2 

Comment regarding the drinking water assessment 

Reference: Page 4, Section 4.2 (also noted on Page 17, Section 8.1.2). Document states: The 

Health Effects Division (HED) recommends the normal and reverse acid are appropriate to 

include in the drinking water assessment along with parent dithiopyr. 

DAS Comment: The mono-acid degradates are formed only at significant levels (e.g., >10% 

applied radioactivity) in the aqueous photolysis study [5]. Since photolysis is only possible in 

surface water, these degradates would be unlikely to be formed in ground water, and therefore 

the mono-acids are not applicable to the drinking water assessment. This conclusion is 

supported by the New York ground water monitoring study [3], in which neither normal nor 

reverse monoacid metabolites were detected. This study can be provided to the EPA upon 

request. 

References: 

MRID 40638628. Solsten, R. T. The Photolysis of MON 7200, 3,5-pyridine- dicarbothioic acid, 

2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluromethyl)-S,S- dimethyl ester in Water and in 

Soil. Monsanto unpublished report number MSL7049, 1988. 

Petrecz, T. and W. Zogorski. Ground Water Monitoring Study for Dithiopyr in Nassau and 

Suffolk Counties, New York: Final Report. Dow AgroSciences unpublished report number 

110487, 2005. 

EFED Response: In addition to percent formation, degradate toxicity is also a determinant in 

selecting degradates of concern for the drinking water assessment. The toxicities of the normal 

and reverse acids are assumed to be comparable to that of dithiopyr because they maintain the 

core pyridine structure of dithiopyr (structure comparison: page 7 of the problem formulation). 

Since both acids are more polar than parent dithiopyr and are formed in aerobic terrestrial 

environments (MRIDs 40638629, 41001517), they have the ability to move freely through the 

soil column into groundwater. In the absence of toxicity data, the normal and reverse acids will 

be added into the tolerance expression along with the parent dithiopyr as recommended by the 

Health Effects Division. EFED requests DAS to submit the mentioned groundwater monitoring 

study for dithiopyr. 

Comments regarding ecological incidents 

Reference: Pages 15-16, Section 6.1 and Table 6-3. Document states: In the risk assessment, 

the incidents will be further evaluated to determine if the reported incidents represent current 

patterns of use for dithiopyr. Examples of additional considerations are mitigation (e.g., reduced 

application rates), product cancellations, and changes in use patterns that have occurred since the 

date of the reported incident(s). 

DAS Comment: The majority of the listed incidents occurred in 1992 when Monsanto was the 

primary technical registrant. It is important to note that the use sites indicated in 28 of the 32 
3 

incidents in Table 6-3 relate to applications made to golf greens. DAS labels for dithiopyr either 

1) explicitly prohibit uses on putting greens or 2) clearly lay out detailed restrictions and 

procedures for effective use on putting greens. The development and implementation of this 

label language is intended to ensure undesirable effects do not occur. Based on the ecological 

incidents noted in Table 6-3 of the EFED Problem Formulation document, DAS believes it is 

reasonable to assume that the absence of reported incidents relating to putting greens for over 20 

years is an indicator of the positive effect of the label language used. 

Of the remaining four incidents, the two that occurred in 2003 and 2005 involved tank mixes and 

therefore it cannot be concluded that dithiopyr was the cause. 

One incident was the result of accidental misuse which has not been repeated and the legality of 

the remaining incident is undetermined. Therefore, DAS concludes that the incidents listed are 

not representative of registered uses of dithiopyr. 

Reference: Pages 15-16, Section 6.1 and Table 6-3. Document states: For dithiopyr, registrants 

have reported 6 minor fish and wildlife incidents, 27 minor plant incidents, and 0 other nontarget incidents. Unless additional information on these aggregated incidents becomes available, 

they will be assumed to be representative of registered uses of dithiopyr in the risk assessment. 

DAS Comment: In our review of aggregated incident reports, DAS believes there are specific 

examples of incidents due to off-label use. Additionally some claims of adverse effects are 

unlikely based on available relevant scientific studies. Further, there appears to be a relatively 

low number of reported incidents over the 23 year history of use in the United States. There 

should not be an adverse effects concern with registered uses of dithiopyr products based on 

current Dow AgroSciences product labels. DAS is willing to discuss in more detail if the 

Agency has specific questions or concerns on this topic. 

EFED Response: The total number of actual incidents associated with the use of dithiopyr may 

be higher than what is reported to the Agency. Incidents may go unreported since effects may 

not be immediately apparent and/or readily attributed to the use of a chemical. Therefore, a lack 

of reported incidents should not be taken as an indicator of little or no ecological risk. 

As stated in the problem formulation, the incidents will be further evaluated at the time of the 

risk assessment to determine if the reported incidents represent current patterns of use for 

dithiopyr. The Agency will consider the registrant’s comments during this evaluation. 

Comments regarding the environmental fate of dithiopyr and associated studies 

Reference: Page 19, Table 11-1, Row 5. Document states: 835.4200 Anaerobic soil metabolism 

- Stability will be assumed in the absence of data. 

DAS Comment: DAS agrees with the Agency assessment and assumption to be made in the 

absence of data. Stability is assumed, and a default half-life of 1000 days will be acceptable in 

the risk assessment. 
4 

EFED Response: Stability will be assumed in the absence of anaerobic soil metabolism data. 

In the current environmental exposure models, the input value is zero, which represents stable 

compounds. Please note, the anaerobic soil metabolism data will continue to be a data gap and 

OCSPP Guideline 835.4200 will remain outstanding. 

Reference: Page 19, Table 11-1, Row 6. Document states: EPA has identified the following 

data gap: 835.4300 Aerobic aquatic metabolism. 

DAS Comment: A flooded soil metabolism study was conducted for dithiopyr in 1989 [6]. The 

study examined two unique flooded soils and demonstrated an average half life of 440 days (490 

d in Arkansas clay soil, 390 d in volcanic ash-paddy soil). This study can be provided to the 

EPA upon request. 

Reference: 

Pantano, L. K. The Flooded Aerobic Soil Metabolism of MON 7200, 3,5- Pyridinedicarbothioic 

Acid, 2-(difluoromethyl)-4-(20methylpropyl)-6- (trifluoromethyl)-,S,S-dimethyl Ester. 

Monsanto unpublished report number MSL-8196, 1989. 

EFED Response: EFED requests DAS to submit the mentioned flooded aerobic soil metabolism 

study to the Agency for review and consideration. 

Reference: Page 19, Table 11-1, Row 7. Document states: 835.4400 Anaerobic aquatic 

metabolism - Stability will be assumed in the absence of data. 

DAS Comment: DAS agrees with the Agency assessment and assumption to be made in the 

absence of data. Stability is assumed, and a default half-life of 1000 days will be acceptable in 

the risk assessment. 

EFED Response: Stability will be assumed in the absence of anaerobic aquatic metabolism data. 

In the current environmental exposure models, the input value is zero, which represents stable 

compounds. Please note, the anaerobic aquatic metabolism data will continue to be a data gap 

and OCSPP Guideline 835.4400 will remain outstanding. 

Reference: Page 19, Table 11-1, Row 9. Document states: 835.1410 Volatility–Field - The 

registrant should submit analytical data for the soil and turf grass surfaces to confirm the 

application rate and the rate of volatilization. If this data cannot be provided, a new study is 

requested. 

DAS Comment: Although a field volatility (FV) study is not a required guideline study for 

dithiopyr registration, the initial registrant (Monsanto) conducted a FV study to obtain a more 

complete picture of the dissipation of dithiopyr under field conditions [7]. FV studies were 

conducted at three sites; one near Atlanta, GA, one near Columbus, OH and one near Cleveland, 
5 

OH. Applications of the EC formulation of dithiopyr were made on fescue turf plots at a rate of 

1 pound of active ingredient per acre (2x the current maximum label rate) at all three locations. 

Air samples were collected at the center of the treated plots at heights of 30.5, 61, and 183 cm 

above ground surface and analyzed for dithiopyr. Samples were collected at eight times; 0, 3h, 

1, 2, 3, 7, 14, and 30 days after dithiopyr application. Samples were collected at the warmest 

time of the day to ensure that the maximum volatilization rates would be observed. Cumulative 

dithiopyr emissions were calculated to be 38.2, 19.5 and 12.5% of applied at Atlanta, Columbus, 

and Cleveland, respectively. As much as 10.9% volatilized from the Atlanta site in the first day 

and helps explain the low ‘day 0’ recoveries observed in the terrestrial field dissipation study. 

The maximum concentration of dithiopyr observed at the 30.5 cm height was 7.82 μg/m3 at the 

Atlanta site, 2.6 μg/m3 at the Columbus site and 5.13 μg/m3 at the Cleveland site, on the day of 

application. Concentrations then declined monotonically to < 0.1 μg/m3 by 30 days after 

application at all three sites. 

The maximum concentration at the 30.5 cm height at the Atlanta site is approximately 10% of 

the saturated vapor concentration (SVC) of dithiopyr (86.2 μg/m3), and represents a reasonable 

worst case concentration for use in an inhalation risk assessment. This is consistent with the 

EPA SAP’s [8] comments; “Volatile flux from crops will result in concentrations that are 

substantially below the saturation vapor pressure of most vapors.” Therefore using the SVC of 

dithiopyr as the exposure endpoint for an inhalation risk assessment is extremely conservative, 

and using the maximum concentration observed 30.5 cm directly over the treated field represents 

a realistic worst case scenario for inhalation exposure. 

The EPA’s request for the soil and turf grass analytical data was addressed by Monsanto [9] in 

which they presented the data that could be used to confirm the application rate of dithiopyr in 

the FV studies. The data are summarized again here: 

1. The same spray mixture of dithiopyr MON15104 EC formulation and water and the same 

spraying equipment were used to spray the plots in the FV study and the foliar dissipation study. 

The MON15104 formulation contains 1 pound of dithiopyr per gallon. The application rate to all 

plots was one pound of dithiopyr per acre. 

2. The assay of the lot number of the MON 15104 formulation used and the volumes of water 

and formulation mixed together were presented in the volatility report, MRID 416151601. 

3. The volume of MON 15104 and water in the tank mix, and the volume of tank mix remaining 

after each application along with the area of the plot were used to calculate the application rate 

and confirm that the nominal application rate of 1 pound of dithiopyr per acre was obtained at 

each site. These data are presented in attachments 2, 3, and 4 in MRID 43043301. 

4. It was noted that the EPA Pesticide Rejection Rate Analysis for field volatility studies 

discusses the rejection factor of inadequate soil data to confirm the application rate and states on 

page 142: “Therefore, soil analysis or other means such as known weights or volumes of 

pesticide applied to the soil may be used to confirm the rate of application, provided that the 

calibration and performance of the application equipment is verified.” 
6 

Because the Pesticide Treatment Calculations reported in Attachments 2, 3, and 4 in MRID 

43043301 provide the data needed to calculate the application rate and also to verify the 

performance of the spray equipment, the registrant believes that the Agency has adequate 

information to conclude that the application rate was one pound of dithiopyr per acre. 

Reference: 

MRID 416151601. Fleschar, E. M. Volatilization of Dithiopyr Residues Following 

Applications to Turfgrass. Monsanto unpublished report number MSL-9686, 1990. 

EPA, 2009. FIFRA SCIENTIFIC ADVISORY PANEL (SAP), December 1 – 3, 2009. Docket 

Number: EPA-HQ-OPP-2009-0687 http://www.epa.gov/scipoly/sap/

MRID 43043301. Fleschar, E. M. Response to Agency Reviews of the Previously Submitted 

Study: "Volatilization of Dithiopyr Residues Following Applications to Turfgrass" MRID 

416151601. Monsanto unpublished report number MSL-9686, 1993. 

EFED Response: EFED appreciates the data furnished by DAS confirming the application rate 

and the rate of volatilization of dithiopyr.

References: Page 19, Table 11-1, Row 9. Document states: 835.1410 Volatility–Field - The 

registrant should submit analytical data for the soil and turf grass surfaces to confirm the 

application rate and the rate of volatilization. If this data cannot be provided, a new study is 

requested. 

Page 3, Section 1 (also noted on Page 5, Table 1, Row 4 of the Preliminary Work Plan). 

Document states: Major findings include data needs and the uncertainty described below: 

OCSPP Guideline 835.1410 (TGAI) Volatility-laboratory. 

DAS Comment: DAS would like to seek clarity on the perceived uncertainties, proposed data 

gaps, and data needed to conduct the risk assessment relating to volatility. 40 CFR §158.1300 

cites field and lab volatility studies as conditionally required for terrestrial nonfood use patterns 

and not required for residential outdoor use patterns. DAS believes that a clear understanding of 

exposure and toxicity have been outlined from existing studies/publications [7-10], supportive 

factual information (Henry’s law constant and vapor pressure, as defined in Table 5-1 of EFED 

Problem Formulation), and additional comments made in this document (Comment #7 to EFED 

Problem Formulation, Comment #2 to HED Scoping Document). 

Reference: 

MRID 416151601. Fleschar, E. M. Volatilization of Dithiopyr Residues Following 

Applications to Turfgrass. Monsanto unpublished report number MSL-9686, 1990. 

EPA, 2009. FIFRA SCIENTIFIC ADVISORY PANEL (SAP), December 1 – 3, 2009. Docket 

Number: EPA-HQ-OPP-2009-0687 http://www.epa.gov/scipoly/sap/
7 

MRID 43043301. Fleschar, E. M. Response to Agency Reviews of the Previously Submitted 

Study: "Volatilization of Dithiopyr Residues Following Applications to Turfgrass" MRID 

416151601. Monsanto unpublished report number MSL-9686, 

1993. 

MRID 40638610. Yoshida, M. MON 7200 Acute Inhalation Toxicity Study in Rats. Monsanto 

unpublished report number ET-87-124, 1988. 

EFED Response: EFED appreciates the data furnished by DAS concerning field volatility. The 

data provided will be reviewed by the Agency to characterize the volatility of dithiopyr for 

environmental risk assessments. 

Comments regarding the ecological effects of dithiopyr and associated studies 

Reference: Page 22, Table 11-2, Row 1. Document states: 850.1350 Saltwater invertebrates life 

cycle - In the absence of these data, estuary/marine invertebrate chronic toxicity data for 

surrogate pesticides – starting with data for closely related pesticides (e.g., pyridine herbicides) 

and proceeding to data for less related pesticides – may be used to characterize the chronic 

toxicity of dithiopyr to estuary/marine invertebrates. 

Reference: Page 23, Table 11-2, Row 1. Document states: 850.1350 Saltwater invertebrates life 

cycle - In the absence of these data, sediment-dwelling invertebrate chronic toxicity data for 

surrogate pesticides – starting with data for closely related pesticides (e.g., pyridine herbicides) 

and proceeding to data for less related pesticides – may be used to characterize the chronic 

toxicity of dithiopyr to sediment-dwelling invertebrates. 

Reference: Page 23, Table 11-2, Row 2. Document states: 850.4400 Aquatic plant toxicity test 

using Lemna spp. - In the absence of these data, Lemna toxicity data for surrogate pesticides – 

starting with data for closely related pesticides (e.g., pyridine herbicides) and proceeding to data 

for less related pesticides – may be used to characterize the chronic toxicity of dithiopyr to 

aquatic vascular plants. 

Reference: Page 23, Table 11-2, Row 3. Document states: 850.4500 Algal toxicity - In the 

absence of these data, diatom toxicity data for surrogate pesticides – starting with data for closely 

related pesticides (e.g., pyridine herbicides) and proceeding to data for less related pesticides – 

may be used to characterize the chronic toxicity of dithiopyr to freshwater and marine diatoms. 

Reference: Page 24, Table 11-2, Row 1. Document states: 850.4550 Cyanobacteria - In the 

absence of these data, cyanobacteria toxicity data for surrogate pesticides – starting with data for 

closely related pesticides (e.g., pyridine herbicides) and proceeding to data for less related 

pesticides – may be used to characterize the chronic toxicity of dithiopyr to cyanobacteria. 

Reference: Page 24, Table 11-3, Row 1. Document states: 850.2100 Avian oral toxicity - In the 

absence of these data, passerine toxicity data for surrogate pesticides – starting with data for 
8 

closely related pesticides (e.g., pyridine herbicides) and proceeding to data for less related 

pesticides – may be used to characterize the chronic toxicity of dithiopyr to passerine species. 

DAS Comment: DAS is in favor of not sacrificing more organisms in toxicity testing if EPA 

can determine that the use of surrogate data will not significantly impact the current available use 

patterns of dithiopyr. In the absence of such assertion, DAS will conduct the tests upon a request 

from EPA. 

EFED Response: As stated in the executive summary of the problem formulation, EFED 

considers the above studies to be “data needs” for conducting the Registration Review ecological 

risk assessment. Only in the absence of data specific to dithiopyr will data for surrogate 

pesticides be used to characterize the toxicity of dithiopyr to terrestrial and aquatic organisms. 

The use of data for surrogate pesticides may result in conservative assumptions regarding the 

toxicity of dithiopyr. Therefore, how the use of data for surrogate pesticides will affect risk 

conclusions for dithiopyr is unknown. In response to a Data Call-In (DCI) for an ecological 

effects study, the registrant may submit a waiver request that includes an analysis of surrogate 

toxicity data for consideration towards fulfilling the need for dithiopyr-specific data. 

 

Reference: Page 22, Table 11-2, Row 3. Document states: 850.1400 Saltwater fish early-life 

stage - In the absence of these data, an acute-to-chronic ratio derived from toxicity data for 

freshwater fish may be used to estimate the chronic toxicity of dithiopyr to estuarine/marine fish. 

This methodology may yield a conservative estimate of the chronic toxicity of dithiopyr to 

estuarine/marine fish given the non- definitive acute toxicity endpoint from sheepshead minnow. 

DAS Comment: DAS is in favor of not sacrificing more organisms in toxicity testing if EPA 

can determine that the use of an acute-to-chronic ratio derived from toxicity data for freshwater 

fish will not significantly impact the current available use patterns of dithiopyr. In the absence 

of such assertion, DAS will conduct the tests upon a request from EPA. 

EFED Response: EFED anticipates using an acute-to-chronic ratio derived from toxicity data 

for freshwater fish to estimate the chronic toxicity of dithiopyr to estuarine/marine fish. As 

noted in the problem formulation, this methodology may yield a conservative estimate of the 

chronic toxicity of dithiopyr to estuarine/marine fish given the non-definitive acute toxicity 

endpoint for sheepshead minnow. The registrant may submit an analysis of existing toxicity data 

for dithiopyr and/or surrogate pesticides to be considered for characterizing the chronic toxicity 

of dithiopyr to estuarine/marine fish. 

 

Reference: Page 25, Table 11-3. Document states: EPA has identified the following data needed 

to conduct the risk assessment. ...adult honey bee acute oral toxicity data are not available. 

DAS Comment: An adult honey bee acute oral and a further contact toxicity study were 

conducted in 2013 according to OECD guidelines 213 and 214 and GLP standards in order to 

satisfy the requirements for China [11]. The acute oral and acute contact 48- hour LD50 of 

dithiopyr technical was found to be > 119 μg a.i./bee and > 100 μg a.i./bee, respectively. These 
9 

endpoints provide evidence that dithiopyr is practically non-toxic to adult honey bees. This 

study can be provided to the EPA upon request. 

Reference: 

Verge, E. Acute Oral and Contact Toxicity to the Honeybee Apis mellifera L. (Hymenoptera, 

Apidae) under Laboratory Conditions. Eurofins unpublished report number S13-00206, 2013. 

EFED Response: EFED encourages the registrant to submit the above referenced study (i.e., 

Verge, 2013) for consideration towards fulfilling the need for honey bee acute oral toxicity data. 

Reference: Page 25, Table 11-3. Document states: The “White Paper in Support of the Proposed 

Risk Assessment Process for Bees” as submitted to the FIFRA Scientific Advisory Panel 

proposes a screening-level risk assessment process that includes acute contact and oral toxicity 

tests with adult bees and an acute oral toxicity test with larval bees. Larval and adult honey bee 

acute oral toxicity data are not available. In the absence of these data, honey bee oral toxicity 

data for surrogate pesticides – starting with data for closely related pesticides (e.g., pyridine 

herbicides) and proceeding to data for less related pesticides – may be used to characterize the 

oral toxicity of dithiopyr to honey bees. 

DAS Comment: As dithiopyr has a herbicide mode of action and is not an insect growth 

regulator, it is not expected to impact terrestrial invertebrates. This is supported by the 850.3020 

compliant study referenced in EPA-HQ-OPP-2013-0750-0003 [12], as well as two additional 

GLP studies available [11], which provide evidence that dithiopyr is practically non-toxic to 

adult honey bees and does not cause sub-lethal effects or behavioral differences. Therefore it is 

reasonable to assume that if residues are present in nectar and pollen, during flowering, those 

residues would similarly be practically non-toxic to adult honey bees and that any residues taken 

back to the hive would also be practically non toxic to honey bee larvae. 

Additionally, dithiopyr is a pre-emergence and early post-emergence herbicide not applied 

during flowering growth stages and, therefore, applications are expected to have negligible 

impact on foraging worker bees. 

DAS disagrees that an acute oral toxicity test with larval bees is necessary based on the available 

information. 

References: 

MRID 40638626. Hoxter, K. A. MON 7200 An Acute Contact Toxicity Study with the Honey 

Bee. Wildlife International unpublished report number 139-236, 1987. 

Verge, E. Acute Oral and Contact Toxicity to the Honeybee Apis mellifera L. (Hymenoptera, 

Apidae) under Laboratory Conditions. Eurofins unpublished report number S13-00206, 2013. 

EFED Response: At this point, EFED does not have an understanding of the relative sensitivity 

of the adult and larval life stages of honey bees. Even though the mode of action of dithiopyr on 
10 

target species would not be of concern for adult bees, it may have an unintended mode of action 

that could impact larvae. 

In regards to exposure, DAS did not provide enough information to preclude an exposure to 

larvae. There may be exposure if it is applied to areas adjacent to plants that are flowering. In 

addition, since dithiopyr is systemic, there may be exposure if it is persistent enough in treated 

plants to still be present at the time of flowering (and therefore be present in nectar and pollen).