UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D. C. 20460

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION

ENVIRONMENTAL RISK BRANCH III

ENVIRONMENTAL FATE & EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

PC CODE: 060101

 DP BARCODE: 410836 

MEMORANDUM DATE: April 10, 2014

SUBJECT: EFED Risk Assessment for the Proposed New Uses of Thiabendazole as a Seed 

Treatment for Several Crop Groups, and an Increased Application Rate for 

Soybean Seed Treatment

TO: Tony Kish, Product Manager (RM 23)

Rosemary Kearns, Reviewer

Fungicide Branch

Registration Division (7505P)

FROM: Nick Mastrota, Ph.D., Wildlife Biologist

James K. Wolf, Ph.D., Environmental Scientist

Environmental Risk Branch III

Environmental Fate and Effects Division (7507P)

THRU: Dana Spatz, Branch Chief

James Hetrick, Ph.D., Senior Science Advisor

Melissa Panger, Ph.D., Senior Biologist

Rosanna Louie-Juzwiak, RAPL

Environmental Risk Branch III

Environmental Fate and Effects Division (7507P)

EFED has completed an assessment of the ecological exposure and potential risks associated 

with the proposed additional seed treatment uses of thiabendazole on root vegetables (except 

sugar beet) (Subgroup1B); onion, bulb (Subgroup 3-07A); brassica, head and stem (Subgroup 

5A); cucurbit (Group 9); alfalfa; spinach (including baby), and small grains cereals (wheat, 

barley, oats, rye, and triticale), as well as an increase in the maximum seed treatment rate for 

soybean. 

The ecological risk assessment concluded that all of the proposed new uses could pose a risk to 

mammals because of potential chronic toxic effects resulting from consumption of treated seeds

(risk quotients (RQ) of 1.1 to 44.2). All of the proposed new uses except soybean and some root 
2

vegetables (e.g., radish) may also pose a risk to birds because of chronic toxic effects (RQs 1.43 

to 11.5), although these RQs may overestimate risk because they were based on NOAECs from 

chronic avian tests that were conducted at dietary residue levels less than those predicted on

many of the treated seeds. These chronic avian studies set the NOAEC at the highest dietary

concentration tested, but testing at higher levels could establish higher NOAECs, which would 

decrease the RQ values. A few cucurbit uses (e.g., cucumbers and muskmelons) also may pose 

an acute risk to birds. For many of the new uses, RQs for acute toxicity exceed the endangered 

species level of concern for birds, but not for mammals. These conclusions are based on 

exposure through direct consumption of treated seeds. Species that do not consume seeds would 

have much less exposure and may not be at risk.

The proposed seed treatment uses are expected to result in very little exposure to aquatic 

organisms and nontarget terrestrial plants. Risk to all aquatic organisms and all terrestrial plants 

is expected to be minimal.

Available ecotoxicity data were adequate to assess the ecological risks associated with the 

proposed new seed treatment uses. However, it should be noted that information about avian 

acute oral toxicity to passerines (850.2100) and effects on avian reproduction (850.2300) at 

dietary concentrations above 400 mg a.i./kg is not available from the current thiabendazole 

ecotoxicity database.
3

ECOLOGICAL RISK ASSESSMENT FOR THE

Registration of New Uses for Thiabendazole

PC Code: 060101

Proposed New Seed –Treatment Use: 

 Root vegetables (except sugar beet), Subgroup1B

 Onion, bulb; Subgroup 3-07A

 Brassica, head and stem; Subgroup 5A

 Cucurbit, Group 9

 Alfalfa

 Spinach (including baby)

 Small grains cereals (wheat, barley, oats, rye, and triticale)

 Soybean (increased application rate)

End Use Product:

Mertect®340-F
4

Table	of	Contents

I. Problem Formulation ................................................................................................................5

A. Nature of Chemical Stressor ..............................................................................................5

B. Mode of Action ..................................................................................................................5

C. Structure and Physical-Chemical Characteristics ..............................................................5

D. Overview of Pesticide Usage .............................................................................................6

E. Uncertainties.......................................................................................................................8

II. Environmental Fate and Transport............................................................................................9

A. Transformation Products..................................................................................................10

B. Bioaccumulation...............................................................................................................11

III. Estimation of Exposure........................................................................................................11

A. Aquatic Exposure Modeling.............................................................................................11

B. Water Monitoring Data ....................................................................................................12

C. Terrestrial Exposure Modeling.........................................................................................12

1. Oral Exposure from Consumption of Treated Seed .....................................................12

2. LD50 per Square Foot Analysis.....................................................................................13

3. Exposure Modeling for Non-Target Plants ..................................................................14

IV. Ecological Effects Characterization.....................................................................................14

V. Risk Characterization..............................................................................................................17

A. Aquatic Organisms...........................................................................................................17

B. Terrestrial Organisms.......................................................................................................17

1. Oral Exposure from Consumption of Treated Seeds...................................................17

2. LD50 per Square Foot Analysis.....................................................................................22

3. Terrestrial Plants...........................................................................................................22

VI. Risk Discussion and Conclusions........................................................................................23

VII. References............................................................................................................................26

Appendix A. Thiabendazole degradates identified in Environmental Fate Studies ......................27

Appendix B. Thiabendazole and Its Environmental Degradates...................................................28

Appendix C. GENEEC2 Model Outputs.......................................................................................30

Appendix D. Output from Terrestrial Exposure Model (T-REX) .................................................31

Appendix E. Output from Terrestrial Plant Risk Model (TerrPlant).............................................33

Appendix F. Ecotoxicity Data........................................................................................................34
5

I. Problem	Formulation

A. Nature of Chemical Stressor

Thiabendazole is a fungicide used to control fungal diseases in germinating seeds. It is currently

registered for treatment of seeds of clover, corn, garbanzo (including chickpea), lentil, peas, and 

soybean, and treatment of seed pieces of potato and sweet potato. This ecological risk assessment 

evaluates expanding the crop group uses registered for the product Mertect®340-F Fungicide 

(EPA Reg. No. 100-889) to include the following crops and crop groups:

 Root vegetables (except sugar beet), Subgroup1B

 Onion, bulb; Subgroup 3-07A

 Brassica, head and stem; Subgroup 5A

 Cucurbit, Group 9

 Alfalfa

 Spinach (including baby)

 Small grains cereals (wheat, barley, oats, rye, and triticale)

In addition, the risk assessment evaluates the proposed increase of the maximum seed treatment 

application rate for soybean from 0.005 lb a.i./100 lb seed to 0.020 lb a.i./100 lb seed (or 0.0087 

lb a.i./A to 0.033 lb a.i./A).

B. Mode of Action

Thiabendazole is a systemic fungicide. The mode of action of thiabendazole inhibits the mitosis 

of plant pathogenic fungi species by preventing the polymerization of beta-tubulins. Because 

dimers of beta-tubulin and alpha-tubulin polymerize to form microtubule structures inside the 

cells, thiabendazole acts as an inhibitor of microtubuline polymerization, terminating cell 

division.

C. Structure and Physical-Chemical Characteristics

The CAS name for thiabendazole is 2-(4-Thiazolyl)-1H-benzimidazole, the CAS No. 148-79-8. 

The IUPAC name is 4-(1H-benzimidazol-2-yl)-1,3-thiazole. Figure 1 shows the chemical 

structure of thiabendazole. Select physical-chemical characteristics are provided for 

thiabendazole in Table 1.
6

Figure 1. Chemical Structure of Thiabendazole

Table 1. Physical and Chemical Properties of Thiabendazole

Parameter Value Description Source

(MRID #)

Selected Physical/Chemical Parameters

Molecular mass 201.1 g/mol -- (Calculated)

Molecular formula C10H7N3S --

Vapor pressure (25°C) 3.45 x 10-9 mm Hg 

4.0 x 10-9 mm Hg

-- 48354601

Thiabendazole 

RED (2002)

Henry’s Law Constant (25°C) 3.26 x10-11 atm-m

3

/mol -- (Calculated)

Dissociation Constants pKa1 = 4.5

pKa2 = 12.01

-- 48354604

40947302

Aqueous solubility Buffered solution (25C) at 

pH 5: 30 mg/L

pH 7: 28 mg/L

pH 9: 28 mg/L

Water: 22.74 mg/L

Buffer solution (20C) at:

pH4: 79.82 mg/L

pH7: 22.77 mg/L

pH10: 23.75 mg/L

-- 40947301

48354601

48354604

Log octanol-to-water partition 

coefficient (log KOW)

1.61 @ Room temperature = 16.6°C 48254601

48354604

a Transformation rates of thiabendazole: pH 5 (-0.0019 days-1

); pH 7 (-0.0035 days-1); pH 9 (-0.0026 days-1)

D. Overview of Pesticide Usage

Mertect®340-F (42.3% thiabendazole) is applied to crop seeds as a water-based slurry using 

standard seed treatment equipment. Treated seeds may be planted at any time of the year. Only 

one application (i.e., one planting of treated seed) may be made per crop cycle; however, in some 
7

regions, more than one may occur per year. The maximum seed treatment rates defined on the 

label are given in Table 2.

Table 2. Maximum Application Rates for Proposed New Seed Treatment Uses of Thiabendazole

Crop or Crop Group Crop Group ID

Max. Seed Treatment Rate

mg a.i./seed fl oz product per 

100 lb seed

lb a.i. per

100 lb seed

Alfalfa NA 0.004 5.8 0.18578

Cereals, Small Graina

 Barley NA 0.03 2.6 0.08328

 Oat NA 0.03 3.4 0.10891

 Rye NA 0.03 3.2 0.1025

 Triticale NA 0.03 2.6 0.08328

 Wheat NA 0.03 3.1 0.09930

Brassica, head and stem Subgroup 5A 0.002 NDb ND

Cucurbit Group 9 0.1 ND ND

Onion, bulb Subgroup 3-07A 0.002 ND ND

Root vegetables 

(except sugar beet) Subgroup 1B 0.002 ND ND

Spinach NA 0.006 ND ND

a

“Small grains” is not designated as an EPA crop group.

b

 “ND” designates that the value is not defined by the product label.

In addition to adding these new uses, the proposed label for Mertect®340-F Fungicide would 

increase the maximum seed treatment rate for soybean (for control of Fusarium spp. fungal 

diseases) by approximately four times. The maximum seed treatment rate for soybeans would 

increase from 0.0052 lb a.i./100 lb seed to 0.020 lb a.i./100 lb seed. Assuming a maximum 

seeding rate of 166.7 soybean seeds per acre (USEPA, 2011), the maximum rate for soybean seed 

treatment would change from 0.0087 lb a.i./A to 0.033 lb a.i./A.

The seed treatment rates provided on the label define the maximum amount of active ingredient 

that can be applied per seed. To convert this to rates in terms of pounds of active ingredient per 

acre, these seed treatment rates need to be multiplied by the seeding rates (number of seeds per 

acre). Maximum seeding rates for a variety of crops were defined by the Biological and 

Economical Analysis Division (USEPA, 2011). These values are used in the conversion 

calculations and the resulting application rates (in lb a.i./A) are shown in Table 3. It is EFED’s 

understanding that the registrant of Mertect®340-F (Syngenta Crop Protection) has agreed to put 

on the label a statement which would restrict the maximum allowable annual application rate for 

any seed treatment to 0.15 lb a.i./A per year. This is similar to the maximum single application 

rate on the current label for wheat (assuming the maximum seeding rate for wheat proposed by 

Syngenta), whereas the maximum single rates for other seed treatment crops is expected to be less 

than this. Therefore, this risk assessment will use the calculated rates shown in Table 3 as the 

maximum single application rate, and 0.15 lb a.i./A as the maximum annual rate for all crops.
8

Table 3. Conversion of Proposed Seed Treatment Rates to Pounds of Active Ingredient per Acre

Crop Group

Max. Seed Treatment Rate Representative 

Crops Seeding Rates

(# seeds/acre)

b

Max. Appl. Rate

(lbs a.i./acre)

c fl. oz. product/

100 lb seeda mg a.i./seeda

Alfalfa 5.8 0.004 Alfalfa 3,405,000 0.030

Cereals (small 

grains)

2.6 0.03 Barley 1,222,000 to 

1,300,000

0.086

3.4 0.03 Oats 1,305,000 0.0863

3.2 0.03 Rye 1,962,000 0.130

2.6 0.03 Triticale 1,744,000 0.115

3.1 0.03 Wheat

Wheat, winter

2,250,000d

600,000 to 1,350,000

0.149

0.089

Brassica, head and 

stem

- 0.002 Broccoli 8,297 to 210,845 0.00093

Cabbage 7,260 to 98,010 0.00043

Bulb Vegetables - 0.002 Onion 87,120 to 1,254,528 0.0055

Garlic 261,360-392,040 0.0017

Cucurbit 

Vegetables

- 0.1 Cucumber 4,840 to 139,392 0.031

Muskmelon 2,420 to 34,848 0.0077

Pumpkin 908 to 7,260 0.0016

Squash, summer 1,452 to 15,374 0.0034

Squash, winter 2,723 to 7,260 0.0016

Watermelon 605 to 4,356 0.00096

Spinach - 0.006 196,020 to 653,400 0.0086

Root Vegetables 

(except sugarbeet)

- 0.002 Beet, garden 261,360 to 2,090,880 0.0092

Carrot 104,544 to 2,090,880 0.0092

Horseradish 9,680 to 10,249 0.00005

Radish 418,176 to 1,045,440 0.0046

Parsley 52,272 to 1,568,160 0.0069

Parsnip 87,120 to 435,600 0.0019

Turnip 29,040 to 261,360 0.0012

Soybeans 1.05 0.0302 Soybean 50,000 to 250,000 0.017

a Proposed label MERTEC 340-F 889 MAS 0313 HILITE –dt/df 3/15/13 (Density = 4.1 lb a.i./gallon of product; 42.3 % active 

ingredient (a.i.).[e-submission #4085; MRID 4907800]

b USEPA, 2011.

c The maximum application rates were calculated based on the maximum expected seeding rates as follows: mg a.i./seed * 

seeds/acre * (1 lb/453.59 g) * 1 g/1000 mg = lb a.i./acre.

d Conservative maximum seeding rate proposed by Syngenta.

E. Uncertainties

The assessment assumes that 100 percent of the a.i. comes off the seed and is available for runoff 

or leaching (EFED, 19991). Thus, the amount of “a.i.” in runoff and leachate are expected to be 

conservative. 

 

1 EFED, 1999. Standard Method of Determining the Leachability of pesticides from treated seeds. July 30, 1999, USEPA, OPP, EFED, 

Arlington, VA.
9

The avian acute toxicity studies yielded non-definitive “greater than” values for the acute oral 

LC50 and the dietary LC50. These data were adequate for concluding that the acute risk is below 

the LOC for risk to nonlisted species, but in some cases are not adequate for concluding that the 

acute risk is below the LOC for endangered species.

For chronic risk, the avian reproduction studies with the bobwhite (MRID 00123327) and the 

mallard (MRID 00123328) tested only up to 400 mg/kg-diet. No statistically significant effects 

were observed at the highest test concentration. The concentrations of thiabendazole on treated 

seeds of some crops are expected to be much greater than this amount, up to 4590 mg/kg-diet for 

muskmelon seeds. Therefore, these results are not adequate for assessing chronic risk to seedeating birds. Without additional testing at higher test levels, the assessment must calculate RQ’s 

based on the NOAEC equal to the highest concentration tested (400 mg a.i./kg-diet), which yield 

chronic avian RQs that exceed the LOC.

Tier 1 terrestrial plant toxicity data for thiabendazole failed to establish an NOAEL for some plant 

species. However, the data do show that thiabendazole has relatively low toxicity to terrestrial 

plants. Exposure to the relatively high rate of 0.23 lb a.i./A (which is higher than the maximum 

allowable seasonal application rate of 0.15 lb ai/A) resulted in growth inhibition of no more than 

25%. Likewise, aquatic plant tests with thiabendazole are supplemental, but are adequate to show 

that it has low toxicity. Considering that seed treatment uses result in very little exposure to 

nontarget terrestrial and aquatic plants, additional testing with plants is not warranted.

Toxicity data are not available for saltwater animals except for one acute test with the saltwater 

mysid. However, seed treatment use of thiabendazole is expected to result in little exposure to 

aquatic habitats, and based on toxicity data for freshwater species, risk to aquatic species is 

expected to be minimal. Furthermore, the mysid study indicates that the toxicity of thiabendazole 

to saltwater species is similar to that of freshwater species. Therefore, additional testing with 

saltwater species is not needed to assess these proposed uses.

II. Environmental	Fate	and	Transport	

There have been no additional environmental fate data submitted since the previous assessments. 

The environmental fate studies show that thiabendazole is persistent and immobile (Table 4). 

Thiabendazole is stable to hydrolysis and soil photolysis and degrades slowly in aerobic (half-life 

~ 700 days) and anaerobic soil environments (>200 days). Aquatic photolysis, with a half life of 

1.2 days in clear water, appears to be the only significant degradation pathway. Under anaerobic 

aquatic conditions thiabendazole showed no evidence of abiotic or biotic degradation but moved 

rapidly from water phase to the sediment phase. Thiabendazole binds tightly to soil due to its high 

soil/water partitioning coefficients (Kocs > 1,000 mL/goc), thus limiting the amount available for 

leaching into ground water and for solution phase runoff into surface water. With the high 

sorption capacity, thiabendazole would be transported on eroded sediment. Sorption was weakly 

correlated to organic carbon. Terrestrial field dissipation studies showed thiabendazole to be 

extremely persistent with no leaching. Vapor pressure and Henry’s Constant indicate that there is 

a low potential for thiabendazole to volatilize from soil and water. These studies are discussed in 

more detail in the most recent Reregistration Eligibility Decision (RED) (DP Barcode D245780). 
10

There are no degradation products that are expected to form at levels that are of ecological 

concern.

Table 4. Environmental Fate Parameters of Thiabendazole

Parameter Value Description Source

(MRID #)

Persistence

Hydrolysis half-life (22C) Stable @ pHs 5, 7, 9 Ratesa

of transformation were 

extrapolated resulting in t1⁄2s (200 

days) far beyond the length of the 

study (30 days)

41265301

(D245780; 

02/04/99)

Aqueous photolysis half-life (22.5 to 

23.2C)

1.2 days 96 hour study @ pH5 43328305

Soil photolysis half-life (25C) Stable -- 41297301

41397302

Aerobic soil metabolism half-life 

(25°C)

688 days EFED Registration Document 

D245780

41791201

Anaerobic soil metabolism half-life 

(25°C)

211 day Only degradate detected was 

benzimidazole in a 90 day study.

(max 13.7 % AR;).

41559601 

Anaerobic aquatic metabolism half-life No evidence of biotic or 

abiotic degradation.

Rapidly moved from water phase to 

the sediment phase.

122888

070753

Aerobic aquatic metabolism half-life 

[dissipation half-life A] (25C)

-- Study requirement waived 41559601

(D169687; 

12/30/92)

Mobility

Freundlich soil-water partition 

coefficients (KF); organic carbonnormalized Freundlich coefficients 

(KFOC)

KF (mL/g)/ Koc(mL/goc):

(soil textural class)

2.49/1,104: (sand)

15.97/3,992: (sandy loam)

21.75/1,812: (silt loam)

269.6/22,467: (clay)

Mean = 7,343 mL/goc

Median = 2,902 mL/goc

41170102

Field Dissipation

Terrestrial field dissipation half-life in 

upper 6 inch soil layer; (max. depth of 

observed leaching)

1100 days; (6 inches).

990 and 1444 days; (12 

inches)

833 and 1093 days: (6 

inches).

Winter wheat/loamy sand/WA

Soybean and bare ground/sandy 

loam/IL

Soybean and bare ground/sandy 

loam/GA

43187201

43187202

43187201

Octanol/Water Partition Coefficient 

(logPow)

1.61 -- 48354604

a Transformation rates of thiabendazole: pH 5 (-0.0019 days-1

); pH 7 (-0.0035 days-1); pH 9 (-0.0026 days-1)

A. Transformation Products

Three degradates were identified in the aqueous photolysis study (benzimidazole-2-carboxamide 

as a major degradate; benzimidazole and benzimidazole-2-carboxylic acid as minor degradates), 

two minor degradates were identified in the aerobic soil metabolism study (benzimidazole and 5-

hydroxythiabendazole), and one major degradate was identified in the anaerobic soil metabolism 
11

study (benzimidazole) (Appendix A). The metabolite 5-hydroxythiabendazle and benzimidazole 

are included in the tolerance dietary exposure for food (40 CFR 180.2422), but not included in

drinking water. The common name, chemical names, CAS No., molecular formula and mass, 

chemical structure, and maximum and final percent of applied radioactivity (AR) of the major 

degradates are summarized in Appendix B.

B. Bioaccumulation 

A BCF study with the rainbow trout measured a BCF value 86.5 (whole); 19.7 (fillet), 747 

(viscera). Three-day depuration was 96% in viscera and 50% in edible tissue (MRIDs 41631801; 

42022701). These results indicate that the potential of thiabendazole to bioconcentrate is low.

III. Estimation	of	Exposure	

A. Aquatic Exposure Modeling 

This exposure assessment was completed using the GENEEC model. Exposure was estimated for 

a seed treatment application at 0.15 lb ai/A, which is to be the maximum annual application rate 

allowed on the Mertect®340-F label for any seed treatment use. Model input parameters used are 

given in Table 4. A description of this model can be found on the EPA’s website at:

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

Table 4. Summary of Input Parameter Values for Modeling Aquatic Exposure of Thiabendazole

Using the GENEEC Model

Environmental Fate 

Parameter Valuea

Justification Source

Solubility 28 ppm (mg/L) - MRID 40947301

Adsorption Coefficient (Koc) 7,343 mL/goc Average Koc MRID 41170102

Aerobic Soil Metabolism

(ASM) 2064 days 3 x single ASM half-life MRID 41791201

Aerobic Aquatic Metabolism

(Days) Stable (0) No Data and Persistence observed in ASM No Data

Aqueous Photolysis 1.2 days - MRID 43328305

Hydrolysis (@pH7) Stable (0) - MRID 41265301

GENEEC Management Options

Wetted-In No Not specified on label Label

Method of Application Granular

It is assumed there is no 

spray drift for treated seed 

and granular is the nondrift option in GENEEC 

-

 

2

http://www.gpo.gov/fdsys/pkg/CFR-2005-title40-vol23/xml/CFR-2005-title40-vol23-sec180-242.xml
12

Environmental Fate 

Parameter Valuea

Justification Source

Incorporation Depth 1 and 2 inch Range of planting depths 

for a variety of cropsb,c.

-

a

http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm

b http://www.channel.com/Agronomics/Pages/Plant-Soybeans-to-the-Right-Depth.aspx

c http://www.omafra.gov.on.ca/english/crops/pub811/4planting.htm

Maximum generic estimated environmental concentrations (EECs) for seed-treatment use at 0.15 

lb a.i./A are given in Table 5. These values represent the upper bound of concentrations expected 

for the proposed new use on the label. This modeling indicates that peak EECs for all uses are 

expected to be less than 1.0 μg a.i./L (1 ppb). A representative GENEEC model output is listed in 

Appendix C.

Table 5. Generic aquatic estimated environmental concentrations (EECs) of thiabendazole for

proposed maximum annual application rate of 0.15 lb a.i./A and two seeding depths.

Crop Scenario

Aquatic EECs (μg a.i./L)

Peak 21-Day Mean 60-Day Mean

All crops, 

0.15 lb a.i./A

Generic granular/seed 

treatment scenario with 1-

inch soil incorporation

0.88 0.73 0.53

Generic granular/seed 

treatment scenario with 2-

inch soil incorporation

0.44 0.37 0.27

B. Water Monitoring Data

There are no detections of thiabendazole reported in the California Department of Pesticide 

Regulation (CDPR) Surface Water3 Monitoring Database available and it is not included in CDPR 

Groundwater Protection List4. Thiabendazole is not included in the surface water database 

collected in the USGS National Water Quality Assessment (NAWQA)5Program. There were no 

detections of thiabendazole reported in groundwater (49 samples from CA, DE, FL,MD, NJ) in 

USGS NAWQA6data warehouse with limits of detection <0.06 μg/L. 

C. Terrestrial Exposure Modeling

1. Oral	Exposure	from	Consumption	of	Treated	Seed

Oral exposure from use of thiabendazole as seed treatment was estimated by modeling the dose to 

birds and mammals that feed directly on the treated seeds or seed pieces. EPA’s T-REX program 

(version 5.1) was used to predict the dose of pesticide that would be ingested by birds and 

mammals of various sizes. A sample of the T-REX model output is provided in Appendix D. 

 

3

http://www.cdpr.ca.gov/docs/emon/surfwtr/index.htm; searched January 27, 2014.

4

http://www.cdpr.ca.gov/docs/legbills/calcode/040101.htm#a6800; searched January 27, 2014. 

5

http://infotrek.er.usgs.gov/nawqa_queries/jsp/swmaster.jsp; searched January 27, 2014.

6

http://infotrek.er.usgs.gov/nawqa_queries/jsp/gwmaster.jsp; search January 27, 2014.
13

These predictions are calculated based on average daily food intake and the concentration of 

active ingredient on a treated seed. The model makes the assumption that 100% of the animal’s 

diet is comprised of treated seed. Maximum concentrations on seeds were calculated by dividing 

the maximum seed treatment rate (mg a.i./seed) by the minimum seed weights (mg/seed). The 

minimum seed weights were derived from the maximum seeds per pound counts reported by the 

Biological and Economical Analysis Division (BEAD) (USEPA, 2011). Input values for this 

analysis are presented in Table 6.

Table 6. Input values for the analysis of risk to terrestrial animals from consumption of 

thiabendazole-treated seeds.

Crop Group

Seed Treatment Rate

Representative 

Crops

Max. No. of Seeds 

per Pound

Max Seed 

Concentration

(mg a.i./seed)

fl. oz. 

product/100 lb 

seed

mg a.i./seed

Alfalfa 5.8 0.004 Alfalfa 227,000 2,002

Cereals (small 

grains)

2.6 0.03 Barley 14,000 926

3.4 0.03 Oats 18,144 1,200

3.2 0.03 Rye 18,000 1,190

2.6 0.03 Triticale 22,680 1,500

3.1 0.03 Wheat 18,000 1,190

Brassica, head and 

stem

- 0.002 Broccoli 150,000 661

Cabbage 165,000 728

Bulb Vegetables - 0.002 Onion 130,000 573

Cucurbit Vegetables - 0.1 Cucumber 18144 4,000

Muskmelon 20,800 4,586

Pumpkin 6,400 1,411

Squash, summer 6,400 1,411

Squash, winter 6,400 1,411

Watermelon 9,600 2,116

Spinach - 0.006 Spinach 45,360 600

Root Vegetables 

(except sugarbeet)

- 0.002 Carrot 400,000 1,764

Radish 50,000 220

Parsley 296,500 1,307

Turnip 167,000 736

Soybeans 0.624 0.0302 Soybean 3,600 240

2. LD50 per	Square	Foot	Analysis

In addition to exposure through a dietary route, risk to terrestrial animals was assessed using the 

LD50-per-square-foot risk index. This analysis does not assume any explicit exposure route, but 

rather relates the amount of active ingredient available to an animal within one square foot of 

treated habitat to the acute oral LD50. The LD50-per-square-foot is considered a proxy for an RQ. 

For seed treatment uses of thiabendazole, the maximum application rate in terms of lb a.i./A were

calculated by multiplying the maximum allowable seed treatment rate (i.e., amount of a.i. per 

seed) by the maximum expected seeding rates (seeds per acre) expected for each crop (Table 3).
14

Seeding rates were assumed to be at the maximum of the range of seeding rates provided by the 

BEAD (USEPA, 2011). As a conservative screen, this analysis assumed no soil incorporation of 

the seed. It is recognize, however, that most seeds will be soil incorporated, which would reduce 

the exposure to terrestrial animals. The amount of AI available on one square foot of ground 

planted with treated seeds were then divided by the body-weight adjusted LD50 for small, 

medium, and large birds and mammals. Equations for these calculations are given in the User 

Manual for the T-REX Mode (USEPA, 2012).

3. Exposure	Modeling	for	Non-Target	Plants

Exposure to terrestrial and semi-aquatic plants was estimated using the OPP’s TerrPlant model

(version 1.2.2). Sample output from the TerrPlant model is provided in Appendix E. The 

application rate was assumed to be 0.15 lb a.i./A, the maximum annual application rate allowed 

on the proposed Mertect®340-F label. Seeds were assumed to be planted at a depth of 1 inch. 

Because the new uses are seed treatments, the spray drift fraction was set at zero. The predicted 

terrestrial plant EECs for the new seed treatment uses are summarized in Table 7. 

Table 7. EECs for Thiabendazole Seed Treatment Uses with an Application Rate of 0.15 lb a.i./A

Exposure Description EEC (lb a.i./A)

Runoff to dry areas 0.0075

Runoff to semi-aquatic areas 0.075

Spray drift 0

Total for dry areas 0.0075

Total for semi-aquatic areas 0.075

IV. Ecological Effects Characterization

Table 8 provides a summary of the toxicity of thiabendazole to aquatic organisms. Thiabendazole 

is highly toxic to fish and aquatic invertebrates on an acute basis (LC50s 0.1-1.0 mg a.i./L). With 

chronic exposure, the Daphnia magna was the most sensitive species, with chronic effects 

observed at concentrations as low as 87 μg a.i./L (NOAEL: 42 μg a.i./L). Table 9 provides a 

summary of the toxicity of thiabendazole to terrestrial animals. On an acute basis, thiabendazole 

is classified as practically nontoxic to birds, mammals, and the honey bee. While the avian acute 

oral tests with the bobwhite and mallard determined the LD50s were greater than the highest test 

dose of 4640 mg ai/kg-bw, one mortality and some sublethal toxic effects were observed in the 

test with the mallard. One duck died on day 2 at 4640 mg ai/kg-bw, and lethargy, lower limb 

weakness, and loss of coordination was observed in ducks at this level during the first 5 days after 

exposure (MRID 00123313). In the avian dietary tests with the bobwhite and mallard, no 

mortality was observed at any test level, but reduced food consumption and reduced body weight 

gain were observed in the bobwhite test at 5620 mg ai/kg-bw (MRID 41025003) and in the 

mallard test at 3160-5620 mg ai/kg-bw (MRID 41025004). No reproductive or developmental 

effects were observed in chronic toxicity studies with birds at a concentration of 400 mg a.i./kgdiet. For chronic toxicity to mammals, a two-generation rat reproductive study established the 

NOAEL at 10 mg a.i./kg/day based on reduction in body weight gain and food consumption at the 
15

next higher concentration of 30 mg a.i./kg/day (MRID 43190301). This NOAEL is roughly

equivalent to a dietary dose of approximately 200 mg a.i./kg-diet. Previous ecological risk 

assessments used a higher NOAEC of 1800 mg ai/kg-diet, but that was based only on the lack of 

reproductive effects observed in this study. Other studies also have noted sublethal toxic effects at 

dietary exposures at or near 600 mg a.i./kg-diet, including liver hypertrophy (MRID 43593201) 

and histopathological changes in bone marrow, liver, and thyroid (MRID 42942802). In a 

developmental rat study, a growth effect was also observed in developing young (decreased fetal 

body weight) at the slightly higher dietary concentration of 800 mg ai/kg-diet (MRID 42942803). 

Table 8. Toxicity of Thiabendazole to Aquatic Organisms

Taxonomic 

Group Test Species Principal Acute Toxicity Endpoint

Acute Toxicity 

Categories

Principal Chronic Toxicity 

Endpoint

Freshwater Fish Rainbow Trout

(Oncorhynchus 

mykiss)

96-hr LC50 = 0.56 ppm 

a.i.

Highly toxic NOAEC: 0.11 ppm a.i.

LOAEC: 0.23 ppm a.i.

(Reduced weight)

Freshwater 

Invertebrates

Water Flea 

(Daphnia magna)

48-hr EC50 = 0.31 ppm a.i. Highly toxic NOAEL: 0.042 ppm a.i.

LOAEL: 0.087 ppm a.i.

(Reduced survival and number 

of young per female)

Marine/

Estuarine 

Invertebrates

Mysid 

(Americamysis 

bahia)

96-hr EC50 = 0.34 ppm a.i.

(acceptable)

Highly toxic No Data Available

Aquatic 

nonvascular 

plants

Duckweed 

(Lemna gibba)

7-day EC50 = 2.32 ppm a.i.

NOAEC: 0.26 ppm a.i.

(supplemental)

NA1 NA 2

Aquatic vascular 

plants

Freshwater Green 

Algae 

(Pseudokirchneriella 

subcapitata)

EC50 = 3.06 ppm a.i.

NOAEC: 0.532 ppm a.i.

(supplemental)

NA1 NA 2

1. This item is not applicable (NA) because the EPA does not assign acute toxicity characterizations to aquatic plant 

toxicity endpoints.

2. This item is not applicable (NA) because the EPA does not characterize aquatic plant toxicity tests as being 

“acute” or “chronic.”

Table 9. Toxicity of Thiabendazole to Terrestrial Animals

Taxonomic 

Group Test Species Principal Acute Toxicity Endpoint

Acute 

Toxicity 

Categories

Principal Chronic Toxicity 

Endpoint

Birds Bobwhite

(Colinus virginianus)

and mallard (Anas 

platyrhynchos)

Acute oral LD50 > 4,460

mg a.i./kg-bw

(One mortality and sublethal 

effects, see text)

Acute dietary LC50 > 5,620

mg a.i./kg-diet

(No mortality but sublethal 

effects, see text)

Practically 

nontoxic

NOAEC: 400 ppm a.i.

LOAEC: >400 ppm a.i.

(No observed toxic effects)
16

Mammals Laboratory rat

(Mus musculus)

LD50 = 5,070 mg a.i./kg (M)

LD50 = 4,734 mg a.i./kg (F)

Practically 

nontoxic

NOAEC = 

 10 mg a.i./kg/day

LOAEL = 

 30 mg a.i./kg/day

(Reduced bodyweight gain 

and food consumption)

Honey Bee Honey bee 

(Apis mellifera)

Acute Contact LD50 > 34 μg 

a.i./bee

Practically 

nontoxic

NA

1. This item is not applicable (NA) because the Agency does not assign toxicity characterizations to terrestrial plant 

toxicity endpoints.

2. This item is not applicable (NA) because there no guideline terrestrial plant chronic toxicity studies.

Table 10 provides a summary of the toxicity of thiabendazole to terrestrial plants. Information on 

the phytotoxicity of thiabendazole is limited because only tier-1 terrestrial plant toxicity tests are 

available. Based on these studies, the EC25 values were determined to be equal to or greater than 

the tested rate of 0.23 lb a.i./A, which is greater than the maximum annual application rate 

allowed on the label (0.23 lb a.i./A). However, statistically significant adverse effects were 

observed at this test level in both the seedling emergence study and the vegetative vigor study. In 

the seedling emergence study, cucumber exhibited significant reduction in emergence and 

survival (22% reduction compared to the control) at 0.23 lb a.i./A (MRID 48996306). In the 

vegetative vigor study, significant inhibition was observed in growth, as measured by shoot 

length, in ryegrass (9% inhibition), tomato (11% inhibition), and soybean (13% inhibition). 

Therefore, these studies did not determine NOAEL values for toxicity of thiabendazole to 

terrestrial plants.

The toxicity data given in these tables represent the most sensitive toxicity endpoints available

from acceptable studies. A complete listing of all available toxicity data for thiabendazole can be 

found in Appendix F

Table 10. Toxicity of Thiabendazole to Terrestrial Plants

Taxonomic 

Group

Study Type Surrogate 

Species

Toxicity Value

(all units in terms of measured 

active ingredient)

Source 

Terrestrial 

plants

Seedling 

Emergence

Monocot* NOAEL = 0.23 lb/A

EC25 > 0.23 lb/A 48996306 (Acceptable, 

except Supplemental for 

Dicot bean and soybean) –

cucumber

NOAEL < 0.23 lb/A

EC25 ≥ 0.23 lb/A

Endpoint: emergence and 

survival

Vegetative 

Vigor

Monocot –

Ryegrass

NOAEL < 0.23 lb/A

EC25 > 0.23 lb/A

Endpoint: shoot length

48996307 (Acceptable)

Dicot* NOAEL = 0.23 lb/A

EC25 > 0.23 lb/A

* Because no significant inhibition was observed in any test species, the most sensitive species was not determined in 

this study.
17

V. Risk	Characterization

A. Aquatic Organisms

For aquatic risk, a screening-level assessment was conducted by calculating risk quotients for the 

maximum annual application rate for all proposed seed treatment crops, which the label restricts 

to 0.15 lb ai/A. At this application rate, none of the acute or chronic RQs exceed the acute or 

chronic LOC (Table 11). While data are not available to assess acute or chronic risk to saltwater 

fish, or chronic risk to saltwater invertebrates, the very low aquatic RQs for freshwater species 

suggest that risk to saltwater species is unlikely. Therefore, all of the proposed new seed treatment 

uses of thiabendazole are presumed to pose minimal risk to all aquatic plants and animals.

Table 11. RQs for Aquatic Organisms Exposed to Thiabendazole from the Proposed New Seed 

Treatment Uses

Risk Type Taxa EECa

(μg a.i./L)

Toxicity

(μg a.i./L) RQ

Aquatic Animals

Acute Freshwater Fish 0.88 560 <0.01

Freshwater Invertebrate 0.88 310 <0.01

Saltwater Invertebrate 0.88 340 <0.01

Chronic Freshwater Fish 0.53 110 <0.01

Freshwater Invertebrate 0.73 42 0.017

Saltwater Invertebrate 0.73 No Data --

Aquatic Plants

Non-listed 

Species

Vascular Plants 0.88 2,320 <0.01

Non-vascular Plants 0.88 3,060 <0.01

Listed Species Vascular Plants 0.88 260 <0.01

Non-vascular Plants 0.88 532 <0.01

a

From Table 5.

B. Terrestrial Organisms

1. Oral	Exposure	from Consumption	of	Treated	Seeds

a) Birds

Acute RQs for risk to birds are given in 
18

Table 13. The values shown were calculated using the nondefinitive “greater than” toxicity values 

(i.e., the highest level tested in the acute studies). These RQs are conservative because if the 

actual toxicity levels were determined, the RQs would be less than the values shown. For all 

crops except two cucurbit crops, cucumbers and muskmelons, the conservative RQs indicate that 

the acute risk is below the LOC for nonendangered species. For cucumber and muskmelons, the 

acute risk to nonendangered species is uncertain because the conservative RQs slightly exceed the 

acute risk LOC of 0.5 None of the acute dose-based RQs for non-endangered species exceed the 

LOC. Most of the uses had one or more conservative RQ that exceed the LOC for threatened and 

endangered species (0.10). Again, however, acute risk for listed species is uncertain since the 

actual RQs would be less than the values shown. Only radish, and soybean had RQs which were 

all below the endangered species LOC.

Table 12. Avian Acute RQs for the Proposed New Uses of Thiabendazole

Crop Group

Seed Treatment Rate

Representative 

Crops

Max Seed 

Concentration

(mg a.i./seed)

DietaryBased

RQ

Dose-Based Acute RQ

fl. oz. 

product/

100 lb 

seeda

mg 

a.i./seeda Small Med. Large

Alfalfa 5.8 0.004 Alfalfa 2,002 0.36* 0.15* 0.07 0.02

Cereals (small 

grains)

2.6 0.03 Barley 926 0.16* 0.07 0.03 0.01

3.4 0.03 Oats 1,200 0.21* 0.09 0.04 0.01

3.2 0.03 Rye 1,190 0.21* 0.09 0.04 0.01

2.6 0.03 Triticale 1,500 0.27* 0.11* 0.05 0.02

3.1 0.03 Wheat 1,190 0.21* 0.09 0.04 0.01

Brassica, head 

and stem

- 0.002 Broccoli 661 0.12* 0.05 0.02 0.01

Cabbage 728 0.13* 0.06 0.02 0.01

Bulb 

Vegetables

- 0.002 Onion 573 0.10* 0.04 0.02 0.01

Cucurbit 

Vegetables

- 0.1 Cucumber 4,000 0.71** 0.30* 0.14* 0.04

Muskmelon 4,586 0.82** 0.35* 0.16* 0.05

Pumpkin 1,411 0.25* 0.11* 0.05 0.02

Squash, summer 1,411 0.25* 0.11* 0.05 0.02

Squash, winter 1,411 0.25* 0.11* 0.05 0.02

Watermelon 2,116 0.38* 0.16* 0.07 0.02

Spinach - 0.006 Spinach 600 0.11* 0.05 0.02 0.01

Root 

Vegetables 

(except 

sugarbeet)

- 0.002 Carrot 1,764 0.31* 0.13* 0.06 0.02

Radish 220 0.04 0.02 0.01 <0.01

Parsley 1,307 0.23* 0.10* 0.04 0.01

Turnip 736 0.13* 0.06 0.02 0.01

Soybeans 0.624 0.0302 Soybean 240 0.04 0.02 0.01 <0.01

 * RQ exceeds the LOC for presumption of acute risk to threatened and endangered species.

** RQ exceeds the LOC for presumption of acute risk to non-endangered species. 

RQs for chronic risk to birds (
19

Table 13) are considerably higher than the acute RQs. The chronic RQ exceeds the LOC (1.0) for 

chronic risk for all crops evaluated except for radish and soybean. For crops which exceeded the 

LOC, chronic RQs generally ranged from approximately 1.5 to 5.0, but were 10 or more for 

cucumber and muskmelons.
20

Table 13. Avian Chronic RQs for the Proposed New Uses of Thiabendazole

Crop Group

Seed Treatment Rate

Representative 

Crops

Max Seed 

Concentration

(mg a.i./seed)

fl. oz. product/ Chronic RQ

100 lb seeda

mg 

a.i./seeda

Alfalfa 5.8 0.004 Alfalfa 2,002 5.00*

Cereals (small 

grains)

2.6 0.03 Barley 926 2.31*

3.4 0.03 Oats 1,200 3.00*

3.2 0.03 Rye 1,190 2.98*

2.6 0.03 Triticale 1,500 3.75*

3.1 0.03 Wheat 1,190 2.98*

Brassica, head 

and stem

- 0.002 Broccoli 661 1.65*

Cabbage 728 1.82*

Bulb Vegetables - 0.002 Onion 573 1.43*

Cucurbit 

Vegetables

- 0.1 Cucumber 4,000 10.00*

Muskmelon 4,586 11.46*

Pumpkin 1,411 3.53*

Squash, summer 1,411 3.53*

Squash, winter 1,411 3.53*

Watermelon 2,116 5.29*

Spinach - 0.006 Spinach 600 1.50*

Root Vegetables 

(except sugarbeet)

- 0.002 Carrot 1,764 4.41*

Radish 220 0.55

Parsley 1,307 3.27*

Turnip 736 1.84*

Soybeans 0.624 0.0302 Soybean 240 0.60

* RQ exceeds the LOC for presumption of chronic risk to endangered and non-endangered species.

b) Mammals

Acute RQs for risk to mammals are given in Table 14. Acute RQs were all below the LOC for 

risk to non-endangered species (0.5), as well as the LOC for possible direct effects to listed 

species (0.05). Therefore, all of the proposed new seed-treatment uses of thiabendazole is 

presumed to pose minimal acute risk to mammals.

Table 14. Mammalian Acute RQs for the Proposed New Uses of Thiabendazole

Crop Group

Seed Treatment Rate

Representative 

Crops

Max Seed 

Concentration

(mg a.i./seed)

Dose-Based Acute RQ

fl. oz. 

product/

100 lb seeda

mg 

a.i./seeda Small Med. Large

Alfalfa 5.8 0.004 Alfalfa 2,002 0.04 0.03 0.02

Cereals (small 

grains)

2.6 0.03 Barley 926 0.01 <0.01 <0.01

3.4 0.03 Oats 1,200 0.02 <0.01 <0.01

3.2 0.03 Rye 1,190 0.02 <0.01 <0.01

2.6 0.03 Triticale 1,500 0.02 <0.01 <0.01

3.1 0.03 Wheat 1,190 0.01 <0.01 <0.01

Brassica, head 

and stem

- 0.002 Broccoli 661 <0.01 <0.01 <0.01

Cabbage 728 <0.01 <0.01 <0.01
21

Crop Group

Seed Treatment Rate

Representative 

Crops

Max Seed 

Concentration

(mg a.i./seed)

Dose-Based Acute RQ

fl. oz. 

product/

100 lb seeda

mg 

a.i./seeda Small Med. Large

Bulb Vegetables - 0.002 Onion 573 <0.01 <0.01 <0.01

Cucurbit 

Vegetables

- 0.1 Cucumber 4,000 0.05 0.02 0.04

Muskmelon 4,586 0.07 0.03 0.04

Pumpkin 1,411 <0.01 <0.01 <0.01

Squash, summer 1,411 <0.01 <0.01 <0.01

Squash, winter 1,411 <0.01 <0.01 <0.01

Watermelon 2,116 0.02 0.01 0.02

Spinach - 0.006 Spinach 600 0.01 <0.01 <0.01

Root Vegetables 

(except 

sugarbeet)

- 0.002 Carrot 1,764 0.02 0.01 0.02

Radish 220 <0.01 <0.01 <0.01

Parsley 1,307 0.01 <0.01 0.01

Turnip 736 0.02 <0.01 <0.01

Soybeans 0.624 0.0302 Soybean 240 <0.01 <0.01 <0.01

Chronic RQs are given in Table 15. Dose-based chronic RQs range from 0.88 for turnip to 44.2 

for muskmelon. All dose-based chronic RQs exceeds the LOC (1.0) except for those for large 

mammals for use on turnips. Thus, the risk assessment concludes chronic risk to mammals from 

all of the proposed seed treatment uses. 

Table 15. Mammalian Chronic RQs for the Proposed New Uses of Thiabendazole

Crop Group

Seed Treatment Rate

Representative 

Crops

Max Seed 

Concentration

(mg a.i./seed)

Dose-Based Chronic RQ

fl. oz. 

product/

100 lb seeda

mg 

a.i./seeda Small Med. Large

Alfalfa 5.8 0.004 Alfalfa 2,002 19.3* 16.5* 8.8*

Cereals (small 

grains)

2.6 0.03 Barley 926 8.9* 7.6* 4.1*

3.4 0.03 Oats 1,200 11.6* 9.9* 5.3*

3.2 0.03 Rye 1,190 11.5* 9.8* 5.3*

2.6 0.03 Triticale 1,500 14.5* 12.4* 6.6*

3.1 0.03 Wheat 1,190 11.5* 9.8* 5.3*

Brassica, head 

and stem

- 0.002 Broccoli 661 6.4* 5.4* 2.9*

Cabbage 728 7.0* 6.0* 3.2*

Bulb 

Vegetables

- 0.002 Onion 573 5.5* 4.7* 2.5*

Cucurbit 

Vegetables

- 0.1 Cucumber 4,000 38.6* 32.9* 17.7*

Muskmelon 4,586 44.2* 37.8* 20.2*

Pumpkin 1,411 13.6* 11.6* 6.2*

Squash, summer 1,411 13.6* 11.6* 6.2*

Squash, winter 1,411 13.6* 11.6* 6.2*

Watermelon 2,116 20.4* 17.4* 9.3*

Spinach - 0.006 Spinach 600 5.8* 4.9* 7.8*

Root - 0.002 Carrot 1,764 17.0* 14.5* 1.0*
22

Vegetables 

(except 

sugarbeet)

Radish 220 2.1* 1.8* 5.8*

Parsley 1,307 12.6* 10.8* 3.3*

Turnip 736 7.1* 6.1* 0.88

Soybeans 0.624 0.0302 Soybean 240 2.3* 2.0* 1.1*

* RQ exceeds the LOC for presumption of chronic risk to endangered and non-endangered species.

2. LD50 per	Square	Foot	Analysis

To further evaluate the potential risk posed the proposed new seed treatment uses of 

thiabendazole, the LD50-per-square-foot index was calculated to compare the amount of active 

ingredient expected in one square foot of ground to the acute oral toxicity of thiabendazole for 

birds and mammals. This index was calculated for all crops shown in Table 3 based on the 

maximum application rates (lb a.i./A) given in that table. For all of the crops, the LD50-persquare-foot index was less than 0.01. Thus, for all proposed seed treatment crops, the index was 

well below all levels of concern.

3. Terrestrial	Plants

RQs for risk to nontarget terrestrial plants are given in Table 16. In tier-1 phytotoxicity tests, 

plants were exposed to an application rate (0.23 lb a.i./A) which was greater than the maximum 

annual application rate allowed by the proposed product label for Mertect 340-F (0.15 lb a.i./A). 

At this rate, the observed inhibition of emergence and plant growth was less than 25% for all 

monocot and dicot plants. Results from the TerrPlant model indicates that the RQs for 

nonendangered species are well less than 1, and thus less the LOC. For threatened and endangered 

species, however, the conclusions are less certain. Statistically significant effects were observed 

in test some species in the seedling emergence and the vegetative vigor studies. As the percent 

inhibition was no greater than 22% with exposure at 0.23 lb a.i./A (see Section IV), and the 

proposed new seed-treatment uses are expected to result in nontarget plant exposure at no more 

than 0.075 lb a.i./A (Table 16Table 7), direct adverse effects to nontarget plants are expected to 

be small. Additional factors that suggest little risk to nontarget plants are that 

1. Seed treatment uses would result in no spray drift, and thus there would be minimal foliar 

exposure

2. Thiabendazole is immobile in soil, and therefore off-site movement from planted fields 

would be minimal

3. The widespread use of thiabendazole as a fungicidal seed treatment and lack of plant 

incidents suggest that it does not have significant phytotoxicity at the application rates 

used.

Therefore, while limitations of the phytotoxicity data results in some uncertainty, the EPA does 

not expect the proposed seed treatment uses of thiabendazole to result in effects to terrestrial 

plants.
23

Table 16. RQs for Terrestrial Plants Exposed to Thiabendazole from the Proposed SeedTreatment Uses

Taxa

EECs

(lb a.i./A)

Non-listed 

Species 

Endpoint

(EC25 in lb a.i./A)

Listed Species 

Endpoint

(NOAEC in lb

a.i./A)

Non-Listed 

Species RQ

Listed 

Species RQ

Monocots

Uplands 0.0015 >0.23 <0.23 <0.07 >0.007

Wetlands 0.015 >0.23 <0.23 <0.07 >0.007

Dicots

Uplands 0.0015 >0.23 0.23 <0.07 <0.007

Wetlands 0.015 >0.23 0.23 <0.07 <0.07

VI. Risk	Discussion	and	Conclusions

The proposed seed treatment uses are expected to result in very little exposure to aquatic 

organisms and nontarget terrestrial plants. The application rates of the new uses in terms of 

pounds of active ingredient per acre is small (0.00004 to 0.15 lb a.i./A). These application rates 

are not higher than those of previously registered seed treatment uses. No spray drift would occur 

with seed treatment uses, which reduces off-site transport of the pesticide. Furthermore, because 

the seeds generally are buried and because thiabendazole has very little mobility in soil, off-site 

movement of thiabendazole in runoff is expected to be very limited. As a result, exposure to 

aquatic organisms and to nontarget terrestrial plants is expected to be very limited, and is not 

expected to reach levels which may be toxic.

The acute oral honeybee test classifies thiabendazole as practically nontoxic to the honey bee 

(LD50 > 34 μg a.i./bee). Therefore, no warning statement is needed on the product labels to 

protect bees.

In contrast, wildlife that feed on the treated seeds in fields planted with treated seeds may have 

significant exposure to thiabendazole. Based on an assumption of a diet comprised entirely of 

treated seeds, the ecological risk assessment concluded that all of the proposed new uses pose a 

risk to mammals due to chronic toxicity (RQs of 1.1 to 44.2). All of the proposed new uses except 

soybean and some root vegetables (e.g., radish) also pose a risk to birds due to chronic toxicity

(RQs 1.43 to 11.5). A few cucurbit uses (e.g., cucumbers and muskmelons) also may pose a risk 

to birds because of acute toxicity, although this conclusion is uncertain because the avian acute 

toxicity endpoints are non-definitive (i.e., “greater than” values). For many of the new uses, RQ 

values also indicate that the endangered species acute toxicity level of concern is exceeded for 

birds, but not for mammals.

The potential for acute effects to birds is uncertain because the assessment is based on nondefinitive “greater than” estimates of acute toxicity endpoints. The conclusions for chronic risk 

are also uncertain because the available avian reproduction studies only tested up to 400 mg 

a.i./kg-diet, which is lower than some of the estimated exposure values. Since the chronic avian 

toxicity of thiabendazole is above 400 mg a.i./kg-diet, the level of risk is uncertain for all crops 
24

for which the seed concentration exceeds this amount. The chronic risk conclusions for mammals 

are more certain as they are based on studies in which toxic effects were observed and thus both 

NOAEC and LOAEC values were established.

The conclusions of risk to terrestrial wildlife are based on exposure through direct consumption of 

treated seeds. Exposure would be much less for species that do not consume seeds, or for which 

seeds comprise only a small portion of their diet. Some thiabendazole may be absorbed by the 

plant and translocated to emerged portions of the plant, and thus may result in some exposure to 

herbivorous animals which feed on the crop soon after it emerges. Likewise, some thiabendazole 

could leach into the upper soil and expose soil arthropods, and thereby would expose 

insectivorous animals which feed on them. However, these routes of exposure are expected to be 

much less significant than direct consumption of treated seeds. Therefore, risk to herbivorous and 

insectivorous animals is uncertain. Acute RQs for birds and mammals derived from the LD50-

per-square-foot analysis were very low (<0.01), which indicates that acute risk to wildlife that do 

not feed on seeds might be low.

Since birds serve as a surrogate for reptiles and terrestrial phases of amphibians, the conclusion 

that the proposed uses of thiabendazole pose risk to birds also apply to terrestrial reptiles and 

amphibians with terrestrial phases. However, reptiles and amphibians are generally insectivorous 

or herbivorous, and therefore risks to these taxa are uncertain.

To further characterize the chronic risk to seed eating mammals, an analysis was done to 

determine the number of seeds that small mammals of various sizes would need to eat for the 

chronic RQ to reach 1, the LOC for concluding chronic risk (Table 17). Since most seed-eating 

mammals are small rodents, the results for small to medium mammals are most relevant. The 

number of seeds required for mammals of these sizes range from 3.3 to 311. None of the numbers 

for these weight sizes appear to be so large to be considered impossible, although the numbers for 

Brassica, bulb vegetable, and root crops appear unlikely. The numbers for small grains, soybeans, 

and cucurbits would be easy for small mammals to reach. This analysis agrees with that of the 

previous RQ analysis in indicating that chronic mammalian risk is highest for cucurbit vegetable 

crops (See Table 15). This is because the seed treatment rate is higher for this crop group than for 

other crops and crop groups.

Table 17. Number of Seeds that Must Be Eaten for Oral Exposure to Reach a Mammalian 

Chronic LOC

Crop Group

Seed Treatment 

Rate

(mg a.i./seed)

Size BW (g)

WeightAdjusted 

NOAEL

Number of Seeds 

Alfalfa 0.004 Small 15 21.978 82

Medium 35 17.783 156

Large 1000 7.6916 1923

Cereals (small grains) 0.03 Small 15 21.978 11

Medium 35 17.783 21

Large 1000 7.6916 256

Brassica, head and 

stem

0.002 Small 15 21.978 165

Medium 35 17.783 311

Large 1000 7.6916 3846
25

Crop Group

Seed Treatment 

Rate

(mg a.i./seed)

Size BW (g)

WeightAdjusted 

NOAEL

Number of Seeds 

Bulb Vegetables 0.002 Small 15 21.978 165

Medium 35 17.783 311

Large 1000 7.6916 3846

Cucurbit Vegetables 0.1 Small 15 21.978 3

Medium 35 17.783 6

Large 1000 7.6916 77

Spinach 0.006 Small 15 21.978 55

Medium 35 17.783 104

Large 1000 7.6916 1282

Root Vegetables 

(except sugarbeet)

0.002 Small 15 21.978 165

Medium 35 17.783 312

Large 1000 7.6916 3846

Soybeans 0.0302 Small 15 21.978 11

Medium 35 17.783 20

Large 1000 7.6916 255

The chronic mammalian RQ analysis was based on an NOAEC of 200 mg/kg-diet, which was 

established by observations of reduced weight gain in adult rats and reduced food consumption in 

the next highest treatment level of 600 mg a.i./kg (MRID 43190301). Previous ecological risk 

assessments used the NOAEC obtained in this study for reproductive endpoints only, which was 

1800 mg ai/kg-diet. However, consistent with current EFED policy, the NOAEC used in this 

assessment is based on the most sensitive endpoint affecting the reproduction, survival, or growth, 

and effects on weights of the adults is considered a growth effect. Other studies have also noted 

toxic effects at dietary exposures at or near 600 mg a.i./kg-diet based on organ-level effects, such 

as liver hypertrophy (MRID 43593201) and histopathological changes in bone marrow, liver, and 

thyroid (MRID 42942802). In a developmental rat study, a growth effect was also observed in 

developing young (decreased fetal body weight) at the slightly higher dietary concentration of 800 

mg ai/kg-diet (MRID 42942803). Taken together, these results support use of 200 mg/kg-diet as 

the appropriate NOAEC for chronic effects to mammals.

The primary ecological risk from the proposed thiabendazole seed-treatment uses (as well as 

existing uses) is to birds and small mammals which could feed directly on the treated seeds. 

Exposure, and thus risk, to these animals depend on two factors, the maximum seed treatment rate 

(i.e., the maximum amount of active ingredient that can applied to a seed) and the size of the seed. 

Greater risk is associated with crops for which the seed treatment rate is higher and the seed size 

is smaller. The maximum seed treatment rate for cucurbit vegetables (0.1 mg a.i./seed) is far 

greater than for any other crop or crop group, making the risk to seed-eating animals greatest for 

this crop groups. Among the crops within this group, ones with relatively small seeds, such as 

cucumbers and melons, have the highest risk. Risk to seed-eating animals is also high for alfalfa 

and for certain root vegetables, such as carrots. While the seed treatment rates for these crops are 

fairly low, the seeds are extremely small, making the concentration on these seeds fairly high. 

RQs are also fairly high for small grains (barley, oat, rye, triticale, and wheat), which have the

second highest seed treatment rate (0.03 mg a.i./seed) and moderately small seeds.
26

VII. References

USEPA, 2011. Acres Planted per Day and Seeding Rates of Crops Grown in the United States. 

Memorandum from the Biological and Economic Analysis Division to the Environmental 

Fate and Effects Division. Office of Pesticide Programs. U.S. Environmental Protection 

Agency. Washington, DC. March 17, 2011.
27

Appendix	A.	 Thiabendazole	degradates	identified	in	Environmental	Fate	

Studies

Degradates Hydrolysis Photolysis 

(water)

Photolysis 

(soil)

Aerobic 

Soil

Anaerobic 

Soil

Terrestria

l Field

Benzimidazole X X X

a

5-hydroxy 

thiabendazole X 

Benzimidazole-2-

carboxamide X

a

Benzimidazole-2-

carboxylic acid X

Unidentified [14C] X X

Unknown 1 X

14CO2 X

a

- major degradates (>10%)
28

Appendix	B.		Thiabendazole	and	Its	Environmental	Degradates

Name

Chemical Name CAS 

No.

Formula

Molecular Mass

Chemical Structure Study Type Maximum %AR 

(day)

Final %AR 

(study length)

PARENT

Thiabendazole 2-(4-thiazolyl)-1Hbenzimidazole

CAS No.: 148-79-8

Formula: C10H7N3S

MW: 201.25 g/mol

Aerobic Soil Metabolism

Anaerobic Soil Metabolism

56.8%

78% (@ day 90)

TRANSFORMATION PRODUCTS

Thiabendazole-5-hydroxy 5-hydroxythiabendazole;

2-(4-Thiazolyl)-5-

benzimidazolol

CAS No.: 948-71-0

Formula: C10H7N3S

MW: 217.25g/mol

Aerobic Soil Metabolism
29

Name

Chemical Name CAS 

No.

Formula

Molecular Mass

Chemical Structure Study Type Maximum %AR 

(day)

Final %AR 

(study length)

Benzimidazole 1H-Benzimidazole

CAS No.: 962-28-7 

Formula: C7H6N2

MW: 118.05 g/mol

Aq. Photolysis (minor)

Aerobic Soil Metabolism

Anaerobic Soil Metabolism

Terrestrial Field Dissipation

-

2.2%

13.7%

-

6.49% (avg.)

0.17%

5.5% (@ day 90

-

Benzimidazole 

carboxylic acid

1H-Benzimidazole-2-

carboxylic acid 

CAS No.:

Formula: C8H6N2O2

MW: 162.15 g/mol

Aq. Photolysis (minor) 9.98% @ 96 hrs

Benzimidazole-2-

carboxamide

1H-Benzimidazole-2-

carboxamide

CAS NO. 5805-52-7

Formula: C8H7N3O

MW: 161.16 g/mol

Aq. Photolysis (major) 10.2% @ 96 hrs
30

Appendix	C.		GENEEC2 Model Outputs

 RUN No. 1 FOR thiabendazole ON seed treat * INPUT VALUES * 

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

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN)

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

 .150( .150) 1 1 7343.0 28.0 GRANUL( .0) .0 1.0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

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

 2064.00 2 N/A 1.20- 148.80 .00 148.80

 GENERIC EECs (IN NANOGRAMS/LITER (PPTr)) Version 2.0 Aug 1, 2001

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

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

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

 885.85 860.43 734.13 530.42 427.70

 RUN No. 2 FOR thiabendazole ON seed treat * INPUT VALUES * 

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

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN)

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

 .150( .150) 1 1 7343.0 28.0 GRANUL( .0) .0 2.0

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

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

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

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

 2064.00 2 N/A 1.20- 148.80 .00 148.80

 GENERIC EECs (IN NANOGRAMS/LITER (PPTr)) Version 2.0 Aug 1, 2001

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

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

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

 442.93 430.22 367.07 265.21 213.85
31

Appendix	D.		Output	from	Terrestrial	Exposure	Model	(T-REX)

Chemical: Thiabendazole

Name of 

seed 

treatment 

formulation: Mertect 340-F

Percent AI 

in 

formulation: 42% 0

Density of 

product 

(lbs/gal): 9.692670395

Endpoints Reported Tested Body Adjusted LD50 Size class for adjusted LD50

Weight (g)

Avian LD50: 4640.00 178 3342.79 Small (20g)

Avian repro. 

NOAEC: 400.00 4255.54 Medium (100g)

6011.11 Large (1000g)

Mammalian 

LD50: 4734.00 350 10404.53 Small (15g)

Mammalian 

NOAEL: 200.00 8418.37 Medium (35g)

3641.21 Large (1000g)

Adjusted NOAEL for Mammals

Small (15g) 21.98

Medium (35g) 17.78

Large (1000g) 7.69

Animal Size Crop

Maximum 

Application 

Rate

Maximum 

Seed 

Application 

Rate

Avian Nagy 

Dose

Mammalian 

Nagy Dose

Available 

AI

(lbs ai/A)

(mg ai/kg 

seed)

(mg ai/kgbw/day)(mg ai/kgbw/day) (mg ai ft-2)

Small

cucumber 0.031 2645.55

669.56 560.52

Medium 381.81 387.39 0.32

Large 170.94 89.82

Crop

Risk Quotients†

Avian (20 g) Mammalian (15 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic
32

cucumber

0.2003 0.00480 6.61 0.0539 0.00205 25.50

Avian (100 g) Mammalian (35 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

cucumber

0.0897 0.00075 6.61 0.0460 0.00109 21.78

Avian (1000 g) Mammalian (1000 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

cucumber

0.0284 0.00005 6.61 0.0247 0.00009 11.68

† Acute RQ #1 = (mg ai /kg-bw/day) / LD50

Acute RQ #2 = mg ai ft-2 /(LD50*bw)

Avian Chronic RQ = mg kg-1 seed / NOAEL

Mammalian Chronic RQ = mg a.i./kg-bw/day / adjusted NOAEL

Animal

Nagy allometry

Food ingestion value 

g/day

20 g Bird 5.061777181

15 g Mammal 3.178078065

100 g Bird 14.4321986

35 g Mammal 5.125113265

1000 g Bird 64.6148732

1000 g 

Mammal 33.95072796
33

Appendix	E.		Output	from	Terrestrial	Plant	Risk	Model	(TerrPlant)

TerrPlant v. 1.2.2

Table 1. Chemical Identity.

Chemical Name Thiabendazole

PC code 60101

Use Seed treatments

Application Method Ground

Application Form Seed treatments

Solubility in Water 

(ppm) 22.74

Table 2. Input parameters used to derive EECs.

Input Parameter Symbol Value Units

Application Rate A 0.15 lb ai.A

Incorporation I 1 none

Runoff Fraction R 0.05 none

Drift Fraction D 0 none

Table 3. EECs for Thiabendazole. Units in lb ai.A.

Description Equation EEC

Runoff to dry areas (A/I)*R 0.0075

Runoff to semi-aquatic areas (A/I)*R*10 0.075

Spray drift A*D 0

Total for dry areas ((A/I)*R)+(A*D) 0.0075

Total for semi-aquatic areas ((A/I)*R*10)+(A*D) 0.075
34

Appendix	F.	 Ecotoxicity	Data

Table F.1. Aquatic Toxicity Profile for Thiabendazole

Taxon Exposure 

Duration

Most Sensitive 

Species Tested Toxicity Value Toxicity 

Category

MRID /

Classification 

Fish

Freshwater 

Fish

Acute Rainbow trout

Oncorhynchus mykiss 96-hr LC50 = 0.56 mg/L Highly toxic 41025005 (Acceptable)

Chronic Rainbow trout

NOAEC = 0.11 mg/L

LOAEC = 0.23 mg/L

Endpoint: Reduced weight

--

42508901

(Acceptable)

Estuarine/ 

Marine Fish

Acute Sheepshead minnow

Cyprinodon variegatus LC50 > 10 mg/L3

Slightly toxic 41192003 

(Supplemental)

Chronic No data No data -- --

Aquatic Invertebrates

Freshwater 

Invertebrates

Acute Water Flea

Daphnia magna 48-hr EC50 = 0.31 mg/L2 Highly toxic 00123326 (Acceptable)

Chronic Water Flea

NOAEC = 0.042 mg/L

LOAEC = 0.087 mg/L

Endpoint: survival, 

offspring production

--

00094645 

(Acceptable)

Estuarine/ 

Marine 

Invertebrates

Acute

Mysid shrimp 

(Americamysis bahia)

96-hr LC50 = 0.34 mg/L Highly toxic 41192002 

(Acceptable)

Eastern Oyster 

(Crassostrea virginica)

IC50 > 0.26 mg/L

(22% inhibition at 0.26 

mg/L)

Endpoint: shell growth

Not determined 41192004 

(Supplemental)

Chronic No data No data -- --

Freshwater 

Benthic 

Invertebrates

Acute No data

Chronic No data

Aquatic Plants

Vascular 

plant Acute Duckweed

Lemna gibba

7-day EC50 = 2.32 mg/L 

NOAEC = 0.26 mg/L Not defined 48996308 (Supplemental)

Non-vascular 

plant Acute

Green algae

Selenastrum 

capricornutum

96-hr EC50 = 3.06 mg/L 

NOAEC = 0.532 mg/L Not defined 49146601(Supplemental)
35

Table F.2. Terrestrial Toxicity Profile for Thiabendazole

Taxon Exposure 

Duration

Most Sensitive 

Species Tested Toxicity Value Toxicity Category

MRID

Reference

Terrestrial Vertebrates

Mammals

Acute Laboratory Rat

Rattus norvegicus

LD50 = 5,070 mg ai/kg bw (M)

LD50 = 4,734 mg ai/kg bw (F)

Practically 

nontoxic

41258201

(acceptable)

Chronic Laboratory Rat

NOAEL = 10 mg ai/kg diet

LOAEL = 30 mg ai/kg diet

(Reduced bodyweight gain and 

food consumption)

Not applicable 43190301

(acceptable)

NonPasserine 

Birds

Acute Oral

Bobwhite

Colinus virginianus LD50 > 2250 mg ai/kg bw Practically nontoxic

410250-02

(acceptable)

Bobwhite LD50 > 4640 mg ai/kg bw Practically 

nontoxic

00123313

(acceptable)

Mallard

Anas platyrhynchos LD50 > 4640 mg ai/kg bw Practically nontoxic

00137948

(acceptable)

Acute 

Dietary

Bobwhite and 

Mallard LC50 > 5620 mg ai/kg diet Practically 

nontoxic

41025003

41025004

(acceptable)

Chronic

Bobwhite and 

Mallard NOAEC = 400 mg ai/kg diet

LOAEC >400 mg ai/kg diet Not applicable

00123327 

00123328 

(acceptable)

Passerine 

Birds Acute No data

Terrestrial Arthropods

Terrestrial 

Insects Acute Honey BeeApis mellifera LD50 > 34 μg a.i./bee Practically nontoxic

48996305

(acceptable)

Terrestrial Plants

Terrestrial 

Plants -

Seedling 

Emergence

Survival 

and growth

Monocots NOAEL = 0.23 lb/A

EC25 > 0.23 lb/A NA 48996306 

(Acceptable, 

except 

Supplemental 

for bean and 

soybean)

Dicots

NOAEL < 0.23 lb/A

EC25 ≥ 0.23 lb/A

Endpoint: emergence and 

survival

NA

Terrestrial 

Plants -

Vegetative 

Vigor

Survival 

and growth

Monocots

NOAEL < 0.23 lb/A

EC25 > 0.23 lb/A

Endpoint: shoot length

NA 48996307 

(Acceptable)

Dicots NOAEL = 0.23 lb/A

EC25 > 0.23 lb/A NA