 Introduction

This is the Environmental Protection Agency’s (EPA or the Agency)
Final Work Plan (FWP) for the registration review of tribufos.  The FWP
includes the expected registration review timeline.  The FWP also
addresses public comments received concerning the Preliminary Work Plan
(PWP) in the Summary Document which was posted in the tribufos
registration review docket (EPA-HQ-OPP-2008-0883) 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 that 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  
HYPERLINK "http://www.epa.gov/oppsrrd1/registration_review/" 
http://www.epa.gov/oppsrrd1/registration_review/ .

Tribufos was the subject of a 2000 Interim Reregistration Eligibility
Decision (IRED), and in July 2006 the Reregistration Eligibility
Decision (RED) for tribufos was finalized upon completion of the
Organophosphate (OP) Cumulative Risk Assessment.  Tribufos is an
organophosphate defoliant used on cotton crops.  There are no
residential or public recreational uses of tribufos.

Comments Received on the Preliminary Work Plan

In the Summary Document for tribufos, EPA solicited comments on the
topics of environmental justice, water body impairment, and potential
trade irritants, in addition to questions specific to tribufos
concerning use, usage, benefits, and available alternatives.  No
comments or information were received during the public comment period. 
Therefore, the Agency’s intent to conduct the risk assessments
described in the Summary Document, and the timeline for tribufos
registration review described in the PWP, remain the same. 

Anticipated Risk Assessment and Data Needs:

The Agency anticipates updating and revising the ecological risk
assessment for tribufos (including an endangered species risk
assessment) and updating and revising the human health risk assessment. 
Additional data that the Agency anticipates needing to complete these
assessments are specified below.

Ecological Risk:

An ecological risk assessment (1996) and an updated drinking water
assessment (2000) were completed for the IRED (2000).  Eight ecological
studies were required during reregistration to address the data gaps. 
These data have been submitted; however, one of the studies (850.1400
– Early Life Stage Toxicity of Tribufos to the Fathead Minnow) was
determined to be unacceptable for risk quotient (RQ) calculations and
remains an outstanding data gap.

The environmental fate database for tribufos is currently complete with
two exceptions; an aerobic aquatic metabolism study is needed to
describe the fate of tribufos in surface water bodies, and an
octanol-air partition coefficient (KOA) study is also required in order
to describe the fate of tribufos in the atmosphere in support of
bioaccumulation modeling.  

While they will not be used for an exposure assessment, submission of
analytical methods in water and in sediment (and associated independent
laboratory validations) are       needed to support enforcement and
monitoring at the federal, state, and local levels.  The analytical
method in sediment data is needed, since tribufos is expected to
partition predominately to sediment in surface water bodies.  If data
are submitted indicating that the previously submitted analytical method
in soil is appropriate for measuring residues in sediment, then a
separate analytical method in sediment should not be necessary.  

In order to conduct a comprehensive risk assessment during registration
review, the following data are needed: avian acute oral toxicity
(passerine); terrestrial plant toxicity; and sediment toxicity for
invertebrates.  

Although acceptable acute avian oral toxicity data were submitted for
mallard duck and bobwhite quail, data are not available for a passerine
species.  The available acute oral toxicity data for mallard ducks and
bobwhite quails indicates that tribufos may be moderately toxic to birds
on an acute oral exposure basis, and there is evidence from other OPs
(e.g., dimethoate) that passerines are significantly more sensitive to
at least some OPs when compared to the upland and game bird species
tested. 

While acceptable Tier II terrestrial plant studies are available for
tribufos, the plant species studied (i.e., annual dicots and monocots)
are not expected to be as sensitive to tribufos as other plants (e.g.,
woody deciduous trees and shrubs) due to its mechanism of action. 
Available toxicity studies were conducted on plants that do not form
abscission zones, and data from these studies may underestimate the
potential toxicity of tribufos to non-target plants that do form
abscission zones.  Therefore, additional terrestrial plant toxicity
studies on plants expected to be sensitive to tribufos’ mode of action
are required.  

The fate properties of tribufos indicate it can potentially partition to
the sediment, and no sediment toxicity data are available for review;
therefore, toxicity to benthic invertebrates is unknown.  Sediment
toxicity data are required to assess risk to the benthic invertebrate
community from tribufos.  

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 tribufos’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 Service (the
Services), as appropriate.

The Agency completed endangered species assessments evaluating the
potential effects of tribufos on the California red-legged frog (Rana
aurora draytonii) consistent with court orders and settlements, Center
for Biological Diversity v. Johnson, et al.,No. 02-1580  (N.D. Cal.,
October 20, 2006).  The Agency concluded in October 2008 that tribufos
is Likely to Adversely Affect the California red-legged frog and
initiated formal consultation with the U.S. Fish and Wildlife Service
regarding this determination.

EPA anticipates needing the following guideline studies in order to
conduct a complete ecological risk assessment for the registration
review of tribufos:  

835.4300 - Aerobic Aquatic Metabolism

835.6200 - Analytical Methods (and Associated Independent Laboratory
Validations) for Residues in Water and Sediment

850.2100 - Avian Acute Oral Toxicity Test (Passerine)

850.1735 - Sediment Toxicity Testing on Freshwater Invertebrates

850.1740 - Sediment Toxicity Testing on Marine Invertebrates

Non-guideline - Partition Coefficient (n-Octanol/Air)

Non-guideline - Terrestrial Plant Toxicity Test

Please refer to the December 10, 2008 document “Problem Formulation
for Ecological Risk, Endangered Species, and Drinking Water Exposure
Assessments in Support of the Registration Review of Tribufos” in the
registration review docket (EPA-HQ-OPP-2008-0883) for a detailed
discussion of the anticipated risk assessment needs.

Human Health Risk: 

The human health risk assessment conducted in support of the 2000 IRED
was the primary source used for the 2009 “Tribufos:  Revised Human
Health Assessment Scoping Document in Support of Registration Review.”
  EPA completed its reevaluation of tribufos with the release of the OP
Cumulative Risk Assessment (CRA) in July 2006.  The CRA did not result
in any changes to tribufos registrations.

Should the Agency determine that new information on tribufos is
available which could potentially impact the CRA, the Agency will
revisit the CRA.

The toxicity database for tribufos is largely complete, with the
exception of a comparative cholinesterase (CCA) study and an
immunotoxicity study.  The CCA study is an outstanding requirement from
the IRED. 

A new drinking water exposure assessment is anticipated to be needed for
registration review to incorporate the latest modeling methods.

A new dietary risk assessment is anticipated to be needed for
registration review, which will include updated drinking water estimated
environmental concentrations (EECs), updated percent crop treated
information, and revised anticipated residue levels taking into account
the revised dietary burden to ruminants.  Additionally, any changes to
toxicity endpoints or safety factors will also be incorporated.  

The updated dietary assessment will also include a reassessment of
tribufos tolerance levels for livestock commodities.  The only
tribufos-treated livestock feedstuff, cotton gin byproducts, has had a
significant reduction in the percent of diet (25 to 5%), resulting in a
lower dietary burden, which may in turn result in lower tolerance levels
than previously recommended.  

Since there are no currently registered residential or public
recreational uses for tribufos, a residential risk assessment is not
required at this time.

The most recent aggregate assessment (Human Health Risk Assessment for
Tribufos, June 2000) considered only food and water, as there are no
residential uses of tribufos.  

A new aggregate risk assessment may be needed in registration review due
to the identified data gaps in the toxicity database and likely
revisions to drinking water estimates, percent crop treated information,
and anticipated residue levels.  

Occupational handler and post-application risk estimates (inhalation and
dermal exposure) have been updated in the February 20, 2009 “Tribufos.
 Revised Human Health Assessment Scoping Document in Support of
Registration Review” based on current labels incorporating mitigation
measures required in the 2000 IRED. 

During reregistration, a biomonitoring study was required to obtain
exposure values to refine occupational risk estimates.  However, the
registrants requested, and the Agency agreed, to postpone the study
until after the required developmental neurotoxicity (DNT) study was
submitted and reviewed.  The Agency has completed its review of
submitted studies, and concludes the toxicity data did not eliminate the
need for additional exposure data.  Therefore, a biomonitoring study or
a similar study that provides exposure data that can be used to refine
occupational handler and post-application risk estimates is required and
was called in by letter to the technical registrant in February, 2009.  

A new occupational assessment is anticipated to be needed for
registration review, which will include risk estimates based on the
required tribufos biomonitoring or equivalent data, as well as any new
toxicity data that apply.

There are no maximum residue levels (MRLs) established for tribufos in
Canada, Mexico, or through Codex; thus, there are no international
harmonization issues.

Please refer to the February 20, 2009 “Tribufos:  Revised Human Health
Assessment Scoping Document in Support of Registration Review” in the
registration review docket (EPA-HQ-OPP-2008-0883) for a detailed
discussion of the anticipated risk assessment needs.

Timeline:

EPA has created the following estimated timeline for the completion of
the tribufos registration review.

Registration Review for Tribufos – Projected Registration Review
Timeline

Activities	Estimated Yr/Mon

Opening the docket

Open Docket and Public Comment Period	2009 – March

Close Public Comment	2009 – May

Case Development

Develop Final Work Plan (FWP)	 2009 – Aug.

Issue DCI	 2010 – April - June

Data Submission	 2012 – April - June 

Preliminary Risk Assessments and Open Public Comment Period	 2013 –
Oct. - Dec. 

Close Public Comment Period	 2014 – Jan. - March

Registration Review Decision

Proposed Registration Review Decision 	 2014– April - June

Public Comment Period 	 2014 – July - Sept.

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

Total (years)	 6

	

As noted above, the Agency plans to require a number of ecological fate
and effects studies, an immunotoxicity study, and a CCA study through a
data call-in, which is expected to be issued in 2010.  This new
information will be used to conduct a comprehensive ecological risk
assessment, including an endangered species assessment, as well as a
full human health assessment, for all tribufos pesticidal uses.

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