disulfoton (Di-Syston) Chemical Fact Sheet 12/84
CHEMICAL FACT SHEET FOR:
- FACT SHEET NUMBER: 43
DATE ISSUED: DECEMBER 31, 1984
1. DESCRIPTION OF CHEMICAL
- Generic Name: O,O-diethyl S-[2-(ethylthio)ethyl] phosphorodithioate
- Common Name: disulfoton
- Trade Name: Di-Syston
- EPA Shaughnessy Code: 032501
- Chemical Abstracts Service (CAS) Number: 298-04-4
- Year of Initial Registration: 1958
- Pesticide Type: Insecticide/Acaricide
- Chemical Family: Organophosphate
- U.S. and Foreign Producers: Mobay Chemical Corp.
2. USE PATTERNS AND FORMULATIONS
- Application sites: Grain crops, nut crops, cole crops, root crops,
pome, strawberry and pineapple fruits, forage, field and vegetable
crops, sugarcane, seed crops, forest plantings, ornamentals, and
potted plants (including houseplants)
- Types of formulations: Emulsifiable concentrates, granulars,
pelleted/tableted, and ready-to-use liquids
- Types of methods of application: Soil incorporation of granulars,
ground and aerial spray, and granular broadcast applications
- Application rates: Range from 0.25 lbs. a.i./A to 8 lbs. a.i./A for
broadcast applications and 0.25 oz. a.i./1,000 ft. row to 10 oz.
a.i./1,000 foot row for band treatment; also individual potted
plant soil treatment uses at various rates depending on plant and
- Usual carriers: Synthetic clays, various solvents, fertilizers
3. SCIENCE FINDINGS
- Physical state: liquid
- Color: pale yellow
- Odor: unknown
- Boiling point: 62 degrees C at 0.01 mm/Hg
- Vapor pressure: 1.8 x 10(-4) millibars at 20 degrees C
- Flash point: >180 degrees F (TOC)
- Acute oral: 1.9-6.2 mg/kg, Toxicity Category I
- Acute dermal: 3.6-15.9 mg/kg, Toxicity Category I
- Primary dermal irritation: NA since chemical's toxicity would
preclude testing for this requirement.
- Acute inhalation: One study, which did not meet Agency standards,
indicated toxicity at 0.2 mg/l, which would place it in Toxicity
- Neurotoxicity: One study was submitted which did not meet Agency
standards. The study did not indicate delayed neurotoxic effects.
- Oncogenicity: Two studies have been evaluated; one was acceptable
and did not suggest oncogenic potential.
- Teratogenicity: Two studies were evaluated; one was acceptable,
and the other did not fully meet the Agency standards. The
chemical is not teratogenic at 0.3 mg/kg/day.
- Reproduction - 2 generation: Data gap
- Metabolism: The available studies suggest that disulfoton is
rapidly absorbed and may undergo sequential oxidation steps that
enhance anti-cholinesterase properties. Excretion is complete
and rapid via urine. Major metabolites include the O-analog
of disulfoton, and the sulfoxide and sulfone derivatives of both
disulfoton and its O-analog. Data to further describe the nature
and dynamics of this process are necessary.
- Mutagenicity: Contradictory reports are available on the mutagenic
potential of disulfoton. The Agency has concluded that the
mutagenic potential is not adequately defined and further testing
Physiological and Biochemical Behavioral Characteristics
- Mechanism of pesticidal action: A plant systemic insecticide
which is active by contact, ingestion, and vapor action. Disulfoton
and its major metabolites are potent cholinesterase inhibitors
primarily attacking acetylcholinesterase. Poisoning and death
result from excessive stimulation of both the parasympathetic and
central nervous systems, and the consequent myoneural junction
effect as a result of acetylcholinesterase accumulation.
- Symptoms of poisoning: headache, dizziness, extreme weakness,
ataxia, tiny pupils, twitching, tremor, nausea, slow heartbeat,
pulmonary edema, and excessive sweating. Continual daily
absorption at intermediate doses may cause influenza-like illness
characterized by weakness, anorexia, and malaise.
- Metabolism and persistence in plants and animals:
- The metabolism of disulfoton in plants is adequately understood.
The major plant metabolite appears to be disulfoton sulfone.
Consequently, the Agency believes that the tolerances for
disulfoton residues should be expressed as disulfoton sulfone.
- The metabolism in animals is not well understood. More data
are required to quantify and qualify animal metabolites, and to
quantify plant metabolites.
- Available data are insufficient to assess the environmental fate of
disulfoton. Data gaps exist for virtually all.required studies.
In order to characterize the potential of the chemical to contam-
inate groundwater, adsorption and leaching studies are being
requested by the Agency.
- Droplet size spectrum testing and drift field evaluation studies
are being requested in order to determine the magnitude of
exposure to non-target organisms.
- Avian oral: Mallard duck - 6.54 mg/kg, bobwhite quail - 12-31
- Avian dietary: Mallard duck - 510-692 ppm, bobwhite quail -
541-715 ppm, ring-necked pheasant - 634 ppm
- Freshwater fish: Coldwater fish (rainbow trout) - 3.0 ppm, warm-
water fish (bluegill sunfish) - 0.039 ppm
- Acute freshwater invertebrates: (All studies listed were not
conducted according to Agency standards.)
- Acute estuarine and marine organisms: data gaps
- Precautionary language is being required to mitigate hazards to
birds, fish, and aquatic organisms. Additional labeling to protect
identified endangered species may be required at a later date.
Because of the lack of environmental fate and field monitoring data
to quantify exposure of disulfoton to these organisms, the Agency
cannot quantify the hazard potential. Additional chronic toxicity
studies may be required, depending on the results of the environ-
mental fate and field monitoring data.
- The Agency is unable to complete a tolerance reassessment because
of certain residue chemistry and toxicology data gaps.
- Tolerances: alfalfa (fresh) - 5.0 ppm; alfalfa (hay) - 12.0 ppm;
asparagus - 0.1 ppm; barley (fodder, green) - 5.0 ppm; barley
(grain) - 0.75 ppm; barley (straw) - 5.0 ppm; beans (dry) - 0.75
ppm; beans (lima) - 0.75 ppm; beans (snap) - 0.75 ppm; beans
(vines) - 5.0 ppm; beets, sugar (roots) - 0.5 ppm; beets, sugar
(tops) - 2.0 ppm; broccoli - 0.75 ppm; Brussels sprouts - 0.75
ppm; cabbage - 0.75 ppm; cauliflower - 0.75 ppm; clover (fresh) -
5.0 ppm; clover (hay) - 12.0 ppm; coffee beans - 0.3 ppm; corn,
field (fodder) - 5.0 ppm; corn, field (forage) - 5.0 ppm; corn,
grain - 0.3 ppm; corn, pop - 0.3 ppm; corn, pop (fodder) - 5.0 ppm;
corn, pop (forage) - 5.0 ppm; corn, sweet (fodder) - 5.0 ppm;
corn, sweet (forage) - 5.0 ppm; corn, sweet. grain (kernels plus
cob with husks removed) - 0.3 ppm; cottonseed - 0.75 ppm; hops -
0.5 ppm; lettuce - 0.75 ppm; oats (fodder, green) - 5.0 ppm;
oats (grain) - 0.75 ppm; oats (straw) - 5.0 ppm; peanuts - 0.75 ppm;
peanuts (hay) - 5.0 ppm; peanuts (hull) - 0.3 ppm; peas - 0.75 ppm;
peas (vines) - 5.0 ppm; pecans - 0.75 ppm; peppers - 0.1 ppm;
pineapples (forage) - 5.0 ppm; potatoes - 0.75 ppm; rice - 0.75 ppm;
rice (straw) - 5.0 ppm; sorghum (fodder) - 5.0 ppm; sorghum (forage)
- 5.0 ppm; sorghum (grain) - 0.75 ppm; soybeans - 0.1 ppm; soybeans
(forage) - 0.25 ppm; soybeans (hay) - 0.25 ppm; spinach - 0.75 ppm;
sugarcane - 0.3 ppm; tomatoes - 0.75 ppm; wheat (fodder. green) -
5.0 ppm; wheat (grain) - 0.3 ppm; wheat (straw) - 5.0 ppm.
- Based on established tolerances, the theoretical maximum residue
contribution (TMRC) for disulfoton residue in the human diet is
calculated to be 0.2544 mg/day. The provisional acceptable daily
intake (PADI) of disulfoton is 0.0025 mg/kg/day. The maximum
permissible intake (MPI) for a 60-kg person is 0.15 mg/day. The
percent of the ADI utilized is 169%. However, a reassessment of
the current tolerances based on actual constituents of the plant
residues (metabolites) is necessary, as well as toxicity data on
the most toxic metabolite. Conformity of U.S. tolerances with
Canada and Codex Alimentarius tolerances is withheld pending
receipt and evaluation of appropriate data referred to above.
- U.S. tolerances for most raw agricultural commodities are not
supported by current residue data. More data are required.
Summary Science Statement
- Disulfoton is very highly toxic to all mammalian systems by all
routes of exposure and is assigned to Toxicity Category I, on the
basis of acute toxicity requiring the most stringent labeling
precautions and use restrictions. It is not considered to be
oncogenic, mutagenic, or teratogenic based upon existing data.
However, additional studies in a second species are being requested
to fully assess oncogenic and teratogenic potential. Additional
mutagenicity studies are also being required. Reproduction data
is lacking and is required.
- Due to the high acute toxicity and cholinesterase inhibition of
disulfoton, the Agency is imposing a 24-hour reentry restriction
for crop uses until appropriate reentry studies and dermal
absorption data are submitted and evaluated and a decision is
reached whether a different time interval is more appropriate.
- Data are insufficient to assess the environmental fate of
disulfoton. The Agency is requesting necessary data to make this
assessment and also to specifically assess whether or not disulfoton
will leach into groundwater. Considering the high acute toxicity of
disulfoton, spray drift data are being required to measure human and
non-target organism exposure resulting from spray applications, and
dermal and inhalation exposure data are being required to measure
worker exposure in outdoor applications.
- Disulfoton is very highly toxic to fish and mammals, highly toxic to
birds, and moderately toxic to honeybees. Full field monitoring
studies are required for the terrestrial uses to assess the exposure
potential. Based on these results and on the results of the out-
standing environmental fate data, chronic studies for both aquatic
and terrestrial species may be required, as well as full field
monitoring studies for the aquatic uses. Use precautions and
restrictions are being imposed in the interim to reduce potential
- A number of terrestrial and aquatic endangered species have been
identified as at risk from the use of a number of chemicals,
including disulfoton on certain crops. This issue is currently
being addressed as part of a cluster approach. Interim labeling to
protect these species may be necessary if the cluster analysis is
not completed by 1986.
- A full tolerance reassessment cannot be completed. The previous ADI
was established using a rat chronic feeding study which was found to
be unacceptable. The present Provisional ADI was based on a dog
chronic feeding study. The percent of the PADI utilized is 169%.
A second rat chronic feeding study is required, as well as animal
metabolism data to quantify and qualify disulfoton oxidation
metabolites in meat, milk, poultry, and eggs, and residue data on
numerous commodities. The Agency is requiring that when the toler-
ance reassessment is made, after receipt of the requested data, all
tolerances are to be calculated and expressed in terms of disulfoton
sulfone, the major metabolite, rather than as demeton (which is how
the Agency previously expressed tolerances for disulfoton).
4. SUMMARY OF REGULATORY POSITION AND RATIONALE
- The Agency has determined that it should continue to allow the
registration of disulfoton. Adequate studies are available to assess
the acute toxicological effects of disulfoton to humans. None of the
criteria for unreasonable adverse effects listed in Section 162.11(a)
of Title 40 of the U.S. Code of Federal Regulations have been met or
exceeded. However, because of certain gaps in the data base, a full
risk assessment of disulfoton cannot be completed.
- Also, a full tolerance reassessment cannot be completed because of
certain residue chemistry and toxicology data gaps.
- The Agency is concerned about whether or not the potential total human
exposure to disulfoton and its metabolites, both from direct and
indirect human contact and the exceeded ADI, poses any unacceptable
hazards. To resolve this concern, additional residue, metabolism,
and exposure data are required, and until it is resolved no new uses
will be granted.
- All end-use products formulated at greater than 2% are classified for
Restricted Use, pending receipt and evaluation of appropriate acute
toxicity data. Acute toxicity data on products 2% and less are being
required in order to determine the appropriateness of a Restricted
Use classification. These steps are being taken due to the extreme
toxicity of disulfoton and the lack of product specific acute toxicity
- A federal 24-hour reentry interval is established for treated crop
areas until reentry and dermal absorption data are submitted, as
required, and the Agency decides on the most appropriate time interval
- Available data are insufficient to fully assess the environmental fate
of disulfoton. The Agency is requesting data to determine if
disulfoton will contaminate groundwater.
- Toxicity data available for disulfoton indicates that it is highly
toxic to aquatic, terrestrial, and avian species. Data to assess the
extent of the potential exposure is currently lacking and is required
to complete the hazard assessment.
- Compliance dates for revised labeling:
- For addition of Restricted Use classification to product formula-
tions containing greater than 2% disulfoton: all such products
released for shipment after September 1, 1985 must bear Restricted
Use labeling. All such products in the channels of trade after
September 1, 1986 must be labeled for Restricted Use.
- For intrastate products, the Agency is requiring submission of
applications for full registration of all intrastate products
containing disulfoton by December 31, 1985. Holders of such
intrastate products who request withdrawal or who fail to respond
to the notification may not distribute or sell the intrastate
product after December 11, 1985. Products already in the channels
of trade as of that date may continue to be distributed and sold
by dealers and retailers until June 30, 1986. Any product found in
the channels of trade after June 30, 1986 will be considered to be
in violation of FIFRA Section 23(2)(1)(A).
5. SUMMARY OF MAJOR DATA GAPS
- Additional residue data on various raw agricultural and processed
commodities are being required. Also, additional chronic toxicity,
oncogenicity, and mutagenicity data are needed to better define the
long-term effects of this chemical. Plant and animal metabolism.
exposure, spray drift, reentry, and subchronic toxicity data are
required to better qualify and quantify human exposure to residues of
disulfoton and its metabolites, both from dietary and non-dietary
- Other requirements:
- Acute inhalation
- Acute oral, dermal, and inhalation studies on formulating
intermediates and end-use products
- Acute delayed neurotoxicity
- Dermal absorption study
- Product integrity study
- Hydrolysis study
- Photodegradation studies
- Soil and water metabolism studies
- Mobility studies
- Volatility studies
- Dissipation studies
- Accumulation studies
- Large scale field monitoring studies
- Acute freshwater invertebrates testing
- Acute estuarine and marine organisms testing
- Honeybee toxicity of residues on foliage study
6. Contact Person at EPA
George T. LaRocca
Product Manager (15)
Registration Division (TS-767C)
Office of Pesticide Programs
Environmental Protection Agency
401 M Street S.W.
Washington, DC 20460
Office location and telephone number:
Room 204, Crystal Mall #2
1921 Jefferson Davis Highway
Arlington, VA 22202
DISCLAIMER: THE INFORMATION PRESENTED IN THIS CHEMICAL INFORMATION FACT
SHEET IS FOR INFORMATIONAL PURPOSES ONLY AND NOT TO BE USED TO FULFILL
DATA REQUIREMENTS FOR PESTICIDE REGISTRATION AND REREGISTRATION.