccumulation is not a concern. 

 Table 3.1. Tebuthiuron and Its Environmental Degradates. 

Parent / Degradate Name and 

Structure

Percent of Applied 

Maximum Day MRID # Study Type 

104: N-[5-(1,1-dimethylethyl)-1,3,4-

thiadiazol-2-yl]-N-methylurea ND*

 N/A** N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

6.8 14 41050201 soil photolysis 

6.9 270 41328001 aerobic soil metabolism 

2.9 60 41328002 anaerobic soil metabolism 

1.5 21 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

22.9 408 43318101 terrestrial field dissipation 

2.2 (edible) 21 40819501 bioaccumulation in fish 

105: N-[5-(1,1-dimethylethyl)-1,3,4-

thiadiazol-2-yl]-N=-methylurea 

ND N/A N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

3.5 19 41050201 soil photolysis 

0.4 90 41328001 aerobic soil metabolism 

0.2 60 41328002 anaerobic soil metabolism 

0.4 28 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

NH2

O

N

CH3

N N

SH

H3C

H3C CH3

MolWt: 214.29 C8 H14 N4 O1 S1 

 

CH3

NH

O

NH

N N

SH

H3C

H3C CH3

MolWt: 214.29 C8 H14 N4 O1 S1 

 
Page 21 of 121 

Parent / Degradate Name and 

Structure

Percent of Applied 

Maximum Day MRID # Study Type 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

ND N/A 43318101 terrestrial field dissipation 

4.7 (edible) 21 40819501 bioaccumulation in fish 

106: N-[5-(1,1-dimethylethyl)-1,3,4-

thiadiazol-2-yl] urea ND N/A N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

2.7 5 41050201 soil photolysis 

ND N/A 41328001 aerobic soil metabolism 

ND N/A 41328002 anaerobic soil metabolism 

ND N/A 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

ND N/A 43318101 terrestrial field dissipation 

ND N/A 40819501 bioaccumulation in fish 

107: 5-(1,1-Dimethylethyl)-2-

methylamino-1,3,4-thiadiazole 

ND N/A N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

ND N/A 41050201 soil photolysis 

1.1 270 41328001 aerobic soil metabolism 

ND N/A 41328002 anaerobic soil metabolism 

0.3 21 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

ND N/A 43318101 terrestrial field dissipation 

ND N/A 40819501 bioaccumulation in fish 

108: 2-dimethylethyl-5-amino-1,3,4-

thiadiazole 

ND N/A N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

H2N

O

NH

N N

SH

H3C

H3C CH3

MolWt: 200.26 C7 H12 N4 O1 S1 

 

NH

CH3

N N

SH

H3C

H3C CH3

MolWt: 171.26 C7 H13 N3 S1 

 
Page 22 of 121 

Parent / Degradate Name and 

Structure

Percent of Applied 

Maximum Day MRID # Study Type 

ND N/A 41050201 soil photolysis 

0.6 270 41328001 aerobic soil metabolism 

ND N/A 41328002 anaerobic soil metabolism 

0.1 7 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

ND N/A 43318101 terrestrial field dissipation 

ND N/A 40819501 bioaccumulation in fish 

109: N-[5-(1,1-dimethylethyl)-1,3,4-

thiadiazol-2-yl]-N'-hydroxymethylN-methylurea 

ND N/A N/A hydrolysis 

ND N/A 41305101 aqueous photolysis 

ND N/A 41050201 soil photolysis 

ND N/A 41328001 aerobic soil metabolism 

0.2 60 41328002 anaerobic soil metabolism 

0.3 7 41372501 aerobic aquatic metabolism 

ND N/A 41913101 anaerobic aquatic metabolism 

ND N/A 40768401 batch equilibrium 

N/A N/A N/A laboratory volatility 

N/A N/A N/A field volatility 

ND N/A 43318101 terrestrial field dissipation 

40.1 (edible) 21 40819501 bioaccumulation in fish 

*: Not determined. 

**: Not applicable. 

NH2

N N

SH

H3C

H3C CH3

MolWt: 157.23 C6 H11 N3 S1 

 

OH

NH

O

N

CH3

N N

SH

H3C

H3C CH3

MolWt: 258.34 C10 H18 N4 O2 S1 

 
Page 23 of 121 

Table 3.2 lists the environmental fate properties of tebuthiuron in the submitted environmental 

fate and transport studies. 

Table 3.2. Summary of tebuthiuron Environmental Fate Properties 

Study/Parameter Value (units)

MRID # 

Or 

Source

Study Status 

and Comments

Hydrolysis Stable Acc. No 00020779 Acceptable 

Solubility 2500 (mg/L @ 25oC) Tomlin CDS, ed (2004-05) N/A 

Vapor Pressure 3.0 x 10-7 (torr) Tomlin CDS, ed (2004-05) N/A 

log Kow 1.79 (estimation) BioByte. 1995. ClogP for Windows 

Program (v1.0). BioByte Corp., 

Claremont, CA. 

N/A 

Direct Aqueous 

Photolysis 

Stable 41305101 Acceptable 

Aerobic Soil 

Metabolism 

t1/2 = 1062 days 41328001 Acceptable 

Anaerobic Soil 

Metabolism 

Stable 41328002 Acceptable 

Aerobic Aquatic 

Metabolism 

683 41372501 Acceptable 

Anaerobic Aquatic 

Metabolism 

> 1 year 41913101 Acceptable 

Kd-ads 0.11, 0.62, 0.82, and 1.82 

mL/g 

40768401 Acceptable 

Terrestrial Field 

Dissipation 

t1/2 = 1-2 years 43318101 Acceptable 

Fish 

Bioaccumulation 

Bioconcentration factor BCF= 

1.98, 3 .40, and 2.63 for edible 

tissue, non-edible tissue, 

and whole fish, respectively. 

40819501 Acceptable 

 3.1.2. Measures of Aquatic Exposure 

Aquatic exposures are quantitatively estimated for all assessed uses using scenarios that represent 

high exposure sites for tebuthiuron use. Each of these sites represents a 10-hectare field that drains 

into a 1-hectare pond that is 2 meters deep and has no outlet. Exposure estimates generated using 

the standard pond are intended to represent a wide variety of vulnerable water bodies that occur at 

the top of watersheds including prairie pot holes, playa lakes, wetlands, vernal pools, man-made 

and natural ponds, and intermittent and first-order streams. As a group, there are factors that make 

these water bodies more or less vulnerable than the standard surrogate pond. Static water bodies 

that have larger ratios of drainage area to water body volume would be expected to have higher 

peak EECs than the standard pond. These water bodies will be either shallower or have large 

drainage areas (or both). Shallow water bodies tend to have limited additional storage capacity, 

and thus, tend to overflow and carry pesticide in the discharge whereas the standard pond has no 

discharge. As watershed size increases beyond 10 hectares, at some point, it becomes unlikely that 

the entire watershed is planted to a single crop, which is all treated with the pesticide. Headwater 

streams can also have peak concentrations higher than the standard pond, but they tend to persist 

for only short periods of time and are then carried downstream. 
Page 24 of 121 

Tebuthiuron environmental fate data used for generating model parameters are listed in Table 3.3. 

Chemical property input values were chosen in accordance with current input parameter guidance 

(USEPA, 20091). Using meteorological files information associated with each PRZM scenario, 

application dates were chosen in according with the product label: “For optimum results, 

applications should be made prior to the resumption of active seasonal growth in the spring or 

before expected seasonal rainfall”. 

The maximum labeled single and annual application rate for tebuthiuron is at 6 lb a.i./A for 

broadcast applications via spray or granules. Only one application per year or every three years is 

assumed. However, a maximum application rate of 4 lb a.i./A is also expected for several of the 

same uses that is applied at 6 lb a.i./A. Thus, the application rate of 4 lb a.i./A was included in 

the assessment to evaluate whether the risk picture would improve significantly when the 6 lb 

a.i./A application rate is reduced to 4 lb a.i./A. The application methods assumed are as follows: 

aerial, ground, shaker can, spreader, hand held spray gun, backpack sprayer, and band sprayer. 

For this assessment, aerial and ground broadcast applications involve the highest application 

rates and are used to assess the potential risk to wildlife exposed to tebuthiuron. EECs for spot 

treatment applications are not available; however, exposure from spot treatment using backpack 

sprayers or granules will be subsequently discussed for any potential risks to non-target wildlife 

and plants after the assessment on broadcast applications as a liquid spray or with granules. 

Table 3.3. Summary of PRZM/EXAMS Environmental Fate Data Used for Aquatic Exposure 

Inputs for Tebuthiuron 

Fate Property Value (unit) MRID (or source) Comment 

Molecular Weight 228.3 Tomlin CDS, ed (2004-05) 

Henry’s constant 1.2E-10 (atm-m3/mol) Tomlin CDS, ed (2004-05) 

Vapor Pressure 3.0E-7 (torr) Tomlin CDS, ed (2004-05) 

Solubility in Water 2500 (mg/L) Tomlin CDS, ed (2004-05) 

Application Rate 6.72 (kg a.i./ha) 

4.48 (kg/ha) 

EPA Reg No. 62719-122 

Number of 

Applications 1 EPA Reg No. 62719-122 

Interval between 

Applications (days) Not applicable EPA Reg No. 62719-122 

Photolysis in Water Stable MRID 41305101 

Aerobic Soil 

Metabolism Half-life 270.6 (day) MRID 41328001

3X aerobic soil metabolism 

half-life for total residues (a 

single aerobic soil metabolism 

half-life value is available); 

input parameter per Input 

Parameter Guidance (1) 
Page 25 of 121 

Fate Property Value (unit) MRID (or source) Comment 

Hydrolysis Stable Acc. No 00020779 

Aerobic Aquatic 

Metabolism (water 

column) 

2050 (day) MRID 41372501

3X aerobic aquatic metabolism 

half-life for total residues (a 

single aerobic aquatic 

metabolism half-life value is 

available); input parameter per 

Input Parameter Guidance (1) 

Anaerobic Aquatic 

Metabolism (benthic) Stable MRID 41913101 

Koc 85.2 (mg/L) MRID 40768401 

Binding is correlated to organic 

carbon content. Mean Koc; 

input parameter per Input 

Parameter Guidance (1) 

Chemical Application 

Method (CAM) 2 pre-plant or pre-emergence, unincorporated 

1 Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides; 

Version 2.1, October 22, 2009. 

Results from each PRZM scenario are presented in Table 3.4. Output that generates peak EECs 

are reported in Appendix C. A typical available grass PRZM scenario used as pasture is alfalfa. 

Table 3.4. Tier II Concentrations of Tebuthiuron in Surface Water 

Tier II 

PRZM/EXAMS Scenario

Application rate

6 lbs a.i./A 4 lbs a.i./A 

(ppb) 

TXalfalfaOP 

Aerial 1109 736.3 

Ground 1018 679 

Granular 997.3 664.9 

MNalfalfaOP 

Aerial 558.6 372.7 

Ground 376 251 

Granular 330.8 220.8 

NCalfalfaOP 

Aerial 604 403 

Ground 453.1 301.5 

Granular 414.7 276.8 

PAalfalfaOP 

Aerial 693.9 462.9 

Ground 534.8 355.9 

Granular 494.8 329.9 

CAalfalfa_WirrigOP 

Aerial 498.9 332 

Ground 367.9 245 

Granular 335 223 

Because of the persistence of this compound, the 1 in 10 year PRZM/EXAMS EECs are 

expected to represent 30 years of residue accumulation in the pond. This fact was presented by 

plotting time against the EECs from the following scenarios modeled: TXalfalfaOP (aerial), and 
Page 26 of 121 

PAalfalfaOP (aerial) (ones with highest EECs) show a continuous accumulation of tebuthiuron in 

pond (Appendix D). Such residue accumulation is driven mostly by the persistence of the 

compound as well as by the lack of flow out of the pond. In cases where a flowing water exists, 

the EEC over the long run would not be as high. 

 3.1.3. Measures of Terrestrial Exposure 

3.1.3.1. Terrestrial Animals 

Exposure estimates for terrestrial animals assumed to be in the target area or in an area exposed 

to spray drift or granules are derived using the T-REX2

 model (version 1.5.2.). This model 

incorporates the Kenaga nomograph, as modified by Fletcher et al. (1994), which is based on a 

large set of actual field residue data. The upper limit values from the nomograph represent an 

approximation of the highest residue value observed in the data set (Hoerger and Kenaga 1972). 

Consideration is given to different types of feeding strategies for mammals and birds, including 

herbivores, insectivores and granivores. For dose-based exposures, three weight classes of birds 

(20, 100, and 1000 g) and mammals (15, 35, and 1000 g) are considered. A default foliar 

dissipation half-life of 35 days is used based on the work of Willis and McDowell (1987). 

 Ingestion of Food Items Based on Foliar Application 

The same tebuthiuron application and seasonal rate limitations as evaluated in previous 

assessments (i.e., maximum broadcast application rates of 6.0 and 4.0 lb a.i. and one application 

per year) are taken from active labels and are used to estimate terrestrial exposure for this 

assessment. The dietary-based EECs on a variety of food items from the use of wettable powder 

and water dispersible granule formulations of tebuthiuron applied at the maximum label rate are 

provided in Table 3.5; dose-based EECs are provided in Table 3.6. 

Table 3.5. T-REX Calculated Dietary-Based EECs of Tebuthiuron on Food Residues.

Application Rate 

(lb a.i./A) Food Type Maximum Dietary-based EECs (mg/kg-diet) 

6.0 

Short Grass 1440 

Tall Grass 660 

Broadleaf plants 810 

Fruits/pods/seeds 90 

Arthropods 564 

4.0 

Short Grass 960 

Tall Grass 440 

Broadleaf plants 540 

Fruits/pods/seeds 60 

Arthropods 376 

 

2

 USEPA. 2013. Pesticides: Science and Policy. Terrestrial Models, T-REX Version 1.5.2. 

http://www.epa.gov/oppefed1/models/terrestrial/ 
Page 27 of 121 

Table 3.6. T-REX Calculated Dose-Based EECs of Tebuthiuron on Food Residues. 

Application 

Rate (lb a.i./A) Food Type 

Dose-based EECs for 

Birds 

(mg/kg-bw) 

Dose-based EECs for Mammals 

(mg/kg-bw) 

Small 

(20 g) 

Medium 

(100 g) 

Large 

(1000 g) 

Small 

(15 g) 

Medium 

(35 g) 

Large 

(1000 g) 

6.0 

Short grass 1640 935 419 1373 949 220 

Tall grass 752 429 192 629 435 101 

Broadleaf plants 922 526 236 772 534 124 

Fruits/pods 103 58 26 86 59 14 

Arthropods 642 366 164 538 372 86 

Seeds 

(granivore) 23 13 6 19 13 3 

4.0 

Short grass 1093 623 279 915 633 147 

Tall grass 501 286 128 420 290 67 

Broadleaf plants 615 351 157 515 356 83 

Fruits/pods 68 39 17 57 40 9 

Arthropods 428 244 109 358 248 57 

Seeds 

(granivore) 15 9 4 13 9 2 

Ingestion of granules based on broadcast applications 

T-REX includes the capability to calculate the LD50/ft2

 risk index values for granular applications. 

Conceptually, an LD50/ft2 is the amount of a pesticide estimated to kill 50% of exposed animals in 

each square foot of applied area. Although a square foot does not have defined ecological 

relevance, and any unit area could be used, risk presumably increases as the LD50/ft2 value 

increases. For additional information on the LD50/ft2

 risk index, please refer to USEPA (1992). 

The LD50/ft2 value is calculated using a toxicity value (adjusted LD50) and the EEC (mg a.i./ft2) 

and is directly compared with the Agency’s levels-of-concern (LOCs). In addition, pellets and 

granules are considered equivalent. 

Assuming a maximum of 40% active ingredient in pelleted/tableted formulations, the EEC for 

tebuthiuron-treated pelleted/tableted formulations applied at the maximum label rate of 6 and 4 

lb a.i./A is 25 and 16.7 mg a.i./ft2

, respectively. For granular formulations which contains a 

maximum of 5% tebuthiuron, the EEC at 6 and 4 lb a.i./A is 3.12 and 2.08 mg a.i./ ft2

, 

respectively. 

3.1.3.2. Terrestrial Plants 

Tebuthiuron exposure to terrestrial and semi-aquatic plants is estimated using the TerrPlant3

(version 1.2.2) model. The model generates EECs for plants residing near a use area that may be 

exposed via runoff and/or spray drift from foliar and granular applications. The EECs are 

generated from one application at the maximum rate for a particular use and compound-specific 

 

3

 USEPA. 2013a. Pesticides: Science and Policy. Terrestrial Models, TerrPlant Version 1.2.2. 

http://www.epa.gov/oppefed1/models/terrestrial/ 
Page 28 of 121 

solubility information. Only a single application is considered because it is assumed that for plants, 

toxic effects are likely to manifest shortly after the initial exposure and that subsequent exposures 

do not contribute to the response. Hence, the model estimates EECs based on application rate, the 

solubility limit, and default assumptions of drift. The EECs for terrestrial and semi-aquatic plants 

for a single application of tebuthiuron at 6 and 4 lb a.i./A are presented in Table 3.7. 

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

AgDRIFT (v. 2.1.1) were used to estimate EECs for off-site spray drift exposure in dry and semiaquatic areas and water bodies (i.e., standard farm pond) (Table 3.8) for wettable powder and 

water dispersible granule formulations. For aerial applications, EFED used the American Society 

of Agricultural and Biological Engineers (ASABE, formerly ASAE) Fine-to-Medium droplet size 

category in its modeling. This drop size category is EFED’s default distribution when labels do 

not specify application parameters. For ground applications, EFED used the ASABE Very Fineto-Fine droplet size category with a high boom height. Again, this droplet size category is EFED’s 

default distribution when labels do not specify the application parameters for ground applications. 

Negligible amount of drift is likely from granular applications; thus, drift EECs for granular 

formulations are not included in the AgDRIFT modeling. 

Table 3.7. TerrPlant EECs for Terrestrial Plants Located Adjacent to Tebuthiuron Treated Sites. 

Broadcast 

Application Rate 

(lb a.i./A) 

Application 

Method 

 TerrPlant EECs (lbs a.i./A) 

Total Loading to 

Dry Areas Adjacent 

to Treated Areas1

Total Loading to SemiAquatic Areas Adjacent 

to Treated Areas2

Drift to 

Adjacent Areas3

6 Aerial Unincorp. 

Spray 

0.6 3.3 0.3 

4 0.4 2.2 0.2 

6 Ground Unincorp. 

Spray 

0.36 3.06 0.06 

4 0.24 2.04 0.04 

6 Granular 0.3 3 0.0 

4 0.2 2 0.0 

1 EEC = Sheet Runoff + Drift (1% for ground) 

2

 EEC = Channelized Runoff + Drift (1% for ground) 

3 EEC for ground (appl. rate x 1% drift) 

Table 3.8. Maximum Drift Loadings at Specific Distances for Terrestrial and Semi-Aquatic Areas and 

Ponds Near Tebuthiuron Use Areas. 

Broadcast 

Application 

Rate 

(lb a.i./A) 

Application 

Method 

Distance from Edge of 

Field 

(ft) 

Drift EEC 

Dry Areas 

(lb a.i./A) 

Wetland 

(lb a.i./A) 

Pond 

(mg/L) 

6.0 Ground 

0 6.0 0.4 0.02 

10 1.6 0.2 0.01 

50 0.3 0.1 0.007 

100 0.1 0.08 0.004 

500 0.02 0.02 0.001 

997 0.009 0.007 0.0004 

6.0 Aerial 

0 3 0.8 0.04 

10 2 0.6 0.036 

50 1 0.4 0.025 

100 0.6 0.3 0.017 
Page 29 of 121 

The Screening Tool for Inhalation Risk (STIR v.1.0, November 19, 2010) was used to calculate 

an upper-bound estimate of wildlife inhalation exposure using tebuthiuron’s vapor pressure and 

molecular weight for vapor phase exposure as well as the maximum application rate and method 

of application for spray drift. STIR incorporates results from several toxicity studies including 

acute oral and inhalation rat toxicity endpoints obtained from the “six-pack” of core studies, which 

is a series of six guideline studies that are submitted to the Registration Division of the Office of 

Pesticide Programs for technical and formulated products of a pesticide (rat LD50 = 388 mg/kgbw; rat inhalation LC50 = 3.696 mg/L) as well as the most sensitive acute oral avian toxicity 

endpoint (mallard LD50 >2000 mg/kg-bw). Based on the results of the STIR model (Appendix 

E), exposure through inhalation of spray drift or the vapor phase of tebuthiuron is not determined 

to be a potential pathway of concern for either avian or mammalian species on an acute exposure 

basis. 

 

The analysis of the inhalation route in STIR does not consider that aggregation with other exposure 

pathways such as dietary, dermal, or drinking water may contribute to a total exposure that has a 

potential for effects to non-target animals. However, the Agency does consider the relative 

importance of other routes of exposure in situations where data indicate that pesticide exposures 

through other routes may be potentially significant contributors to wildlife risk (USEPA 2004). 

The risk characterization section (Section 4.2), discusses the impact of consideration of other 

routes of exposure that have been identified as potentially important and the degree of certainty 

associated with screening-level risk assessment conclusions. Detailed information about STIR 

v.1.0, as well as the tool, can be found on EPA’s website 

(http://www.epa.gov/pesticides/science/models_pg.htm#terrestrial). 

The Screening Imbibition Program (SIP v.1.0, released June 15, 2010) was used to calculate an 

upper-bound estimate of wildlife drinking water exposure using tebuthiuron’s solubility (i.e., 2500 

mg/L at 20o

C), and compared to the most sensitive acute and chronic avian and mammalian 

toxicity endpoints. Upon results of the screening model SIP, drinking water exposure alone is a 

concern for birds and mammals (Appendix F). Tebuthiuron has a very high solubility, which is 

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

sensitive to tebuthiuron on either an acute and/or chronic basis. This pathway will be explored 

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

For a sample of the output generated by SIP v.1.0, please see Appendix F. Detailed information 

500 0.1 0.1 0.005 

997 0.07 0.07 0.004 

4.0 Ground 

0 4.0 0.25 0.013 

10 1.0 0.15 0.008 

50 0.2 0.08 0.004 

100 0.1 0.05 0.003 

500 0.02 0.01 0.0007 

997 0.006 0.005 0.0003 

4.0 Aerial 

0 2 0.5 0.028 

10 1.3 0.4 0.024 

50 0.7 0.3 0.02 

100 0.4 0.2 0.01 

500 0.07 0.06 0.004 

997 0.05 0.04 0.003 
Page 30 of 121 

about the SIP v.1.0, as well as the tool, can be found on the EPA’s website at 

http://www.epa.gov/pesticides/science/models_pg.htm#terrestrial. 

3.3.2. Ecological Effects Characterization 

In screening-level ecological risk assessments, effects characterization describes the types of 

effects a pesticide can have on aquatic and terrestrial organisms. This characterization is based on 

the registrant-submitted toxicity data for birds, mammals, fish, invertebrates, and plants for the 

active ingredient selected for this ecological risk assessment phase of Registration Review. 

This assessment evaluates the potential for tebuthiuron to directly or indirectly affect Federallylisted species or modify designated critical habitat. As discussed, assessment endpoints include 

direct toxic effects on the survival, reproduction, and growth of listed entities, as well as indirect 

effects, such as reduction of the prey base or modification of habitat. In addition, potential 

modification of critical habitat is assessed by evaluating effects to the principle constituent 

elements (PCEs), which are components of the critical habitat areas that provide essential life cycle 

needs of listed entities. 

As described in the Agency’s Overview Document (USEPA 2004), the most sensitive endpoint 

for each taxon is used for risk estimation. For this assessment, evaluated taxa include freshwater 

and estuarine/marine fish (freshwater fish also a surrogate for aquatic phase amphibians), 

freshwater and estuarine/marine invertebrates, aquatic plants, birds (also surrogate for terrestrialphase amphibians and reptiles), mammals, honeybees, and terrestrial plants. 

Toxicity endpoints are established based on data generated from guideline studies submitted by 

the registrant, and from open literature studies that meet the criteria for inclusion into the ECOTOX 

database maintained by EPA/Office of Research and Development (ORD) (U.S. EPA, 2004). In 

general, effects data in the open literature that indicate effects at lower concentrations than the 

registrant-submitted data are considered. The degree to which open literature data are 

quantitatively or qualitatively characterized is dependent on whether the information is relevant to 

the assessment endpoints (survival, reproduction, and growth). For example, endpoints such as 

behavior modifications are likely to be qualitatively evaluated, because quantitative relationships 

between modifications and reduction in species survival, reproduction, and/or growth are not 

available. On December 24, 2013, none of the open literature that passed the ECOTOX screen 

obtained effects at lower concentrations than the submitted studies and thus are not used in this 

risk assessment. 

In addition to registrant-submitted and open literature toxicity information, other sources of 

information, including use of the acute probit dose-response relationship to establish the 

probability of an individual effect and reviews of the Ecological Incident Information System 

(EIIS), are conducted to further refine the characterization of potential ecological effects associated 

with exposure to tebuthiuron. A summary of the available aquatic and terrestrial ecotoxicity 

information and use of the probit dose-response relationship for tebuthiuron are provided in 

Section 4, respectively. 
Page 31 of 121 

A review of the Ecological Incident Information System (EIIS) maintained by the Agency’s Office 

of Pesticide Programs (OPP) indicates a total of 4 reported ecological incidents associated with 

the use of tebuthiuron. The reported incidents involved damage to terrestrial plants. Two incidents 

were associated with misuse use of tebuthiuron, one to a registered use, and the legality of the used 

in another was not determined. In addition, there is one aggregate minor fish and wildlife incident 

reported for tebuthiuron in 2003 that was reported by the pesticide registrant. 

Summary of Ecological Incidents Associated with Tebuthiuron Uses, By Certainty. 

Incident 

type 

Incident 

ID Use Type Legality 

Certainty 

All 

(excluding 

unlikely) 

Unlikely Possible Probable Highly 

Probable 

Damage 

to 

terrestrial 

plants 

1003503-

001 

Power line Misuse 

(accidental) 

 X 

1015921-

002 

Agricultural 

area 

Undetermined X 

1-13603-

002 

Rangeland Registered 

use 

 X 

100512-

001 

Right-ofway, road Misuse 

(accidental) 

 X 

 

Similarly, a search of the Avian Incident Monitoring System (AIMS; 

http://www.abcbirds.org/abcprograms/policy/toxins/aims/aims/index.cfm) was conducted on 

December 24, 2013. AIMS is a database administered by the American Bird Conservancy that 

contains publicly available data on reported avian incidents involving pesticides. No incidents 

involving tebuthiuron were found. 

A major degradate of tebuthiuron, identified as compound 104, was measured in a terrestrial field 

dissipation study accounting for up to 23% of the mass applied and was concluded to be the only 

degradate of toxicological concern due to the structural similarity to tebuthiuron. While similar to 

the parent chemical, it lacks an N-methyl group; however, it may still have the same herbicidal 

toxic mode of action. As of an uncertainty since the toxicity of compound 104 to plants is 

unknown, EFED presumes that the degradate has the equivalent toxicity of its parent compound 

to terrestrial plants. Previous risk assessments indicate the parent compound resulted in LOC 

exceedances for terrestrial plants; thus, under this presumption, LOC exceedances would be 

expected with degradate compound 104. Additional data testing with the degradate compound 104 

(e.g., OCSPP 850.4100 and/or 850.4150 Tier II terrestrial plant toxicity test) would be needed to 

reduce the uncertainty associated with this presumption. 

3.3.2.1. Aquatic Effects Characterization 

The most sensitive acute and chronic effects measurement endpoints associated with tebuthiuron 

exposure to freshwater and estuarine/marine species are summarized in Table 3.9. Selected effects 

endpoints for the risk assessment are from acceptable and supplemental studies for tebuthiuron 

and are briefly described below. 

Based on the available toxicity data, tebuthiuron is classified as practically non-toxic to aquatic 

organisms with the exception of the slightly toxic toxicity classification to estuarine/marine 
Page 32 of 121 

invertebrates. In addition, freshwater fish are surrogates for aquatic-phase amphibians in the 

absence of amphibian-specific data; therefore, using the acute toxicity data with freshwater fish, 

tebuthiuron would be classified as practically non-toxic to aquatic-phase amphibians. Chronic 

toxicity data were only available for freshwater fish and invertebrates, for which effects on 

reproduction and growth were observed in the invertebrate study and growth in the fish study. 

Acute-to-Chronic Ratio calculations were used to estimate the NOAEC for chronic effects to 

estuarine/marine fish using rainbow trout data and to invertebrates using data from the daphnid 

studies. 

Summaries of the ecological toxicity studies are provided in Appendix G. 
Page 33 of 121 

Table 3.9. Most Sensitive Acute and Chronic Toxicity Data for Aquatic Organisms 

Exposed to Tebuthiuron. 

Exposure 

Scenario

Species Affected 

Endpoint

Toxicity Measurement MRID

(Classification)

Freshwater Fish

Acute Bluegill sunfish

(Lepomis 

macrochirus) 

Mortality 96-hr LC50 = 106 mg a.i./L 40098001

(acceptable)

Chronic Fathead minnow

(Pimephales 

promelas)

Sub-lethal 33-day NOAEC = 9.3 mg a.i./L

(length)

00090084

(acceptable)

Freshwater Invertebrates

Acute Water flea

(Daphnia spp.)

Immobility 48-hr EC50 = 297 mg a.i./L 00041694

(acceptable)

Chronic Sub-lethal 21-day NOAEC = 21.8 mg a.i./L 

(growth, fecundity, time of first 

brood release, young per adult)

00138700

(acceptable)

Estuarine/Marine Fish

Acute Sheepshead 

minnow

(Cyprinodon 

variegates)

Mortality 96-hr LC50>98 mg a.i./L 48722702

(acceptable)

Chronic No data 

available

Estimated NOAEC >18 mg 

a.i./L

ACR1

Estuarine/Marine Invertebrates

Acute Pink shrimp

(Penaeus 

duorarum)

Mortality 96-hr LC50 = 62 mg a.i./L 00041684

(acceptable)

Chronic No data 

available

Estimated NOAEC = 4.56 mg 

a.i./L

ACR1

Aquatic Plants

7-day 

exposure

Duckweed

(Lemna gibba)

Frond count 7-day EC50 = 0.13 mg a.i./L

NOAEC = 0.05 mg a.i./L

41080404

(supplemental)

96-hour 

exposure

Marine diatom

(Skeletonema 

costatum)

Cell density 96-hr EC50 = 0.05 mg a.i./L

NOAEC = 0.038 mg a.i./L

41080402

(supplemental)

1 Estimated value based on Acute to Chronic Ratio (ACR) method using rainbow trout data; ACR = 5.5.

Sheepshead minnow NOAEC = (LC50 of >98 mg/L ÷ 5.5) = >18 mg/L

2 Estimated value based on Acute to Chronic Ratio (ACR) method using daphnid data; ACR = 13.6. Shrimp 

NOAEC = (LC50 of 62 mg/L ÷ 13.6) = 4.56 mg/L 

 

Freshwater Fish, Acute Effects 

Fish toxicity studies using the technical grade active ingredient (TGAI) were submitted to establish 

the acute toxicity of tebuthiuron to fish. The preferred test species are the rainbow trout and bluegill 

sunfish. Results of the acute studies with freshwater fish indicate the LC50 value of 106 mg a.i./L 

for the sunfish is the most sensitive endpoint for fish (Table 3.10). As a result, tebuthiuron is 
Page 34 of 121 

categorized as practically nontoxic to fish. The sunfish LC50 will be used to evaluate potential 

acute effects of tebuthiuron to freshwater fish. 

Freshwater Fish, Chronic Effects 

Laboratory toxicity studies using the TGAI were submitted to establish the chronic toxicity of 

tebuthiuron to freshwater fish. The preferred test species is the most sensitive fish of the freshwater 

fish acute toxicity tests; when a 96-hour LC50 is not available, rainbow trout or fathead minnow is 

preferred. Results of the chronic studies with the TGAI to freshwater fish indicate the lowest 

NOAEC value to be 9.3 mg a.i./L (Table 3.11). Thus, the NOAEC of 9.3 mg a.i./L will be used to 

evaluate potential chronic effects of tebuthiuron to freshwater fish. 

Table 3.11. Freshwater Fish Chronic Toxicity Profile for Tebuthiuron. 

Species % 

a.i. 

LOAEC / 

NOAEC 

(mg a.i./L)

Endpoint 

Affected 

MRID No. 

Author/Year 

Study 

Classification

Rainbow trout

(Oncorhynchus 

mykiss)

98 52/26 Adult survival 

and length

00090083

Sauter, Meyerhoff, 

Todd et al. (1981)

Acceptable

Fathead minnow

(Pimephales 

promelas)

98 18/9.3 Length

00090084

Sauter, Meyerhoff, 

Todd et al. (1981)

Acceptable

Table 3.10. Freshwater Fish Acute Toxicity Profile for Tebuthiuron. 

Species % a.i. LC50

(mg a.i./L) 

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification

Rainbow trout

(Oncorhynchus mykiss) 98 143A Practically 

nontoxic

00020661

Anon. (1972) Acceptable

Bluegill sunfish 

(Lepomis macrochirus) 98 106B Practically nontoxic

00020661

Anon. (1972) Acceptable

Goldfish

(Carassius auratus) 98 >160 Practically 

nontoxic

00020661

Anon. (1972) Supplemental1

Fathead minnow

(Pimephales promelas)

98

>180 Practically 

nontoxic

00041685

Hamelink and 

Kehr (1976)

Supplemental 80 2

20 

(pellet) A Trout 95% Confidence Interval (C.I.) = 118-224 mg a.i./L; slope = N/A (binominal) B Sunfish 95% C.I. = 87-120 mg a.i./L; slope = N/A (binomial) 

1

 Dilution water, pH and dissolved oxygen were not measured; goldfish is not a recommended species. 

2

 Inadequate reporting of test conditions and aeration during the test. 
Page 35 of 121 

Freshwater Invertebrates, Acute Effects 

A toxicity study using the TGAI was submitted to establish the acute toxicity of tebuthiuron to 

freshwater invertebrates. Daphnids are the preferred test species. Results of the acute study with 

freshwater invertebrates indicate an EC50 value of 297 mg a.i./L (Table 3.12), which categorizes 

tebuthiuron as practically nontoxic to freshwater invertebrates. Thus, the EC50 will be used to 

evaluate potential acute effects of tebuthiuron to freshwater invertebrates. 

Freshwater Invertebrates, Chronic Effects 

A laboratory toxicity study using the TGAI was submitted to establish the chronic toxicity of 

tebuthiuron to freshwater invertebrates. The preferred test species is the water flea. Results of the 

reproduction study with freshwater invertebrates indicate a NOAEC value of 21.8 mg a.i./L (Table 

3.13). The NOAEC will be used to evaluate potential chronic effects of tebuthiuron to freshwater 

invertebrates. 

Table 3.13. Freshwater Invertebrate Chronic Toxicity Profile for Tebuthiuron. 

Species % 

a.i. 

LOEAC / 

NOAEC 

(mg a.i./L)

Endpoint 

Affected 

MRID No. 

Author/Year 

Study 

Classification

Water flea

(Daphnia 

magna)

97.4 44.2/21.8

Time to first 

brood release, 

offspring per 

adult and length

000138700

Grothe and 

Meyerhoff. (1983)

Acceptable

Estuarine/Marine Fish, Acute Effects 

An estuarine/marine fish toxicity study using the TGAI was submitted to establish the acute 

toxicity of tebuthiuron to estuarine/marine fish. The preferred test species is the Atlantic 

silversides; however, the registrant submitted an acceptable species, the Sheepshead minnow. 

Results of the acute study indicate an LC50 value of >98 mg a.i./L (Table 3.14). With no mortality 

at and up to 98 mg a.i./L, tebuthiuron is categorized as practically nontoxic to estuarine/marine 

fish. The LC50 value of >98 mg a.i./L will be used to evaluate potential acute effects of tebuthiuron 

to estuarine/marine fish. 

Table 3.12. Freshwater Invertebrate Acute Toxicity Profile for Tebuthiuron. 

Species % a.i. EC50 

(mg a.i./L)

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification

Water flea

(Daphnid magna) 99.2 297A Practically 

nontoxic

00041694

Hamelink, Todd, 

Brannon et al. 

(1978)

Acceptable

A D. magna 95% Confidence Interval = 279-316 mg a.i./L; Probit slope = 13.3 (9.06-17.6) 
Page 36 of 121 

Estuarine/Marine Fish, Chronic Effects 

No chronic ecological effects studies of tebuthiuron using estuarine/marine fish were submitted. 

The Acute-to-Chronic Ratio (ACR) approach was used to estimate the chronic toxicity to 

estuarine/marine fish in absence of data. The ACR using rainbow trout acute and chronic data is 

5.5. Employing the ACR of 5.5 to the sheepshead LC50 of >98 mg a.i./L calculates a chronic 

toxicity value of >18 mg a.i./L for estuarine/marine fish. The NOAEC of >18 mg a.i./L will be 

used to evaluate potential chronic effects of tebuthiuron to estuarine/marine fish. 

Estuarine/Marine Invertebrates, Acute Effects 

Toxicity studies using the TGAI were submitted to establish the acute toxicity of tebuthiuron to 

estuarine/marine invertebrates. The preferred test species are the mysid shrimp and Eastern oyster 

(shell deposition). Results of the acute studies indicate the lowest LC50 value for estuarine/marine 

invertebrates to be 62.3 mg a.i./L (Table 3.15). As of a result, tebuthiuron is categorized as slightly 

toxic to estuarine/marine invertebrates. The LC50 of 62.3 mg a.i./L will be used to evaluate 

potential acute effects of tebuthiuron to estuarine/marine invertebrates. 

Table 3.15. Estuarine/Marine Invertebrate Acute Toxicity Profile for Tebuthiuron. 

Species % a.i. LC50/EC50 

 (mg a.i./L)

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification

Pink shrimp

(Penaeus duorarum)

98

62.3A Slightly 

toxic

00041684

Heitmuller and 

Parrish (1976)

Acceptable

Fiddler crab

(Uca pugilator) >100 Practically 

nontoxic Supplemental1

Eastern oyster 

[embryos]

(Crassostrea 

virginica)

>100 B Practically 

nontoxic Acceptable2

Eastern oyster

[shell deposition]

(Crassostrea 

virginica)

99.8 >95 Practically 

nontoxic

48722701

York (2011) Acceptable

A Shrimp 95% Confidence Interval = 38.7-90.1 mg a.i./L; slope = 4.5 (2.1-6.9) 

Table 3.14. Estuarine/Marine Fish Acute Toxicity Profile for Tebuthiuron. 

Species % a.i. LC50 

(mg a.i./L)

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification 

Sheepshead minnow

(Cyprinodon 

variegates)

99.8 >98 Practically 

nontoxic

48722702

Fournier (2011) Acceptable
Page 37 of 121 

Table 3.15. Estuarine/Marine Invertebrate Acute Toxicity Profile for Tebuthiuron. 

Species % a.i. LC50/EC50 

 (mg a.i./L)

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification B The 48-hour LC50 is actually between 180 and 320 mg a.i./L due to 100% mortality at 320 mg a.i./L and 0% 

mortality at 180 mg a.i./L. Because a definite LC50 could not be calculated, the EPA reviewer determined a LC50 of 

>100 mg a.i./L which still classifies tebuthiuron as practically nontoxic to oyster embryos. 

1

 Not a recommended species 

2 Bivalve acute toxicity study (OCSPP 850.1055; embryo-larval)

Estuarine/Marine Invertebrates, Chronic Effects 

No chronic ecological effects studies of tebuthiuron using estuarine/marine invertebrates were 

submitted. The Acute-to-Chronic Ratio (ACR) approach was used to estimate the chronic 

toxicity to estuarine/marine invertebrates in absence of data. The ACR using Daphnia acute and 

chronic data is 13.6. Employing the ACR of 13.6 to the shrimp LC50 of 62.3 mg a.i./L calculates 

a chronic toxicity value of 4.56 mg a.i./L for estuarine/marine invertebrates. The NOAEC of 4.56 

mg a.i./L will be used to evaluate potential chronic effects of tebuthiuron to estuarine/marine 

invertebrates. 

Effects to Aquatic Vascular and Non-vascular Plants 

Tebuthiuron studies were submitted for a freshwater vascular plant (duckweed, Lemna gibba) and 

four phytoplanktonic nonvascular plants: green algae (Selenastrum capricornutum), cyanobacteria 

(Anabaena flos-aquae), freshwater diatom (Navicula pelliculosa) and marine alga (Skeletonema 

costatum) (Table 3.16). 

Aquatic plant toxicity studies using the TGAI were submitted to establish the toxicity of 

tebuthiuron to vascular and non-vascular species. The aquatic vascular plant duckweed EC50 and 

NOAEC of 0.13 and 0.05 mg a.i./L, respectively, and the most sensitive EC50 of aquatic 

nonvascular plants studies using the marine diatom EC50 and NOAEC of 0.05 and 0.038 mg a.i./L, 

respectively, will be used to evaluate potential effect of tebuthiuron to aquatic plants. While the 

green algae and marine diatom, when based on the EC50, were the most sensitive nonvascular 

plants; the No Effect level for the freshwater green algae may be more sensitive than the marine 

diatom; however, the results of the algae study could not be verified, the NOAEC for marine 

diatom was determined to be more reliable than the algae’s NOAEC. 

EC50 values of 0.13 and 0.05 mg a.i./L (Table 3.16) will be used to evaluate potential effects of 

tebuthiuron to non-listed aquatic vascular and non-vascular plants, respectively. Corresponding 

NOAEC values of 0.05 and 0.038 mg a.i./L will be used to evaluate potential effects of tebuthiuron 

to listed aquatic vascular and nonvascular plants, respectively. 
Page 38 of 121 

Table 3.16. Aquatic Plant Toxicity Profile for Tebuthiuron. 

Species % a.i. EC50 

(mg a.i./L)

NOAEC 

(mg a.i./L) 

MRID No. 

Author/Year 

Study 

Classification

Vascular Plants

Duckweed

(Lemna gibba) 99.08 0.13A 0.05

41080404

Negilski and 

Cocke (1989)

Supplemental1

Non-Vascular Plants

Marine diatom

(Skeletonema 

costatum)

99.08 0.05B 0.038

41080402

Negilski and 

Cocke (1989)

Supplemental1

Freshwater blue-green 

algae

(Anabaena flos-aquae)

99.08 0.81C

<0.31 

(EC05 = 

0.03)

41080401

Negilski, Grothe 

and Cocke (1989)

Supplemental1

Freshwater diatom

(Navicula pelliculosa) 99.08 0.09D 0.056

41080403

Negilski and 

Cocke (1989)

Supplemental1

Freshwater green 

algae

(Selenastrum 

capricornutum)

98 0.05 0.013

00138697

Todd, Meyerhoff, 

Grothe, et al.

(1983)

Supplemental1,2

A Duckweed 95% Confidence Interval (C.I.) = 0.06-0.26 mg a.i./L

B Marine diatom 95% C.I. = 0.04-0.06 mg a.i./L 

C Blue-green algae 95% C.I. = 0.59-1.12 mg a.i./L 

D Freshwater diatom 95% C.I. = 0.08-0.14 mg a.i./L 

1

 Three replicates per treatment level were used. EPA requires four replicates per level. 

2

 Results could not be verified since raw data were not provided. Results are based on a 5-day exposure. 

3.3.2.2. Terrestrial Effects Characterization 

The most sensitive acute and chronic effects measurement endpoints associated with tebuthiuron 

exposure to terrestrial species are summarized in Table 3.17. Selected effects endpoints for the 

risk assessment are from acceptable and supplemental studies for tebuthiuron and are described 

below. 

Overall, tebuthiuron is practically nontoxic to mallard ducks on an acute basis. Tebuthiuron caused 

no mortality or sub-lethal effects up to and including 2,000 mg a.i./kg bw and 5093 mg a.i./kg diet 

to waterfowl via oral and dietary routes, respectively; and up to 500 mg a.i./kg bw and 5113 mg 

a.i./kg diet for upland game birds. On the other hand, tebuthiuron is slightly toxic to small birds 

(i.e., passerines) on a dietary basis. In addition, birds also act as surrogates for reptiles and 

terrestrial-phase amphibians in the absence of taxa specific data; therefore, using the acute toxicity 
Page 39 of 121 

data with passerines, tebuthiuron would be classified as slightly toxic to reptiles and terrestrialphase amphibians of similar body weight. Observations of reproductive effects in birds were as 

low as 500 mg a.i./kg diet; the resulting NOAECs for two avian species were both 100 mg a.i./kg 

diet. 

Tebuthiuron is moderately toxic to mammals on an acute oral basis (female LD50 = 387.5 mg 

a.i/kg-bw; male LD50 = 477.5 mg a.i/kg-bw). In the chronic 2-generation studies, reproductive 

effects are as low as 30 mg a.i./kg/day (NOAEL = 14 mg a.i./kg/day), based on decreased body 

weight and decreased body weight gains for female rats. 

Results of the honeybee acute contact test with tebuthiuron indicate the TGAI is practically 

nontoxic to honeybees (LD50 = >100 μg a.i./bee). 

As may be anticipated with an herbicide, terrestrial plants were affected at the seedling and the 3-

leaf plant stages with the lowest EC25’s at 0.018 and 0.16 lb a.i./A, respectively. 

Table 3.17. Most Sensitive Acute and Chronic Toxicity Data for Terrestrial Animals 

and Plants Exposed to Tebuthiuron.

Exposure 

Scenario 

Species Exposure 

Duration 

Toxicity Reference 

Value 

MRID 

(Classification)

Mammals

Acute

(Dose-based) Laboratory rat

(Rattus 

norvegicus)

Single oral 

dose

Female LD50 = 387.5 mg 

a.i./kg-bw

40583901

(acceptable)

Chronic

(Dietarybased)

OnegenerationLOAEL = 400 ppm

NOAEL = 200 ppm (14 

mg/kg/day)

00090108

(acceptable)

Birds

Acute 

(Dose-based)

Mallard duck

(Anas 

platyrhynchus)

14-day 

single oral 

dose

LD50 >2000 mg a.i./kg bw 00041692

(acceptable)

Acute 

(Dietarybased)

Zebra finch

(Taeniopygia 

guttata)

8-day 

dietary

LC50 = 1465 mg a.i./kgdiet 48928201

(acceptable)

Chronic 

(Dietarybased)

Mallard duck

(Anas 

platyrhynchus)

OnegenerationNOAEC = 100 mg a.i./kgdiet000104243

(acceptable)

Terrestrial Plants

Seedling 

emergence

Ryegrass 

(monocot)

14 days

EC25 = 0.27 lbs a.i./A 

NOAEC = 0.25 lbs a.i./A

48722703

(supplemental)

Carrot

(dicot)

EC25 = 0.018 lbs a.i./A 

EC05 = 0.0032 lbs a.i./A*

Vegetative 

Vigor

Ryegrass

(monocot)

EC25 = 0.30 lbs a.i./A 

NOAEC = 0.12 lbs a.i./A

48722704 

(supplemental)

Sugarbeet

(dicot)

EC25 = 0.16 lbs a.i./A 

NOAEC = 0.062 lbs a.i./A
Page 40 of 121 

Table 3.17. Most Sensitive Acute and Chronic Toxicity Data for Terrestrial Animals 

and Plants Exposed to Tebuthiuron.

Exposure 

Scenario 

Species Exposure 

Duration 

Toxicity Reference 

Value 

MRID 

(Classification)

Beneficial Insects

Contact Honey bee 48 hours LD50 >100 μg a.i./bee 40840401 

(acceptable)

* The NOAEC was above the EC25; thus, the EC05 was used instead. 

Birds, Acute Oral 

An acute oral toxicity study using the TGAI is required to establish the acute toxicity of tebuthiuron 

to birds. The preferred guideline bird test species is either mallard duck (a waterfowl) or Northern 

bobwhite quail (an upland game bird), and a passerine bird. The submitted acute data indicate that 

the TGAI is categorized as practically nontoxic to waterfowls (LD50 >2000 mg a.i./kg bw, the 

maximum dose tested), and no effects to domestic chickens and upland game birds were observed 

at and up to 500 mg a.i./kg bw (Table 3.18). 

However, for passerines, regurgitation occurred at all test doses of an acute oral range-finding 

study, preventing the estimation of an oral LD50;therefore the registrant opted to conduct a dietarybased study using OCSPP 850.2200 with food consumption closely monitored. The study design 

switch was intended to estimate an oral LD50 value for passerines using the LC50 data from the 

passerine dietary study (MRID 48928201; Table 3.19). An oral LD50 could not be estimated due 

to variability in the study results; thus, the dose-based oral toxicity of tebuthiuron to passerines is 

unknown. The toxicity of the active ingredient to passerines is discussed in more detail in the avian 

acute dietary toxicity profile following this section. 

Thus, with no mortality at doses as high as 2,000 mg a.i./kg-bw, the LD50 of >2000 mg a.i./kg-bw 

will be used to evaluate potential acute effects of the TGAI to birds via the oral route. The available 

avian acute oral toxicity data are summarized in Table 3.18.

Table 3.18. Avian Acute Oral Toxicity Profile for Tebuthiuron. 

Species % a.i. LD50 

(mg a.i./kg 

bw) 

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification 

Domestic 

chicken 

(Gallus sp.)

98 >500

No effects 

up to 500 

ppm

00020661

Todd, Kehr, West and et 

al. (1972)

Supplemental1

Mallard duck

(Anas 

platyrhynchos)

98 >2000 Practically 

nontoxic

00041692

Kehr, Hamelink, Todd 

and et al. (1978)

Acceptable

98 >500

No effects 

up to 500 

ppm

00020661

Todd, Kehr, West and et 

al. (1972)

Supplemental1
Page 41 of 121 

Table 3.18. Avian Acute Oral Toxicity Profile for Tebuthiuron. 

Species % a.i. LD50 

(mg a.i./kg 

bw) 

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification 

Northern 

bobwhite quail

(Colinus 

virginianus)

98 >500

No effects 

up to 500 

ppm

00020661

Todd, Kehr, West and et 

al. (1972)

Supplemental1

1 The highest concentration was below the recommended limit dose of 2000 mg a.i./kg bw. 

Birds, Subacute Dietary 

Dietary studies using the TGAI are required to establish the subacute toxicity of tebuthiuron to 

birds. The preferred test species are mallard duck (waterfowl) and bobwhite quail (upland game 

bird). The submitted data indicate no mortality occurred and that the 8-day acute dietary LC50 for 

duck and quail were >5093 and >5113 mg a.i./kg diet, respectively, the maximum concentrations 

tested. The TGAI is categorized as practically non-toxic to upland game birds and waterfowl on a 

subacute dietary basis (Table 3.19). 

While results indicate the duck and quail were equally insensitive to the TGAI; passerines were 

slightly sensitive to tebuthiuron as mortality and extremely low food consumption were observed 

in the three highest concentrations (1590, 2870, and 5380 mg a.i./kg-diet). At the 1590 mg a.i./kgdiet treatment level, 8 birds died (80%). At the 2870 mg a.i./kg-diet treatment level, two birds were 

taken off of treated feed before the full five day exposure was complete due to extremely reduced 

feed consumption and were counted as mortalities. Of the 8 birds that stayed on the treated diet, 7 

birds died (87.5%). At the highest treatment level, two birds died on Day 2 of the test and the other 

eight birds were switched to untreated feed due to overall extreme reduction in feed consumption. 

One bird died on Day 4. All other surviving birds on untreated food were normal in appearance 

and behavior from Day 4 to test termination but were considered to have died (100%) due to 

insufficient caloric intake to sustain life had the birds remained on treated food. Clinical signs of 

toxicity were observed as follows: at 497 mg a.i./kg-diet, up to 50% of birds displayed slightly 

ruffled appearance and wing droop between Days 2 and 6; at 895 mg a.i./kg-diet, up to 60% of 

birds displayed ruffled appearance and wing droop between Days 2 and 7; at 1590 mg a.i./kg-diet, 

all birds displayed at least one clinical sign of toxicity (ruffled appearance, lethargy, and wing 

droop) by Day 4; at 2870 mg a.i./kg-diet, all birds were noted with at least one clinical sign of 

toxicity (ruffled appearance, lethargy, wing droop, loss of coordination, prostrate posture, and loss 

of righting reflex) or died by Day 3; and at 5380 mg a.i./kg-diet, all birds were noted with ruffled 

appearance by the afternoon of Day 1. Due to the mortality pattern that occurred, there was not a 

dose-response effect. Not all mortalities were related to the toxicity of tebuthiuron but were a result 

of avoidance of the treated feed. Assuming all birds removed from treated diets because of reduced 

feed consumption would have died, the resulting LC50 is 1465 mg a.i./kg diet for passerines. 

Therefore, the LC50 value of 1465 mg a.i./kg-diet will be used to evaluate potential acute effects 

of the TGAI to birds via the dietary route. The available avian subacute dietary studies are 

summarized in Table 3.19. 
Page 42 of 121 

Table 3.19. Avian Acute Dietary Toxicity Profile for Tebuthiuron 

Species % a.i. Nominal 

LC50 

(mg a.i./kg 

diet) 

Toxicity 

Category 

MRID No. 

Author/Year 

Study 

Classification 

Zebra finch

(Taeniopygia 

guttata)

99.8 1465A Slightly toxic

48928201

Hubbard, Martin 

and Beavers (2012)

Acceptable

Mallard duck

(Anas 

platyrhynchos)

98 >2500

No effects up 

to 2500 mg 

a.i./kg diet

00041680/ 

(00041693; 

addendum)

West and 

Hamelink (1976) / 

Kehr, Brannon, 

Todd and et al. 

(1978)

Supplemental1

99.1 >5093 Practically 

nontoxic

40601001

Negilski, Grothe & 

Meyeroff (1988)

Acceptable

Northern bobwhite 

quail (Colinus 

virginianus)

98 >2500

No effects up 

to 2500 mg 

a.i./kg diet

00041681/ 

(00041693; 

addendum)

West and 

Hamelink. (1976) / 

Kehr, Brannon, 

Todd and et al. 

(1978)

Supplemental1

99.1 >5113 Practically 

nontoxic

40601002

Negilski, Grothe & 

Meyeroff (1988)

Acceptable

A Finch 95% Confidence Interval = 1145 to 1883 mg a.i./L; slope = 6.5 (2.997-10.03)

1 The concentrations used (when adjusted for active ingredient) did not produce a precise LD50 and the highest 

concentration was below 5000 mg a.i./kg bw.

Birds, Reproduction 

Avian reproduction studies using the TGAI are required to establish the chronic toxicity of 

tebuthiuron to birds (Table 3.20). The preferred test species are mallard duck and bobwhite quail. 

The lowest reproduction NOAEC of 100 mg a.i./kg diet will be used to evaluate potential chronic 

effects of the TGAI to birds. The available avian reproduction studies are summarized in Table 

3.20. 

Bobwhite quail showed dose-dependent effects for offspring survival. At 903 and 1550 mg a.i./kg 

diet, the offspring survival (14-day hatchlings per number hatched) was slightly (4-7%), but 

significantly reduced, compared to the control (p<0.05; Jonckheere-Terpstra test). Also, at the 

1550 mg a.i./kg diet level, egg production was reduced 23%, from an average of 44.8 eggs laid per 
Page 43 of 121 

pen in the control group to 34.7 eggs laid per pen in the treated group (p<0.05; Jonckheere-Terpstra 

test); both the study author and reviewer concluded that this effect was treatment-related, despite 

the non-monotonicity of the data. No statistically significant differences in eggshell thickness 

were observed between the control group and any of the treatment groups. Adult male body weight 

gain was significantly reduced in a dose-dependent manner 53-121% at all treated levels, relative 

to the negative control (p<0.05, William’s test). Adult female body weight was significantly 

reduced at the 1550 mg a.i./kg diet level, relative to the control (p<0.01; Jonckheere-Terpstra test). 

No statistically significant differences were observed between the control group and any of the 

treated groups for food consumption. Offspring body weights were significantly reduced in a 

dose-dependent manner, with reductions from control detected at the 903 and 1550 mg a.i./kg diet 

levels for hatchling body weights (p<0.01; Jonckheere-Terpstra test) and at the 1550 mg a.i./kg 

diet level for 14-day old survivor weight (p<0.05; William’s test). 

In the mallard study, a statistically significant (p<0.05) dose-dependent reduction in hatchling 

body weight and eggshell thickness was observed at 1550 mg a.i./kg diet compared to the 

controls. There were statistically significant (p<0.05; Williams test, 26-41%) reductions from 

control in egg production (eggs laid per pen) in the 500, 903 and 1550 mg a.i./kg diet groups. A 

slight (<1%) reduction from control in eggs not cracked per eggs laid was detected at the lowest 

treatment level (p = 0.04), but was not considered to be treatment-related. 

Table 3.20. Avian Reproduction Toxicity Profile for Tebuthiuron. 

Species % 

a.i. 

NOAEC / LOAEC 

(mg a.i./kg-diet) 

Affected endpoints MRID no. 

Author/Year 

Study 

classification 

Northern 

bobwhite quail

(Colinus 

virginianus)

96.4 100/>100 None

00104243

Fink, Beavers 

& Brown 

(1978)

Acceptable

99.8 <500/500

Eggs laid per pen, 14-

day hatchlings per 

number hatched, 

hatchling body 

weight, 14 day 

survivor weight, 

female weight gain 

and male weight gain.

48928203

Frey et al. 

(2012)

Supplemental1

Mallard duck

(Anas 

platyrhynchos)

96.4 100/>100 None

00093690

Fink, Beavers 

& Brown 

(1978)

Acceptable

99.8 <500/500

Eggs laid per pen, 

hatchling weight, and 

eggshell thickness

48928202

Frey et al. 

(2012)

Supplemental1

1 An NOAEC was not available.
Page 44 of 121 

Mammals, Acute Oral 

Wild mammal testing is required on a case-by-case basis, depending on the results of lower Tier 

laboratory mammalian studies, intended use pattern and pertinent environmental fate 

characteristics. In most cases, rat or mouse toxicity values obtained from the Agency's Health 

Effects Division (HED) substitute for wild mammal testing. These toxicity values are reported 

below in Table 3.21. 

The results indicate tebuthiuron is categorized as moderately toxic (female LD50 = 387.5 mg a.i/kgbw; male LD50 = 477.5 mg a.i/kg-bw) to small mammals on an acute oral basis. The LD50 value 

of 387.5 mg a.i./kg-bw will be used to evaluate potential acute effects to mammals. 

Table 3.21. Mammalian Acute Oral Toxicity Profile for Tebuthiuron. 

Species % 

a.i.

Test 

Type 

LD50 

(mg a.i./kg 

bw)

Affected 

Endpoints

MRID No. 

Author, Year

Toxicity 

Category

Rat

(Fischer 

344)

99.1 acute oral Females = 387.5

Males = 477.5 Mortality

40583901

Negilski and 

Hawkins (1988)

Moderately toxic

Mammals, Reproduction 

In the 2-generation reproduction study (Table 3.22), rats exposed to the TGAI showed no 

reproductive or offspring effects; however, reduced body weight and decreased body weight gains 

were observed. Parental female rats in the 200 and 400 ppm groups had mean weekly body weights 

7-9% (not biologically significant) and 8-13% (p<0.01 or <0.05), respectively, less than the control 

group throughout the premating period starting with day 21 for the 200 ppm group and day 7 for 

the 400 ppm group. Weight gain over the entire premating period was 7% (not significant) less 

than control for 200 ppm group and 14% (p<0.05) less for the 400 ppm group. The lowest NOAEC 

value of 200 ppm (14 mg a.i./kg-bw/day) will be used to evaluate potential chronic effects of the 

TGAI to mammals. The available mammalian reproduction studies are summarized in Table 3.22. 

Table 3.22. Mammalian Reproductive Toxicity Profile for Tebuthiuron.

Species % 

a.i.

Test 

Type

Toxicity 

ppm (mg a.i./kg bw/day)

Affected 

Endpoints MRID No. 

Rat 

(Wistar) 98 2-generation 

Parental 

Female NOAEL = 200 (14) 

Male NOAEL = 400 (30) 

Female LOAEL = 400 (30) 

Male LOAEL = >400 (>30) 

Decreased body 

weight and body 

weight gains for 

female rats 

00090108 

Reproductive NOAEL = 400 (30) 

LOAEL = >400 (>30) None 

Offspring NOAEL = 400 (30) 

LOAEL = >400 (>30) None 
Page 45 of 121 

Beneficial Insects, Acute 

A honeybee acute contact study using tebuthiuron is required to establish the toxicity of the TGAI 

to beneficial insects because of outdoor uses that will result in honeybee exposure (Table 3.23). 

The acute contact LD50, using the honey bee, Apis mellifera, is derived from a laboratory study 

designed to estimate the quantity of toxicant required to cause 50% mortality in a test population 

of bees. Results of the acute contact study with honeybees indicate tebuthiuron is practically nontoxic to bees on a contact exposure basis. The available honeybee acute contact study is 

summarized in Table 3.23. 

Table 3.23. Honeybee Acute Contact Toxicity Profile for Tebuthiuron.

Species % 

a.i. 

LD50

(μg a.i./bee) 

Toxicity 

Category 

MRID No. Study 

Classification 

Honey bee 

(Apis 

mellifera) 

99.1 >100 Practically 

nontoxic 

40840401 

Hoxter and 

Jaber (1988) 

Acceptable 

Terrestrial Plants, Seedling Emergence and Vegetative Vigor 

Terrestrial plant studies are required for all terrestrial outdoor pesticides. Tier II terrestrial plant 

toxicity studies were conducted to establish the toxicity of tebuthiuron (Table 3.24) to non-target 

terrestrial plants tested at 4 lb a.i./A. The recommendations for seedling emergence and vegetative 

vigor studies are for testing of (1) six species of at least four dicotyledonous families, one species 

of which is soybean (Glycine max), and the second of which is a root crop, and (2) four species of 

at least two monocotyledonous families, one of which is corn (Zea mays). The tests indicate 

seedlings and young plants for each species were generally equally sensitive when exposed to 

tebuthiuron, except for dicots in which seedlings were more sensitive. EC50 values were used to 

evaluate potential effects of tebuthiuron to non-listed terrestrial plant species. Corresponding 

NOAEC values were used to evaluate potential effects of tebuthiuron to listed terrestrial plant 

species. 

Results of the seedling emergence test indicate the most sensitive monocot species, based on fresh 

weight, was ryegrass with an EC25 of 0.27 lb a.i./A and a NOAEC of 0.25 lb a.i./A. The most 

sensitive dicot species, based on fresh weight, was carrot with an EC25 of 0.018 lb a.i./A and a 

NOAEC of 0.031 lb a.i./A. 

In the vegetative vigor test, the most sensitive monocot species, based on fresh weight, was 

ryegrass with an EC25 of 0.30 lb a.i./A and a NOAEC of 0.12 lb a.i./A. The most sensitive dicot 

species, based on fresh weight, was sugarbeet with an EC25 of 0.16 lb a.i./A and a NOAEC of 

0.062 lb a.i./A 
Page 46 of 121 

Both studies are scientifically sound but do not fulfill the guidelines (OCSPP 850.4100 and 

850.4150) and are classified as supplemental because fresh weight instead of dry weight was 

measured, and only the three highest test levels instead of all test levels were verified. 

EC25 values will be used to evaluate potential effects of tebuthiuron to non-listed plants, 

respectively. Corresponding NOAEC values will be used to evaluate potential effects of 

tebuthiuron to listed plants, respectively. 

The description and classification of the available terrestrial plants studies are summarized in 

Table 3.24. 

Table 3.24. Summary of Tier II Seedling Emergence and Vegetative Vigor Toxicity 

Profile for Tebuthiuron. A 

Seedling Emergence * Vegetative Vigor ** 

Species Endpoint NOAE

C

EC05 EC25 Endpoint NOAEC EC05 EC25

Onion Fresh weight 0.25 0.23 0.46 Fresh weight 0.25 0.25 0.57 

Corn Shoot length 2.0 1.8 3.1 Fresh weight 2.0 1.8 2.6 

Oat Fresh weight 0.50 0.27 0.52 Shoot length 0.5 0.16 0.93 

Ryegrass Fresh weight 0.25 0.12 0.27 Fresh weight 0.12 0.065 0.30 

Carrot Fresh weight 0.031 0.0032 0.018 Fresh weight 0.50 0.30 0.52 

Cucumber Fresh weight 0.12 0.13 0.33 Fresh weight 0.12 0.097 0.28 

Cabbage Fresh weight 0.12 0.14 0.23 Fresh weight 0.12 0.18 0.32 

Soybean Fresh weight 1.0 0.79 1.2 None 0.12/0.50

/2.0B

NC NC 

Sugarbeet Fresh weight 0.12 0.11 0.20 Fresh weight 0.062 0.053 0.16 

Tomato Fresh weight 0.062 0.13 0.26 Fresh weight 0.25 0.25 0.43 

* 22.1% a.i; MRID 48722703, Bergfield 2012. 

** 22.1% a.i; MRID 48722704, Bergfield 2012. 

NC – Not calculated or converged. 

A Measured concentrations for sugarbeet, tomato, and cabbage were 0.0020, 0.0039, 0.0078, 0.016, 0.031, 0.062, 

0.12, 0.25, 0.50, and 1.0 lb a.i./A. For soybean, measured concentrations were 0.0020, 0.0039, 0.0078, 0.016, 

0.031, 0.062, 0.12, 0.25, 0.50, 1.0, and 2.0 lb a.i./A. For onion, ryegrass, oat, carrot, and cucumber, measured 

concentrations were 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb a.i./A. The measured 

concentrations for corn were 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb ai/A. 

B NOAEC values for fresh weight, shoot length, and survival, respectively.

4. Risk Characterization

4.1. Risk Estimation 

Toxicity data and exposure estimates are used to evaluate the potential for adverse ecological 

effects on non-target species. This assessment for tebuthiuron relies on the deterministic RQ 

method to provide a metric of potential risks. The RQ provides a comparison of exposure estimates 

to toxicity endpoints (i.e., the estimated exposure concentrations are divided by acute and chronic 

toxicity values). The resulting RQs are compared to the Agency’s LOCs. LOCs are used by the 

Agency to indicate when the use of a pesticide, as directed by the label, has the potential to cause 

adverse effects to non-target organisms. 
Page 47 of 121 

4.1.1 Risk Quotient Calculations for Aquatic Organisms 

There is a potential for exposure of the active ingredient to aquatic organisms (fish and 

invertebrates); toxicity information is used to estimate the risks to aquatic organisms as a result of 

surface runoff and spray drift from tebuthiuron uses. The LD50 and EC50 are used to estimate acute 

risk for adverse effects on survival to fish and invertebrates, respectively, and the NOAEC is used 

to estimate chronic risk for adverse effects to both fish and invertebrates. A single application at 4 

and 6 lb a.i./A was modeled. Applications from aerial, ground and granular uses were modeled. 

Tables 4.1 and 4.2 list acute and chronic RQs calculated for aquatic organisms (freshwater fish 

also surrogates for aquatic-phase amphibians) exposed to tebuthiuron based on the EECs generated 

from PRZM-EXAMS modeling. 

Results of the assessment on aquatic organisms indicate no acute or chronic LOC exceedances for 

fish and invertebrates exposed to tebuthiuron (Table 4.1 – freshwater fish; Table 4.2 – freshwater 

invertebrates; and Table 4.3 – estuarine/marine invertebrates); acute and chronic RQs were 

generally <0.01 and <0.1, respectively. Maximum acute and chronic RQs were 0.02 and 0.2, 

respectively, using the surrogate scenario of aerial applications in Texas, which exposure from 

aerial applications is expected to be the highest. Because RQs did not exceeded the LOCs for the 

Texas scenario with aerial applications, at which exposure is the highest; it is anticipated RQs for 

ground and granular applications along with the remaining aerial application scenarios modeled to 

be below the LOCs as well. Acute and chronic RQs for estuarine/marine fish were not calculated 

since mortality and sub-lethal effects were not established (LC50 >98 mg a.i./L; ACR-derived 

NOAEC >18 mg a.i./L). 

For listed species, LOC exceedances require an additional in-depth listed species evaluation of the 

potential co-occurrence of listed species in tebuthiuron-treated areas to characterize risks. 

Likewise, the probability of an acute effect (i.e., mortality) to an endangered species based on the 

acute RQ value was calculated. Based on results, the probability of an acute effect to aquatic 

animals was minimal (ranges from 1 in 9x1018 to 1 in >3x10155). 

The potential risks to fish and invertebrates are discussed further in Section 4.2. 
Page 48 of 121 

Table 4.1. Risk Quotients for Freshwater Fish

Crop Scenario 

App Rate 

in lbs a.i./A 

(Method of 

Application) 

1-in-10-

year 

Peak EEC 

(mg/L) 

Freshwater Fish 

LC50 = 106 mg a.i./L (slope = NA) 1-in-10

60-day 

(mg

Acute RQs 

Probability of Acute 

Effect1

(1 in ...) 

Rangeland/ 

Pastures 

CAalfalfa_WirrigOP 

6.0 

(Aerial) 

0.5 <0.01 >9x1018 0.

MNalfalfaOP 0.56 <0.01 >9x1018 0.5

NCalfalfaOP 0.60 <0.01 >9x1018 0.6

PAalfalfaOP 0.69 <0.01 >9x1018 0.6

TXalfalfaOP 1.1 0.01 9x1018 1.

CAalfalfa_WirrigOP 

4.0 

(Aerial) 

0.33 <0.01 >9x1018 0.3

MNalfalfaOP 0.37 <0.01 >9x1018 0.3

NCalfalfaOP 0.40 <0.01 >9x1018 0.4

PAalfalfaOP 0.46 <0.01 >9x1018 0.4

TXalfalfaOP 0.74 <0.01 >9x1018 0.7

Abbreviations: App. Application; NA Not available 

A bold cell indicates that the RQ meets or exceeds the aquatic LOC for acute risk to listed species (LOC = 0.05), restricted use (LOC 

species (LOC = 0.5) or chronic risk to listed and non-listed species (LOC=1). 

1 Probability of individual mortality based on actual screening level RQ value for tebuthiuron. A default concentration-response slop

generate the probability value when slope values were not available from the available test data (USEPA 2004). 
Page 49 of 121 

Table 4.2. Risk Quotients for Freshwater Invertebrates

Crop Scenario 

App Rate 

in lbs a.i./A / 

(Method of 

Application) 

1-in-10-

year 

Peak EEC 

(mg/L) 

Freshwater Invertebrates 

LC50 = 297 mg a.i./L (slope = 13.3) 1-in-10

21-day 

(mg/

Acute RQs 

Probability of Acute 

Effect1

(1 in ...) 

Rangeland/ 

Pastures 

CAalfalfa_WirrigOP 

6.0 

(Aerial) 

0.5 <0.01 >3x10155 0.5 

MNalfalfaOP 0.56 <0.01 >3x10155 0.5

NCalfalfaOP 0.60 <0.01 >3x10155 0.6

PAalfalfaOP 0.69 <0.01 >3x10155 0.6

TXalfalfaOP 1.1 <0.01 >3x10155 1.

CAalfalfa_WirrigOP 

4.0 

(Aerial) 

0.33 <0.01 >3x10155 0.3

MNalfalfaOP 0.37 <0.01 >3x10155 0.3

NCalfalfaOP 0.40 <0.01 >3x10155 0.4

PAalfalfaOP 0.46 <0.01 >3x10155 0.4

TXalfalfaOP 0.74 <0.01 >3x10155 0.7

Abbreviations: App. Application. 

A bold cell indicates that the RQ meets or exceeds the aquatic LOCs. 

1 Probability of individual mortality based on actual screening level RQ value for tebuthiuron. A concentration-response slope value o

the probability value (USEPA 2004). 
Page 50 of 121 

Table 4.3. Risk Quotients for Estuarine/marine Invertebrates

Crop Scenario 

App Rate 

in lbs a.i./A / 

(Method of 

Application) 

1-in-10-

year 

Peak EEC 

(mg/L) 

Estuarine/marine Invertebrates 

LC50 = 62 mg a.i./L (slope = N/A) 1-in-10

21-day 

(mg/

Acute RQs 

Probability of Acute 

Effect1

(1 in ...) 

Rangeland/ 

Pastures 

CAalfalfa_WirrigOP 

6.0 

(Aerial) 

0.5 <0.01 >9x1018 0.5 

MNalfalfaOP 0.56 <0.01 >9x1018 0.5

NCalfalfaOP 0.60 <0.01 >9x1018 0.6

PAalfalfaOP 0.69 0.01 9x1018 0.6

TXalfalfaOP 1.1 0.02 10x1013 1.

CAalfalfa_WirrigOP 

4.0 

(Aerial) 

0.33 <0.01 >9x1018 0.3

MNalfalfaOP 0.37 <0.01 >9x1018 0.3

NCalfalfaOP 0.40 <0.01 >9x1018 0.4

PAalfalfaOP 0.46 <0.01 >9x1018 0.4

TXalfalfaOP 0.74 0.01 9x1018 0.7

Abbreviations: App. Application. 

A bold cell indicates that the RQ meets or exceeds the aquatic LOCs. 

1 Probability of individual mortality based on actual screening level RQ value for tebuthiuron. A default concentration-response slo

generate the probability value when slope values were not available from the available test data (USEPA 2004). 
Page 51 of 121 

4.1.2 Risk Quotient Calculations for Aquatic Plants 

There is a potential for exposure of the active ingredient to aquatic vascular and nonvascular plant 

species. Toxicity information are used to estimate the risks to aquatic plants as a result of surface 

runoff and spray drift from broadcast ground applications via spray or granules. The IC50 is used 

to estimate risk for adverse effects on growth to non-listed aquatic plants and the NOAEC (or EC05

when a NOAEC is not available) is used to estimate risk for adverse effects on growth to listed

aquatic plants. 

According to peak PRZM/EXAMS EECs and the range of application rates for tebuthiuron uses 

every year and toxicity to aquatic plants, the non-listed and listed RQs for aquatic plants (both 

vascular and nonvascular species) are above Agency’s plant LOC of 1. The RQs were 2-29 for 

aquatic plants exposed to tebuthiuron uses (Table 4.4). 

The potential risks to aquatic plants are discussed further in Section 4.2. 
Page 52 of 121 

Table 4.4. Risk Quotients for Aquatic Plants

Crop Scenario App Rate (lbs/A) 

1-in-10 

year Peak 

EEC4

(mg/L) 

Aquatic Vascular Plants 

IC50 = 0.13 mg a.i./L 

NOAEC = 0.05 mg a.i./L 

Aquatic Nonvasc

IC50 = 0.05 m

NOAEC = 0.03

Non-listed

RQs Listed RQs Non-listed RQs 

Rangeland / 

Pastures 

CAalfalfa_WirrigOP 

6.0 

(Aerial) 

0.5 4 10 10 

MNalfalfaOP 0.56 4 11 11 

NCalfalfaOP 0.60 5 12 12 

PAalfalfaOP 0.69 5 14 14 

TXalfalfaOP 1.1 8 22 22 

CAalfalfa_WirrigOP 

4.0 

(Aerial) 

0.33 3 7 7 

MNalfalfaOP 0.37 3 7 7 

NCalfalfaOP 0.40 3 8 8 

PAalfalfaOP 0.46 4 9 9 

TXalfalfaOP 0.74 6 15 15 

CAalfalfa_WirrigOP 

6.0 

(Ground) 

0.37 3 7 7 

MNalfalfaOP 0.38 3 8 8 

NCalfalfaOP 0.45 3 9 9 

PAalfalfaOP 0.53 4 11 11 

TXalfalfaOP 1.0 8 20 20 

CAalfalfa_WirrigOP 

4.0 

(Ground) 

0.25 2 5 5 

MNalfalfaOP 0.25 2 5 5 

NCalfalfaOP 0.30 3 6 6 

PAalfalfaOP 0.36 3 7 7 

TXalfalfaOP 0.68 5 14 14 

CAalfalfa_WirrigOP 6.0 

(Granules) 

0.34 3 7 7 

MNalfalfaOP 0.33 3 7 7 

NCalfalfaOP 0.41 3 8 8 
Page 53 of 121 

Table 4.4. Risk Quotients for Aquatic Plants

Crop Scenario App Rate (lbs/A) 

1-in-10 

year Peak 

EEC4

(mg/L) 

Aquatic Vascular Plants 

IC50 = 0.13 mg a.i./L 

NOAEC = 0.05 mg a.i./L 

Aquatic Nonvasc

IC50 = 0.05 m

NOAEC = 0.03

Non-listed

RQs Listed RQs Non-listed RQs 

PAalfalfaOP 0.49 4 10 10 

TXalfalfaOP 1.0 8 20 20 

CAalfalfa_WirrigOP 

4.0 

(Granules) 

0.22 2 4 4 

MNalfalfaOP 0.22 2 4 4 

NCalfalfaOP 0.28 2 6 6 

PAalfalfaOP 0.33 3 7 7 

TXalfalfaOP 0.66 5 13 13 

Abbreviations: App. Application A bold cell indicates that the RQ meets or exceeds the LOC for non-listed or listed plants (LOC = 1). 
Page 54 of 121 

4.1.3. Risk Quotient Calculations for Terrestrial Animals 

For this screening-level risk assessment with birds and mammals, acute and chronic RQs are 

derived based on ecological toxicity data for tebuthiuron, and then directly compared to the 

dietary-based EECs generated from T-REX. For dose-based RQs, the EECs and toxicity values 

are first adjusted based on food intake and body weight differences of the terrestrial animals prior 

to the assessment. Size classes are not used in dietary-based RQs. The LD50 and LC50 are used to 

estimate acute risk for adverse effects on survival to both birds and mammals; the 

NOAEC/NOAEL is used to estimate chronic risk for adverse effects on reproduction and growth 

to birds and mammals. Dietary-based exposure modeling for granular consumption was not 

available. Therefore, dietary-based RQs for granular uses could not be calculated; it is presumed 

the dietary-based RQs for granular uses are equivalent to the RQs for spray uses. Dose-based 

exposure modeling for granular consumption is available; however, all of the dose-based toxicity 

values for birds were non-definitive; thus, the dose-based RQs for granular uses were not 

calculated for birds. On the other hand, the mammal dose-based toxicity value was definitive, and 

RQs for mammals consuming granules were calculated. The potential risk for granular exposure 

to birds and mammals is discussed in the Risk Description section. 

Similar to aquatic organisms, the probability of an acute effect to terrestrial organisms was 

estimated for the additional endangered species assessment and is included in the RQ tables below, 

where applicable. As of a result, the probability of an acute effect to terrestrial animals ranges from 

negligible (1 in 2x1019) to absolute certainty (1 in 1). 

Birds

There is a potential for acute exposure to birds via the oral route, but acute dose-based RQs were 

not calculated because definitive toxicity endpoints for upland game bird and waterfowl were not 

established (mallard and northern quail oral LD50s >2000 mg a.i./kg-bw), and there were 

difficulties in obtaining a dose-based value for the passerine (e.g., severe regurgitation). 

At the maximum application rate of 6 lb a.i./A applied broadcast as a liquid spray, the acute dietarybased RQs of 0.38 – 0.98 for birds based on a LC50 of 1465 mg a.i./kg-diet for the zebra finch were 

3-10x above the listed species LOC of 0.1, for birds that consume vegetation and insects in 

tebuthiuron-treated sites (Table 4.5). Only birds consuming fruits/pods/seeds did not exceeded the 

LOC (Highest RQ = 0.06). There is an uncertainty with the RQs calculated for the bobwhite quail 

and mallard that exceeded the LOC when the zebra LC50 of 1465 mg a.i./kg-diet was used in the 

calculations. If the toxicity of the upland game bird (LC50 >5113 mg a.i./kg diet) or mallard (LC50 

>5093 mg a.i./kg-diet) was used instead, the acute RQs would be <0.01 or 10x greater than the 

highest EECs for the mallard and quail species, suggesting the effects to upland game and 

waterfowl birds would be minimal. 

At 4 lb a.i./A applied broadcast as a liquid spray, the acute dietary-based RQs of 0.26-0.66 were 

2-7x above the listed species LOC of 0.1 for birds that consume vegetation and insects in 

tebuthiuron-treated sites (Table 4.5). 
Page 55 of 121 

As stated above, the dietary-based acute RQs could not be calculated for granular or pellet uses, it 

is presumed the acute RQs would be the same for liquid sprays if sprayed at 6 or 4 lb a.i./A. Thus, 

granule uses would be 3-10x above the LOC at 6 lb a.i./A and 2-7x above the LOC at 4 lb a.i./A. 

This is an uncertainty since the modeling for liquid spraying assumes 100% of the active ingredient 

when the pelleted and granular formulations actually has only 40% and 5% of tebuthiuron, 

respectively. 

For liquid sprays at 6 lb a.i./A, chronic RQs of 6-14 for birds based upon an NOAEC of 100 mg 

a.i./kg-diet were 6-14x above the LOC of 1 for potential risk to non-listed and listed birds 

consuming vegetation and insects with tebuthiuron residues (Table 4.6). Only birds consuming 

fruits/pods/seeds did not exceed the chronic LOC (Highest RQ = 0.9). 

For liquid sprays at 4 lb a.i./A, chronic RQs were 4-10 for birds; which is 4 to 10x above the LOC 

of 1. 

No methodology is available to model chronic exposure to birds from granular or pellet uses. Thus 

it is presumed the chronic RQs for granular uses would be the same of the RQs for liquid sprays 

which is 6-14x and 4-10x above the chronic LOC for 6 and 4 lb a.i./A, respectively. 

The potential for risk to birds is discussed in the Risk Description section. 
Page 56 of 121 

Table 4.5. Acute Avian Dietary-Based Risk Quotients Site 

LC50(mg a.i./kgdiet) 

EECs and RQs Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds ArthroEEC RQ IEC3 EEC RQ IEC EEC RQ IEC EEC RQ IEC EEC 

Rangeland1 1465 1440 0.98 2.1 660 0.45 83 810 0.55 22 90 0.06 1x1015 564 Rangeland2 1465 960 0.66 8.3 440 0.30 2950 540 0.37 400 60 0.04 2x1019 376 Size class not used for dietary risk quotients Bold values indicate terrestrial LOC exceedances for acute risk to listed species (LOC = 0.1), restricted use (LOC = 0.2), o

species (LOC = 0.5) or chronic risk to listed and non-listed species (LOC=1). 

1 Exposure scenario based on 1 broadcast spray at 6 lb a.i./A 2 Exposure scenario based on 1 broadcast spray at 4 lb a.i./A 3 Probability of individual mortality based on actual screening level RQ value for tebuthiuron (1 in ....). A concentration-re

value of 6.5 is used to generate the probability value (USEPA 2004). 

Table 4.6. Chronic Avian Dietary-Based Risk Quotients Site 

NOAEC (mg a.i./kgdiet) 

EECs and RQs Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthro

EEC RQ EEC RQ EEC RQ EEC RQ EEC 

Rangeland1 100 1440 14 660 7 810 8 90 0.9 564 Rangeland2 100 960 10 440 4 540 5 60 0.6 376 Size class not used for dietary risk quotients Bold values indicate LOC exceedances (exceeds the LOC of 1 for chronic risk) 1 Exposure scenario based on 1 broadcast spray at 6 lb a.i./A 2 Exposure scenario based on 1 broadcast spray at 4 lb a.i./A 
Page 57 of 121 

Terrestrial-phase Amphibians and Reptiles 

EFED currently uses surrogate data (birds) for terrestrial-phase amphibians and reptiles when data 

are not available. There were LOC exceedances for birds; these exceedances apply to terrestrialphase amphibian and reptiles. Risks are discussed in the Risk Description section. 

Mammals 

Acute dose-based RQ values of 0.19-1.6 for mammals based on toxicity data for the laboratory rat 

(female LD50 = 385 mg a.i./kg-bw) were above the LOC for mammals that eat vegetation and 

insects exposed to tebuthiuron from ground or aerial applications at 4 and 6 lb a.i./A (Table 4.7). 

These RQs were 2-16x higher than the listed species LOC of 0.1 and 3x higher than the non-listed 

species LOC of 0.5. The RQ of 0.1 was at the LOC for 15-g mammals that consume 

fruits/pods/seeds for the 6 lb a.i./A application scenario. RQs for 35-g and 1000-g mammals that 

consume fruits/pods/seeds for the 6 lb a.i./A application and for 15-g mammals that eat 

fruits/pods/seeds for the 4 lb a.i./A application were not above the acute listed LOC (Highest RQ 

= 0.09). RQs for granivorous mammals that eat grain and seeds were not above the LOC (Highest 

RQ = 0.02). 

For granular and pellet consumption, RQs for 15-g and 35-g mammals exceeded the acute LOCs, 

while the RQs for 1000-g mammals do not exceeded the LOCs. However, for granular uses at 4 lb 

a.i./A, the RQ of 0.09 for 35-g mammals does not exceed the acute listed species LOC of 0.1. The 

RQs that exceeded the listed species LOC of 0.1 ranged from 0.13 to 1.96, which were 1 to 19x 

higher than the listed species LOC. 

The probability for an acute effect to 15-g mammals that consume tebuthiuron-exposed short 

grass, using the highest application scenario, at 6 lb a.i./A with broadcast liquid sprays is 1 in 

1.22; for 35-g mammals, it is 1 in 1.34; and for 1000-g mammal it is 1 in 4. Table 4.7 provides 

more details on the probability of an acute effect to 15-g, 35-g, 1000-g mammals that consume 

these selected feed items treated with tebuthiuron. For pellet uses at 6 lb a.i./A with 40% of 

tebuthiuron in the formulation, the probability for an acute effect to 15-g mammals is 1 in 1.1; 

for 35-g mammals, it is 1 in 1.88; and for 1000-g mammals it is 1 in 3x106

. For granular 

applications at 6 and 4 lb a.i./A with 5% of tebuthiuron in its formulation, the probability to 15-g 

mammals is 1 in 378 and 1 in 5850, respectively. 
Page 58 of 121 

Table 4.7. Acute Mammalian Dose-Based Risk Quotients 

Site Size Class (grams) Adjusted7 LD50 EECs and RQs for Spray Uses Short Grass Tall Grass 

EEC7 RQ IEC (1 in ..)9 EEC RQ IEC (1 in ..) EEC 

Rangeland 

(6 lb/A broadcast spray) 1

15 851.66 1373 1.6 1.22 629 0.74 4 772 

35 689.08 949 1.4 1.34 435 0.64 5 534 

1000 298.05 220 0.7 4 101 0.34 57 124 

Rangeland 

(4 lb/A broadcast spray) 2

15 851.66 915 1.1 1.74 420 0.50 11 515 

35 689.08 633 0.9 2 290 0.42 22 356 

1000 298.05 147 0.5 11 67 0.23 491 83 

Site Size Class (grams) Adjusted7 LD50 EECs and RQs for Spray Uses Fruits/Pods/ Seeds Arthropods 

EEC RQ IEC (1 in ..) EEC RQ IEC (1 in ..) EEC 

Rangeland 

(6 lb/A broadcast spray) 1

15 851.66 86 0.10 3x105 538 0.64 5 19 

35 689.08 59 0.09 8x105 372 0.54 9 13 

1000 298.05 14 0.05 4x108 86 0.29 129 3 

Rangeland 

(4 lb/A broadcast spray) 2

15 851.66 57 0.07 10x106 358 0.42 22 13 

35 689.08 40 0.06 5x107 248 0.36 44 8.8 

1000 298.05 9 0.03 3x1011 57 0.19 1710 2.1 

Site Size Class (grams) Adjusted7 LD50 EECs and RQs for Granular Uses EEC8 RQ IEC (1 in ..) 

Rangeland 

(6 lb/A pellet broadcast) 3

15 851.66 25 1.96 1.1 

35 689.08 25 1.04 1.88 

1000 298.05 25 0.08 3x106

Rangeland 

(4 lb/A pellet broadcast) 4

15 851.66 16.7 1.31 1.43 

35 689.08 16.7 0.69 4.27 

1000 298.05 16.7 0.06 5x107

Rangeland 

(6 lb/A granular broadcast) 5

15 851.66 3.12 0.24 378 

35 689.08 3.12 0.13 3x104

1000 298.05 3.12 0.01 9x1018

Rangeland 

(4 lb/A granular broadcast) 6

15 851.66 2.08 0.16 5850 

35 689.08 2.08 0.09 8x105

1000 298.05 2.08 0.01 6x1021

Bold values indicate terrestrial LOC exceedances 
Page 59 of 121 

1 Exposure scenario based on 1 broadcast spray at 6 lb a.i./A 2 Exposure scenario based on 1 broadcast spray at 4 lb a.i./A 3 Exposure scenario based on 1 broadcast pellet application at 6 lb a.i./A with 40% of tebuthiuron 4 Exposure scenario based on 1 broadcast pellet application at 4 lb a.i./A with 40% of tebuthiuron 5 Exposure scenario based on 1 broadcast granular application at 6 lb a.i./A with 5% of tebuthiuron 6 Exposure scenario based on 1 broadcast granular application at 4 lb a.i./A with 5% of tebuthiuron7 mg a.i./kg-bw 8 mg a.i./ft2 9 Probability of individual mortality based on actual screening level RQ value for tebuthiuron. A default concentration-response slope 

generate the probability value when slope values were not available from the available test data (IEC model V.1.1 USEPA 2004).
Page 60 of 121 

Chronic RQ values for mammals are presented in Tables 4.8 and 4.9. Based on the rat chronic 

NOAEL of 14 mg a.i./kg-bw/day (200 mg/kg-diet) for exposure to tebuthiuron, both dietary and 

dose-based RQ values exceed the chronic risk LOC (LOC = 1) for all size classes of mammals that 

consume short grass, tall grass, broadleaf plants, fruits and pods (dose-based only), and arthropods 

(RQs ranged 1.9-45). These chronic RQs were 2-45x higher than the chronic LOC of 1.0. Chronic 

dietary-based RQs for applications of 4 and 6 lb a.i/A tebuthiuron do not exceed the chronic risk 

LOC for mammals consuming fruits/pods/seeds (Highest RQ = 0.45). Chronic dose-based RQs do 

not exceed the chronic risk LOC for all size classes of mammals that consume seeds (Highest RQ 

= 0.62) and 1000 g mammals that consume fruits/pods/seeds (Highest RQ = 0.85). 

Chronic RQs for granular and pelleted formulations are not calculated. 

The potential for risk to mammals is further discussed in the Risk Description section. 

Table 4.8. Chronic Mammalian Dietary-Based Risk Quotients 

Site 

NOAEL 

(mg 

a.i./kgdiet) 

EECs and RQs 

Short Grass Tall Grass 

Broadleaf 

Plants Fruits/Pods/ 

Seeds Arthropods 

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ 

Rangeland 1 200 1440 7.2 660 3.3 810 4.1 90 0.45 564 2.8 

Rangeland 2 200 960 4.8 440 2.2 540 2.7 60 0.30 376 1.9 

Size class not used for dietary risk quotients 

Bold values indicate LOC exceedances (exceeds the LOC of 1 for chronic risk) 1 Exposure scenario based on 1 broadcast spray at 6 lb a.i./A 

2

 Exposure scenario based on 1 broadcast spray at 4 lb a.i./A 
Page 61 of 121 

Table 4.9. Chronic Mammalian Dose-Based Risk Quotients 

Site 

Size 

Class 

(grams) 

Adjusted 

NOAEL 

EECs and RQs 

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/ Seeds Arthropods Gra

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC 

Rangeland 1

15 30.77 1373 45 629 20 773 25 86 3 538 17 19 

35 24.90 949 38 435 17 534 21 59 2 372 15 13 

1000 10.77 220 20 101 9 124 11 14 1 86 8 3 

Rangeland2

15 30.77 915 30 420 14 515 17 57 2 358 12 13 

35 24.90 633 25 290 12 356 14 40 2 248 10 9 

1000 10.77 147 14 67 6 83 8 9 0.9 57 5 2 

Bold values indicate LOC exceedances (exceeds the LOC of 1 for chronic risk) 1 Exposure scenario based on 1 broadcast spray at 6 lb a.i./A 2 Exposure scenario based on 1 broadcast spray at 4 lb a.i./A 
Page 62 of 121 

Beneficial Insects 

Tebuthiuron is classified as “practically nontoxic” to non-target beneficial insects including 

honeybees (honeybee acute contact LD50 >100 μg a.i./bee) on an acute exposure basis. The need 

for precautionary label language for beneficial insects is not required and is discussed further in 

the Risk Description section. 

Terrestrial Invertebrates 

EFED currently uses surrogate data (bees) for terrestrial invertebrates when data are not available. 

There is a potential for acute exposure to terrestrial invertebrates, but acute RQs were not 

calculated because tebuthiuron toxicity information for the honeybee was not established 

(honeybee acute contact LD50s >100 μg a.i./bee). Risks to terrestrial invertebrates are discussed in 

the Risk Description section. 

4.1.4. Risk Quotient Calculations for Terrestrial Plants in Terrestrial 

and Semi-aquatic Environments

For this risk assessment with terrestrial monocots and dicots, non-listed and listed plant species 

RQs are derived based on ecological toxicity data for the active ingredient and then compared to 

TerrPlant EECs for plants in non-target area receiving surface runoff combined with spray drift 

adjacent to the target area. Details of the TerrPlant model and EECs are presented in Table 2.11. 

There is a potential for exposure of tebuthiuron to terrestrial plants. Toxicity information on the 

TEP (Typical End-use Product) are used to estimate the risks to non-target terrestrial plants 

inhabiting dry or semi-aquatic areas adjacent to a treated field as a result of surface runoff and/or 

spray drift. The EC25 was used to estimate risk for adverse effects on growth to non-listed plant 

species while the NOAEC (or EC05 when a NOAEC is not available) was used to estimate risk for 

adverse effects on growth to listed plant species. Note: TerrPlant does not consider exposures to 

plants from multiple pesticide applications; thus, results are based on single pesticide applications.

Table 4.10 presents RQs for exposure of terrestrial plants to tebuthiuron via ground and aerial 

spray applications as well as granular applications. 

RQs (1.11-1031) for non-listed and listed monocots and dicots inhabiting dry and semi-aquatic 

areas adjacent to a treated site exposed to tebuthiuron through a combination of surface water 

runoff and spray drift exceed the plant LOC of 1.0 for broadcast applications by aerial, ground, 

and granular at 4 and 6 lb a.i./A. However, when reduced to 4 lb a.i./A from 6 lb a.i./A, non-listed 

and listed monocots inhabiting dry areas adjacent to a treated site exposed to tebuthiuron through 

runoff and drift via ground or granular applications were no longer above the LOC of 1.0; the 

highest RQ was 0.96. 

There is no LOC exceedance for non-listed and listed monocots exposed to drift alone from aerial 

uses at 4 lb a.i./A tebuthiuron or from ground or granular applications at 4 and 6 lb a.i./A 

tebuthiuron; the highest RQ is 0.74. 
Page 63 of 121 

For granular uses, no drift is produced; thus, there are no exceedances for non-listed and listed

plants exposed to spray drift alone. 

The potential risks to terrestrial plants are discussed further in the Risk Description section. 

Table 4.10. Terrestrial Plant Risk Quotient 1, 2, 3, 4 

Scenario 

Non-listed RQs Listed RQs 

Terrestrial 

Adjacent area 

Semi-aquatic 

Adjacent 

Area 

Drift Terrestrial 

Adjacent area

Semi-aquatic 

Adjacent 

Area 

Drift 

One Aerial Spray at 6 lb a.i./A

Monocot 2.22 12.2 1.11 2.4 13.2 1.2 

Dicot 33.3 183 16.7 188 1031 94 

One Aerial Spray at 4 lb a.i./A 

Monocot 1.48 8.15 0.74 1.6 8.8 0.8 

Dicot 22.2 122 11.1 125 688 62.5 

One Ground Spray at 6 lb a.i./A 

Monocot 1.3 11.3 0.22 1.4 12.2 0.24 

Dicot 20 170 3.3 113 956 18.8 

One Ground Spray at 4 lb a.i./A

Monocot 0.89 7.56 0.15 0.96 8.16 0.16 

Dicot 13.3 113 2.22 75 638 12.5 

One Granular Application at 6 lb a.i./A 

Monocot 1.11 11.1 N/A 1.2 12 N/A 

Dicot 16.67 167 N/A 94 938 N/A 

One Granular Application at 4 lb a.i./A 

Monocot 0.74 7.41 N/A 0.8 8 N/A 

Dicot 11.1 111 N/A 62.5 625 N/A 1 RQs for aerial and ground spray and granular applications in this table were calculated using the maximum 

application rates for tebuthiuron uses. 

2

 Non-listed toxicity thresholds (EC25) were 0.27, 0.018, 0.3, and 0.16 lb a.i./A for seedling emergence monocot, 

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

3

 Corresponding listed toxicity thresholds (NOAEC) were 0.25, 0.0032, 0.12, and 0.62 lb a.i./A for seedling 

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

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

species). 

4.2. Risk Description 

The risk hypothesis states that the use of tebuthiuron as an herbicide for terrestrial crop sites has 

the potential to adversely affect survival, reproduction, and/or growth of non-target aquatic and 

terrestrial animals and plants, including Federally-listed endangered and threatened species. Based 

on the available ecotoxicity data and predicted environmental exposures, the hypothesis is 

confirmed for potential adverse effects to birds, terrestrial-phase amphibians, reptiles, mammals, 

terrestrial plants, terrestrial invertebrates and aquatic plants (vascular and algae/diatoms) based on 

the registered uses for tebuthiuron. Results of the risk estimation indicate aquatic animals are not 
Page 64 of 121 

expected to be affected. Label protections for beneficial insects are not required. The following 

sections discuss the potential risks of tebuthiuron to these taxonomic groups. 

4.2.1. Risks to Aquatic Animals and Plants 

In the conceptual model, spray drift and surface runoff/leaching to adjacent bodies of water were 

predicted as the most likely sources of exposure of tebuthiuron to non-target aquatic organisms 

and plants. Risks to aquatic species (i.e., fish, invertebrates, and plants) were assessed based on 

modeled estimated environmental concentrations (EECs) and available toxicity data. The aquatic 

EECs for the ecological exposure to the TGAI were estimated using the PRZM/EXAMS model to 

evaluate whether the potential for exposure is nationwide or limited to certain locations (Table 

3.8). 

 Risk to Aquatic Organisms 

Available acute toxicity data indicates that tebuthiuron is practically non-toxic to fish and generally 

practically non-toxic to invertebrates on an acute basis. The pink shrimp was the only aquatic 

animal to show slight sensitivity to tebuthiuron. The other invertebrates exposed to tebuthiuron 

were the daphnid, fiddler crab, and oyster bivalve; none of them were affected up to the highest 

concentration tested. The acute endpoint for pink shrimp, the most sensitive aquatic animal from 

the available studies, was compared to PRZM/EXAMS peak EECs for non-target fish and 

invertebrates inhabiting waterbodies adjacent to fields applied with tebuthiuron. The highest acute 

RQ was 0.02, which did not exceed the Agency’s endangered species LOC of 0.05. With a 

maximum RQ of 0.02 for acute effects to aquatic animals, the probability for acute effect is 1 in 

9x1018 or lower. 

The lowest test level of no observed adverse reproduction effects for aquatic animals (i.e., 9.3 mg 

a.i./L based on reduced length in fathead minnow) was compared to PRZM/EXAMS 60-day EECs 

for non-target organism exposed to tebuthiuron as a result of surface water runoff and spray drift. 

The highest chronic RQ was 0.2, which is below the chronic LOC of 1.0. Aquatic animals, 

including aquatic-phase amphibians for which freshwater fish served as a surrogate, are not 

anticipated to be at risk for adverse effects on survival, reproduction, and growth when exposed to 

tebuthiuron. 

Bluegills exposed to tebuthiuron concentrations of 5.0 ppm demonstrated bioconcentration factors 

of 1.98, 3.40, and 2.63 for edible tissue, nonedible tissue, and whole fish, respectively. 

Accumulated residues depurated rapidly from fish tissue with depuration half-lives of 0.33 and 

0.51 days reported for edible and nonedible tissue, respectively, which indicates tebuthiuron is not 

expected to bioaccumulate in fish. An assessment using the Agency’s KABAM (KOW (based) 

Aquatic BioAccumulation Model) v1.0 model was not conducted since both the environmental 

properties of tebuthiuron and results of bioconcentration studies indicated bioaccumulation in fish 

and biomagnification through the food chain is unlikely. 

 
Page 65 of 121 

 Risk to Aquatic Vascular and Non-Vascular Plants 

The most sensitive EC50’s derived from the duckweed (Lemna gibba) study for aquatic vascular 

plants and from available studies for aquatic non-vascular plants (marine diatom) were compared 

to EECs from several PRZM/EXAMs exposure scenarios for non-target, non-listed plants 

inhabiting bodies of water adjacent to fields treated with tebuthiuron. Non-listed plant RQs for 

aquatic vascular and non-vascular plants exceeded the non-listed plant species LOC of 1 following 

one broadcast application of tebuthiuron at 4 and 6 lb a.i./A (RQs = 2-22). 

The corresponding NOAECs derived from the duckweed study for aquatic vascular plants and 

from the marine diatom study for aquatic non-vascular plants were compared to several 

PRZM/EXAMs exposure scenarios for non-target Federally-listed plants inhabiting bodies of 

water adjacent to fields treated with tebuthiuron. Listed plant RQs for aquatic vascular and nonvascular plants all exceeded the listed plant species LOC of 1 for tebuthiuron uses (RQs = 4-29). 

To assess the range of sensitivity in aquatic non-vascular plant species when exposed to 

tebuthiuron, a comparison of the least sensitive species (blue-green algae EC50 of 0.81 mg a.i./L) 

and the most conservative PRZM/EXAMS EEC using the TX alfalfa scenario (1.1 mg a.i./L) yields 

an RQ of 1.4 which does exceed the plant LOC of 1.0. Thus it is anticipated that all algae would 

be adversely affected when exposed to tebuthiuron. Aquatic vascular plant data were limited to the 

duckweed study; therefore, the range of sensitivity in aquatic vascular plants adversely affected by 

tebuthiuron applications could not be assessed. 

In addition, the labels state that tebuthiuron is a thiadiazolylurea herbicide that is readily absorbed 

through the roots and translocated. Once it enters the plant, it acts by inhibiting photosynthesis in 

the plant. Thus, some herbicidal effects to aquatic plants are anticipated. 

The AgDRIFT spray drift model was used to assess the distance from the edge of a site at which 

RQs for spray drift fall below the aquatic plant LOC of 1 (Table 4.11). The RQs were calculated 

by comparing the amount of deposition at a specific distance (EEC) and the toxicity endpoints 

for duckweed and marine diatom (EC50). As the distance increases, the chance for exposure to 

non-target species outside a treated site increases. This spray drift exposure analysis indicates 

that when aerial-based application methods are used (which produces the highest exposures) at 

the maximum label rate, exposure falls below the LOC for the most sensitive aquatic vascular 

and non-vascular plants at 0 and 10 feet, respectively, off-site. At 4 lb a.i./A, there were no LOC 

exceedances from spray drift exposures via aerial or ground application for aquatic plants 

inhabiting waterbodies (e.g. pond) adjacent to the edge of a site. Therefore, adverse effects to 

non-target aquatic plants in waterbodies adjacent to treated site exposed to spray drift alone at or 

greater than 10 feet are not likely. This suggest aquatic plants inhabiting water bodies adjacent to 

treated sites subjected to surface water runoff with tebuthiuron residues would more likely be 

adversely affected than when exposed to off-site spray drift alone. 
Page 66 of 121 

Table 4.11. Spray Drift (Aquatic) Assessment for Non-Target Listed and Non-Listed 

Aquatic Vascular and Non-Vascular Plant Species 

No. of 

Applications and 

Application Rate

(lb a.i./A)

Distance from 

Edge of Field

% of 

Application 

Rate

EEC

(mg 

a.i./L)

Non-listed

Plant

RQ 

(EEC/EC50)

Listed

Plant

RQ (EEC/ 

NOAEC)

Spray 

Method

Aquatic vascular plants inhabiting pond a

1 @ 6 0 feet 13 0.04 0.3 0.8 Aerial

Aquatic non-vascular plants inhabiting pond b

1 @ 6 0 feet 13 0.04 0.8 1 Aerial 1 @ 6 10 feet 11 0.036 0.7 0.95

1 @ 6 0 feet 2.7 0.009 0.2 0.2 Ground 1 @ 6 10 feet 1.4 0.005 0.1 0.1

1 @ 4 0 feet 13 0.03 0.6 0.8 Aerial 1 @ 4 10 feet 11 0.024 0.5 0.6

1 @ 4 0 feet 2.7 0.006 0.1 0.2 Ground 1 @ 4 10 feet 1.4 0.003 0.06 0.08 a Based on duckweed EC50 of 0.13 mg a.i./L and NOAEC of 0.05 mg a.i./L. (MRID #41080404) 

b

 Based on marine diatom EC50 and NOAEC of 0.05 and 0.038 mg a.i./L, respectively. (MRID #41080402) 

*Value in bold indicates the LOC (=1) is exceeded. 

4.2.2. Risks to Terrestrial Animals and Plants 

In the conceptual model, dietary ingestion of tebuthiuron residues on vegetative matter, insects 

and granules on treated areas are predicted as the most likely sources of tebuthiuron exposure to 

terrestrial animals. Spray drift, runoff and wind erosion of soil particles with resulting residues on 

upland and/or wetland foliage and soil are the most likely sources of tebuthiuron exposure to nontarget terrestrial plants, including Federally-listed endangered and threatened species. Risks to 

terrestrial species (i.e., birds, mammals and plants) are assessed based on modeled estimated 

environmental concentrations (EECs) and available toxicity data. Terrestrial EECs for ecological 

exposure to the TGAI are estimated using T-REX (Tables 3.9, 3.10, and 3.11) and TerrPlant 

(Table 3.12). 

 Risk to Terrestrial Animals 

Available avian acute toxicity data indicate that tebuthiuron is practically non-toxic to upland game 

birds and waterfowl on an acute basis for both oral and dietary routes (e.g., >2000 mg a.i./kg bw 

and >5113 mg a.i./kg diet). However, the TGAI is slightly toxic to passerine birds via the dietary 

route; the dietary-based LC50 for passerines is 1465 mg a.i./kg diet as a result of counting extremely 

low food consumption as mortalities since the birds would not survive throughout the test, 

including those birds that died. Dietary-based risk quotient calculations focusing on the most 

conservative T-REX scenario using the short grass EEC of 1440 mg a.i./kg diet was 10 times 

higher than EPA’s LOC of 0.1 for potential risk to Federally-listed endangered and threatened 

birds and 2 times higher than EPA’s LOC of 0.5 for potential risk to non-listed birds. Using the 

maximum RQ of 0.98 for short grass based upon a broadcast application of 6 lb a.i./A and 20 g 

birds, this indicates 1 in 2 small-sized birds that consume tebuthiuron-treated short grass would be 
Page 67 of 121 

affected. At 4 lb a.i./A of the same scenario with highest RQ at 0.66, 1 in 8.3 small-sized birds 

would be affected. 

Meanwhile, since dietary concentrations that caused no mortality or acute sub-lethal effects in 

bobwhite quail and mallard are 3.5x environmentally relevant concentrations, acute risk to upland 

and waterfowl birds is anticipated to be low. 

Chronic avian RQs for ground spray of tebuthiuron were above the chronic risk LOC of 1, with a 

calculated maximum RQ of 14.4. 

Acute and chronic RQs for granular and pellet applications were not available. All acute dosebased endpoints were non-definitive and tools to model chronic EECs were not available in this 

screening-level assessment; thus, the RQs for granular/pellet and spray uses were assumed 

equivalent. However, an uncertainty is introduced since this assumption is that all formulations 

are 100% tebuthiuron. With granular and pelleted formulations containing up to 5% and 40% of 

tebuthiuron, respectively, it is uncertain the RQs for granular and pellet uses are as high as the 

spray uses. 

No mortalities were observed in the acute studies with the upland game bird and waterfowl, yet 

significant sub-lethal effects were observed in the reproduction studies at 500 mg a.i./kg diet and 

higher. These same large birds demonstrated no regurgitation or food avoidance of the test material 

in the acute oral studies. On the other hand, smaller birds such as passerines were more acutely 

sensitive to tebuthiuron due to severe regurgitation and extreme food avoidance, which these 

observations were counted as mortalities in the dietary study. This suggests the acute effects of 

tebuthiuron may be observed more often in smaller birds compared to larger birds when exposed 

to tebuthiuron. While chronic reproduction effects were not evaluated for small birds (i.e., 

passerines), as growth and reproduction (chronic) effects were observed in other avian species, 

chronic effects could be anticipated in several bird class sizes. 

Available mammalian acute toxicity data indicate that tebuthiuron is moderately toxic to mammals 

on an acute oral basis. Dose-based RQ calculations focusing on the most conservative T-REX 

scenario indicate that the short grass EEC of 1440 mg a.i./kg diet is 16 times higher than EPA’s 

LOC of 0.1 for potential risk to Federally-listed endangered and threatened mammals and 3 times 

higher than EPA’s LOC of 0.5 for potential risk to non-listed mammals. In addition, rabbits are 

slightly more acutely sensitive to tebuthiuron than rats; RQs calculated using the rabbit toxicity 

endpoint would be more than 16 times higher than the LOC. These acute LOC exceedances 

indicate mammals may be at risk for adverse effects on survival from exposure to tebuthiuron. 

Using the maximum RQ of 1.6 for short grass and 15 g mammals, 1 in 1.22 small-sized mammals 

that consume tebuthiuron-treated short grass would be affected. A maximum RQ of 4.9 for 

granules and 15 g mammals indicates that 1 in 1 small-sized mammals that consume granules 

would be affected. 

Based on the most sensitive mammalian 2-generation NOAEL and upper bound T-REX EECs, the 

dose-based RQs of 13-45 exceed the chronic LOC for 15 g, 35 g and 1000 g mammals foraging 

on short grass, tall grass, broadleaves and arthropods following one tebuthiuron application at 4 or 
Page 68 of 121 

6 lb a.i./A. For the dietary-based risk estimations, the results were similar; however, there were no 

LOC exceedances for mammals foraging on fruits/seeds/pods (highest RQ = 0.45). Thus, these 

exceedances indicate mammals may be at potential risk for adverse effects on reproduction and 

growth from exposure to tebuthiuron as a result of the labeled uses of the herbicide. 

Those adverse effects discussed above would be observed for those terrestrial vertebrates that 

remain in a tebuthiuron-treated site all the time. The less amount of time a bird or mammal remains 

in the treated site, the less the risk to terrestrial vertebrates. Also, the smaller the application site 

(i.e. driveway vs. agricultural field) is, the less chance for terrestrial vertebrates to be at risk. 

Considering the maximum single tebuthiuron application at 6.0 lbs a.i./A, the potential for 

adversely affecting the reproduction and growth of terrestrial animals in treated sites is likely. 

Reducing the application rate would mitigate risks to birds and mammals residing inside the treated 

site. A reduced application rate that would not result in an acute LOC exceedance for birds would 

be 0.35 lb a.i./A. For mammals, it would be 0.14 lb a.i./A. At 6.0 lbs a.i./A, the mitigated 

applications for birds and mammals need to be reduced 17x and 40x, respectively, of the maximum 

application rate. Using the maximum application rate as an example, Graphs 4-1 and 4-2 illustrate 

the days needed to exceed the chronic LOC for adverse effects to birds and mammals, respectively, 

foraging on food resources in fields/sites treated with tebuthiuron. 

Graph 4-1 below illustrates one tebuthiuron application at the maximum application rate of 6.0 lb 

a.i./A applied as a broadcast application to selected feed items that terrestrial vertebrates rely as a 

food resource.. Pictured are the EECs for short grass, tall grass, broadleaf plants, fruits/pods/seeds, 

and arthropods following one application of tebuthiuron. Based on the graph, the upper-bound 

Kenaga dietary-based EECs exceed the LOC of 1 (NOAEC = 100 mg a.i./kg diet) for birds 

foraging in tebuthiuron-treated sites following application. Likewise, the number of days in 

exceedance of the LOC for reproduction and growth effects to birds that forage on vegetation in 

tebuthiuron-treated sites lasts approximately 80-140 days, the longest when foraging on short 

grass. Arthropods covered with tebuthiuron residues may be toxic to birds for 55 days. T-REX 

provides graphs for dietary-based assessments only; graphs for dose-based assessments are not 

available. 
Page 69 of 121 

Graph 4-1. Number of applications and days needed to exceed the LOC of 1 for adverse effects of growth and 

reproduction to birds following tebuthiuron application at 6 lb/A on rangeland and pastures as per label 

directions. 

In Graph 4-2, the upper-bound Kenaga dietary-based EECs exceed the LOC of 1 (NOAEC = 200 

mg a.i./kg diet) for mammals foraging in tebuthiuron-treated sites following application. Likewise, 

the number of days in exceedance of the LOC for reproduction and growth effects to mammals 

that forage on short grass, broadleaves, tall grass, and arthropods in tebuthiuron-treated sites lasts 

approximately 100, 75, 60, and 50 days, respectively. 
Page 70 of 121 

Graph 4-2. Number of applications and days needed to exceed the LOC of 1 for adverse effects of growth and 

reproduction to mammals following tebuthiuron application at 6 lb/A on rangeland and pastures as per label 

directions. 

AgDRIFT was used to assess whether the LOC exceedances should apply to non-target birds and 

mammals that inhabits areas outside tebuthiuron-treated sites. Table 4.12 shows the spray drift 

distances estimated using LOCs divided by risk quotients (calculated based on a single application) 

as the “Fraction of Applied” in AgDRIFT to estimate the spray drift distance for terrestrial animals. 

The spray drift exposure analysis indicates that for aerial-based applications, the RQ falls below 

the LOC for the non-listed bird and mammal at 138 and 479 feet, respectively, off-site. For groundbased applications, the RQ falls below the LOC for bird and mammal at 39 and 125 feet, 

respectively. This suggests that effects to non-target terrestrial vertebrates could occur outside the 

site of application. 

 
Page 71 of 121 

Risk to Terrestrial Monocots and Dicots 

Based on the available data, monocots are less sensitive to tebuthiuron than dicots. The RQs for 

runoff to semi-aquatic areas result in the highest RQs compared to dry areas. This scenario 

represents a 10 acre area running off into a one acre area. The highest RQ for a combination of 

runoff and spray drift to semi-aquatic areas is 1031, which exceeds the Agency’s LOC of 1 for 

potential risk to plants. Since the herbicide is absorbed through the roots and translocated 

throughout, the plant the LOC exceedance for potential risk to non-listed and listed plants 

inhabiting semi-aquatic areas cannot be discounted. 

AgDRIFT was used to assess whether the LOC exceedances should apply to non-target terrestrial 

plants in dry (Table 4.13) and semi-aquatic (Table 4.14) areas adjacent to tebuthiuron-treated 

sites. The spray drift exposure analysis indicates that for aerial-based applications, exposure falls 

below the LOC for non-listed and listed terrestrial plant exposed to tebuthiuron as a result of spray 

drift at and greater than 997 feet off-site. However, for ground-based applications, exposure falls 

below the LOC at approximately 500 and >997 feet for non-listed and listed plants, respectively. 

This suggests that effects to non-target terrestrial plants could occur outside the site of application. 

Table 4.12. Spray Drift (Terrestrial) Assessment for Non-Listed Terrestrial Vertebrate 

Species Exposed to 6 lb a.i./A Tebuthiuron 

Taxa Type of 

Endpoint

Highest Risk 

Quotient LOC*

Fraction of 

Applied 

(LOC/RQ)

Distance from 

Edge of Field 

Where RQ 

Falls below 

LOC

Aerial Applications

Birds

Acute 0.98 0.5 0.51 0 feet

Chronic 14 1 0.07 138 feet

Mammals

Acute 1.6 0.5 0.3 16 feet

Chronic 45 1 0.02 479 feet

Ground applications

Birds

Acute 0.98 0.5 0.51 7 feet

Chronic 14 1 0.07 39 feet

Mammals

Acute 1.6 0.5 0.3 10 feet

Chronic 45 1 0.02 125 feet

* The LOCs shown are for non-listed species. 
Page 72 of 121 

In addition to the AgDRIFT assessment using the most sensitive plant species, an assessment was 

conducted on the least sensitive plant species and the highest exposure scenario to observe whether 

all or some terrestrial plants would be affected when exposed. Corn was used as the least sensitive 

species with seedling emergence EC25 and NOAEC values of 3.1 and 2.0 lb a.i./A, respectively. 

For plants inhabiting dry area adjacent a treated site, AgDRIFT indicates exposure to drift from 

aerial and ground broadcast applications using the corn EC25 and NOAEC values does not exceed 

the LOC for both non-listed and listed plant species beyond a distance of 50 feet from the edge of 

a site (Table 4.15), suggesting exposure is limited to the treated site. Nevertheless, with the least 

sensitive NOAEC (corn) of the 10 test species not exceeding the LOC for listed species inhabiting 

semi-aquatic areas beyond the edge of a treated site as well (Table 4.16), it is anticipated that 

potential risk to not all but some listed terrestrial plants exposed to off-site drift is likely. 

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

monocots and dicots, including Federally-listed endangered and threatened species, that inhabit 

dry and semi-aquatic areas adjacent to tebuthiuron-treated fields through a combination of runoff 

and spray drift or by drift alone. Spray drift from granular/pelleted/tableted uses is expected to be 

negligible; however, risk to plants from runoff-borne residues alone cannot be discounted 

(Discussed in Table 4.10). 

Table 4.13. Spray Drift (Terrestrial) Assessment for Non-target Listed and Non-listed 

Terrestrial Plant Speciesa

No. of 

Applications and 

Application Rate

(lb a.i/.A)

Distance from 

Edge of Field

% of Application 

Rate

Dry Area 

EEC

(lb a.i./A)

Non-listed 

Plant

RQ

Listed

Plant 

RQ

Spray 

Method

Terrestrial plants inhabiting dry areas

1 @ 6 0 feet 0.5 3 167 938

Aerial 1 @ 6 50 feet 0.17 1 56 313

1 @ 6 500 feet 0.02 0.1 6 31

1 @ 6 997 feet 0.01 0.07 4 22

1 @ 6 0 feet 1 6.0 352 1978

Ground 1 @ 6 50 feet 0.05 0.3 17 94

1 @ 6 500 feet 0.004 0.02 1 6

1 @ 6 997 feet 0.001 0.0085 0.7 3 a Based on carrot EC25 of 0.018 lb a.i./A and EC05 of 0.0032 lb a.i./A. 

*Value in bold indicates the LOC is exceeded. 
Page 73 of 121 

Table 4.14. Spray Drift (Aquatic) Assessment for Listed and Non-Listed Terrestrial Plant 

Species a

No. of 

Applications and 

Application Rate

(lb a.i./A)

Distance from 

Edge of Field

% of Application 

Rate

 Semi-Aquatic 

EEC

(lb a.i./A)

Non-listed

Plant

RQ

Listed 

Plant

RQ

Spray 

Method

Terrestrial plants inhabiting semi-aquatic areas

1 @ 6 0 feet 0.1 0.8 44 250

Aerial 1 @ 6 50 feet 0.07 0.4 22 125

1 @ 6 500 feet 0.02 0.1 6 31

1 @ 6 997 feet 0.01 0.07 4 22

1 @ 6 0 feet 0.06 0.37 21 116

Ground 1 @ 6 50 feet 0.02 0.1 6 31

1 @ 6 500 feet 0.003 0.02 1 6

1 @ 6 997 feet 0.001 0.007 0.4 2 a Based on carrot EC25 of 0.018 lb a.i./A and EC05 of 0.0032 lb a.i./A 

*Value in bold indicates the LOC is exceeded. 

Table 4.15. Spray Drift (Terrestrial) Assessment for Least Sensitive Terrestrial Plant 

Species a

No. of 

Applications and 

Application Rate

(lb a.i/.A)

Distance from 

Edge of Field

% of Application 

Rate

Dry Area 

EEC

(lb a.i./A)

Non-listed 

Plant

RQ

Listed

Plant 

RQ

Spray 

Method

Terrestrial plants inhabiting dry areas

1 @ 6 0 feet 50 3 0.97 1.5 Aerial 1 @ 6 50 feet 17 1 <0.1 0.5

1 @ 6 0 feet 100 6 1.9 3 Ground 1 @ 6 50 feet 1.7 0.1 0.03 0.05

1 @ 4 0 feet 50 2 0.6 1 Aerial 1 @ 4 50 feet 17 0.7 0.2 0.4

1 @ 4 0 feet 100 4 1.3 2 Ground 1 @ 4 50 feet 1.7 0.07 0.02 0.04 a Based on corn EC25 of 3.1 lb a.i./A and NOAEC of 2.0 lb a.i./A. 

*Value in bold indicates the LOC is exceeded. 
Page 74 of 121 

Table 4.16. Spray Drift (Aquatic) Assessment for Least Sensitive Terrestrial Plant Speciesa

No. of 

Applications and 

Application Rate

(lb a.i/.A)

Distance from 

Edge of Field

% of Application 

Rate

Semi-aquatic 

Area EEC

(lb a.i./A)

Non-listed 

Plant

RQ

Listed

Plant 

RQ

Spray 

Method

Terrestrial plants inhabiting semi-aquatic areas

1 @ 6 0 feet 13 0.75 0.2 0.4 Aerial 1 @ 6 50 feet 7.3 0.44 0.1 0.2

1 @ 6 0 feet 6.2 0.37 0.1 0.2 Ground 1 @ 6 50 feet 1.9 0.11 0.04 0.06

1 @ 4 0 feet 13 0.5 0.2 0.3 Aerial 1 @ 4 50 feet 7.3 0.29 0.09 0.1

1 @ 4 0 feet 6.2 0.25 0.08 0.1 Ground 1 @ 4 50 feet 1.9 0.08 0.03 0.04 a Based on corn EC25 of 3.1 lb a.i./A and NOAEC of 2.0 lb a.i./A. 

*Value in bold indicates the LOC is exceeded. 

Risk to Beneficial Insects and Terrestrial Invertebrates 

 

The available beneficial insect toxicity data, based on tests with honeybees, suggest that the TGAI 

is practically non-toxic to bees on an acute contact basis. The LD50 value is >100 μg a.i./bee. Risk 

to beneficial insects in the direct treatment area exposed to tebuthiuron is expected to be minimal; 

consequently, precautionary labeling for honeybee protection is not required at this time. 

For the assessment on terrestrial invertebrates, no mortality occurred in the honeybee study at 

concentrations as high as 100 μg a.i./bee. However, two studies (Doerr, T.B. 1980 and Zavaleta, 

J. 2012) suggest tebuthiuron does not affect terrestrial invertebrates, but increases the abundance 

and number of families in treated areas compared to untreated areas. However, Zavaleta reported 

that selected individual invertebrate taxonomic groups reacted differently to treatment with a 200% 

increase in grasshoppers and a 92% decrease in caterpillar larvae; treehoppers were not affected. 

No toxicity endpoints were provided; thus, it was not possible to assess whether the reduction in 

the larvae of caterpillars was a result of tebuthiuron treatment or a reduction in forb cover. Hagen 

et al. (2005) and Jamison (2002) suggest invertebrate abundance is dependent on herbaceous cover 

and structure, with invertebrates increasing in treated areas with more forb than in untreated areas. 

Regardless of the effects, the adverse effects of the use patterns outside the target area would 

extend no farther than 100 feet off site. According to AgDRIFT and bee toxicity as a surrogate 

value, the potential for risk to non-target terrestrial invertebrates adjacent to treated sites exposed 

to tebuthiuron falls below the interim LOC of 0.05 for terrestrial invertebrates at approximately 

100 feet off-site from the edge of the field as a result of aerial application. (Table 4.17). Until 

definitive endpoints are available for listed terrestrial invertebrates, it is uncertain whether all, 

some, or none of the listed terrestrial invertebrates would be adversely affected when exposed 

directly to tebuthiuron. 
Page 75 of 121 

In addition, there is an uncertainty of the LOC exceedances for terrestrial invertebrates. While the 

RQ of 0.36 may exceed the LOC of 0.05, it is uncertain whether there are actually any adverse 

effects for terrestrial invertebrates since no effects occurred at the highest dose tested. The highest 

RQ could be much lower than 0.36. 

Table 4.17. Spray Drift (Terrestrial) Assessment for Listed Terrestrial Invertebrate Species a

No. of 

Applications and 

Application rate

(lb a.i/.A)

Distance from 

Edge of Field

% of Application 

Rate

Dry Area 

EEC

(lb a.i./A)

Listed

Terrestrial 

Invertebrate RQ

Spray Method

Terrestrial invertebrates inhabiting dry areas

1 @ 6 0 feet 0.5 3 0.36

Aerial 1 @ 6 50 feet 0.17 1 0.12

1 @ 6 100 feet 0.1 0.6 0.07

1 @ 6 500 feet 0.02 0.1 <0.01 a Based on honeybee LD50 of >100 μg a.i./bee where 0% mortality was observed. This is equivalent to an exposure of 

780 mg/kg body weight or 8.3 lb T-REX/A assuming 1.0 lb/A = 94 mg T-REX arthropod/kg for terrestrial 

invertebrates. 

*Value in bold indicates the LOC of 0.05 is exceeded. 

Taxa Exposed to Tebuthiuron As a Result of Spot Treatment Applications 

There is uncertainty with the risk assessment for spot treatment uses. EFED has modified the 

risk assessment for spot treatment uses using an assumption that the rates are broadcast (Table 

4.18). While aerial applications is permitted in some labels; however, to characterize the risks 

from spot treatment uses, only ground and granular application scenarios were used. EFED 

believes that exposure estimates for resources are overly conservative for spot treatments 

because the underlying assumption in the models is that 100% of the field is treated with the 

pesticide. In order to better characterize the importance of the 100% area treated assumption on 

the overall risk conclusions; an estimate of the area threshold of spot treatment for each taxon 

where the LOC would no longer be exceeded is given. 

As an example, it is estimated that for the highest application rate (i.e. 6 lbs a.i./acre) the area 

treated would need to exceed 10% for the LOC to be exceeded for birds. However, EFED does 

not have information on typical spot treatment area percentages. Also, the labels that do specify 

spot treatment and the total area the container can cover (square feet) can be compared with 

Table 4.18 to determine whether effects are possible from these uses. 
Page 76 of 121 

Table 4.18 Area Percentage of Spot Treatment Uses (Spray or Granules) Needed to Exceed 

the LOC 

Taxa Application 

rates 

(lb a.i./A) 

RQs 

assuming 

100% area 

treated 

(highest to 

lowest)* 

LOC Area Percentage 

of application 

needed to exceed 

LOC1

Birds

(acute effects, liquid 

spray, broadcast)

6 0.98

Acute = 0.1

10% 

4 0.66 15% 

Mammals

(acute effects, liquid 

spray, broadcast)

6 1.6 6% 

4 1.1 9% 

Birds

(acute effects, 

granules and pellets, 

broadcast)

6 No granule or 

pellets RQs 

were 

calculated; 

assumed 

equivalent to 

liquid spray 

RQs

--

4 --

Mammals

(acute effects, pellets, 

broadcast)

6 1.96 5%

4 1.31 8%

Mammals

(acute effects, 

granules, broadcast)

6 0.24 42%

4 0.16 63%

Birds

(chronic effects, 

broadcast, dietarybased)

6 14

Chronic = 1.0

7%

4

10 10%

Mammals

(chronic effects, 

liquid spray, 

broadcast, dietarybased)

6 7 14%

4 5

20%

Mammals

(chronic effects, 

broadcast, liquid 

spray, dose-based)

6 45 2%

4 30

3%

Terrestrial Plants

(non-listed effects, 

liquid spray, ground, 

broadcast)

6 170

Non-listed and listed 

plants = 1.0

0.6%

4 113

0.9%

Terrestrial Plants 6 956 0.1%
Page 77 of 121 

Table 4.18 Area Percentage of Spot Treatment Uses (Spray or Granules) Needed to Exceed 

the LOC 

Taxa Application 

rates 

(lb a.i./A) 

RQs 

assuming 

100% area 

treated 

(highest to 

lowest)* 

LOC Area Percentage 

of application 

needed to exceed 

LOC1

(listed effects, liquid 

spray, ground, 

broadcast)

4 638

0.2%

Terrestrial Plants

(non-listed effects, 

granules, ground, 

broadcast)

6 167 0.6%

4 111

0.9%

Terrestrial Plants

(listed effects, 

granules, ground, 

broadcast)

6 938 0.1%

4 625

0.2%

Aquatic vascular 

plants 

(non-listed effects, 

liquid spray, ground, 

broadcast)

6 8 13%

4 5 20%

Aquatic vascular 

plants 

(listed effects, liquid 

spray, ground, 

broadcast)

6 20 5%

4 14 7%

Aquatic nonvascular 

plants

(non-listed effects, 

liquid spray, ground, 

broadcast)

6 20 5%

4 14 7%

Aquatic nonvascular 

plants

(listed effects, liquid 

spray, ground, 

broadcast)

6 26 4%

4 18 6%

Aquatic vascular 

plants 

(non-listed effects, 

granules, ground, 

broadcast)

6 8 13%

4 5 20%

Aquatic vascular 

plants 

6 20 5%

4 13 8%
Page 78 of 121 

Table 4.18 Area Percentage of Spot Treatment Uses (Spray or Granules) Needed to Exceed 

the LOC 

Taxa Application 

rates 

(lb a.i./A) 

RQs 

assuming 

100% area 

treated 

(highest to 

lowest)* 

LOC Area Percentage 

of application 

needed to exceed 

LOC1

(listed effects, 

granules, ground, 

broadcast)

Aquatic nonvascular 

plants

(non-listed effects, 

granules, ground, 

broadcast)

6 20 5%

4 13 8%

Aquatic nonvascular 

plants

(listed effects, 

granules, ground, 

broadcast)

6 26 4%

4 17 6%

*Bold values indicate LOC exceedance 

1 The threshold percentage is assessed to determine how much percent of exposure that RQ needs to exceed the LOC. 

For example, the highest acute RQ for birds inhabiting a field treated at 6 lb a.i./A is 0.98, which is up to 10x the 

listed species LOC of 0.1; to assess the % of exposure needed at which the LOC exceedance occurred, results 

indicate 10% is needed for the RQ to exceed the LOC. A “no effect” or “may affect” determination was not made 

since it is unknown what % of an actual area is typical spot treatment. 

5. Federally Threatened and Endangered (Listed) Species Concerns 

Section 7 of the Endangered Species Act, 16 U.S.C. Section 1536(a)(2), requires all federal 

agencies to consult with the National Marine Fisheries Service (NMFS) for marine and 

anadromous listed species, and/or the United States Fish and Wildlife Service (USFWS) for listed 

wildlife and freshwater organisms, if they are proposing an "action" that may affect listed species 

or their designated critical habitat. Each federal agency is required under the Act to ensure that 

any action they authorize, fund, or carry out is not likely to jeopardize the continued existence of 

a listed species or result in the destruction or adverse modification of designated critical habitat. 

To jeopardize the continued existence of a listed species means "to engage in an action that 

reasonably would be expected, directly or indirectly, to reduce appreciably the likelihood of both 

the survival and recovery of a listed species in the wild by reducing the reproduction, numbers, or 

distribution of the species" (50 C.F.R. § 402.02). 

To facilitate compliance with the requirements of the Endangered Species Act (subsection (a)(2)), 

the Office of Pesticide Programs has established procedures to evaluate whether a proposed 

registration action may directly or indirectly appreciably reduce the likelihood of both the survival 

and recovery of a listed species in the wild by reducing the reproduction, numbers, or distribution 
Page 79 of 121 

of any listed species (USEPA, 2004). After the Agency’s screening level risk assessment is 

conducted, if any of the Agency’s listed species’ LOCs are exceeded for either direct or indirect 

effects, an analysis is conducted to determine if any listed or candidate species may co-occur in 

the area of the proposed pesticide use or areas downstream or downwind that could be 

contaminated from drift or runoff/erosion. If listed or candidate species may be present in the 

proposed action area, further biological assessment is undertaken. The extent to which listed 

species may be at risk is considered, which then determines the need for the development of a 

more comprehensive consultation package, as required by the Endangered Species Act. 

The federal action addressed herein is the proposed renewed registration of tebuthiuron on 

agricultural use sites. It is expected that its use could occur nationwide. 

5.1. Action Area 

For listed species assessment purposes, the action area is considered to be the area affected directly 

or indirectly by tebuthiuron use and not merely the immediate area where tebuthiuron is applied. 

At the initial baseline, the risk assessment considers broadly described taxonomic groups and 

conservatively assumes that listed species within those broad groups are co-located with the 

pesticide treatment area. This means that listed terrestrial plants and wildlife are assumed to be 

located on or adjacent to the treated site and listed 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. 

 5.2. Taxonomic Groups Potentially at Risk 

 

If the assumptions associated with the baseline 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 on listed species 

that depend upon the taxonomic group for which the RQ was calculated. 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 are considered to determine the extent to which screening assumptions regarding an action 

area apply to a particular listed organism. These subsequent refinement steps will consider how 

this information would impact the action area for a particular listed organism and potentially 

include areas of exposure that are downwind and downstream of the pesticide use site. 

Assessment endpoints, exposure pathways, and the conceptual models addressing tebuthiuron 

uses, and the associated exposure and effects analyses conducted for the tebuthiuron baseline risk 

assessment are in Sections 2 to 3. The assessment endpoints used in the baseline risk assessment 

include those defined operationally as reduced survival and reproductive impairment for both 
Page 80 of 121 

aquatic and terrestrial animal species and survival, reproduction, and growth of aquatic and 

terrestrial plant species from both direct acute and chronic exposures. These assessment endpoints 

are intended to address the standard set forth in the Endangered Species Act requiring federal 

agencies to ensure that any action they authorize does not appreciably reduce the likelihood of 

both the survival and recovery of a listed species in the wild by reducing the reproduction, 

numbers, or distribution of the species. Risk estimates (RQs) which, integrating exposure and 

effects, are calculated for broad based taxonomic groups in the screening-level risk assessment 

presented in Section 4.

Both acute endangered species and chronic risk LOCs are considered in the baseline risk 

assessment to identify direct and indirect effects to taxa of listed species. This section identifies 

direct effect concerns, by taxa that are triggered by exceeding endangered LOCs in the baseline 

risk assessment, with an evaluation of the potential probability of individual effects for exposures 

that may occur at the established endangered species LOC. Data on exposure and effects collected 

under field and laboratory conditions are evaluated to make determinations on the predictive utility 

of the direct effect screening assessment findings to listed species. Potential for direct effects to 

listed taxa are described in Table 5.1. 

Table 5.1. Potential Effects to Federally-Listed Taxa Associated with Direct Effects from the 

Registered Uses of Tebuthiuron. 

Listed Plant Taxon Potential Direct Effects 

Terrestrial and semi-aquatic plants – 

monocots and dicots Yes* 

Aquatic nonvascular plants Yes* 

Aquatic vascular plants Yes* 

Listed Animal Taxon Potential Direct Effects 

Acute Chronic 

Terrestrial invertebrates Uncertain1 N/A2

Mammals Yes* Yes*

Birds 

Yes* for small-sized classes; 

No effect for mid- and largesized classesYes* (all size classes)

Reptiles3 Yes* Yes* 

Terrestrial-phase amphibians3 Yes* Yes* 

Freshwater fish No Effect No Effect 

Aquatic-phase amphibians4 No Effect No Effect 

Freshwater invertebrates No Effect No Effect

Estuarine/marine fish No Effect No Effect 

Estuarine/marine invertebrates No Effect No Effect 
Page 81 of 121 

6. Literature Cited 

Tomlin CDS, ed. Tebuthiuron (34014-18-1). In: The e-Pesticide Manual, 13th Edition Version 

3.1 (2004-05). Surrey UK, British Crop Protection Council.

USEPA, 2009. Registration Review -Preliminary Problem Formulation for Ecological Risk 

 and Environmental Fate, Endangered species and Drinking Water Assessments for 

 Tebuthiuron (PC Code 105501; DP Barcode D363982). 

USEPA, 2004. Tier II Aquatic Exposure Assessment for Selected Tebuthiuron Uses on 

 Rangeland, Pastureland, and Other Non-Crop Lands in the Pacific Northwest: 

 Endangered Species (ES) Consultation Package. 

USEPA, 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. Office of Prevention, Pesticides, and Toxic Substances. Office of Pesticide 

Programs. Washington, D.C. January 23, 2004. 

USEPA. 2013. Pesticides: Science and Policy. Terrestrial Models, T-REX Version 1.5.2. 

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

USEPA. 2013a. Pesticides: Science and Policy. Terrestrial Models, TerrPlant Version 1.2.2. 

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

Zavaleta, J. 2012. Community Response to use of Prescribed Grazing and Tebuthiuron Herbicide 

for Restoration of Sand Shinnery Oak Communities. M.S. Thesis. Texas Tech University. 

Doerr, T. (1980). Effects of tebuthiuron on lesser prairie chicken habitat and food supplies. 

M.S. Thesis, Texas Tech University, Lubbock. 

Doerr, T., & Guthery, F. (1980). Effects of shinnery oak control on lesser prairie chicken habitat. 

pp. 59-63. In: P. A. Vohs and F. L. Knopf, editors, Proceedings of the Prairie 

Grouse Symposium. Stillwater: Oklahoma State University. 89 pp. 

Doerr, T., & Guthery, F. (1983). Effects of tebuthiuron on lesser prairie chicken habitat and 

foods. Journal of Wildlife Management 47: 1138-1142 

Hagen, C. A, G. C. Salter, J. C. Pitman, R. J. Robel, and R. D. Applegate. 2005. Lesser prairiechicken brood habitat in sand sagebrush: invertebrate biomass and vegetation. 

Wildlife Society Bulletin 33(3): 1080-1091. 

Jamison, B. E., R. J. Robel, J. S. Pontius, and R. D. Applegate. 2002. Invertebrate biomass: 

associations with lesser prairie-chicken habitat use and sand sagebrush density in 

southwestern Kansas. Wildlife Society Bulletin 30: 517-526 
Page 82 of 121 

Submitted Ecological Effects Studies 

MRID 00041679 

Todd, G.C.; Kehr, C.C.; West, H.C.; et al. (1972) The Acute Toxicity of EL-103 in Mice, Rats, 

Rabbits, Cats, Dogs, Quail, Ducks, Chickens, and Fish. Unpublished study prepared by Lilly 

Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 00020661 

Todd, G.C.; Kehr, C.C.; West, H.C.; et al. (1972) The Acute Toxicity of EL-103 in Mice, Rats, 

Rabbits, Cats, Dogs, Quail, Ducks, Chickens, and Fish. Unpublished study prepared by Lilly 

Research Laboratories in cooperation with Bionomics, Inc. Submitted by Elanco Products Co., 

Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00041692 

Kehr, C.C.; Hamelink, J.L.; Todd, G.C.; et al. (1978) The Acute Oral Toxicity of Compound 

75503 (EL-103) in Mallard Ducks: Study 7021-77. Unpublished study prepared by Lilly 

Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 00041680 

West, H.C.; Hamelink, J.L. (1976) The Acute Toxicity of Compound 75503 (EL-103) in 

Mallards (Anas platyrhynchos): Eight Day Dietary Study 9007-6. Unpublished study prepared by 

Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 00041681 

West, H.C.; Hamelink, J.L. (1976) The Acute Toxicity of Compound 75503 (EL-103) in 

Bobwhites (Colinus virginianus): Eight Day Dietary Study 9012-6. Unpublished study prepared 

by Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 00041693 

Kehr, C.C.; Brannon, D.R.; Todd, G.C.; et al. (1978) Addendum to Section J: Fish and Wildlife 

Safety: EPA Submission (EL-103): Pasture and Rangeland. Unpublished study prepared by Lilly 

Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 40601001 

Negilski, D.; Grothe, D.; Meyerhoff, R. (1988) The Toxicity of Tebuthiuron to Juvenile 

Bobwhite in a Five-day Dietary Study. Laboratory Project ID A00188. Unpublished study 

prepared by Lilly Research Laboratories. Submitted by Elanco Products Company, Div. of Eli 

Lilly and Co., Indianapolis, Ind. 

MRID 40601002 
Page 83 of 121 

Negilski, D.; Grothe, D.; Meyerhoff, R. (1988) The Toxicity of Tebuthiuron to Juvenile Mallards 

in a Five-day Dietary Study. Laboratory Project ID A00288. Unpublished study prepared by 

Lilly Research Laboratories. Submitted by Elanco Products Company, Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 48928201 

Hubbard, P.M, K.H. Martin, and J.B. Beavers. 2012. Tebuthiuron: An LD50/LC50 Study Using 

Dietary Exposure with the Zebra Finch (Taeniopygia guttata). Unpublished study performed by 

Wildlife International, Ltd., Easton, MD. Laboratory Project No. 379-264. Study sponsored by 

Dow Chemical Company, Midland, MI for Dow AgroSciences, Indianapolis, IN. Study initiated 

July 27, 2012 and completed August 31, 2012. 

MRID 00093690 

Fink, R.; Beavers, J.B.; Brown, R. (1978) Final Report: One-generation Reproduction Study with 

Mallard Duck: Compound 75503, Tebuthiuron. Laboratory Project No. 151-105. Unpublished 

study prepared by Wildlife International, Ltd., Easton, MD. Submitted by Elanco Products Co., 

Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00104243 

Fink, R.; Beavers, J.B.; Brown, R. (1978) Final Report: One-generation Reproduction Study with 

Bobwhite Quail: Compound 75503, Tebuthiuron. Laboratory Project No. 151-104. Unpublished 

study prepared by Wildlife International, Ltd., Easton, MD. Submitted by Elanco Products Co., 

Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00041685 

Hamelink, J.L.; Kehr, C.C. (1976) The Acute Static Toxicity of Two Formulations of Compound 

75503, EL-103, to Fathead Minnows (Studies no. 1012-6, 1013-6, 1014-6). Unpublished study 

prepared by Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly 

and Co., Indianapolis, Ind. 

MRID 00041683 

Hamelink, J.L.; Froman, R.O. (1976) An Acute Study of EL-103 to Daphnia magna Expressed as 

Tolerance Limit (TL50). Unpublished study prepared by Lilly Research Laboratories. Submitted 

by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00041694 

Hamelink, J.L.; Todd, G.C.; Brannon, D.R.; et al. (1978) Acute Toxicity of Compound 75503 

(EL-103) to Daphnia magna. Study No. 5058-77. Unpublished study prepared by Lilly Research 

Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00041684 

Heitmuller, T.; Parrish, R. (1976) Acute Toxicity of EL-103 to Embryos of Eastern Oysters 

(Crassostrea virginica), to Pink Shrimp (Penaeus duorarum), and to Fiddler Crabs (Uca 

pugilator). Unpublished study prepared by Lilly Research Laboratories. Submitted by Elanco 

Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind. 
Page 84 of 121 

MRID 48722701 

York, D. (2011) Tebuthiuron TGAI - Acute Toxicity to Eastern Oyster (Crassostrea virginica) 

under Flow-Through Conditions, Following OPPTS Guideline (Draft) 850.1025. Project 

Number: 12550/6630, 110205. Unpublished study prepared by Smithers Viscient Laboratories, 

Wareham, MA. Study sponsored by Dow Chemical Company, Midland, MI for Dow 

AgroSciences, Indianapolis, IN. 

MRID 48722702 

Fournier, A.E. 2011. Tebuthiuron - Acute Toxicity to Sheepshead Minnow (Cyprinodon 

variegatus) Under Static-Renewal Conditions, Following OPPTS Draft Guideline 850.1075. An 

unpublished study performed by Smithers Viscient, Wareham, MA. Laboratory report number: 

12550.6629. Study sponsored by Dow Chemical Company, Midland, MI for Dow AgroSciences, 

Indianapolis, IN. Sponsor report number: 110204. Study initiated March 18, 2011 and 

completed on November 28, 2011. 

MRID 00020735 

Hamelink, J.L.; Jordan, W.H.; Kitchen, D.N.; et al. (1978) Final Data Report: Aquatic MiniEcosystem Evaluation of EL-103 (Compound 75503, Tebuthiuron). Unpublished study prepared 

by Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 00041682 

Hamelink, J.L.; Kehr, C.C. (1976) The Acute Toxicity of Compound 75503 (EL-103), Lot B30-

23-149, to Rainbow Trout (Salmo gairdneri) and Bluegills (Lepomis macrochirus). Unpublished 

study prepared by Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli 

Lilly and Co., Indianapolis, Ind. 

MRID 00090083 

Sauter, S.; Meyerhoff, R.D.; Todd, G.C.; et al. (1981) The Toxicity of Tebuthiuron (EL-103, 

Compound 75503) in Water to Rainbow Trout (Salmo gairdneri) in a 45-day Embryo-larvae 

Study. Study No. F14580. Unpublished study prepared by Lilly Research Laboratories. 

Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00090084 

Sauter, S.; Meyerhoff, R.D.; Todd, G.C.; et al. (1981) The Toxicity of Tebuthiuron (EL-103, 

Compound 75503) in Water to Fathead Minnows (Pimephales promelas) in a 33-day Embryolarvae Study. Study No. F08381. Unpublished study prepared by Lilly Research Laboratories. 

Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00138700 

Grothe, D.; Meyerhoff, R.; Todd, G.; et al. (1983) The Toxicity of Tebuthiuron (EL-103, 

Compound 75503) to Daphnia magna in a 21-day Static Renewal Full Life-Cycle Study. Study 

No. C02882. Unpublished study prepared by Lilly Research Laboratories. Submitted by Elanco 

Products Co., Div. of Eli Lilly and Co., Indianapolis, IN. 
Page 85 of 121 

MRID 00041686 

Hamelink, J.L.; Mohr, R.M.; Todd, G.C.; et al. (1977) Progress Report for EL-103 (Tebuthiuron) 

Mini-Ecosystem Study. Study No. 7001-6. Unpublished study prepared by Lilly Research 

Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, Ind, 

MRID 00041695 

Hamelink, J.L.; Todd, G.C.; Brannon, D.R.; et al. (1978) Progress Report Number Two: Aquatic 

Mini-Ecosystem Evaluation of EL-103. Study No. 7001-6. Unpublished study prepared by Lilly 

Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, Ind. 

MRID 41066901 

Waldrep, T. (1988) Influence of Tebuthiuron on Seedling Emergence and Vegetative Vigor of 

Ten Crop Plants. Laboratory Project ID 61988008. Unpublished study prepared by Lilly 

Research Laboratories. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., 

Indianapolis, IN. 

MRID 41066902 

Waldrep, T. (1988) Influence of Tebuthiuron on the Germination of Seeds of Ten Crop Plants. 

Laboratory Project ID 61988007. Unpublished study prepared by Lilly Research Laboratories. 

Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, IN. 

MRID 48722703 

Bergfield, A. 2012. Spike 20P (Tebuthiuron 200 g a.s./kg WG): Effects on the Seedling 

Emergence and Growth of Non-Target Terrestrial Plants (Tier II). Unpublished study performed 

by ABC Laboratories, Inc., Columbia, Missouri. ABC Study Number: 67305. Study sponsored 

by The Dow Chemical Company, Midland, Michigan, for Dow AgroSciences, LLC, 

Indianapolis, Indiana. Study initiated October 10, 2011 and completed January 16, 2012. 

MRID 48722704 

Bergfield, A. 2012. Spike 20P (Tebuthiuron 200 g a.s./kg WG): Effects on the Vegetative Vigor 

of Non-Target Terrestrial Plants (Tier II). Unpublished study performed by ABC Laboratories, 

Inc., Columbia, Missouri. ABC Study Number: 67306. Study sponsored by The Dow Chemical 

Company, Midland, Michigan, for Dow AgroSciences, LLC, Indianapolis, Indiana. Study 

initiated October 10, 2011 and completed January 16, 2012. 

MRID 41080401 

Negilski D.S., Grothe D.W., and Cocke P.J. 1989. Toxicity of Tebuthiuron to the Blue-Green 

Alga (Anabaena flos-aquae) in a Static Test System. Unpublished study performed by Lilly 

Research Laboratories, Greenfield, IN. Laboratory Report No. J00489. Sponsored by Elanco 

Products Company. Study initiated on January 5, 1989 and completed on March 31, 1989. 

MRID 41080402 
Page 86 of 121 

Negilski, D.S. and P.J. Cocke. 1989. Toxicity of Tebuthiuron to Marine Diatom (Skeletonema 

costatum) in a Static Test System. Conducted by Lilly Research laboratories, Greenfield, 

Indiana. Laboratory Project No. J00389. Sponsored by Elanco Products Company. Study 

initiated on January 6, 1989 and completed on April 6, 1989. 

MRID 41080403 

Negilski D.S. and Cocke P.J. 1989. Toxicity of Tebuthiuron to a Freshwater Diatom (Navicula 

pelliculosa) in a Static Test System. Unpublished study performed by Lilly Research 

Laboratories, Greenfield, IN. Laboratory Report No. J00888. Sponsored by Elanco Products 

Company. Study initiated on November 23, 1988 and completed on April 7, 1989. 

MRID 41080404 

Negilski, D.S. and P.J. Cocke. 1989. Toxicity of Tebuthiuron to Duckweed (Lemna gibba) in a 

Static Renewal Test System. Conducted by Lilly Research laboratories, Greenfield, Indiana. 

Laboratory Project No. J00588. Sponsored by Elanco Products Company. Study initiated on 

October 26, 1988 and completed on April 20, 1989. 

MRID 00138697 

Todd, G.; Meyerhoff, R.; Grothe, D.; et al. (1983) The toxicity of Tebuthiuron (EL-103, 

Compound 75503) to Algae (Selenastrum capricornutum) in a 14-Day Study. Laboratory Project 

No. J00383. Unpublished study prepared by Lilly Research Laboratories. Submitted by Elanco 

Products Co., Div. of Eli Lilly and Co., Indianapolis, IN. 

MRID 40840401 

Hoxter, K.; Jaber, M. (1988) The Acute Contact Toxicity of Tebuthiuron to the Honey Bee: 

Wildlife International Ltd. Project No.: 151-108. Unpublished study prepared by Wildlife 

International Ltd. Submitted by Elanco Products Co., Div. of Eli Lilly and Co., Indianapolis, IN. 

MRID 00090099 

Kline, R.M.; Preston, D.A.; Turner, J.R. (1977) The Effect of Tebuthiuron on Microorganisms. 

Unpublished study prepared by Lilly Research Laboratories. Submitted by Elanco Products Co., 

Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00090100 

Kline, R.M.; Peloso, J.S. (1978) Effect of Tebuthiuron on Microorganisms: Nitrogen Fixation. 

Unpublished study prepared by Lilly Research Laboratories. Submitted by Elanco Products Co., 

Div. of Eli Lilly and Co., Indianapolis, Ind. 

MRID 00138699 

Grothe, D.; Todd, G.; Kehr, C.; et al. (1982) The Toxicity of Soil Incorporated Tebuthiuron (EL103, Compound 75503) to Earthworms in a 14-Day Study. Study No. W00682. Unpublished 

study prepared by Lilly Research Laboratories. Submitted by Elanco Products Co., Div. of Eli 

Lilly and Co., Indianapolis, IN. 

MRID 43329501 
Page 87 of 121 

Crosswhite, F.; Feldman, W.; Minch, E. (1993) Impact of Pesticides on Cacti: Lab Project 

Number: E-009572-01-0. Unpublished study prepared by University of Arizona and Arizona 

Dept. of Agriculture. 

MRID 47736301 

Havens, P. (2009) Tebuthiuron - Use of Bureau of Land Management Assessments to Fulfill 

ESA Consultation Requirements. Project Number: 091092. Unpublished study prepared by Dow 

AgroSciences LLC. 

MRID 00034756 

Davis, E.A., A. Loh, W.L. Sullivan (1980) April 17, 1980. Tebuthiuron Residues in Stream 

Water Following the Spot Treatment of Chaparral Watershed in Arizona. Prepared by Lilly 

Research Laboratories, Greenfield, Indiana 46140. Submitted by Elanco Products Co., Div. of 

Eli Lilly and Co., Indianapolis, Ind. 

MRID 00049579 

Elanco Products Company (1980) Proposed Field Monitoring Study with Graslan for Rangeland 

Brush Control. (Unpublished Study Received Nov. 6, 1980 under 1471-109; CDL: 243738-A). 

MRID 00090105 

Loh, A., G.R. Stephenson, M.D. Hammond, B.J. Eaton (1981) September 1981. Sagebrush to 

Grass Conversion in Southwest Idaho Wateshed Study with Tebuthiuron (Experiment MDH80-

0). Prepared by Lilly Research Labs., Greensfield, IN. Submitted by Elanco Products Co., Div. 

of Eli Lilly and co., Indianapolis, IN. 
Page 88 of 121 

Appendix A 

Environmental Fate Studies Summary 

Hydrolysis (MRID 00020779) 

14C tebuthiuron (radiochemical purity~ 99.5%), at 10 and 100 ppm, did not degradf during 64 

days of incubation in sterile aqueous solutions at pH 3, 6, and 9 in the dark at 25 °C. 

Photodegradatoin in water (MRID 41328001)

Tebuthiuron did not photodegrade in sterile aqueous buffered (pH 5) solutions that were 

Continuously irradiated for 33 days With a xenon light source at approximately 25 °C. 

Tebuthiuron was the only compound identified in the irradiated and dark control solutions at 

all sampling intervals. At 33 days posttreatment, tebuthiuron comprised 96.3-97. % and 

99.7-100% of the recovered radioactivity in the irradiated and dark control solutions, 

respectively. The major products of photodegradation were soil bound residues and five minor 

products (each less than 7% of applied radioactivity ). 

Photodegradatoin in soil (MRID 41050201)

Asieved (2mm) sandy loam soil (60% sand, 22 % silt , 12 % clay , 1.4% organic matter , 11.86% 

field moisture capacity at 0.33 bar , 5.7 pH, 4.9 meq/100g CEC and 1.28 g/ml bulk density) was 

used in this study . Thin layers of soil in petri dishes were fortified with 28.0 ppm of thiadiazol 

ring - labeled 14C-tebuthiuron ( specificactivity , 17.1 uCi/mg and 97.2% 

radio purity) and non radio labeled tebuthiuron at a rate o f 588.5 ppm. tebuthiuron photolyzed on 

soil with a half-life of 39.7 days. 

I 

Aerobic soil metabolism (MRID 41328001)

In a 9-month study, thiadiazole-labeled 14C tebuthiuron, at a concentration of 6 ppm in sandy 

loam soil incubated in darkness at 24 °C and 75% field moisture capacity, degraded with!a 

half-life (calculated by the registrant) of 35.4 months. The degradates identified by twodimensional TLC were N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (degradate 

104), N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N'-methylurea (compound 105), 5-(1,1-

dimethylethyl)-2 methylamino-1,3,4-thiadiazol (compound 107), and 5-(1,1- dimethylethyl)-2 

amino-1,3,4-thiadiazol (compound 108). The concentration of degradate 104, which accounted for 

6.9% of the applied radioactivity after 9 months of incubation, appeared to be increasing at the end 

of the experiment. 

Anaerobic soil metabolism (MRID41328002)

Following 30 days of aerobic incubation at 24 ± 1 °C and 75% of 0.33 bar moisture capacity 
Page 89 of 121 

and 60 days under flooded conditions in a sandy loam soil, thiadiazole-labeled 14C tebuthiuron 

(nominal concentration 6 ppm) exhibited very little metabolism. After 60 days of anaerobic 

incubation, the concentration of parent tebuthiuron had decreased 4.7% from the concentration at 

initiation of flooding. Degradates identified were N-[5-:(1,1-dimethylethyl)- 1,3,4-thiadiazol-2-

yl]-N-methylurea (compound·104), N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N'-

methylurea (compound 105), and N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N'-

(hydroxymethyl)-N-methylurea (compound 109). 

Anaerobic aquatic metabolism (MRID 41913101)

Tebuthiuron degrad~d with a half-life (calculated by the registrant) of> 1 year in anaerobic 

system containing pond water and sediment incubated for 365 days in darkness at 25.5 ± 0.8 

°C. During the study there was very little degradation oftebuthiuron, with 93.7% the applied 

radiocarbon remaining as parent material at day 365. Degradates were repqrted to comprise 

approximately 1.4% of the applied radioactivity at the termination of the study.

Aerobic aquatic metabolism (MRID 41372501) 

Tebuthiuron did not degrade appreciably in pond water and sediment that was incubated in 

darkness at 24 ± 1 °C fro 4 weeks under aerobic conditions. After 4 weeks of incubation, 95.2% 

of the applied radioactivity was present in parent tebuthiuron. Degradates identified by twodimensional TLC were N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (compound 

1 04), N-[5-(1, 1-dimethylethyl)-1 ,3,4-thiadiazol-2-yl]-N' -methyl urea (compound 

105), 5-(1,1-dimethylethyl)-2 methylamino-1,3,4-thiadiazol (compound 107), 5-(1,;1- 

Dimethylethyl)-2-amino-1,3,4-thiadiazol (compound 108), and N-[5-(1-1, dimethylethyl)-1,3,4-

thiadiazol-2-yl]-N’-(hydroxymethyl)-N-methylurea (compound 109).

Leaching, Adsorption/Desorption (MRID 40768401) 

Tebuthiuron is mobile in all 4 soils and has a Kads for sand, sandy loam, loam, boe!dmbz loam 

of 0.11, 0.62, 0.82, and 1.82, respectively. The corresponding Koc values are 38, 716! 75, and 

152, respectively. 

Bioaccumulation in fish (MRID 40819501)

In a 28-day flow-through study in which bluegill sunfish were exposed to a nominal tebuthiuron 

concentratio11 of 5.0 ppm, bioconcentration factors of 1.98, 3.40, and 2.63 were reported for 

edible tissue, nonedible tissue, and whole fish, respectively. Residues in the tissues consisted 

primarily of tebu thiuron and two metabolites N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N'-

(hydroxymethyl)-N-methylurea (compound 109) and compound 103(OH) (hydroxylated form of 

parent tebuthiuron). Accumulated residues depurated rapidly from fish tissue with depuration halflives of0.33 and 0.51 days reported for edible and lqnedible tissue, respectively. Based on the 
Page 90 of 121 

reported octanollwater coefficient (log Kow=1.79), there is slight potential for tebuthiuron 

residues to accumulate in fish.

Terrestrial Field Dissipation (MRID 43318101) 

Tebuthiuron was broadcast applied once 'as a spray at nominal application rates of 6.6 lb ai/ A 

on bareground plots of Lakeland fine sand (Florida), Watsonville sandy loam (California), and 

Sharpsburg silty clay loam (Nebraska). EFED calculated soil dissipation half-lives for Florida 

Of 385 days(r2 = 0.66) for the total parent (summed across 0 to 72 inches) and 123 days (r2 = 

0.92) for the surface layer (0 to 6 inches), for California of 770 days (r2 = 0.89) for the total 

parent (summed across 0 to 72 inches) and 462 days (r2

 = 0.97) for the surface layer (0 to 6 inches), 

and for Nebraska of 575 days (r2 = 0.89) for the total parent (summed across 0 to 72 

inches) and 433 days (r2 = 0.91) for the surface layer (0 to 6 inches). The registrant calculated 

halflives (total parent profile) of383 days for Florida, 806 days for California, and 575 days 

for Nebraska.

Tebuthiurori was detected in the deepest soil sample (60 to 72 inches) at the Florida site 

indicating that migration to ground water is a concern. · Tebuthiuron was not detected below 

24 to 30 inches at the California site or below 30 to 36 inches at the Nebraska site, although 

these depths indicate that tebuthiuron is leaching. The metabolite (Compound 109383) was 

detected at all three sites but was not observed to have migrated below 6 to 12 inches in 

Florida, 12 to 18 inches in California, and 0 to 6 inches in Nebraska. The registrant did not 

calculate the half life for the metabolite. EFED estimated the half life for the metabolite from 

each study by performing linear regression on the decline pattern (as determined by peak 

concentration of metabolite). EFED calculated half lives for the total profile for Florida, 

California, and Nebraska, respectively were 81 days (r2 = 0.92), 495 days (r2 =0.96), and 385 

days (r2 = 0.57). 

Finally, it should be noted that evaporation exceeded water input at the Califonia and 

Nebraska sites and therefore the vertical movement (leaching potential) of tebuthiuron and its 

degradate may be underestimated at these two sites. The reported data indicate that tebuthiuron is 

mobile and persistent. Based on the data provided, leaching to ground water is likely to be a 

significant route of dissipation. 

Small Scale Retrospective Ground Water Monitoring Study (MRID 42390901) 

A small-scale retrospective ground water monitoring study was performed on a ran ch near 

Sarita, Texas, that has last been treated with tebuthiuron on March 24, 1986. The results of 

the study indicated that tebuthiuron was mobile enough to leach at least 15 feet to the water 

table, then still be present above minimum detection levels more than 4 years after the application. 

Soil sampling at and near the study site showed convincingly that tebuthiuron 

can persist at relatively high concentrations in soil and soil water if restrictive layer blocks 

leaching to the ground water.
Page 91 of 121 

Appendix B 

Chemical structures for the parent (tebuthiuron) and degradates 104, 106, 

108, and 109 

Tebuthiuron

 N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-z-yl]- 

 N,N-dimethylurea 

 

 
Page 92 of 121 

 

 
Page 93 of 121 

Appendix C 

PRZM-EXAMS output files 

Eco modeling 

outputs.zip
Page 94 of 121 

Appendix D 

Tebuthiuron pond accumulation with time 

TXalfalfaOP (aerial) 

SUMMARY OUTPUT                

          

Regression Statistics        

Multiple R 0.933674633              

R Square 0.871748319              

Adjusted R Square 0.867167902              

Standard Error 123.7752708              

Observations 30

           

ANOVA           

   df SS MS F

Significance

F    

Regression 1 2915774 2915774 190.32073 5.189E‐14      

Residual 28 428968.89 15320.318          

Total 29 3344742.9                

           

   Coefficients

Standard

Error t Stat P‐value Lower 95% Upper 95%

Lower

95.0%

Upper

95.0%

Intercept 210.068046 46.350473 4.5321661 9.954E‐05 115.12341 305.01269 115.12341 305.01269

X Variable 1 36.01862069 2.6108627 13.795678 5.189E‐14 30.670511 41.36673 30.670511 41.36673
Page 95 of 121 

SUMMARY OUTPUT              

                

Regression Statistics             

Multiple R 0.9337579              

R Square 0.8719039              

Adjusted R

Square 0.867329              

Standard Error 123.20537              

Observations 30

                

ANOVA                

   df SS MS F

Significance

F    

Regression 1 2893009.8 2893009.8 190.5858 5.101E‐14      

Residual 28 425027.79 15179.564          

Total 29 3318037.6                

                

   Coefficients

Standard

Error t Stat P‐value Lower 95% Upper 95%

Lower

95.0%

Upper

95.0%

Intercept 209.06 46.137062 4.5312811 9.98E‐05 114.55251 303.56749 114.55251 303.56749

X Variable 1 35.877742 2.5988415 13.805283 5.1E‐14 30.554256 41.201227 30.554256 41.201227
Page 96 of 121 

SUMMARY OUTPUT          

              

Regression Statistics           

Multiple R 0.9342              

R Square 0.8727              

Adjusted R Square 0.8681              

Standard Error 122.3              

Observations 30

              

ANOVA              

   df SS MS F

Significance

F     

Regression 1 2869847 2869847 191.8721 4.7E‐14      

Residual 28 418798.3 14957.08          

Total 29 3288645                

              

   Coefficients Standard Error t Stat P‐value Lower 95%

Upper

95%

Lower

95.0%

Upper

95.0%

Intercept 205.85 45.79771 4.494775 0.00011 112.038 299.6627 112.038 299.6627

X Variable 1 35.734 2.579726 13.85179 4.7E‐14 30.44949 41.01815 30.44949 41.01815
Page 97 of 121 

PAalfalfaOP (aerial) 

SUMMARY OUTPUT              

                

Regression Statistics             

Multiple R 0.9745036            

R Square 0.9496573              

Adjusted R

Square 0.9478594              

Standard Error 48.564216              

Observations 30

                

ANOVA                

   df SS MS F

Significance

F    

Regression 1 1245722.9 1245722.9 528.1882 1.028E‐19      

Residual 28 66037.525 2358.483          

Total 29 1311760.5                

                

   Coefficients

Standard

Error t Stat P‐value Lower 95% Upper 95%

Lower

95.0%

Upper

95.0%

Intercept 80.422943 18.185978 4.42225 0.000134 43.170655 117.67523 43.170655 117.67523

X Variable 1 23.54295 1.0243928 22.982346 1.03E‐19 21.444576 25.641323 21.444576 25.641323
Page 98 of 121 

SUMMARY OUTPUT              

                

Regression Statistics             

Multiple R 0.9748338              

R Square 0.9503009              

Adjusted R

Square 0.9485259              

Standard Error 1.997309              

Observations 30

                

ANOVA                

   df SS MS F

Significance

F    

Regression 1 2135.8012 2135.8012 535.3901 8.584E‐20      

Residual 28 111.69881 3.9892432          

Total 29 2247.5                

                

   Coefficients

Standard

Error t Stat P‐value Lower 95% Upper 95%

Lower

95.0%

Upper

95.0%

Intercept 1957.5453 0.8573801 2283.1707 2.49E‐75 1955.789 1959.3015 1955.789 1959.3015

X Variable 1 0.0403955 0.0017458 23.138498 8.58E‐20 0.0368194 0.0439716 0.0368194 0.0439716
Page 99 of 121 

SUMMARY OUTPUT              

                

Regression Statistics             

Multiple R 0.9751028              

R Square 0.9508254              

Adjusted R

Square 0.9490692              

Standard Error 1.9867413              

Observations 30

                

ANOVA                

   df SS MS F

Significance

F    

Regression 1 2136.9801 2136.9801 541.3995 7.397E‐20      

Residual 28 110.51994 3.9471408          

Total 29 2247.5                

                

   Coefficients

Standard

Error t Stat P‐value Lower 95% Upper 95%

Lower

95.0%

Upper

95.0%

Intercept 1957.5685 0.8517472 2298.2975 2.07E‐75 1955.8238 1959.3132 1955.8238 1959.3132

X Variable 1 0.040447 0.001738 23.26799 7.4E‐20 0.036886 0.044008 0.036886 0.044008

 
Page 100 of 121 

Appendix E 

STIR Results for Tebuthiuron 

Welcome to the EFED 

Screening Tool for Inhalation Risk

This tool is designed to provide the risk assessor with a rapid method for determining the potential

significance of the inhalation exposure route to birds and mammals in a risk assessment.

Input

Application and Chemical Information

Enter Chemical Name Teb

Enter Chemical Use Rangeland

Is the Application a Spray? (enter y or n) y

If Spray What Type (enter ground or air) air

Enter Chemical Molecular Weight (g/mole) 228.3

Enter Chemical Vapor Pressure (mmHg) 3.00E-07

Enter Application Rate (lb a.i./acre) 6

Toxicity Properties

Bird

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

Enter Mineau Scaling Factor 1.15

Enter Tested Bird Weight (kg) 1.58

Mammal

Enter Lowest Rat Oral LD50 (mg/kg bw) 388

Enter Lowest Rat Inhalation LC50 (mg/L) 3.696

Duration of Rat Inhalation Study (hrs) 4

Enter Rat Weight (kg) 0.35

Output

Results Avian (0.020 kg )

Maximum Vapor Concentration in Air at Saturation (mg/m3) 3.69E-03

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 4.63E-04

Adjusted Inhalation LD50 7.66E+01

Ratio of Vapor Dose to Adjusted Inhalation LD50 6.05E-06 Exposure not Likely Significant

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 5.76E-01

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 7.53E-03 Exposure not Likely Significant

Results Mammalian (0.015 kg )

Maximum Vapor Concentration in Air at Saturation (mg/m3) 3.69E-03

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 5.82E-04

Adjusted Inhalation LD50 2.20E+02

Ratio of Vapor Dose to Adjusted Inhalation LD50 2.65E-06 Exposure not Likely Significant

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 7.25E-01

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 3.29E-03 Exposure not Likely Significant

**NOTE**: When entering values, press

order to update linked cells.
Page 101 of 121 

Appendix F 

SIP Results for Tebuthiuron 
Page 102 of 121 

Table 1. Inputs

Parameter Value

Chemical name Teb

Solubility (in water at 25o

C; mg/L) 2500

Mammalian LD50 (mg/kg-bw) 388

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian species

Mammalian NOAEL (mg/kg-bw) 20

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian species

Avian LD50 (mg/kg-bw) >2000

Avian test species mallard duck

Body weight (g) of "other" avian species

Mineau scaling factor 1.15

Mallard NOAEC (mg/kg-diet) 100

Bobwhite quail NOAEC (mg/kg-diet) 100

NOAEC (mg/kg-diet) for other bird species

Body weight (g) of other avian species

NOAEC (mg/kg-diet) for 2nd other bird species

Body weight (g) of 2nd other avian species

Table 2. Mammalian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 430.0000 430.0000

Adjusted toxicity value (mg/kg-bw) 298.4343 15.3832

Ratio of exposure to toxicity 1.4409 27.9526

Conclusion*

Exposure through drinking 

water alone is a potential 

concern for mammals

Exposure through drinking 

water alone is a potential 

concern for mammals

Table 3. Avian Results

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 2025.0000 2025.0000

Adjusted toxicity value (mg/kg-bw) #VALUE! 4.9613

Ratio of exposure to acute toxicity #VALUE! 408.1627

Conclusion* #VALUE!

Exposure through drinking 

water alone is a potential 

concern for birds

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, 

when aggregated with other routes (i.e ., diet, inhalation, dermal), pesticide exposure through drinking water may 

contribute to a total exposure t
Page 103 of 121 

Appendix G 

Tebuthiuron Ecological Effects Summaries 

OCSPP 850.1010 – Daphnia Acute Toxicity Test 

MRID 00041694

The acute toxicity of tebuthiuron to 10 hours old first instar daphnia was assessed for 48 hours. 

Nominal concentrations of the test material were set at 0 (control), 225, 300, and 400 mg a.i./L 

with 9, 10, 11 daphnia placed in each test vessel. All treatment and control groups were 

triplicated. There were 6, 50, and 97% mortality in the 225, 300, and 400 mg a.i./L treatment 

groups, respectively. Affected daphnia were reported light colored and lethargic swimming 

behavior followed by prostration, immobilization and death. The 48-hour EC50 (95% C.I.) was 

297 (279-316) mg a.i./L, which would classify tebuthiuron to be practically nontoxic to trout, in 

accordance with the toxicity classification of USEPA. 

The study is classified as scientifically sound and satisfies the guideline requirement for a 

freshwater invertebrate acute toxicity test with daphnia magna. The study is classified as 

acceptable. 

OCSPP 850.1025 – Estuarine/marine Mollusk Shell Deposition 

MRID 48722701 

The 96-hour acute toxicity of tebuthiuron to the Eastern oyster, Crassostrea virginica, was 

studied under flow-through conditions. Oysters were exposed to the test material at nominal 

concentrations of 0 (negative control), 6.3, 13, 25, 50, and 100 mg a.i./L. Mean-measured 

concentrations were <LOD, 6.5, 14, 26, 50, and 95 mg a.i./L. Mortality and shell deposition were 

observed daily. No mortality was observed in the dilution water control or treatment groups. 

After 96 hours, the reviewer-calculated mean percent inhibition in shell growth compared to 

negative control was 26, 16, 7, 21, and 23% in the 6.5, 14, 26, 50, and 95 mg a.i./L test 

concentrations, respectively. The 96-hour EC50 based on shell deposition and mean measured 

concentrations was >95 mg a.i./L, categorizing tebuthiuron as practically non-toxic to the 

Eastern oyster. 

The study is scientifically sound and satisfies the guideline requirements for an acute 

estuarine/marine invertebrate toxicity study with the Eastern oyster (Crassostrea virginica). This 

study is classified as acceptable. 

OCSPP 850.1035 – Mysid Acute Toxicity Test 

MRID 00041684 
Page 104 of 121 

In a 96-hr acute toxicity study, Fiddler crab (Uca pugilator) a crustacean similar to the mysid 

shrimp were exposed to tebuthiuron at nominal concentrations of 0 (control), 10, 32, 100, 180, 

and 320 mg a.i./L. Mortality was 0, 0, 10, 0, 0, 20% in the 0 (control), 10, 32, 100, 180, and 320 

mg a.i./L treatment groups, respectively, at test termination. The 96-hour LC50 was >320 mg 

a.i./L, which would classify tebuthiuron to be practically nontoxic to the Fiddler crabs, in 

accordance with the toxicity classification of USEPA. 

This study is scientifically sound but does not satisfy the guideline requirement for a mysid 

shrimp acute toxicity study with Fiddler crab. This study is classified as supplemental since the 

fiddler crab is not a recommended test species. 

OCSPP 850.1045 – Penaeid Acute Toxicity Test 

MRID 00041684 

In a 96-hr acute toxicity study, pink shrimp (Penaeus duorarum) were exposed to tebuthiuron at 

nominal concentrations of 0 (control), 10, 32, 100, 180, and 320 mg a.i./L. Mortality in the 

highest concentration was 100% at 24 hours, 100% in the 2nd highest concentration at 48 hours, 

and 11% and 77% in the 32 and 100 mg a.i./L treatment groups, respectively, at test termination. 

The 96-hour LC50 with 95% confidence intervals was 62 (39-90) mg a.i./L, which would classify 

tebuthiuron to be slightly toxic to pink shrimp, in accordance with the toxicity classification of 

USEPA. 

This study is scientifically sound and satisfies the guideline requirement for a penaeid acute 

toxicity study with pink shrimp. This study is classified as acceptable. 

OCSPP 850.1055 – Bivalve Acute Toxicity Test (embryo larval) 

MRID 00041684 

The acute toxicity of tebuthiuron to normal Eastern oyster (Crassostrea virginica) embryos was 

assessed for 48 hours. Nominal concentrations of tebuthiuron were 0 (seawater control), 32, 56, 

100, 180, and 320 mg a.i./L. Three replicates per level. Each test replicate was inoculated with an 

estimated 27,000 ± 1350 embryos. The highest test concentration at 320 mg a.i./L nominal was 

the only level to experience mortality with 100% embryos dead. All other treatment groups 

embryos were normal at test termination. Since there were no concentration at which the percent 

reduction was between 100 and 0%, a definite LC50 could not be calculated. The 48-hour LC50 

for eastern oyster embryos is estimated to be >180 and < 320 mg a.i./L. With an LC50 greater 

than 100 mg a.i./L, tebuthiuron is classified as practically nontoxic to Eastern oyster embryos, in 

accordance with the toxicity classification of USEPA. 

This study is classified as scientifically sound and fulfills the guideline requirement for a bivalve 

(embryo larval) acute toxicity test with Eastern oyster. This study is classified as acceptable. 

OCSPP 850.1075 – Freshwater and Estuarine/marine Fish Acute Toxicity 

MRID 00020661 (Goldfish) 
Page 105 of 121 

In a 96-hr acute toxicity study, goldfish (Carassius auratus) were exposed to tebuthiuron at 

nominal concentrations of 0 (control), 5, 10, 20, 40, 80, and 160 mg a.i./L without a solvent in 

the first test and at nominal concentrations of 0 (control), 6.25, 12.5, 25, 50, and 100 mg a.i./L 

with the solvent added in the second test. Goldfish were also exposed to the solvent alone at 

nominal concentrations of 0.33, 0.67, and 1.33 ml/L. Ten goldfish per replicate with two, three, 

four, or 6 replicates, depending on which test was performed. One fish died throughout the tests. 

The 96-hour LC50 was >160 mg a.i./L, which would classify tebuthiuron to be practically 

nontoxic to goldfish, in accordance with the toxicity classification of USEPA. 

This study is scientifically sound however does not satisfy the guideline requirement for a 

freshwater fish acute toxicity study with Carassius auratus, due to several departures from 

guideline protocols. This study is classified as supplemental. 

MRID 00020661 (Rainbow trout) 

In a 96-hr acute toxicity study, rainbow trout (Oncorhynchus mykiss) were exposed to 

tebuthiuron at nominal concentrations of 0 (control), 1, 10, 50, 87, 120, and 160 mg a.i./L. Ten 

fish per test level. 10% mortality was observed in the highest level at 24 hours, which increased 

to 70% at 96 hours. Mortality was 20% in the 120 mg a.i./L test level at test termination. No fish 

died in the 87 mg a.i./L test level but dark coloration and lethargy in the fish were observed. The 

author reported the behavior of fish succumbing to the test material was generally similar to a 

syndrome characteristic of chemically poisoned fish. The gills of the trout at 160 mg a.i./L 

became irritated and inflamed exhibiting a red color although hemorrhaging was not prevalent. 

Moribund fish were then observed to become dark, lose equilibrium and then drop to the bottom 

of the test vessel where they eventually expired. The 96-hour LC50 (95% C.I.) was 143 (118-224) 

mg a.i./L, which would classify tebuthiuron to be practically nontoxic to trout, in accordance 

with the toxicity classification of USEPA. 

This study is scientifically sound and satisfies the guideline requirement for a freshwater fish 

acute toxicity study with Rainbow trout, a warmwater fish. This study is classified as acceptable. 

MRID 00020661 (Bluegill sunfish) 

In a 96-hr acute toxicity study, bluegill sunfish (Lepomis macrochirus) were exposed to 

tebuthiuron at nominal concentrations of 0 (control), 1, 10, 50, 87, 120, and 160 mg a.i./L. Ten 

fish per test level. At 24 hours, mortality was 0, 0, 0, 0, 0, 10, and 10% in the 0 (control), 1, 10, 

50, 87, 120, and 160 mg a.i./L test levels. At 96 hours, 90 and 100% were observed in the 120 

and 160 mg a.i./L test group, respectively. Dark coloration and lethargy were observed in fish of 

the 87 mg a.i./L test level. The author reported the behavior of fish succumbing to the test 

material was generally similar to a syndrome characteristic of chemically poisoned fish. 

Moribund fish were then observed to become dark, lose equilibrium and then drop to the bottom 

of the test vessel where they eventually expired. The 96-hour LC50 (95% C.I.) was 106 (87-120) 

mg a.i./L, which would classify tebuthiuron to be practically nontoxic to bluegill sunfish, in 

accordance with the toxicity classification of USEPA. 
Page 106 of 121 

This study is scientifically sound and satisfies the guideline requirement for a freshwater fish 

acute toxicity study with Lepomis macrochirus, a coldwater fish. This study is classified as 

acceptable. 

MRID 00041685 

In a 7-day acute toxicity study, fathead minnow (pimephales promelas) were exposed to a 98% 

technical formulation, an 80% wettable powder, and a 20% pelleted formulation of tebuthiuron 

at nominal concentrations equivalent to 0 (control), 70, 90, 110, 140, and 180 mg a.i./L. Ten fish 

per test level. Mortality was minimal in the tests however there were signs of sub-lethal effects. 

Sub-lethal effects observed were exploratory behavior and hypoactivity in concentrations at and 

greater than 140 mg a.i./L. Fish treated with 70, 90, 110 mg a.i./L of tebuthiuron appeared 

normal; however, periods of less aggressive feeding activity were observed compared to the 

control group throughout the test. The 96-hour LC50 was >180 mg a.i./L, which would classify 

tebuthiuron to be practically nontoxic to fathead minnow, in accordance with the toxicity 

classification of USEPA. 

This study is scientifically sound but does not satisfy the guideline requirement for a freshwater 

fish acute toxicity study with Pimephales promelas, a warmwater fish. This study is classified as 

supplemental since study deviates from recommended protocol and inadequate reporting of the 

data. 

MRID 48722702

In a 96-hr acute toxicity study, sheepshead minnows (Cyprinodon variegates), an 

estuarine/marine fish, were exposed to tebuthiuron at nominal concentrations of 0 (negative 

control), 6.3, 13, 25, 50, and 100 mg a.i./L under static-renewal conditions. Mean measured 

concentrations were <0.42 (<LOQ, negative control), 6.5, 13, 25, 50, and 98 mg a.i./L. There 

was no mortality observed in this study. The 96-hr LC50 was >98 mg a.i./L. Based on sub-lethal 

effects (loss of equilibrium) observed at 98 mg a.i./L, the NOAEC was determined to be 50 mg 

a.i./L, based on mean-measured concentrations. Based on the results of this study, tebuthiuron 

would be classified as practically non-toxic to Cyprinodon variegatus up to the exposure levels 

tested, in accordance with the classification system of the U.S. EPA. 

This study is scientifically sound and satisfies the guideline requirement for an estuarine/marine 

fish acute toxicity study with Cyprinodon variegatus. This study is classified as acceptable. 

OCSPP 850.1300 – Freshwater Invertebrate Life Cycle 

MRID 00138700 

In a 21-day static renewal full life cycle study, <24-hours first instar water fleas (daphnia 

magna) were exposed to tebuthiuron at nominal concentration of 0 (control), 5.63, 11.25, 22.5, 

45, and 90 mg a.i./L. The mean-measured concentrations were 0 (control), 5.47, 11, 21.8, 44.2, 

and 90.2 mg a.i/L. Ten replicates per treatment level. Seven replicates with one daphnia each 

were used for fecundity and three replicates of five daphnia each were used for survival data. 

Two sets of replicates were used for control. No significant levels of mortality occurred at any of 

the levels tested. Daphnia from the 44.2 and 90.2 mg a.i./L treatment groups were significantly 
Page 107 of 121 

shorter than control daphnia. Daphnia from the 44.2 and 90.2 mg a.i./L treatment groups had 

significant reductions in number of broods per reproducing adult and number of offspring per 

adult when compared to control daphnia. Daphnia from the 90.2 mg a.i./L treatment group 

required a significantly longer time to release their first brood. As of a result of significant 

differences in growth and fecundity at the 44.2 and 90.2 mg a.i./L treatment levels, the NOAEC 

is 21.8 mg a.i/L. The LOAEC is 44.2 mg a.i./L. 

This study is scientifically sound and satisfies the guideline requirement for a freshwater 

invertebrate life cycle study with Daphnia magna. This study is classified as acceptable. 

OCSPP 850.1500 – Fish Full Life Cycle Toxicity Test 

MRID 00090083 

In a 45-day flow-through fish embryo-larval study, a total of 50 eyed embryos of Rainbow trout, 

Oncorhynchus mykiss, in each of two replicates per treatment were exposed to tebuthiuron at 

nominal concentrations of 0 (control), 3.1, 6.2, 12.5, 25, and 50 mg a.i./L. Mean-measured 

concentrations were 0 (control), 3.1, 6.3, 12.5, 25, and 52 mg a.i./L. On Day 2 when the hatching 

of all embryos was complete, a total of 25 normal larvae were transferred from the two egg cups 

in each replicate aquarium. These larvae were continuously exposed to the same treatment level 

for 43 days. 

Hatching was 100% successful in all replicate aquaria. Survival was 100, 100, 98, 94, 92, and 

80% in the control, 3.1, 6.3, 12.5, 26, and 52 mg a.i./L groups. Only exposure to the highest test 

concentration resulted in a statistically and biologically significant reduction of larvae survival at 

days 30 and 45. The behavior and feeding response of larvae in the treatment groups were 

considered normal when compared to control larvae. When compared to control larvae, the total 

length of the larvae in the highest test concentration was significantly reduced by test days 30 

and 45. This reduction in mean length correlated well with an average weight of control larvae. 

Statistically significant differences in total length of <6% were found on Day 30 between control 

larvae exposed in the 6.3, 12.5, and 25 mg a.i./L treatment groups. No weights were recorded on 

day 30 to substantiate this size difference and by day 45 statistically significant length 

differences at these concentrations did not exist. The differences noted on day 30 were therefore 

considered to be biologically insignificant. 

Based on significant reduction in the survival and size of rainbow trout larvae at 52 mg a.i./L, the 

highest test concentration; the NOAEC is 26 mg a.i./L. This fish embryo-larval study was 

request in exchange of a fish full life cycle test. This study is scientifically sound and shows that 

a chronic exposure to tebuthiuron as low as 52 mg a.i./L can result in impaired reproduction and 

reduced growth in rainbow trout. This study is classified as acceptable. 

MRID 00090084 

In a 33-day flow-through fish embryo-larval study, a total of 50 embryos of fathead minnow, 

Pimephales promelas, in each of two replicates per treatment were exposed to tebuthiruon at 

nominal concentrations of 0 (control), 5, 10, 20, 40, and 80 mg a.i./L. Mean-measured 
Page 108 of 121 

concentrations were 0 (control), 4.7, 9.3, 18, 38, and 76 mg a.i./L. Four replicates were used for 

the water control. After hatch at day 5, larvae were transferred to the aquaria and were 

continuously exposed to tebuthiuron for 28 days. 

Hatching was 93, 90, 88, 86, 90, and 95% in the control, 4.7, 9.3, 18, 38, and 76 mg a.i./L 

groups. Survival was 88, 90, 88, 85, 92, and 89% in these same groups. No significant reduction 

in hatching or survival of minnows was observed as high as 76 mg a.i./L. When compared to 

control larvae and the three highest concentrations, the larvae in the 18, 38 and 76 mg a.i./L 

groups were statistically significantly shorter. The NOAEC was 9.3 mg a.i./L. 

This fish embryo-larval study with the fathead minnow was request in exchange of a fish full life 

cycle test. This study is scientifically sound and shows that a chronic exposure to tebuthiuron as 

low as 18 mg a.i./L can result in reduced growth but not reproductive effects in fathead 

minnows. 

OCSPP 850.2100 – Avian Acute Oral 

MRID 00041692 

The acute oral toxicity of Tebuthiuron to 9-month-old mallard ducks (Anas platyrhynchos) was 

assessed over 14 days. Tebuthiuron was administered to the birds by gavage at nominal 

concentrations of 0 (control), 1000, and 2000 mg a.i./kg bw. No mortality was observed 

throughout the test in the treatment groups or control group. The 14-day acute oral LD50 was 

>2000 mg a.i/kg bw. According to the US EPA toxicity classification, tebuthiuron would be 

classified as practically nontoxic to mallard ducks on an acute oral basis. 

Body weight was taken on days 3, 7 and 14; however, results were not reported. Food 

consumption was not observed or measured, the results are unknown. Signs of toxicity of 

anorectic, low appetite in females, hypoactivity, and lethargy were reported; however, by day 

two, birds were no longer hypoactivity or lethargy. 

 

This toxicity study is classified as scientifically sound and satisfies the OCSPP 850.2100 

guideline requirement for acute oral toxicity study for the mallard duck. This study is classified 

as acceptable. 

MRID 00020661 (Bobwhite quail) 

The acute oral toxicity of tebuthiuron to Northern bobwhite quail (Colinus virginianus) was 

assessed for 14 days. Tebuthiuron was administered to the birds by gavage at the limit dose of 

500 mg a.i./kg bw nominal. No evidence of toxicity was observed during the test period and all 

birds survived. The behavior, appearance and appetite of the birds remained normal during the 

test period and each of the birds gained weight. The 14-day acute oral LD50 was >500 mg a.i/kg 

bw. No mortality occurred up to and at 500 mg a.i./kg bw, the limit dose; however, because the 

test concentration was not tested as high as 2,000 mg a.i./kg bw, a firm toxicity classification 

could not be made this time. As of a result, the study is classified as supplemental as the test did 

not comply with protocols recommended by the guideline for the following reasons: 1. The age 

of the birds was not reported, 2. Individual body weight and mean food consumption were not 
Page 109 of 121 

reported, and the test did not established a definite LD50, neither does it show that the LD50 is 

greater than 2000 mg a.i./kg bw. 

MRID 00020661 (Mallard duck) 

The acute oral toxicity of tebuthiuron to one-year old mallard duck (Anas platyrhynchos) was 

assessed for 14 days. Tebuthiuron was administered to the birds by gavage at the limit dose of 

500 mg a.i./kg bw nominal. No evidence of toxicity was observed during the test period and all 

birds survived. The behavior, appearance, and appetite of the birds remained normal during the 

test period and each of the birds gained weight. The 14-day acute oral LD50 was >500 mg a.i/kg 

bw. No mortality occurred up to and at 500 mg a.i./kg bw, the limit dose; however, because the 

test concentration was not tested as high as 2,000 mg a.i./kg bw, a firm toxicity classification 

could not be made this time. As of a result, the study is classified as supplemental as the test did 

not comply with protocols recommended by the guideline for the following reasons: Individual 

body weight and mean food consumption were not reported; the test did not established a 

definite LD50, neither does it show that the LD50 is greater than 2000 mg a.i./kg bw. 

MRID 00020661 (Domestic chicken) 

The acute oral toxicity of tebuthiuron to adult chickens (White Rock Cross) was assessed for 14 

days. Tebuthiuron was administered to the birds by gavage at the limit dose of 200 mg a.i./kg 

and in a second study at 500 mg a.i./kg bw nominal. In the 200 mg a.i./kg bw limit dose study, a 

slight hypoactivity and mild anorexia were noted 24 hours post-treatment; however, appeared 

normal thereafter. All birds survived and gained weight during the test period. In the 500 mg 

a.i./kg bw limit dose study, all chickens appeared normal, survived, and gained weight during the 

test period. The 14-day acute oral LD50 was >500 mg a.i/kg bw. No mortality occurred up to and 

at 500 mg a.i./kg bw, the limit dose; however, because the test concentration was not tested as 

high as 2,000 mg a.i./kg bw, a firm toxicity classification could not be made this time. As of a 

result, the study is classified as supplemental as the test did not comply with protocols 

recommended by the guideline for the following reasons: Individual body weight and mean food 

consumption were not reported; the test did not established a definite LD50, neither does it show 

that the LD50 is greater than 2000 mg a.i./kg bw. 

OCSPP 850.2200 – Avian Acute Dietary 

MRID 00041680 

The acute dietary toxicity of Tebuthiuron to 7 days old mallard ducks (Anas platyrhynchos) was 

assessed for 8 days. Tebuthiuron was administered to the ducks in the diet at nominal 

concentrations of 0 (control), 400, 1000, and 2500 mg a.i./kg diet. The 8-day acute dietary LC50

was >2500 mg a.i./kg diet, the highest concentration tested. No mortality occurred up to and at 

2500 mg a.i./kg diet; however, because the test concentration was not tested as high as 5,000 mg 

a.i./kg diet, a firm toxicity classification could not be made this time. 

Ten birds per test levels. The author reported there were no difference in behavior, appearance, 

and posture in the treated ducks when compared to the control group ducks. None of the groups 

appeared reluctant to eat the compound containing diets. With additional data submitted (MRID 

41693) on food consumption and body weight gain, a significant reduction in food consumption 
Page 110 of 121 

in all treatment groups compared to the control was noted from day 0 to 5. With no significant 

reduction in body weight gain between treated and control groups and the small difference in 

food consumption, the apparent reduction in food consumption has probably not affected the 

outcome of the test significantly. 

This toxicity study is classified as scientifically sound but does not satisfy the OCSPP 850.2200 

guideline requirement for acute dietary toxicity study with the mallard duck. This study is 

classified as supplemental. Test concentrations were not tested as high as 5,000 mg a.i/kg diet. 

MRID 00041681 

The acute dietary toxicity of Tebuthiuron to 7 days old bobwhite quails (Anas platyrhynchos) 

was assessed for 8 days. Tebuthiuron was administered to the chicks in the diet at nominal 

concentrations of 0 (control), 400, 1000, and 2500 mg a.i./kg diet. The 8-day acute dietary LC50

was >2500 mg a.i./kg diet, the highest concentration tested. No mortality occurred up to and at 

2500 mg a.i./kg diet; however, because the test concentration was not tested as high as 5,000 mg 

a.i./kg diet, a firm toxicity classification could not be made this time. 

Ten chicks per test level were used. The author reported there were no difference in behavior, 

appearance, and posture in the treated quails when compared to the control group quails. None of 

the groups appeared reluctant to eat the compound containing diets. With additional data 

submitted (MRID 41693) on food consumption and body weight gain, a significant reduction in 

food consumption in all treatment groups compared to the control was noted from day 0 to 5. 

With no significant reduction in body weight gain between treated and control groups and the 

small difference was not pronounced in food consumption, the apparent reduction in food 

consumption has probably show that the test material is not toxic or has repellent effects. 

This toxicity study is classified as scientifically sound but does not satisfy the OCSPP 850.2200 

guideline requirement for acute dietary toxicity study with the bobwhite quail. This study is 

classified as supplemental. Test concentrations were not tested as high as 5,000 mg a.i/kg diet. 

MRID 40601001 

The acute dietary toxicity of Tebuthiuron to 11-day-old Northern bobwhite quail (Colinus 

virginianus) was assessed over 8 days. Tebuthiuron was administered to the birds in the diet at 

nominal concentrations of 0 (control), 600, 1200, 2500, and 5000 mg a.i./kg diet. Meanmeasured concentrations were 0 (<LOQ, control), 636, 1210, 2573, and 5113 mg a.i./kg diet, 

respectively. No significant mortality or behavioral signs of toxicity were noted throughout the 

test. The 8-day acute dietary LC50 was determined to be >5113 mg a.i./kg diet. According to the 

US EPA classification, Tebuthiuron would be classified as practically non-toxic to bobwhite 

quails on an acute dietary basis. 

One bird in the 1210 mg a.i./kg diet test group and one bird in the 5113 mg a.i./kg diet test group 

died during the three-day period that the birds were fed untreated diet. Both birds were the 

smallest in their pens and one was the smallest in the study. Both birds gained weight while 

feeding on the treated diet and no signs of toxicity were noted. It is possible that the stress of 

handling these small birds, for weighing on day 5 contributed to their deaths. No other 
Page 111 of 121 

mortalities or signs of toxicity were recorded during the study. Birds in the 2573 and 5113 mg 

a.i./kg diet test groups gained significantly less weight as compared with the control group 

during the 5-day treatment phase. However, there were no significant differences between mean 

body weight gain values of control and treatment birds during the 3-day basal diet phase. There 

were no significant differences between the mean food consumption of control birds and any of 

the treatment group birds during any phase of the study. 

This toxicity study is classified as scientifically sound and satisfies the OCSPP 850.2200 

guideline requirement for acute dietary toxicity study with the bobwhite quail. This study is 

classified as acceptable. 

MRID 40601002 

The acute dietary toxicity of Tebuthiuron to 4-day-old mallard ducks (Anas platyrhynchos) was 

assessed over 8 days. Tebuthiuron was administered to the ducks in the diet at nominal 

concentrations of 0 (control), 600, 1200, 2500, and 5000 mg a.i./kg diet. Mean-measured 

concentrations were 0 (<LOQ, control), 583, 1176, 2578, and 5093 mg a.i./kg diet, respectively. 

No mortality or behavioral signs of toxicity were noted throughout the test. The 8-day acute 

dietary LC50 was determined to be >5093 mg a.i./kg diet. According to the US EPA 

classification, Tebuthiuron would be classified as practically non-toxic to mallard ducks on an 

acute dietary basis. 

During the five-day treatment phase, there was no significant difference between the mean body 

weight gain values of control birds and birds at the 583 mg a.i./kg diet level. However, ducks 

exposed to diets containing tebuthiuron concentrations of >1176 mg a.i./kg diet gained 

significantly less weight as compared with the control group. During the 3-day basal diet phase, 

there were no significant differences in mean body weight gain values of control and treat ducks. 

Ducks in the 2578 and 5093 mg a.i./kg diet mean-measured treatment levels ate significantly less 

food than control ducks. Ducks at the 1176 mg a.i./kg diet treatment level also consumed less 

food than control ducks, although this difference was not significantly significant. During the 3-

day basal diet phase, ducks at the 2578 and 5093 mg a.i./kg diet test levels consumed less food 

than control ducks; however, these differences were not statistically significant. 

This toxicity study is classified as scientifically sound and satisfies the OCSPP 850.2200 

guideline requirement for acute dietary toxicity study with the mallard duck. This study is 

classified as acceptable. 

MRID 48928201 

The acute dietary toxicity of Tebuthiuron to 5-7 month-old Zebra Finches (Taeniopygia guttata) 

was assessed over 8 days. Tebuthiuron was administered to the birds in the diet at nominal 

concentrations of 0 (control), 562, 1000, 1780, 3160, and 5620 mg a.i./kg diet. Mean-measured 

concentrations were <50 (<LOQ, control), 497, 895, 1590, 2870, and 5380 mg a.i./kg diet, 

respectively. The 8-day acute dietary LC50 (with 95% C.I.) was 1465 (1145 to 1883) mg a.i./kg 

diet. According to the US EPA classification, Tebuthiuron would be classified as slightly toxic

to Zebra Finches on an acute dietary basis. 
Page 112 of 121 

Mortality was 80, 90, and 100% in the mean-measured 1590, 2870, and 5380 mg a.i./kg diet 

levels, respectively. During the exposure period, some birds were taken off of treated feed before 

the full 5-day exposure period was complete due to extremely reduced food consumption values. 

These birds were counted as mortalities. 

Treatment-related decreases in body weight gains were observed at 895, 1590, and 2870 mg 

a.i./kg diet during the exposure period (Days 0-5). Body weight gains at these doses were 

comparable or greater than controls during the post-dosing period (Days 6-8). However, overall 

mean body weights were still reduced compared to controls in the 1590 and 2870 mg a.i./kg diet 

survivors. A direct comparison of body weights and body weight gains could not be made in the 

5380 mg a.i./kg diet group as all birds were taken off of treated feed after only 2 days of 

exposure. 

There was a slight reduction in food consumption noted in the 497 mg a.i./kg diet group. 

Treatment-related reductions in food consumption were observed at ≥895 mg a.i./kg diet during 

the exposure period (Days 0-5). Post-exposure food consumption was comparable to controls at 

all doses. Mean daily dietary doses were 115.6, 170.5, 211.3, 250.2, and 175.8 mg a.i./kg 

bw/day in the mean-measured 497, 895, 1590, 2870, and 5380 mg a.i./kg diet levels, 

respectively. Clinical signs of toxicity were observed as follows: at 497 mg a.i./kg, up to 50% 

of birds displayed slightly ruffled appearance and wing droop between Days 2 and 6; at 895 mg 

a.i./kg, up to 60% of birds displayed ruffled appearance and wing droop between Days 2 and 7; 

at 1590 mg a.i./kg, all birds displayed at least one clinical sign of toxicity (ruffled appearance, 

lethargy, and wing droop) by Day 4; at 2870 mg a.i./kg, all birds were noted with at least one 

clinical sign of toxicity (ruffled appearance, lethargy, wing droop, loss of coordination, prostrate 

posture, and loss of righting reflex) or died by Day 3; and at 5380 mg a.i./kg, all birds were noted 

with ruffled appearance by the afternoon of Day 1.

This toxicity study is classified as scientifically sound and satisfies the OCSPP 850.2200 

guideline requirement for acute dietary toxicity study with the Zebra Finch. This study is 

classified as acceptable. 

OCSPP 850.2300 – Avian Reproduction 

MRID 00093690

The one-generation reproductive toxicity of tebuthiuron to 5-month-old mallard ducks (Anas 

platyrhynchos) was assessed over 22 weeks. Tebuthiuron was administered to the ducks in the 

diet at nominal concentrations of 0 (control), 20 and 100 mg a.i./kg diet. Measured 

concentrations in the diet were not significantly different. 

There was no treatment-related mortality, signs of toxicity, or effects on body weight or food 

consumption at any dietary level. No significant reduction in the reproductive parameters of egg 

laid, eggs cracked, viable embryos, live three-week embryos, normal hatchlings, 14 day-old 

survivors, and eggshell thickness were observed. A statistically significant but non-dose –related 

reduction in 14-day old survivors was observed at the 20 mg a.i./kg-diet. With no tebuthiuron-
Page 113 of 121 

related effects observed in mallard ducks up to and at 100 mg a.i./kg diet, the NOAEC is 100 mg 

a.i./kg-diet, the highest concentration tested. 

This study is scientifically sound and fulfills the guideline requirement for a mallard duck (Anas 

platyrhynchos) reproductive toxicity study. This study is classified as acceptable. 

MRID 00104243 

The one-generation reproductive toxicity of tebuthiuron to 6-month-old bobwhite quails (Colinus 

virginianus) was assessed over 22 weeks. Tebuthiuron was administered to the chicks in the diet 

at nominal concentrations of 0 (control), 20 and 100 mg a.i./kg diet. Measured concentrations in 

the diet were not significantly different. 

There was no treatment-related mortality, signs of toxicity, or effects on body weight or food 

consumption at any dietary level. No significant reduction in the reproductive parameters of egg 

laid, eggs cracked, viable embryos, live three-week embryos, normal hatchlings, 14 day-old 

survivors, and eggshell thickness were observed in the treatment groups compared to control 

group. With no tebuthiuron-related effects observed in bobwhite quails up to and at 100 mg 

a.i./kg diet, the NOAEC is 100 mg a.i./kg-diet, the highest concentration tested. 

This study is scientifically sound and fulfills the guideline requirement for a bobwhite quail 

(Anas platyrhynchos) reproductive toxicity study. This study is classified as acceptable. 

MRID 48928202 

The one-generation reproductive toxicity of Tebuthiuron to 16 pairs per level of 22-week old 

mallards (Anas platyrhynchos) was assessed over 20 weeks. Tebuthiuron was administered to 

the birds in the diet at nominal concentrations of 0 (negative control), 500, 900, and 1500 mg 

a.i./kg diet. Mean-measured concentrations were <50 (<LOQ, control), 500, 903, and 1550 mg 

a.i./kg diet. 

There were no treatment-related mortalities, signs of toxicity, or effects on body weight or food 

consumption at any dietary level. A statistically significant (p<0.05) dose-dependent reduction 

in hatchling body weight and eggshell thickness was observed at 1550 mg a.i./kg diet compared 

to the controls. There were statistically significant (p<0.05; Williams test, 26-41%) reductions 

from control in egg production (eggs laid per pen) in the 500, 903 and 1550 mg a.i./kg diet 

groups. A slight (<1%) reduction from control in eggs not cracked per eggs laid was detected at 

the lowest treatment level (p = 0.04), but was not considered to be treatment-related. No 

statistically significant differences in the other endpoints were observed between the control 

group and any of the treatment groups. Based on the dose-dependent reductions in eggs laid, the 

NOAEC could not be determined in this study (i.e., <500 mg a.i./kg diet). 

This study is scientifically sound but does not satisfy the guideline requirement for a mallard 

(Anas platyrhynchos) reproductive toxicity study. The study is classified as supplemental due to 

significant effects in eggs laid at all test levels that a NOAEC could not be determined; while this 

deviation impacted the acceptability of the study, the reviewer felt this study provides useful 

information that it should not be rejected. This study was conducted with progressively higher 
Page 114 of 121 

concentrations of what was used in the original study (MRID 00093690) at which no effects in 

birds were observed at and up to 100 mg a.i./kg diet. Combining the studies with test levels at 20, 

100, 500, 903, and 1550 mg a.i./kg diet, the resulting LOAEL and NOAEL would be 500 and 

100 mg a.i./kg diet, respectively. 

MRID 48928203 

The one-generation reproductive toxicity of Tebuthiuron to 16 pairs per level of 36-week old 

Northern bobwhite quail (Colinus virginianus) was assessed over 20 weeks. Tebuthiuron was 

administered to the birds in the diet at nominal concentrations of 0 (negative control), 500, 900, 

and 1500 mg a.i./kg diet. Mean-measured concentrations were <50 (<LOQ, control), 500, 903, 

and 1550 mg a.i./kg diet. 

No treatment-related mortalities were observed at any treatment level. Three control birds were 

found dead during Weeks 6, 13, and 14. One male bird in the 1550 mg a.i./kg group was 

euthanized on Day 4 of Week 16 due to fractured right leg. Adult male body weight gain was 

significantly reduced in a dose-dependent manner 53-121% at all treated levels, relative to the 

negative control (p<0.05, William’s test). Adult female body weight was significantly reduced at 

the 1550 mg a.i./kg diet level, relative to the control (p<0.01; Jonckheere-Terpstra test). No 

statistically-significant differences were observed between the control group and any of the 

treated groups for food consumption. Offspring body weights were significantly reduced in a 

dose-dependent manner, with reductions from control detected at the 903 and 1550 mg a.i./kg 

diet levels for hatchling body weights (p<0.01; Jonckheere-Terpstra test) and at the 1550 mg 

a.i./kg diet level for 14-day old survivor weight (p<0.05; William’s test). 

No treatment-related effects on reproductive performance parameters were observed in the 500 

mg a.i./kg diet group, but dose-dependent effects were detected for offspring survival. At 903 

and 1550 mg a.i./kg diet, the offspring survival (14-day hatchlings per number hatched) was 

slightly (4-7%), but significantly reduced, compared to the control (p<0.05; Jonckheere-Terpstra 

test). Also, at the 1550 mg a.i./kg diet level, egg production was reduced 23%, from an average 

of 44.8 eggs laid per pen in the control group to 34.7 eggs laid per pen in the treated group 

(p<0.05; Jonckheere-Terpstra test); both the study author and reviewer concluded that this effect 

was treatment-related, despite the non-monotonicity of the data. No statistically significant 

differences in egg shell thickness were observed between the control group and any of the 

treatment groups. The reviewer concluded that the NOAEC could not be determined in this 

study due to dose-dependent reductions in adult male body weight gain (i.e., NOAEC<500 mg 

a.i./kg diet).

This study is scientifically sound but does not satisfy the guideline requirement for a Northern 

bobwhite quail (Colinus virginianus) reproductive toxicity study. The study is classified as 

supplemental due to significant effects in male weight gain at all test levels that a NOAEC could 

not be determined; while this deviation impacted the acceptability of the study, the reviewer felt 

this study provides useful information that it should not be rejected. This study was conducted 

with progressively higher concentrations of what was used in the original study (MRID 

00104243) at which no effects in birds were observed at and up to 100 mg a.i./kg diet. If the 
Page 115 of 121 

studies were combined with test concentrations at 20, 100, 500, 903, and 1550 mg a.i./kg diet, 

the resulting LOAEL and NOAEL would be 500 and 100 mg a.i./kg diet, respectively. 

OCSPP 850.3020 – Honeybee Acute Contact Toxicity Test 

MRID 40840401

The acute contact toxicity of technical tebuthiuron to honey bees (Apis mellifera) was tested in 

the laboratory. In the 48-hour acute contact test, bees were exposed to technical tebuthiuron 

administered topically to the thorax, at an application rate of 0 (negative control), 0 (solvent 

control), 13, 22, 36, 60, and 100 μg a.i./bee. No treatment-related effects were noted throughout 

the study period. Mortality rates did not exceed 11% in any of the treatment groups; mortality in 

the negative and solvent controls was 5% and 7%, respectively. The 48-hour acute contact LD50

was >100 μg a.i./bee, the highest dose tested. According to the US EPA classification, 

Tebuthiuron would be classified as practically non-toxic to honeybees on an acute contact basis. 

The study is scientifically sound and satisfies the guideline requirements for a honeybee acute 

contact toxicity test. This study is classified as Acceptable. 

OCSPP 850.4100 – Seedling Emergence and Seedling Growth Toxicity Test 

MRID 41066901 

The effect of the technical grade of tebuthiuron on the seedling emergence of monocot (corn, Zea 

mays; wheat, Triticum aestivum; sorghum, Sorghum bicolor; and rice, Oryza sativa) crops at 

0.04, 0.08, 0.16, 0.32, 0.64, and 1.28 lb a.i./A and dicot (cabbage, Brassicacea oleracea; cotton, 

Gossypium hirsutum; cucumber, Cucumis sativus; radish, Raphanus sativus; soybean, Glycine 

max; and sunflower, Helianthus annuus) crops at 0.02, 0.04, 0.08, 0.16, 0.32, 0.64 lb a.i./A was 

studied for 21 days. 

Tebuthiuron did not interfere with seed emergence of any of the species tested. One week after 

emergence, radish was determined to be extremely sensitive to tebuthiuron at 0.08 lb a.i./A. 

After three weeks, radish, cucumber, cabbage, and wheat were severely injured. At the high rate 

tested, seedlings of these species were killed. Corn, rice, cotton, and sunflower were intermediate 

in susceptibility and were injured 50% or more at the highest test level tested. The study author 

determined a NOAEC of 0.02 lb a.i./A for dicots and 0.04 lb a.i./A for monocots three weeks 

after treatment. Early injury symptoms were chlorosis and stunting of plants followed by burning 

of leaves and eventual plant death of some species at the highest test level. All species were 

reduced in height as the application rate was increased. Cucumber, radish, cabbage, and wheat 

were reduced the most. 

The most sensitive monocot species was wheat based on fresh weight, with NOAEC and EC25

values of 0.04 and 0.07 lb a.i./A, respectively. The most sensitive dicot species was cabbage 

based on fresh weight, with NOAEC and EC25 values of 0.02 and 0.03 lb a.i./A, respectively. 
Page 116 of 121 

This toxicity study is scientifically sound and satisfies the guideline requirement for a Tier II 

seedling emergence toxicity study. This study is classified as Acceptable. 

MRID 41066902 

The effect of the technical grade of tebuthiuron on the seed germination of monocot (corn, Zea 

mays; wheat, Triticum aestivum; sorghum, Sorghum bicolor; and rice, Oryza sativa) crops and 

dicot (cabbage, Brassicacea oleracea; cotton, Gossypium hirsutum; cucumber, Cucumis sativus; 

radish, Raphanus sativus; soybean, Glycine max; and sunflower, Helianthus annuus) crops at the 

maximum application rate of 6.0 lb a.i./A was studied for 5 days. 

Analysis of the data was by visual observation. No significant differences were noted between 

treated and control groups. Corn, sorghum, wheat, rice, soybean, cucumber, sunflower, and 

radish seeds were not affected, while cotton and cabbage germination was 2.8% and 3.8% less 

than the controls. The germination percentage for corn, sorghum, wheat, rice, soybean, 

cucumber, sunflower, cabbage, cotton, and radish was 100%, 95%, 99%, 76%, 88%, 99%, 96%, 

73%, 97%, and 100%, respectively. 

This study is no longer required by EPA; however, provides useful information on tebuthiuron to 

terrestrial plants. 

MRID 48722703 

The effect of Spike 20P (AI: Tebuthiuron) on the seedling emergence of monocot (onion, 

Allium cepa; corn, Zea mays; oat, Avena sativa; and ryegrass, Lolium perenne) and dicot (carrot, 

Daucus carota; cucumber, Cucumis sativus; cabbage, Brassicacea oleracea; soybean, Glycine 

max; sugarbeet, Beta vulgaris; and tomato, Lycopersicon esculentum) crops was studied at the 

following nominal concentrations: 

Sugarbeet, tomato, and cabbage: 

0 (negative control), 0.0020, 0.0039, 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, and 1.0 lb 

a.i./A 

Soybean: 

0 (negative control), 0.0020, 0.0039, 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, 1.0, and 2.0 lb 

a.i./A 

Onion, ryegrass, oat, carrot, and cucumber: 

0 (negative control), 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb a.i./A 

Corn: 

0 (negative control), 0.062, 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb a.i./A. 

The growth medium used in the seedling emergence test was natural soil mixed with silica sand 

(loam, pH 6.5, organic carbon 1.0%). On day 21 the surviving plants per pot were recorded and 

cut at soil level for measuring the plant height and fresh weight. 

Fresh weight and height were significantly affected in every species tested. Survival (% 

survived of planted) was significantly affected in all species except soybean. Emergence was 
Page 117 of 121 

only significantly affected in cabbage and tomato. The % inhibition in seedling emergence in the 

treated species as compared to the control ranged from 3 to 52%. The most sensitive monocot 

species, based on fresh weight, in the seedling emergence test was ryegrass with an EC25 of 0.27 

lb a.i./A and a NOAEC of 0.25 lb a.i./A. The most sensitive dicot species, based on fresh weight, 

was carrot with an EC25 of 0.018 lb a.i./A and a NOAEC of 0.031 lb a.i./A. 

However, the study author reported analytical recoveries of the test material from nominal 1.0, 

2.0, and 4.0 lb a.i./A test levels yielded pre-application recoveries ranging from 37 to 76% of the 

nominal concentrations. At the first post-application, recoveries ranged from 63 to 66% of the 

nominal concentrations. At the second post-application, recoveries ranged from 39 to 50% of the 

nominal concentrations. The lower test concentrations were not verified. The study author 

averaged all of the application data and averaged all the percent recoveries to obtain a single 

recovery value of 56% of nominal. If the reviewer’s results are adjusted for the 56% average 

recoveries, the most sensitive monocot species was ryegrass based on fresh weight, with 

NOAEC and EC25 values of 0.14 and 0.15 lb a.i./A, respectively. The most sensitive dicot 

species was carrot based on fresh weight, with NOAEC and EC25 values of 0.017 and 0.010 lb 

a.i./A, respectively. 

There were compound related phytotoxic effects observed in every species tested, with a doseresponse relationship expressed in the upper 4-5 test levels. Based on the phytotoxicity rating 

system used by the study author (0 = no injury, 100 = complete mortality), soybean and corn had 

maximum effects of 63 and 70. All other species tested except carrot and cucumber had 

maximum ratings ranging from 85 to 97. Carrot and cucumber experienced complete mortality, 

with a rating of 100. 

This toxicity study is scientifically sound but does not satisfy the guideline requirement for a 

Tier II seedling emergence toxicity study. This study is classified as Supplemental. 

OCSPP 850.4150 – Vegetative Vigor Toxicity Test 

MRID 48722704 

The effect of Spike 20P (AI: Tebuthiuron) on the vegetative vigor of monocot (onion, Allium 

cepa; corn, Zea mays; oat, Avena sativa; and ryegrass, Lolium perenne) and dicot (carrot, Daucus 

carota; cucumber, Cucumis sativus; cabbage, Brassicacea oleracea; soybean, Glycine max; 

sugarbeet, Beta vulgaris; and tomato, Lycopersicon esculentum) crops was studied at the 

following nominal concentrations up to 4.0 lb a.i./A: 

Sugarbeet, tomato, and cabbage: 

0 (negative control), 0.0020, 0.0039, 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, and 1.0 lb 

a.i./A 

Soybean: 

0 (negative control), 0.0020, 0.0039, 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, 1.0, and 2.0 lb 

a.i./A 

Onion, ryegrass, oat, carrot, and cucumber: 

0 (negative control), 0.0078, 0.016, 0.031, 0.062, 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb a.i./A 
Page 118 of 121 

Corn: 

0 (negative control), 0.12, 0.25, 0.50, 1.0, 2.0, and 4.0 lb a.i./A. 

The growth medium used in the vegetative vigor test was natural soil mixed with silica sand 

(loam, pH 6.5, organic carbon 1.0%). On day 21 the surviving plants per pot were recorded and 

cut at soil level for measuring the plant height and fresh weight. 

In the vegetative vigor test, the plant fresh weight and shoot length were significantly affected by 

the test material. Survival was significantly affected in onion, oat, carrot, cucumber, and 

sugarbeet. 

The most sensitive monocot species, based on fresh weight, in the vegetative vigor test was 

ryegrass with an EC25 of 0.30 lb a.i./A and a NOAEC of 0.12 lb a.i./A. The most sensitive dicot 

species, based on fresh weight, was sugarbeet with an EC25 of 0.16 lb a.i./A and a NOAEC of 

0.062 lb a.i./A. 

However, the study author reported analytical recoveries of the test material in the nominal 1.0, 

2.0, and 4.0 lb a.i./A test levels yielded pre-application recoveries ranging from 50 to 84% of the 

nominal concentrations for all species except cucumber, and 38 to 71% of nominal for cucumber. 

At post-application, recoveries ranged from 55 to 84% (all species except cucumber), and from 

44 to 88% (cucumber) of the nominal concentrations. The lower test concentrations were not 

verified. The study author averaged all of the application data and averaged all the percent 

recoveries to obtain a single recovery value of 61% of nominal. If the reviewer’s results are 

adjusted for the 61% average recoveries, the most sensitive monocot species was ryegrass based 

on fresh weight, with NOAEC and EC25 values of 0.07 and 0.18 lb a.i./A, respectively. The most 

sensitive dicot species was sugarbeet based on fresh weight, with NOAEC and EC25 values of 

0.04 and 0.10 lb a.i./A, respectively. 

There were compound related phytotoxic effects observed in every species tested, with a doseresponse relationship expressed in the upper 3-5 test levels. Based on the phytotoxicity rating 

system used by the study author (0 = no injury, 100 = complete mortality), corn and soybean had 

maximum effects of 36 and 46, respectively. Ryegrass, tomato, sugarbeet, and cabbage had 

maximum ratings ranging from 54 to 76. Onion, oat, carrot, and cucumber had maximum ratings 

ranging from 86 to 98. 

This toxicity study is scientifically sound but does not satisfy the guideline requirement for a 

Tier II vegetative vigor toxicity study. This study is classified as Supplemental. 

OCSPP 850.4400 – Aquatic Plant Toxicity Test Using Lemna spp. 

MRID 41080404 

In a 14-day acute toxicity study, freshwater aquatic vascular plants, duckweed (Lemna gibba), 

were exposed to tebuthiuron mean-measured concentrations of <0.005 (<LOD), 0.005, 0.0096, 

0.049, 0.091, 0.19, 0.38, and 0.78 mg a.i./L. 
Page 119 of 121 

The percent inhibition of frond density after 7 and 14 days ranged from 15% to 86% and -6% 

(stimulation) to 98%, respectively. The percent inhibition of yield at 7 and 14 days, calculated 

from frond density, ranged from 16% to 90% and -6% to 100%, respectively. The percent 

inhibition of growth rate at 7 and 14 days, calculated from frond density, ranged from 6% to 61% 

and -1% to 99%, respectively. The percent inhibition of frond dry weight (biomass) after 14 days 

ranged from -21% to 98%. The percent inhibition of yield after 14 days, calculated from frond 

dry weight, ranged from -22% to 100%. The percent inhibition of growth rate after 14 days, 

calculated from frond dry weight, ranged from -5% to 100%. The 7-day EC50 reviewercalculated for yield and growth rate of frond density was 0.192 and 0.495 mg a.i./L, respectively. 

The 14-day EC50 reviewer-calculated for yield and growth rate of frond dry weight was 0.126 

and 0.205 mg a.i./L, respectively. Yield in frond dry weight was the most sensitive endpoint, its 

corresponding NOAEC was 0.091 mg a.i./L due to significant differences between the negative 

control and the three highest test concentrations. 

 

This toxicity study is scientifically sound and does not fulfill the guideline requirement for a Tier 

II aquatic vascular plant toxicity study with duckweed, Lemna gibba. This study is classified as 

supplemental. 

OCSPP 850.4500 – Algal Toxicity Test 

MRID 00138697 

In a 14-day toxicity study, freshwater green alga, Selenastrum capricornutum, were exposed to 

tebuthiuron nominal concentrations of 0 (negative control), 5, 10, 15, 20, 40, 80, 160, and 320 

μg/L under static conditions. The mean-measured test concentrations were 0 (control; LOD <0.5 

μg/L), 5, 10, 13, 16, 33, 79, 168, and 338 μg a.i./L. After 5 days, the percent inhibition in 

biomass relative to the control ranged from 0% to 83%. The 5-day EC50 and NOAEC was 50 and 

13 μg a.i./L, respectively. 

This toxicity study is scientifically sound and does not fulfill the guideline requirement for a Tier 

II algal toxicity study with freshwater green algae, Selenastrum capricornutum. This study is 

classified as supplemental. 

 

MRID 41080402 

In a 7-day toxicity study, cultures of the marine diatom, Skeletonema costatum, were exposed to 

tebuthiuron nominal concentrations of 0 (negative control), 0.002, 0.01, 0.02, 0.04, 0.08, 0.16, 

and 0.32 mg a.i./L under static conditions. Mean-measured concentrations were 0.0018, 0.0092, 

0.018, 0.038, 0.076, 0.16, and 0.3 mg a.i./L. 

After 96-hr and 168-hr, the percent inhibition in yield, calculated from cell density, relative to 

the control ranged from 38% to 105% and -36% to >100%, respectively. The percent inhibition 

in AUC (area under the curve), calculated from cell density, relative to the control ranged from -

4% to 82% and -21% to 98% at these respectively times. The percent inhibition in growth rate, 

calculated from cell density, relative to the control at 96-hr and 168-hr ranged from 15% to 

>100% and -9% to >100%, respectively. The 96-hr EC50 for yield, AUC and growth rate was 

0.05 mg a.i./L, 0.06 mg a.i./L, and not calculable, respectively. With yield as the most sensitive 
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endpoint, its corresponding NOAEC was 0.038 mg a.i./L based upon significant reductions in 

yield detected between the control and the three highest test concentrations of the seven test 

concentrations tested. 

This toxicity study is scientifically sound but does not satisfy EPA guideline requirements for a 

Tier II aquatic nonvascular plant toxicity study with the marine diatom, Skeletonema costatum. 

This toxicity study is classified as supplemental. 

MRID 41080403 

In a 7-day toxicity study, cultures of the freshwater diatom, Navicula pelliculosa, were exposed 

to tebuthiuron nominal concentrations of 0 (negative control), 0.005, 0.01, 0.05, 0.1, 0.2, 0.4, and 

0.8 mg a.i./L. Mean-measured concentrations were <0.005 (<LOD), 0.0012, 0.011, 0.056, 0.11, 

0.22, 0.46, and 0.89 mg a.i./L, 82-107% of nominal concentrations. 

After 96-hr and 168-hr, the reviewer-calculated percent inhibition in yield based on cell density 

relative to the control ranged from 11% to 100% and -17% to 100%, respectively. The percent 

inhibition in area under the growth curve, calculated from cell density, relative to the control 

ranged from -5% to 109% at 96 hours and -19% to 98% at 168 hours. The percent inhibition in 

growth rate relative to the control at 96 and 168 hours ranged from 4% to 100% and -3% to 

112%, respectively. The reviewer-calculated 96-hour EC50 for yield, area under the growth 

curve, and growth rate was 0.09, 0.1, and 0.18 mg a.i./L. Significant reductions in yield, the most 

sensitive endpoint, were detected between the control and the four highest test concentrations; as 

of a result, the corresponding NOAEC was 0.056 mg a.i./L. 

This toxicity study is scientifically sound but does not fulfill EPA guideline requirements for a 

Tier II aquatic nonvascular plant toxicity study with the freshwater diatom, Navicula pelliculosa. 

This toxicity study is classified as supplemental. 

OCSPP 850.4550 – Cyanobacteria (Anabaena flos-aquae) Toxicity Test 

MRID 41080401 

In a 7-day toxicity study, cultures of the freshwater blue-green algae, Anabaena flos-aquae, were 

exposed to tebuthiuron nominal concentrations of 0 (negative control), 0.31, 0.62, 1.25, 2.5, 5, 

and 10 mg a.i./L. Mean-measured concentrations were <0.012 (<LOD), 0.31, 0.62, 1.32, 2.62, 

5.49, and 11.05 mg a.i./L, 98-105% of nominal concentrations. 

At 96-hr and 168-hr, the reviewer-calculated percent inhibition in yield based on cell density 

relative to the control ranged from 13% to 88% and -2% (growth) to 88%, respectively. The 

percent inhibition in area under the growth curve based on cell density relative to the control 

ranged from 10% to 87% and 7% to 87% at these respectively times. The percent inhibition in 

growth rate based on cell density relative to the control at 96-hr and 168-hr ranged from 3% to 

43% and -0.2% (growth) to 39%, respectively. The study author based it conclusions upon test 

termination that lasted 168 hours; however, OSCPP guideline 850.4500 prefers 96 hours toxicity 

endpoints. The reviewer-calculated 96-hour EC50 for yield, area under the growth curve, and 

growth rate was 0.81, 1.09, and >11.05 mg a.i./L, respectively. Significant reductions in yield, 
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AUC, and growth rate were detected between the control and all test concentrations, its NOAEC 

could not be established. With yield as the most sensitive endpoint, its corresponding EC05 was 

0.03 mg a.i./L. 

This toxicity study is scientifically sound however does not fulfill EPA guideline requirements 

for a Tier II aquatic nonvascular plant toxicity study with the freshwater blue-green algae, 

Anabeana flos-aquae. This toxicity study is classified as Supplemental.