UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 060101 

060102 

DP Barcode: 421319 

August 11, 2014 

MEMORANDUM 

SUBJECT: 

FROM: 

THROUGH: 

TO: 

EFED Response to Comments on the Problem Formulation for Thiabendazole and 

Thiabendazole Hypochloride 

James 

Nicholas 

Wolf, 

Mastrota, 

Environmental 

Biologist 

Engineer 

~ C"h~ ~ /( W~ Environmental Risk Branch 3 O"v··· . Environmental Fate and Effect Division 

Office of Pesticide Programs 

Dana Spatz, Branch Chief ~ ~ ........ <:::::::.__ ~ c _ \\ _ ZJ) \ LJ 

Environmental Risk Branch 3 ~ ~ 1::) \ 

Environmental Fate and Effects Division 

Neil Anderson, Branch Chief 

Ricardlo Jones, Chemical Reviewer 

Risk Management and Implementation Branch 1 

Pesticide Re-evaluation Division {7508P) 

This memo communicates the response of the Environmental Fate and Effect s Division (EFED) 

to comments received on the Preliminary Work Plan for the Registration Review of 

thiabendazole {060101) and thiabendazole hypochloride (060102). It addresses comments that 

the registrant, Syngenta, made on the document Problem Formulation for the Ecological Risk 

and Drinking Water Exposure Assessments to be Conducted in Support of the Registration 

Review of Thiabendazole and its Hypophosphite Salt, which EFED completed in March, 2014. 
2 

Syngenta Comment #1

PAGE #, Paragraph# EPA statement

Page 5, Paragraph 2 “Residues of thiabendazole that accumulated during mushroom cultivation

potentially could remain in the SMC (spent mushroom compost) and be released into

the environment at this time. Grogan and Jukes (2003) found that thiabendazole is

highly persistent in mushroom casing soil, and thus some thiabendazole residues are

expected to persist in SMC.”

Syngenta Comment: Grogan and Jukes (2003) observed that “By day 35, levels for all

treatments (thiabendazole) had fallen between 15 and 34 g a.i. 100m‐2, representing 10‐

15% of what had been applied. By this stage, the crop was no longer being harvested and

the casing soil had begun to dry out.”

Spent mushroom compost (SMC) may be stored for extended periods and is steamed

before use as fertilizer/mulch to eliminate any pests, pathogens, and weed seeds. The

processing and storage of SMC could markedly reduce thiabendazole residues, as observed

by Grogan and Jukes (2003), before SMC is applied in the environment for agricultural

purposes.

Grogan HM, Jukes AA. 2003. Persistence of the fungicidesthiabendazole,

carbendazim and prochloraz‐Mn in mushroom casing soil. Pest Manag 59:

1225‐1231.

EFED Response #1:

We will consider the registrants comment in the risk assessment. However, it is not possible to

determine a half‐life or degradation rate with the information presented in the published

paper. As noted in the paper by Grogan and Jukes (and Syngenta), the SMC has been permitted

to dry out and lower thiabendazole concentrations were measured. Due to the soil drying and

the high sorption coefficient (Koc 1,100 to 22,400 mL/goc), the decreased thiabendazole

concentration my reflect limitation of the extraction agent (methanol). Syngenta indicates that

“Spent mushroom compost (SMC) may be stored for extended periods and is steamed before use

as fertilizer/mulch to eliminate any pests, pathogens, and weed seeds.” However, there is

currently no requirement on how SMC is stored , and thus the storage and steaming of the

SMC may not occur. Also, there is no assurance that the SMC would be stored for 35 days or

more before it is used.

   
3 

Syngenta Comment #2

PAGE #, Paragraph# EPA statement

Page 6, Paragraph 3 “However, there is uncertainty regarding risk from use in mushroom culture, and

exposuresfrom application of spent mushroom compost could potentially be much

greater than for seed treatment uses. When considering the application rate

expressed in terms of pounds a.i./acre, exposure would be 10 times greater than for

seed treatments...”

Syngenta Comment: Assumptions about potential exposure associated with spent mushroom

compost (SMC) are highly conservative and do not consider the potential degradation/dilution

of thiabendazole. Grogan and Jukes (2003) observed that, while thiabendazole is stable in

casing soils used to cultivate mushrooms, once compost dries after mushrooms are harvested

the measured levels of thiabendazole in SMC are only between 10‐15% of what had been

applied. This observation suggests that thiabendazole residues may be markedly reduced

during the processing and storage of SMC.

EFED Response #2:

EFED agrees that the exposure may be conservative, but will consider any additional data

Syngenta submits that may help to refine the exposure assessment prior to preparing the risk

assessment. The exposure must consider not only the concentration of thiabendazole, but the

volume (or mass) of SMC placed in fields.  Without further information to refine our

assumptions, EPA plans to assess risk from the use of the SMC soon after harvest, without

processing, and with limited amount of drying. The EPA may also characterize risk from a more

typical exposure scenario for SMC, but this would be done in addition to the upper bound

scenario.

Syngenta Comment #3   

PAGE #, Paragraph# EPA statement

Page 9, Table 3 Terrestrial field dissipation half‐life in upper 6 inch soil layer; (max. depth of observed

leaching); t1/2= 1100 days (6 inches, Winter wheat/loamy sand/WA, MRID 43187201),

t1/2= 900 and 1444 days (12 inches, Soybean and bare ground sandy loam/IL, MRID

43187202), t1/2= 833 and 1093 days (6 inches, Soybean and bare ground sandy

loam/GA, MRID 43187201).

Syngenta Comment: There appears to be a typographical error; MRID 43187203 should be

listed as the data source for the values of t1/2= 833 and 1093 days (6 inches, Soybean and bare

ground sandy loam/GA), rather than MRID 43187201.

EFED Response #3:

The MRID # does appear to be incorrect. The correct number will be used.  
4 

Syngenta Comment #4  

PAGE #, Paragraph# EPA statement

Page 14, Table 6 Freshwaterfish, Chronic (Early Life‐stage, Rainbow trout, NOAEC = 0.11

mg/L; LOAEC = 0.23 mg/L; MRID 42508901

Syngenta Comment: Study referenced is with fathead minnow (Pimephales promelas) rather

than rainbow trout.

EFED Response #4:

Syngenta is correct; the study was with the fathead minnow.  The correction will be made.

Syngenta Comment #5  

PAGE #, Paragraph# EPA statement

Page 14, Table 6 Referenced endpointsfor aquatic non‐vascular and vascular plant studies are

presented as cell density and/or frond density, respectively.

Syngenta Comment: Effective concentrations(ECx) calculated based on growth rate are

considered more robust than those based on standing crop (cell density and biomass) for a

given time‐pointsince growth rate is independent of exposure time (EPA 2012). For aquatic

plants, study duration, conditions of temperature, light, and nutrients, and reported

endpoints can vary considerably among studies. Other resource limiting factors,such as

nutrient depletion and self‐shading, can complicate the interpretation of reported plant

ECxs. These factors may affect study results, which could result in biased estimatesin

certain cases for some species(EPA2012). Using absolute biomass can potentially result in

particularly misleading ECx’s when growth rates are modest, and a departure from

exponential growth will be most pronounced in the treatments with the highest growth

rates (EPA 2012). With respect to toxicity testing, differencesin design of the study can

result in complicated growth dynamics and relationshipsthat are difficult to interpret and

apply (EPA 2012).

As indicated by Bergtold and Dohmen (2011), the Organization for Economic Cooperation

and Development (OECD) indicates a clear preference for growth rate as the appropriate

endpoint in the lemna and algal toxicity test guideline (OECD 2006a, b). Specifically, the

OECD guideline for testing growth inhibition in freshwater alga and cyanobacteria (OECD

2006a) states:

“ECx values based upon average specific growth rate (ErCx) will generally be higher

than results based upon yield (EyCx) if the test conditions of this Guideline are

adhered to, due to the mathematical basis of the respective approaches. This should

not be interpreted as a difference in sensitivity between the two response variables,

simply that the values are different mathematically. The concept of average specific
5 

growth rate is based on the general exponential growth pattern of algae in non‐

limited cultures, where toxicity is estimated on the basis of the effects on the growth

rate, without being dependent on the absolute level of the specific growth rate of

the control,slope of the concentration‐response curve or on test duration. In

contrast, results based upon the yield response variable are dependent upon all

these other variables. [Yield] is dependent on the specific growth rate of the algal

species used in each test and on the maximum specific growth rate that can vary

between species and even different algal strains. This response variable should not

be used for comparing the sensitivity to toxicants among algal species or even

differentstrains.”

Based on the aforementioned information, relative growth rate is the most appropriate

aquatic plant endpoint for ecological risk assessment.

Bergtold, M. and G.P. Dohmen. 2011. Biomass or growth rate endpoint for

algae and aquatic plants: Relevance for the aquatic risk assessment of

herbicides. Integr. Environ. Assess. Manag. 7: 237‐247.

OECD (Organization for Economic Cooperation and Development).

2006a. OECD Guideline 201: Freshwater alga and cyanobacteria,

growth inhibition test. Adopted 23

March 2006. Paris.

OECD (Organization for Economic Cooperation and Development).

2006b. OECD Guideline 221: Lemna sp. growth inhibition test.

Adopted 23 March 2006. Paris.

US EPA, 2012. Proposed Methodology for Specifying Atrazine Levels of

Concern for Protection of Plant Communitiesin Freshwater Ecosystems.

Report To: Environmental Fate and Effects Division, Office of Pesticide

Programs, U.S. Environmental Protection Agency, Washington, DC.

EPA/600/R‐12/019. Mid‐Continent Ecology Division, National Health and

Ecological Effects Laboratory, Office of Research and Development, U.S.

Environmental Protection Agency, Duluth, MN. March 5, 2012.

EFED Response #5:

EFED agrees that there may be benefits to analyzing the growth rate rather than yield

measurements such as cell density or frond density. It is noted that thiabendazole has low

toxicity to aquatic plants and low mobility in soil, and thus the risk assessment is expected to

conclude low risk to aquatic plants (as did past assessments) regardless of which endpoint is

used to analyze results from the aquatic plant toxicity tests.

   
6 

Syngenta Comment #6   

PAGE #, Paragraph# EPA statement

Page 18, Paragraph 4 ”A preliminary evaluation indicatesthat this route could be significantsince the use

rate on mushroom compost is much greater than that associated with seed

treatment uses, and the high persistence of thiabendazole makes it likely that

residues will persist in spent mushroom compost when it is disposed or applied to

soil.”

Syngenta Comment: Assumptions about potential exposure associated with spent

mushroom compost (SMC) are highly conservative and do not consider the potential

degradation/dilution of thiabendazole. Grogan and Jukes (2003) observed that while

thiabendazole is stable in casing soils used to cultivate mushrooms, once compost dries

after mushrooms are harvested, measured levels of thiabendazole in SMC are markedly

lower. This observation suggeststhat thiabendazole residues will be reduced during the

processing and storage of SMC. Syngenta requests an opportunity to discuss potential

environmental exposure with the Agency following mushroom compost use.

EFED Response #6:

The first part of this comment is identical to Comment #2.  EFED would welcome the

opportunity to discuss potential environmental exposure associated with spent mushroom

compost.

Syngenta Comment #7   

PAGE #, Paragraph# EPA statement

Page 18, Paragraph 5 “For tree‐injection of thiabendazole hypophosphite, a limited amount of water

contaminationmay occur from residuesthat leach out of shed leaves and fallen

trunks and branches. This route of exposure is expected to be minor since release of

residuesthrough these routes would be slow, and because of the high cost and labor

of tree injection, only a small fraction of trees in a watershed are expected to be

treated with thiabendazole.”

Syngenta Comment: Syngenta agrees that the use of thiabendazole hypophosphite as a tree‐

injection is unlikely to lead to appreciable concentrations of residuesin the environment. It is

also important to note that the tree‐injection use is highly specialized and the label (Arbotect®,

100‐892) limits use to only a few tree speciesfor control of specific diseases. “Arbotect 20‐S is a

systemic fungicide for use as a flare root injection for prevention of Dutch elm disease

(Ophiostoma ulmi and O. novo‐ulmi) on elms (Ulmus spp.), treatment of sycamore anthracnose

(Apiognomonia platani) on sycamores and London plane trees (Platanusspp.) and oak

anthracnose on oak trees.”
7 

   
8 

EFED Response #7:

Please see the response to Comment #9.

Syngenta Comment #8  

PAGE #, Paragraph# EPA statement

Page 22, Paragraph 2 “Based on information on use of SMC (spent mushroom compost) provided by the

Oregon State University Extension Service and the American Mushroom Institute, it is

estimated that 2 to 3 inches of SMC will be applied to soil.”

Syngenta Comment: The American Mushroom Institute website

(www.mushroomcompost.org/Corn.pdf) states that “it is recommended to apply a 2‐inch depth

of fresh mushroom compost and then till into soil.” Incorporation of spent SMC should be

considered in modeling/estimating of environmental concentrations.

EFED Response #8:

Response: EFED will consider incorporation depth as well “thickness of application (to estimate

an application)” when conducting the modeling (i.e., chemical application method, etc).  It is

noted that SMC is also sometimes applied as a mulch or top‐dressing, and thus would not

always be soil incorporated. The EPA will conduct a risk assessment for both incorporated and

unincorporated uses of SMC to characterize risk.

Syngenta Comment #9

PAGE #, Paragraph# EPA statement

Page 23, Paragraph 2 “”Trees that are treated with thiabendazole hypophosphite will assume to

surround a standard one‐acre pond.........For a conservative screening‐level

assumption, all of the thiabendazole hypophosphite injected into the trees will be

assumed to be translocated into the leaves, and released into the pond water when

the leavesfall and decay. This amount will be divided by the volume of the water in

the pond to derive a conservative aquatic EEC for this use.”

Syngenta Comment: The assumption that all trees surrounding a one‐acre pond will be treated

with thiabendazole hypophosphite is highly conservative. As mentioned on page 18 of the

Preliminary Problem Formulation for thiabendazole, “only a small fraction of trees in a

watershed are expected to be treated with thiabendazole.” While the thiabendazole tree‐

injection treatment is highly effective at treating and preventing diseases, it is not a wide‐

spread option for forest management due to costs associated with materials and the high labor

demand of the tree‐injection process. The assumptionsthat all thiabendazole hypophosphite
9 

injected into trees will be translocated to leaves, all the leaves from all treated‐trees will enter

the pond, and that all thiabendazole hypophosphite in leaves that enter the pond will be

released are highly conservative. Therefore, the proposed screening‐level evaluation will

markedly over‐predict concentrations of thiabendazole in surface water associated with the

tree‐injection use.

EFED Response #9:

EFED recognizes that this is conservative. This approach was suggested only as a screen with

the hopes that it would be sufficient to rule out risk.  EFED believes that the likelihood that all

the trees around the pond are of a species that may be treated with thiabendazole

hypochloride, and that all of these trees would be treated, is low.  However, EFED does not

have adequate information for refining the estimate of the maximum number of trees around a

single pond that would likely be treated.

Any data or specific information that Syngenta can provide on typical tree‐injection use of

thiabendazole hypochloride that may improve these risk assessment assumptions would be

welcomed.

Syngenta Comment #10

PAGE #, Paragraph# EPA statement

Page 25, Paragraph 1 Endangered Species Assessments

Syngenta Comment: Syngenta suggests that the best available data and risk assessment

approaches should be implemented in the Thiabendazole endangered species assessment. A

summary of the types of higher‐tier data and assessmentrefinements available to more

accurately and realistically evaluate potential risks to endangered speciesis detailed in MRID#

49200601 (Risk Assessment Considerations Concerning the Potential Impacts of Pesticides on

Endangered Species).

EFED Response #10:

EPA has reviewed the comments from Syngenta on the registration review docket openings for

Thiabendazole and plans to address many of the concerns raised by Syngenta as part of the

implementation plan for assessing the risks of pesticides to listed species based on the

recommendations of the April 2013 National Academy of Sciences (NAS) report.  EPA will

address concerns specific to thiabendazole in connection with the development of its

registration review decision for thiabendazole.  

   
10 

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.    

Syngenta Comment #11

PAGE #, Paragraph# EPA statement

Page 26, Table 9.

Submitted Environmental

Fate Data for

Thiabendazole and Data

Gaps

Ready biodegradability test or one of three biodegradation in activated sludge

simulation tests; Transport to wastewater treatment plant and aquatic environment.

Additional data needed.

Syngenta Comment: Syngenta no longer supports end‐use products registered for

antimicrobial uses of thiabendazole. The antimicrobial end‐use products were transferred to

Lanxess.

EFED Response #11:

This information will also be used to evaluate exposure from post‐harvest uses.

   
11 

Syngenta Comment #12

PAGE #, Paragraph# EPA statement

Page 26, Table 9.

Submitted Environmental Fate

Data for Thiabendazole any

Data Gaps

Environmental Chemistry Methods(ECM) and Independent Laboratory

Validation (ILV); Water (no study).

Syngenta Comment: A water method and corresponding validation for thiabendazole have

been performed by Syngenta. These studies are included in this submission in response to the

data gap indicated.

EFED Response #12:

EFED notes that two reports were submitted on May 28, 2014 (MRIDs 49393802 and

49393803).