UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

 OFFICE OF 

 CHEMICAL SAFETY AND

 POLLUTION PREVENTION

 PC Code: 074801

DP Barcode: D424574

Date: September 3, 2015

MEMORANDUM

SUBJECT: Transmittal of the Preliminary Environmental Fate and Ecological Risk 

Assessment for the Registration Review of the Organophosphate Tribufos

TO: Marianne Mannix, Chemical Review Manager

Tracy Perry, Team Leader

Kelly Sherman, Branch Chief

Risk Management and Implementation Branch III

Pesticide Re-Evaluation Division (7508P)

FROM: Katrina White, Ph.D., Risk Assessment Process Leader

Cheryl Sutton, Ph.D., Environmental Scientist

Environmental Risk Branch IV

Environmental Fate and Effects Division (7507P)

REVIEWED

BY: Thomas M. Steeger, Ph.D., Senior Science Advisor

James Carleton, Ph.D., Senior Fate Scientist

Environmental Risk Branch IV

Environmental Fate and Effects Division (7507P)

THROUGH: Jean Holmes, D.V.M., M.P.H., Branch Chief

Environmental Risk Branch IV

Environmental Fate and Effects Division (7507P)

This memorandum transmits the preliminary ecological risk assessment for the registration 

review of tribufos (S,S,S-tributyl phosphorotrithioate; CAS No.: 78-48-8; PC Code 074801). 

After reviewing the most recent environmental fate and effects data for tribufos, the assessment 

concludes that registered uses of the defoliant tribufos may result in risks to both federally 

threatened and endangered (listed) and non-listed species of wildlife and aquatic vertebrates and 

invertebrates. 
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Environmental Fate and Ecological Risk Assessment 

for the Registration Review of Tribufos

 

Tribufos

(S,S,S-tributyl phosphorotrithioate)

CAS Registry Number 74-48-8

PC Code 074801

Prepared by:

Katrina White, Ph.D., Biologist

Cheryl Sutton, Ph.D., Environmental Scientist

U.S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division (EFED)

Environmental Risk Branch IV

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460 Reviewed by:

Thomas Steeger, Ph.D., Senior Science Advisor

James Carleton, Ph.D., Senior Fate Scientist

Jean Holmes, D.V.M., M.P.H., Branch Chief

September 3, 2015
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Table of Contents

1. Executive Summary.............................................................................................................. 5

2. Problem Formulation ......................................................................................................... 10

2.1. Nature of Regulatory Action..................................................................................................................10

2.2. Stressor Source and Distribution ..........................................................................................................11

2.2.1. Nature of the Stressor........................................................................................................................11

2.2.3. Overview of Pesticide Use ................................................................................................................11

2.3. Receptors.................................................................................................................................................12

2.3.1. Aquatic and Terrestrial Effects..........................................................................................................12

2.3.2. Ecosystems Potentially at Risk..........................................................................................................12

2.4. Assessment Endpoints............................................................................................................................12

2.5. Conceptual Model ...................................................................................................................................13

2.5.1. Risk Hypotheses................................................................................................................................14

2.5.2. Conceptual Pathways Table ..............................................................................................................14

2.6. Analysis Plan ...........................................................................................................................................15

2.6.1. Residues of Concern..........................................................................................................................15

2.6.2. Preliminary Identification of Data Gaps and Uncertainties...............................................................16

2.6.4. Measures of Exposure .......................................................................................................................16

2.6.5. Measures of Effect.............................................................................................................................18

2.6.6. Integration of Exposure and Effects..................................................................................................20

3. Analysis............................................................................................................................. 21

3.1. Use Characterization..............................................................................................................................21

3.2. Exposure Characterization....................................................................................................................22

3.2.1. Environmental Fate and Transport Characterization.........................................................................22

3.2.2. Measures of Aquatic Exposure..........................................................................................................27

3.2.3. Measures of Terrestrial Exposure......................................................................................................30

3.3. Ecological Effects Characterization......................................................................................................37

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

4.1. Risk Quotient (RQ) Values ..........................................................................................................................46

4.2. Incident Database Review ............................................................................................................................54

4.3. Risk Description............................................................................................................................................55

4.3.1 Risk to Aquatic Organisms................................................................................................................56

4.3.2. Risk to Terrestrial Organisms............................................................................................................59

Non-target terrestrial plants..............................................................................................................................63

4.4. Uncertainties and Data Gaps.................................................................................................................64

4.4.1. Fate and Transport.............................................................................................................................64

4.4.2. Effects................................................................................................................................................65

5. Endocrine Disruptor Screening Program (EDSP)........................................................... 66

6. Federally Threatened and Endangered (Listed) Species Concerns............................... 66

7. Conclusions.......................................................................................................................... 67

8. References............................................................................................................................ 69

8.1 Submitted Product Chemistry and Environmental Fate Studies .............................................................72

8.2. Submitted Ecotoxicity Studies (Retrieved from OPPIN April 2015) .......................................................72
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Appendices

Appendix A. Selected SWCC Output Files................................................................................. 85

Appendix B. Toxicity Categories and Levels of Concern .......................................................... 92

Appendix C. Summary of Ecological Toxicity Data ................................................................... 93

Appendix D. Example T-REX (v. 1.5.2) Input and Output for Tribufos .................................. 100

Appendix E. STIR Results......................................................................................................... 102

Appendix F. Screening Imbibition Program (SIP) Version 1.0 Inputs and Results.................. 103

Appendix G. Accepted ECOTOX Data Table (2008 to 2015 Refresh Report)......................... 104

Appendix H. Ecosar Analysis for Parent and Degradate........................................................... 107

Appendix I. Example KABAM Inputs and Outputs.................................................................. 111

Appendix J. Aquatic Plant Toxicity Endpoints In the EFED ECOTOX Database ................... 115
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1. Executive Summary

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

risk assessment for the Registration Review of tribufos (S,S,S-tributyl phosphorotrithioate; CAS 

No.: 78-48-8; PC code 074801) and its end-use products. The potential for direct effects to 

federally threatened and endangered (listed) and non-listed organisms was assessed.

Tribufos is an organophosphate chemical that, when applied to cotton plants, is taken up by its 

leaves, resulting in premature abscission of whole green leaves. Information was not available to 

determine whether tribufos is transported in the xylem and phloem of plants. Tribufos is 

registered for use on cotton, and is used in a ground or aerial spray application prior to harvest.

This preliminary risk assessment examines the potential for adverse effects on non-target 

organisms following exposure (i.e., ecological risks) associated with this single labeled use of 

tribufos based on available scientific and commercial information on the use, environmental fate 

and transport, and effects of tribufos on nontarget organisms. 

Tribufos is a semi-volatile, moderately soluble compound that has low mobility in soil. Based 

on submitted data, it is stable to the abiotic processes of hydrolysis and photolysis. It is expected 

to rapidly degrade if present in the atmosphere. Tribufos is persistent in aerobic and anaerobic 

soil, and slightly persistent in anaerobic aquatic environments. Therefore, there is potential for 

tribufos to accumulate in soil with repeated applications. Tribufos is expected to sorb to soil, and 

not to appreciably leach to groundwater or move to surface waters in solution phase via runoff. 

Surface water contamination may occur though through spray drift and/or transport of residues 

sorbed to sediment in runoff (i.e., erosion). In either aquatic or terrestrial environments, tribufos 

may bioaccumulate in organisms exposed to it (BCF=730 L/kg-whole fish); however, 

metabolism and rapid depuration will help to mitigate this exposure pathway. While residues of 

tribufos bioconcentrate in fish, as indicated by its BCF, the BCF is not high enough to trigger the 

Office of Chemical Safety and Pollution Prevention (OCSPP) criteria (i.e., BCF >1000) for 

being considered bioaccumulative1

. 

Tribufos is the only residue of concern identified for aquatic and terrestrial organisms. Oxon 

transformation products were not identified in laboratory or field studies. Aquatic exposure 

estimates for tribufos are presented for the parent compound as the single residue of concern. 

Acute 1-in-10 year estimated environmental concentrations (EECs) in surface water for modeled 

cotton use of tribufos ranged from 11 μg/L to 28 μg/L. Tribufos monitoring data are limited. 

Surface water monitoring conducted by the California Department of Pesticide Regulation 

(DPR) between 1991 and 2003 reported detections (at the limit of quantification [LOQ] of 0.01 

ppb) in only 2 of 810 surface water samples from 15 counties (DPR, 2004b). However, the DPR 

data do not reflect targeted monitoring and it is uncertain whether the frequency and quantity of 

material in surface water would be higher if sampling were conducted at or close to the time/site 

of application. 

Tribufos is characterized as very highly toxic to aquatic invertebrates, and highly toxic to aquatic 

 1 http://www.pbtprofiler.net/criteria.asp
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vertebrates (fish and aquatic-phase amphibians) on an acute exposure basis. The compound is

highly toxic to mammals on a subacute dietary exposure basis and is moderately toxic to

mammals and birds, reptiles and terrestrial-phase amphibians on an acute oral exposure basis

while it is slightly toxic to birds, reptiles and terrestrial-phase amphibians on a subacute dietary 

exposure basis. Chronic effects observed across taxa include effects on survival, growth, and 

reproduction. Table 1 summarizes the taxa with a potential for direct adverse effects from 

exposure to tribufos along with the lines of evidence used in assessing risk. 

Risk was evaluated at both maximum and average application rates. The average application 

rate is derived by dividing the average pounds of active ingredient applied based on proprietary 

usage survey data by the total area treated (USEPA, 2014). Thus, the average rate may not 

reflect an actual application rate used in the field and is lower than the rate used by some.

Table 1. Potential for Direct Effects on the Use of Tribufos on Cotton1.

Taxa Exposure Duration and 

Endpoint RQ2

Potential for Direct 

Effects? Data gaps/ Refinements/

Lines of evidence Listed Nonlisted

Aquatic vascular 

plants Frond Count 0.02-0.21 No- No- None

Aquatic nonvascular plants Measures of biomass 0.14-0.25 NA Yes* Data gaps on non-vascular 

aquatic plants, see text.

Dicot terrestrial 

plants Measures of biomass 0.21-0.53 Yes* Yes* Defoliation observed in 

plants that form abscission 

zones at maximum labeled 

application rates. Effects 

observed for cotton at 0.42 

lbs a.i./A.

Monocot terrestrial 

plants Measures of biomass 0.10-0.26 Yes* Yes*

Freshwater 

Vertebrates

Acute: mortality 0.07–0.10 Yes+ NoSome LOCs exceeded at 

average and maximum 

labeled application rates.

Chronic: NOAEC 

extrapolated using ACR 

for E/M fish

4.09-7.17 Yes+

Estuarine/ marine 

fish

Acute: mortality 0.03-0.04 No- NoChronic: growth, post 

hatch survival 1.30-2.28 Yes+

Freshwater 

invertebrates

Acute: mortality 3.9-5.5 Yes+ Yes+ LOCs exceeded at average 

and maximum labeled 

application rates.

Chronic: Reduced 

number of young/adult/day 

and reduced growth

10.8-17.4 (water 

column)

3.9-24 (benthic)

Yes+

Estuarine/ marine 

Invertebrates

Acute: mortality 1.8-2.6 Yes+ Yes+ LOCs exceeded at average 

and maximum labeled 

application rates. Chronic: mortality

2.4-3.8

(water column)

4.4-8.4 (benthic)

Yes+

Mammals

Acute oral dose: mortality

Subacute dietary:

mortality

<0.01-1.02

0.05-2.80, 52.6-114 Yes+ Yes+

LOCs exceeded at average 

and maximum labeled 

application rates and with 

upper-bound and meanKenaga EECs. LOCs 

exceeded for mammals and 

Chronic: increased 

number of litters with still 

born pups

0.53-14.04, 32.6-

70.5 Yes+
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Taxa Exposure Duration and 

Endpoint RQ2

Potential for Direct 

Effects? Data gaps/ Refinements/

Lines of evidence Listed Nonlisted

Birds

Acute oral dose: mortality

Sub-Acute dietary:

mortality, based on BCF

<0.01-4.71

0.01-0.30, 5.57-

12.1 Yes+ Yes+

birds consuming aquatic 

organisms using both low 

and high EECs and BCF 

values.

Chronic: egg-shell 

thickness, based on BCF

0.19-3.00, 32.6-

70.5 Yes+

Terrestrial 

invertebrates

Individual survival:

Acute contact: 3% 

mortality at level

<0.21 NoTier 1 dietary and chronic 

data are not available to 

evaluate risk. An open 

literature study shows 

effects on terrestrial 

invertebrates at maximum 

labeled application rates.

Individual survival:

Acute and chronic dietary No data

Brood Size and success:

Acute and chronic larval No data

ACR=Acute-to-chronic; RQ=risk quotient; LOC=level of concern; E/M=estuarine/marine; NOAEC=No observable adverse effect concentration; 

BCF=bioconcentration factor; EEC=estimated environmental concentration

1 ‘Taxa’ cells and ‘potential for direct effects’ cells are shaded as follows: 

- red and plus sign (+) indicates high confidence in a high likelihood of direct adverse effects (e.g., multiple lines of evidence, RQs 

exceed LOCs, RQs based on reliable data and exposure estimate);

- yellow and asterisk (*) indicates lower confidence in a high likelihood of direct adverse effects (e.g., low confidence in data used to 

calculate RQs, lack of data, or few lines of evidence support the conclusion); and

- green and minus sign (-) indicates a high confidence in a low likelihood of direct adverse effects (e.g., full data set, high confidence in 

exposure estimates and toxicity dataset). 

- When data were not available to evaluate risk and there are no lines of evidence suggesting a potential for direct effects, the cell was 

not shaded. 2 The RQ range reflects the RQs for the range of exposure estimates for maximum labeled application rates.

Lines of Evidence

• Exposure Considerations

o Tribufos is persistent and slightly to hardly mobile. It has the potential to accumulate in 

soils with repeated applications and be transported into surface water and benthic 

sediments via erosion and spray drift.

o Tribufos is taken up into leaves, but it is uncertain whether it is transported in the xylem 

and phloem or absorbed by roots.

o Based on BCF value of 730 L/kg-wet weight, tribufos may bioconcentrate and 

bioaccumulate in aquatic and terrestrial organisms; however, it is metabolized and 

depurates rapidly in fish. The upper trophic BCF and BAF estimated using the ArnotGobas method are 277 and 281, respectively (EPI SuiteTM, v.4.11, 2012). 

Aquatic Environment

• Freshwater and Estuarine/marine Aquatic Invertebrates:

o There is a potential for acute mortality and reduced shell growth, reduced growth, and 

reduced number of young following chronic exposure for aquatic invertebrates 

inhabiting the water-column and benthic zones (i.e., RQs> acute listed, acute non-listed, 

and chronic risk LOCs). RQs exceed acute listed and non-listed and chronic risk LOCs 

for freshwater fish and chronic risk LOCs for estuarine/marine fish for both maximum 

and average application, with the exception that the chronic risk LOC for 

estuarine/marine invertebrates inhabiting the water column is not exceeded for 

California and Arizona average application rates simulated with the California cotton 
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scenario.

• Freshwater and Estuarine/marine aquatic vertebrates (including aquatic-phase amphibians 

and reptiles)

o There is a potential for acute mortality and reduced growth, and reduced survival of 

young following chronic exposure for freshwater aquatic vertebrates from use of 

tribufos on cotton (i.e., RQs> acute listed and chronic risk LOCs). Acute and chronic 

risk LOCs are exceeded for both maximum and average application rates.

o There is a potential for reduced growth, and reduced survival of young following 

chronic exposure for estuarine/marine aquatic vertebrates from use of tribufos on cotton 

(i.e., RQs> chronic risk LOCs). Acute and chronic risk LOCs are exceeded for both 

maximum and average application rates with the exception that the chronic risk LOC for 

estuarine/marine vertebrates is not exceeded for California and Arizona average 

application rates simulated with the California cotton scenario.

o There is one incident of a fish kill associated with an approved use of tribufos.

• Aquatic Plants

o RQs do not exceed LOCs for risk to listed or non-listed aquatic plants. However, a full 

dataset is not available to assess the risk to non-vascular aquatic plants, and based on 

aquatic plant data for other organophosphates, it is possible that RQs could exceed 

LOCs if additional data were available.

• Monitoring data confirm the identified transport pathways. Non-targeted monitoring data 

for surface water (peak=0.3 μg/L) and sediment (peak=5.1 μg/kg) are approximately a 

factor of 10 lower than the most sensitive toxicity endpoints for aquatic invertebrates 

(Daphnia magna median lethal concentration = 5.4 μg/L; D. magna 21-day no-observableadverse-effect-concentration, NOAEC=1.56 μg/L; Hyalella azteca 10-day sediment 

NOAEC=71 μg /kg-sediment) and aquatic vertebrates (Lepomis macrochirus estimated 

NOAEC=4.1μg/L. However, these data are not expected to capture maximum

concentrations because the sampling was infrequent and not targeted to an area of tribufos 

use.

• Spray Drift

o There is a potential for adverse effects to aquatic invertebrates when exposure was 

estimated for runoff only. Therefore, a spray drift buffer will reduce exposure but will 

not eliminate risk from runoff in vulnerable areas.

o When considering exposure to spray drift alone, there is a potential for direct adverse 

effects to listed aquatic invertebrates located in surface waters greater than 1000 feet 

from the edge of the field for aerial applications and 170 to 577 feet from the edge of the 

field for ground applications. For non-listed aquatic invertebrates, adverse effects may 

occur up to 262 feet from the edge of the field due to exposure to spray drift, depending 

on the application rate and type of application.

Terrestrial Environment

• Birds (surrogates for terrestrial-phase amphibians and reptiles) and Mammals

o There is a potential for acute mortality and reduced egg shell thickness, reduced egg 

production, reduced survival of hatchlings, reduced growth, and increased number of 

females with regressed ovaries following chronic exposure for birds consuming short 

grass, tall grass, broadleaf plants/small insects, and arthropods (i.e., RQs> acute listed, 

acute non-listed, and chronic risk LOCs). Some RQs exceed acute and chronic risk 
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LOCs for both maximum and average application rates. 

o There is a potential for acute mortality and increased number of litters born with 

stillborn pups, increased pup death through lactation, decreased growth (bodyweight) in 

F1 and F2, and an increased F1 gestation period following chronic exposure for 

mammals consuming short grass, tall grass, broadleaf plants/small insects, and 

arthropods (i.e., RQs> acute listed, acute non-listed, and chronic risk LOCs). Some RQs 

exceed acute and chronic risk LOCs for both maximum and average application rates. 

o There is a potential for adverse effects due to acute and chronic dietary exposure to 

birds, reptiles, terrestrial-phase amphibians and mammals consuming aquatic organisms 

(i.e., RQs>acute listed, acute non-listed, and chronic risk LOCs). This risk is mainly in 

vulnerable areas such as areas where tribufos residues may persist in water through 

erosion or spray drift.

• Terrestrial Invertebrates

o Acute-contact honey bee (Apis mellifera) toxicity data indicate that the likelihood of 

adverse effects from exposure to tribufos following its use on cotton is low (i.e., 

RQ<LOC). Acute and chronic oral toxicity data for young adult bees and acute and 

chronic larval honey bee toxicity data are not available to fully evaluate the potential for 

effects to individual adult honey bees or their brood (eggs, larvae, and pupae). 

o Given tribufos is an organophosphate and there is evidence of insect mortality at lower 

application rates than average and maximum labeled rates, there is a potential for 

adverse effects to terrestrial invertebrates from exposure to the compound.

• Terrestrial Plants

o There is a potential for adverse effects (defoliation) to non-target plants that form 

abscission zones (e.g., deciduous shrubs and trees) based on the tribufos being a known 

defoliant. Data are available showing that effects to non-target plants do occur at the 

maximum labeled application rates. 

o There is one incident to terrestrial plants associated with the use of tribufos and diuron. 

This incident may have been caused by diuron, as it is more toxic to plants than tribufos.

There are fate and ecotoxicity data gaps discussed in the Section 4.4. The understanding of the 

microbial (biotic) metabolism of tribufos and the model estimates for aerobic and anaerobic soil 

metabolism and anaerobic aquatic metabolism rely on only one study with each of these studies 

testing one soil or sediment to derive model parameters. Consistent with EPA’s model input 

parameter guidance, the available half-life for each of these studies is multiplied by an 

uncertainty factor (3X) to derive the model input values.

The main uncertainties in the toxicity dataset involve risk to honey bees, benthic aquatic 

invertebrates, and aquatic non-vascular plants as data are not available for these taxa to fully 

evaluate the potential for direct effects. There is also some uncertainty on the risk to freshwater 

vertebrates from chronic exposure to tribufos. 

A ten-day sediment toxicity study is not available for the freshwater benthic invertebrate midge 

(Chironomus dilutus). There are 10-day sediment toxicity studies available for the freshwater 

(H. azteca) and estuarine/marine amphipod (Leptocheirus plumulosus). The midge and amphipod 

have different life histories, occupy different taxonomic groups (crustaceans versus insects) and 

generally interact with sediments in different ways. Chironomid larvae are more infaunal 
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(submerged in sediment) whereas amphipods are more epibenthic (moving in and out of surficial 

sediment layer). Additionally, the 10-day sediment toxicity tests do not examine the potential for 

reproductive effects, which were observed in toxicity studies where other taxa were exposed to 

tribufos. The current risk assessment for sediment may underestimate the potential for risk to 

benthic aquatic invertebrates.

Pollinator risk assessment guidance has recently been finalized (see Section 3.2.3.4) and 

recommends the following honey bee data for a Tier I screening-level risk assessment for honey 

bees:

• acute oral toxicity to adult honey bees;

• acute oral toxicity to larval honey bees;

• chronic oral toxicity to adult honey bees; and,

• chronic oral toxicity to larval honey bees.

If Tier I studies identify the potential for a risk concern, additional studies may be requested. As 

these data are currently unavailable for tribufos, the potential for adverse effects to individual 

honey bees and their brood could not be fully assessed.

Toxicity data from chronic exposure to tribufos are not currently available for the species of 

freshwater fish that are most acutely sensitive to tribufos (i.e., bluegill sunfish; L. macrochirus). 

Therefore, the chronic effects endpoint used in this assessment for freshwater fish is based on an 

acute-to-chronic ratio (ACR) using toxicity data from the estuarine/marine sheepshead minnow 

(Cyprinodon variegatus). Because the ACR was determined using endpoints from an 

estuarine/marine species, chronic risks to freshwater fish may be over- or under-estimated, 

depending on the relative sensitivity of those species.

2. Problem Formulation

The purpose of problem formulation is to provide the foundation for the environmental fate and 

ecological risk assessment for the Registration Review of the organophosphate pesticide tribufos 

(S,S,S-tributyl phosphorotrithioate; CAS No.: 78-48-8; PC code 074801) and its end-use 

products. The problem formulation sets the objectives for the risk assessment, establishes a risk 

hypothesis and conceptual model depicting potential routes of exposure, biological receptors and 

their measurement endpoints, and it provides a plan for analyzing the data and characterizing the 

risk associated with the use of tribufos (USEPA, 1998). 

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). To determine whether a pesticide can be registered, the EPA evaluates its 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 Review2

. The 

 2 http://www.epa.gov/oppsrrd1/registration_review/
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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.

2.2. Stressor Source and Distribution

2.2.1. Nature of the Stressor

Tribufos, S,S,S-tributyl phosphorotrithioate, is an organophosphate chemical that, when applied 

to cotton plants and absorbed by its leaves, injures the cells in the layer immediately beneath the 

leaf cuticle (via esterase inhibition), causing stress to the plant, which stimulates the production 

of ethylene (USEPA, 2000). The increased ethylene concentrations result in the premature 

abscission of whole green leaves. Due to tribufos’ mechanism of action, effects to terrestrial 

plants that form abscission zones (e.g., some woody, deciduous plants) could occur as a 

consequence of exposure. Like other organophosphates there is evidence that tribufos inhibits 

brain, plasma, and erythrocyte acetylcholinesterase (ChE) activity in animals after acute and 

chronic exposures (USEPA, 2000). Table 2 summaries the chemical/physical characteristics of 

tribufos.

Table 2. Chemical Profile of Tribufos.

Common name Tribufos

Chemical name S,S,S-tributyl phosphorotrithioate; CAS No.: 78-48-8; PC code 074801

Structure (A3 form of the 

molecule, the most toxic of 

the 8 homologues)

SMILES Notation O=P(SCCCC)(SCCCC)SCCCC

Pesticide type Pre-harvest cotton defoliant

Chemical class Organophosphate

CAS number CAS No.: 78-48-8

Molecular mass 314.54 g mol-1

2.2.3. Overview of Pesticide Use 

Tribufos was first registered in the United States in 1961 for use on cotton and is formulated as 

an emulsifiable concentrate that is applied by ground or aerial spray as a pre-harvest treatment as 

a defoliant to facilitate cotton harvesting and to reduce the pieces of dry leaves and other inert 

matter in harvested cotton. Three technical products and four end-use products containing 

tribufos are registered and all products only contain the single active ingredient. This assessment 

evaluates risk based on currently labeled maximum application rates and average application 
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rates. The average application rate is derived by dividing the average pounds of active ingredient 

applied by the total area treated (USEPA, 2014). Thus, the average rate may not reflect an actual 

application rate used in the field. Additionally, usage at rates higher than the average application 

rate does occur. More information on use and usage is available in the analysis plan.

2.3. Receptors

2.3.1. Aquatic and Terrestrial Effects

The receptor is the biological entity that is exposed to the stressor (USEPA, 1998). Consistent 

with the process described in the Overview Document (USEPA, 2004a), this risk assessment uses 

a surrogate species approach. Toxicity data generated from surrogate test species, which are 

intended to be representative of broad taxonomic groups, are used to extrapolate potential effects 

on a variety of species (receptors) included under these taxonomic groupings. 

Acute and chronic toxicity data from studies submitted by pesticide registrants along with the 

available open literature are used to evaluate potential direct effects to aquatic and terrestrial 

receptors. The open literature studies are located through EPA’s ECOTOXicity database 

(ECOTOX; http://cfpub.epa.gov/ecotox/), which provides a source for locating single chemical 

toxicity data for aquatic life, terrestrial plants, and wildlife. 

2.3.2. Ecosystems Potentially at Risk

The ecosystems potentially at risk are often extensive in scope; therefore, it may not be possible 

to identify specific ecosystems during the development of a nation-wide ecological risk 

assessment. However, in general terms, terrestrial ecosystems potentially at risk could include 

the treated field or tree(s) and immediately adjacent areas that may receive drift or runoff. Areas 

adjacent to the treated field or tree(s) could include cultivated fields, fencerows and hedgerows, 

meadows, fallow fields or grasslands, woodlands, riparian habitats, and other uncultivated areas. 

Aquatic ecosystems potentially at risk include water bodies adjacent to or downstream from use 

sites, and might include natural or impounded lentic water bodies such as ponds, lakes and 

reservoirs, or flowing waterways such as streams or rivers including all adjacent off-channel 

habitats that are permanently or intermittently connected to flowing waters. For uses in coastal 

areas, aquatic habitat also includes marine ecosystems, including estuaries, embayments, and 

near-shore environments such as tidal marshes.

2.4. Assessment Endpoints

Assessment endpoints represent the actual environmental value that is to be protected, defined by 

an ecological entity (species, community, or other entity) and its attribute or characteristics 

(USEPA, 1998). For tribufos, ecological entities of potential concern include the following: 

birds, terrestrial- and aquatic-phase amphibians, reptiles, mammals, freshwater fish and 

invertebrates, estuarine/marine fish and invertebrates, non-target terrestrial plants, terrestrial 

invertebrates, and aquatic vascular and non-vascular plants. The attributes for each of these 

entities include growth, survival, and reproduction.
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The assessment endpoints are intended to reflect population sustainability and community 

structure within ecosystems and hence relate back to ecosystems at risk. If risks are expected for 

given species/taxa based on the screening-level assessment, then risks might be expected to 

translate to higher levels of biological organization.

2.5. Conceptual Model

For a pesticide to pose an ecological risk, it must reach ecological receptors in biologically 

significant concentrations. An exposure pathway is the means by which a pesticide moves in the 

environment from a source to an ecological receptor. For an ecological pathway to be complete, 

it must have a source, a release mechanism, an environmental transport medium, a point of 

exposure for ecological receptors, and a feasible route of exposure. The conceptual model is 

intended to provide a written description and visual representation of the predicted relationships 

between tribufos, potential routes of exposure, and the predicted effects for the assessment 

endpoint. The conceptual model consists of two major components: the risk hypotheses and a 

conceptual model (USEPA, 1998).

The conceptual model used to depict the potential ecological risk associated with the proposed 

and existing uses of tribufos relies on previous assessments and the Agency’s current 

understanding of the environmental fate and ecological effects of the chemical. Consistent with 

the intended use of tribufos as a defoliant, potential risk to non-target terrestrial and aquatic 

plants is expected. Effects on plants, which represent the primary producers in an ecosystem, 

may result in secondary effects on consumers which may include a broad range of taxa. 

Based on the Interim Reregistration Eligibility Decision (IRED) (USEPA, 2000), acute and 

chronic risks to birds, mammals, and aquatic invertebrates (both freshwater and 

estuarine/marine) are expected from the registered use of tribufos on cotton. Acute risks to 

estuarine/marine fish, but not freshwater fish, were also identified. Neither risks to fish from 

chronic exposure to tribufos nor risks to plants could be fully evaluated previously due to a lack 

of data. Several studies were submitted after the IRED was completed. These studies were 

considered by the EPA in an endangered species risk assessment for tribufos and the endangered 

California red-legged frog (Rana aurora draytonii) (CRLF) that was submitted to the U.S. Fish 

and Wildlife Service as part of formal Section 7 consultation on October 20, 2008. EPA 

determined in its endangered species risk assessment that use of tribufos on cotton in California 

was likely to adversely affect (LAA) the CRLF. Based on the previous risk assessments, the 

current risk hypothesis evaluated in this assessment is that the use of tribufos on cotton may 

result in exposure and subsequent adverse effects to terrestrial and aquatic animals.

Due to the potential direct adverse effects to the listed and non-listed groups of organisms 

described above, listed species in all taxa may potentially be affected indirectly due to alterations 

in their habitat (e.g., food sources, shelter, and areas to reproduce) if exposure to tribufos occurs.
-Page 14 of 116-

2.5.1. Risk Hypotheses

For tribufos, the following generic ecological risk hypotheses are being employed for this risk 

assessment:

Given the currently labeled uses of tribufos and its environmental fate properties, there is 

a likelihood of exposure to non-target terrestrial and aquatic organisms. 

When used in accordance with the label, tribufos may result in potential adverse effects 

upon the survival, growth, and reproduction of non-target terrestrial and aquatic 

organisms. 

 2.5.2. Conceptual Pathways Table

The environmental fate properties of tribufos indicate that for foliar applications, spray drift and

runoff (including of eroded sediment) are potential transport mechanisms to aquatic habitats 

where non-target organisms may be exposed. It is expected that non-target terrestrial organisms 

can be exposed via foliar applications of tribufos through consumption of exposed plants and 

invertebrates on treated fields. Additionally, tribufos may reach terrestrial environments off the 

field via spray drift, and via application of irrigation water containing residues of tribufos. A 

summary of the transport pathways and the models used for those pathways in the assessment are 

provided in Table 3. 

Tribufos is a semi-volatile, moderately soluble compound that has low mobility in soil. It is 

stable to abiotic degradation and is persistent in soil. It is expected to sorb to soil and sediments, 

and is slightly persistent in anaerobic aquatic environments. Surface water contamination may

occur due to spray drift and/or transport of residues sorbed to sediment in runoff (i.e., erosion). 

In either aquatic or terrestrial environments, tribufos may bioaccumulate in organisms exposed to 

it (BCF =730 L/kg-wet weight fish); however, it is metabolized and depurates rapidly (71-88% 

of residues depurated over 14-days) once exposure ends. 

Additionally, the Screening Tool for Inhalation Risk (STIR) version 1.0 (November 23, 2010) 

indicates that exposure via inhalation is not likely to be a risk concern for birds and mammals 

(Appendix G). Sorption coefficients of tribufos indicate that risk to sediment-dwelling 

organisms should be evaluated.

The exposure pathways may result in exposure to various aquatic and terrestrial organisms. 

With exposure, the following attribute changes have the potential to occur: 

• effects to individual organisms;

• effects to the food chain (reduction in prey and food, modification of primary constituent 

elements (PCE) related to prey availability); and,

• effects to habitat integrity (reduction in primary productivity, reduced cover, community 

change, and modification of PCE related to habitat).
-Page 15 of 116-

Table 3. List of the Various Models and the Related Taxa for which the Models will be 

Used to Assess Risk.

Environment Taxa of 

Concern

Exposure

Media

Exposure 

Pathway

Model(s) or 

Pathway Attribute Change

Aquatic

Vertebrates/ 

Invertebrates

(including 

sediment 

dwelling) Surface water and 

sediment

Runoff and spray 

drift to water and 

sediment

SWCC

Individual 

Organisms

Food Chain

Habitat Integrity

Aquatic Plants 

(vascular and 

nonvascular)

Food Chain

Habitat Integrity

Riparian plants See terrestrial exposure pathways Habitat Integrity

Terrestrial

Vertebrate

Dietary items

Ingestion of 

residues in/on 

dietary items as 

a result of direct 

application

T-REX

Individual 

Organisms

Food Chain

Habitat Integrity

Consumption of 

aquatic organisms

Residues taken 

up by aquatic 

organisms

KABAM

Individual 

Organisms

Food Chain

Habitat Integrity

Plants

Spray drift/runoff Runoff and spray 

drift to plants TERRPLANT

Food Chain

Habitat Integrity Surface water

Residues in 

irrigation water

Not a major 

transport 

pathway Groundwater

Bees and other 

terrestrial 

invertebrates1

Contact

Dietary items

Spray contact 

and ingestion of 

residues in/on 

dietary items as 

a result of direct 

application

See honey bee 

risk assessment 

guidance 

(USEPA et al., 

2014)

Individual 

Organisms

Colony/Population 

Integrity

All 

Environments All

Movement through 

air to aquatic and 

terrestrial media

Spray drift

AgDRIFT (Spray 

drift) Individual 

Organisms

Food Chain

Habitat Integrity

AgDISP (Spray 

drift)

Atmospheric 

transport

Not a major 

transport 

pathway

Text in italics represent transport pathways that are not of concern. 1 See pollinator SAP white paper for full list of modelling approaches for evaluating exposure used in this 

assessment (USEPA, 2012b).

2.6. Analysis Plan

2.6.1. Residues of Concern

The only major degradate observed in the aerobic soil metabolism study was 1-butane sulfonic 

acid (1-BSA), identified at approximately 10% of the applied radioactivity 9 months after 

treatment. The 1-BSA degradate was a minor degradate in anaerobic soil, but was a major 
-Page 16 of 116-

degradate in the anaerobic aquatic (i.e., flooded sediment) system, at up to 29.5-30.6% of the 

applied radioactivity 6 to 9 months after treatment. No toxicity data are available on this 

degradate, but it was determined to be of no toxicological concern in previous risk assessments

(USEPA, 2008). Additionally, 1-BSA does not retain the organophosphate moiety of the parent 

compound; the Ecological Structure Activity Relationship (ECOSAR; 

http://www.epa.gov/oppt/newchems/tools/21ecosar.htm) tool indicates 1-BSA will likely be less 

toxic than parent (Appendix H). Therefore, the degradation product is not considered a residue 

of concern. Oxon degradates are often a concern for organophosphates. Tribufos itself is an 

oxon and additional oxon transformation products did not occur in laboratory or field studies (the 

single major degradate was 1-BSA). Therefore, only the parent compound is considered the 

residue of concern. 

2.6.2. Preliminary Identification of Data Gaps and Uncertainties 

This assessment is based upon available data. There are no significant data gaps in the 

environmental fate database submitted by the registrant. However, there is some uncertainty 

introduced by the use of single half-life values. Laboratory metabolism data are only available 

for single soils or sediments for aerobic and anaerobic soil metabolism, and anaerobic aquatic 

metabolism. There are no data available for aerobic aquatic metabolism as that data requirement 

was waived, and tribufos is simply assumed to be stable in aerobic aquatic systems.

Ecotoxicity data gaps are discussed further in the Risk Description. Ecotoxicity data gaps 

include toxicity data for benthic invertebrates, freshwater fish, honey bees, and aquatic nonvascular plants. 

The main ecotoxicity uncertainties in the risk assessment involve risk to honey bee, as data are 

not available to fully assess risk, and benthic aquatic invertebrate risk may be underestimated 

based on available data. While the chronic toxicity endpoint for freshwater vertebrates is 

estimated, it is consistent with available non-quantitative results. Finally, a full dataset to assess 

risk to non-vascular aquatic plants is not available.

2.6.4. Measures of Exposure

Using EFED’s standard suite of models, surface water estimated environmental concentrations 

(EECs) were generated for tribufos for the registered use on cotton. The Surface Water 

Concentration Calculator (SWCC, v 1.106, May, 2014) is used to generate estimated 

environmental concentrations (EEC)s that may occur in surface water. The SWCC simulates 

pesticide transformation on and runoff from an agricultural field (in addition to loadings from 

spray drift), following applications to crops. The model then simulates resulting concentrations 

in an adjacent surface water body (i.e., the standard pond). The measure of exposure for aquatic 

species in surface water bodies receiving runoff and/or spray drift is a return frequency of 1-in10 years of the EEC, based on 30-year simulations. The maximum 1-in-10 year return frequency 

concentration is used for estimating acute exposures to aquatic organisms (plants and animals); 

the 1-in-10 year 21-day mean concentration is used for assessing aquatic invertebrate chronic 

exposure; and the 1-in-10 year 60-day mean concentration is used for assessing chronic exposure 

for fish (and aquatic-phase amphibians for which freshwater fish serve as surrogates). EECs for 
-Page 17 of 116-

tribufos include only the parent compound as the single residue of concern. Additional details on 

the model can be found at http://www.epa.gov/oppefed1/models/water/.

Terrestrial wildlife are potentially exposed to tribufos via consumption of residues on food items 

from spray applications on cotton. For spray applications, the T-REX model (Terrestrial 

Residue EXposure model; v. 1.5.2; June 6, 2013) is used to predict dietary exposure to tribufos 

residues on foliar surfaces and insects using the Kenaga nomogram as modified by Fletcher 

(Fletcher et al., 1994; Hoerger and Kenaga, 1972). In this assessment, the default foliar 

dissipation half-life of 35 days was used for terrestrial modeling purposes. Additionally, the 

influence of lower foliar dissipation half-live values on risk is explored. Estimated exposures of 

terrestrial insects to tribufos are evaluated. Additional details on the model may be found at 

http://www.epa.gov/oppefed1/models/terrestrial/trex/trex_v1_5_webversion.xlsm.

The TERRPLANT (v. 1.2.2; October 29, 2009) model is used to derive EECs relevant to 

terrestrial and wetland plants for the proposed new uses of tribufos. The model assumes that 

default fractions of the intended application will be transported to an adjacent field through 

runoff and spray drift. Measures of exposure to terrestrial plants are expressed as a fraction of 

the mass of the tribufos applied to a treated field. Additional details on the model may be found 

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

To characterize potential non-target animal and plant exposure from spray drift alone, Tier 1 

modules in AgDRIFT (v. 2.1.1, December 2011) were used to estimate how far off-site spray 

drift exposure might occur at a level known to cause adverse effects based on aquatic and 

terrestrial toxicity endpoints. Additional details on the model may be found at 

http://www.agdrift.com/AgDRIFt2/DownloadAgDrift2_0.htm.

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 tribufos’ 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 (LD50 = 192

mg/kg-bw/day, MRID 41954903; and inhalation LC50 >4.65 mg/L, MRID 41782301, 

respectively) as well as the most sensitive acute oral avian toxicity endpoint (Bobwhite quail; 

Colinus virginianus; LD50 =151 mg/kg-bw, MRID 00120771). Based on the results of the STIR 

model (Appendix E), exposure through inhalation of spray drift or the vapor phase of tribufos is 

not considered a primary pathway of concern for either avian or mammalian species on an acute 

inhalation exposure basis. Additional details on the model may be found at 

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

 

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 
-Page 18 of 116-

(USEPA, 2004a). The risk characterization section (Section 4.3), 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 tribufos’ solubility (i.e., 2.3

mg/L at 20oC, MRID 41618803), and compared to the most sensitive acute and chronic avian 

and mammalian toxicity endpoints (see Table 19). Based on this analysis, drinking water 

exposure alone is not considered a primary pathway of concern for birds or mammals on either 

an acute or chronic exposure basis. For a sample of the output generated by SIP v.1.0, please see 

Appendix F. Additional details on the model may be found at

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

Because there is evidence of the potential for bioaccumulation of tribufos in aquatic organisms3, 

an additional exposure pathway that is considered in this assessment is the consumption of 

contaminated fish or aquatic invertebrates that have residues of tribufos from uptake from the 

water and diet. The potential risk from this pathway will be evaluated and discussed further in 

the “Risk Description” section of the document. Multiple lines of evidence will be used to 

evaluate bioaccumulation potential, including measured bioconcentration factors (BCF), 

bioaccumulation factors (BAF) and a food web bioaccumulation model (Kow-Based Aquatic 

Bioaccumulation Model or KABAM version 1.0). The bioaccumulation assessment relies on 

predicted water and sediment concentrations from SWCC to estimate concentrations of tribufos

in the tissues of aquatic organisms. In order to evaluate potential risk these estimated tissue 

concentrations will be compared to toxicity values for various taxonomic groups that may eat 

aquatic organisms. The current version of KABAM estimates risks only to terrestrial birds and 

mammals consuming aquatic prey.

2.6.5. Measures of Effect 

Measures of effect are obtained from a suite of registrant-submitted guideline studies that were 

conducted with a limited number of surrogate test species (Table 4). The test species are not 

intended to be representative of the most sensitive species but rather were selected based on their 

ability to thrive under laboratory conditions. Toxicity testing reported in this risk assessment 

utilizes surrogate species to represent all freshwater fish (2000+) and bird (680+) species in the 

U.S. In addition, the ECOTOX database (USEPA, 2009a) was searched in March 2015 to 

provide more ecological effects data for tribufos. Data were further examined if endpoints were 

more sensitive (lower) than endpoints available from submitted studies or were available for 

amphibians, freshwater mollusks, or reptiles (USEPA, 2011). Data meeting the OPP’s standard

for inclusion in risk assessment (http://www.epa.gov/pesticides/science/literature-studies.html) 

were available in ECOTOX that showed greater sensitivity than results from submitted studies 

 3 The measured BCF values for whole fish was 730 L/kg-wet weight fish. Additionally, the log octanol-water 

partition coefficient of 5.5, indicates that the bioaccumulation pathway should be assessed. However, evidence 

indicates that tribufos is metabolized and accumulates less than expected based on its KOW value and that tribufos 

rapidly depurates (71-88% depurated over 14-days) once fish are transferred to clean water.
-Page 19 of 116-

for freshwater fish (Mayer and Ellersieck, 1986, MRID 40098001, ECOTOX Number 6797) and 

daphnids (Brausch et al., 2010, ECOTOX Record Number 156445; Mayer and Ellersieck, 1986, 

MRID 40098001, ECOTOX Number 6797). Additionally, ECOTOX data were examined for 

data on amphibians, freshwater mollusks, and reptiles. 

The acute measures of effect used in this screening-level assessment are the LD50 (Lethal Dose), 

LC50 (Lethal Concentration), EC50 (Effects Concentration), and IC50 (Inhibition Concentration). 

These are measures of acute toxicity which result in 50% of the respective effect in tested 

organisms. The endpoints for chronic measures of exposure are the NOAEC (No Observed 

Adverse Effects Concentration) and the NOAEL (No Observed Adverse Effects Level). 

Toxicity studies were submitted for freshwater fish and invertebrates, estuarine/marine fish and 

invertebrates, aquatic and terrestrial plants, birds, mammals, and honey bees. The endpoints 

used for risk characterization were derived from studies which underwent review and were 

classified as “acceptable” (conducted under guideline conditions and considered to be 

scientifically valid) or “supplemental” (conditions deviated from guidelines but the results are 

considered to be scientifically valid). 

Table 4 lists the measures of environmental exposure and ecological effects used to assess the 

potential risks of tribufos to non-target organisms. The methods used to assess the risk are 

consistent with those outlined in the document “Overview of the Ecological Risk Assessment 

Process in the Office of Pesticide Programs” (USEPA, 2004a).

Table 4. Measures of Exposure and Measures of Effect Used in Assessing Potential Risks 

Associated with the Proposed Uses of Tribufos.

Assessment Endpoint Surrogate Species and Measures of 

Ecological Effect1 Measures of Exposure

Birds2

Survival

Lowest acute LD50 (single oral dose test) 

and LC50 (subacute dietary test)

Upper-bound residues on 

food items Reproduction 

and Growth

Lowest NOAEC

(21-week reproduction test)

Mammals

Survival Lowest acute LD50 (single oral dose test)

and LC50 (subacute dietary test)

Reproduction 

and Growth

Lowest NOAEC

(2-generation reproduction test)

Aquatic Animals (Fish and 

invertebrates)3

Survival Lowest tested LC50 or EC50

(acute toxicity test)

Peak EECs4

Reproduction 

and Growth

Lowest NOAEC (early life-stage, full 

life-cycle tests, 10-day sediment tests, 

or chronic sediment tests)

21-day EECs for invertebrates 

and 60-day EECs for fish4

Terrestrial plants5 Survival and 

growth

Lowest EC25 (for non-listed plants) and 

corresponding NOAEC (for listed 

plants)

(endpoints derived for monocots and 

dicots from seedling emergence and 

vegetative vigor studies)

Estimates of runoff and spray 

drift to non-target areas

Insects6

Survival Lowest honey bee LD50 (acute contact 

test)

Upper-bound residues on tall 

grass7
-Page 20 of 116-

Table 4. Measures of Exposure and Measures of Effect Used in Assessing Potential Risks 

Associated with the Proposed Uses of Tribufos.

Assessment Endpoint Surrogate Species and Measures of 

Ecological Effect1 Measures of Exposure

Aquatic plants (vascular 

and non-vascular)

Survival and 

growth

Lowest EC50 (for non-listed plants) and 

corresponding NOAEC or EC05 (for 

listed plants)

Peak EECs4

1 The most sensitive species tested within taxonomic groups is used for screening-level risk assessments. 2 Birds represent surrogates for terrestrial-phase amphibians and reptiles. 3 Freshwater fish represent surrogates for aquatic-phase amphibians. 4 Aquatic EECs are based on modeling described in Section 3.2.2.1. 5 Four species of two families of monocots - one is corn, six species of at least four dicot families, of which one is soybeans. 6 Risk to terrestrial invertebrates from tribufos use is quantitatively evaluated against the acute listed terrestrial invertebrate LOC of 0.4. 7 The tall grass residue value from the T‐REX model (v. 1.5.2) is used as a surrogate for pesticide concentrations in nectar and pollen.

2.6.6. Integration of Exposure and Effects

The exposure and toxicity effects data are integrated in order to evaluate the risks of adverse 

ecological effects on non-target species. For the risk assessment of tribufos, the risk quotient 

(RQ) method is used to compare estimated exposure and measured toxicity values. The RQ 

method involves dividing estimated environmental concentrations (EEC) by acute and chronic 

toxicity values. The resulting RQs are then compared to the Agency’s Levels of Concern (LOC) 

(USEPA, 2004a) (Table 5). The LOCs are used to indicate when applications of tribufos, as 

directed on the label, have the potential to cause adverse effects to non-target organisms from 

exposure to tribufos.

Table 5. Agency Risk Quotient (RQ) Metrics and Levels of Concern (LOC) Per Risk Class.

Risk Class Risk Description RQ LOC

Aquatic Animals (fish and invertebrates)

Acute Potential for effects to non-listed animals from acute 

exposures

Peak EEC/LC501 0.5

Acute 

Restricted Use

Potential for effects to animals from acute exposures

Risks may be mitigated through restricted use 

classification

Peak EEC/LC501 0.1

Acute Listed 

Species

Listed species may be potentially affected by acute 

exposures

Peak EEC/LC501 0.05

Chronic Potential for effects to non-listed and listed animals 

from chronic exposures

60-day EEC/NOAEC (fish)

21-day EEC/NOAEC 1

(invertebrates)

Aquatic Plants

Non-Listed Potential for effects to non-listed plants from 

exposures

Peak EEC/LC501 1

Listed Potential for effects to listed plants from exposures Peak EEC/NOAEC 1

Terrestrial Animals (mammals and birds)2

Acute Potential for effects to non-listed animals from acute 

exposures

EEC/LC50 (Dietary) 0.5

EEC/LD50 (Dose)

Acute 

Restricted Use

Potential for effects to animals from acute exposures

Risks may be mitigated through restricted use 

classification

EEC/LC50 (Dietary)

0.2

EEC/LD50 (Dose)
-Page 21 of 116-

Risk Class Risk Description RQ LOC

Aquatic Animals (fish and invertebrates)

Acute Listed 

Species

Listed species may be potentially affected by acute 

exposures

EEC/LC50 (Dietary) 0.1

EEC/LD50 (Dose)

Chronic Potential for effects to non-listed and listed animals 

from chronic exposures EEC/NOAEC 1

Honey bees

Acute Potential for effects to individual bees or brood due 

to contact and dietary exposure.

Contact EEC/acute contact 

LD50

Diet EEC/ acute oral LD50

Diet EEC/ Larval LD50

0.4

Chronic Potential for effects to individual bees or brood due 

to contact and dietary exposure.

Diet EEC/ adult NOAEL

Diet EEC/ larval NOAEL 1.0

Terrestrial and Semi-Aquatic Plants

Non-Listed Potential for effects to non-target, non-listed plants 

from exposures EEC/ EC25 1

Listed Plant Potential for effects to non-target, listed plants from 

exposures

EEC/ NOAEC

1

EEC/ EC05

1 LC50 or EC50. 2 EEC based on upper bound Kenaga nomogram values for foliar exposure.

3. Analysis

3.1. Use Characterization

Use patterns 

Tribufos was first registered in the United States in 1961 for use on cotton and is formulated as 

an emulsifiable concentrate that is applied by ground or aerial spray as a pre-harvest treatment as 

a defoliant to facilitate cotton harvesting and to reduce the pieces of dry leaves and other inert 

matter in harvested cotton. Three technical products and four end-use products containing 

tribufos are registered and all products only contain one active ingredient. End-use products 

include FOLEX® 6EC (70.5% a.i.), Def® 6 Emulsifiable Defoliant

® (70.5% a.i.), DFT® 6 EC 

Cotton Defoliant (70.9% a.i.), and Tribufos® 6 (70.9% a.i.; liquid). Table 6 provides details on 

the registered use rates and typical use rates.

The maximum annual label application rate is 1.875 lb a.i./A in California and Arizona, and the

maximum annual label application rate is 1.125 lb a.i./A in all other states. In all states, two 

applications are allowed, not to exceed the annual maximum application rate, although it is 

usually used in a single application (BEAD, 2014). The reapplication interval is not specified on 

the current labels other than as “2-6 weeks apart” for rank cotton (CA and AZ). The pre-harvest 

interval is listed as 7-10 days for CA and AZ. Applications are limited to foliar applications via 

ground and aerial spray equipment; applications via irrigation systems (chemigation) are not 

allowed. All formulations are liquid formulations.

Table 6. Tribufos Use on Cotton: Maximum Application Rates On Labels and Average 

Application Rates Applied in the Field
-Page 22 of 116-

Rates Shown Area 

Allowed

Single App 

Rate 

(lbs a.i./A)

Seasonal 

Rate2

(lbs a.i./A)

Number of 

Applications Comment

Maximum Rates 

on Labels

All except 

CA and AZ 1.125 1.125 1 ˗

CA and AZ 1.875 1.875 1 ˗

Average Rates 

Used in the Field 

Recommended 

by BEAD1

All except 

CA and AZ 0.54 Not provided 1 Average over 2008 to 2012

CA 0.64 Not provided 1.2 Average over 2008 to 

2012 (excluding 2011)

AZ 0.77 Not provided 1.6 Average over 2008 to 

2012 1

Averate rates in the field were provided by BEAD (USEPA, 2014). 2Cotton will only be grown once per year on a given production field because it is a warm season crop with a growing cycle of 130 to 160 days 

(USEPA, 2014). Therefore, the seasonal rate is equal to the annual rate. Two applications are allowed, as long as the maximum annual 

application rate is not exceeded. Generally tribufos is applied as a single application, but may be used twice such as for rank cotton. Rank cotton 

refers to tall, vegetative cotton growth; often a result of late planting, excessive nitrogen fertilizer, fertile soils, or excessive moisture. Rank 

growth often renders cotton plants more attractive and susceptible to late-season insects, more susceptible to boll rot, and more difficult to 

defoliate.

Though labels indicate potential use of tribufos, usage data allow for more accurate 

characterization of actual usage of tribufos. The Screening Level Usage Analysis (SLUA)

prepared by the Biological and Economic Analysis Division (BEAD), estimated the average 

percent crop treated as 30% nationally, with a maximum estimate of 35% (USEPA, 2014). The 

average annual pounds applied estimated for the SLUA was 2.3 million pounds per year. 

Tribufos is used throughout the cotton belt which extends from California to Florida and as far 

north as Missouri, Tennessee, and Virginia (USEPA, 2014).

BEAD used available proprietary data to estimate the average a.i. rate and number of 

applications both nationally and for California and Arizona. National usage data from 2008 –

2012 show that the average rate for tribufos is 0.54 lb a.i. per acre, and the average number of 

applications per year on acreage is 1.2 per year. Arizona and California have higher labelled 

application rates. For Arizona from 2008 - 2012, the average rate for tribufos is 0.77 lbs a.i. per 

acre, and the average number of applications is 1.6 per year. From 2008 – 2012 (excluding 2011 

due to anomalous data), the average rate for California was 0.64 pounds a.i. per year, and an 

average number of applications on treated acreage is 1.2 per year. BEAD indicated that data for 

2011 in California are outliers and should not be considered in the 5-year average because they 

are not considered by BEAD to be representative. BEAD recommended using an average rate of 

0.64 lbs a.i./A for California, which excludes usage data from 2011.

This assessment evaluates exposure and consequent risk based on currently labeled maximum 

application rates and average application rates. The average application rate is derived by 

dividing the average pounds of active ingredient applied by the total area treated (USEPA, 2014). 

Thus, the average rate may not reflect an actual application rate used in the field and is lower 

than the rate used by some. 

3.2. Exposure Characterization

3.2.1. Environmental Fate and Transport Characterization

The general physical/chemical properties of tribufos are summarized in Table 7. Environmental 
-Page 23 of 116-

fate and transport-related properties of tribufos are characterized in further detail in the following 

sections. The Agency’s understanding of these properties is limited by the available dataset. 

Table 7. General Physical and Chemical Properties of Tribufos.

Property Value Reference

Molecular mass 

(molecular formula) 314.54 g/mol MRID 41618803

Vapor pressure (20°C) 1.7 x 10-6 Torr MRID 41618803

Water solubility 

(20°C) 2.3 mg/L MRID 41618803

Henry’s Law constant 

(20°C) 3.1 x 10-7 atm-m3

/mol (calculated)

Octanol-air partition 

coefficient (KOA) 2.6 x 1010 EPI Suite (v.4.11, 2012), estimate

Octanol-water partition 

coefficient (Kow; Log 

Kow) (20°C)

3.31 x 105

5.5 (log Kow)

5.7

MRID 41618803

EPI Suite (v.4.11, 2012), experimental database

Fish Bioconcentration 

Factor (35 d)

300X (edible)

1300X (inedible)

730X (whole fish)

71-88% depurated after 14 days

113-277 (estimated BCFs)

MRID 41618811

MRID 43080401

EPI Suite (v.4.11, 2012), estimates

Tribufos is a semi-volatile, moderately soluble compound that has low mobility in soil. Based 

on submitted data, tribufos is stable to the abiotic processes hydrolysis and photolysis (MRIDs

41618814, 41618816, 41719401). Estimates of atmospheric oxidation (half-life of 1.6 days) 

indicate that it would rapidly degrade if present in the atmosphere (EPI Suite, v.4.11, 2012). It is 

persistent in aerobic and anaerobic soil, and slightly persistent in anaerobic aquatic 

environments. Therefore, there is potential for tribufos to accumulate in soil with repeated 

applications if the compound reaches the soil surface and does not dissipate from such through 

volatilization. Tribufos is expected to sorb to soil, and not to appreciably leach to groundwater 

or move to surface waters in solution phase via runoff. To the extent that it occurs, surface water 

contamination may be expected to arise predominantly as a consequence of spray drift and/or 

transport of residues sorbed to sediment entrained in runoff (i.e., erosion). In either aquatic or 

terrestrial environments, tribufos may bioconcentrate and/or bioaccumulate in organisms 

exposed to it. Additional discussion on the environmental fate of tribufos is presented below. 

Table 8 contains the submitted environmental fate data for the pesticide.

Table 8. Environmental Fate Data Summary for Tribufos.

Parameter Value Source/Comments

Persistence

Hydrolysis half-life (50°C) No evidence of degradation MRID 41618814
-Page 24 of 116-

Parameter Value Source/Comments

Persistence

Aqueous photolysis half-life (25°C) No evidence of degradation MRID 41719401

Soil photolysis half-life (20°C) No evidence of degradation MRID 41618816

Aerobic soil metabolism half-life (25°C)

745 d (sandy loam)

MRID 42007204

Single half-life 

available.

Anaerobic soil metabolism half-life (25°C) 389 d (sandy loam) MRID 42007205

Aerobic aquatic metabolism half-life No data Data requirement

waived.

Anaerobic aquatic metabolism half-life (25°C) 100 d (flooded silty clay sediment) MRID 43325504

Mobility

Freundlich soil-water partition coefficients (KF); 60.6 mL/g (sandy loam)

66.8 mL/g (sand)

74.3 mL/g (silt loam)

106 mL/g (clay loam)

MRID 41618817

Freundlich organic carbon-normalized coefficients 

(KFOC) 10,465 mL/g (sandy loam)

12,684 mL/g (sand)

4,870 mL/g (silt loam)

9,115 mL/g (clay loam)

MRID 41618817

Does not follow Koc

model (USEPA, 

2008, DP Barcode 

357536)

Field Dissipation

Terrestrial field dissipation half-life 13.9 d (not detected below 15 cm) MRID 45863701

3.2.1.1. Transport and Mobility

Tribufos is expected to be relatively immobile in soil, based on Freundlich soil adsorption values 

that range from 61 to 106 L/kg (MRID 41618817). Because the coefficient of variation (COV) 

for Freundlich soil adsorption (KF) values (0.26) is less than that for values normalized for 

organic carbon (KFOC) (0.36), tribufos does not conform to the organic carbon partitioning model 

(USEPA, 2006). However, surface water contamination may occur via spray drift and/or from 

the transport of residues sorbed to sediment that is suspended in runoff (i.e., erosion). Based on 

the tendency of the compound to bind to soil, significant groundwater contamination as a result 

of tribufos use is not expected. 

Tribufos may dissipate in the environment partially through volatilization, although the potential 

for volatilization from soil and water is expected to be low to moderate. Tribufos’ moderately 

low vapor pressure (1.7 x 10-6 torr at 20oC; MRID 41618803) and Henry’s Law constant 

(calculated to be 3.1 x 10-7 atm-m3/mol at 20°C) indicate that the compound is semi-volatile. In 

a submitted foliar washoff study, 7.6% of the applied tribufos volatilized during the 15 days after 

foliar application. However, volatilization was not observed in other submitted environmental 

fate studies. Based on a quantitative structure-activity relationship (QSAR) estimation method, 

volatilized (vapor phase) tribufos is predicted to be degraded in the atmosphere by reaction with 

photochemically-produced hydroxyl radicals, with an estimated atmospheric half-life of 1.6

hours (EPI Suite v.4.11, 2012). 
-Page 25 of 116-

The potential for moderate volatilization has been reviewed by the California Department of 

Pesticide Regulation (CDPR) in which a panel reviewed the open literature on ambient air 

concentrations of tribufos as of August 1998 (Scientific Review Panel, 1999). The panel 

highlighted two monitoring studies in which air concentrations in California Central Valley 

towns near cotton fields ranged from nondetectable (i.e., less than 1.1 ng/m3) to 548 ng/m3, 

indicating that photodegradation in the atmosphere may not consistently remove all of the 

pesticide. Furthermore, CDPR has found that tribufos is readily photodegraded to the volatile 

transformation product, n-butyl mercaptan, and possibly to n-butyl disulfide (CADPR, 1999). 

However, these photodegradates were not observed in environmental fate studies submitted to 

the Agency. CDPR listed the end-use product DEF (i.e., tribufos) as a Toxic Air Contaminant 

(TAC) due to a slight carcinogenic risk from tribufos in ambient air.

3.2.1.2. Degradation

Tribufos degraded very slowly (745-day half-life) in a single aerobic sandy loam soil (25°C; 

MRID 42007204). Thus, tribufos is apparently more persistent than is typical for most 

organophosphate chemicals. The only major degradate observed in the aerobic soil metabolism 

study was 1-butane sulfonic acid (1-BSA), identified at approximately 10% of the applied 

radioactivity 9 months after treatment. 

A half-life of 389 days was calculated for tribufos in anaerobic sandy loam soil (MRID 

42007205), and a half-life of approximately 100 days was calculated for tribufos in anaerobic 

flooded silty clay sediment (MRID 43325504). The primary degradate, 1-BSA, was a minor 

degradate in anaerobic soil. However, it was a major degradate in the anaerobic aquatic (i.e., 

flooded sediment) system, at up to 29.5% to 30.6% of the applied radioactivity 6 to 9 months 

after treatment. Anaerobic aquatic metabolism followed an atypical biphasic pattern, with 

degradation occurring slowly during the first 2 months, and more rapidly thereafter. The 

observed degradation half-life (DT50) was 4 to 6 months; however, the linearly regressed half-life 

for the duration of the study was approximately 2 months. No explanation was provided for the 

atypical degradation pattern. A single, nonlinearly regressed half-life of 100 days was selected 

to represent the benthic zone degradation rate in modeling.

3.2.1.3. Field Studies

In a supplemental terrestrial field dissipation study conducted on a bare ground plot at a single 

site in Mississippi (MRID 45863701), the half-life for dissipation of tribufos in the top 15 cm of 

sandy loam soil was 13.9 days. The dissipation pattern was biphasic in this study, with an initial 

rapid decline through approximately 28 days post-treatment, followed by slower dissipation for 

the duration of the study period. Tribufos did not leach below the 15-cm depth. The major route 

of dissipation of tribufos under terrestrial field conditions was not reported. The patterns of 

formation and decline of transformation products were not addressed and the transformation 

products were not identified in the study; therefore, it could not be determined whether the study 

duration was adequate to capture such information. In addition, only one site representative of 

the intended tribufos use site (cotton) was studied. The dissipation half-life from this study is not 

consistent with the laboratory fate data and suggests that there are dissipation pathways (such as 
-Page 26 of 116-

volatilization) that have not been addressed in laboratory fate studies. A cropped plot (with foliar 

treatment) was not studied.

3.2.1.4. Monitoring Data

Monitoring data are available for tribufos. Groundwater monitoring was conducted by the

CDPR, the Department of Health Services and the U.S. Geological Survey (USGS) between 

1986 and 1994 (CADPR, 2004b). All of the samples from 465 wells in 16 California counties 

yielded non-detectable residues (< the LOQ of 0.01 ppb).

Surface water monitoring conducted by CDPR between 1991 and 2003 yielded positive 

detections (at the LOQ of 0.01 ppb) in only 2 of 810 samples from 15 counties (CADPR, 2004a). 

However, data from more recent years (e.g., 2007, 2009) from USGS’s National Water Quality 

Assessment (NAWQA) program show multiple detections in CA surface water samples, at up to 

0.3 μg/L; tribufos was also detected in surface water samples from Texas at up to that maximum 

level. There are NAWQA surface water monitoring data available for CA, TX, AL, AK, AZ, 

CO, CT, FL, GA, IA, ID, IL, IN, KY, MD, MI, MN, MO, MS, NC, NE, NJ, NM, NE, NY, OH, 

OR, PA, SC, TN, TX, UT, VA and WA. Generally tribufos levels in surface water are <0.035 

μg/L, although there are multiple detections greater than that in several states including CA and 

TX. NAWQA sediment monitoring data are reported for IA, IL, IN, KS, KY, MN, NE, OH, SD, 

TX through 2013. The maximum level reported in sediment is 5.1 μg/kg for a sample from 

Texas. Generally tribufos levels in sediment are <2.2 μg/kg.

3.2.1.5. Bioconcentration Data

Bioconcentration factors (BCF) for tribufos residues in bluegill sunfish were determined to be 

300X for edible tissue, 1300X for inedible tissue, and 730X for whole fish tissues (MRIDs 

41618811, 43080401). The 730 L/kg BCF value is based on measured radioactivity associated 

with tribufos in the water and tissue with 28 days of exposure. This was the highest BCF value 

reported in the study for whole fish with a 35-day exposure period, confirming that equilibration 

was likely achieved. After 14 days of depuration, 71-88% of residues were eliminated from fish 

tissues. Parent tribufos was the only compound detected in the edible tissue and water samples. 

In inedible tissue, tribufos parent was the major extractable residue at 33 to 46.2% of 

radioactivity. In addition to parent, 47 other metabolites (radio-labeled compounds) were 

extracted from the inedible tissue; however, only three were identified, at between 3% and 4% of 

the measured radioactivity. Unidentified unextracted radioactivity accounted for 17 to 20% of 

the radioactivity extracted from fish tissue. The BCF values indicate that tribufos has some

potential to bioconcentrate, although this may be limited by the metabolism and excretion of the 

compound. 

Consistent with these BCF values, an octanol-water partition coefficient (KOW) of 3.31 x 105 (log 

KOW of 5.52) was determined for tribufos, indicating that tribufos is oleophilic (MRID 

41618803). Quantitative structure-activity relationship (QSAR) estimates for tribufos 

determined using the Estimation Program Interface (EPI) Suite

(http://www.epa.gov/oppt/exposure/pubs/episuite.htm) yielded an octanol-air partition 

coefficient (KOA) of 2.6 x 1010 (log octanol-air partition coefficient, KOA, of 10.41). Kelly and 
-Page 27 of 116-

Gobas (2003) and Armitage and Gobas (2007) concluded that chemicals with log KOW >2 and 

log KOA >5 have the potential to biomagnify in organisms in terrestrial habitats. Therefore, 

tribufos has the potential to biomagnify in organisms in terrestrial habitats.

3.2.1.5. Environmental Degradates

The major degradate resulting from environmental degradation processes is 1-butane sulfonic 

acid (1-BSA; Table 9). This transformation product was identified in at least one of the 

environmental fate studies at concentrations >10% of the applied radioactivity and would 

typically be considered in the environmental exposure assessment. However, because it does not 

retain the organophosphate moiety of the parent compound, 1-BSA is not expected to be as toxic 

as tribufos and the degradate is not included in EECs. 

Table 9. Table of Transformation Products Formed in the Environmental Fate Studies

Chemical 

Name Chemical Structure Study Type MRID(s) Maximum %AR (day)

Final %AR 

(study length

or last interval 

detected)

Tribufos

S,S,Stributyl 

phosphorotrithioate

Aerobic Soil 

Metabolism

42007204 ̶ 62.3-66.8% 

(360 days)

Anaerobic Soil 

Metabolism 42007205 ̶

73.3-84.4% (60 

days postflooding)

Anaerobic 

Aquatic 

Metabolism

43325504 ̶ 0.9% (12 

months)

Terrestrial 

Field 

Dissipation

45863701 ̶

0.014 μg a.i./g 

(28 days; 0-15 

cm)

1-BSA

1-butane 

sulfonic 

acid

Aerobic Soil 

Metabolism

42007204 6.9-9.9% (272 

days) ̶

Anaerobic Soil 

Metabolism 42007205 3.4% (31 days post-flooding)

3.5% (60 days 

post-flooding)

Anaerobic 

Aquatic 

Metabolism

43325504 29.5-30.6% (6-9 

months) ̶

3.2.2. Measures of Aquatic Exposure

3.2.2.1. Surface Water and Sediment Exposure to Tribufos

Surface Water and Sediment Exposure to Tribufos

Using EFED’s standard suite of models, surface water, sediment pore water, and bulk sediment

EECs were generated for tribufos for the registered use on cotton. The Surface Water 

Concentration Calculator (SWCC, v 1.106, May, 2014), which incorporates the Pesticide Root 
-Page 28 of 116-

Zone Model (PRZM) and the Exposure Analysis Model System (EXAMS), is used to generate 

EECs that may occur in surface water and sediment. The SWCC simulates pesticide 

transformation on and runoff from an agricultural field (in addition to loadings from spray drift), 

following applications to crops. The model then simulates resulting concentrations in an 

adjacent surface water body. The EXAMS model simulates pesticide concentrations in water 

due to loading via runoff, erosion, and spray drift, assuming a standard 1-ha pond, 2-m deep 

(20,000 m3) with no outlet that borders a 10-ha treated field. Simulations are for 30 years, from 

which the Agency estimates peak values that are expected to recur on average once every ten 

years, based on time series of daily estimated concentrations. The coupled PRZM and EXAMS 

models and user’s manuals are available from the U.S. Environmental Protection Agency Water 

Models web-page (USEPA, 2011b).

Exposure estimates generated using the standard pond are intended as surrogates for potential 

concentrations in a wide variety of vulnerable water bodies, including prairie potholes, playa 

lakes, vernal pools, other wetlands, man-made and natural ponds, and ephemeral and first-order 

streams. Multiple site-specific factors will tend to make such water bodies more or less 

vulnerable than the standard pond. Static water bodies that have larger ratios of pesticide-treated 

drainage area to water body volume are generally expected to have higher peak concentrations 

than the standard pond’s EECs. Such water bodies may be smaller in size, and/or have larger 

drainage areas. However such water bodies may be more likely to overflow and discharge 

pesticide in their effluent, whereas the EFED standard pond has no discharge. As watershed size 

increases, it becomes increasingly unlikely that the entire watershed is planted with a single crop 

that is all treated simultaneously with a pesticide. Headwater streams may have peak 

concentrations that are higher than those predicted for the standard pond, but such concentrations 

will likely persist for only short periods of time or distance, as they are carried downstream and 

diluted with water originating elsewhere.

The general chemical and environmental fate data for tribufos listed in Table 8 and 9 were used 

for generating model input parameter values for SWCC (listed in Table 10 and 11). Chemical 

specific and model input values were chosen in accordance with current divisional guidance 

(USEPA, 2009). Modeling was conducted to generate EECs in surface water of tribufos 

following a foliar application to cotton 7 days prior to harvest (harvest date varies with the 

standard scenario used). Modeling was conducted using SWCC with an application rate of 1.125

(for all scenarios except CA) or 1.875 lb a.i./A (CA scenario only), applied as either a single 

aerial or single ground spray application. Modeling was also conducted for the average 

application rates described in Section 2 on Overview of Pesticide Use and Usage. Modeling 

results are reported in Table 12. 

Table 10. SWCC Chemical Specific Input Parameter Values for Modeling Tribufos Use on 

Cotton.

INPUT PARAMETER (units) VALUE(S) SOURCE COMMENT

Molecular weight (g/mol) 314.54 g/mol MRID 

41618803 −

Henry’s Law Constant (atmm3

/mol; 20oC)

3.1 x 10-7 atmm3

/mol

MRID 

41618803 −
-Page 29 of 116-

INPUT PARAMETER (units) VALUE(S) SOURCE COMMENT

Vapor Pressure (Torr; @ 20 oC) 1.7 x 10-6 Torr MRID 

41618803 −

Solubility in Water (mg/L @ 

20oC; pH 7) 2.3 MRID 

41618803 −

Soil partition coefficient KFD

(mL/g)

76.9 mL/g MRID 

41618817

Mean KFD (x=4), per EFED 

guidance.1 Does not follow Koc

model.

Hydrolysis (t1⁄2 in days; pH 7) Stable MRID 

41618814 −

Aqueous Photolysis (t1⁄2 in days; 

pH 7) Stable MRID 

41719401 −

Aerobic Soil metabolism halflife (25°C; t1⁄2 in days) 2235 (sandy loam) MRID 

42007204

Input value is three times the study 

half-life to account for uncertainty in 

using a single half-life, per EFED 

guidance.1

Anaerobic aquatic metabolism 

half-life (25°C, t1⁄2 in days) 300 MRID 

43325504

Input value is three times the study 

half-life (100 days) to account for 

uncertainty in using a single half-life, 

per EFED guidance.1

Aerobic Aquatic metabolism 

half-life (25°C; t1⁄2 in days) 4470 MRID 

42007204

Input value is two times the aerobic 

soil input value, per EFED guidance.1

1

Input Parameter Guidance found at http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm.

Table 11. SWCC Scenario(s) and Inputs for Modeling Tribufos Use on Cotton.

Geographic 

Area Type of Rate SWCC Scenario

Applic. Rate

(lbs a.i./A) 

[kg/ha]

Applic. Date Source/Comments1

National Max label MS Cotton 1.125

[1.26]

Sept. 15 (single 

application)

Aerial spray/foliar

Ground spray/foliar

National Max label NC Cotton 1.125

[1.26]

Oct. 25 (single 

application)

Aerial spray/foliar

Ground spray/foliar

National Max label TX Cotton OP 1.125

[1.26]

Sept. 9 (single 

application)

Aerial spray/foliar

Ground spray/foliar

California 

and Arizona Max label CA Cotton w/irrig 1.875[2.1]

Nov. 4 (single 

application)

Aerial spray/foliar

Ground spray/foliar

National Average MS Cotton 0.54

[0.61]

Sept. 15 (single 

application Aerial spray, foliar

California Average (CA 

rate) CA Cotton w/irrig 0.64[0.72]

Nov. 4 (single 

application) Aerial spray/foliar

Arizona Average (AZ 

rate) CA Cotton w/irrig 0.77[0.86]

Nov. 4 (single 

application) Aerial spray/foliar

1 The aerial spray drift fraction for the EPA pond is 0.125 (USEPA, 2013). The ground spray drift fraction for the EPA pond is 0.062 (USEPA, 

2013). 

The modeled aquatic EECs resulting from the tribufos use on cotton (listed in Table 12) were 

used for risk estimation in this assessment. Bolded values indicate the maximum EECs for 
-Page 30 of 116-

maximum and average application rates, both of which are associated with aerial spray. Selected 

model input and output files supporting these values are presented in Appendix A. 

Table 12. Modeled Aquatic Surface Water 1-in-10-year EECs for Tribufos on Cotton.

Crop Scenario 

(application 

date)

App Rate 

(lbs 

a.i./A)

1-in-10 year Surface Water 

(μg/L0

1-in-10 year PoreWater1-in-10 year BulkSediment (μg/kg)

peak 

daily

peak 21-

day 

average

peak 60-

day 

average

peak 

daily

peak

21-day 

average

peak 

daily

peak

21-day 

average

Aerial Spray- Max Labeled Rate

MS Cotton, 

(9/15) 1.125 29.7 27.1 25.1 22.4 22.4 1732 1732

CA Cotton 

w/Irrigation 

(11/4)

1.875 21.2 16.8 14.3 11.7 11.7 904 904

Aerial Spray – Average Application Rates

MS Cotton 

(9/15) 0.54 14.2 12.9 11.9 10.7 10.6 827 819

CA Cotton 

w/Irrig., CA 

avg. rate (11/4)

0.64 7.3 5.8 4.9 4.0 4.0 309 309

CA Cotton 

w/Irrig., AZ 

avg. rate (9/15)

0.77 8.7 6.9 5.9 4.8 4.8 370 370

Ground Spray- Max label rate

MS Cotton 

(9/15)

1.125 29.0 26.1 23.9 21.5 21.5 1662 1662

1.125 

runoff 

only

26.5 24.0 21.8 19.8 19.6 1531 1515

Ground Spray – Average Application Rate

MS Cotton 

(9/15) 0.54 14.0 12.6 11.6 10.4 10.4 804 804

3.2.3. Measures of Terrestrial Exposure

3.2.3.1. Ingestion of Surface Residues by Birds and Mammals

Terrestrial wildlife exposure estimates are typically calculated for birds and mammals, 

emphasizing a dietary exposure route for uptake of pesticide active ingredients. Avian exposures 

are considered surrogates for exposures to terrestrial-phase amphibians and reptiles. For 

exposure to terrestrial organisms, such as birds and mammals, pesticide residues on food items 

are estimated, based on the assumption that organisms are exposed to pesticide residues in a 

given exposure use pattern. For tribufos, application methods for the registered uses include 

foliar spray. For terrestrial animals, the T-REX model (Version 1.5.2)4 is used to calculate 

dietary- and dose-based EECs of tribufos for mammals and birds feeding on the site of 

application. Input values for T-REX include the maximum single application rates, number of 

applications, and retreatment interval for a given use and are located in Table 13. In this 

assessment, EFED uses a default foliar dissipation half-life of 35 days as an input for terrestrial 

exposure modeling in T-REX. Additionally, foliar dissipation half-life values between 1 and 35 

 4 http://www.epa.gov/oppefed1/models/terrestrial/
-Page 31 of 116-

days were explored as inputs for terrestrial exposure modeling in T-REX.

Upper-bound Kenaga nomogram values based on Hoerger and Kenaga (1972) as modified by 

Fletcher et al. (1994) are used to derive EECs for tribufos exposures to terrestrial mammals and 

birds based on dietary- and dose-based exposures from foliar applications of tribufos (Table 13). 

A one-year time period is simulated. Consideration is given to different types of feeding 

strategies for mammals and birds, including herbivores, insectivores and granivores. For dosebased exposures, three weight classes of mammals (15, 35, and 1000 g) and birds (20, 100, and 

1000 g) are considered. For more information on estimating exposure to terrestrial organisms, 

see the T-REX User’s Guide5. EECs derived based on mean Kenaga nomogram values were 

also evaluated.

 5 http://www.epa.gov/oppefed1/models/terrestrial/trex/t_rex_user_guide.htm
-Page 32 of 116-

Table 13. T-REX Calculated EECs as Food Residues for Terrestrial Animals from Tribufos Uses.

Food Type

Dietary-Based

EEC (mg/kgdiet)Dose-Based EEC (mg/kg-body weight)

Mammals and 

Birds Birds Mammals

All Sizes 

Classes Small (20 g) Medium (100 g) Large(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

Max Label Rate for Cotton Across U.S Except AZ and CA: 1.125 lbs a.i./A, 1 application

Short grass 270.0 307.5 175.4 78.51 257.4 177.9 41.25

Tall grass 123.8 140.9 80.37 35.98 118.0 81.54 18.91

Broadleaf plants/small insects 151.9 173.0 98.64 44.16 144.8 100.1 23.20

Fruits/pods/(seeds, dietary only) 16.88 19.22 10.96 4.907 16.09 11.12 2.578

Arthropods 105.8 120.4 68.68 30.75 100.8 69.68 16.16

Seeds (granivore) N/A 4.271 2.435 1.090 3.575 2.471 0.5729

Max Label Rate for Cotton in AZ and CA: 1.875 lbs a.i./A, 1 application

Short grass 450.0 512.5 292.3 130.85 429.0 296.5 68.75

Tall grass 206.3 234.9 133.95 59.97 196.6 135.91 31.51

Broadleaf plants/small insects 253.1 288.3 164.39 73.60 241.3 166.8 38.67

Fruits/pods/(seeds, dietary only) 28.13 32.03 18.27 8.178 26.82 18.53 4.297

Arthropods 176.3 200.7 114.47 51.25 168.0 116.14 26.93

Seeds (granivore) N/A 7.118 4.059 1.817 5.959 4.118 0.9549

Average Rate Used on Cotton Across U.S. Except CA and AZ: 0.54 lbs a.i./A, 1 application

Short grass 129.6 147.6 84.17 37.68 123.6 85.40 19.80

Tall grass 59.40 67.65 38.58 17.27 56.63 39.14 9.075

Broadleaf plants/small insects 72.90 83.03 47.34 21.20 69.50 48.04 11.14

Fruits/pods/(seeds, dietary only) 8.100 9.225 5.261 2.355 7.723 5.337 1.238

Arthropods 50.76 57.81 32.97 14.76 48.40 33.45 7.755

Seeds (granivore) N/A 2.050 1.169 0.5234 1.716 1.186 0.2750

Average Rate Used on Cotton in CA: 0.64 lbs a.i./A, 1 application*

Short grass 153.6 174.9 99.76 44.66 146.4 101.21 23.47

Tall grass 70.40 80.18 45.72 20.47 67.12 46.39 10.756

Broadleaf plants/small insects 86.40 98.40 56.11 25.12 82.38 56.93 13.20

Fruits/pods/(seeds, dietary only) 9.600 10.933 6.235 2.791 9.153 6.326 1.467

Arthropods 60.16 68.52 39.07 17.49 57.36 39.64 9.191

Seeds (granivore) N/A 2.430 1.385 0.6203 2.034 1.406 0.3259

Average Rate Used on Cotton in AZ: 0.77 lbs a.i./A, 1 application+

Short grass 184.8 210.5 120.02 53.73 176.2 121.77 28.23

Tall grass 84.70 96.46 55.01 24.63 80.75 55.81 12.940
-Page 33 of 116-

Food Type

Dietary-Based

EEC (mg/kgdiet)Dose-Based EEC (mg/kg-body weight)

Mammals and 

Birds Birds Mammals

All Sizes 

Classes Small (20 g) Medium (100 g) Large(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

Broadleaf plants/small insects 103.95 118.39 67.51 30.23 99.11 68.50 15.88

Fruits/pods/(seeds, dietary only) 11.550 13.154 7.501 3.358 11.012 7.611 1.765

Arthropods 72.38 82.43 47.01 21.05 69.01 47.69 11.058

Seeds (granivore) N/A 2.923 1.667 0.7463 2.447 1.691 0.3921

* An average of 1.2 applications a year was estimated by BEAD. This estimate gives a lower bound of average exposure. 

+ An average of 1.6 applications a year was estimated by BEAD. This estimate gives a lower bound of average exposure.
34

3.2.3.2. Residues in Aquatic Food Items for Mammals and Birds 

The KABAM model (KOW (based) Aquatic BioAccumulation Model) version 1.0 was used to 

evaluate the potential exposure and risk of direct effects to birds and mammals via 

bioaccumulation and biomagnification in aquatic food webs. KABAM is used to estimate 

potential bioaccumulation of hydrophobic organic pesticides in freshwater aquatic ecosystems 

and risks to mammals and birds consuming aquatic organisms which have bioaccumulated these 

pesticides. The bioaccumulation portion of KABAM is based upon work by Arnot and Gobas 

(2004) who parameterized a bioaccumulation model based on PCBs and some pesticides (e.g., 

lindane, DDT) in freshwater aquatic ecosystems (Arnot and Gobas, 2004). KABAM relies on a 

chemical's octanol-water partition coefficient (KOW) to estimate uptake and elimination constants 

through respiration and diet of organisms in different trophic levels. Pesticide tissue residues are 

calculated for organisms at different levels of an aquatic food web. The model then uses 

pesticide tissue concentrations in aquatic animals to estimate dose- and dietary-based exposures 

and associated risks to mammals and birds (surrogate for amphibians and reptiles) consuming 

aquatic organisms. Seven different trophic levels including phytoplankton, zooplankton, benthic 

invertebrates, filter feeders, small-sized (juvenile) forage fish, medium-sized forage fish, and 

larger piscivorous fish, are used to represent an aquatic food web. Importantly, chemical 

metabolism by biota is assumed to be zero in KABAM unless evidence indicates such 

metabolism is likely to affect the model predictions substantially. Measured BCF values are 

lower than what would be expected based on tribufos’ log KOW of 5.5 and metabolites were 

observed in the BCF studies, indicating that tribufos is metabolized in fish. Input scenarios and 

parameters were chosen to represent the range of exposures from high to low and are presented 

in Table 14. Example output from the bioaccumulation model is provided in Appendix I.

Table 14. Bioaccumulation Model Input Values for Tribufos.

Parameter Input Value Source

Pesticide Name Tribufos --

Log KOW 5.52 MRID 41618803

KOC 9284

Mean KOC value from MRID 41618817. Because the coefficient of 

variation for Freundlich soil adsorption (KF) values (0.26) is less than 

that for values normalized for organic carbon (KFOC) (0.36), tribufos 

does not conform to the organic carbon partitioning model (USEPA, 

2006). Therefore, there is uncertainty in the KABAM model output.

Pore water EEC 

(μg/L) 22.4 Maximum 1-in-10 year 21-day average value from surface water 

modeling. The estimated time to reach steady state was 93 days.

Water column EEC 

(μg/L) 25.1 Maximum 1-in-10 year 60-day average value surface water modeling. 

The estimated time to reach steady state was 93 days.

Based on KABAM results, estimated concentrations of tribufos residues in the tissue of 

organisms in the different trophic levels following application on cotton range from 356 to 2,250

mg/kg-tissue (Table 15). The measured bioconcentration potential in whole fish of tribufos in 

bluegill sunfish was 730 L/kg, and is lower than BCF values estimated in KABAM modeling

(Cleveland and Hamilton, 1983). The maximum BCF from KABAM was 15,912 L/Kg in the 

whole fish, suggesting that the model estimates exaggerate residues in fish and that fish may be 

able to metabolize tribufos.
35

Table 15. Predicted concentrations of tribufosin aquatic organism tissues at different trophic 

levels (μg/kg).

Use Scenario Phytoplankton Zooplankton Benthic 

Invertebrates

Filter 

Feeders1

Small 

Fish

Medium 

Fish

Large 

Fish

MSCotton (1.1 

lbs a.i./A) 355,660 306,175 363,261 238,200 669,880 1,027,363 2,250,493

1 Filter feeders include clams, krill, sponges, whales, and many fish and may be vertebrates or invertebrates.

Because a reliable metabolism rate constant cannot be generated to parameterize KABAM, the 

empirical BCF values for fish was used to estimate tribufos concentrations in aquatic organisms. 

The empirical bioconcentration factors (BCF) for tribufos is 730 L/kg for whole fish. Maximum 

1-in-10-year 60-day average EECs for the scenarios explored for aquatic modeling ranged from 

11.6 to 25.1 μg/L. The estimated concentrations in aquatic organisms resulting from this range of 

EECs are used in combination the measured BCF to bracket the potential concentrations of 

tribufos in aquatic vertebrate tissues (at steady state). It should be noted that tissue 

concentrations in aquatic vertebrates will likely be limited by toxicity of tribufos on these 

organisms. For instance, the NOAEC for aquatic vertebrates exposed to tribufos is 4.1 μg a.i./L. 

At this concentration, there were reductions in growth and post-hatch survival. Given that we 

expect a range of sensitivities in different aquatic invertebrates, some sensitive aquatic 

vertebrates would be expected to experience effects at the estimated concentrations while less 

sensitive species could carry residues in tissue without showing effects. Assuming EECs of 11.6

to 25.1 μg a.i./L and a BCF value of 730 L/kg , tribufos concentrations in aquatic vertebrate 

tissues may range from 8.5 to 18 mg a.i./kg (diet) (BCF x EEC = Concentration in fish). 

3.2.3.4. Exposure to Honey bees

Potential risk to bees is assessed in this document according to the tiering process described in 

the Guidance for Assessing Pesticide Risks to Bees (USEPA et al., 2014). As part of the Tier I 

risk assessment, screening-level exposures are estimated in pollen and nectar using generic 

residue data generated from other chemicals as well as other plant parts as described Table 16. 

For dietary exposures resulting from foliar applications, it is assumed that pesticide residues on 

tall grass (from the Kenaga nomogram of T-REX) are a suitable surrogate for residues in pollen 

and nectar of flowers that are directly sprayed during application. For contact exposure, 

exposure is based on the work of Koch and Weisser6(Koch and Weißer, 1997). For soil 

applications, pesticide concentrations in pollen and nectar are assumed to be consistent with 

chemical concentrations in the xylem of barley (calculated using the Briggs’ model). For seed 

treatments, pesticide concentrations in pollen and nectar are based on concentrations in leaves 

and stems of treated plants (based on the European and Mediterranean Plant Protection 

Organization (EPPO) default value discussed in the White Paper), assumed to be 1 milligram per 

 6 Koch, H. and P. Weißer. 1997. Exposure of honey bees during pesticide application under field conditions. 

Apidologie, 28: 439-447.
36

kilogram (mg/kg) or 1 part per million (ppm). More details on these methods are available in the 

in the T-REX User’s Guide7

. 

The Tier I method is intended to generate “reasonably conservative” estimates of pesticide 

exposure to honey bees which are then refined using available empirical residue data from a 

variety of crops; however, these data are not available for tribufos. The Tier I exposure method is 

intended to account for the major routes of pesticide exposure that are relevant to bees (i.e., 

through diet and contact). Exposure routes for bees differ based on application type. Under the 

approach used in this assessment, bees foraging in a field treated with a pesticide through foliar 

spray could potentially be exposed to the pesticide through direct spray, i.e., contact, as well as 

through consuming tribufos residues in pollen and nectar. Table 16 summarizes exposure 

estimates for honey bees for the Tier I assessment.

 Table 16. Screening-level EECs for honey bees based on foliar applications.

Use Exposure 

Route Exposure Estimate Calculation1 Lifestage

EEC

(μg 

a.i./bee/day)

Max Label Rate for Cotton 

Across U.S Except AZ and 

CA: 1.125 lbs a.i./A, 1 

application

Contact APP*(2.7 μg a.i./bee) Adults 3.03

Diet APP*(110 μg a.i /g)*(0.292 g/day) Adults 36.1

Diet APP*(110 μg a.i /g)*(0.124 g/day) Brood 15.3

Max Label Rate for Cotton in 

AZ and CA: 1.9 lbs a.i./A, 1 

application

Contact APP*(2.7 μg a.i./bee) Adults 5.06

Diet APP*(110 μg a.i /g)*(0.292 g/day) Adults 60.2

Diet APP*(110 μg a.i /g)*(0.124 g/day) Brood 25.6

Average Rate Used on Cotton 

Across U.S. Except CA and 

AZ: 0.54 lbs a.i./A, 1 

application

Contact APP*(2.7 μg a.i./bee) Adults 1.46

Diet APP*(110 μg a.i /g)*(0.292 g/day) Adults 17.3

Diet APP*(110 μg a.i /g)*(0.124 g/day) Brood 7.37

Average Rate Used on Cotton 

in CA: 0.64 lbs a.i./A, 1 

application*

Contact APP*(2.7 a.i./bee) Adults 1.73

Diet APP*(110 μg a.i /g)*(0.292 g/day) Adults 20.6

Diet APP*(110 μg a.i /g)*(0.124 g/day) Brood 8.73

Average Rate Used on Cotton 

in AZ: 0.77 lbs a.i./A, 1 

application+

Contact APP*(2.7 a.i./bee) Adults 2.08

Diet APP*(110 μg a.i /g)*(0.292 g/day) Adults 24.7

Diet APP*(110 μg a.i /g)*(0.124 g/day) Brood 10.5

APP = application rate in lbs ai/A

* An average of 1.2 applications a year was estimated by BEAD. One application was modeled based on the recommendation from BEAD that 

more than one application per year is not expected to occur.

+ An average of 1.6 applications a year was estimated by BEAD. One application was modeled based on the recommendation from BEAD that 

more than one application per year is not expected to occur. 1

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

 

3.2.3.2. Runoff and Spray Drift to Terrestrial and Semi-Aquatic Plants

TERRPLANT (Version 1.2.2)8 is used to calculate EECs for non-target plants that inhabit dry 

and semi-aquatic areas. In this assessment, exposure to non-target plants is calculated based on 

the potential runoff and spray drift of foliar applications of tribufos and potential runoff after soil 

and seed treatment applications (Table 17). TERRPLANT does not account for particulate drift.

 7 http://www.epa.gov/oppefed1/models/terrestrial/trex/t_rex_user_guide.htm

8 http://www.epa.gov/oppefed1/models/terrestrial/
37

Table 17. EECs for Non-target Terrestrial and Semi-aquatic Plants for Tribufos (based on 

TERRPLANT).

Uses App

Technique

Single Max. 

App. Rate

(lbs a.i./A)

EECs (lbs a.i./A)

Semi-Aquatic 

Areas (Total)

Dry Areas 

(Total)

Max Label Rate for Cotton Across U.S

Except AZ and CA Foliar 1.125 0.16875 0.0675

Max Label Rate for Cotton in AZ and CA Foliar 1.875 0.28125 0.1125

Average Rate Used on Cotton Across U.S. 

Except CA and AZ Foliar 0.54 0.081 0.0324

Average Rate Used on Cotton in CA* Foliar 0.64 0.096 0.0384

Average Rate Used on Cotton in AZ+ Foliar 0.77 0.1155 0.0462

App = Application

* An average of 1.2 applications a year was estimated by BEAD. This estimate gives a lower bound of average exposure. 

+ An average of 1.6 applications a year was estimated by BEAD. This estimate gives a lower bound of average exposure.

3.3. Ecological Effects Characterization 

The ecological effects characterization for tribufos is based upon registrant-submitted toxicity 

data for the technical grade active ingredient (TGAI) and for formulations. The ecotoxicity data 

for tribufos and its associated products have been reviewed previously in multiple ecological risk 

assessments (USEPA, 2000) (USEPA, 2008) and in a Preliminary Problem Formulation for 

Registration Review (USEPA, 2008, DP Barcode 357536). These data are summarized briefly in 

Sections 3.3.1 and 3.3.2. Various studies with terrestrial and aquatic plants, birds, and aquatic 

animals exposed to either the TGAI or formulated tribufos have been received since the Problem

Formulation was issued in 2008; the results of these studies are described briefly in this section. 

The results of a 2015 review of the Ecological Incident Information System (EIIS) are described 

in Section 3.3.3 (USEPA, 2012a). A search of the public ECOTOXicology database in March 

2015 and the EFED ECOTOX refresh report (February 2014), yielded no new data from suitable 

studies with more sensitive (lower) toxicity endpoints than those previously used in risk 

assessments9. Data meeting the OPP’s standard for inclusion in risk assessment 

(http://www.epa.gov/pesticides/science/literature-studies.html) were available in ECOTOX that 

showed greater sensitivity than results from submitted studies for freshwater fish (Mayer and 

Ellersieck, 1986, MRID 40098001, ECOTOX Number 6797) and daphnids (Brausch et al., 2010, 

ECOTOX Record Number 156445; Mayer and Ellersieck, 1986, MRID 40098001, ECOTOX 

Number 6797). The Mayer and Ellersick (1986) results for freshwater fish and daphnid were 

included in previous risk assessments; however, the endpoints were updated in this risk 

assessment because the raw data were obtained and statistically analyzed, resulting in the most

sensitive toxicity endpoints for freshwater fish and daphnid. The endpoints from the ECOTOXcatalogued open literature studies are provided in Appendix G, for reference. Tables 

summarizing registrant-submitted ecological toxicity data, and open literature results found to be 

more sensitive to registrant submitted studies or that provided additional information on 

 9 There were some endpoints that were lower in the ECOTOX report; however, the endpoints were not considered 

reliable for use in risk assessment.
38

mollusks, amphibians, or reptiles are available in Appendix C.

3.3.1. Aquatic Toxicity Assessment

A summary of the most sensitive toxicity endpoints for aquatic organisms is shown in Table 18. 

These endpoints will be used to calculate risk quotients.

The available data indicate that tribufos TGAI is highly toxic to freshwater fish and highly toxic 

to estuarine/marine fish on an acute exposure basis. Tribufos is very highly toxic to highly toxic 

to freshwater invertebrates and highly toxic to estuarine/marine invertebrates on an acute 

exposure basis. Acute toxicity studies on the toxicity of formulated product DEF6® (72.3% a.i.)

were submitted for freshwater fish and aquatic invertebrates. The studies conducted with TGAI 

resulted in lower (more sensitive) endpoints. Effects on growth were observed in chronic fish 

studies at 19 μg a.i./L and effects on post-hatch survival were observed at 39 μg a.i./L. Effects 

on reduced number of young/adult/day and reduced adult length were observed in chronic water 

flea studies at 3.5 μg a.i./L. Effects on frond count were observed in the vascular aquatic plant

duckweed (Lemna gibba; EC05=140 μg a.i./L) and inhibited growth was observed when 

nonvascular freshwater green alga (Pseudokirchneriella subcapitata) were exposed to tribufos at 

254 μg a.i./L. 

Table 18. Summary of Most Sensitive Toxicity Data for Aquatic Organisms Exposed to 

Tribufos (TGAI) and Formulations.

Common Name

(Species)

Study Type

and

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect Observed 

at Endpoint)

Toxicity Value

(95% CI)

μg a.i./L3

(Acute Toxicity 

Category)

MRID

Study 

Classification1

Freshwater Vertebrates

Bluegill sunfish

(Lepomis macrochirus)

Static

(96-hour) TGAI LC50

(Survival)

283 (136-378)

Slope=6.23

(highly toxic)

40098001

SupplementalQuantitative

Bluegill sunfish

(Lepomis macrochirus) Chronic -- NOAEC 4.1 Estimated2

Estuarine/Marine Vertebrates

Sheepshead minnow

(Cyprinodon 

variegatus)

Acute 

Flow-through 

96-hour

TGAI 

(98.6%)

LC50

(Survival)

767 (560-1370)

(highly toxic)

41896302

Acceptable

Sheepshead minnow

(Cyprinodon 

variegatus)

Early Life 

Cycle/

Flow-through

(36-day)

TGAI 

(99.4%)

NOAEC/ LOAEC

(growth) 11/ 19 45863707

Acceptable

Freshwater Aquatic Invertebrates

Water flea

(Daphnia magna)

Static

(48-hour)

TGAI

Not 

available

EC50

(Immobility)

5.4 (3.5-7.7)

Slope=2.5

(very highly toxic)

40098001

SupplementalQuantitative

Water flea

(Daphnia magna)

Static-renewal

(21-days) TGAI

(97.2%)

NOAEC/ LOAEC

(reduced number of 

young/adult/day 

and reduced adult 

length)

1.56/ 

3.234

43978201

Acceptable
39

Common Name

(Species)

Study Type

and

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect Observed 

at Endpoint)

Toxicity Value

(95% CI)

μg a.i./L3

(Acute Toxicity 

Category)

MRID

Study 

Classification1

Hyalella

(Hyalella azteca)

(Amphipod)

Spiked 

sediment

(10-day)

TGAI 

(98.2%)

NOAEC LOAEC

(Growth)

71/ 220 μg a.i./kgsediment

2.81/8.27 μg/L porewater

49164802

Acceptable

Estuarine/marine Aquatic Invertebrates

Mysid shrimp 

(Americamysis bahia

formerly Mysidopsis 

bahia) 

Flow-through

(96-hour)

TGAI 

(96.4%)

LC50

(Survival)

11.53* 

(9.30-15.11) 

(very highly toxic)

41896301

Acceptable

Mysid shrimp 

(Americamysis bahia

formerly Mysidopsis 

bahia)

Flow-through

(96-hour) TGAI NOAEC/ LOAEC

(None) 0.26/ >0.26* 42083201

Acceptable

Marine Amphipod

(Leptocheirus 

plumulosus)

Spiked 

sediment

(10-day)

TGAI 

(98.2%)

NOAEC/

LOAEC

(Mortality)

207 /773 μg/kgsediment

7.10/22.7 μg a.i./L

pore-water

49164801

Acceptable

Aquatic Plants

Duckweed 

(Lemna gibba)

Static Renewal

14-day

TGAI 

(99.4%)

EC50

(Frond Count) 1100 (760-1700) 45863705

SupplementalEC05 140 (35-570) quantitative

Freshwater green alga

(Pseudokirchneriella

subcapitata1

)

7-day TGAI 

(99.9%)

EC50

(Percent inhibition) 148 41618813Acceptable

NOAEC/ LOAEC 118/ 254* 41618813

Acceptable

TGAI=technical grade active ingredient:

*Represents the most sensitive toxicity endpoint to be used in RQ calculations.

1 Supplemental studies may or may not be suitable to calculate RQs. Quantitative values are suitable for calculation of RQ. Qualitative values 

are not considered suitable for calculation of RQs but may be useful in understanding the weight of evidence for evaluating the likelihood of risk. 2 This NOAEC is based on the acute-to-chronic ratio using sheepshead minnow endpoints [767/11=69.7 (ACR)] and the 96-hour LC50 of the 

bluegill sunfish. 3

 Endpoints are for water-column, unless otherwise stated. 4 The ACR for daphnid is 3.5 (5.4/1.56).

3.3.1.1. Toxicity to Aquatic Vertebrates

On an acute exposure basis, tribufos is highly toxic to freshwater fish, with 96-hour LC50 values 

ranging from 245 to 920 μg a.i./L across two tested species. Only a single estuarine/marine fish 

acute study was provided for tribufos on the sheepshead minnow (Cyprinodon variegatus; MRID

41896302), which resulted in a 96-hour LC50 of 767 μg a.i./L.

Two freshwater fish early-life cycle studies were submitted; however, the results from the tests 

are not considered suitable for calculation of risk quotients due to either the presence of tribufos 

near the limit of detection in controls or statistically significant (p<0.05) effects observed with 

the solvent control relative to the negative control. In order to derive a chronic freshwater fish 

toxicity endpoint for calculation of risk quotients, an acute-to-chronic ratio (ACR) based upon 
40

data from the sheepshead minnow was applied to the bluegill sunfish acute toxicity endpoint10

resulting in a chronic toxicity value (NOAEC) of 4.1 μg a.i./L. The consistency of the acute 

toxicity values for saltwater and freshwater species suggests that saltwater species are not 

considerably more or less sensitive to tribufos than freshwater species.

Available data (Appenidix C) indicate that DEF6® is not substantially (more than an order of 

magnitude) more toxic to freshwater fish as compared to tribufos TGAI on an acute exposure 

basis. 

Chronic toxicity to estuarine/marine fish was evaluated in a 36-day early life stage test with the 

sheepshead minnow (Cyprinodon variegatus; MRID 45863707), the NOAEC and LOAEC were 

11 and 19 μg a.i./L, respectively, based on effects observed on growth. For all growth endpoints, 

statistically significant (p<0.05) differences were observed at the mean-measured concentration 

of 19, 39, and 81 μg a.i./L. The percent juvenile survival of 51% at 39 μg a.i./L was 

significantly different (p<0.05) than the juvenile survival at 19 μg a.i./L of 86%, resulting in a 

NOAEC and LOAEC of 19 and 39 μg a.i./L, respectively.

3.3.1.2. Toxicity to Aquatic Invertebrates

On an acute exposure basis, tribufos is highly to very highly toxic to freshwater invertebrates, 

with water-column 48-hour LC50 values ranging from 5.4 to 110 μg a.i./L for the water flea 

(Daphnia magna). Estuarine/marine invertebrate acute toxicity studies were provided for 

tribufos on the mysid shrimp (Americamysis bahia formerly Mysidopsis bahia; MRID 

41896300) and the 96-hour LC50 value was 11.5 μg a.i/L. The 96-hour EC50 for inhibition of

shell growth for the Eastern oyster (Crassostrea virginica; MRID 45863702) was 39.6 μg a.i./L. 

These endpoints indicate that tribufos is very highly toxic to estuarine/marine invertebrates on an 

acute exposure basis.

Chronic toxicity data are available for the water flea and the mysid shrimp, the most sensitive 

species based on acute toxicity data. For the water flea, the 21-day NOAEC/ LOAEC was 1.56 

/3.23 μg a.i./L (MRID 43978201) based on statistically significant (p<0.05) reduction in number 

of young/adult/day and reduced adult length. For the mysid shrimp, the 28-day 

NOAEC/LOAEC was 0.26/>0.26 μg a.i./L because no treatment related effects were observed in 

the study.

Sediment toxicity studies were submitted since the previous risk assessments and problem 

formulations were completed. In a 10-day spiked sediment test for the freshwater amphipod 

(Hyalella azteca), NOAEC/ LOAEC for reduced growth were 2.81/8.27 μg a.i./L in pore water 

and 71/220 μg a.i./kg-sediment. There was a 39% reduction in weights at the LOAEC. In a 10-

day spiked sediment test for the marine amphipod (Leptocheirus plumulosus), the NOAEC/ 

LOAEC were 207/ 773 μg a.i./kg-sediment and 7.10/ 22.7 μg a.i./L pore water based on 

mortality. There was 74% survival at 22.7 μg a.i./L pore water, as compared to 89 to 90% 

survival in other treatments and controls. These endpoints will be used to evaluate risk to 

 10 The ACR was calculated as the sheepshead minnow 96-hour LC50 of 767 μg a.i./L divided by the 35-day NOAEC 

for the sheepshead minnow of 11 μg a.i./L resulting in an ACR of 69.7. The chronic bluegill sunfish endpoint was 

calculated by dividing the bluegill sunfish 96-hour LC50 of 283 μg a.i./L divided by the ACR of 69.7.
41

sediment-dwelling freshwater and estuarine/marine invertebrates. Because a definitive 

estuarine/marine chronic endpoint for aquatic invertebrates is not available in the water-column, 

the pore water NOAEC of 7.10 μg a.i./L for the estuarine/marine amphipod, will be used to 

evaluate risk in the water column for estuarine/marine aquatic invertebrates. 

Typically, sediment data are also recommended for the freshwater midge (Chironomus dilutus). 

The midge, a Dipteran, and Hyalella, an amphipod, have different life histories, occupy different 

taxonomic groups (insects versus crustaceans) and generally interact with sediments in different 

ways. Chironomid larvae are more infaunal (submerged in sediment) whereas amphipods are 

more epibenthic (moving in and out of surficial sediment layer). This is an uncertainty in the 

sediment toxicity dataset. 

3.3.1.3. Toxicity to Aquatic Plants

Data on one aquatic vascular plant species and one nonvascular plant species are available. In a 

14-day toxicity study with vascular aquatic plant duckweed, L. gibba (MRID 45863705) exposed 

to tribufos TGAI (99.9%), statistically significant (p<0.05) effects in frond count were observed. 

The 14-day EC50 was 1100 μg a.i./L and the corresponding EC05 was 140 μg a.i./L. The percent 

growth inhibition based on frond count (the most sensitive endpoint) in the treated culture as 

compared to the control ranged from 26.2 to 69.5%. 

In a 7-day toxicity test, the nonvascular freshwater green alga was exposed to tribufos TGAI 

(99.4%, MRID 41618813), a statistically significant (p<0.05) reduction in percent inhibition was 

observed at 254 μg a.i./L. The NOAEC and EC50/IC50 values based on percent inhibition were 

118 and 148 μg a.i./L, respectively. 

Data on a marine diatom (e.g., Skeletonema costatum), freshwater diatom (e.g., Navicula 

pelliculosa), and cyanobacterium (e.g., Anabaena flos-aquae) are not available.

3.3.2. Terrestrial Effects Characterization

A summary of the most sensitive terrestrial animal toxicity data for tribufos, based on a current 

EPA review of all submitted data, is provided in Table 19 and is discussed further in Sections 

3.3.2.1 to 3.3.2.3. The available data indicate that tribufos TGAI ranges from practically nontoxic to moderately toxic to five species of birds including passerines and is moderately toxic to 

mammals on an acute oral exposure basis and highly toxic to mammals on an subacute-dietary 

exposure basis. Tribufos is practically nontoxic to young adult honey bees on an acute contact 

exposure basis.

Bobwhite quail (Colinus virginianus) fed diets containing tribufos for 20-weeks had a reduced 

eggshell thickness relative to controls at 280 mg a.i./kg-diet, but no mortality or overt signs of 

toxicity were observed. Laboratory rats fed diets containing tribufos (260 mg a.i./kg-diet) had an 

increased number of litters with stillborn pups relative to controls. Additional endpoints affected 

include increased pup death through lactation, decreased F1 and F2 pup body weights, and 

increased F1 gestation period.
42

The available data for terrestrial plants exposed to the formulated product DEF6® (70.8% a.i.)

provided in Section 3.3.2.4, indicate that tribufos exposure to seeds in treated soils resulted in 

reduced plant height at application rates equivalent to 0.22 lbs a.i./A and exposure to foliage 

resulted in reduced plant height and dry weight at application rates equivalent to 0.93 lbs a.i./A. 

In a non-guideline study examining plant defoliation, exposure of deciduous tree saplings and 

shrubs resulted in a reduction in the number of old leaves left on the plant and an increased 

weight of fallen leaves at 1.125 and 1.875 lbs a.i./A (NOAEC=0.56 lbs a.i./A). 

Table 19. Summary of the Most Sensitive Toxicity Data for Terrestrial Animals Exposed 

to Tribufos.

Common Name

(Species)

Study Type

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect at 

endpoint)

Toxicity Value

(95% CI)

(Acute Toxicity 

Category)

MRID

Study 

Classification

Birds

Canary 

(Serinus canaria)

Acute Oral (Oral 

gavage with 14-

day observation)

TGAI 

(98.2%)

ED50

(Mortality and 

Regurgitation)

111 mg/kg-bw (73.7-

168)*

(moderately toxic)

49121303

Supplemental due to 

regurgitation within 

one hour of dosing.

Bobwhite quail

(Colinus 

virginianus)

Acute Oral (Oral 

gavage with 14-

day observation)

TGAI 

(92%)

LD50

(Mortality)

151 mg a.i./kg-bw

(128-178)*

Slope=7.9 (4.3-11.5)

(moderately toxic)

00120771

Acceptable

Bobwhite quail

(Colinus 

virginianus)

Subacute Dietary

(5-day)

TGAI

(98.9%)

LD50

(Mortality)

1519 mg a.i./kg-diet

(slightly toxic)

41618804

Acceptable

Bobwhite quail

(Colinus 

virginianus)

Reproduction

(20 week) TGAI

(98.7%)

NOAEC/

LOAEC 

(egg shell 

thickness)

150/280 mg a.i./kg-diet* 40757101

Acceptable

Mammals2

Laboratory rat

(Rattus 

norvegicus)

Acute Inhalation

(4-hour)

TGAI LC50

(Mortality)

4650 mg/m3

4.65 mg/L 41782301

Laboratory rat

(Rattus 

norvegicus)

Acute Oral TGAI LD50

(Mortality)

192 mg/kg-bw 

(practically non-toxic) 41954903

White footed 

mouse 

(Peromyscus 

leucopus)

Sub-acute 

Dietary

(5-day)

TGAI LC50

(Mortality)

161 mg/kg-diet (28.3-

293.7)

Slope=1.73 (0.66-2.79)

45863708

SupplementalQuantitative

Laboratory rat

(Rattus 

norvegicus)

Two-generation 

Reproduction TGAI

NOAEL/

LOAEL

(increased 

number of 

litters with still 

born pups, see 

comments)

32 mg/kg-diet*

1.7 mg/kg/day/

260 mg/kg-diet

15 mg/kg/day

42040201

Honey bees
43

Common Name

(Species)

Study Type

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect at 

endpoint)

Toxicity Value

(95% CI)

(Acute Toxicity 

Category)

MRID

Study 

Classification

Honey Bee

(Apis mellifera)

Acute Contact

(48-hour)

Variable., 

Mixed 

with 

pyrolite 

dust

LD50

(Mortality)

>24.17 μg a.i./bee*

(practically nontoxic)

00001999

Acceptable

* Most sensitive endpoint; will be used in RQ calculations.

2 Most mammalian toxicity endpoints were provided and reviewed by EPA/OCSPP/OPP Health Effects Division

3.3.2.1. Toxicity to Terrestrial Animals

Acute oral LD50 values for four bird species (including a passerine species) ranged from 111 to 

871 mg/kg-body weight, indicating that tribufos is moderately to slightly toxic to birds. The 111 

mg/kg-body weight value was based on the effect on both survival and regurgitation in the 

passerine canary (Serinus canaria). Subacute dietary 5-day LC50 values for two species ranged 

from 150 to >5000 mg/kg-diet, indicating that tribufos ranged from practically nontoxic to 

moderately toxic to birds. Clinical signs of toxicity observed in these studies were ruffled 

feathers, underactivity, incoordination, recumbency, body tremors, panting, salivation, and/or 

regurgitation. 

In 20-week reproductive toxicity study on the Northern bobwhite quail, the NOAEC and 

LOAEC were 150 and 280 mg a.i./kg-diet, based on effects observed on egg shell thickness. 

There was also a statistically significant (p<0.05) reduction in egg production (71% reduction) 

and hatchling survival (56% reduction) at 410 mg/kg-diet relative to controls. 

Available data indicate that tribufos is moderately toxic to laboratory rats (Rattus norvegicus) on 

an acute oral exposure basis, with an LD50 of 192 mg a.i./kg-bw (MRID 41954903). On a 

subacute-dietary exposure basis, tribufos is classified as highly toxic based on a 5-day LC50 of 

161 mg/kg-diet for the white footed mouse (Peromyscus leucopus). Chronic exposures to 

tribufos during a two-generation reproduction study in rats (MRID 42040201) resulted in a 

statistical significant (p<0.05) increase in the number of litters with stillborn pups at 260-mg/kgdiet (15 mg/kg/day), resulting in a NOAEL of 32 mg/kg-diet (1.7 mg/kg/day). Effects on pup 

death through lactation, decreased F1 and F2 pup weights, and increased F1 gestation period 

were also observed at 260 mg/kg-diet.

In an acute contact toxicity test, young adult honey bees were exposed to tribufos mixed with 

pyrolite dust (MRID 00001999). There was 3% mortality observed at 24.17 μg a.i./bee. Based

on this limited dataset, tribufos is classified as practically non-toxic to young adult honey bees on 

an acute-contact exposure basis.11

As some tier I honey bee toxicity studies are currently unavailable for tribufos, there is

uncertainty in the risk assessment on the potential for adverse effects on to honey bees from 

 11 More information on pollinator data gaps is available in the executive summary.
44

exposure to tribufos.

An additional study identified from the open literature (Greenberg et al., 2004, ECOTOX 

Reference Number 92450) includes some information on effects of tribufos on terrestrial 

invertebrates [specifically boll weevils (Anthonomus grandis)]. This study is considered 

supplemental and not adequate for RQ calculation (e.g., it is a non-guideline study; no controls 

were used in portions of the study; only one concentration was tested). In the laboratory portion 

of the study, boll weevils sprayed directly with tribufos at an application rate of 0.42 lb a.i./acre 

(using the DEF® 6 formulation), which is 22% of the maximum labeled rate, had statistically 

significantly (p<0.05) higher mortality than controls 72-hrs post-spray [42.5% (standard 

deviation ± 2.2) mortality, versus 2.5% (standard deviation ± 5.0), respectively]. 

3.3.2.4. Toxicity to Terrestrial Plants

Terrestrial plant toxicity studies are required for pesticides that have terrestrial use patterns or 

may move off of the application site. The NOAEL and IC25 (concentration that causes an 

inhibitory effect in 25% of the population for the binary endpoints such as survival and 

emergence or 25% of an effect for continuous endpoints) values are established for markers of

seedling emergence and vegetative vigor. 

The submitted Tier II seedling emergence study (MRID 45863404) indicated that onion (Allium 

cepa) was the most sensitive monocot species tested (NOAEC 0.45 lb a.i./A; EC25 >2.0 lb a.i./A) 

based on reduced plant height. Soybean (Glycine max) was the most sensitive dicot species 

tested, also based on reduced plant height, with a NOAEC of 0.22 lb a.i./A (LOAEC=0.45 lbs 

a.i./A) and an EC25 of >2.0 lb a.i./A. All other species (monocots and dicots) had NOAEC and 

EC25 values at 2.0 lb a.i./A and >2.0 lb a.i./A (the highest rate tested) with the exception of turnip

(Brassica rapa), a dicot species with a NOAEC of 0.94 lb a.i./A and LOAEC of 1.9 lbs a.i./A. 

The Tier II vegetative vigor study (MRID 45863704) indicated that corn (monocot; Zea mays) 

and buckwheat (dicot; Fagopyrum sagiitatum) were the most sensitive species based on reduced 

dry weight. The NOAEC and EC25 values for corn were 0.48 lb a.i./A and 1.8 lb a.i./A, 

respectively. The NOAEC and EC25 values for buckwheat were 0.48 lb a.i./A and 1.3 lb a.i./A, 

respectively. All other species tested showed that dry weight was the most sensitive endpoint for 

all species, though ryegrass (monocot; Lolium perenne) and sunflower (dicot; Helianthus 

annuus) did not exhibit effects at any of the rates tested.

Because tribufos affects the abscission zone in plants, plants that do not form abscission zones 

are not expected to be as sensitive to tribufos as plants that form abscission zones. While some of 

the plants tested in guideline vegetative vigor and seedling emergence studies do form abscission 

zones, the studies may not be sensitive to the defoliation endpoint. A non-guideline plant 

defoliation study (MRID 48728101) was conducted to better characterize risk to sensitive plant 

species. The effect of the tribufos formulation Folex® 6EC (70.5% a.i.) along with an adjuvant 

(0.1% Activator 90®) on non-target woody, deciduous shrubs (flowering currant, Ribes 

sanguineum [ribes], forsythia, Forsythia intermedia, viburnum, Viburnum spp.) and trees (maple, 

Acer spp, and oak, Quercus spp) was studied at nominal application rates of 0 (negative control), 

0.141, 0.281. 0.563, 1.125, and 1.875 lb a.i./acre (0, 0.158, 0.315, 0.63, 1.26, 2.10 kg a.i./ha). 
45

Both adjuvant and negative controls were run concurrently with treatments.

The study was designed to measure health, growth and survival of woody, deciduous plants 

exposed to a foliar spray of the formulated product. Eight individually potted shrubs of forsythia, 

flowering currant, and viburnum and seven individually potted saplings of maple and oak were 

selected for each treatment rate. After treatment, each pot was placed in a sheltered outdoor area. 

Plants were bottom-watered as needed and fertilized once per week. Each day, fallen leaves were 

collected, counted and dried before being weighed. Each plant was inspected daily over the 28-

day study period for phytotoxicity (chlorosis, necrosis, wilting, leaf and stem deformation) and 

plant mortality. 

Application of the formulated tribufos product Folex® 6EC at rates of up to 1.875 lb a.i./acre and 

in combination with the adjuvant Activator 90® (0.1%) resulted in statistically significant 

(p<0.05) leaf fall in viburnum and maple at treatment rates equivalent to 1.125 and 1.875 lb 

a.i./acre and in oak at 1.875 lb a.i./acre (NOAEC = 0.563 lbs a.i./A). These effects were 

accompanied by a reduction in the numbers of old leaves remaining on the plants at the end of 

the 28-day study period. There were no apparent treatment-related effects on the development of 

new leaves in the three shrub species, Forsythia, Ribes and Viburnum at any of the treatment 

rates in comparison to the water- and adjuvant-treated controls, or in the limited number of 

replicates of the two tree species where new growth was recorded. 

One study identified from the open literature (Greenberg et al., 2004, ECOTOX Reference 

Number 92450) describes effects (defoliation) to cotton from a tribufos application at a rate of 

0.42 lb a.i./acre (using the DEF6®) formulation). At this application rate (which is 22% of the 

maximum labeled rate), an average of 80.2% (± SD of 6.1) of the leaves per plant (N = 9) were 

dropped 7 days post-application; whereas, in the untreated control plot, 3.3% (± SD of 1.6) of the 

leaves per plant (N = 10) were dropped in the same time period.

Table 20. Summary of Terrestrial Plants Toxicity Data for Tribufos Formulations.

Crop Formulation 

(% a.i.) Species EC25(lbs a.i./A)

NOAEC

or IC05/ 

LOAEC

(lbs a.i./A)

Endpoint 

Affected

MRID

Study Classification

Seedling Emergence

Monocots DEF6®

(70.8%)

Onion

(Allium 

cepa)

>2.0* 0.45/0.93* Plant height

45863404

Acceptable

Dicots

Soybean

(Glycine 

max)

>2.0* 0.22/0.46* Plant height

Vegetative Vigor

Monocots DEF6®

(70.8%)

Corn (Zea 

mays) 1.8* 0.48/0.93* Dry weight, plant height 45863704

Acceptable

Dicots

Buckwheat

(Fagopyrum 

sagiitatum)

1.3* 0.48/0.99* Dry weight

Plant Defoliation Study

Deciduous 

shrubs

Folex6EC®

Cotton 

Defoliant

Forsythia 

(Ribes 

sanguineum)

Responses not 

sufficient to 

reliably 

1.875

No significant 

response observed 

at highest dose 

48728101

Supplemental
46

Crop Formulation 

(% a.i.) Species EC25(lbs a.i./A)

NOAEC

or IC05/ 

LOAEC

(lbs a.i./A)

Endpoint 

Affected

MRID

Study Classification

Activator90®

adjuvant

(0.1%)

calculate a 

dose-response 

curve

tested

Viburnum 

(Viburnum 

spp.)

0.563*/ 

1.125

Number/weight of 

fallen leaves

Deciduous 

Tree 

samplings

Maple (Acer 

spp.)

0.563*/ 

1.125

Number of old 

leaves left on plant

Oak 

(Quercus 

Spp.)

0.563*/ 

1.125

Number of old 

leaves left on plant

* Most sensitive endpoint used for risk estimation (i.e., RQ calculations) or risk description (for non-definitive endpoints).

4. Risk Characterization

4.1. Risk Quotient (RQ) Values

This assessment of the labeled uses of tribufos 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., estimated exposures divided by acute and/or chronic toxicity endpoints expressed 

in the same units as exposures, respectively). The resulting unitless RQ values, calculated in the 

Risk Estimation Section) are compared to the Agency’s LOCs (see Table 5). The LOCs are 

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

potential to result in exposure levels sufficient to cause adverse effects to non-target organisms. 

For endangered species, LOC exceedances require an additional in-depth listed species 

evaluation to characterize risks for the potential co-occurrence of listed species and areas in 

which crops are grown. In this approach, RQs that exceed non-listed species LOCs necessarily 

also exceed the corresponding listed species LOCs. Acute risk LOCs are different for listed and 

non-listed taxa; however, the chronic LOC is 1.0 across all animals. For plants, unlike for 

animals, RQ values are not presented for acute versus chronic risk; instead, RQ values are 

presented for listed and non-listed species based on a comparison of a given EEC to NOAEL and 

EC25 values (for terrestrial plants) and EC50 values (for aquatic plants), respectively. The LOC 

for all plants is 1.0. A discussion of the RQ values for tribufos and of other information that 

provides context for the interpretation of potential risk to various taxa is presented in the Risk 

Description in Section 4.2. 

Risk quotients were estimated for maximum labeled rates and for average application rates. Risk 

evaluated based on maximum labeled rates reflects the potential for adverse effects from 

exposure if tribufos is used at the maximum amount allowed on the label. Risk evaluated based 

on average application rates reflects the potential for adverse effects from exposure if tribufos is 

used at the average application rate. Some individuals will use tribufos at higher-than-average 

application rates, and in such cases the risk evaluation based upon average application rates may 

not be protective for these uses.

 
47

4.1.1. RQ Values for Aquatic Organisms

The SWCC was used to calculate water-column, pore water, and bulk sediment 1-in-10-year 

peak, 21-day average, and 60-day average EECs for aquatic organisms. Toxicity endpoints used 

to calculate risk quotients are the most sensitive acute and chronic endpoints for each taxon and 

are specified in the table showing the RQ value.

Freshwater fish serve as surrogates for aquatic-phase amphibians. RQ values based on both 

maximum and average application rates for freshwater fish exposed to tribufos are at or above 

(i.e., exceed) the LOCs (0.05) for acute risk for listed but not non-listed species LOC (0.5) 

(Table 21). Similarly, chronic RQ values for freshwater fish exceed the chronic risk LOC (1.0)

for both maximum label rates and average application rates. When the contribution of spray drift 

to exposure is not simulated and at the maximum labeled rate, RQs still exceed the acute risk to

listed species LOC and chronic risk LOC for freshwater vertebrates.

RQ values based on both maximum label and average application rates for estuarine/marine fish 

exposed to tribufos are below the LOCs (0.05) for acute risk for listed and non-listed species 

(LOC = 0.5) (Table 21). Chronic RQ values for estuarine/marine fish exceed the chronic risk 

LOC of 1.0 for both maximum label rates and average application rates, except for average 

application rate simulated using the Califonria cotton scenario and average application rates in 

Arizona and California. When the contribution of spray drift to exposure is not simulated and at 

the maximum labeled rate, RQs still exceed chronic risk LOCs for estuarine/marine vertebrates.

Table 21. RQs for Direct Effects to Fish from Tribufos Use on Cotton.

Geographic area 

and Type of 

Application

App. 

Rate

(lbs 

a.i./A)

Peak

EECs

(μg 

a.i./L)

60-day 

average

EECs

Risk Quotients

Freshwater Fish Estuarine/Marine Fish

Acute

LC50 = 283 

μg a.i./L

Chronic

NOAEC = 

4.1 μg 

a.i./L1

Acute

EC50 = 767 

μg a.i./L

Chronic

NOAEC = 11 

μg a.i./L

Maximum Label Rates – Runoff and Spray Drift Simulated

National Aerial 1.125 29.7 25.1 0.10* 6.12* 0.04 2.28*

CA and AZ Aerial 1.875 21.2 14.3 0.07* 3.49* 0.03 1.30*

National Ground 1.125 29.0 23.9 0.10* 5.83* 0.04 2.17*

Average Application Rates – Runoff and Spray Drift Simulated

National Aerial 0.54 14 11.9 0.05* 2.90* 0.02 1.08*

CA Aerial 0.64 7.3 4.9 0.03 1.20* 0.01 0.45

AZ Aerial 0.77 8.7 5.9 0.03 1.44* 0.01 0.54

National Ground 0.54 14 11.6 0.05* 2.83* 0.02 1.05*

Maximum Label Rates – Runoff Only

National Ground 1.125 26.5 21.8 0.09* 5.31* 0.03 1.98*

CA Ground 1.875 7.53 6.07 0.03 1.48* 0.01 0.55*

Abbreviations: App.= Application

A bold value with an asterisk indicates that the RQ meets or exceeds the LOC for acute risk (0.05) to listed species or meets or exceeds the 

chronic risk LOC (1) for listed and non-listed species. The acute risk LOC (0.5) for non-listed species is not exceeded. 1 Estimated using the freshwater fish 96-hr LC50 of 283 μg a.i./L and an ACR from estuarine/marine fish. 

All RQ values based on maximum and average application rates for freshwater and aquatic 

invertebrates exposed to tribufos on an acute exposure basis exceed LOCs for risk to listed 
48

(LOC=0.05) and non-listed (LOC=0.5) species (Table 22). All RQ values for freshwater aquatic 

invertebrates, inhabiting both the water-column and benthic zone, based on chronic exposure to 

tribufos, exceed LOCs for chronic risk (Table 22 and 23). When the contribution of spray drift 

to exposure is not simulated at the maximum labeled rate, RQs still exceed the acute risk to listed 

and non-listed species LOC and chronic LOCs for freshwater invertebrates.

RQ values based on maximum and average application rates for estuarine/marine aquatic 

invertebrates exposed to tribufos on an acute basis exceed LOCs for risk to listed (LOC=0.05) 

and non-listed (LOC=0.5) species (Table 22). In the chronic water-column study with 

estuarine/marine invertebrate mysids, no effects were observed at the highest concentration 

tested of 0.26 μg a.i./L. Chronic EECs are higher than the highest level tested. There are two 

alternative endpoints that could be used to evaluate the potential for effects due to chronic 

exposure for estuarine/marine aquatic invertebrates, 1) the benthic estuarine/marine aquatic 

invertebrate NOAEC of 7.1 μg a.i./L for pore-water, or 2) an estimated chronic endpoint using 

an ACR for the daphnid12 and the 96-hr EC50 for the mysid, resulting in an estimated NOAEC 

of 3.3 μg ai./L13. The toxicity value for the benthic invertebrate was used in RQ calculations. 

Chronic RQ values for estuarine/marine aquatic invertebrates, inhabiting both the water-column 

and benthos, exceed the LOC of 1.0 for chronic risk (Table 22 and 24), except for California and 

Arizona average application rates simulated using the Califonria cotton scenario. When the 

contribution of spray drift to exposure is not simulated at the maximum labeled rate, RQs still 

exceed the acute risk to listed and non-listed species LOC and chronic risk LOCs for some 

aquatic invertebrates.

Table 22. RQs for Direct Effects to Aquatic Invertebrates Inhabiting the Water-column 

from Tribufos Use on Cotton.

Geographic area 

and Type of 

Application

App. 

Rate

(lbs 

a.i./A)

Peak

EECs

(μg a.i./L)

21-day 

average

EECs

Risk Quotients

Freshwater 

Invertebrates E/M Invertebrates

Acute

EC50 = 5.4 

μg a.i./L

Chronic

NOAEC = 

1.5 μg 

a.i./L

Acute

EC50 = 

11.53 μg 

a.i./L

Chronic

NOAE = 7.1 

μg a.i./L2

Maximum Label Rates – Runoff and Spray Drift Simulated

National Aerial 1.125 29.7 27.1 5.5* 17.4* 2.6* 3.8*

CA and AZ Aerial 1.875 21.2 16.8 3.9* 10.8* 1.8* 2.4*

National Ground 1.125 29.0 26.1 5.4* 16.7* 2.5* 3.7*

Average Application Rates – Runoff and Spray Drift Simulated

National Aerial 0.54 14.2 12.9 2.6* 8.3* 1.2* 1.8*

CA Aerial 0.64 7.3 5.8 1.4* 3.7* 0.6* 0.82

AZ Aerial 0.77 8.7 6.9 1.6* 4.4* 0.8* 0.97

National Ground 0.54 14.0 12.6 2.6* 8.1* 1.2* 1.8*

Maximum Label Rates – Runoff Only

National Ground 1.125 26.5 24.0 4.9* 15.4* 2.3* 3.4*

CA Ground 1.875 7.53 6.51 1.4* 4.2* 0.78* 0.9

 12 ACR=48-hr EC50/NOAEC=5.4 μg a.i./L/1.56 μg a.i./L=3.5 13 NOAEC = 96-hr LC50 / ACR = 11.53 μg a.i./L /3.3 μg a.i./L = 3.3 μg a.i./L
49

Abbreviations: App.= Application.; E/M=estuarine/marine

All RQs exceed acute and chronic risk LOCs for listed and non-listed species. A bold value with an asterisk indicates that the RQ meets or 

exceeds the acute non-listed, acute listed, and chronic risk LOC.. 2 The NOAEC used to calculate risk quotient for estuarine/marine invertebrates inhabiting the water column was taken from a 10-day spiked 

sediment test with the estuarine/marine amphipod (MRID 49164801). A definitive chronic endpoint from a water-column study was not 

available for estuarine/marine invertebrates.

Table 23. Chronic RQs for Direct Effects to Benthic Invertebrates from Tribufos Use on 

Cotton.

Geographic area 

and Type of 

Application

App. 

Rate

(lbs 

a.i./A)

21-day Average EEC Chronic Risk Quotients

Porewater

(μg a.i./L)

Bulksediment

(μg a.i./kg)

Freshwater Invertebrates Estuarine/Marine 

Invertebrates

NOAEC =

2.81 μg a.i./L

NOAEC =

71 μg a.i./kg

NOAEC =

7.10 μg a.i./L

NOAEC =

207 μg a.i./kg

Maximum Label Rates – Runoff and Spray Drift Simulated

National Aerial 1.125 22.4 1732 7.97* 24* 3.15* 8.36*

CA and AZ Aerial 1.875 11.7 904 4.16* 13* 1.65* 4.37*

National Ground 1.125 21.5 1662 7.65* 23* 3.03* 8.03*

Average Application Rates – Runoff and Spray Drift Simulated

National Aerial 0.54 10.6 819 3.77* 11.5* 1.49* 3.96*

CA Aerial 0.64 4.00 309 1.42* 4.35* 0.56 1.49*

AZ Aerial 0.77 4.80 370 1.71* 5.21* 0.68 1.79*

National Ground 0.54 10.4 804 3.70* 11* 1.46* 3.88*

Maximum Label Rates – Runoff Only

National Ground 1.125 19.6 1515 6.98* 21 2.76* 7.32*

CA Ground 1.875 5.29 409 1.88* 0.64 0.745* 1.98*

Abbreviations: App.= Application

A bold value with an asterisk indicates that the RQ meets or exceeds the chronic risk LOC (1) for listed and non-listed species. 

None of the RQ values based on maximum application rates for aquatic vascular and nonvascular plants exposed to tribufos exceeded LOCs (1.0) for risk to listed or non-listed plants

(Table 24). Therefore, based on these studies the likelihood of adverse effects to aquatic plants 

from exposure to tribufos in surface water is considered low.

Table 24. RQ Values for Direct Effects to Aquatic Vascular and Nonvascular Plants from

Tribufos Use on Cotton.

Geographic area 

and Type of 

Application

App. 

Rate

(lbs 

a.i./A)

Peak water

EEC

(μg a.i./L)

Risk Quotients

Aquatic Vascular Plants Aquatic Non-vascular Plants

Listed

EC05 =

140 μg a.i./L

Non-listed

EC50=

1100 μg a.i./L

Listed

NOAEC =

118 μg a.i./L

Non-listed

EC50 =

148 μg a.i./L

Maximum Label Rates – Runoff and Spray Drift Simulated

National Aerial 1.125 29.7 0.21 0.03 0.25 0.20

CA and AZ Aerial 1.875 21.2 0.15 0.02 0.18 0.14

National Ground 1.125 29.0 0.21 0.03 0.25 0.20

Abbreviations: App.= Application

RQs do not exceed the aquatic plant risk LOC of 1. 

4.1.2. RQ Values for Terrestrial Organisms

Exposure estimates for birds and mammals were calculated using upper-bound Kenaga values 

calculated using T-Rex version 1.5.2, as well as though ingestion of contaminated prey estimated 
50

using KABAM based on BCF values, and 1-in-10 year 60-day water-column concentrations 

estimated using the SWCC. As noted previously, birds are the surrogate species for reptiles and 

terrestrial-phase amphibians. Toxicity endpoints used to calculate risk quotients are the most 

sensitive for the taxa evaluated and are described in the tables showing the RQ values.

For maximum and average applications rates for use of tribufos on cotton, the acute dose-based 

RQ values for birds exceed the LOC for acute risk to listed (RQ≥0.1) species for three or more 

types of dietary items (Table 25). In addition, for maximum label rates, the acute dose-based 

RQ values for birds exceed the LOC for acute risk to non-listed (RQ≥0.5) species for the all size 

classes of birds feeding on three or more types of dietary items. When evaluating risk based on 

average application rates, acute-dose based RQs still exceed LOCs for risk to listed and nonlisted birds, but not for as many size classes and dietary categories. The chronic risk LOC 

(RQ≥1) is exceeded for birds consuming short grass, tall grass, or broadleaf plants/small insects 

at the maximum label rate in California and Arizona. At the national maximum label rate, the 

chronic risk LOC is exceeded for birds consuming short grass and broadleaf plants/small insects. 

When evaluating risk based on average application rates, the chronic risk LOC is exceeded for 

birds consuming short grass in California and Arizona but not for the average national rate of 

0.54 lbs a.i./A.

Table 25. Acute and Chronic RQ for Birds Exposed to Tribufos as a Result of Use on 

Cotton.

Food Type

Acute Dose-Based RQs1

Subacute Dietary 

RQs2

Chronic

Dietary-Based RQs Small 3

(20 g)

Medium 

(100 g)

Large

(1000 g)

California and Arizona Maximum Label Rate

1.875 lbs a.i./A, 1 application, Foliar

Short grass 4.71* 2.11* 0.67* 0.30* 3.00*

Tall grass 2.16* 0.97* 0.31* 0.14* 1.38*

Broadleaf plants/small 

insects 2.65* 1.19* 0.38* 0.17* 1.69*

Fruits/pods/(seeds, 

dietary only) 0.29* 0.13* 0.04 0.02 0.19

Arthropods 1.85* 0.83* 0.26* 0.12* 1.18*

Seeds (granivore) 0.07 0.03 0.01 NA NA

National Maximum Label Rate

1.125 lbs a.i./A, 1 application, Foliar

Short grass 2.83* 1.27* 0.40* 0.18* 1.80*

Tall grass 1.30* 0.58* 0.18* 0.08 0.83

Broadleaf plants/small 

insects 1.59* 0.71* 0.23* 0.10* 1.01*

Fruits/pods/(seeds, 

dietary only) 0.18* 0.08 0.03 0.01 0.11

Arthropods 1.11* 0.50* 0.16* 0.07 0.71

Seeds (granivore) 0.04 0.02 0.01 NA NA

Arizona Average Use Rate

0.54 lbs a.i./A, 1 application, Foliar

Short grass 1.36* 0.61* 0.19* 0.09 0.86

Tall grass 0.62* 0.28* 0.09 0.04 0.40

Broadleaf plants/small 

insects 0.76* 0.34* 0.11* 0.05 0.49
51

Food Type

Acute Dose-Based RQs1

Subacute Dietary 

RQs2

Chronic

Dietary-Based RQs Small 3

(20 g)

Medium 

(100 g)

Large

(1000 g)

Fruits/pods/(seeds, 

dietary only) 0.08 0.04 0.01 0.01 0.05

Arthropods 0.53* 0.24* 0.08 0.03 0.34

Seeds (granivore) 0.02 0.01 <0.01 NA NA

National Average Use Rate

0.77 lbs a.i./A, 1 application, Foliar

Short grass 1.93* 0.87* 0.27* 0.12* 1.23*

Tall grass 0.89* 0.40* 0.13* 0.06 0.56

Broadleaf plants/small 

insects 1.09* 0.49* 0.15* 0.07 0.69

Fruits/pods/(seeds, 

dietary only) 0.12* 0.05 0.02 0.01 0.08

Arthropods 0.76* 0.34* 0.11* 0.05 0.48

Seeds (granivore) 0.03 0.01 <0.01 NA NA

California Average Use Rate

0.64 lbs a.i./A, 1 application, Foliar

Short grass 1.61* 0.72* 0.23* 0.10* 1.02*

Tall grass 0.74* 0.33* 0.10* 0.05 0.47

Broadleaf plants/small 

insects 0.90* 0.41* 0.13* 0.06 0.58

Fruits/pods/(seeds, 

dietary only) 0.10* 0.05 0.01 0.01 0.06

Arthropods 0.63* 0.28* 0.09 0.04 0.40

Seeds (granivore) 0.02 0.01 <0.01 NA NA

NA=not applicable

Bolded values with an asterisk meet or exceed the Level of Concern (LOC) for acute risk to listed (RQ > 0.1) and/or non-listed (RQ > 0.5) birds 

or chronic risk to birds (RQ > 1). 1 Acute dose-based RQ values are based on the bobwhite quail LD50 value of 151 mg ai/kg-bw (MRID 00120771). There is an acute dose-based 

ED50 for the canary of 111 mg a.i./kg-bw based on both mortality and regurgitation (MRID 49121303). When you correct for body weight, the 

RQ values based on the bobwhite quail and canary endpoint are different by 0.01. 2 Acute dietary RQ values are based on the bobwhite quail 5-day LC50 value of 1519 mg a.i./kg diet (MRID 41618804). 3 Chronic RQ values are based on the bobwhite quail NOAEC value of 150 mg a.i./kg diet (MRID 40757101).

For maximum and average applications rates for use of tribufos on cotton, the acute dose-based 

RQ values for all size classes of mammals exceed the LOC for acute risk to listed (RQ≥0.1) 

species for three or more types of dietary items (Table 26). In addition, for maximum label 

rates, the acute dose-based RQ values for mammals exceed the LOC for acute risk to non-listed 

(RQ≥0.5) species for medium and small size classes of mammals for one or more dietary classes. 

When evaluating risk based on average application rates, acute dose-based RQs still exceed 

LOCs for risk to listed and non-listed species for some dietary items. Subacute dietary RQs for 

maximum label rates meet or exceed the risk to listed species LOCs for two or more dietary 

classes. Chronic dietary RQs exceed the chronic risk LOC of 1.0 for at least two dietary classes.

The chronic risk LOC (RQ≥1) is exceeded for mammals consuming short grass, tall grass, 

broadleaf plants/small insects, and arthropods for RQs based on maximum label rates in 

California and Arizona and for mammals consuming short grass and broadleaf plants/insects for 

that national maximum label rate. Chronic risk LOCs for mammals consuming short grass are 

also exceeded for RQs calculated based on average application rates in California and Arizona 

but not the national average application rate.
52

Table 26. Acute and Chronic RQ for Mammals Exposed to Tribufos as a Result of Use on

Cotton.

Food Type

Acute Dose-Based RQs1 Subacute Dietary 

RQs2

Acute Dose

Chronic

Dietary- Based RQs3

Acute Dietary

Small

(15 g)

Medium 

(35 g)

Large

(1000 g)

California and Arizona Maximum Label Rate

1.875 lbs a.i./A, 1 application, Foliar

Short grass 1.02* 0.87* 0.47* 2.80* 14.06*

Tall grass 0.47* 0.40* 0.21* 1.28* 6.45*

Broadleaf plants/small 

insects 0.57* 0.49* 0.26* 1.57* 7.91*

Fruits/pods/(seeds, 

dietary only) 0.06 0.05 0.03 0.17* 0.88

Arthropods 0.40* 0.34* 0.18* 1.09* 5.51*

Seeds (granivore) 0.01 0.01 0.01 NA NA

National Maximum Label Rate

1.125 lbs a.i./A, 1 application, Foliar

Short grass 0.61* 0.52* 0.28* 1.68* 8.44*

Tall grass 0.28* 0.24* 0.13* 0.77* 3.87*

Broadleaf plants/small 

insects 0.34* 0.29* 0.16* 0.94* 4.75*

Fruits/pods/(seeds, 

dietary only) 0.04 0.03 0.02 0.10* 0.53

Arthropods 0.24* 0.20* 0.11* 0.66* 3.30*

Seeds (granivore) 0.01 0.01 <0.01 NA NA

Arizona Average Use Rate

0.54 lbs a.i./A, 1 application, Foliar

Short grass 0.29* 0.25* 0.13* 0.80* 4.05*

Tall grass 0.13* 0.11* 0.06 0.37* 1.86*

Broadleaf plants/small 

insects 0.16* 0.14* 0.08 0.45* 2.28*

Fruits/pods/(seeds, 

dietary only) 0.02 0.02 0.01 0.05 0.25

Arthropods 0.11* 0.10* 0.05 0.32* 1.59*

Seeds (granivore) <0.01 <0.01 <0.01 NA NA

National Average Use Rate

0.77 lbs a.i./A, 1 application, Foliar

Short grass 0.42* 0.36* 0.19* 1.15* 5.78*

Tall grass 0.19* 0.16* 0.09 0.53* 2.65*

Broadleaf plants/small 

insects 0.23* 0.20* 0.11* 0.65* 3.25*

Fruits/pods/(seeds, 

dietary only) 0.03 0.02 0.01 0.07 0.36

Arthropods 0.16* 0.14* 0.07 0.45* 2.26*

Seeds (granivore) 0.01 <0.01 <0.01 NA NA

California Average Use Rate

0.64 lbs a.i./A, 1 application, Foliar

Short grass 0.35* 0.30* 0.16* 0.95* 4.80*

Tall grass 0.16* 0.14* 0.07 0.44* 2.20*

Broadleaf plants/small 

insects 0.20* 0.17* 0.09 0.54* 2.70*

Fruits/pods/(seeds, 

dietary only) 0.02 0.02 0.01 0.06 0.30
53

Food Type

Acute Dose-Based RQs1 Subacute Dietary 

RQs2

Acute Dose

Chronic

Dietary- Based RQs3

Acute Dietary

Small

(15 g)

Medium 

(35 g)

Large

(1000 g)

Arthropods 0.14* 0.12* 0.06 0.37* 1.88*

Seeds (granivore) <0.01 <0.01 <0.01 NA NA

NA=not applicable

Bolded values with an asterisk meet or exceed the Level of Concern (LOC) for acute risk to listed (RQ > 0.1) and/or non-listed (RQ > 0.5) birds 

or chronic risk to birds (RQ > 1). 1 Acute dose-based RQ values are based on the laboratory rat LD50 value of 192 mg a.i./kg-bw (MRID 41954903). 2 Subacute dietary RQ values are based on the white footed mouse 5-day LC50 value of 161 mg a.i./kg diet (MRID 45863708). 3 Chronic RQ values are based on the laboratory rat NOAEC value of 32 mg a.i./kg-diet (MRID 42040201). 4 Probability of individual mortality based on actual screening-level RQ value for small mammals. The default acute dose-response slope of 4.5

and a concentration-response slope value of 2.79 was used to generate the probability values. A dose-response slope value was not calculable 

from the available acute dose test data (Urban and Cook, 1986; USEPA, 2004b). Probability values are shown for the highest and lowest RQ 

values.

Not enough data are available to calculate a reliable metabolism rate constant to parameterize 

KABAM; therefore, the empirical BCF values for fish were used to estimate tribufos 

concentrations in aquatic organisms. The empirical BCF value for tribufos is 730 L/kg in whole 

fish and was measured for the bluegill sunfish (Lepomis macrochirus). Maximum 1-in-10-year 

60-day average EECs for the scenarios explored for aquatic modeling ranged from 11.6 to 25.1 

μg/L. The estimated concentrations in aquatic organisms resulting from this range of EECs are 

used with the BCFs to bracket the potential concentrations of tribufos in aquatic vertebrate 

tissues (at steady state) in areas vulnerable to runoff. It should be noted that tissue 

concentrations in aquatic vertebrates will likely be bound by toxicity of tribufos on these 

organisms. For instance, the NOAEC for aquatic vertebrates exposed to tribufos is 4.1 μg a.i./L. 

At this concentration, there were reductions in growth and post-hatch survival. Given that we 

expect a range of sensitivity in different aquatic invertebrates, some sensitive aquatic vertebrates 

would be expected to have effects at the estimated concentrations while less sensitive species 

could carry residues in tissue without showing effects. Assuming EECs of 11.6 to 25.1 μg a.i./L 

and a BCF value of 730 L/kg for whole fish, tribufos concentrations in aquatic vertebrate tissues 

may range up to 8.5 to 18 mg a.i./kg (diet) (BCF x EEC = concentration in fish). All of these 

concentrations are higher than dietary subacute and chronic dietary endpoints for mammals and 

birds. Acute RQs range from 5.57 to 12.1 for birds and 52.6 to 114 for mammals. Chronic RQs 

range from 56.5 to 122 for birds and 32.6 to 70.5 for mammals.

Table 27. RQ Values for Birds and Mammals Consuming Fish.

Use 

Scenario

WaterColumn 

60-day 

average 

EEC (μg/L)

BCF 

(L/kg)

Tribufos 

conc in 

fish 

(μg/kg)

Subacute Dietary RQ Chronic RQ

Birds

5-day LC50=

1519 mg a.i./kgdiet

Mammals

5-day LC50=

161 mg/kg-diet

Birds

NOAEC=

150 mg/kgdiet

Mammals

NOAEC=

260 mg/kgdiet

1.875 lbs 

a.i./A, no 

runoff, 

California

11.6 730 8468 5.57 52.6 56.5 32.6

1.1 lbs 

a.i./A, 

Mississippi

25.1 730 18323 12.1 114 122 70.5

Conc=concentration
54

4.1.2.3. Terrestrial Invertebrates 

EECs for terrestrial invertebrates were calculated using the equations described in Section

3.2.3.4. Tribufos is classified as practically nontoxic to honey bees on an acute contact exposure 

basis. The only study available to evaluate potential effects to honey bees is currently an acute 

contact study where only 3% mortality was observed at the highest dose tested of 24.7 μg 

a.i./bee. The highest acute contact EECs of 5.06 estimated for the maximum label rate of 1.875

lbs a.i./A is lower than the highest dose tested where only 3% mortality occurred. Since the 

acute contact toxicity study for honey bees exposed to tribufos was non-definitive (i.e., LD50

greater than highest concentration tested), RQs are not calculated. The risk to insect pollinators 

is discussed in greater detail in the Risk Description (Section 4.2). 

4.1.2.4. Non-target Terrestrial Plants

EECs for terrestrial plants were calculated using TERRPLANT (Version 1.2.2). RQ values for 

terrestrial plants are calculated using the most sensitive endpoints for monocot and dicot species 

in vegetative vigor and seedling emergence tests with tribufos typical end-use product (TEP) 

(Table 28). For all labeled uses of tribufos, RQ values for monocot and dicot plants exposed to 

runoff are lower than (i.e., do not exceed) the risk to non-listed and listed species LOC (LOC 

=1). 

RQ values are not calculated for non-listed terrestrial plants due to runoff because definitive 

EC25 values were not available from the submitted seedling emergence tests with TEP (i.e., no 

dicot species/endpoints were inhibited by 25% or more at the application rates used). The 

potential for adverse effects to listed and non-listed species of terrestrial and semi-aquatic plants 

from exposure to formulated tribufos is discussed in greater detail in the Risk Description 

(Section 4.2). In the risk description, the distance from the edge of the field where effects may 

occur due to exposure to spray drift is reported based on the results of a non-guideline 

defoliation study.

Table 28. RQs for Non-target Terrestrial Plants Adjacent to Tribufos Use on Cotton 

(Aerial Application).

Plant Type Listed Status Risk Quotients

Dry Semi-Aquatic

Monocot non-listed Not calculated* Not calculated*

Monocot listed 0.10 0.26

Dicot non-listed Not calculated* Not calculated*

Dicot listed 0.21 0.53

*Runoff RQ values for nontarget plants are based on the seedling emergence NOAEC of 0.45 lbs a.i./A for monocots and 0.22 lbs a.i./A for 

dicots, corresponding EC25 values were estimated to be >2 lbs a.i./A and an RQ was not calculated for non-listed species. The most sensitive 

NOAEC for vegetative vigor is 0.48 lbs a.i./A for both monocots and dicots. 

4.2. Incident Database Review

The Ecological Incident Information System (EIIS), which is maintained by the Agency’s Office 

of Pesticide Programs, was searched to determine if ecological incidents have been reported for 

tribufos. Based on a search of EIIS conducted in April 2015. There is only one incident in the 
55

EIIS database associated with tribufos (I016036-024). This incident involved damage to 53 acres 

of lettuce in Riverside, California, in 2004. The damage was described as ‘uniform throughout’ 

the field, although the specific type of damage to the lettuce was not specified in the incident 

report. Damage was attributed to spray drift of tribufos and diuron that had been legally applied 

by air to a cotton field adjacent to the damaged lettuce field. The damaged lettuce tested positive 

for tribufos (50 ppb). No other residue information was provided in the report. 

The American Bird Conservancy Avian Incident Monitoring System was searched in April 2015 

and returned no records of incidents associated with tribufos14.

Based on a search of the U.S. EPA Office of Pesticide Programs Incident Data System (IDS), 

which compiles aggregate incidents, there were a total of four incidents involving tribufos 

between 1993and 1997, of which four were associated with minor plant damage and one was 

associated with a fish kill in a pond. There were also two misuse incidents associated with a fish 

kill.

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

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

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

impaired reproduction, are rarely received. Available incident information indicates that use of 

tribufos on cotton has the potential to result in effects to nontarget plants. 

4.3. Risk Description

 

This screening-level assessment of the labeled uses of tribufos on cotton relies on the 

deterministic RQ method to provide a metric of potential risks. RQs exceed LOC values for:

• freshwater and estuarine/marine fish (acute listed and chronic risk LOCs);

• freshwater and estuarine/marine invertebrates (acute listed, acute non-listed, and chronic

risk LOCs);

• birds (acute listed, acute non-listed, and chronic risk LOCs); and,

• mammals (acute listed, acute non-listed, and chronic LOCs).

While RQs for terrestrial plants are not exceeded, incidents for terrestrial plants have been 

reported and non-guideline toxicity studies indicate that defoliation may occur in sensitive plants 

such as cotton and deciduous trees and shrubs. This will be discussed in more detail in this 

section. 

Limited available toxicity data for honey bees suggest that LOCs would not be exceeded on an 

acute-contact basis; however, there is uncertainty in risk to bees via oral exposure.

RQs for aquatic plants do not exceed LOCs for listed or non-listed plants. Therefore, the 

likelihood of risk to aquatic plants is considered low.

 14 http://www.abcbirds.org/abcprograms/policy/toxins/aims/aims/index.cfm
56

4.3.1 Risk to Aquatic Organisms

Aquatic Exposure

Surface water and benthic sediment pore-water EECs resulting from applications of tribufos to 

cotton are estimates of exposure for the use of tribufos as a broadcast herbicide, i.e., defoliant. 

Application practices were modeled using the maximum annual application rate with a single 

application and with average application rates assuming a single application. Exposure 

estimated for average application rates will produce EECs lower than those that may occur when 

tribufos is used at a higher rate. Use of tribufos at rates higher than the average rate is expected 

to commonly occur. Environmental fate data were conservatively represented in keeping with 

EFED’s Input Parameter Guidance 

(http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm) by using an 

uncertainty factor of 3X applied to the single soil and sediment metabolism half-life estimates. 

Finally, scenario selection was conservatively performed by modeling runoff prone soils that 

drain to a small pond without an outlet. When EECs were calculated assuming that spray drift 

would not occur (e.g., that the spray drift fraction was zero), EECs were 35 to 89% of the 

maximum EECs modeled with spray drift included. In runoff vulnerable areas, the contribution 

of spray drift to the final EEC was minimal (i.e., EECs calculated with and without simulation of 

spray drift were similar).

Aquatic Animals

Based on the most sensitive endpoints, one major concern for aquatic organisms in this 

assessment is for freshwater and estuarine/marine invertebrates inhabiting both the water-column 

and benthic environments. Tribufos is characterized as highly toxic to very highly toxic to 

aquatic invertebrates. Acute risk (listed and non-listed) and chronic risk LOCs for freshwater 

and estuarine/marine invertebrates are exceeded for the EECs simulated for maximum label rates 

and average application rates15. Effects on survival and shell deposition were observed in acute 

toxicity studies at concentrations lower than those estimated in the environment from the use of 

tribufos on cotton. Acute RQ values range from 0.65 to 5.5 for all exposure scenarios simulated 

(including maximum and average application rates, and simulations excluding spray drift). This 

indicates a potential for acute mortality to occur among aquatic invertebrates from exposure as a 

result of the current labeled use of tribufos in areas vulnerable to runoff. With chronic exposure, 

a reduced number of young/adult/day (LOAEC=3.23 μg a.i./L for the water flea), reduced, 

growth (LOAEC=8.27 μg a.i./L in pore-water for the freshwater amphipod), and reduced 

survival (LOAEC=22.7 μg a.i./L in pore-water for the estuarine/marine amphipod) were 

observed at concentrations lower than those predicted in vulnerable areas when tribufos is used 

on cotton (21-day average EECs range from 6.51 to 27.1 μg a.i./L in the water-column and 5.29

to 22.4 in pore-water) (Table 29). Chronic risk LOCs are still exceeded, after the contribution of 

spray drift is removed from exposure estimates. While any buffer between an application and 

aquatic water body is expected to reduce exposure and risk, a methodology is not available to 

estimate the reduction in EECs due to transport in runoff because channelized runoff may occur. 

Therefore, there is a potential for adverse effects (e.g., mortality, reduced growth, reductions in 

 15 The chronic risk LOC for esturarine/marine invertebrates inhabiting the water column is not exceeded for average 

application rates simulated with the California cotton scenario.
57

the number offspring, and reduced shell growth) to aquatic invertebrates (including mollusks16)

inhabiting both the water-column and benthic zones from use of tribufos on cotton.

Table 29. Comparison of LOAEC Values to the Lowest and Highest Estimated EECs for 

Aquatic Invertebrates.

21-day 

average

EECs

LOAEC

Ratio of 

EEC 

/LOAEC

Species Effect Observed at LOAEC

10.4*

3.23 3.22 Water flea 10% reduction in number of young/adult/day, 

2-6% reduced growth

8.27 1.26 Freshwater amphipod 39% reduction in growth

22.7 0.46 Estuarine/marine amphipod 16% reduction in survival

27.1+

3.23 8.39 Water flea 10% reduction in number of young/adult/day, 

2-6% reduced growth

8.27 3.28 Freshwater amphipod 39% reduction in growth

22.7 1.19 Estuarine/marine amphipod 16% reduction in survival

* Lowest 1-in-10 year 21-day average EEC in pore-water for 0.54 lbs a.i./A applied via ground application.

+ Highest 1-in-10 year 21-day average EEC in the water-column for 1.125 lbs a.i./A applied via aerial application.

Based on available information, tribufos is categorized as highly toxic to aquatic vertebrates (fish 

and aquatic-phase amphibians) on an acute exposure basis. Risk quotients for freshwater fish,

based on acute exposure and the most sensitive endpoints available, exceed LOCs for risk to

listed species but not non-listed species. Risk quotients for estuarine/marine fish based on acute 

exposure basis do not exceed acute risk LOCs. Risk quotients based on a chronic exposure 

exceed chronic risk LOCs for both freshwater and estuarine/marine vertebrates. These LOCs are 

exceeded when simulating both maximum labeled rates and average application rates and when 

simulating runoff only in areas highly vulnerable to runoff, except for the chronic risk LOC for 

estuarine/marine vertebrates for the California and Arizona average application rates simulated 

using the Califonria cotton scenario. Effects to growth and post hatch-survival were observed in 

early life cycle tests with the sheepshead minnow at the 19 μg a.i./L treatment level. One-in-10-

year 60-day average EECs for maximum label rates ranged from 14.3 to 25.1 and LOAECs 

range from 4.83 to 19 μg a.i./L. One-in-10 year 60-day average EECs estimated for average 

application rates are lower than the LOAECs but still exceed NOAECs. Similar effects to 

freshwater vertebrates would be expected to occur based on an estimated toxicity endpoint. 

Additionally, there is one incident associated with an approved use of tribufos involving a fish 

kill in a pond, although, diuron17 was also present in the formulation and is also toxic to fish. 

Therefore, there is a potential for adverse effects (e.g., mortality, reduced growth, and reduced 

survival of young) to aquatic vertebrates from use of tribufos on cotton.

Non-targeted monitoring data for surface water (peak=0.3 μg/L) and sediment (peak=5.1 μg/kg) 

confirm the identified transport pathways and are approximately a factor of 10 lower than the 

most sensitive toxicity endpoints for aquatic invertebrates (Daphnid 48- LC50, =5.4 μg/L; 

 16 Toxicity data are only available for estuarine/marine mollusks. Unless additional data are provided showing that 

freshwater mollusks are less sensitive than estuarine/marine mollusks, the risk conclusion for estuarine/marine 

mollusks is also assumed for freshwater mollusks.

17 According to the EFED ECOTOXITY Database, the lowest diruon acute toxicity endpoint for fish is a 96-hour 

LC50 for the Cutthroat trout (Oncorhynchus clarkii) of 0.71 mg/L. 
58

Daphnid 21-day No-observable-adverse-effect-concentration, NOAEC=1.56 μg/L; Hyalella

azteca 10-day sediment NOAEC=71 μg a.i./kg-sediment) and aquatic vertebrates (Lepomis 

promelas estimated NOAEC=4.1μg/L). These monitoring studies are not expected to capture 

peak concentrations because the sampling was infrequent and not targeted to an area of tribufos 

use. These data add support to the potential for exposure at concentrations that could result in 

adverse effects to aquatic invertebrates and vertebrates.

Formulation toxicity data for tribufos (DEF6®) resulted in higher (less sensitive) aquatic toxicity 

values as compared to tribufos TGAI. Therefore, evaluation of risk based on TGAI, is expected 

to be protective of risk due to exposure to formulations.

Aquatic Plants

Based on available data, the likelihood for adverse effects to listed and non-listed vascular plants 

is considered low, and possible for nonvascular aquatic plants. RQ values for non-listed and 

listed vascular and nonvascular aquatic plants ranged from 0.02 to 0.25 and were all lower than 

LOCs of 1.0. However, there is some uncertainty with this conclusion for non-vascular plants as 

data are only available for freshwater green algae. Data are also required for marine diatom, 

freshwater diatom, and cyanobacterium and it is possible that these species could be more 

sensitive than freshwater green algae. Data from the EFED ECOTOX database on 

organophosphate chemicals was compiled (see Appendix J) to better understand the potential 

risk to plants from exposure to tribufos. Results indicate that the EC50 endpoints for 

organophosphates commonly vary by a factor of 10. Therefore, it is possible additional data for 

non-vascular aquatic plants could change risk conclusions.

Aquatic Spray Drift Exposure 

Spray drift exposure to tribufos was determined using AgDRIFT version 2.1.1 (USEPA, 2013). 

The distance to water body from edge-of-field was determined for an initial average 

concentration of 0.27 μg/L for listed species and 2.7 μg/L for non-listed species in Tier I Ground 

and Tier I Aerial aquatic assessments. The default ASAE Fine to Medium droplet size 

distribution was assumed for aerial applications. For ground applications, a high boom, ASAE 

very fine to fine drop size distribution, and the 90th data percentile was assumed. These are the 

default spray drift inputs recommended for modeling spray drift (USEPA, 2013).

While runoff only EECs exceed LOCs for aquatic organisms and therefore, a spray drift buffer 

will not eliminate the potential for adverse effects to aquatic organisms from exposure through 

runoff (both water and erosion), it is useful to know how far from the edge of the field spray drift 

exposure alone could result in risk to aquatic organisms. Concentrations of 0.27 μg/L in water 

exceed the acute risk to listed species LOC (based on the D. magna acute toxicity endpoint of 5.4

μg/L). The aquatic spray drift distance (i.e., the distance from the edge of the field where spray 

drift exposure could result in RQs that exceed LOCs) for risk to listed and non-listed aquatic 

invertebrates are summarized in Table 30. For listed species, the buffer distance for aerial

applications is greater than 1000 feet from the edge of the field for the lower average application 

rate of 0.54 lbs a.i./A. For ground applications, the spray drift distance ranges from 170 to 577 

feet from the edge of the field. For non-listed species, the spray drift distances are lower. 
59

Table 30. Aquatic Drift Distance for the Use of Tribufos on Cotton.

Application rate

Distance from Edge of Field Where Spray Drift Could Result in RQs greater than 

LOCs for Aquatic Invertebrates (feet)1,2

Listed aquatic invertebrates Non-listed aquatic invertebrates

Aerial Application (Tier I, Fine to Medium DSD)

1.875 >1000 262

1.125 >1000 131

0.54 >1000 30

Ground Application (Tier I, Very fine to fine DSD, 90th data percentile, high boom)

1.875 577 30

1.125 374 7

0.54 170 0

DSD=drop size distribution

1 Estimated based on a the 48-hour LC50 of 5.4 μg a.i./L for daphnid and the listed (0.05) and non-listed (0.5) LOCs, initial average concentrations 

were 270 ng/L for listed species and 2700 ng/L for non-listed species. 2 Runoff only RQs exceed LOCs, therefore, a spray drift buffer alone will not completely eliminate the potential for risk to aquatic invertebrates.

4.3.2. Risk to Terrestrial Organisms

Terrestrial Exposure

To evaluate the potential for terrestrial exposure from the use of tribufos on cotton, application 

practices were modeled by using maximum application rates and average applications and 

assuming a single application. Exposure estimated for average application rates results in EECs 

that are lower than those that may occur when tribufos is used at a higher rate. Exposure was 

estimated using upper-bound and mean Kenaga values, unless otherwise stated, from T-REX 

version 1.5.2 and using BCF values. RQs considering T-REX and BCF values are discussed 

separately. The spray drift model AgDrift version 2.1.1 provides helpful bounds which define

the spatial extent of risk concern resulting from off-field drift at currently registered tribufos

application rates. 

Birds and Mammals

Tribufos is categorized as being slightly toxic to moderately toxic to birds on an acute oral 

exposure basis and practically non-toxic to slightly toxic birds on a subacute dietary exposure 

basis. Tribufos is categorized as being moderately toxic to mammals on an acute oral exposure 

basis and highly toxic on a subacute dietary exposure basis. 

In this assessment, RQs based on acute oral exposure to birds exceed the acute risk to listed

species LOC of 0.1 and acute risk to non-listed species LOC of 0.5 for at least three of the 

dietary classes for both average application rates and maximum application rates. Risk to nonlisted species LOCs are exceeded for four dietary classes for small and medium-sized birds at 

maximum labeled rates. RQs calculated based on a subacute dietary exposure to birds exceed 

LOCs for risk to listed birds at the maximum label rates but not for the average application rates. 

RQs for birds based on chronic dietary exposure exceed the chronic risk LOC for three dietary 

classes at the maximum application rate of 1.875 lbs a.i./A, for two dietary classes at 1.125 lbs 

a.i./A, for one dietary class at 0.64 and 0.77 lbs a.i./A, and there are no exceedances at the 

average application rate of 0.54 lbs a.i./A. Bobwhite quail exposed to tribufos in the diet at 280 

mg a.i./kg-diet for 20 weeks, had a reduced egg shell thickness. At 410 mg/kg-diet there was a 

reduced egg production, reduced survival of hatchlings, and reduced growth, an increased 
60

number of females with a regressed or regressing ovary, and slight to moderate distension of the 

gastrointestinal tract. Dietary EECs range from 176 to 450 mg/kg-diet for maximum label rates 

and are lower than the LOAEC (280 mg a.i./kg-diet) for average application rates. Standard RQs 

are calculated based on upper-bound Kenaga values. When considering EECs based on mean 

Kenaga exposure estimates, some RQs exceed acute risk to listed, acute risk non-listed, and 

chronic risk LOCs at the 1.875 lbs a.i./A application rate. When considering EECs based on 

mean Kenaga exposure estimates and the 1.125 lbs a.i./A application rate, some RQs exceed 

acute risk listed and acute risk to non-listed species LOCs but chronic risk LOCs are not

exceeded. 

For tribufos use on cotton, RQs based on acute oral exposure to mammals exceed the acute risk 

to listed species LOC of 0.1 and acute non-listed species LOC of 0.5 for at least three of the 

dietary classes for both average application rates and maximum application rates. Non-listed

species LOCs are exceeded for short grass for small and medium-sized mammals at maximum 

label rates. RQs calculated based on sub-acute dietary exposure to mammals exceed the acute

risk to listed species and chronic risk LOCs at the maximum label rates and average application 

rates for at least four dietary forage categories. 

Laboratory rats exposed to tribufos in the diet at 260 mg a.i./kg-diet in the 2-generation 

mammalian reproduction study, had an increased number of litters with still born pups, increased 

pup death through lactation, decrease in F1 and F2 pup body weights, and an increased F1 

gestation period. Dietary EECs range from 176 to 450 mg/kg-diet for maximum label rates and 

are lower than the LOAEC, for average application rates. Standard RQs are calculated based on 

upper-bound Kenaga values. When considering EECs based on mean Kenaga dietary exposure 

estimates, some RQs exceed acute risk to listed and non-listed species, and chronic risk LOCs at 

the maximum labeled application rates. When considering EECs based on mean Kenaga 

exposure estimates and the 0.54 lbs a.i./A application rate, some RQs exceed acute risk to listed 

and non-listed species LOCs, but chronic risk LOCs are no longer exceeded.

The potential for risk to birds and mammals from proposed foliar uses were evaluated in this 

assessment using the default foliar dissipation half-life of 35 days, since no additional data were

available to determine the decline in tribufos residues in dietary items. However, since LOCs for 

acute risk to listed (RQ≥0.05) and non-listed (RQ≥0.1) birds and chronic risk (RQ≥1) to 

mammals are exceeded in this assessment, the impact of different half-lives on RQs was 

examined for purposes of characterization (Table 31). Based on this analysis, at maximum 

labeled application rates, both acute and chronic risk LOCs are exceeded even when a half-life of 

1 day is considered. Additionally, at maximum label rates and assuming the default 35 day foliar 

dissipation rates, LOCs are exceeded for 30 to more than 56 days. Therefore, potential risks of 

concern to birds and mammals from use of tribufos on cotton would not be precluded based on 

refinements to the default foliar dissipation half-life.
61

Table 31. Impact of Different Foliar Dissipation Half-lives on the Number of Days RQs 

Based on Dietary Exposure Exceed LOCs.

Application Rate 

(lbs a.i./A)

Number of Days There is a Dietary LOC exceedance1

Avian Mammalian

Acute Chronic Acute Chronic

35 day foliar dissipation rate

1.875 56 56 56 56

1.125 30 56 56 56

0.54 56 0 56 55

1 day foliar dissipation rate

1.875 6 2 4 7

1.125 5 1 3 7

0.54 4 0 2 6 1 The analysis is only carried out to a maximum of 56 days based on limitations in T-REX.

Based on EECs of 11.6 to 25.1 μg a.i./L and using a BCF value of 730 L/kg, tribufos 

concentrations in aquatic vertebrate tissues range from 8,468 to 18,323μg a.i./kg(diet) (BCF x 

EEC = concentration in fish). These concentrations are higher than acute-dietary LC50 values

and chronic dietary NOAECs for birds and mammals for all scenarios explored in aquatic 

modeling. Risk quotients range from 5.57 to 122. Therefore, there is a potential for adverse 

effects to birds and mammals from exposure to aquatic prey items in which residues may have 

accumulated/concentrated. However, tribufos has been shown to rapidly depurate, and once fish 

are in clean water, 71-88% of tribufos residues are expected to dissipate over 14-days. 

Additionally, the EECs were estimated for areas vulnerable to runoff and risk is only expected to 

occur in these areas that would be highly vulnerable to runoff. When evaluating risk using the 

peak single value from a grab sample, (0.3 μg/L), tribufos concentrations in aquatic vertebrate 

tissues drop to 219 μg a.i./kg-diet, and only exceed the subacute dietary endpoint for mammals 

(EEC/NOAEC=219/161=1.4) and chronic NOAEC for birds (EEC/NOAEC=219/150=1.5). It 

would take several days of exposure at 0.3 μg/L for residues in fish to accumulate concentrations 

where effects would be observed.

Although tribufos is characterized as semi-volatile, the STIR model output indicates that 

inhalation is not likely to be a significant exposure pathway of concern for avian and mammalian 

species. 

Terrestrial Invertebrates

In an acute contact toxicity test in which young adult honey bees were exposed to tribufos mixed 

with pyrolite dust, there was 3% mortality observed at 24.17 μg a.i./bee. Based on these limited 

data, tribufos is classified as practically non-toxic to young adult honey bees on an acute contact 

exposure basis. The acute dietary exposure EEC for tall grass range from 1.46 to 5.06 μg 

a.i./bee/day, the highest ratio of the EEC to the highest level tested with 3% mortality 

(24.17/5.06=0.21) is less than the acute risk LOC (0.4) for individual bees. 

Since tribufos is used on cotton and since cotton is used by some beekeepers to produce honey, 

the proximity of bee colonies to tribufos use areas may be high especially given that cotton is an 

interderminant blooming crop, i.e., blooms over a protracted period of time. In a survey of 

pesticide residues in various colony matrices (bees, wax, pollen) from managed migratory bee 
62

colonies (Mullin et al., 2010)18, tribufos residues were reported in 2.9% of wax samples, ranging 

from 7.6 to 59.0 ppb (mean=25.1 ppb) in samples with a positive detection (Table 32). The limit 

of detection was 2.0 ppb. This provides evidence that honey bees will be exposed in the field 

and that residues can be brought back to the colony where developing brood may in turn be 

exposed. Tribufos is taken up into plants; however, it is unknown whether it is transported in the 

xylem and phloem (USEPA, 2008) and whether bees may be exposed to tribufos taken up into 

plant materials such as pollen, nectar, etc. As tribufos is used close to harvest, the potential for 

honey bee exposure may be lessened; however, the detections in wax and pollen samples, 

indicate that exposure can still occur (either through late blooms on cotton or through exposure 

to residues on nontarget blooming plants adjacent to the cotton fields).

Table 32. Summary of Pesticide Detections in Wax and Pollen Samples from North 

American Honey Bee Colonies (Reported in Mullin et al.,(2010))

Media Detects Samples % Tribufos concentrations (ppb)

High Low 95%tile Mean

Wax 6 208 2.9 59.0 7.6 51.5 25.1

Pollen 1 350 0.3 3.5 3.5 3.5 3.5

There are no reported incidents of bee kills for tribufos. However, there are honey bee incidents 

for other organophosphate insecticides. The absence of bee kill incident data cannot be 

construed as the absence of incidents. 

Based on the acute contact study, risk to individual honey bees would be considered low; 

however, a full dataset is not available to fully evaluate risk to honey bees. Pollinator risk 

assessment guidance has recently been finalized and recommends the following honey bee data 

be available for a Tier I screening level risk assessment for honey bees:

• acute oral toxicity to adult honey bees;

• acute oral toxicity to larval honey bees;

• chronic oral toxicity to adult honey bees; and,

• chronic oral toxicity to larval honey bees.

If Tier I studies identify the potential for a risk concern, additional studies may be requested.

Until these data are available, the risk to honey bees cannot be fully evaluated. Risk to 

individual adult honey bees exposed in their diet to tribufos could not be evaluated. 

Additionally, data are not available to evaluate potential effects to brood. 

Additional information from the open literature (Greenberg et al., 2004; ECOTOX Ref.: 92450) 

indicates mortality to individuals and population effects in boll weevils exposed to tribufos at an 

application rate of 0.42 lb a.i./acre (using the DEF 6 formulation), which is 22 to 37% of the 

maximum allowable labeled rates. This line of evidence indicates that mortality to similarly 

sensitive insects in the field will occur at average and maximum labeled application rates. 

Additionally, tribufos is an organophosphate (intended for use as a defoliant) and does inhibit

acetylcholine esterase (USEPA, 2009b). Other organophosphates are commonly used as 

insecticides and terrestrial invertebrates may be sensitive to toxicity from acetylcholine esterase 

 18 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0009754
63

inhibition.

Non-target terrestrial plants

Based on the available registrant-submitted toxicity data, RQs do not exceed the Agency’s LOC

for risk to terrestrial plants (Table 28). Because the studies from which these data are drawn 

were not conducted on sensitive plant species (e.g., deciduous trees and shrubs), these data are 

expected to underestimate the potential toxicity of tribufos to non-target plants that do form 

abscission zones (e.g., some woody, deciduous plants). 

Application of the formulated tribufos product Folex® 6EC at rates of up to 1.9 lb a.i./acre and in 

combination with the adjuvant Activator 90® (0.1%) resulted in statistically significant (p<0.05) 

leaf fall in viburnum and maple at treatment rates equivalent to 1.125 and 1.875 lb a.i./acre and 

in oak at 1.875 lb a.i./acre (NOAEC = 0.563 lbs a.i./A). These effects were accompanied by a 

reduction in the numbers of old leaves remaining on the plants at the end of the 28-day study 

period. However, there were no apparent treatment-related effects on the development of new 

leaves in the three shrub species, Forsythia, Ribes and Viburnum at any of the treatment rates in 

comparison to the water- and adjuvant-treated controls, or in the limited number of replicates of 

the two tree species where new growth was recorded. This indicates that there is a potential for 

effects to non-target plants that form abscission zones to occur at the maximum labeled rates and 

average application rates. Some plants will be able to replace dropped leaves. Assuming an 

aerial application, ASAE fine to medium drop size distribution, and 1.9 lbs a.i./A application 

rate, exposure at 0.56 lbs a.i./A would occur up to 13 feet from the edge of the field. Effects 

would not occur off the field at 1.125 lbs a.i./A. 

One study identified from the open literature (Greenberg, 2004) describes effects (defoliation) to 

cotton from a tribufos application at a rate of 0.42 lb a.i./acre using the DEF 6® formulation. At 

this application rate (which is 22 to 37% of the maximum allowable labeled rate), an average of 

80.2% (± SD of 6.1) of the leaves per plant (N = 9) were dropped 7 days post-application;

whereas, in the untreated control plot, 3.3% (± SD of 1.6) of the leaves per plant (N = 10) were 

dropped in the same time period. It is possible that other plants will have a similar sensitivity as 

cotton.

One terrestrial plant incident in the EIIS database is associated with tribufos (I016036-024). 

This incident, which damaged 53 acres of lettuce in Riverside, California, also involved the 

herbicide diuron. Although the damaged lettuce tested positive for tribufos (50 ppb) and no other

residues were identified, the fact that an herbicide that is more toxic to plants was also involved 

in the incident makes it difficult to determine what specific effects, if any, were due to tribufos 

alone, diuron alone, and/or both chemicals together. The diuron vegetative vigor EC25 for the 

dicot tomato (Lycopersicon esculentum) is 0.002 lb a.i./A (MRID 44113401) suggests, that 

diuron may have been the cause of the incident. However, based on the fact that tribufos is a 

known defoliant, has a mechanism of action that could result in effects to plants (e.g., deciduous 

shrubs and trees), there is a potential for tribufos to directly affect terrestrial plants such as 

deciduous shrubs and trees.
64

Terrestrial Spray Drift Distance

It is useful to know how far from the edge of the field spray drift exposure alone could result in 

risk to terrestrial organisms. Similar to what was done for aquatic organisms, spray drift 

exposure to tribufos was determined using AgDRIFT version 2.1.1 (USEPA, 2013). The 

terrestrial spray drift distance was determined using Tier I Ground and Tier I Aerial terrestrial 

point deposition estimates using a fraction of applied based on the acute-dose based RQ for birds 

and the acute listed and acute non-listed LOC. The default ASAE Fine to Medium droplet size 

distribution was assumed for aerial applications. For ground applications, a high boom, ASAE 

very fine to fine drop size distribution, and the 90th data percentile was assumed. These are the 

default spray drift inputs recommended for modeling spray drift (USEPA, 2013).

The acute-dose based RQ value combined with the acute risk to listed and acute risk to non-listed

species LOCs, results in the greatest spray drift distance for terrestrial organisms. The terrestrial

spray drift distance (i.e., the distance from the edge of the field where spray drift exposure could 

result in RQs that exceed LOCs) for risk to listed and non-listed birds are summarized in Table 

33. For listed species, the terrestrial spray drift distances for aerial applications range from 135 

to 460 feet. For ground applications, the spray drift distance ranges from 170 to 577 feet from 

the edge of the field. For non-listed species, the spray drift distances range from 10 to 89 feet. 

These spray drift distances are lower for ground applications. The terrestrial spray drift distance 

estimated for birds is protective of other terrestrial organisms from effects due to exposure to 

spray drift. These spray drift distances are not protective for birds and mammals that consume 

aquatic organisms because they are exposed through ingestion of residues in aquatic organisms 

rather than dietary items close to the field. 

Table 33. Aquatic Drift Distance for the Use of Tribufos on Cotton.

Application rate

Distance from Edge of Field Where Spray Drift Could Result in RQs greater than 

LOCs for Terrestrial Organisms (feet)1,2

Listed Birds Non-listed Birds

Aerial Application (Tier I, Fine to Medium DSD)

1.875 460 89

1.125 233 46

0.54 135 10

Ground Application (Tier I, Very fine to fine DSD, 90th data percentile, high boom)

1.875 125 27

1.125 62 16

0.54 40 7

DSD=drop size distribution

1 Estimated based on acute-dose based RQ values for the small bird consuming short grass and acute listed LOC (0.1) and acute non-listed LOC 

(0.5).

4.4. Uncertainties and Data Gaps

4.4.1. Fate and Transport

T-Rex EECs are based on residues on surfaces of dietary items. Tribufos is taken up into leaves

and it is uncertain whether tribufos is taken up by roots and/or distributed in xylem and/or 

phloem. Exposure based on this exposure route was not assessed.
65

Applications were only assumed to occur once per year; however, labels allow for two 

applications a year and usage data indicate that more than one application may occur in some 

cases (e.g., for rank cotton), if the maximum application rate is not used. The maximum single 

application rate and maximum annual application rate are the same. The assumption of one 

application per year was based on a recommendation from BEAD, stating that cotton will only 

be grown once per year on a given production field because it is a warm season crop with a 

growing cycle of 130 to 160 days (USEPA, 2014) and tribufos is applied pre-harvest. The main 

uncertainty occurs when using the risk estimates based on average application rates, where more 

than one application could legally occur and do occur based on market survey data. 

Additionally, the “average application rate” used to refine EECs is derived by dividing the 

average pounds of active ingredient applied by the total area treated (USEPA, 2014). Thus, the 

average rate modeled may not reflect an actual application rate used in the field. 

4.4.2. Effects 

Toxicity data from chronic exposure to tribufos are not currently available for the species of 

freshwater fish that are most acutely sensitive to tribufos (i.e., bluegill sunfish). Therefore, the 

chronic effects endpoint used in this assessment for freshwater fish is estimated based on an 

acute-to-chronic ratio using toxicity data from sheepshead minnows. Because the ACR was 

determined by using endpoints from an estuarine/marine species, chronic risks to freshwater fish 

may be over- or under-estimated, depending on the relative sensitivity of those species. The 

Agency’s LOCs are intentionally set low, and conservative estimates are made in the risk 

assessment to account for these types of uncertainties. The potential for risk evaluated based on 

average application rates is not considered to be conservative because it may underestimate 

exposure under use conditions that are expected to occur.

A ten-day sediment toxicity study is not available for the freshwater insect C. dilutus although 

data are available for the freshwater benthic amphipod Hyalella. The midge and amphipod have 

different life histories, occupy different ecological niches, belong to different taxonomic groups 

(insects versus crustaceans) and generally interact with sediments in different ways. Chironomid 

larvae are more infaunal (submerged in sediment) whereas amphipods are more epibenthic 

(moving in and out of surficial sediment layer). The 10-day toxicity study for the C. dilutus is a 

standard data requirement and lack of this data is an uncertainty that could result in 

underestimating risk. Because RQ values based on the 10-day sediment tests exceed the chronic 

risk LOCs, there is uncertainty whether or not these data are reflective of the sensitivity of 

organisms which may spend a greater percentage of time within benthic sediments. Also, the 10-

day sediment tests do not examine the potential for reproductive effects, which were observed in 

toxicity studies where other taxa were exposed to tribufos. The current analysis for sediment 

may underestimate the potential for risk to benthic aquatic invertebrates.

Pollinator risk assessment guidance has recently been finalized and recommends a suite of 

laboratory-based studies extending beyond the acute contact study with adult bees. As these data 

are currently unavailable for tribufos, the potential risk to individual honey bees and brood 
66

success could not be fully assessed.

Data on a marine diatom (Skeletonema costatum), freshwater diatom (Navicula pelliculosa), and 

cyanobacterium (Anabaena flos-aquae) are not available to evaluate the potential risk to aquatic 

plants. As these data are currently unavailable for tribufos, the potential risk to non-vascular 

aquatic plants could not be fully assessed. 

5. Endocrine Disruptor Screening Program (EDSP)

Tribufos was not included in the first group of 67 chemicals issued an order to conduct Tier 1 

EDSP testing. For additional information on the EDSP, visit http://www.epa.gov/endo/.

6. Federally Threatened and Endangered (Listed) Species Concerns

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

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

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

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

agencies in response to the National Academy of Sciences’ (NAS) recommendations and reflect 

a common approach to risk assessment shared by the agencies as a way of addressing scientific 

differences between the EPA and the Services. The NAS report outlines recommendations on 

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

that EPA and the Services must conduct in connection with their obligations under the

Endangered Species Act (ESA) and FIFRA. 

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

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

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

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

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

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

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

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

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

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

Report,” dated November 1, 2013. 

Given that the agencies are continuing to develop and work toward implementation of the 

Interim Approaches to assess the potential risks of pesticides to listed species and their 

designated critical habitat, this preliminary risk assessment for tribufos does not contain a 

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

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

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

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

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

the application of tribufos. This screening level assessment will allow EPA to focus its future 
67

evaluations on the types of species where the potential for effects exists once the scientific 

methods being developed by the agencies have been fully vetted. This screening-level risk 

assessment for tribufos indicates potential risks of direct effects to listed dicot terrestrial plants, 

monocot terrestrial plants, freshwater and estuarine/marine vertebrates and invertebrates, 

mammals, birds, and terrestrial invertebrates for tribufos use on cotton. Listed species of semiaquatic plants may also be affected through indirect effects because of the potential for direct 

effects on listed and non-listed species upon which such species may rely. Potential direct 

effects on listed dicot terrestrial plants, monocot terrestrial plants, freshwater and 

estuarine/marine vertebrates and invertebrates, mammals, birds, and terrestrial invertebrates from 

the use of tribufos may be associated with modification of Primary Constituent Elements (PCEs) 

of designated critical habitats, where such designations have been made. Once the agencies have 

fully developed and implemented the scientific methods necessary to complete risk assessments 

for endangered and threatened (listed) species and their designated critical habitats, these 

methods will be applied to subsequent analyses for tribufos as part of completing this registration 

review. 

Table 34. Preliminary Conclusions for Potential Direct Effects to Federally Listed Taxa 

Associated with the Registered Uses of Tribufos.

Listed Plant Taxon Potential Direct Effects

Terrestrial and semi-aquatic plants –

monocots and dicots Yes, for sensitive plants that form abscission zones

Aquatic vascular plants No

Terrestrial Plants Yes, for plants that form abscission zones

Listed Animal Taxon Potential Direct Effects

Acute Notes Chronic Notes

Terrestrial invertebrates Yes

Effects 

observed in 

open 

literature

No data

No data on 

chronic 

exposure to 

honey bees

Mammals Yes RQ>LOC Yes RQ>LOC

Birds Yes RQ>LOC Yes Reptiles RQ>LOC

Amphibians

Freshwater fish Yes RQ>LOC Yes RQ>LOC

Freshwater invertebrates Yes RQ>LOC Yes RQ>LOC

Mollusks Yes RQ>LOC No data

Estuarine/marine fish No RQ<LOC Yes RQ>LOC

Estuarine/marine invertebrates Yes RQ>LOC Yes RQ>LOC

7. Conclusions

The risk hypothesis identified in the problem formulation stated, 
68

Given the currently labeled uses of tribufos and its environmental fate properties, there is 

a likelihood of exposure to non-target terrestrial and aquatic organisms. 

When used in accordance with the label, tribufos may result in potential adverse effects 

upon the survival, growth, and reproduction of non-target terrestrial and aquatic 

organisms. 

After reviewing the most recent environmental fate and effects data for tribufos, the screeninglevel assessment concludes that registered uses of the defoliant tribufos may result in risks to both 

listed and non-listed species of wildlife and aquatic vertebrates and invertebrates. 

With respect to exposure, tribufos is characterized as a semi-volatile, moderately soluble 

compound that has low mobility in soil. Based on submitted data, it is stable to the abiotic 

processes hydrolysis and photolysis. Tribufos is persistent in aerobic and anaerobic soil, and 

slightly persistent in anaerobic aquatic environments. Therefore, there is potential for tribufos

(which is applied to foliage) to accumulate in soil with repeated applications if it reaches the soil 

surface and is not dissipated through volatilization. Tribufos is expected to sorb to soil, and not 

to appreciably leach to groundwater or move to surface waters in solution phase via runoff. 

Surface water contamination may also occur due to spray drift and/or transport of residues 

sorbed to sediment in runoff (i.e, erosion). In either aquatic or terrestrial environments, tribufos 

may bioaccumulate in organisms exposed to it. 

The available data indicate that tribufos TGAI is highly toxic to freshwater and estuarine/marine 

fish and invertebrates on an acute exposure basis. Effects on growth and on post-hatch survival 

were observed in chronic studies with fish while reduced survival and growth were observed in 

chronic studies of aquatic invertebrates. Effects on frond count were observed in vascular 

aquatic plants and inhibited growth was observed in nonvascular aquatic plants. 

Tribufos TGAI ranges from practically non-toxic to moderately toxic to birds and is moderately 

toxic to mammals on an acute oral exposure basis and highly toxic to mammals on a subacutedietary exposure basis. However, tribufos is practically nontoxic to young adult honey bees on 

an acute contact exposure basis.

Terrestrial plants exposed to formulated product experienced reduced plant height and dry 

weight based on guideline studies and in a reduction in the number of old leaves left on the plant 

and an increased weight of fallen leaves based on a non-guideline study.

There is a potential for direct adverse effects to the following taxa based on LOC exceedances:

• Mammals (acute listed, acute non-listed, and chronic risk LOCs exceeded)

• Birds, terrestrial-phase amphibians, and reptiles (acute listed, acute non-listed, and 

chronic risk LOCs exceeded)

• Freshwater and estuarine/marine invertebrates (acute listed, acute non-listed, and chronic

risk LOCs exceeded)

• Freshwater vertebrates (aquatic-phase amphibians) (acute non-listed and chronic risk

LOCs exceeded)

• Estuarine/marine vertebrates (chronic risk LOCs exceeded)
69

While RQs do not exceed LOCs for risk to terrestrial plants, tribufos is a defoliant based on its 

intended use as well as a non-guideline study where defoliation was observed; therefore, there is 

a potential for direct adverse effects to terrestrial plants that form abscission zones such as 

deciduous trees and shrubs from exposure to tribufos from its registered use on cotton. 

Limited available toxicity data for honey bees suggest that acute risk LOCs are not exceeded on 

an acute contact basis; however, data are not available to evaluate potential for effects to 

individual bees and their brood exposed via the diet.

RQs for aquatic plants do not exceed LOCs for risk to listed or non-listed plants. However, a full 

dataset is not available to assess the risk to non-vascular aquatic plants, and based on aquatic 

plant data for other organophosphates, it is possible that these RQs could exceed LOCs if 

additional data were available.

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2015).

Mullin, C. A., Frazier, M., Frazier, J. L., Ashcraft, S., Simonds, R., VanEngelsdorp, D., et al. 

2010. High levels of miticides and agrochemicals in north american apiaries: 

implications for honey bee health. PLoS One, 5(3), e9754. 

NIH. 2015. Hazardous Substance Data Bank (HSDB). U.S. National Library of Medicine. 

Available at http://toxnet.nlm.nih.gov/newtoxnet/hsdb.htm (Accessed March 27, 2015).

Scientific Review Panel. 1999. Findings of the Scientific Review Panel on Evaluation of DEF as 

a Toxic Air Contaminant. April 13, 1999. California Department of Pesticide Regulation. 

Available at http://www.cdpr.ca.gov/docs/emon/pubs/def/srp_find.pdf (Accessed April 

14, 2015).

Urban, D. J., & Cook, N. J. 1986. Ecological Risk Assessment. EPA 540/9–85-001. Office of 

Pesticide Programs. U.S. Environmental Protection Agency. 

USEPA. 1998. Guidelines for Ecological Risk Assessment. EPA/630/R-95/002F. Risk 

Assessment Forum. Office of Research and Development. U.S. Environmental 

Protection Agency. Available at http://www.epa.gov/raf/publications/guidelinesecological-risk-assessment.htm (Accessed November 7, 2012).

USEPA. 2000. Interim Registration Eligibility Decision (IRED): Tribufos. EPA 738-R-00-022. 

September 2000. Office of Prevention, Pesticides, and Toxic Substances. U.S. 

Environmental Protection Agency. Available at 

http://www.epa.gov/opp00001/chem_search/reg_actions/reregistration/ired_PC074801_1-Sep-00.pdf (Accessed March 25, 2015).

USEPA. 2004a. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs, United States Environmental Protection Agency (USEPA) (pp. 100). 

Washington, D.C.: Government Printing Office. Available at 

http://www.epa.gov/espp/consultation/ecorisk-overview.pdf (Accessed June 19, 2009).

USEPA. 2004b. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. Environmental Fate and Effects Division. Office of Pesticide Programs. U.S. 

Environmental Protection Agency. Available at 

http://www.epa.gov/espp/consultation/ecorisk-overview.pdf (Accessed June 19, 2009).

USEPA. 2006. Reregistration Eligibility Decision (RED) Tribufos. July 2006. Office of 
71

Prevention, Pesticides, and Toxic Substances. U.S. Environmental Protection Agency. 

Available at http://www.epa.gov/pesticides/reregistration/REDs/tribufos_red.pdf

(Accessed March 27, 2015).

USEPA. 2008.DP Barcode 357536. Registration Review: Preliminary Problem Formulation for 

Ecological Risk, Endangered Species, and Drinking Water Exposure Assessments for 

Tribufos. DP Barcode 357536. December 10, 2008. Environmental Fate and Effects 

Division. Office of Pesticide Programs. U.S. Environmental Protetion Agency. 

USEPA. 2008. Risks of Tribufos Use to Federally Threatened California Red-legged Frog. 

Memorandum From to Environmental Fate and Effects Division. Office of Pesticide 

Programs. U.S. Environmental Protection Agency. Available at 

http://www.epa.gov/espp/litstatus/effects/redleg-frog/tribufos/determination.pdf

(Accessed March 27, 2015).

USEPA. 2009a. ECOTOXicology Database, United States Environmental Protection Agency 

(USEPA). Available at http://cfpub.epa.gov/ecotox/ (Accessed March 27, 2015).

USEPA. 2009b. Tribufos. Revised Human Health Assessment Scoping Document in Support of 

Registration Review. D. B. 361529. February 2009. Health Effects Division. Office of 

Pesticide Programs. U.S. Environmental Protection Agency. 

USEPA. 2011. Evaluation for Ecological Toxicity Data in Open Literature. May 9, 2011. 

Environmental Fate and Effects Division. Office of Pesticide Programs. Office of 

Chemical Safety and Pollution Prevention. U.S. Environmental Protection Agency. 

Available at 

http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/endangered_s

pecies_reregistration_workgroup/esa_evaluation_open_literature.htm (Accessed April 3, 

2015).

USEPA. 2012a. Ecological Incident Information System. Environmental Fate and Effects 

Division. United States Environmental Protection Agency. 

USEPA. 2012b. White Paper in Support of the Proposed Risk Assessment Process for Bees. 

September 11-14, 2012. September 11, 2012. U.S. Environmental Protection Agency. 

Pest Management Regulatory Agency. California Department of Pesticide Regulation. 

Available at http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-

0543-0004 (Accessed June 22, 2014).

USEPA. 2013. Guidance on Modeling Offsite Deposition of Pesticides Via Spray Drift for 

Ecological and Drinking Water Assessment. Environmental Fate and Effects Division. 

Office of Pesticide Programs. Office of Chemical Safety and Pollution Prevention. U.S. 

Environmental Protection Agency. Available at 

http://www.regulations.gov/#!docketDetail;D=EPA-HQ-OPP-2013-0676 (Accessed 

April 11, 2014).

USEPA. 2014. Tribufos (074801) Use and Usage Data for Draft Risk Assessment. November 6, 

2014. Biological and Economic Analysis Division. Office of Chemical Safety and 

Pollution Prevention. U.S. Environmental Protection Agency. 

USEPA, Health Canada PMRA, & California Department of Pesticide Regulation. 2014. 

Guidance for Assessing Pesticide Risks to Bees. June 23, 2014. U.S. Environmental 

Protection Agency. Health Canada Pest Management Regulatory Agency. California 

Department of Pesticide Regulation. Available at http://www2.epa.gov/pollinatorprotection/pollinator-risk-assessment-guidance (Accessed April 10, 2015).
72

8.1 Submitted Product Chemistry and Environmental Fate Studies

MRID 41618803. Talbott, T. 1990. Product Chemistry of DEF Technical: Lab Project Number: 90606: 94500: 

94682. Unpublished study prepared by Mobay Corp. 29 p.

MRID 41618811. Cohle, P. 1990. Uptake, Depuration and Bioconcentration of (Carbon 14)-DEF by Bluegill 

(Lepomis macrochirus): Lab Project Number: 38466: 100236. Unpublished study prepared by Analytical 

Bio- Chemistry Laboratories, Inc. 48 p.

MRID 41618814. Schocken, M. and I. Philippson. 1987. Stability of DEF in Sterile Aqueous Buffer Solutions: 

Lab Project Number: 94918. Unpublished study prepared by Mobay Corp. 23 p.

MRID 41618816. Jackson, S., A. Kesterson, L. Lawrence. 1988. Soil Surface Photolysis of [14C]DEF in Natural 

Sunlight: Lab Project Number: 1153: 206: 95673. Unpublished study prepared by Pharmacology and 

Toxicology Research Laboratory. 67 p.

MRID 41618817. Daly, D. 1987. Soil Adsorption/Desorption with [14C]DEF: Lab Project Number: 36356: 95600. 

Unpublished study prepared by Analytical Bio-Chemistry Laboratories, Inc. 50 p.

MRID 41719401. Carpenter, M. 1990. Determination of the Aqueous Photolysis Rate of DEF: Lab Project 

Number: 38876. Unpublished study prepared by Analytical Bio-Chemistry Laboratories, Inc. 41 p.

MRID 42007204. Stevenson, T., W. Leimkuehler, A. Mathew. 1991. The Metabolism of Tribufos in Soil Under 

Aerobic Conditions: Lab Project Number : DE042101: 100338. Unpublished study prepared by Mobay 

Corp., R & D Dept. 33 p.

MRID 42007205. Stevenson, T., W. Leimkuehler. 1990. The Metabolism of Tribufos in Soil Under Anaerobic 

Conditions: Lab Project Number: 100333. Unpublished study prepared by Mobay Corp., R & D Dept. 31 

p.

MRID 43080401. Leimkuehler, W., K. Moor. 1993. Identification and Characterization of Radioactive Residues 

of [14C] Tribufos in Bluegill Sunfish (Lepomis macrochirus): Lab Project Number: DE040301. 

Unpublished study prepared by Miles Inc. 84 p.

MRID 43325504. Schmidt, J. 1994. Anaerobic Aquatic Metabolism of 14C-Tribufos: Lab Project Number: 

DE042402: 106767. Unpublished study prepared by ABC Lab., Inc. 61 p.

MRID 45863701. Wood, S. 2002. Terrestrial Field Dissipation of Tribufos on Mississippi Soil, 2000: Lab Project 

Number: DE022102: 110590. Unpublished study prepared by Stoneville R & D, Inc., A & L Great Lakes 

Laboratories, and Bayer Corporation. 84 p.

8.2. Submitted Ecotoxicity Studies (Retrieved from OPPIN April 2015)

71-1 Avian Single Dose Oral Toxicity

49258 Lamb, D.W.; Jones, R.E. (1972) Acute Oral Toxicity of DEF Technical to Bobwhite Quail and Mallard Ducks: 

Report No. 35016. (Unpublished study received Mar 22, 1976 under 3125-71; submitted by Mobay Chemical 

Corp., Kansas City, Mo.; CDL:224114-P)
73

120771 Lamb, D.; Jones, R. (1972) Acute Oral Toxicity of DEF Technical to Bobwhite Quail and Mallard Ducks: 

Report No. 35016. (Unpublished study received May 7, 1973 under 3125-71; submitted by Mobay Chemical 

Corp., Kansas City, MO; CDL:007020-O)

92034001 Toll, P. (1990) Mobay Corporation Phase 3 Summary of MRID 00049258 and Related MRIDs 00120771. 

Acute Oral Toxicity of DEF Technical to Bobwhite Quail and Mallard Ducks: Reference No. 72-35; Report 

No. 35016-1. Prepared by Chemagro Div. of Baychem. 12 p.

71-2 Avian Dietary Toxicity

41618804 Grau, R. (1990) DEF Technical Grade: 5-day Dietary LC50 to Bobwhite Quail: Lab Project Number: BV-007: 

E2950351-5: 100223. Unpublished study prepared by Bayer Ag. 36 p.

41618805 Grau, R. (1990) DEF Technical Grade: 5-day Dietary LC50 to Mallard Duck: Lab Project Number: VE-002: 

E2970426-0: 100224. Unpublished study prepared by Bayer Ag. 43 p.

71-4 Avian Reproduction

40757101 Beavers, J.; Marselas, G.; Jaber, M. (1988) DEF: A One-generation Reproduction Study with the Bobwhite 

(Colinus virginianus): Wildlife International Ltd. Project No. 149-127. Unpublished study prepared by Wildlife 

International Ltd. 101 p.

92034002 Toll, P. (1990) Mobay Corporation Phase 3 Summary of MRID 40757101. A One-generation Reproduction 

Study with the Bobwhite (Colinus virginianus): WIL Project No.149-127. Prepared by Wildlife International 

Ltd.. 16 p.

71-5 Simulated or Actual Field Testing

49259 Lamb, D.W.; Jones, R.E. (1972) Toxicity of DEF 6 Lbs/Gal Spray Concentrate to Bobwhite Quail and New 

Zealand Rabbits Applied under ULV Simulated Field Conditions: Report No. 35030. (Unpublished study 

received Mar 22, 1976 under 3125-71; submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:224114-

Q)

72-1 Acute Toxicity to Freshwater Fish

63387 Howell, J.H. (1963) Letter sent to Lewis F. Stevens dated Jan 4, 1963 Chemicals related to Bayer 73|: Report 

No. 11144. (U.S. Fish and Wildlife Service, Bureau of Commercial Fisheries, Hammond Bay Biological 

Station; unpublished study; CDL:130685-H)

74016 Borash, A.J.; Moorefield, H.H. (1963) The Toxicity of Sodium cis- 3-Chloroacrylate (UC 20299) to Goldfish: 

Project No. 141E41. (Unpublished study received Dec 10, 1963 under unknown admin. no.; submitted by 

Union Carbide Corp., Arlington, Va.; CDL: 129278-B)

83208 Borash, A.J.; Moorefield, H.H. (1963) The Toxicity of Sodium cis-3- Chloroacrylate (UC 20299) to Goldfish: 

Project No. 141E41. (Unpublished study received Dec 11, 1963 under PP0410; submitted by Union Carbide 

Corp., New York, N.Y.; CDL:090442-F)

120762 Howell, J. (19??) Toxicity of Chemicals: | 11144. (Unpublished study received May 7, 1973 under 3125-71; 

prepared by U.S. Fish and Wildlife Service, Bureau of Commercial Fisheries, Hammond Bay Biological 

Station, submitted by Mobay Chemical Corp., Kansas City, MO; CDL:007020-B)

120764 Walker, C. (1963) Chemical Toxicity to Fish: DEF 12947. (Unpublished study received May 7, 1973 under 

3125-71; prepared by U.S. Fish and Wildlife Service, Fish Control Laboratory, submitted by Mobay Chemical 

Corp., Kansas City, MO; CDL: 007020-D)
74

120767 Marking, L. (1967) Concentrations Producing All-or-none Effects on a Preliminary Basis at 24 Hours and 12C: 

?Various Chemagro Compounds 17363. (Unpublished study received May 7, 1973 under 3125-71; prepared by 

U.S. Fish and Wildlife Service, Fish Control Laboratory, submitted by Mobay Chemical Corp., Kansas City, 

MO; CDL:007020-G)

120770 Lamb, D.; Roney, D. (1972) Acute Toxicity of DEF 6 Lbs/Gal Spray Concentrate to Fish: Report No. 33400. 

(Unpublished study received May 7, 1973 under 3125-71; submitted by Mobay Chemical Corp., Kansas City, 

MO; CDL:007020-N)

41618806 Grau, R. (1990) DEF Technical Grade: Acute Toxicity to Bluegill Sunfish in a Flow-through Test: Lab Project 

Number: FB-003: E2860137: 100222. Unpublished study prepared by Bayer Ag. 36 p.

41618807 Grau, R. (1990) DEF 6: Acute Toxicity to Bluegill Sunfish in a Flow-through Test: Lab Project Number: FB006: E02860355-9: 100219. Unpublished study prepared by Bayer Ag. 40 p.

41618808 Grau, R. (1990) DEF Technical Grade: Acute Toxicity to Rainbow Trout in a Flow-through Test: Lab Project 

Number: FF-286: E2810136-1: 100221. Unpublished study prepared by Bayer Ag. 36 p.

41618809 Grau, R. (1990) DEF 6: Acute Toxicity to Rainbow Trout in a Flow- through Test: Lab Project Number: FF228: E2510429: 100220. Un- published study prepared by Bayer Ag. 39 p.

45863706 Kern, M.; Lam, C. (2002) Acute Toxicity of Tribufos Technical to the Fathead Minnow (Pimpephales 

promelas) Under Flow-Through Conditions: Lab Project Number: DE831202: 200144. Unpublished study 

prepared by Bayer Corporation. 34 p.

72-2 Acute Toxicity to Freshwater Invertebrates

111775 Lowe, J. (1964) Effects of Pesticides: Telodrin and Other Com- pounds|. (U.S. Fish and Wildlife Service, 

Bureau of Commercial Fisheries, Biological Laboratory; unpublished study; CDL: 123712-A)

41689901 Heimbach, F. (1989) Acute Toxicity of DEF (Technical) to Waterfleas (Daphnia magna): Lab Project Number: 

99630. Unpublished study prepared by Bayer AG. 32 p.

41689902 Heimbach, F. (1990) Acute Toxicity of DEF 6 to Waterfleas (Daphnia magna): Lab Project Number: 

HBF/DM96: E 320 0403-2: 100208. Unpublished study prepared by Bayer AG. 33 p.

72-3 Acute Toxicity to Estuarine/Marine Organisms

66341 U.S. Environmental Protection Agency, Environmental Research Laboratory (1981) Acephate, Aldicarb, 

Carbophenothion, DEF, EPN, Ethoprop, Methyl Parathion, and Phorate: Their Acute and Chronic Toxicity, 

Bioconcentration Potential, and Persistence as Related to Marine Environments: EPA-600/4-81-023. 

(Unpublished study)

103754 Butler, P. (1966) The Problem of Pesticides in Estuaries. Pages 110-115 In Symposium on Estuarine Fishes|; 

1964, Atlantic City, NJ. Washington, DC. (Also In unpublished submission received Oct 6, 1969 under 

unknown admin. no.; submitted by Shell Chemical Co., Washington, DC; CDL:120338-K)

41896301 Ward, G. (1991) DEF: Acute Toxicity to The Mysid, Mysidopsis bahia, Under Flow-Through Test Conditions: 

Lab Project Number: J900802- 4B: 101245. Unpublished study prepared by Mobay Corp. 45 p.

41896302 Gagliano, G. (1991) Acute Toxicity of DEF Technical to the Sheepshead Minnow (Cyprinodon variegatus) 

Under Flow-Through Conditions: Lab Project Number: DE831201: 101240. Unpublished study prepared by 

Mobay Corp. 31 p.
75

42083201 Wheat, J.; Ward, G. (1991) DEF Technical: Acute Effect on New Shell Growth of, The Eastern Oyster, 

Crassostrea virginica: Lab Project Number: 101959: J9008024C. Unpublished study prepared by Toxikon 

Environmental Sciences. 53 p.

72-4 Fish Early Life Stage/Aquatic Invertebrate Life Cycle Study

48320 Butler, P.A. (1963) Commercial Fishery Investigations. Pages 5-28, In Pesticide-Wildlife Studies, 1963: A 

Review of Fish and Wildlife Service Investigations during the Calendar Year. By U.S. Fish and Wildlife 

Service. Washington, D.C.: U.S. Fish and Wildlife Service. (Circular 199; submitter report no. 16229h, 

published study; CDL:226511-H)

49257 Lamb, D.W.; Roney, D.J. (1972) Acute Toxicity of DEF 6 Lbs/Gal Spray Concentrate to Fish: Report No. 

33400. (Unpublished study received Mar 22, 1976 under 3125-71; submitted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:224114-O)

51525 Howell, J.H. (1963) Letter sent to Lewis F. Stevens dated Jan 4, 1963 Toxicity of chemicals to fish and larvae|: 

Report No. 11144. (U.S. Fish and Wildlife Service, Bureau of Commercial Fisheries, Hammond Bay Biological 

Station, unpublished study; CDL:224114-B)

51526 Walker, C.R. (1963) Letter sent to Lewis F. Stevens dated Apr 24, 1963 Screening results on various 

compounds|: Report No. 11401. U.S. Fish and Wildlife Service, unpublished study; CDL:224114-C)

66341 U.S. Environmental Protection Agency, Environmental Research Laboratory (1981) Acephate, Aldicarb, 

Carbophenothion, DEF, EPN, Ethoprop, Methyl Parathion, and Phorate: Their Acute and Chronic Toxicity, 

Bioconcentration Potential, and Persistence as Related to Marine Environments: EPA-600/4-81-023. 

(Unpublished study)

111775 Lowe, J. (1964) Effects of Pesticides: Telodrin and Other Compounds|. (U.S. Fish and Wildlife Service, Bureau 

of Commercial Fisheries, Biological Laboratory; unpublished study; CDL: 123712-A)

41618810 Heimbach, F. (1990) Influence of DEF Technical on the Reproduction Rate of Water Fleas: Lab Project 

Number: HBF/RDM29: E3210393-1: 100235. Unpublished study prepared by Bayer Ag. 56 p.

43978201 Bowers, L. (1996) Chronic Toxicity of (carbon 14)-DEF to the Waterflea (Daphnia magna) Under Static 

Renewal Conditions: Lab Project Number: DE840701: 107327. Unpublished study prepared by Bayer Corp. 64 

p.

45863702 Blankinship, A.; Kendall, T.; Krueger, H. (2002) Tribufos Technical: A Flow-Through Life-Cycle Toxicity 

Test with the Saltwater Mysid (Mysidopsis bahia): Lab Project Number: 149A-133A: DE843101: 200363. 

Unpublished study prepared by Wildlife International, Ltd. 77 p. 

45863703 Kern, M.; Lam, C. (2002) Early Life Stage Toxicity of Tribufos to the Fathead Minnow (Pimephales 

promelas): Lab Project Number: DE841201: 200308. Unpublished study prepared by Bayer Corporation. 79 p.

45863707 Palmer, S.; Kendall, T.; Krueger, H. (2002) Tribufos Technical: An Early Life-Stage Toxicity Test with the 

Sheepshead Minnow (Cyprinodon variegatus): Lab Project Number: 149A-134: DE842801: 200336. 

Unpublished study prepared by Wildlife International, Ltd. 82 p. 
76

47958201 Matlock, D.; Lam, C. (2010) Early Life Stage Toxicity of Tribufos Technical to the Fathead Minnow 

(Pimephales promelas) Under Flow-Through Conditions. Project Number: EBTRL001, M/360947/01/1. 

Unpublished study prepared by Bayer CropScience. 96 p.

72-6 Aquatic org. accumulation

41618818 Cohle, P. (1990) Uptake, Depuration and Bioconcentration of (Carbon 14)-DEF by Bluegill (Lepomis 

Macrochirus): Lab Project Number: 38466: 100234. Unpublished study prepared by Mobay Corp. 45 p.

81-1 Acute oral toxicity in rats

45983 Crawford, C.R. (1971) Treatment Studies for DEF Intoxication: Report No. 30093. (Unpublished study 

received Jul 19, 1971 under unknown admin. no.; submitted by Chemagro Corp., Kansas City, Mo.; 

CDL:103419-A)

49241 Crawford, C.R.; Anderson, R.H. (1972) The Acute Oral Toxicity of DEF (Special) 6 lbs/gal Spray Concentrate 

to Rats: Report No. 33127. (Unpublished study received Mar 22, 1976 under 3125- 71; submitted by Mobay 

Chemical Corp., Kansas City, Mo.; CDL: 224109-F)

62031 Dow Chemical Company (1965) Results of Acute Oral Toxicity Testing of Ronnel Insecticide in Paired 

Combination with Other Cholinesterase Inhibitors When Administered to Female Rats. (Unpublished study 

received Apr 23, 1969 under 9F0800; CDL:091378-H)

74871 Schoenig, G. (1966) Report to Chevron Chemical Company: Dibrom Potentiation Studies with Six Organic 

Phosphates: IBT No. A4629. (Unpublished study received Sep 20, 1966 under 7F0532; prepared by Industrial 

Bio-Test Laboratories, Inc., submitted by Chevron Chemical Co., Richmond, Calif.; CDL:090645-G)

75911 Shoenig, G. (1966) Report to Chevron Chemical Company: Desethyl- phosphamidon--Potentiation Studies 

with Six Organic Phosphates: IBT No. A4734. (Unpublished study received Nov 1, 1966 under 7F0546; 

prepared by Industrial Bio-Test Laboratories, Inc., submitted by Chevron Chemical Co., Richmond, Calif.; 

CDL: 090671-G)

79900 Connor, N.D. (1965) Letter sent to H.S. Bryan dated Mar 9, 1965: DEF and Banol acute oral potentiation study 

in rats. (Unpublished study received Feb 3, 1966 under 9156-1; submitted by Upjohn Co., Kalamazoo, Mich.; 

CDL:100650-W)

87417 Murphy, S.D.; DuBois, K.P. (1959) Toxicity and anticholinesterase activity of tributyl phosphorotrithioate 

(DEF). American Medical Association Archives of Industrial Health 20(Aug):161-166. (Submitter 4242; also

In unpublished submission received Jan 18, 1969 under 9F0737; submitted by Mobay Chemical Corp., Kansas 

City, Mo.; CDL:091270-J)

87420 DuBois, K.P. (1964) The Acute Toxicity of Two Samples of S,S,S- Tributyl Phosphorothioate to Adult, Female 

Rats: 14565. (Unpublished study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, Dept. of 

Pharmacology, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-M)

99099 Schoenig, G. (1966) Report to Chevron Chemical Company: Phosphamidon--Potentiation Studies with Six 

Organic Phosphates: IBT No. A4630. (Unpublished study received Nov 1, 1966 under 7F0546; prepared by 

Industrial Bio-Test Laboratories, Inc., submitted by Chevron Chemical Co., Richmond, Calif.; CDL: 090671-E)
77

118756 Connor, N. (1965) Letter sent to H. Bryan dated Mar 9, 1965: DEF and Banol: Acute oral potentiation study in 

rats. (Unpublished study received Feb 3, 1966 under 9156-1; submitted by Upjohn Co., Kalamazoo, MI; 

CDL:005694-W)

142425 Crawford, C. (1971) Treatment Studies for DEF Intoxication: Report No. 30093. Unpublished study prepared 

by Chemagro Corp. 7 p.

41954903 Sheets, L. (1991) Acute Oral Toxicity Study with Technical Grade Tribufos (DEF) in Rats: Lab Project 

Number: 90-012-ES: 100697. Unpublished study prepared by Mobay Corp. 20 p.

42408101 Astroff, A. ; Phillips, S. (1992) Experimental Acute Oral Toxicity Study with HBM Sulfone (a Metabolite of 

Tribufos, DEF) in Female Rats: Lab Project Number: 92-912-OD. Unpublished study prepared by Miles, Inc. 

18 p.

42682401 Sheets, L.; Phillips, S. (1992) Acute Oral Toxicity Study with DEF 6 in Rats: Lab Project Number: 92-012-NO: 

102678. Unpublished study prepared by Miles Inc. 22 p.

45863708 Mach, J. (2002) Tribufos: Acute Dietary Toxicity Test with the White-Footed Mouse (Peromyscus leucopus): 

Final Report: Lab Project Number: 02005: DE732601: 110770. Unpublished study prepared by Genesis 

Laboratories, Inc. 91 p. 

81-2 Acute dermal toxicity in rabbits or rats

45978 DuBois, K.P. (1957) The Dermal Toxicity of S,S,S-Tributyl phosphorotrithioate to Rats (DEF; B-1776): Report 

No. 1990. (Unpublished study received Feb 8, 1960 under unknown admin. no.; prepared by Univ. of Chicago, 

Dept. of Pharmacology, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:103421-D)

49243 Crawford, C.R.; Anderson, R.H. (1972) The Acute Dermal Toxicity of DEF ULV to Rabbits: Report No. 

34480. (Unpublished study received Mar 22, 1976 under 3125-71; submitted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:224109-H)

87420 DuBois, K.P. (1964) The Acute Toxicity of Two Samples of S,S,S- Tributyl Phosphorothioate to Adult, Female 

Rats: 14565. (Unpublished study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, Dept. of 

Pharmacology, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-M)

87424 Dubois, K.P.; DiGiacomo, R.; Root, M.; et al. (1966) The Subacute Dermal Toxicity of Tributyl 

Phosphorotrithioate (DEF) to Rats: 19563. (Unpublished study received Jan 18, 1969 under 9F0737; prepared 

by Univ. of Chicago, Toxicity Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 

091270-Q)

41954902 Sheets, L.; Phillips, S. (1991) Acute Dermal Toxicity Study with Technical Grade Tribufos (DEF) in Rabbits: 

Lab Project Number: 90-025-FE: 100698. Unpublished study prepared by Mobay Corp. 20 p.

42682402 Astroff, A.; Phillips, S. (1992) Acute Dermal Toxicity Study with DEF 6 in Rats: Lab Project Number: 92-022-

NZ: 102679. Unpublished study prepared by Miles Inc. 22 p.

81-3 Acute inhalation toxicity in rats

49238 DuBois, K.P.; Meskauskas, J. (1968) The Acute Inhalation Toxicity of a DEF Formulation to Mice: Report No. 

23805. (Unpublished study received Mar 22, 1976 under 3125-71; prepared by Univ. of Chicago, Toxicity 

Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:224109-C)
78

49242 Kimmerle, G.; Lorke, (1972) DEF ULV 6 Lb./Gal. S.C.: Acute Inhalation Toxicity: Report No. 3487; Report 

No. 34161. (Unpublished study received Mar 22, 1976 under 3125-71; prepared by Farbenfabriken Bayer, AG, 

submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:224109-G)

87858 Mobay Chemical Corporation (1981) Pulmonary Exposure of Mice to DEF: 54054. (Unpublished study 

received Sep 10, 1981 under 3125-282; CDL:246511-B)

151227 Thyssen, J. (1978) DEF: Studies on Acute Inhalation Toxicity (Rats): 80-T-014: Report No. 7636. Unpublished 

Mobay Report No. 66481 prepared by Bayer AG. 6 p.

41782301 Warren, D. (1990) Acute Four-Hour Inhalation Toxicity Study with Technical Grade DEF in Rats: Lab Project 

Number: 90-042-HQ. Unpublished study prepared by Mobay Corp. 33 p.

42682403 Warren, D.; Tran, A. (1992) Acute Four-Hour Inhalation Toxicity Study with DEF 6 in Rats: Lab Project 

Number: 91-042-MD: 102669. Unpublished study prepared by Miles Inc. 35 p.

81-4 Primary eye irritation in rabbits

49239 Crawford, C.R. (1971) The Eye and Dermal Irritating Properties of DEF 6 Lbs/Gal Spray Concentrate to 

Rabbits: Report No. 30814. (Unpublished study received Mar 22, 1976 under 3125- 71; submitted by Mobay 

Chemical Corp., Kansas City, Mo.; CDL: 224109-D)

49244 Crawford, C.R.; Anderson, R.H. (1972) The Skin and Eye Irritation Properties of DEF Technical to Rabbits: 

Report No. 35006. (Unpublished study received Mar 22, 1976 under 3125-71; submit- ted by Mobay Chemical 

Corp., Kansas City, Mo.; CDL:224109-I)

42441601 Sheets, L.; Phillips, S. (1992) Primary Eye Irritation Study with Technical Grade Tribufos in Rabbits: Lab 

Project Number: 91-335-MN. Unpublished study prepared by Miles Inc. 17 p.

92034003 Sheets, L. (1990) Mobay Corporation Phase 3 Summary of MRID 00049244. The Skin and Eye: Irritation 

Properties of DEF Technical to Rabbits: Project: 72-38 (Mobay Report 35006). Prepared by Mobay Corp.. 7 p.

81-5 Primary dermal irritation

49239 Crawford, C.R. (1971) The Eye and Dermal Irritating Properties of DEF 6 Lbs/Gal Spray Concentrate to 

Rabbits: Report No. 30814. (Unpublished study received Mar 22, 1976 under 3125- 71; submitted by Mobay 

Chemical Corp., Kansas City, Mo.; CDL: 224109-D)

49244 Crawford, C.R.; Anderson, R.H. (1972) The Skin and Eye Irritation Properties of DEF Technical to Rabbits: 

Report No. 35006. (Unpublished study received Mar 22, 1976 under 3125-71; submit- ted by Mobay Chemical 

Corp., Kansas City, Mo.; CDL:224109-I)

41896303 Sheets, L.; Fuss, M. (1991) Primary Dermal Irritation Study with Technical Grade Tribufos (DEF) in Rabbits: 

Lab Project Number: 90-325-FS. Unpublished study prepared by Mobay Corp. 16 p.

42289301 Sheets, L.; Phillips, S. (1992) Primary Dermal Irritation Study with DEF 6 in Rabbits: Lab Project Number: 91-

325-MG. Unpublished study prepared by Miles Inc. 28 p.

42891202 Zorbas, M. (1993) Primary Dermal Irritation Study with Technical Grade Tribufos (DEF) in Rabbits: Lab 

Project Number: 90-325-FS; 100694-1. Unpublished study prepared by Miles, Inc. 8 p.

81-6 Dermal sensitization

41618812 Sheets, L. (1990) Dermal Sensitization Study with Technical Grade Tribufos (DEF) in Guinea Pigs: Lab 

Project Number: 90-324-GK: 100268. Unpublished study prepared by Mobay Corp. 24 p.

42891201 Zorbas, M. (1993) Dermal Sensitization Study with Technical Grade Tribufos (DEF) in Guinea Pigs: Lab 

Project Number: 90-324-GK: 100268-1. Unpublished study prepared by Miles, Inc. 9 p.
79

43936601 Warren, D.; Meier, M. (1996) Dermal Sensitization Study with DEF 6 in Guinea Pigs: Lab Project Number: 

107182: 95-324-EJ. Unpublished study prepared by Bayer Corp. 25 p.

81-7 Neurotoxicity study in hens

144098 Thyssen, J.; Schilde, B.; Kimmerle (1976) DEF: Neurotoxicity studies on Dermally Treated Hens: Report No. 

6255. Unpublished study prepared by Bayer AG. 17 p.

144940 Thyssen, J.; Mohr, U. (1976) DEF: Neurotoxicity Studies on Hens: Report No. 5948. Unpublished Mobay 

report 47932 prepared by Bayer Ag and Medizinische Hochschule Hannover, Abteillung fuer Experimentelle 

Pathologie, W. Ger. 21 p.

146338 Thyssen, J.; Wechsler, W.; Schilde, B. (1977) DEF: Neurotoxicity Studies on Hens: Thirty-day Feeding 

Experiments: Report No. 6941. Unpublished study prepared by Bayer AG. 39 p.

151228 Abou-Donia, M. (1978) Study of the Low Level Effects of Organophosphorus Esters with Specific References 

to the Delayed Neurotoxic Syndrome: Quarterly Report 4: Effect of a Single Oral Dose and Subchronic (90-

day) Oral Administration of Technical DEF to Adult Hens. Unpublished study prepared by Duke University 

Department of Pharmacology. 35 p.

151229 Abou-Donia, M. (1979) Late acute effect of S,S,S-tributyl phosphorotrithioate (DEF) in hens. Toxicology 

Letters (4): 231-236.

151230 Wilson, B.; Cisson, C.; Randall, W.; et al. (1980) Organophosphate risk assessment: Field testing of DEF with 

the scaleless chicken. Bull. Environm. Contam. Toxicol. (24):921-928.

151231 Abou-Donia, M.; Graham, D.; Timmons, P.; et al. (1979) Delayed neurotoxic and late acute effects of S,S,Stributyl phosphorotrithioate on the hen: Effect of route of administration. Neurotoxicology 1:425-447.

151232 Abou-Donia, M.; Graham, D.; Abdo, K.; et al. (1979) Delayed neurotoxic, late acute and cholinergic effects of 

S,S,S-tributyl phosphorotrithoate (DEF): Subchronic (90 days) administration in hens. Toxicology 14:229-243.

158404 Abdo, K.; Timmons, P.; Abou-Donia, M. (1983) Effects of a dermal dose of S,S,S,-Tri-n-butyl 

phosphorotrithioate on brain acetyl- cholinesterase, acid phosphatase, and 2,3'-cyclic nucleotide-3' 

phosphohydrolase and plasma butyrlcholinesterase in hens: Report No. 88977. P. 499-502 in Elsevier Science 

Publishers B.V.- Development in the Science and Practice of Toxicology.

164992 Thyssen, J.; Schilde, B. (1976) DEF: Neurotoxicity Studies on Hens (30-day Feeding Study): Report No. 6440. 

Unpublished Mobay Report No. 90493 prepared by Bayer AG. 44 p.

40869201 Baron, R.; Johnson, H. (1964) Neurological disruption produced in hens by two organophosphate esters. Brit. J. 

Pharmacol. 23:295- 304.

40879102 Johnson, M. (1970) Organophosphorus and other inhibitors of brain 'neurotoxic esterase' and the development 

of delayed neurotoxicity in hens. Biochem. J. 120:523-531.

81-8 Acute neurotoxicity screen study in rats

44071501 Wu, S.; Casida, J. (1996) Subacute neurotoxicity induced in mice by potent organophosphorus neuropathy 

target esterase inhibitors. Tox. and Appl. Pharmacol. 139:195-202.

45194401 Sheets, L.; Gilmore, R. (2000) An Acute Oral Neurotoxicity Screening Study with Technical Grade Tribufos 

(DEF) in Wistar Rats: Lab Project Number: 98-412-WF: 109802. Unpublished study prepared by Bayer 

Corporation. 476 p. {OPPTS 870.6200}
80

45464601 Sheets, L.; Gilmore, R. (1999) Verification of Personnel Training to Perform a Functional Observational 

Battery with Rats: (Carbaryl): Lab Project Number: 97-962-LG. Unpublished study prepared by Bayer 

Corporation. 94 p.

82-1 Subchronic Oral Toxicity: 90-Day Study

87421 Doull, J.; Root, M. (1965) Subacute Oral Toxicity of DEF to Male and Female Dogs: 16422. (Unpublished 

study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, Dept. of Pharmacology, submitted 

by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-N)

87423 Root, M.; Taitel, C.; Doull, J. (1965) Subacute Oral Toxicity of DEF to Male and Female Rats: 16823. 

(Unpublished study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, Dept. of 

Pharmacology, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-P)

87425 Root, M.; Doull, J. (1967) Effect of Adding DEF in Combination with Malathion to the Diet of Male and 

Female Dogs: 19773. (Unpublished study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, 

Toxicity Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-R)

97897 Root, M.S.; Doull, J. (1966) Comparative subacute oral toxicity of some organic phosphates in rats and dogs. 

Taken from: Abstracts of Papers of the Fifth Annual Meeting of the Society of Toxicology, 1966. N.P. 

(Abstract 50). (Submitter 17845; also in unpublished submission received Aug 21, 1972 under unknown admin. 

no.; submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:140139-G)

153410 Hayes, R. (1985) Pilot Study on Technical Tribufos (DEF) with Mice: Study No. 85-971-01: 90445. 

Unpublished study prepared by Mobay Chemical Corp. 11 p.

82-2 21-day dermal-rabbit/rat

42007201 Sheets, L.; Phillips, S. (1991) 21-Day Dermal Toxicity Study with Technical Grade Tribufos (DEF) in Rabbits: 

Lab Project Number: 90-125-FP. Unpublished study prepared by Mobay Corp., Toxicology Dept. 445 p.

82-3 90-day dermal-rodent

87424 Dubois, K.P.; DiGiacomo, R.; Root, M.; et al. (1966) The Subacute Dermal Toxicity of Tributyl 

Phosphorotrithioate (DEF) to Rats: 19563. (Unpublished study received Jan 18, 1969 under 9F0737; prepared 

by Univ. of Chicago, Toxicity Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 

091270-Q)

82-4 90-day inhal.-rat

80037 Thyssen, J.; Lorke, D.; Mohr, U. (1978) DEF: Subacute Inhalation Study on Rats: Report No. 7649; 66435. 

(Unpublished study received Aug 21, 1981 under 3125-282; prepared by Bayer AG, West Germany, submitted 

by Mobay Chemical Corp., Kansas City, Mo.; CDL:245782-A)

149213 Thyssen, J.; Mohr, V.; Lorke, D. (1978) DEF: Subacute Inhalation Study on Rats: Report No. 7649. 

Unpublished study prepared by Bayer AG. 63 p.

41954901 Pauluhn, J. (1991) DEF: Tribufos; Tribufate: Range-Finding Study of the Subacute Inhalation Toxicity to Rats 

(Exposure: 10 x 6h): Lab Project Number: 100695: 91-T-036: 19903. Unpublished study prepared by Bayer 

AG. 212 p.
81

42399801 Pauluhn, J. (1992) DEF (Common Name: Tribufos): Study of the Subchronic Inhalation Toxicity to Rats in 

Accordance with OECD Guideline No. 413: Lab Project Number: 102697. Unpublished study prepared by 

Bayer Ag, Wuppertal, Germany. 1050 p.

82-5 Subchronic Neurotoxicity: 90-Day Study

80026 Thyssen, J.; Schilde, B.; Lorke, D. (1978) DEF, Studies of Subacute Dermal Neurotoxic Effects on Poultry: 

Report No. 8081; 67075. (Unpublished study received Aug 21, 1981 under 3125-282; pre- pared by Bayer AG, 

West Germany, Submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:245780-A)

132875 Hansen, L.; Francis, B.; Metcalf, R.; et al. (1982) Effects of Chronic Oral and Dermal Administration of 

Pesticides on Laying Hens, Including Delayed Neuropathy: I. Organophosphorus Pesticides: Project No. XX82-275. (Unpublished study received Nov 30, 1983 under 524-124; prepared by Univ. of Illinois, College of 

Veterinary Medicine and Institute of Environmental Studies, submitted by Monsanto Co., Washington, DC; 

CDL:251840-C)

151228 Abou-Donia, M. (1978) Study of the Low Level Effects of Organophosphorus Esters with Specific References 

to the Delayed Neurotoxic Syndrome: Quarterly Report 4: Effect of a Single Oral Dose and Subchronic (90-

day) Oral Administration of Technical DEF to Adult Hens. Unpublished study prepared by Duke University 

Department of Pharmacology. 35 p.

151232 Abou-Donia, M.; Graham, D.; Abdo, K.; et al. (1979) Delayed neurotoxic, late acute and cholinergic effects of 

S,S,S-tributyl phosphorotrithoate (DEF): Subchronic (90 days) administration in hens. Toxicology 14:229-243.

42007202 Sheets, L. (1991) Subchronic Delayed Neurotoxicity Study with Technical Grade Tribufos (DEF) in Hens: Lab 

Project No: 89-428-CS. Unpublished study prepared by Mobay Corp., Toxicology Dept. 189 p.

82-7 Subchronic Neurotoxicity

45369101 Sheets, L.; Gilmore, R. (2001) A Subchronic Neurotoxicity Screening Study with Technical Grade Tribufos 

(DEF) in Wistar Rats: Lab Project Number: 99-N72-BB: 110242: 9465. Unpublished study prepared by Bayer 

Corp. and Pathology Associates International. 538 p.

83-1 Chronic Toxicity

87426 Hibbs, C.; Nelson, D.L. (Year not reported) Chronic Oral Toxicity of DEF to Male and Female Rats: 20462. 

(Unpublished study received Jan 18, 1969 under 9F0737; submitted by Mobay Chemical Corp., Kansas City, 

Mo.; CDL:091270-S)

87428 DuBois, K.P.; Root, M.; Meskauskas, J.; et al. (1967) Chronic Oral Toxicity of DEF to Male and Female Dogs: 

Submitter| 21556. (Unpublished study received Jan 18, 1969 under 9F0737; prepared by Univ. of Chicago, 

Toxicity Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL:091270-U)

145268 Root, M.; Meskauskas, J.; Doull, J. (1967) Chronic Oral Toxicity of DEF to Male and Female Rats. 

Unpublished Mobay report 20462 prepared by Univ. of Chicago, Toxicity Laboratory. 18 p.

145868 Root, M.; Meskanskas, J.; Doull, J. (1967) Chronic Oral Toxicity of DEF to Male and Female Rats. 

Unpublished study prepared by University of Chicago. 18 p.

42007203 Christenson, W. (1991) Chronic Feeding Toxicity Study of Technical Grade Tribufos (DEF) with Dogs: Lab 

Project Number: 88-274-AB. Unpublished study prepared by Mobay Corp., Toxicology Dept. 585 p.
82

83-2 Oncogenicity

40273801 Hayes, R. (1987) Interim Report on Tribufos (DEF) Mouse Oncogenicity Study: Lab Project No.: 94700. 

Unpublished study prepared by Mobay Corp. 4 p.

40436001 Hayes, R. (1987) Oncogenicity Study with Tribufos (DEF) in the Mouse: Interim Report: Laboratory Project 

ID 86-271-01. Unpublished study prepared by Mobay Corporation. 5 p.

40611301 Hayes, R. (1988) Oncogenicity Study with Tribufos (DEF) in the Mouse: Study No. 86-271-01. Unpublished 

study prepared by Mobay Corp. 5 p.

41171001 Hayes, R. (1989) Oncogenicity Study of Technical Grade Tribufos (DEF) with Mice: Study No. 86-271-01. 

Unpublished study pre- pared by Mobay Corp. 2042 p.

92034004 Christenson, W. (1990) Mobay Corporation Phase 3 Summary of MRID 41171001. Oncogenicity Study with 

Technical Grade Tribufos (DEF) with Mice: Project: 86-271-01 (Mobay Report 99175). Prepared by Mobay 

Corp.. 25 p.

83-3 Teratogenicity -- 2 Species

40190601 Kowalski, R. (1986) A Teratology Study with DEF Technical in the Rat: Report No. 87320. Unpublished study 

prepared by Miles Laboratories, Inc. 213 p.

40190602 Clemens, G.; Bare, J.; Hartnagel, R. (1987) Teratology Study in the Rabbit with DEF Technical: Report No. 

MTD0003. Unpublished study prepared by Miles Laboratories, Inc. 120 p.

40879101 Kasymova, R. (1975) Experimental Study of the Embryotoxic Effect of Butiphos. A translation of 

Ekcnepnmehtanbhoe Nyhehne Embpnotok cnheckoto )|enctbnr bytnoca translated by Rainbow Systems, Inc. 9 

p.

92034005 Eigenberg, D. (1990) Mobay Corporation Phase 3 Summary of MRID 40190601. A Teratology Study with 

DEF in the Rat: Project 8536 (Mobay Report 87320). Prepared by Miles Laboratories, Inc.. 27 p.

92034006 Eigenberg, D. (1990) Mobay Corporation Phase 3 Summary of MRID 40190602. Teratology Study in the 

Rabbit with DEF Technical: Project 8605 (Mobay Report 94468). Prepared by Miles Laboratories, Inc.. 34 p.

83-4 2-generation repro.-rat

87427 DuBois, K.P.; Root, M.; DiGiacomo, R.; et al. (1967) Effect of DEF in the Diet on the Reproduction and 

Lactation on Mice: ?Submitter| 21543. (Unpublished study received Jan 18, 1969 under 9F0737; prepared by 

Univ. of Chicago, Toxicity Laboratory, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 091270-

T)

144939 Root, M.; DiGiacomo, R.; Meskauskas, J.; et al. (1967) Effect of DEF in the Diet on the Reproduction and 

Lactation of Mice. Unpublished Mobay report 21543 prepared by Univ. of Chicago, Toxicity Laboratory. 7 p.

42040201 Eigenberg, D.; Elcock, L. (1991) A Two-Generation Reproduction Study in Rats Using Tribufos (DEF): Lab 

Project Number: 88-671- AK. Unpublished study prepared by Mobay Corp. 1025 p.

83-5 Dietary: Combined Chronic Toxicity/Oncogenicity Studies

42335101 Christenson, W. (1992) Technical Grade Tribufos (DEF): A Chronic Toxicity/Oncogenicity/ Neurotoxicity

Feeding Study in the Fisher 344 Rat: Lab Project Number: 88-271-AA. Unpublished study prepared by Miles 

Inc. 3672 p.
83

42553601 Christenson, W. (1992) Technical Grade Tribufos (DEF): A Chronic Toxicity/Oncogenicity/Neurotoxicity 

Feeding Study in the Fischer 344 Rat: A Supplement: Lab Project Number: 88-271-AA: 102675-1. 

Unpublished study prepared by Miles, Inc. 183 p.

45475501 Sheets, L. (2001) A Developmental Neurotoxicity Screening Study with Technical Grade Tribufos (DEF) in 

Wistar Rats: Lab Project Number: 00-D72-AG: 00-D72-AS: 110523. Unpublished study prepared by Bayer 

Corporation. 1281 p. 

45499501 Lake, S. (2001) A Developmental Neurotoxicity Screening with Technical Grade Tribufos (DEF) in Wistar 

Rats: Lab Project Number: 00-D72-AG: 00-D72-AS. Unpublished study prepared by Bayer Corporation. 1281 

p. 

121-1 Phytotoxicity

102298 Mobay Chemical Corp. (1974) Efficacy of Guthion Preparations and Other Compounds for Insect Control on 

Cotton|. (Compilation; unpublished study received Aug 9, 1974 under 3125-271; CDL:009691-B)

112032 Mobay Chemical Corp. (1974) Efficacy Study: DEF, Guthion, & Other Specific Insecticides on Selected Grain 

Crops|. (Compilation; unpublished study received Apr 23, 1974 under 3125-71; CDL: 025713-A)

112149 Olson, T.; Horton, J. (1974) Comparative Drift Study of DEF As ULV and Spray Concentrate in Water: 39400. 

(Unpublished study received Mar 22, 1976 under 3125-282; submitted by Mobay Chemical Corp., Kansas City, 

MO; CDL:232397-C)

139286 Mobay Chemical Corp. (1972) (Phytotoxicity of DEF and DEF-Guthion to Sorghum and Other Crops). 

(Compilation; unpublished study received Mar 22, 1976 under 3125-282; CDL:232397-A)

122-2 Aquatic plant growth

45863705 Kern, M.; Lam, C. (2002) Toxicity of Tribufos Technical to Duckweed (Lemna gibba G3): Lab Project 

Number: DE883701: 200301. Unpublished study prepared by Bayer Corporation. 45 p.

123-1 Seed germination/seedling emergence and vegetative vigor

45863704 Bowers, L. (2002) Tier 2 Seedling Emergence and Vegetative Vigor Nontarget Phytotoxicity Study Using DEF 

6: Lab Project Number: DE451601: DE451602: 200245. Unpublished study prepared by Bayer Corporation. 

105 p.

123-2 Aquatic plant growth

41618813 Hughes, J. (1990) The Toxicity of DEF Technical to Selanastrum capricornutum (Tier 2 Growth and 

Reproduction of Aquatic Plant- s): Lab Project Number: BO59-02-2: 100230. Unpublished study prepared by 

Malcolm Pirnie, Inc. 36 p.

141-1 Honey bee acute contact

49252 University of California, Agricultural Extension Service Toxicity of Pesticides and Other Agricultural 

Chemicals to Honey Bees: Field Study. Riverside, Calif.: UC, ACES. (AXT-251; submitter report no. 21052; 

also in unpublished submission received Mar 22, 1976 under 3125-71; submitted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:224114-I)

165-0 Accumulation Studies -- General

66341 U.S. Environmental Protection Agency, Environmental Research Laboratory (1981) Acephate, Aldicarb, 

Carbophenothion, DEF, EPN, Ethoprop, Methyl Parathion, and Phorate: Their Acute and Chronic Toxicity, 

Bioconcentration Potential, and Persistence as Related to Marine Environments: EPA-600/4-81-023. 

(Unpublished study)
84
85

Appendix A. Selected SWCC Output Files

MS Cotton, Aerial Spray, Maximum Application Rate (1.125 lb ai/A)

Summary of Water Modeling of Tribufos and the USEPA Standard 

Pond

Estimated Environmental Concentrations for Tribufos are presented in Table 1 for the USEPA standard 

pond with the MScottonSTD field scenario. A graphical presentation of the year-to-year peaks is 

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

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

This model estimates that about 5.6% of Tribufos applied to the field eventually reaches the water body. 

The main mechanism of transport from the field to the water body is by runoff (53.9% of the total 

transport), followed by erosion (23.8%) and spray drift (22.3%).

In the water body, pesticide dissipates with an effective water column half-life of 759.1 days. (This value 

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

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

volatilization (effective average half-life = 907.2 days) followed by metabolism (4649.4 days).

In the benthic region, pesticide dissipates very slowly (312.0 days). The main source of dissipation in the 

benthic region is metabolism (effective average half-life = 312 days). The vast majority of the pesticide in 

the benthic region (99.52%) is sorbed to sediment rather than in the pore water.

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

Peak (1-in-10 yr) 29.7

4-day Avg (1-in-10 yr) 29.1

21-day Avg (1-in-10 yr) 27.1

60-day Avg (1-in-10 yr) 25.1

365-day Avg (1-in-10 yr) 21.6

Entire Simulation Mean 15.4

Table 2. Summary of Model Inputs for Tribufos.

Scenario MScottonSTD

Cropped Area Fraction 1

Kd (ml/g) 76.9

Water Half-Life (days) @ 20 °C 4470

Benthic Half-Life (days) @ 20 °C 300

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

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 20 °C 2235

Foliar Half-Life (days)
86

Molecular Wt 314.54

Vapor Pressure (torr) 1.7E-6

Solubility (mg/l) 2.3

Table 3. Application Schedule for Tribufos.

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

9/15 Foliar 1.26 0.95 0.125

Figure 1. Yearly Peak Concentrations

CA Cotton with Irrigation Standard Scenario, Aerial Spray, Max. Applic. Rate (1.875 lb ai/A)

Summary of Water Modeling of Tribufos and the USEPA Standard 

Pond

Estimated Environmental Concentrations for Tribufos are presented in Table 1 for the USEPA standard 

pond with the CAcotton_WirrigSTD field scenario. A graphical presentation of the year-to-year peaks is 

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

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

This model estimates that about 2% of Tribufos applied to the field eventually reaches the water body. 

The main mechanism of transport from the field to the water body is by spray drift (63.6% of the total 

transport), followed by runoff (32.8%) and erosion (3.67%).

In the water body, pesticide dissipates with an effective water column half-life of 830.4 days. (This value 

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

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

volatilization (effective average half-life = 1005 days) followed by metabolism (4781.1 days).

In the benthic region, pesticide dissipates very slowly (320.9 days). The main source of dissipation in the 

benthic region is metabolism (effective average half-life = 320.9 days). The vast majority of the pesticide 

in the benthic region (99.52%) is sorbed to sediment rather than in the pore water.
87

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

Peak (1-in-10 yr) 21.2

4-day Avg (1-in-10 yr) 20.0

21-day Avg (1-in-10 yr) 16.8

60-day Avg (1-in-10 yr) 14.3

365-day Avg (1-in-10 yr) 11.5

Entire Simulation Mean 9.48

Table 2. Summary of Model Inputs for Tribufos.

Scenario CAcotton_WirrigSTD

Cropped Area Fraction 1

Kd (ml/g) 76.9

Water Half-Life (days) @ 20 °C 4470

Benthic Half-Life (days) @ 20 °C 300

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

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 20 °C 2235

Foliar Half-Life (days)

Molecular Wt 314.54

Vapor Pressure (torr) 1.7E-6

Solubility (mg/l) 2.3

Table 3. Application Schedule for Tribufos.

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

11/4 Foliar 2.1 0.95 0.125

Figure 1. Yearly Peak Concentrations
88

MS Cotton, Aerial Spray, Average Application Rate (0.54 lb ai/A)

Summary of Water Modeling of Tribufos and the USEPA Standard 

Pond

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

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

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

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

This model estimates that about 5.6% of tribufos applied to the field eventually reaches the water 

body. The main mechanism of transport from the field to the water body is by runoff (53.9% of 

the total transport), followed by erosion (23.8%) and spray drift (22.3%).

In the water body, pesticide dissipates with an effective water column half-life of 759.1 days. 

(This value does not include dissipation by transport to the benthic region; it includes only 

processes that result in removal of pesticide from the complete system.) The main source of 

dissipation in the water column is volatilization (effective average half-life = 907.2 days) 

followed by metabolism (4649.4 days).

In the benthic region, pesticide dissipates very slowly (312.0 days). The main source of 

dissipation in the benthic region is metabolism (effective average half-life = 312 days). The vast 

majority of the pesticide in the benthic region (99.52%) is sorbed to sediment rather than in the 

pore water.

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

Peak (1-in-10 yr) 14.2

4-day Avg (1-in-10 yr) 13.9

21-day Avg (1-in-10 yr) 12.9

60-day Avg (1-in-10 yr) 11.9
89

365-day Avg (1-in-10 yr) 10.3

Entire Simulation Mean 7.34

Table 2. Summary of Model Inputs for Tribufos.

Scenario MScottonSTD

Cropped Area Fraction 1

Kd (ml/g) 76.9

Water Half-Life (days) @ 20 °C 4470

Benthic Half-Life (days) @ 20 °C 300

Photolysis Half-Life (days) @ 40 

°Lat

0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 20 °C 2235

Foliar Half-Life (days)

Molecular Wt 314.54

Vapor Pressure (torr) 1.7E-06

Solubility (mg/l) 2.3

Table 3. Application Schedule for Tribufos.

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

09/15 Foliar 0.60 .95 .125

Figure 1. Yearly Peak Concentrations
90

CA Cotton with Irrigation Standard Scenario, Aerial Spray, Average Applic. Rate for Arizona 

(0.77 lb ai/A)

Summary of Water Modeling of Tribufos and the USEPA Standard 

Pond

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

standard pond with the CAcotton_WirrigSTD field scenario. A graphical presentation of the 

year-to-year peaks is presented in Figure 1. These values were generated with the Surface Water 

Concentration Calculator (SWCC Version 1.106). Critical input values for the model are 

summarized in Tables 2 and 3.

This model estimates that about 2% of tribufos applied to the field eventually reaches the water 

body. The main mechanism of transport from the field to the water body is by spray drift (63.6% 

of the total transport), followed by runoff (32.8%) and erosion (3.67%).

In the water body, pesticide dissipates with an effective water column half-life of 830.4 days. 

(This value does not include dissipation by transport to the benthic region; it includes only 

processes that result in removal of pesticide from the complete system.) The main source of 

dissipation in the water column is volatilization (effective average half-life = 1005 days) 

followed by metabolism (4781.1 days).

In the benthic region, pesticide dissipates very slowly (320.9 days). The main source of 

dissipation in the benthic region is metabolism (effective average half-life = 320.9 days). The 

vast majority of the pesticide in the benthic region (99.52%) is sorbed to sediment rather than in 

the pore water.

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

Peak (1-in-10 yr) 8.66

4-day Avg (1-in-10 yr) 8.20

21-day Avg (1-in-10 yr) 6.89

60-day Avg (1-in-10 yr) 5.86

365-day Avg (1-in-10 yr) 4.70

Entire Simulation Mean 3.88

Table 2. Summary of Model Inputs for Tribufos.

Scenario CAcotton_WirrigSTD

Cropped Area Fraction 1

Kd (ml/g) 76.9

Water Half-Life (days) @ 20 °C 4470

Benthic Half-Life (days) @ 20 °C 300

Photolysis Half-Life (days) @ 40 

°Lat

0
91

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 20 °C 2235

Foliar Half-Life (days)

Molecular Wt 314.54

Vapor Pressure (torr) 1.7E-06

Solubility (mg/l) 2.3

Table 3. Application Schedule for Tribufos.

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

11/04 Foliar 0.86 .95 .125

Figure 1. Yearly Peak Concentrations
92

Appendix B. Toxicity Categories and Levels of Concern

 

 Table B1. Categories of Toxicity for Aquatic Organisms

LC50 (ppm) Toxicity Category

< 0.1 Very highly toxic

> 0.1 - 1 Highly toxic

> 1 - 10 Moderately toxic

> 10 - 100 Slightly toxic

> 100 Practically nontoxic

Table B2. Categories of Toxicity for Terrestrial Organisms

Oral dose LD50 (mg/kg-bw) Toxicity Category

< 10 Very highly toxic

10 - 50 Highly toxic

51 - 500 Moderately toxic

501 - 2000 Slightly toxic

> 2000 Practically nontoxic

Dietary LC50 (ppm) Toxicity Category

< 50 Very highly toxic

50 - 500 Highly toxic

501 - 1000 Moderately toxic

1001 - 5000 Slightly toxic

> 5000 Practically nontoxic

Table B3. Categories of Toxicity for Honey Bee

Bee Acute Contact LD50 (μg/bee) Toxicity Category

<2 Highly toxic

2 – 10.99 Moderately toxic

> 11 Practically nontoxic

Table B4. Levels of Concern for Terrestrial and Aquatic Organisms

Taxa Acute LOC Chronic LOC

Avian1 (terrestrial phase amphibians) 0.1 1

Mammalian2 0.1 1

Terrestrial plants3 1

Aquatic Animals4 (aquatic phase

amphibians)

0.05 1

Insects 5 0.05 1

Used in RQ calculations: 1 LD50 and estimated NOEL 2 LD50 and NOEC 3 EC25

4 LC/EC50 and estimated and reproductive NOEC 5 LD50 per EFED’s CRLF Steering Committee
93

Appendix C. Summary of Ecological Toxicity Data

Data in this appendix include results from submitted ecological toxicity studies and results from ECOTOX when the test result was

more sensitive than those in submitted toxicity data. Submitted studies that are considered invalid include: 41618808, 41618809 (no 

finalized DER, and 41918810). 

Table C 1. Summary of Toxicity Data for Aquatic Vertebrates Exposed to Tribufos (TGAI) and Formulations.

Common Name

(Species)

Study Type

and

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect 

Observed at 

Endpoint)

Toxicity Value

(95% CI)

μg a.i./L

(Acute Toxicity 

Category)

MRID

Study 

Classification

Comments

Freshwater Vertebrates

Bluegill sunfish

(Lepomis macrochirus)

Static

(96-hour) TGAI LC50

(Survival)

283* (136-378)

Slope=6.23

(highly toxic)

40098001

SupplementalQuantitative

Some deviations from guidelines 

requirements.

Bluegill sunfish

(Lepomis macrochirus)

Flow-through 

(96-hour)

TGAI

(96.2%)

LC50

(Survival)

630 (500-780)

Slope=6.46 (3.18-

9.74)

(highly toxic)

41618806

Acceptable

Precipitate in all test solutions. Not 

filtered prior to analysis. Variation in 

measured concentration. Test levels do 

not overlap but they are close to 

overlapping.

Bluegill sunfish

(Lepomis macrochirus)

Flow-through 

96-hour

DEF® 6

(72.3%)

LC50

(Survival)

490

(highly toxic)

41618807

Acceptable

Binomial

Rainbow trout

(Oncorhynchus mykiss)

Flow-through 

96-hour

TGAI

(96.2%)

LC50

(Survival) --

41618808

SupplementalQualitative

Precipitate observed at all test 

concentrations. Not filtered prior to 

analysis. Concentrations near solubility.

Fathead Minnow 

(Pimephales promelas)

Flow-through 

96-hour

TGAI

(99.4%)

LC50

(Survival)

920

(highly toxic)

45863706

Acceptable

Loss of equilibrium, labored respiration, 

at surface and/or bottom of the vessel at 

0.59 mg a.i./L and above. Binomial.

Fowlers toad 

(Bufo fowleri)

Acute

(96-hr) TGAI LC50

(Survival) 450 40098001Not reviewed

Included in summary because it is data 

specific to amphibians.

Fathead Minnow 

(Pimephales promelas)

Early Life 

Cycle

(36-day)

TGAI

(99.4%)

NOAEC/ 

LOAEC (post 

hatch survival)

3.35/ 8.38

45863703

SupplementalQualitative 

Tribufos residues in negative control days 

0 to 4 near level of quantitation of 0.16 

μg/L. Some overlap in concentrations 

between treatment groups. 
94

Common Name

(Species)

Study Type

and

(Duration of 

Exposure)

Test 

Substance 

(% a.i.)

Endpoint

(Effect 

Observed at 

Endpoint)

Toxicity Value

(95% CI)

μg a.i./L

(Acute Toxicity 

Category)

MRID

Study 

Classification

Comments

Fathead Minnow 

(Pimephales promelas)

Early Life 

Cycle

(35-day)

TGAI

(96.6%)

NOAEC/ 

LOAEC (fry 

survival, length, 

dry weight)

2.49/ 4.83

45863703

SupplementalQualitative 

Significant reduction in average dry 

weight in solvent control. Endpoint may 

not be a result of exposure to TGAI.

Bluegill sunfish

(Lepomis macrochirus) Chronic -- NOAEC 4.1* Estimated

This NOAEC is based on the acute to 

chronic ratio using sheepshead minnow 

endpoints [767/11=69.7 (ACR)]

Estuarine/Marine Vertebrates

Sheepshead minnow

(Cyprinodon 

variegatus)

Acute 

Flow-through 

96-hour

TGAI 

(98.6%)

LC50

(Survival)

767* (560-1370)

(highly toxic)

41896302

Acceptable

Binomial method.

Sheepshead minnow

(Cyprinodon 

variegatus)

Early Life 

Cycle

(35-day)

TGAI 

(99.4%)

NOAEC/ 

LOAEC

(growth, post 

hatch survival)

11/ 19* 45863707

Acceptable

TGAI=technical grade active ingredient 

*Represents the most sensitive toxicity endpoint to be used in RQ calculations.

Table C 2. Summary of Toxicity Data for Aquatic Invertebrates Exposed to TGAI and Formulations Containing Tribufos.

Species

(Common Name)

Study 

Type

(Exposure 

Duration)

Test 

Substanc

e (% a.i.)

Endpoint (Effect 

Observed at 

Endpoint)

Toxicity Value

[95% CI]

μg a.i./L

(Acute Toxicity 

Category)

MRID

Study 

Classification1

Comments

Freshwater Invertebrates

Water flea

(Daphnia magna)

Static

(48-hour)

Not 

available

EC50

(Immobility)

6.63 (5.50-8.01)

(very highly toxic)

40098001

SupplementalQualitative

A negative control was not included in 

the study.

Water flea

(Daphnia magna)

Static

(48-hour)

Not 

available

EC50

(Immobility)

5.4* (3.5-7.7)

Slope=2.5

(very highly toxic)

40098001

SupplementalQuantitative

--

Water flea

(Daphnia magna)

Static

(48-hour)

DEF6 ®

(72.3%)

EC50

(Immobility)

61

(highly toxic)

41689902

Acceptable --
95

Species

(Common Name)

Study 

Type

(Exposure 

Duration)

Test 

Substanc

e (% a.i.)

Endpoint (Effect 

Observed at 

Endpoint)

Toxicity Value

[95% CI]

μg a.i./L

(Acute Toxicity 

Category)

MRID

Study 

Classification1

Comments

Water flea

(Daphnia magna)

Static

(48-hour)

TGAI

(98.7%)

EC50

(Immobility)

110

(highly toxic)

41689901

Acceptable --

Water flea

Staticrenewal

(21-days)

TGAI

(97.2%)

NOAEC/ LOAEC

(reduced number 

of 

young/adult/day 

and reduced adult 

length)

*1.56/ 

3.23

43978201

Acceptable

Freshwater 

amphipod 

(Hyalella azteca)

Spiked 

sediment

(10-day)

TGAI

(98.2%)

NOAEC/ LOAEC 

(Survival)

693/>693 μg a.i./kgsediment

27,000/ >27,000

μg/kg-TOC

34.1/>34.1 pore-water 49164802

Acceptable

--

NOAEC LOAEC

(Growth)

71/ 220 μg a.i./kgsediment

2700/ 8500 μg a.i./kgTOC

2.81/8.27 pore-water

--

Estuarine/Marine Invertebrates

Mysid shrimp 

(Mysidopsis 

bahia) 

Flowthrough

(96-hour)

TGAI

(96.4%)

LC50

(Survival)

11.53* 

(9.30-15.11) 

(very highly toxic)

41896301

Acceptable

Calculated using moving average 

method. Abnormal physical 

appearance and mortality observed at 

lowest test concentration (1.26 μg 

a.i./L).

Eastern Oyster

(Crassostrea 

virginica)

Flowthrough 

(96.4)

TGAI EC50 (shell 

deposition)

39.6 (35-45)

(very highly toxic)

42083201

Acceptable
96

Species

(Common Name)

Study 

Type

(Exposure 

Duration)

Test 

Substanc

e (% a.i.)

Endpoint (Effect 

Observed at 

Endpoint)

Toxicity Value

[95% CI]

μg a.i./L

(Acute Toxicity 

Category)

MRID

Study 

Classification1

Comments

Mysid shrimp 

(Americamysis 

bahia) 

Flowthrough

(28-day)

TGAI

(96.4%)

NOAEC/ LOAEC

(None) 0.26/ >0.26* 45863702

Supplemental

Analytical variation in the measured 

test concentrations exceeded the 20% 

among replicate results. Minimum 

control number of offspring produced 

was less than one. The minimum 

mean requirement is 3 offspring per 

female per day. No treatment related

effects observed.

Amphipod

(Leptocheirus 

plumulosus)

Spiked 

sediment

(10-day)

TGAI

(98.2%)

NOAEC/

LOAEC

(Mortality)

207 /773 μg a.i./kgsediment

7.10/22.7 pore-water

49164801

Acceptable --

TGAI=technical grade active ingredient:

*Represents the most sensitive toxicity endpoint to be used in RQ calculations. A risk quotient is not calculated for non-definitive endpoints, but the information 

is used in the risk description.

1 Supplemental studies may or may not be suitable to calculate RQs. Quantitative values are suitable for calculation of RQ. Qualitative values are not considered 

suitable for calculation of RQs but may be useful in understanding the weight of evidence for evaluating the likelihood of risk.

Table C 3. Summary of Toxicity Data for Vascular and Nonvascular Aquatic Plants Exposed to Tribufos.

Common Name

(Species)

Type of Test

(Exposure 

Duration)

Test Substance 

(% a.i.)

Endpoint

(Effect at 

Endpoint)

Toxicity Value 

(95% CI)

μg a.i./L 

MRID

(Study 

Classification)

Duckweed 

(Lemna gibba)

Static Renewal

14-day TGAI (99.4%)

EC50

(Frond Count)

1100 (760-

1700)* 45863705

EC05 140 (35-570)* Supplemental

NOAEC <17.2

Freshwater green 

alga

(Pseudokirchneriella

Subcapitata1

)

7-day TGAI (99.9%)

EC50

(Percent 

inhibition)

148* 41618813

Acceptable NOAEC/ 

LOAEC 118/ 254*

*Represents the most sensitive toxicity endpoint to be used in RQ calculations

1 Pseudokirchneriella subcapitata are formerly known as Selenastrum capricornutum and also as Raphidocelis subcapitata.
97

Table C 4. Summary of Toxicity Data for Terrestrial Animals Exposed to Tribufos.

Common Name

(Species)

Study Type

(Duration of 

Exposure)

Test Substance 

(% a.i.)

Endpoint

(Effect at 

endpoint)

Toxicity Value

(95% CI)

(Acute Toxicity 

Category)

MRID

Study 

Classification

Sublethal effects

Birds

Zebra Finch 

(Taeniopygia 

guttata)

Acute Oral 

(Oral gavage 

with 14-day 

observation)

TGAI (98.2%)

ED50

(Mortality and 

Regurgitation)

397 mg/kg-bw

(265-588) assuming 

regurgitation was 

equal to mortality

(slightly toxic)

49121302

Supplemental due to 

regurgitation within 

one hour of dosing

Ruffled feathers, underactivity, 

incoordination, recumbency, body 

tremors, panting, salivation and/or 

regurgitation observed at all 

treatment levels.

Canary 

(Serinus canaria)

Acute Oral 

(Oral gavage 

with 14-day 

observation)

TGAI (98.2%)

ED50

(Mortality and 

Regurgitation)

111 mg/kg-bw (73.7-

168)

Assuming 

regurgitation was 

equal to mortality

(moderately toxic)

49121303

Supplemental due to 

regurgitation within 

one hour of dosing

Lethargy, unsteadiness, 

recumbency, body tremors, 

panting, salivation, hunched 

posture, and/or emesis were 

observed at all treatment levels.

Mallard duck

(Anas 

platyrhynchos)

Acute Oral 

(Oral gavage 

with 14-day 

observation)

TGAI (92%) LD50

(Mortality)

871 mg a.i./kg-bw

(468-2892)

Slope=1.67 (0.56-

2.77)

(slightly toxic)

00049258

Supplemental due to 

testing of 8 birds and 

no food 

consumption data

Bobwhite quail

(Colinus 

virginianus)

Acute Oral

(Oral gavage 

with 14-day 

observation)

TGAI (92%) LD50

(Mortality)

151 mg a.i./kg-bw

(128-178)*

Slope=7.9 (4.3-11.5)

(moderately toxic)

00120771

Acceptable

Mallard duck

(Anas 

platyrhynchos)

Subacute 

Dietary

(5-day)

TGAI (99.8%) LC50

(Mortality)

>5000 mg a.i./kg-diet

(practically non-toxic)

41618805

Acceptable

Reduced body weight gain 

observed at all test concentrations 

(313 to 5000 mg a.i./kg-diet).

Bobwhite quail

(Colinus 

virginianus)

Subacute 

Dietary

(5-day)

TGAI

(98.9%)

LD50

(Mortality)

1519 mg a.i./kg-bw

(slightly toxic)

41618804

Acceptable

Reduced body weight gain during 

exposure period was observed at 

the three highest test 

concentrations (lowest was 1135 

mg/kg-bw)

Bobwhite quail

(Colinus 

virginianus)

Reproduction

(20 week) TGAI

(98.7%)

NOAEC/

LOAEC 

(egg shell 

thickness)

150/280 mg a.i./kgdiet*

40757101

Acceptable

Mammals2
98

Common Name

(Species)

Study Type

(Duration of 

Exposure)

Test Substance 

(% a.i.)

Endpoint

(Effect at 

endpoint)

Toxicity Value

(95% CI)

(Acute Toxicity 

Category)

MRID

Study 

Classification

Sublethal effects

Laboratory rat

(Rattus 

norvegicus)

Acute 

Inhalation

(4-hour)

TGAI LC50

(Mortality)

4650 mg/m3

4.65 mg/L 41782301

Laboratory rat

(Rattus 

norvegicus)

Acute Oral TGAI LD50

(Mortality)

192 mg/kg-bw 

(moderately toxic) 41954903

White footed 

mouse 

(Peromyscus 

leucopus)

Acute Diet

(5-day) TGAI LC50(Mortality)

161 mg/kg-diet

(highly toxic)

Slope=1.73 (0.66-

2.79)

45863708

Supplementalquantitative

Droopy eyelids, ataxia, 

hypoactivity, immobility, 

moribund condition observed at 

all treatment levels.

Laboratory rat

(Rattus 

norvegicus)

Twogeneration 

Reproduction

TGAI

NOAEL/

LOAEL

(increased 

number of 

litters with still 

born pups)

32 mg/kg-diet*

1.7 mg/kg/day/

260 mg/kg-diet

15 mg/kg/day

42040201

Additional endpoints affected 

include increased pup death 

through lactation; decrease F1 and 

F2 pup body weights, increased 

F1 gestation period.

Honey bees

Honey Bee

(Apis mellifera)

Acute Contact

(48-hour)

Variable., 

Mixed with 

pyrolite dust

LD50

(Mortality)

>24.17 μg a.i./bee*

(practically nontoxic)

00001999

Acceptable

3% mortality at 24.17 μg a.i./bee. 

* Most sensitive endpoint; will be used in RQ calculations.

2 Mammalian toxicity endpoints for provided and reviewed by EPA/OCSPP/OPP Health Effects Division

Table C 5. Summary of Terrestrial Plants Toxicity Data for Tribufos Formulations.

Crop Formulation 

(% a.i.) Species EC25(lbs a.i./A)

NOAEC

or IC05/ 

LOAEC

(lbs a.i./A)

Endpoint 

Affected

MRID

Study Classification

Seedling Emergence

Monocots DEF 6 

(70.8%)

Onion

(Allium 

cepa)

>2.0* 0.45/0.93* Plant height 45863404

Acceptable

Dicots Soybean

(Glycine >2.0* 0.22/0.46* Plant height
99

Crop Formulation 

(% a.i.) Species EC25(lbs a.i./A)

NOAEC

or IC05/ 

LOAEC

(lbs a.i./A)

Endpoint 

Affected

MRID

Study Classification

max)

Vegetative Vigor

Monocots DEF 6 

(70.8%)

Corn (Zea 

mays) 1.8* 0.48/0.93* Dry weight, plant height 45863704

Acceptable

Dicots

Buckwheat

(Fagopyrum 

sagiitatum)

1.3* 0.48/0.99* Dry weight

Plant Defoliation Study

Deciduous 

shrubs

Folex® 6EC 

Cotton 

Defoliant

Activator90

® adjuvant

(0.1%)

Forsythia 

(Ribes 

sanguineum) Responses not 

sufficient to 

reliably 

calculate a 

dose-response 

curve

1.875

No significant 

response observed 

at highest dose 

tested

48728101

Supplemental

Viburnum 

(Viburnum 

spp.)

0.563 Number/weight of 

fallen leaves

Deciduous 

Tree 

samplings

Maple (Acer 

spp.) 0.563 Number of old leaves left on plant

Oak 

(Quercus

Spp.)

0.563 Number of old 

leaves left on plant

* Most sensitive endpoint used for risk estimation (i.e., RQ calculations) or risk description (for non-definitive endpoints).
100

Appendix D. Example T-REX (v. 1.5.2) Input and Output for Tribufos

Table D.1. T-REX Model Inputs: Chemical Application Scenarios and Body Mass for Assessed

Species.
101

Table D.2. Avian and Mammalian Toxicity Endpoints for RQ Calculation.
102

Appendix E. STIR Results
103

Appendix F. Screening Imbibition Program (SIP) Version 1.0 Inputs and Results
104

Appendix G. Accepted ECOTOX Data Table (2008 to 2015 Refresh Report)

The code list for ECOTOX can be found at: http://cfpub.epa.gov/ecotox/blackbox/help/codelist.pdf

Genus Species

Common 

Name

Effect 

Group Effect Meas Endpt1 Endpt2

Dur 

Preferred 

Mean

Dur Unit 

Preferred

Conc #2 

Purity 

Adjusted in 

Preferred 

Unit Mean

Conc 

Units 

Preferred

% 

Purity Ref #

Americamysis bahia

Opossum 

Shrimp MOR MOR MORT LC50 4 d mg/L 95 56755

Americamysis bahia

Opossum 

Shrimp MOR MOR MORT LC50 4 d mg/L 95 56755

Daphnia magna Water Flea MOR MOR MORT LC50 2 d mg/L 100 156445

Penaeus duorarum

Northern Pink 

Shrimp MOR MOR MORT LC50 4 d mg/L 95 56755

Palaemonetes pugio

Daggerblade 

Grass Shrimp MOR MOR MORT LC50 4 d mg/L 95 56755

Penaeus stylirostris Shrimp MOR MOR MORT LC50 4 d mg/L 95 56755

Leiostomus xanthurus Spot MOR MOR MORT LC50 4 d mg/L 95 56755

Leiostomus xanthurus Spot MOR MOR MORT LC50 4 d mg/L 95 56755

Schizaphis graminum Greenbug MOR MOR MORT LC50 0.3333 d ug/eu 100 103314

Oligonychus pratensis

Banks Grass 

Mite MOR MOR MORT LC50 1 d ug/eu 100 63628

Tetranychus urticae

Two-Spotted 

Spider Mite MOR MOR MORT LC50 1 d ug/eu 100 63628

Musca domestica House Fly MOR MOR MORT LC50 1 d

AI 

mg/m2 >98 93122

Musca domestica House Fly MOR MOR MORT LC50 1 d

AI 

mg/m2 >98 93122

Lagodon rhomboides Pinfish MOR MOR MORT LC50 4 d mg/L 95 56755

Cyprinodon variegatus

Sheepshead 

Minnow MOR MOR MORT LC50 4 d mg/L 95 56755

Ceriodaphnia dubia Water Flea MOR MOR MORT LC50 4 d ug/L 98 158065

Ceriodaphnia dubia Water Flea MOR MOR MORT LC50 4 d ug/L 98 158065

Phasianus colchicus

Ring-Necked 

Pheasant MOR MOR MORT LD50 14 d

mg/kg 

bdwt 92 50386

Anas platyrhynchos Mallard Duck MOR MOR MORT LD50 14 d

mg/kg 

bdwt 92 50386
105

Genus Species

Common 

Name

Effect 

Group Effect Meas Endpt1 Endpt2

Dur 

Preferred 

Mean

Dur Unit 

Preferred

Conc #2 

Purity 

Adjusted in 

Preferred 

Unit Mean

Conc 

Units 

Preferred

% 

Purity Ref #

Tribolium castaneum

Rust-Red Flour 

Beetle MOR MOR MORT LD50 9 d ug/cm2 100 93121

Anthonomus grandis Boll Weevil MOR MOR MORT LD50 2 d ug/org 100 92637

Tribolium castaneum

Rust-Red Flour 

Beetle MOR MOR MORT LD95 9 d ug/cm2 100 93121

Varroa jacobsoni Mite MOR MOR MORT NOAEL 2 d nl/org 100 103663

Varroa jacobsoni Mite MOR MOR MORT NOAEL 2 d nl/org 100 103663

Americamysis bahia

Opossum 

Shrimp MOR MOR MORT NOAEL LOAEL 28 d 0.00068 mg/L 95 56755

Penaeus stylirostris Shrimp MOR MOR MORT

NRLETH 3 d mg/L 95 56755

Americamysis bahia

Opossum 

Shrimp MOR MOR MORT

NRLETH 9 d mg/L 95 56755

Cyprinodon variegatus

Sheepshead 

Minnow MOR MOR MORT

NRZERO 4 d mg/L 95 56755

Penaeus duorarum

Northern Pink 

Shrimp MOR MOR MORT

NRZERO 2 d mg/L 95 56755

Blattella germanica

German 

Cockroach MOR MOR MORT

NRZERO 14 d ppm 100 68639

Aedes sollicitans Mosquito MOR MOR MORT

NRZERO 1 d mg/L 100 119693

Aedes sollicitans Mosquito MOR MOR MORT

NRZERO 1 d mg/L 100 119693

Aedes aegypti

Yellow Fever 

Mosquito MOR MOR MORT

NRZERO 0.1667 d mg/L 100 66934

Helicoverpa armigera

Old World 

Bollworm MOR MOR MORT

NRZERO 2 d ug/org 99 74861

Culex

pipiens ssp. 

pipiens Mosquito MOR MOR MORT

NRZERO 1.1667 d mg/L 100 103374

Culex

pipiens ssp. 

pipiens Mosquito MOR MOR MORT

NRZERO 1.1667 d mg/L 100 103374

Culex

pipiens ssp. 

quinquefasciata Mosquito MOR MOR MORT

NRZERO 1.1667 d mg/L 100 103374

Helicoverpa armigera

Old World 

Bollworm MOR MOR MORT

NRZERO 2.0417 d ug/org 98 119665
106

Genus Species

Common 

Name

Effect 

Group Effect Meas Endpt1 Endpt2

Dur 

Preferred 

Mean

Dur Unit 

Preferred

Conc #2 

Purity 

Adjusted in 

Preferred 

Unit Mean

Conc 

Units 

Preferred

% 

Purity Ref #

Helicoverpa armigera

Old World 

Bollworm MOR MOR MORT

NRZERO 2.0417 d ug/org 98 119665

Heliothis virescens Budworm MOR MOR MORT

NRZERO 1 d ppm 100 92447

Apis mellifera Honey Bee MOR MOR MORT

NRZERO 1 d ug/org 98 119503

Listronotus oregonensis Carrot Weevil MOR MOR MORT

NRZERO 1 d ppm 100 103315

Listronotus oregonensis Carrot Weevil MOR MOR MORT

NRZERO 1 d ppm 100 103315

Spodoptera frugiperda Fall Armyworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Spodoptera frugiperda Fall Armyworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Helicoverpa zea Corn Earworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Helicoverpa zea Corn Earworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Agrotis ipsilon Cutworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Agrotis ipsilon Cutworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Spodoptera frugiperda Fall Armyworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Helicoverpa zea Corn Earworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Agrotis ipsilon Cutworm MOR MOR MORT

NRZERO 2 d ug 99 63706

Musca domestica House Fly MOR MOR MORT

NRZERO 2 d ppm 100 154990

Blattella germanica

German 

Cockroach MOR MOR MORT

NRZERO 2 d ppm 100 154990

Oncopeltus fasciatus

Large 

Milkweed Bug MOR MOR MORT

NRZERO 2 d ppm 100 154990
107

Appendix H. Ecosar Analysis for Parent and Degradate

ECOSAR RUN for Tribufos

ECOSAR Version 1.11 Results Page

SMILES : CCCCSP(=O)(SCCCC)SCCCC

CHEM : 

CAS Num: 

ChemID1: 

MOL FOR: C12 H27 O1 P1 S3 

MOL WT : 314.50

Log Kow: 5.748 (EPISuite Kowwin v1.68 Estimate)

Log Kow: (User Entered)

Log Kow: 5.70 (PhysProp DB exp value - for comparison only)

Melt Pt: (User Entered for Wat Sol estimate)

Melt Pt: -25.00 (deg C, PhysProp DB exp value for Wat Sol est, <-25)

Wat Sol: 0.5384 (mg/L, EPISuite WSKowwin v1.43 Estimate)

Wat Sol: (User Entered)

Wat Sol: 2.3 (mg/L, PhysProp DB exp value)

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

Values used to Generate ECOSAR Profile

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

Log Kow: 5.748 (EPISuite Kowwin v1.68 Estimate)

Wat Sol: 2.3 (mg/L, PhysProp DB exp value)

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

ECOSAR v1.11 Class-specific Estimations

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

Esters

Esters (phosphate)

 Predicted

ECOSAR Class Organism Duration End Pt mg/L (ppm)

=========================== ================== ======== ====== ==========

Esters : Fish 96-hr LC50 0.272

Esters : Daphnid 48-hr LC50 0.379

Esters : Green Algae 96-hr EC50 0.089

Esters : Fish ChV 0.009

Esters : Daphnid ChV 0.088

Esters : Green Algae ChV 0.077

Esters : Fish (SW) 96-hr LC50 0.329

Esters : Mysid 96-hr LC50 0.051

Esters : Fish (SW) ChV 0.093

Esters : Mysid (SW) ChV 0.00181

Esters : Earthworm 14-day LC50 253.805 *

Esters (phosphate) : Fish 96-hr LC50 0.332

Esters (phosphate) : Daphnid 48-hr LC50 0.000471

Esters (phosphate) : Green Algae 96-hr EC50 0.203

Esters (phosphate) : Fish ChV 0.005

Esters (phosphate) : Green Algae ChV 0.751
108

Esters (phosphate) : Fish (SW) 96-hr LC50 0.356

Esters (phosphate) : Mysid (SW) 96-hr LC50 0.002

Esters (phosphate) : Fish (SW) ChV 0.010

Esters (phosphate) : Mysid (SW) ChV 1.32e-005

=========================== ================== ======== ====== ==========

Neutral Organic SAR : Fish 96-hr LC50 0.111

(Baseline Toxicity) : Daphnid 48-hr LC50 0.089

 : Green Algae 96-hr EC50 0.278

 : Fish ChV 0.016

 : Daphnid ChV 0.023

 : Green Algae ChV 0.157

Note: * = asterisk designates: Chemical may not be soluble enough to

 measure this predicted effect. If the effect level exceeds the

 water solubility by 10X, typically no effects at saturation (NES)

 are reported.

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

Class Specific LogKow Cut-Offs

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

If the log Kow of the chemical is greater than the endpoint specific cut-offs

presented below, then no effects at saturation are expected for those endpoints.

Esters:

------

Maximum LogKow: 5.0 (Fish 96-hr LC50; Daphnid LC50, Mysid LC50)

Maximum LogKow: 6.0 (Earthworm LC50)

Maximum LogKow: 6.4 (Green Algae EC50)

Maximum LogKow: 8.0 (ChV)

Esters (phosphate):

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

Maximum LogKow: 5.0 (LC50)

Maximum LogKow: 6.4 (EC50)

Maximum LogKow: 8.0 (ChV)

Baseline Toxicity SAR Limitations:

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

Maximum LogKow: 5.0 (Fish 96-hr LC50; Daphnid LC50)

Maximum LogKow: 6.4 (Green Algae EC50)

Maximum LogKow: 8.0 (ChV)

ECOSAR run for 1-BSA

ECOSAR Version 1.11 Results Page

SMILES : S(O)(=O)(=O)CCCC

CHEM : 

CAS Num: 

ChemID1: 

MOL FOR: C4 H10 O3 S1 

MOL WT : 138.18

Log Kow: -0.908 (EPISuite Kowwin v1.68 Estimate)
109

Log Kow: (User Entered)

Log Kow: (PhysProp DB exp value - for comparison only)

Melt Pt: (User Entered for Wat Sol estimate)

Melt Pt: -15.20 (deg C, PhysProp DB exp value for Wat Sol est)

Wat Sol: 1E+006 (mg/L, EPISuite WSKowwin v1.43 Estimate)

Wat Sol: (User Entered)

Wat Sol: (PhysProp DB exp value)

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

Values used to Generate ECOSAR Profile

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

Log Kow: -0.908 (EPISuite Kowwin v1.68 Estimate)

Wat Sol: 1E+006 (mg/L, EPISuite WSKowwin v1.43 Estimate)

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

ECOSAR v1.11 Class-specific Estimations

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

******************************************************************************

| Not Related to an Existing ECOSAR Class Definition |

| |

| Estimates provided below use the Neutral Organics QSAR equations which |

| represent baseline toxicity potential (minimum toxicity) assuming a simple |

| non-polar narcosis model. Without empirical data on structurally similar |

| chemicals, it is uncertain if this substance will present significantly |

| higher toxicity above baseline estimates. |

******************************************************************************

 Predicted

ECOSAR Class Organism Duration End Pt mg/L (ppm)

=========================== ================== ======== ====== ==========

--> Acid moeity found: Predicted values multiplied by 10

Neutral Organics-acid : Fish 96-hr LC50 4.65e+005

Neutral Organics-acid : Daphnid 48-hr LC50 2.02e+005

Neutral Organics-acid : Green Algae 96-hr EC50 49493.461

Neutral Organics-acid : Fish ChV 33084.156

Neutral Organics-acid : Daphnid ChV 9315.006

Neutral Organics-acid : Green Algae ChV 7127.344

Neutral Organics-acid : Fish (SW) 96-hr LC50 5.75e+005

Neutral Organics-acid : Mysid 96-hr LC50 3.06e+006 *

Neutral Organics-acid : Fish (SW) ChV 10563.744

Neutral Organics-acid : Mysid (SW) ChV 6.27e+005

Neutral Organics-acid : Earthworm 14-day LC50 4811.211

Note: * = asterisk designates: Chemical may not be soluble enough to

 measure this predicted effect. If the effect level exceeds the

 water solubility by 10X, typically no effects at saturation (NES)

 are reported.

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

Class Specific LogKow Cut-Offs
110

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

If the log Kow of the chemical is greater than the endpoint specific cut-offs

presented below, then no effects at saturation are expected for those endpoints.

Neutral Organics:

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

Maximum LogKow: 5.0 (Fish 96-hr LC50; Daphnid LC50, Mysid LC50)

Maximum LogKow: 6.0 (Earthworm LC50)

Maximum LogKow: 6.4 (Green Algae EC50)

Maximum LogKow: 8.0 (ChV)
111

Appendix I. Example KABAM Inputs and Outputs
112
113
114
115

Appendix J. Aquatic Plant Toxicity Endpoints In the EFED ECOTOX Database

CHEMICAL PC_CODE MRID COMMONNAME TAXONOMIC %AI STUDY

TIME

DOSE

TYPE TGL TOXICITY TOXLEVEL

Naled 034401 42529603 Freshwater diatom Navicula pelliculosa 94.4 5 D EC50 0.012 PPM

Naled 034401 42529602 Marine diatom

Skeletonema 

costatum 94.4 5 D EC50 0.015 PPM

Coumaphos 036501 48322802 Green algae

Pseudokirchneriella 

subcapitata 95.7 96 hr EC50 > 0.091 PPM

Chlorpyrifos 059101 40228401 Marine algae Isochrysis galbana 92 96 hr EC50 0.14 PPM

Tribufos 074801 41618813 Green algae

Selenastrum 

capricornutum 99.9 7 D EC50 0.148 PPM

Chlorpyrifos 059101 40228401 Diatom Thalassiosira sp. 92 96 hr EC50 0.15 PPM

Coumaphos 036501 48322801 Duckweed Lemna gibba 95.7 7 D EC50 > 0.166 PPM

Chlorpyrifos 059101 40228401 Marine diatom

Skeletonema 

costatum 92 96 hr EC50 0.3 PPM

EPN 

(Cancelled in 

U.S.) 041801 40228401 Marine diatom

Skeletonema 

costatum 97.2 96 hr EC50 0.34 PPM

Tribufos 074801 40228401 Marine diatom

Skeletonema 

costatum 95 96 hr EC50 0.37 PPM

Naled 034401 42529604 Bluegreen algae Anabaena flos-aquae 94.4 5 D EC50 0.64 PPM

Tribufos 074801 45863705 Duckweed Lemna gibba 99.4 14 D EC50 1.1 PPM

Naled 034401 42529601 Duckweed Lemna gibba 94.4 14 D EC50 > 1.8 PPM

Fenthion 053301 40186713 Marine diatom

Skeletonema 

costatum 96.9 7 D EC50 0.36 PPM

Dicrotophos 035201 00140380 Phytoplankton N.R. 82 4 hr EC50 > 1.0 PPM

Dicrotophos 035201 00140380 Phytoplankton N.R. 82 4 hr EC50 > 1.0 PPM

Terbufos 105001 48939101 Marine diatom Skeletonema grethae 89.3 96 hr EC50 > 1.01 PPM

Fenthion 053301 40186711 Green algae

Selenastrum 

capricornutum 96.9 96 hr EC50 1.1 PPM

Fenthion 053301 40186715 Freshwater diatom Navicula pelliculosa 96.9 7 D EC50 1.1 PPM
116

CHEMICAL PC_CODE MRID COMMONNAME TAXONOMIC %AI STUDY

TIME

DOSE

TYPE TGL TOXICITY TOXLEVEL

Terbufos 105001 48689902 Green algae

Pseudokirchneriella 

subcapitata 89.3 96 hr EC50 > 1.85 PPM

Oxydemetonmethyl 058702 44657701 Green algaeScenedesmus 

subspicatus 51.3 96 hr EC50 > 100 PPM

Dichlorvos 084001 40228401 Green algae Dunaliella tertiolecta 98 48 hr EC50 > 100 PPM

Acephate 103301 48879501 Green algae

Pseudokirchneriella 

subcapitata 98.8 96 hr EC50 > 1035 PPM

Acephate 103301 48879503 Duckweed Lemna gibba 98.8 7 D EC50 1038 PPM

Dimethoate 035001 47784001 Green algae

Selenastrum 

capricornutum 96.7 72 Hr EC50 110 PPM

Dichlorvos 084001 40228401 Marine algae Isochrysis galbana 98 48 hr EC50 14 PPM

Dichlorvos 084001 40228401 Marine diatom

Thalassiosira 

pseudonana 98 48 hr EC50 17 PPM

Fenthion 053301 40186714 Duckweed Lemna gibba 96.9 14 D EC50 > 2.8 PPM

Naled 034401 42529605 Green algae

Selenastrum 

capricornutum 94.4 5 D EC50 20 PPM

Dichlorvos 084001 40228401 Marine diatom

Skeletonema 

costatum 98 48 hr EC50 28 PPM

Diazinon 057801 40509806 Green algae

Selenastrum 

capricornutum 87.7 7 D EC50 3.7 PPM

Fenthion 053301 40186712 Bluegreen algae Anabaena flos-aquae 96.9 7 D EC50 > 3.8 PPM

Terbufos 105001 48689901 Duckweed Lemna gibba 89.3 7 D EC50 > 4.20 PPM

Acephate 103301 40228401 Marine diatom

Skeletonema 

costatum 94 96 hr EC50 > 50 PPM