Docket Number: EPA-HQ-OPP-2009-0846 

www.regulations.gov

Diquat Dibromide Final Work Plan 

Registration Review 

May 2010 

DIQUAT DIBROMIDE FINAL WORK PLAN 

Introduction: 

This is the Environmental Protection Agency’s (EPA’s) Final Work Plan for the 

registration review of diquat dibromide. This work plan includes the expected 

registration review timeline. The work plan also addresses public comments received 

concerning the Preliminary Work Plan in the Summary Document, which was posted in 

the diquat dibromide registration review docket (EPA-HQ-OPP-2009-0846), and any 

other comments concerning the initial docket postings. The Summary Document

provided information on what EPA knows about the pesticide and what additional risk 

analyses and data or information the Agency believes are needed to make a registration 

review decision. 

The Agency is implementing the registration review program and will review each 

registered pesticide every 15 years to determine whether it continues to meet the Federal 

Insecticide, Fungicide, and Rodenticide Act (FIFRA) standard for registration. Changes 

in science, public policy, and pesticide use practices will occur over time. The 

registration review program is intended to make sure that, as the ability to assess risk 

evolves and as policies and practices change, all registered pesticides continue to meet 

the statutory standard. The public phase of registration review begins when the initial 

docket is opened for each case. Information on this program is provided at: 

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

Diquat dibromide (Case 0288) is a non-selective contact algicide, defoliant, desiccant and 

herbicide. As an herbicide/algicide it is registered for use to control broadleaf and grassy 

weeds in non-crop (including residential) and aquatic areas. As a desiccant/defoliant, it is 

registered for use on seed crops and potatoes. 

Comments Received on Preliminary Work Plan: 

Multiple comments were received during the 60-day public comment period on the 

Preliminary Work Plan. These comments are summarized in this document and changes 

to the work plan set out in the Preliminary Work Plan are explained. In the Preliminary 

Work Plan, EPA also solicited comments on three specific topics: environmental justice, 

water body impairment, and trade irritants. No comments or information were received 

during the comment period concerning these issues. 

Risk Assessment and Data Needs: 

The Agency will conduct a comprehensive ecological risk assessment, including an 

endangered species assessment, and intends to update the existing human health risk 

assessments for all uses of diquat dibromide. The Agency will require data in order to 

conduct the planned human health and ecological risk assessments. Below is a summary 

of the ecological and human health issues relevant to the registration review of diquat 

dibromide: 

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Ecological Risk: 

• The most recent ecological assessment for diquat dibromide was completed in 

1995 in support of the Diquat Dibromide Reregistration Eligibility Decision 

(RED). 

• The Agency has not conducted a risk assessment that supports a complete 

endangered species determination. The ecological risk assessment planned during 

registration review will allow the Agency to determine whether diquat 

dibromide’s use has “no effect” or “may affect” federally listed threatened or 

endangered species (listed species) or their designated critical habitats. When an 

assessment concludes that a pesticide’s use “may affect” a listed species or its 

designated critical habitat, the Agency will consult with the U.S. Fish and 

Wildlife Service and/or National Marine Fisheries Services (the Services), as 

appropriate. 

• The Agency will require the following data in order to conduct the planned 

ecological risk assessment, including an endangered species assessment, for all 

uses: 

o OCSPP Guideline 850.1450 - Fish Early-Life Stage Toxicity 

(estuarine/marine) 

o OCSPP Guideline 850.1350 - Aquatic Invertebrate Life Cycle Toxicity 

(estuarine/marine) 

o Special Study - Chronic Toxicity of Estuarine and Marine Sediment 

associated contaminants with the Amphipod Leptocheirus plumulosa. 

o Special Study - Toxicity and Bioaccumulation of Sediment-associated 

contaminants with Freshwater Invertebrates. 

o OCSPP Guideline 850.4150 - Vegetative Vigor Tier II 

o OCSPP Guideline 835.6100 (7) - Environmental Chemistry Analytical 

Methods - soil 

o OCSPP Guideline 835.6200 (7) - Environmental Chemistry Analytical 

Methods - water 

• Please refer to Registration Review - Preliminary Problem Formulation for the 

Ecological Risk Assessment of Diquat Dibromide (Revised), dated April 20, 2010, 

located in the docket, for a detailed discussion of the planned risk assessment 

needs. 

Human Health Risk: 

• The most recent diquat dibromide dietary, residential, and aggregate human health 

assessments were conducted in 2002 in support of the diquat dibromide Tolerance 

Reassessment Progress and Risk Management Decision (TRED). The most 

recent comprehensive occupational assessment was conducted in support of the 

1995 RED. A subsequent analysis of post-application risk from diquat dibromide 

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use was conducted in February, 2002 in response to a request from a registrant to 

reduce personal protective equipment (PPE) and restricted entry interval (REI) 

label requirements. 

• The Agency will update the existing assessments to reflect current methodologies 

for assessing combined dietary and drinking water exposures. Additionally, 

magnitude of the residue studies (Guideline 830.1500) for sorghum and soybean 

aspirated grain fractions were specified as confirmatory data in the TRED and 

were required in a 2005 Data Call-In (DCI) for diquat dibromide. These data 

remain outstanding. However, in a letter dated November 10, 2009, the technical 

registrant submitted a request to voluntarily terminate uses for sorghum and 

soybean. The Agency approved the request in a Cancellation Order published in 

the Federal Register (EPA-HQ-OPP-2009-0979; FRL-8820-1) on April 7, 2010. 

The Agency is in discussion with diquat dibromide registrants whose products 

have these uses listed. Provided the uses for sorghum and soybean are voluntarily 

removed from all diquat dibromide product labels, the magnitude of the residue 

studies for sorghum and soybean aspirated grain fractions will no longer be 

required. 

• The Agency will revise the diquat dibromide residential exposure assessment to 

incorporate updated standard protocols and exposure assumptions. 

• As revisions to the dietary (food and drinking water) and residential assessments 

are expected, the Agency will revise the aggregate exposure (dietary and 

residential) risk assessment to support the registration review of diquat dibromide. 

• The Agency will conduct a new occupational risk assessment to reflect changes in 

assessment standards and assumptions that the Agency has adopted since the last 

occupational risk assessment conducted in support of the 1995 RED. The new 

occupational exposure and risk assessment will include an assessment of diquat 

dibromide use in sewer systems. 

• The toxicity database is complete with the exception of the following study, 

which the Agency will be requiring in order to conduct a complete human health 

assessment: 

o OCSPP Guideline 870.7800 Immunotoxicity 

After the Agency has reviewed the additional toxicity data, the points of departure 

and safety factors for diquat dibrobmide will be reexamined. 

In the Preliminary Work Plan, the Agency had indicated that a subchronic 

neurotoxicity study would be required. However, the Agency received a 

comment suggesting an appropriate study may have already been submitted. 

After review, the Agency found the existing subchronic neurotoxicity study for 

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diquat dibromide acceptable, satisfying the guideline requirement for this study. 

The Agency will not require an additional study. 

• Product chemistry data are required for the Syngenta 41.1% and 37.3% 

formulation intermediates (EPA Reg. Nos. 100-1062 and 100-1063) including: 

 

o OCSPP Guideline 830.1550 Product identity and composition 

o OCSPP Guideline 830.1600 Description of materials used to produce the 

product 

o OCSPP Guideline 830.1620 Description of production process 

o OCSPP Guideline 830.1670 Discussion of formation of impurities 

o OCSPP Guideline 830.1700 Preliminary analysis 

o OCSPP Guideline 830.1750 Certified limits 

o OCSPP Guideline 830.1800 Enforcement analytical method 

o OCSPP Guideline 830.6302 Color 

o OCSPP Guideline 830.6303 Physical state 

o OCSPP Guideline 830.6304 Odor 

o OCSPP Guideline 830.6313 Stability to normal and elevated temperatures, 

metals, and metal ions 

o OCSPP Guideline 830.6314 Oxidation/reduction: chemical 

incompatibility 

o OCSPP Guideline 830.6315 Flammibility 

o OCSPP Guideline 830.6316 Explodability 

o OCSPP Guideline 830.6317 Storage stability 

o OCSPP Guideline 830.6319 Miscibility 

o OCSPP Guideline 830.6320 Corrosion characteristics 

o OCSPP Guideline 830.7000 pH 

o OCSPP Guideline 830.UV/Visible absorption (EPA Reg. No. 100-1063) 

o OCSPP Guideline 830.7100 Viscosity 

o OCSPP Guideline 830.7200 Melting point/melting point range 

o OCSPP Guideline 830.7220 Boiling point/boiling point range 

o OCSPP Guideline 830.7300 Density/relative density/bulk density 

o OCSPP Guideline 830.7370 Dissociation constants in water 

o OCSPP Guideline 830.7550/7560/7570 Partition coefficient (noctanol/water) 

o OCSPP Guideline 830.7840/7860 Water solubility 

o OCSPP Guideline 830.7950 Vapor pressure 

 

• The tolerance expression in 40 CFR § 180.226 will be reviewed during 

registration review to ensure that it appropriately covers the metabolites and 

degradates of diquat dibromide and that it specifies the residues to be measured 

for each commodity. Any proposed revisions would be to the chemical 

description rather than the individual tolerance levels. 

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• Please refer to Diquat dibromide: Human Health Risk Scoping Document in 

Support of Registration Review located in the docket, for a detailed discussion of 

the planned risk assessment needs for human health. 

Endocrine Disruptor Screening Program: 

As required under FFDCA section 408(p), EPA has developed the Endocrine Disruptor 

Screening Program (EDSP) to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the 

Administrator may designate.” The EDSP employs a two-tiered approach to making the 

statutorily required determinations. Tier 1 consists of a battery of 11 screening assays to 

identify the potential of a chemical substance to interact with the estrogen, androgen, or 

thyroid (E, A, or T) hormonal systems. Chemicals that go through Tier 1 screening and 

are found to have the potential to interact with E, A, or T hormonal systems will proceed 

to the next stage of the EDSP where EPA will determine which, if any, of the Tier 2 tests 

are necessary based on the available data. Tier 2 testing is designed to identify any 

adverse endocrine related effects caused by the substance, and establish a quantitative 

relationship between the dose and the E, A, or T effect. 

Between October 2009 and February 2010, EPA and issuing test orders/data call-ins for 

the first group of 67 chemicals, which contains 58 pesticide active ingredients and 9 inert 

ingredients. This list of chemicals was selected based on the potential for human 

exposure through pathways such as food and water, residential activity, and certain postapplication agricultural scenarios. This list should not be construed as a list of known or 

likely endocrine disruptors. 

Diquat dibromide is not among the group of 58 pesticide active ingredients on the initial 

list to be screened under the EDSP. Under FFDCA sec. 408(p) the Agency must screen 

all pesticide chemicals. Accordingly, EPA anticipates issuing future EDSP orders/data 

call-ins for all registration review cases, including those for which EPA has already 

opened a registration review docket for a pesticide active ingredient. 

For further information on the status of the EDSP, the policies and procedures, the list of 

67 chemicals, the test guidelines and the Tier 1 screening battery, please visit our 

website: http://www.epa.gov/endo/. 

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Timeline: 

EPA has created the following estimated timeline for completion of the registration 

review for diquat dibromide. 

Registration Review for Diquat Dibromide – Projected Registration Review Timeline

Activities Estimated Date 

Opening the docket 

Open Docket and Public Comment Period Completed – Dec. 16, 2009 

Close Public Comment Completed – Feb.16, 2010 

Case Development 

Final Work Plan Completed – May 2010 

Issue DCI 2011 – Jan. - Mar. 

Data Submission 2013 – Jan. - Mar. 

Open Public Comment Period for Draft Risk Assessments 2014 – July - Sept. 

Close Public Comment Period 2014 – Oct. - Dec. 

Registration Review Decision 

Open Public Comment Period for Proposed Registration Review 

Decision 

2015 – Jan. – Mar. 

Close Public Comment Period 2015 – April - June 

Registration Review Decision and Begin Post-Decision Follow-up 2015 

Total (years) 6 

Summary of Comments Received During Comment Period: 

The diquat dibromide registration review docket was open for a 60-day comment period 

beginning on December 16, 2009. During the comment period, multiple comments were 

received, which are summarized below. Comments that prompted a change in the work 

plan, or are not addressed by other response documents included in the diquat dibromide 

docket, have responses included below. While some comments did facilitate minor 

changes to the work plan, they did not affect the timeline set out in the Preliminary Work 

Plan. Further, this document makes final the work plan for the diquat dibromide 

registration review process. 

Comments from Syngenta Crop Protection Inc.:

Syngenta Crop Protection, Inc. (Syngenta) submitted comments on both the Registration 

Review – Preliminary Problem Formulation for the Ecological Risk Assessment of 

Diquat Dibromide (USEPA 2009b) and the Diquat Dibromide: Human Health Risk 

Scoping Document in Support of Registration Review (USEPA 2009c) documents. 

Concerning ecological risk, Syngenta’s comments dealt with ecological assessment 

endpoints and Agency anticipated data gaps. Concerning human health risk, Syngenta’s 

comments dealt with the Agency’s intention to update the human health risk assessments 

and Agency anticipated data gaps. 

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Response:

Ecological Risk: 

The Agency has revised the Registration Review – Preliminary Problem Formulation for 

the Ecological Risk Assessment of Diquat Dibromide document, dated November 5, 

2009, to correct two species names which Syngenta’s comments correctly noted were 

misspelled: Cyprinodon variegatus and Pseudokirchneriella subcapitata. Additionally, 

the MRID number for a hydrolysis half-life study referenced for diquat dibromide was 

incorrect and has been corrected, as well as the actual half-life value. The fish 

bioconcentration factor (BCF) of ≤2.5X for non-edible fish portion (viscera) and the 

wholefish BCF (1.03) are now correctly listed as well. 

Human Health Risk: 

In the Preliminary Work Plan, the Agency had indicated that a subchronic neurotoxicity 

study would be required. However, the Agency received a comment suggesting an 

appropriate study may have already been submitted. After review, the Agency found the 

existing subchronic neurotoxicity study for diquat dibromide acceptable, satisfying the 

guideline requirement for this study. The Agency will not require an additional study. 

Comments from Dr. Richard Geller:

Dr. Richard Geller submitted a comment concerning the Agency’s current findings on the 

acute oral, dermal, and inhalation toxicity characteristics of diquat dibromide. 

Response: 

The incident data Dr. Geller submitted to support his assertions on the toxicity 

characteristics of diquat dibromide have been added to the Agency’s Incident Database 

System (IDS). The Agency will consider information in the IDS during development of 

the registration review human health risk assessments for diquat dibromide. Please refer 

to the document Diquat Dibromide: Response to Comments on the Human Health Risk 

Scoping Document in Support of Registration Review, dated April 15, 2010, for the 

Agency’s complete evaluation of Dr. Geller’s comments. 

Comments from Douglas Products and Packaging:

Douglas Products suggests the Agency treat diquat dibromide similarly with regard to 

risk assessment assumptions and personal protective equipment requirements as another 

active ingredient with registrations for sewer root control use, metam sodium. 

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Response:

The Agency plans to conduct a new occupational risk assessment for diquat dibromide. 

Although not stated explicitly in the Preliminary Work Plan, the new occupational risk 

assessment will include an assessment of sewer applications. The workplan has been 

revised to provide additional clarity. 

Comments from FIFRA Endangered Species Task Force (FESTF): 

FESTF stated that its data on endangered species proximity contained in the FESTF 

Information Management System and NatureServe data submissions to the Agency 

(MRIDs 46325901, 46486301, and 47260101) “...fulfill the data requirements spelled 

out in Pesticide Registration Notice 2000-2 and provide the best available data necessary 

for the proximity analysis of Diquat dibromide use and listed species locations.” FESTF 

stated that Syngenta Crop Protection is a current member of FESTF, and as such, is 

entitled to the FESTF data on federally-listed threatened and endangered species. 

Response:

The Agency thanks FESTF for their comments, and will consider this information as it 

conducts the registration review risk assessments for diquat dibromide. 

Availability of Additional Agency Responses to Public Comments: 

Comments relevant to human health have been addressed by the Health Effects 

Division’s document, Diquat Dibromide: Response to comments on the Human Health 

Risk Scoping Document in Support of Registration Review, dated April 15, 2010. 

Comments regarding the ecological fate or effects of diquat dibromide have been 

addressed by the Environmental Fate and Effects Division’s document, EFED Response 

to Public Comments for Diquat Dibromide PF: Docket ID# EPA-HQ-OPP-2009-0846, 

dated April 20, 2010. These documents are available in the diquat dibromide docket 

(EPA-HQ-OPP-2009-0846).

Next Steps: 

The Agency plans to develop a Data Call-In Notice for this pesticide and plans to issue it 

in early 2011. This new information will be used to conduct risk assessments for diquat 

dibromide uses.