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

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 post-harvest 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.  

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   SEQ CHAPTER \h \r 1 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 non-food 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 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.  

-	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.  

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