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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

MEMORANDUM

May 12, 2010

SUBJECT: Diquat Dibromide: Human Health Risk Assessment for Section 3 Use on Canola

PC Code: 032201 DP Barcode: 372014

Decision No.: 423144 Registration No.: 100-1061, 100-1062

Petition No.: 9F7639 Regulatory Action: Section 3 Registration

Risk Assessment Type: Single Chemical Aggregate Case No.: 0288

TXR No.: NA CAS No.: 85-00-7

MRID No.: NA 40 CFR: 180.226

FROM: Becky Daiss, Biologist 

Risk Assessment Branch 4, HED (7509P)

THROUGH: Susan V. Hummel, Branch Senior Scientist

Risk Assessment Branch 4, HED (7509P)

TO: Bethany Benbow, Risk Manager Reviewer

Herbicide Branch Registration Division (7505P)

This document provides Health Effects Division’s (HED’s) assessment of a Section 3 

request for use of diquat dibromide on canola. HED recently completed a risk assessment for a 

Section 18 Emergency Exemption for use of diquat dibromide on canola grown in Oklahoma and 

Kentucky (12/09). The use requested for this Section 3 registration is essentially identical to that 

assessed for the Section 18 request. Since there is no change in anticipated residues and no 

changes in use patterns, no new risk assessment is needed. The Section 18 assessment concluded 

that available data were insufficient to support a permanent tolerance for canola. However,

additional data have been submitted with this petition. The additional data support a tolerance 

and conditional registration for use of diquat dibromide on canola. This assessment summarizes 

the new residue chemistry data submitted with this petition and provides HED’s determination 

that the submitted data support a tolerance and conditional registration for use of diquat 

dibromide on canola. Risk assessments conducted for the Section 18 and for the Tolerance 

Reassessment Eligibility Decision (TRED) issued in April 2002 provide supporting information 

for this assessment. 

Supporting Documents

Risk Assessment for Section 18 – M. Doherty, D355407, 12/3/09

Risk Assessment for TRED – B. Daiss, D281366, 3/6/02

Residue Chemistry Assessment – B. Daiss, 372014, 5/12/10
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Background

This assessment provides information to support an amended Section 3 registration for 

the use of diquat dibromide on canola. The risk assessment recently conducted for a Section 18 

Emergency Exemption for use of diquat dibromide on canola adequately addresses the dietary

and occupational exposure and risk associated with the use requested for this Section 3 

registration. Since there is no change in anticipated residues and no changes in use patterns, no 

new risk assessment is needed. The TRED conducted in April 2002 provides supporting 

information.

Use Profile

Diquat dibromide is a non-selective contact herbicide, algicide, dessicant, and defoliant. 

As a herbicide/algicide, it is used to control aquatic and terrestrial weeds. It is used as a 

preharvest dessicant/defoliant to facilitate the harvest of potatoes and various crops grown for 

seed. Its largest use is as a dessicant on potatoes. Other minor food use applications include use 

as a dessicant on crops grown for seed that is used for feed, i.e., alfalfa, sorghum, soybean, and 

clover. Additionally, use of irrigation water containing residues of diquat may result in diquat 

residues in plants and livestock. Tolerances are established for residues of diquat dibromide 

in/on raw agricultural commodities as defined in 40 CFR §180.226. Non-food use applications 

include use as a preharvest dessicant on carrot, radish and turnip grown for seed; as a postharvest dessicant on cantaloupe, cucumber, pepper, squash, tomato, and watermelon; as a 

dessicant/defoliant for commercial greenhouses and nurseries, ornamental seed crops, and 

commercial and residential landscaping; and as an herbicide to control tree root intrusions in 

drainage systems. Diquat dibromide works as a dessicant by reacting with molecular oxygen to 

produce a superoxide anion in treated plants. The oxidative activity, which occurs subsequent to 

formation of the oxygen radicals, rapidly destroys plant cell membranes. Diquat dibromide is 

formulated as a soluble concentrate and ready-to-use liquid. As an herbicide/algicide used to 

control algae and aquatic weeds, it is applied by direct pouring, hand-held or mechanical sprayer, 

and injection below the water surface. As a terrestrial herbicide and crop dessicant/defoliant, it is 

applied by hand-held sprayer, aircraft, or ground equipment.

Proposed New Use

Under PP#9F7639, Syngenta Crop Protection Inc is supporting the use of diquat 

dibromide as a harvest aid/dessicant on canola. Syngenta has submitted the label Reglone 

Desiccant (EPA Reg. No. 100-1061). Reglone Dessicant is a soluble concentrate containing 3.73 

lbs diquat dibromide salt per gallon equivalent to 2 lbs diquat cation per gallon. An application 

rate of 0.5 lb ai/A is proposed with a limit of one application per season. A 7 day pre harvest 

interval (PHI) is required with harvest no later than 10 days after application. Ground or aerial 

application equipment is permitted. 
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Tolerance Recommendation

There are no residue chemistry issues that would preclude establishing a tolerance and 

granting a conditional registration for the requested use of diquat dibromide or establishment of 

the following tolerances for residues of the diquat cation dibromide:

Canola, seed .....................................................................................2.0 ppm

Canola, meal ....................................................................................6.0 ppm

The recommended tolerance on canola seed has been increased from the proposed 1 ppm 

tolerance to harmonize with Codex. The registration should be conditional based on the 

submission of the US field trial data being prepared by IR4 on preharvest dessicant use of diquat 

dibromide on canola. The current tolerance expression is adequate.

Hazard Characterization

The toxicity data base is adequate to support the Section 3 request for a new use of diquat 

dibromide on canola. There are no residual uncertainties for pre-and/or post natal toxicity. 

There are no indications in the available studies that organs associated with immune function, 

such as the thymus and spleen, are affected by diquat dibromide. Therefore, the default FQPA 

Safety Factor may be removed. However, an immunotoxicity study (870.7800) is required. This 

is a new data requirement under 40 CFR Part 158 for registration of a pesticide (food and nonfood uses). 

Residue Chemistry 

Residues in Crops

Syngenta Crop Protection, Inc. has submitted field trial data for use of diquat dibromide 

as a preharvest dessicant on canola. In eight trials conducted in West Germany and six 

conducted in the United Kingdom, a 20% ai liquid formulation (Reglone 40) was applied to 

canola as a single 0.6 kg ai/ha (0.54 lb ai/A) application using various types of spray equipment. 

Applications were made in spray volumes of 400-500 L/ha in the UK and 600 L/ha in West 

Germany, corresponding to an application concentration of 1-1.2 g ai/L. Applications did not 

include the use of an adjuvant. Treated samples of canola were harvested at 5-8 days after 

treatment (DAT) from each West German site and at 7, 10-13, and 20 DAT from each UK site. 

The HED ChemSAC determined that the EU data would support the establishment of a 

tolerance of 1 ppm on canola seed and the conditional registration of diquat dibromide on canola 

as a harvest aid/desiccant. The tolerance on canola seed has been increased from 1 ppm to 2 ppm 

to harmonize with the Codex MRL, and a tolerance of 6 ppm is recommended for canola meal, 

based on a maximum theoretical concentration factor of 3X. HED is aware that IR-4 is currently

conducting studies in the U.S. to support the preharvest desiccant use of diquat dibromide on 
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canola. Residues of diquat dibromide on canola grown in the U.S. are not expected to differ 

significantly from residues reported in the EU studies, since harvest aid/desiccant applications 

are made late in the growing season with little time between application and harvest. In addition, 

since the recommended tolerances for canola seed and meal have been increased by a factor of 

2X to harmonize with Codex, there is little chance residues in the U.S. trials will exceed these 

tolerances. Nevertheless, since the IR-4 field work has already been completed and the study

reports will be available in July, 2011, HED recommends that these studies be required as a 

condition of registration to confirm the recommended tolerance levels.

Residues in Processed Foods

A maximum theoretical concentration factor of 3X has been applied for the processed 

fraction canola meal. Residue data from field trials conducted in Germany demonstrated that 

diquat residues were not found in any of the oil samples. Therefore no tolerance is needed for 

diquat residues on canola oil.

Analytical Method

Syngenta has proposed a new enforcement method for the determination of residues of 

diquat in crops, including oil seed crops, Analytical Method Number RAM 272/02 “The 

Determination of Residues of Paraquat and Diquat in crops and Soil – A High Performance 

Liquid Chromatographic Method.” Based on HED’s analysis, the proposed analytical method is 

classified as scientifically acceptable. 

Dietary Assessment

HED is recommending tolerances of 2 ppm for canola seed and 6 ppm for canola meal in 

order to harmonize with the Codex maximum residue limit (MRL). The dietary analysis for the 

Section 18 Emergency Exemption assessment assumed a canola seed tolerance of 1 ppm and a 

livestock dietary burden calculated assuming diquat residues of 3 ppm on canola meal. HED 

recalculated the dietary burden for livestock commodities using 6 ppm on canola meal. Based on 

that analysis, the current tolerances on livestock commodities are adequate and anticipated 

residues will not exceed those used in the dietary exposure assessment conducted for the Section 

18. HED reassessed dietary exposure using a 2 ppm tolerance for canola seed. The increased 

tolerance did not change the results of the dietary assessment conducted for the Section 18. 

Therefore a separate dietary exposure assessment is not required for the Section 3 proposed new 

use analysis. 

Data Requirements

- Submission of US field trial data being prepared by IR4 on preharvest dessicant use of 

diquat dibromide on canola is required. 
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- Data on length of storage of canola samples prior to analysis from the W. German and 

UK trials should be submitted. Although the length of storage prior to analysis is 

unknown, based on the structure of diquat, residues in frozen storage are expected to be 

stable much longer than the 6 to 8 months for which storage stability data are available. 

Therefore, lack of data on length of storage is not expected to impact the risk assessment. 

- An immunotoxicity (870.7800) study is required under the revised 40 CFR Part 158 

Toxicology Data Requirements.