UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF 

CHEMICAL SAFETY AND

 POLLUTION PREVENTION 

MEMORANDUM

Date: March 31, 2015

Subject: Response to Public Comments Received Regarding EPA Endangered Species 

Assessment for 2,4-D Choline Salt in Arkansas, Kansas, Louisiana, Minnesota, 

Missouri, Mississippi, Nebraska, Oklahoma, Tennessee, and North Dakotaa

Product Name: Enlist DuoTM

EPA Registration Number: 62719-649

Application Date: November 1, 2011

On October 16, 2014 the Environmental Protection Agency (EPA) published for comment an 

Endangered Species Assessment regarding the proposed use of Enlist Duo in the following 10 

states: Arkansas, Kansas, Louisiana, Minnesota, Missouri, Mississippi, Nebraska, Oklahoma, 

Tennessee, and North Dakota. However, EPA is not going forward at this time with registering 

Enlist Duo for use in Tennessee.

The initial Enlist Duo registration allowed use in 6 states: Illinois, Indiana, Iowa, Ohio, South 

Dakota, and Wisconsin. The only proposed change to the Enlist Duo registration under 

consideration on October 16, 2014 was the addition of the 10 new states, and the only new 

evaluation done by EPA concerning this change was the Endangered Species Assessment 

published for comment on October 16, 2014. EPA did not solicit comments outside the scope of 

the Endangered Species Assessment for the 10 new states. EPA previously considered 

comments on the original Enlist Duo registration and addressed them in the original registration 

decision and response to comments document. The original registration decision, response to 

comments document, and all of the materials supporting the original registration are available in 

Docket ID: EPA-HQ-OPP-2014-0195, as is the Endangered Species Assessment regarding the 

addition of the 10 new states. Comments received during this public participation process not 

 

a Although EPA published for comment, on October 15, 2014, an Endangered Species Assessment regarding the 

proposed use of Enlist Duo in the 10 states of Arkansas, Kansas, Louisiana, Minnesota, Missouri, Mississippi, 

Nebraska, Oklahoma, Tennessee, and North Dakota, EPA is not going forward at this time with registering Enlist 

Duo for use in Tennessee.
related to the Endangered Species Assessment pertaining to the 10 new states are not discussed 

in this response.

The Agency received 34,526 comments in response to this public participation process. The 

EPA welcomes input from the public during the decision process when amending pesticide 

registrations, and is committed to thoroughly evaluating and mitigating any potential risks from 

registered pesticides, consistent with applicable statutory standards. Also, EPA strives to 

document and explain the basis of its regulatory decisions through public documents. 

The majority of the comments received were outside the scope of this action – the Endangered 

Species Assessment for the 10 (now 9) additional states. EPA notes that its previous response to 

comments addressed virtually all of these concerns. Commenters are referred to Docket ID: 

EPA-HQ-OPP-2014-0195 for those responses and any additional information regarding the 

decision to register Enlist Duo. 

Relevant comments concerning the Endangered Species Assessment that were raised for the 

additional states are discussed below

1. “No effect” determinations are arbitrary and capricious because they follow the old 

system. (i.e., species within the action area were found to be not affected if there were no 

indirect or direct effects pertinent to the species) and did not follow the National Academy of 

Sciences report recommendations regarding pesticide mixtures; sublethal, indirect, and 

cumulative effects; and surrogate species.

As stated in a recent report to Congress submitted by EPA, the National Marine Fisheries 

Service (NMFS), the U.S. Fish and Wildlife Service (USFWS), and the U.S. Department of 

Agriculture (USDA):

“EPA scientists used highly conservative and protective assumptions to 

evaluate ecological risks for the new uses of 2,4-D in Enlist Duo. The 

assessments confirm that these uses meet safety standards for pesticide 

registration, and as approved, will be protective of non-target species, 

including endangered species.”

Interim Report to Congress on Endangered Species Act Implementation in Pesticide 

Evaluation Programs (“Interim Report”) at 20 (Dec. 11, 2014), available at

http://www.epa.gov/oppfead1/endanger/2014/esa-reporttocongress.pdf. Additionally, the 

Interim Report shows that the combined agencies acknowledge that EPA’s evaluation of 2,4-

D in Enlist Duo was appropriately based on EPA’s “Overview Document-compliant 

endangered species assessment for new herbicide tolerant crop uses” (Id. at 22), referring to 

EPA’s Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs, U.S. Environmental Protection Agency – Endangered and Threatened Species 

Effects Determinations (“Overview Document”) (January 23, 2004), available at

http://www.epa.gov/oppfead1/endanger/consultation/ecorisk-overview.pdf.
2. EPA’s determinations that listed species are not expected to occur in an action area is 

arbitrary and capricious. EPA may not limit the scope of the action area to the treated field, 

EPA must consult if it is known that the chemical will move off the field.

EPA relied on U.S. Fish and Wildlife and National Marine Fisheries Services (the Services)

location information, habitat information, and the results of the screening assessment (with 

risk mitigation measures in place) to establish the relationship of species locations with the 

expected area where the pesticide exposure would rise to the point where effects were 

reasonably expected to take place for any taxonomic group. The Agency makes no claim 

that drift and runoff do not occur. The Agency characterized the levels of exposure to drift 

and runoff and determined that, with mitigations in place, exposures high enough to cause 

acute or chronic effects are not reasonably expected to occur in aquatic systems adjacent to 

treated fields, nor to any place suitable for wildlife adjacent to fields for any taxonomic 

group. The Agency clearly stated in the screening assessment that exposures were only 

above levels of concern to organisms on treated fields.

EPA used the available terrestrial and aquatic listed species locations by state and the 

available habitat requirement information from the Services to compare with the locations of 

treated use sites. If a species did not occur in a county where the use sites occur, the species 

was reasonably expected to have no effects because it was not co-located with the use site. If 

a species was in a county where the use site was located, it was assumed that it was possible 

it could be co-located with the action areas (the area where effects occur is defined by the 

geographical extent where exposure would elicit effects to any taxonomic group) and that 

further evaluation was necessary to determine if this was actually the case. In such situations

the Agency consulted biological information from the Services to determine if the species 

occurred within the action area. 

If according to the Services, a listed species was an aquatic species without a terrestrial 

phase, it was expected to occupy an aquatic environment, and the screening-level risk 

assessment found no exposures sufficient to elicit effects in aquatic environments. 

Consequently, the Agency concluded aquatic species are not in the action area.

If a listed species was a terrestrial organism in any phase according to the Services, it was 

assumed to occupy habitats that were not otherwise managed to the exclusion of listed 

species (i.e. areas not paved, not enclosed by structures, or areas in row crop). Since the 

steps above found that mitigations prevented pesticide exposure to rise to levels reasonably 

expected to cause effects in areas other than the treated use site, no effects would occur to 

these off-field organisms. Exposure could still occur, just not at levels that would trigger any 

effects concerns for any taxonomic group. For species for which USFWS information 

suggested occurrence on a use site at any time during their life cycle, the Agency turned to 

the biology of the species to determine whether species-specific information would refine the 

screening-level risk conclusions. If the effects expected on the field were not directly related 

to direct effects to the listed species, did not affect any resource the species needed, nor upon 

reassessment of the risk calculations show a species-specific finding that exposures rose to a 

level of concern, the Agency concluded “no effect.”
3. Exposure to the pesticide alone is enough to conclude a “may affect.” Reliance on RQ 

values to make this determination is inappropriate. As noted above in response to Comment 

#1, a recent report to Congress submitted by EPA, NMFS, USFWS, and USA stated:

“EPA scientists used highly conservative and protective assumptions to 

evaluate ecological risks for the new uses of 2,4-D in Enlist Duo. The 

assessments confirm that these uses meet safety standards for pesticide 

registration, and as approved, will be protective of non-target species, 

including endangered species.”

Interim Report at 20 (Dec. 11, 2014). Additionally, the Interim Report shows that the 

combined agencies acknowledge that EPA’s evaluation of 2,4-D in Enlist Duo was 

appropriately based on EPA’s “Overview Document-compliant endangered species 

assessment for new herbicide tolerant crop uses” (Id. at 22), referring to EPA’s Jan. 23, 2004 

“Overview Document” regarding endangered and threatened species effects.

4. The Agency did not use or review ECOTOX data.

The Agency relied on previously evaluated data sets from earlier 2,4-D risk assessments, 

which included ECOTOX data. 

5. The Agency failed to allow for uncertainty in the analysis of effects to honeybees and the 

non-definitive endpoints for other taxa.

The honeybee data and other non-definitive data were used as surrogate toxicology 

information for non-target species, including listed species. Non-definitive data were 

“greater than” values, meaning that the highest toxicity level tested did not result in an acute 

endpoint (i.e., the LD50 is greater than the highest concentration of 2,4-D that was tested). 

The Agency relied on the best available data within the confines of data sets from previous 

2,4-D risk assessments. Exposure estimates were compared to the available data and showed

exposure levels below what was reasonably and conservatively expected to be effects 

endpoints.

6. The Agency did not account for the combined effects of glyphosate and 2,4-D.

The Agency looked at available 2,4-D/glyphosate combined data from the scientific peerreviewed literature and registrant-submitted information for mammal, freshwater 

invertebrates and freshwater fish and found no combined effects that exceeded the expected 

effects of glyphosate or 2,4-D individually.

7. EPA failed to consider glyphosate effects. 

Since the registration of Enlist Duo does not contain any new use for glyphosate, as it has 

been registered for use on genetically engineered corn and soybeans for several years now 

and is presently used on the majority of those crops’ production acres, EPA did not conduct a 

new assessment for the glyphosate portion of Enlist Duo. Also, EPA’s decision did not 
authorize any change in when, where, how, or how much glyphosate can be used from what 

is already authorized under FIFRA. 

8. Indiana bat. EPA arbitrarily used assumptions of farm field foraging frequency to 

reduce exposure levels below the concern threshold. EPA, by admitting that 2,4-D exposure 

would occur, is bound to make a “may affect” determination. 

EPA undertakes a careful and extensive scientific analysis in all of its ESA-related pesticide 

effects analyses, as it did here. The Agency begins its screening level assessments by 

conducting a basic ecological risk assessment. That assessment uses broad default 

assumptions to establish estimated environmental concentrations of particular pesticides. If 

that screening level assessment results in a determination that the pesticide has no effect on 

listed species, EPA preserves its limited resources by stopping there. On the other hand, 

where the screening level assessment does not result in a no effect finding, EPA then uses 

increasingly specific methods and exposure models to refine its estimated environmental 

concentrations. At each screening step, EPA compares the more refined concentrations to 

the toxicity of the pesticide active ingredient to determine whether the pesticide exceeds 

levels of concern established for listed aquatic and terrestrial species. EPA determines that 

there is “no effect” on listed species if, at any step in the screening level assessment, no 

levels of concern are exceeded. If, after performing all the steps in the screening level 

assessment, a pesticide still exceeds the Agency’s levels of concern for listed species, EPA 

then conducts a species-specific assessment to make effects determinations for individual 

listed species. That assessment, unlike the screening level assessment, takes account of 

species’ habitats and behaviors to determine whether any listed species may be affected by 

use of the pesticide.

EPA followed this careful and methodical tiered approach here. In 2013, EPA performed a 

screening level risk assessment of the proposed new uses of 2,4-D in Enlist Duo. The 

Agency subsequently refined its endangered species risk assessment based on spray drift 

mitigation language that Dow AgroSciences added to the Enlist Duo label at EPA’s request. 

Specifically, EPA noted that the spray drift language would limit drift by requiring end users 

to utilize a particular nozzle and mix formulation. This language also required end-users to 

employ a substantial buffer when the wind is blowing toward any area that is not a field in 

crop cultivation, paved area, or area covered by buildings and other structures.

EPA next took into account species-specific biological information as it relates to the timing 

of Enlist Duo applications (and thus potential exposure to 2,4-D). EPA’s refined analysis 

observed that Indiana bats typically forage in closed to semi-open forested habitats and forest 

edges and prefer to forage in wooded areas. A ranking of the foraging use of habitats 

suggested the following order of foraging preference: (a) roads; (b) forests; (c) riparian areas; 

(d) grasslands; (e) agricultural areas. EPA’s refined analysis went on to analyze the 

likelihood that the bat could use agricultural land as a source of prey. EPA concluded that

the bat could make use of agricultural land as a source of prey and roost in patches of 

fragmented forest that are adjacent to corn and soybean fields. The extent of foraging over 

agricultural land, however, would be less than the degree of foraging around the canopies of 

forested areas.
EPA conservatively estimated that 33% of the bat’s diet could be consumed over agricultural 

lands. EPA incorporated this and other data, such as the metabolized energy in bat prey, the 

daily food ingestion rate scaled to individual bat bodyweight, and the agricultural application 

rate of 2,4-D. Taken together, EPA’s analysis established a complete exposure pathway for 

Indiana bats to 2,4-D for those times when the bat does forage over treated fields. EPA 

concluded that no bat would experience even one day of exposure exceeding the applicable 

toxicological threshold. Accordingly, EPA determined that the 2,4-D component of Enlist 

Duo would have no effect on the Indiana bat.

Again, as noted above in response to comment #1 and #3, a recent report to Congress 

submitted by EPA, NMFS, USFWS, and USA stated:

“EPA scientists used highly conservative and protective assumptions to 

evaluate ecological risks for the new uses of 2,4-D in Enlist Duo. The 

assessments confirm that these uses meet safety standards for pesticide 

registration, and as approved, will be protective of non-target species, 

including endangered species.”

Interim Report at 20 (Dec. 11, 2014). Additionally, the Interim Report shows that the 

combined agencies acknowledge that EPA’s evaluation of 2,4-D in Enlist Duo was 

appropriately based on EPA’s “Overview Document-compliant endangered species 

assessment for new herbicide tolerant crop uses” (Id. at 22), referring to EPA’s Jan. 23, 2004 

“Overview Document” regarding endangered and threatened species effects.

9. Louisiana Black Bear. Despite the screening-level assessment trigger for concerns, the 

Agency made up additional components to its illogical assessment to reach a “no effect.” 

These kind of specific biological assumptions are outside of EPA’s expertise and are suspect.

Once the screening level assessment and co-location analysis suggested the bear was 

potentially at risk, the Agency compared available biological/behavioral data identified in the 

literature and in USFWS sources to determine if the exposure assumptions employed in the 

conservative screening assessment were biologically appropriate for the bear. Based on this 

review, more relevant exposure inputs were developed for the bear (i.e., seasonal timing of 

when the bear is expected to use agricultural fields) and the risk assessment was refined to 

reflect these additional lines of evidence. The refined analysis indicated that the exposures 

were overestimated in the screen, and exposures under more biologically relevant conditions 

were below levels of concern.

10. Whooping Crane. EPA did not reproduce the calculations for this 10-state assessment 

that was done for the previous six states. EPA did not conduct a chronic assessment nor 

consider residues on all the food items consumed by the crane.

The chronic assessment for all bird species (including the whooping crane) was included in 

the screening risk assessment and found no risk concerns for birds; only concerns for acute 

effects were identified. The assumptions at the screening-level are intentionally more 
conservative than species-specific analyses; thus, the lack of chronic concerns at the 

screening-level indicated that further refinement would not identify risk and was not 

necessary. EPA did not repeat all the calculations in the 10-state assessment because the 

findings in the previous assessment were based on identical assumptions of exposure and 

toxicity regardless of the state in which the species occurs. Consequently, citing the 

documents in which these analyses were contained was appropriate.

The dietary analysis was performed on the food item that contained the highest 2,4-D 

residues. By assuming that 100 percent of the diet was made up of the food item with the 

highest residues, the analysis is protective of food items with lower residues as well.

11. Mississippi Sandhill Crane. The assessment ignored chronic effects nor did it consider 

residues on all of the food items in the crane’s diet. The RQ fell just below the level of 

concern, so EPA should check its fuzzy math. Given the sensitive status of the crane (located 

on and adjacent to the Mississippi Sandhill Crane National Wildlife Refuge), Enlist Duo 

should not be registered in Mississippi.

See Comment 10 for a response to the chronic effects and dietary analyses. The Agency 

applied the level of concern analysis in an objective manner. The resulting exposure-toeffects ratio is below the concern level that the Agency has used under the evaluation 

approach selected for this assessment. Consequently, “no effects” are expected for the 

Mississippi Sandhill Crane.

12. Lesser Prairie Chicken. The Agency failed to conduct a chronic effects assessment. 

EPA’s attempts to pretend to understand lesser prairie chicken biology by citing a few 

random scientific journal articles is laughable, absurd, and otherwise arbitrary and 

capricious.

See Comment 10 for a response to the chronic effects analysis. EPA relied on publically 

available published information that described the extent to which the lesser prairie chicken

uses agricultural environments. These findings were compared to the assumptions made in 

the screening assessments and all lines of evidence lead to a conclusion that the species 

would not be exposed to 2,4-D choline salt at the levels that were assumed in the screen. 

This initiated a more biologically-relevant exposure estimate that resulted in a finding that 

exposures fell below the concern level for this species. EPA did not find any data showing

agricultural field use by the species from USFWS reports and citations made in those reports 

or reports citing those references. 

13. Gopher Tortoise. The Agency did not complete a chronic assessment. Exposure 

demonstrates that the species is affected.

Birds are used as surrogates for reptiles under the assessment approach when data are 

unavailable for reptiles (USEPA 2004). Birds are likely a conservative surrogate for 

exposure to pesticide residues via the diet due to their higher metabolic rate as warm-blooded 

creatures compared to cold-blooded reptiles, which suggests that food consumption rates, and 
therefore dietary pesticide consumption rates, would be higher for birds. Furthermore, with 

respect to chronic assessment and its reproductive endpoints, both birds and the majority of 

reptiles are egg-layers representing very similar exposure scenarios en-ovo. The screening 

assessment found no chronic concerns for birds and thus no chronic concerns for reptiles.

14. American Burying Beetle. There are no established levels of concern for terrestrial 

invertebrates, presumably because EPA seeks to register pesticides that kill terrestrial 

invertebrates. EPA cites DP411614 as reason for “no effect” and it is not in the record. 

EPA did not consider direct effects (i.e., brushing up against treated plants) to the beetle. 

The Agency has established an interim level of concern of 0.05 that has been used for 

litigation assessments and other endangered species risk assessments. Direct effects to 

terrestrial invertebrates (e.g., the American burying beetle) were assessed in the screeninglevel risk assessment and EPA concluded that direct effects were not a concern. Therefore, 

only indirect effects to the American burying beetle were considered in the refined 

endangered species analysis because direct effects had already been screened out (see 

whooping crane response for more discussion on screening assessments). EPA did not repeat 

the analysis for the American burying beetle in the 10-state assessment because the findings 

in the previous assessment (6-state endangered species assessment, DP411614) were based 

on identical assumptions of exposure and toxicity regardless of the state in which the species 

occurs. Consequently, citing the DP barcode of the document in which this analysis was 

contained was appropriate. DP411614 was placed in the docket in April 2014 (EPA-HQOPP-2014-0195-005). 

15. Spring Creek Bladderpod. By EPA’s own tortured logic, the pesticide cannot be 

approved for use in Wilson County, TN. Enlist Duo should not be registered in Tennessee.

EPA is not going forward at this time with registering Enlist Duo for use in Tennessee. 

16. EPA did not consider populations of its eleven identified listed species that are being 

reintroduced or expanding ranges into areas of the states where corn and soybean are 

grown.

In reaching effects determinations, the Agency referred to species recovery plans, species 5-

year reviews, and United States Fish and Wildlife Service (USFWS) county-level summaries 

of occurrence. A species was considered if USFWS information indicated endangered or 

threatened populations were located within any of the states included in this assessment. 

17. The Agency failed to assess adverse modification or destruction of critical habitat. 

Effects for any biologically mediated process are likely limited to the area where exposure to 

the herbicide rises to the level of affecting one or more taxonomic groups. The Agency 

demonstrated that effects are limited to the agricultural field, and possibly paved areas and 

areas enclosed by structures. Consequently, critical habitat effects for any species would be 

limited to the areas where biological effects on a single individual could be reasonably 
expected to occur (i.e., treated fields, paved areas, and areas enclosed by structures). A 

detailed analysis is provided in the addendum to the 2,4-D choline salt assessment on corn 

and soybean – critical habitat analysis (USEPA 2015, D425283, Docket ID: EPA-HQ-OPP2014-0195-3688). 

18. EPA’s assumption that spray drift will remain confined to the field because of the spray 

drift mitigation language is flawed. Similarly, EPA claims that there will be no 2,4-D 

choline salt in surface waters and that “the proposed 2,4-D choline uses are not expected to 

overlap with rivers, streams, creeks, or other water bodies.”

The Agency makes no claim that spray drift or runoff will not occur; rather it states that with

mitigations in place, exposures high enough to cause acute or chronic effects are not 

reasonably expected to occur in aquatic systems adjacent to treated fields, nor to any place 

suitable for wildlife adjacent to fields for any taxonomic group. The Agency based the spray 

drift mitigation on empirical field data with Enlist Duo as well as the consideration of 

habitats for non-target organisms. Given that 2,4-D choline salt use is restricted to corn and 

soybean fields that are in agricultural production, no water bodies are expected to be within 

the action area (i.e., the field). Consequently, this is a reasonable rationale for the “no 

effects” determinations for aquatic species.

19. Non-target species may move on and off the field during spraying. The Agency has not 

indicated how the 168 endangered plant and animals in its 2,4-D assessment would be 

informed of these field boundaries so as to remain off of the field.

The Agency’s endangered species analysis considered whether a species would be on the 

field at any time, based on species-specific biology and habits, taken from USFWS’ and 

National Marine Fisheries Service’s documents. Consequently, those species that may move 

on and off the field were captured in this analysis and considered as being “on” the field.

20. Effects of the whole product, Enlist Duo (including active ingredients and the additional 

unidentified ingredients) have not been assessed. In addition, synergistic or additive effects 

of Enlist Duo with atrazine or other commonly used herbicides have not been assessed. Risk 

to terrestrial plants and other organisms have been underestimated because formulation data 

are not available. Indirect effects, such as loss of critical food, forage and nesting sites 

resulting from reduced plant diversity associated with increased spraying of the whole 

product, Enlist Duo (separately and in combination with other pesticide products) has not 

been assessed. 

The response to public comments for the initial registration of Enlist Duo on corn and 

soybeans includes a more detailed analysis on the potential for synergy between the 2,4-D 

choline salt and glyphosate (Document ID number EPA-HQ-OPP-2014-0195-2414). In 

summary, if effects data are available for a formulated product containing more than one 

active ingredient, they may be used qualitatively or quantitatively in accordance with the 

Agency’s Overview Document and the Services’ Evaluation Memorandum (USEPA 2004; 
USFWS/NMFS/NOAA 2004). Acute oral rat data were available for the Enlist Duo product 

as formulated and did not indicate any additive or synergistic effects when compared with 

2,4-D by itself. Because the available data showed no indication that the tested results would 

be different for untested species, this conclusion was extended to all animal taxa. For animal 

taxa where Enlist Duo formulation data were not available, the most sensitive 2,4-D 

acid/salt/amine endpoint was used, as a protective surrogate. For terrestrial plants, 

formulation data were available for a number of other 2,4-D products; the most sensitive of 

these was selected to represent the toxicity of Enlist Duo. The Agency does not routinely 

assess tank mixes, consequently, Enlist Duo tank-mixed with atrazine or other commonly 

used herbicides was not assessed. 

The spray drift mitigation language on the label is expected to keep exposure levels below the 

threshold of concern to any species off of the field. Consequently, only indirect effects that 

take place on a treated corn or soybean field needed to be considered. This was included in 

the endangered species risk assessment.

21. The Agency did not follow its own guidance document for honeybee toxicity assessment 

and thereby underestimates the risks to bees and other beneficial insects. Specifically, 

indirect (e.g., effect of reduced flowering plants for foraging) and direct exposures, the 

differential sensitivity of larvae compared to adult bees, sub-lethal effects that may affect 

brood and colony health, and differences in sensitivity when exposed via that contact or oral 

route were not considered. These data gaps should be filled before approving the 

registration of Enlist Duo. EPA inappropriately classifies 2,4-D as “practically non-toxic” 

to honeybees and fails to consider sub-lethal effects, and the importance of these on hive 

health and colony collapse disorder. 2,4-D could also affect other important insect species 

such as monarch butterflies. Glyphosate was not included in the assessment.

The response to public comments for the initial registration of Enlist Duo on corn and 

soybeans includes a honeybee analysis in accord with the Agency’s Pollinator Risk 

Assessment Framework and includes adult acute oral and contact data (ID# EPA-HQ-OPP2014-0195-2414). The analysis indicated that risk concerns are not expected for acute 

exposures to adult honeybees. Sub-lethal signs of toxicity in the contact study included 

lethargy and loss of equilibrium in a total of three bees at the highest treatment level. The 

loss of equilibrium was only observed at the 24 hour observation period and the lethargy was 

only observed at 72 hours. For the acute oral study, reduced coordination was the only sublethal effect noted in the three highest treatments at 4 hours (2, 1, and 5 bees at 23.2, 42.0 and 

62.6 μg ae/bee treatment levels). At the 24 hour and 48 hour observation periods, only 1 bee 

was affected in each of the two highest treatment groups. Overall, the sub-lethal effects 

appear limited and it is unlikely that they would translate into whole colony effects given 

their limited magnitude. 

Agency policy is that any pesticide with an acute LD50 > 11 μg ae/bee is considered 

“practically non-toxic” to honeybees, based on mortality. Generic data gaps for larval 
honeybees and chronic exposures to adults are being considered as part of the ongoing 

Registration Review process. 

The endangered species assessment considered both direct and indirect risk concerns (for 

example, risk concerns for flowering plants) in its analysis, including those to honeybees, 

monarch butterflies, and other terrestrial invertebrates.

Since the registration of Enlist Duo does not contain any new use for glyphosate, as it has 

been registered for use on genetically engineered corn and soybeans for several years now 

and is presently used on the majority of those crops’ production acres, EPA did not conduct a 

new assessment for the glyphosate portion of Enlist Duo. Also, EPA’s decision did not 

authorize any change in when, where, how, or how much glyphosate can be used from what 

is already authorized under FIFRA. 

22. Large-scale spraying of 2,4-D would contaminate and poison plants and animals in the 

vicinity of the sprayed fields, leading to economical and environmental damage. The spray 

drift assessment is impaired by EPA’s reliance on models rather than real-world data.

The Enlist Duo label approved by EPA contains enforceable spray drift mitigation 

requirements that will prevent 2,4-D choline salt from moving off the field in concentrations 

high enough to exceed our levels of concern in non-target species, including listed species. 

EPA uses the best available data and models in its spray drift assessments. The mitigation 

requirements are based on Enlist Duo specific field and laboratory data; the spray drift 

analyses were tailored specifically to the Enlist Duo use patterns, environmental conditions, 

and formulation data. If additional data become available, EPA will consider it for 

incorporation into future analyses, as appropriate.

23. There are multiple gaps in the 2,4-D volatilization and spray drift data that were 

submitted to the Agency. The data that do exist indicate that there may be risks for 

terrestrial plants and for endangered species of mammals and birds from 2,4-D drift off the 

treated fields.

See the response to public comments for the initial registration of Enlist Duo (ID# EPA-HQOPP-2014-0195-2414) for a robust discussion of the spray drift and volatilization data 

available for the 2,4-D choline salt and the lines of evidence used to arrive at conclusions. 

The screening-level assessment was designed to be very conservative and flagged potential 

risk concerns for terrestrial plants, mammals, and birds. Upon refinement of the assessment 

using species specific information (i.e., body weight, species location, timing of species’ use 

of agricultural fields), it was concluded that “no effects” are anticipated for endangered 

species in the 8 states in which the Agency is registering Enlist Duo for use at this time.

24. EPA did not consider the effects of increased use in 2,4-D that will occur if Enlist Duo is 

registered on corn and soybean in AR, KS, LA, MN, MO, MS, NE, ND, OK, and TN.
The ecological risk assessment and endangered species risk assessment assume an organism 

is exposed to the maximum amount of 2,4-D choline salt that is possible (terrestrial and 

aquatic exposure pathways), given that the label directions are followed. Consequently, the 

risk assessment is protective and does not need to be adjusted for the actual number of acres 

that are treated with Enlist Duo.

25. Risks associated with runoff of Enlist Duo are not adequately assessed by EPA.

On October 10, 2014, the Agency issued an addendum that characterized the amount of 

runoff that would be expected from the proposed 2,4-D choline salt uses. Using 

PRZM/EXAMS, it was determined that the amount of runoff expected from the corn and 

soybean uses is <1% of the applied mass. This is well below the assumptions made by the 

TerrPlant model, which was used as a Tier 1 screening-model for risk concerns. Through 

this analysis EPA found that concentrations of 2,4-D choline in runoff will not present a 

concern to terrestrial plants. See USEPA 2014 (D423309) for more details of the analysis.

26. Metabolism of 2,4-D within Enlist corn and soybeans creates a new route of exposure to 

residues and metabolites that EPA does not address for listed and non-listed species. 

EPA did not assess the metabolites of 2,4-D because the toxicity of and exposure to 2,4-D 

acid was expected to be higher, and thus pose greater risk concerns, than the metabolites. 

The 10-state risk assessment (Addendum to 2,4-D Choline Salt Section 3 Risk assessment: 

Refined Endangered Species Assessment for Proposed New Uses on Herbicide-Tolerant 

Corn and Soybean for AR, KS, LA, MN, MS, MO, NE, ND, OK, TN,” found in Docket ID: 

EPA-HQ-OPP-2014-0195-2419) for terrestrial organisms assumes that 100% of residues are 

2,4-D and that the appropriate toxicity endpoints are relative to the free acid. The acute 

lethal and reproduction mammalian endpoints used in assessment were 441 mg ae/kg-bw and 

55 mg ae/kg-bw/day, respectively. For birds, the acute lethal endpoint was 218.7 mg ae/kgbw and the reproduction endpoint was 962 mg ae/kg-diet. By comparison, available 

mammalian data) on the acute lethal endpoint for the metabolite 2,4-dichlorophenol (2,4-

DCP) is 580 mg/kg-bw (1.3 times less toxic) (EHC, 1989) and the reproduction data endpoint 

is 134 mg/kg-bw/day (2.43 times less toxic) (Aoyama et al., 2005). There exist reports for 

more potent mammalian acute measures of 2,4-dichlorphenol for molten product, e.g. 44 

mg/kg (NTIS OTS0534822 1991), but these are not representative of an oral administration 

expected from its conversion in a biological matrix.

Under the most conservative assumption of degradation, one mole of 2,4-D choline (221.04 

g) yields one mole of 2,4-dichlorophenol (163.0 g) for a mass ratio of 0.738. Consequently, 

the conversion of mass to 2,4-dichlorophenol is about 74% of the corresponding residue 

assumption for 2,4-D ae on the field. However, data submitted to the Agency and reviewed 

by the Health Effects Division (HED)[USEPA 2011, D384441 Summary of Analytical 

Chemistry and Residue Data for Use of 2,4-D Choline in/on Herbicide Tolerant Field Corn 

Containing the Aryloxyalkanoate Dioxygenase-1 (ADD-1) Gene. Health Effects Division, 

Office of Pesticide Programs, Environmental Protection Agency”, found in Docket ID: EPAHQ-OPP-2014-0195-0037; USEPA 2013, D400085 Summary of Analytical Chemistry and 

Residue Data for Use of 2,4-D Choline in/on Herbicide Tolerant Soybeans Containing the 
Aryloxyalkanoate Dioxygenase-12 (AAD-12) Gene. Health Effects Division, Office of 

Pesticide Programs, Environmental Protection Agency, found in Docket ID: EPA-HQ-OPP2014-0195-0005] on soybean and corn indicated that no more than 18.4% of recoverable 

residues were 2,4-dichlorophenol and its conjugates in forage, and no more than 9.15% in 

seed (corn was non-detect).

An example of the impacts of these findings to a risk assessment can be seen with the Indiana 

bat. Assuming that 2,4-D choline salt has been converted to 2,4-dichlorophenol (plus 

conjugates) at a rate of 18.4%, the exposure value (excluding any differences in absorption or 

metabolism in the insect prey) would be 18.4% of the assessment for 2,4-D choline. Toxicity 

values would be lower by 9.36 for acute toxicity and higher by a factor of 2.43 for 

reproduction. Therefore, the acute RQ, under the most conservative approach would be:

2,4-dichlorophenol acute RQ assuming total conversion of 2,4-D to 2,4-dichlorophenol:

(RQ for 2,4-D choline 0.06) (1/1.33)(0.184) = 0.008

2,4-dichlorpophenol reproduction RQ assuming total conversion of 2,4-D to 2,4-

dichlorophenol:

(RQ for 2,4-D choline a.e.0.47) (1/2.43)(0.184) = 0.036

Both the acute and chronic RQ are below levels of concern (acute LOC = 0.1; chronic LOC = 

1). This analysis does not consider the potential for differential in absorption of plant 

associated 2,4-dichlorophenol and its conjugates. Pascal-Lorber et al. (2012) compared the 

metabolic fate of [(14)C]-DCP, [(14)C]-residues from radish plants, and purified [(14)C]-

DCP-(acetyl)glucose following oral administration in rats. These authors found that rat 

absorption of plant-associated 2,4-dichlorophenol metabolites was lower from the plant 

matrix than plant-free conjugates and DCP itself. 

The combined effects of lower toxicity and limited residues in corn and soybean forage and 

seed results in risk quotients for the in-plant degradates that are well below levels of concern. 

Assuming all exposures as parent 2,4-D choline or its acid equivalent remains the most 

conservative approach. 

References

Aoyama, Hiroaki; Hojo, Hitoshi; Takahashi, Ken L.; Shimizu, Naoko; Araki, Masayuki; 

Harigae, Michiko; Tanaka, Naoki; Shirasaka, Noriko; Kuwahara, Maki; Nakashima, Nobuaki; 

Yamamoto, Eri; Saka, Machiko; and Teramoto, Shoji. 2005. A Two-Generation Reproductive 

Toxicity Study of 2,4-Dichlorophenol in Rats. Jour. Toxicol. Schi. 30, Special Issues, 59-78.

EHC (1989) Chlorophenols Other Than Pentachlorophenol. Environmental Health Criteria

(EHC) 93, IPCS, section 8.1. http://www.inchem.org/documents/ehc/ehc/ehc093.htm.

National Technical Information Center (NTIS). 1991. Initial submission: toxicologic 

investigation of: 2,4-dichlorophenol (final report) with cover letter. Dated 11/26/91, 11NTIS 

OTS0534822.
Pascal-Lorber S; Despoux S; Jamin EL; Canlet C; Cravedi JP; Laurent FJ. 2012. Metabolic fate 

of 2,4-dichlorophenol and related plant residues in rats. Agric Food Chem. 2012; 60(7):1728-36 

(ISSN: 1520-5118).

Taylor, L. 2015. Personal communications via e-mail from Linda Taylor, USEPA Health 

Effects Division to Meghan Radtke, USEPA Environmental Fate and Effects Division, March 

10 and 11, 2015.

USFWS/NMFS/NOAA. 2004. 50 CFR Part 402. Joint Counterpart Endangered Species Act 

Section 7 Consultation Regulations; Final Rule. Federal Register Volume 69. Number 20.Pages 

47731-47762. August 5, 2004.

USEPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs, U.S. Environmental Protection Agency. Listed and Threatened Species Effects 

Determinations. Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide 

Programs, Washington, D.C. January 23, 2004. 

USEPA. 2011. Summary of Analytical Chemistry and Residue Data for Use of 2,4-D Choline 

in/on Herbicide Tolerant Field Corn Containing the Aryloxyalkanoate Dioxygenase-1 (ADD-1) 

Gene. Health Effects Division, Office of Pesticide Programs, Environmental Protection Agency. 

Docket ID: EPA-HQ-OPP-2014-0195-0037.

USEPA. 2013. Summary of Analytical Chemistry and Residue Data for Use of 2,4-D Choline 

in/on Herbicide Tolerant Soybeans Containing the Aryloxyalkanoate Dioxygenase-12 (AAD-12) 

Gene. Health Effects Division, Office of Pesticide Programs, Environmental Protection Agency. 

Docket ID: EPA-HQ-OPP-2014-0195-0005.

USEPA. 2014. Addendum to 2,4-D Choline Salt Section 3 Risk Assessment: Refined 

Terrestrial Plant Exposure Estimates and Effects Determination. Environmental Fate and Effects 

Division, Office of Pesticide Programs, Environmental Protection Agency. EPA-HQ-OPP-2014-

0195-2413.

USEPA. 2015. Addendum to 2,4-D Choline Salt Section 3 Risk Assessment: Designated 

Critical Habitat Modification Determinations for Proposed New Uses on Herbicide-Tolerant 

Corn and Soybean for AR, IL, IN, IA, KS, LA, MN, MS, MO, NE, ND, OH, OK, SD, TN, and 

WI. Environmental Fate and Effects Division, Office of Pesticide Programs, Environmental 

Protection Agency. Docket ID: EPA-HQ-OPP-2014-0195-2419.