United States
Environmental Protection
Agency
Prevention, Pesticides
And Toxic Substances
(7508C)
EPA-738-F-98-020
December 1998
SERA R.E.D. FACTS
THIODICARB
Pesticide
Reregistration
Use Profile
All pesticides sold or distributed in the United States must be registered
by EPA, based on scientific studies showing that they can be used without
posing unreasonable risks to people or the environment. Because of advances
in scientific knowledge, the law requires that pesticides which were first
registered before November 1, 1984, be reregistered to ensure that they meet
today's more stringent standards.
In evaluating pesticides for reregistration, EPA obtains and reviews a
complete set of studies from pesticide producers, describing the human health
and environmental effects of each pesticide. The Agency develops any mitiga-
tion measures or regulatory controls needed to effectively reduce each pesti-
cide's risks. EPA then reregisters pesticides that can be used without posing
unreasonable risks to human health or the environment.
When a pesticide is eligible for reregistration, EPA explains the basis for
its decision in a Reregistration Eligibility Decision (RED) document. This fact
sheet summarizes the information in the RED document for reregistration case
2675, thiodicarb, dimethyl N,N'-(thiobis((methylimino)carbonyloxy))
bis(ethanimidothioate).
Thiodicarb acts as an insecticide against major Lepidopterous, and
suppresses Coleopterous and some Hemipterous insect pests. Thiodicarb acts
as an ovicide against cotton bollworms and budworms. Thiodicarb is used
primarily on cotton, sweet corn, and soybeans. The remaining usage is spread
among leafy vegetables, cole crops, ornamentals, and other minor use sites.
There are currently 14 tolerances for thiodicarb. Application types for
thiodicarb include aircraft (fixed-wing and helicopter), airblast sprayer,
chemigation, groundboom, high- and low-pressure handwand, backpack
sprayer, and belly-grinder spreader. Thiodicarb is formulated as a liquid,
flowable concentrate, granular, pelleted/tableted, water dispersible granules,
and wettable powder. There are no homeowner uses of thiodicarb.
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Regulatory Thiodicarb was first registered in the United States in 1984 for use as an
History insecticide. Rhone-Poulenc, Inc., is the current manufacturer of thiodicarb. A
data call-in was issued in April 1991. There are 11 thiodicarb products
registered, along with 19 Special Local Needs registrations (SLNs).
Human Health
Assessment
Toxicity
In acute toxicity testing, thiodicarb places in Toxicity Category I (the highest
toxicity category out of four) via the oral route and Toxicity Category II via
the inhalation route. For acute dermal effects and eye irritation thiodicarb is in
Toxicity Category III. For acute skin irritation, thiodicarb produced no
irritation (Category IV).
Thiodicarb has been classified as a Group B2 - probable human carcinogen.
The B2 classification was based on statistically significant increases in tumors
in both sexes of the mouse and statistically significant increases in testicular
interstitial cell tumors in male rats. A linear methodology (Qj*) was applied
for the estimation of human cancer risk and was calculated to be 1.88 x 10"2.
In determining whether to retain, reduce, or remove the lOx FQPA safely
factor for infants and children, EPA uses a weight of evidence approach taking
into account the completeness and adequacy of the toxicity data base, the
nature and severity of the effects observed in pre- and post-natal studies, and
exposure. Although the data provided no indication of increased sensitivity of
rats or rabbits to in utero and/or postnatal exposure to thiodicarb, data gaps
exist for the acute and subchronic neurotoxicity studies. These studies would
have yielded cholinesterase inhibition and field observation behavior data, as
well as histopathology of the central and peripheral nervous system which are
not presently available for evaluation. The Agency determined that the lOx
safety factor to account for increased sensitivity of infants and children should
be reduced from lOx to 3x.
The Agency has determined that thiodicarb has a metabolite, methomyl, which
is also a registered pesticide. Therefore, methomyl residues resulting from
applications of both thiodicarb and methomyl were considered in an aggregate
dietary risk assessment and compared to appropriate toxicological endpoints
for methomyl. In addition, for post application exposure to workers, the
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methomyl short and intermediate-term dermal endpoints were used in the risk
assessment because thiodicarb degrades rapidly to methomyl.
Dietary Exposure and Risk
The RfD for thiodicarb was calculated to be 0.03 mg/kg/day from a chronic rat
toxicity study. The RfD was based on an increased incidence of extramedullary
hemopoiesis in males and decreased RBC cholinesterase in females at the
LOEL. An uncertainty factor of 100 was used for deriving the RfD and
includes lOx for inter-species extrapolation and lOx for intra-species variation.
An FQPA safety factor of 3x (due to data gaps) was applied to derive an
FQPA adjusted RfD of 0.01 mg/kg/day. Chronic dietary exposure to
thiodicarb alone must be less than 100% of the FQPA adjusted RfD to be
considered below EPA's level of concern.
For acute dietary risk assessment for thiodicarb alone, a MOE of 1000 is
required for women 13 years and older, as well as for the general population
including infants and children. This MOE includes the conventional MOE of
100 for inter- and intra-species variation, 3x for FQPA, and another 3x for the
use of a LOEL, instead of a NOEL, in the rat developmental study. The FQPA
Safety Factor (3x) is required because of data gaps (acute and subchronic
neurotoxicity studies). The acute Monte Carlo dietary analysis for thiodicarb
alone indicates that there are adequate margins of exposure for the U.S.
population, women 13 years and older, children 1 to 6 years old and infants.
The results of the chronic dietary risk evaluation system (ORES) analyses, for
thiodicarb alone, indicate that the anticipated residue contribution for the U.S.
Population occupies 68% of the FQPA adjusted RfD. For females (13 years
and older) 67% of the FQPA adjusted RfD is occupied. For children (1 to 6
years old) and infants, 104% and 43%, respectively, of the FQPA adjusted RfD
is occupied. Although for children (1 to 6 years old), the FQPA adjusted RfD
is slightly exceeded, if more refined estimates of dietary exposure were made
(e.g. residues from field trials) significantly lower chronic risk would be
estimated. Therefore, the chronic risk from exposure to thiodicarb from food
sources is not of concern.
For the acute aggregate dietary risk assessment for food, for thiodicarb and
methomyl combined, the endpoint for methomyl was used in the risk
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assessment and compared to residues of methomyl from thiodicarb application
plus residues of methomyl from methomyl application. The results of the acute
aggregate exposure analyses for food, for thiodicarb and methomyl show that
there are adequate margins of exposure for the general U.S. population,
children 1 to 6 years of age, and infants.
For the chronic aggregate dietary risk assessment for food, for thiodicarb and
methomyl combined, the RfD for methomyl was used in the risk assessment
and compared to residues of methomyl from thiodicarb application plus
residues of methomyl from methomyl application. The results of the chronic
aggregate exposure analysis indicate that there are no chronic concerns
associated with potential residues of methomyl on foods as the result of
application of thiodicarb and methomyl.
Thiodicarb degrades rapidly to methomyl in the environment. Therefore, the
Agency has calculated drinking water levels of concern (DWLOCs) for
methomyl. The maximum estimated concentrations of methomyl in surface and
ground water are less than the Agency's levels of concern for methomyl in
drinking water as a contribution to acute aggregate exposure. The estimated
average concentrations of methomyl in surface and ground water are less than
OPP's levels of concern for methomyl in drinking water as a contribution to
chronic aggregate exposure.
A linear methodology (Qj*) was applied for the estimation of human cancer
risk and was calculated to be 1.88 x 10"2. The assessment was conducted for
the total U.S. Population only. The upper bound cancer risk was calculated to
be 3.76 x 10"7. This upper bound risk is below the range the Agency considers
neglible for excess lifetime cancer risk and is not cause for concern.
Occupational Exposure and Risk
Handlers (mixers, loaders, and applicators) of thiodicarb may be exposed to
thiodicarb during and after normal use of liquid and wettable powder
formulations. For dermal exposure, no short- and intermediate-term dermal
risk was seen for thiodicarb. For inhalation exposure, the Agency is requiring
the use of personal protective equipment and/or the use of engineering controls
(water soluble bags). The handler information for thiodicarb has been
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integrated with other considerations, including the toxicity concerns pertaining
to methomyl, a degradate of thiodicarb, in determining the required PPE.
There are no short- or intermediate-term dermal endpoints of concern for
thiodicarb, and a post-application inhalation risk assessment is not warranted.
However, thiodicarb rapidly degrades to methomyl. Therefore, the toxicity
concerns pertaining to methomyl are considered in the post-application risk
assessment. Based on the results of this assessment, the Agency believes an
REI of 48 hours is sufficiently protective of workers following applications of
thiodicarb for the currently registered uses.
Environmental Environmental Fate
Assessment Available environmental fate studies show that thiodicarb degrades rapidly into
methomyl under most conditions. While the parent chemical does not appear
to be very persistent or highly mobile, the degradate methomyl is more
persistent, more mobile, and more toxic. Thiodicarb rapidly degrades (half-
lives on the order of a few days) primarily by metabolism and hydrolysis in
alkaline conditions. It may be more persistent under drier conditions.
Methomyl appears to be moderately persistent and highly mobile in the
environment. The dominant routes of dissipation are metabolism (biologically-
mediated degradation), leaching, and photolysis in clear waters.
While thiodicarb is not expected to have a high potential to contaminate
ground water because of its short persistence, methomyl has fate characteristics
that favor leaching, and it has been detected in ground water in a prospective
ground water monitoring study and in other reported incidences. While it may
reach ground water under certain conditions, methomyl may not persist under
many conditions. Both thiodicarb and methomyl may run off to surface waters
for a few days to several weeks after application. Neither chemical is likely to
persist in clear, shallow waters or in waters with substantial microbiological
populations. However, methomyl may persist in waters where sunlight
penetration is limited (such as in deeper waters or waters with a significant
sediment load or populations of organisms such as algae). Neither chemical is
expected to persist in anaerobic sediments.
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Ecological Effects
Laboratory studies show that thiodicarb is practically non-toxic to birds but
moderately to highly toxic to small mammals on an acute oral basis. Methomyl
is highly toxic to birds and mammals on an acute oral basis but only slightly
toxic to birds on a subacute dietary basis. Thiodicarb may result in chronic isks
to certain species that frequent short grass (e.g, ducks, geese and swans).
Methomyl, as a degradate, poses acute risks to birds and mammals that feed on
short and tall grasses, broadleaf plants, and small insects.
Acute toxicity studies show that thiodicarb is moderately to highly toxic to
freshwater and estuarine/marine fish, respectively, and very highly toxic to
freshwater and estuarine/marine invertebrates. The degradate methomyl is
moderately to highly toxic to freshwater fish and moderately toxic to estuarine
fish. In a chronic early life-stage study, methomyl significantly reduced fish
larvae survival under flow through conditions. Toxicity data suggest that
aquatic invertebrates are much more sensitive to methomyl contamination than
either fresh or salt water fish species. Accumulation of methomyl from
repeated applications contributes to the chronic risks.
Ecological Effects Risk Assessment
EPA is generally concerned about the ecological effects to terrestrial wildlife
and aquatic organisms posed by exposure to thiodicarb. The risk assessment
for thiodicarb and its degradate methomyl shows various levels of concern
regarding avian risk and mammalian risk from multiple applications of
thiodicarb at short intervals. In addition, most agricultural uses present acute
and chronic risks of varying levels to endangered and nonendangered aquatic
organisms. The major concerns for non-target organisms are the chronic risks
posed by the use of thiodicarb to non-target mammalian and freshwater
invertebrate organisms. With risk mitigation measures in place, the Agency
considers these risks acceptable.
Risk Mitigation To lessen ecological and potential water risks posed by thiodicarb and its
degradate methomyl, the Agency is requiring the following mitigation for
thiodicarb containing products.
Thiodicarb will be reclassified as a Restricted Use Pesticide. (Thiodicarb
degrades rapidly to methomyl which is already a restricted use chemical.)
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• Based on the environmental risk assessment for methomyl, the following
advisories are required for thiodicarb: a labeling statement for potential
ground water contamination, a labeling statement to minimize the
potential for surface water contamination and labeling statements on
manufacturing use products and end use products based on the toxicity
to nontarget organisms. A bee hazard statement is also required.
The maximum number of applications on cole groups will be reduced
from 6 to 4 per season, at the maximum rate of 1.0 pounds ai/A. The
number of applications on cotton will be limited to 6.
• A statement supporting the use of an Integrated Pest Management (IPM)
plan must be added to the labels.
Buffer zones have been imposed that will reduce the potential risk to
non-target aquatic organisms from spray drift during aerial or ground
applications.
Additionally, there are risk mitigation measures required in the RED to protect
mixers, loaders, applicators and workers, including water soluble bags,
additional PPE and appropriate REIs. For a detailed list, refer to Chapter V. of
the thiodicarb RED document.
Additional Data The generic data base supporting the reregi strati on of thiodicarb for the above
Required eligible uses has been reviewed and determined to be substantially complete.
For thiodicarb, the following information is being required:
81-8 Acute neurotoxicity study - rat
82-7 Subchronic neurotoxicity study - rat
72-4(a) Estuarine/marine fish early life stage test
72-4(b) Estuarine/marine invertebrate life-cycle tests
164-1 Field Dissipation Study (cotton and corn)
860.1500 Magnitude of residue in cotton (formerly 171-4k)
860.1900 Field Accumulation in Rotational Crops (formerly 165-2)
830.7050 UV/Visible Absorption
Thiodicarb Cooking Study
Product Labeling
All thiodicarb end-use products must comply with EPA's current
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Changes Required pesticide product labeling requirements and with those labeling requirements
imposed in the Thiodicarb RED. For a comprehensive list of labeling
requirements, please see Section V. of the Thiodicarb RED document.
Regulatory
Conclusion
Based on the reviews of the generic data for the active ingredient
thiodicarb, the Agency has sufficient information on the health effects of
thiodicarb and on its potential for causing adverse effects in fish and wildlife
and the environment. The Agency has determined that thiodicarb products,
labeled and used as specified in the Reregi strati on Eligibility Decision, will not
pose unreasonable risks to humans or the environment. Therefore, the Agency
concludes that products containing thiodicarb for all uses are eligible for
reregi strati on.
For More
Information
EPA is requesting public comments on the Reregi strati on Eligibility
Decision (RED) document for thiodicarb during a 60-day time period, as
announced in a Notice of Availability published in the Federal Register. To
obtain a copy of the RED document or to submit written comments, please
contact the Pesticide Docket, Public Response and Program Resources
Branch, Field Operations Division (7506C), Office of Pesticide Programs
(OPP), US EPA, Washington, DC 20460, telephone 703-305-5805.
Electronic copies of the RED and this fact sheet are available on the
Internet. See http://www.epa.gov/REDs.
Printed copies of the RED and fact sheet can be obtained from EPA's
National Center for Environmental Publications and Information
(EPA/NCEPI), PO Box 42419, Cincinnati, OH 45242-0419, telephone 513-
489-8190, fax 513-489-8695.
Following the comment period, the Thiodicarb RED document also will
be available from the National Technical Information Service (NTIS), 5285
Port Royal Road, Springfield, VA 22161, telephone 703-605-6000.
For more information about EPA's pesticide reregi strati on program, the
Thiodicarb RED, or reregi strati on of individual products containing thiodicarb,
please contact the Special Review and Reregi strati on Division (7508C), OPP,
US EPA, Washington, DC 20460, telephone 703-308-8000. For
information about the health effects of pesticides, or for assistance in
recognizing and managing pesticide poisoning symptoms, please contact the
National Pesticide Telecommunications Network (NPTN). Call toll-free 1-
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800-858-7378, between 9:30 am and 7:30 pm Eastern Standard Time, seven
days a week. Their internet address is ace.orst.edu/info/nptn.
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