Pesticide Profile - Disulfoton

Quick Facts:  

• Chemical name: O,O-Diethyl S-[2-(ethylthio)ethyl] phosphorodithioate

• Trade names: Dimaz, Di-Syston, Dithiodemeton, Solvirex, Frumin AL

• Pesticide Type: insecticide, acaricide

• Pesticide class: organophosphate; cholinesterase inhibitor

• Use: systemic and contact insecticide with a broad spectrum of activity; disulfoton is used on a wide variety of food, feed, and nonfood crops. Crops with the highest use are: cotton accounts for 61% of the total use, wheat 16%. 1.7 million pounds of disulfoton were used in 1997.

• Disulfoton bioaccumulates in fish and other organisms.

• Disulfoton has been detected in surface and ground waters in the U.S. It is mobile in soil and persistent in water.

• Disulfoton's degradates, disulfoton sulfone and disulfoton sulfoxide, are more persistent in the environment and as toxic as the parent compound. Another degradate, demeton, is a pesticide which has been cancelled by the EPA due to its toxicity and persistence.

• Disulfoton ranks third among the organophosphate pesticides for percentage of exposed individuals hospitalized for occupational poisoning.

• Disulfoton has been shown to be mutagenic in bacteria.

Chemical Structure

Background

Disulfoton is a broad-spectrum organophosphate insecticide and acaricide used in a wide variety of pesticide products. Approximately 1.7 million pounds are used annually in the U.S. on cotton (61%), wheat (16%), a variety of vegetable crops, plantation-grown Christmas trees, and home gardens. It is formulated as a granular product that is incorporated into the soil, an emulsifiable liquid that can be applied as a foliar treatment and in irrigation water, and as a 95% active ingredient liquid for seed treatment of cotton. It is a systemic insecticide and is absorbed rapidly into plant tissues, conferring toxicity to many or all plant parts. Birds feeding on seeds, as do many of the finches and buntings, or grazing on young plants, like ducks and geese, are directly exposed to toxins after systemic insecticides are applied. Secondary exposure and poisoning occurs after birds feed on insects that have consumed residue-laden plant material. Secondary poisoning is facilitated when normally cryptic or difficult to catch insects are impaired by the effects of pesticides, allowing insectivorous birds to gorge on contaminated insects. High levels of toxins can be attained in this manner and has resulted in avian mortality in connection with disulfoton use.  

Disulfoton degrades into compounds in the environment that have been shown to be just as toxic to target and non-target organisms as the parent compound. It is metabolized in plants to form at least five cholinesterase-inhibiting products. Most of the degradates are more persistent in the environment than the disulfoton, itself. One of the degradates, demeton, is a pesticide which has been cancelled due to its high degree of toxicity and persistence. Residual insecticidal activity may last from 6-8 weeks, as estimated by manufacturers, and this may be a conservative figure. The occurrence of toxic break-down products, many of which have long half-lives, increases the time interval for exposure to target and non-target insects and wildlife. The granular formulation of disulfoton presents multiple dangers to wildlife. The particles can be ingested during foraging. Research has shown that soil comprises up to 30% of the diet of woodcocks and other sandpipers, possibly by consuming soil organisms such as earthworms, which typically contain 20-30% soil. Pools of water that form on fields after irrigation or rain events are attractive to birds and when disulfoton applied to the soil is dissolved in water, the pools become toxic. Birds may drink or bathe in the water, and, as disulfoton is very highly toxic through dermal exposure, this presents a deadly scenario for birds. Disulfoton and breakdown products are persistent in the soil, and may pose these sorts of hazards to birds and other wildlife for an entire growing season or longer.

Environmental Effects

Fate

• Persistence: The data submitted by the registrant is incomplete for a number of key fate measurements for disulfoton and its degradates. In sandy loam soils, disulfoton has been shown to persist for one week. Toxic primary degradates may persist from 8 to 10 weeks, for disulfoton sulfoxide, to greater than 294 days for disulfoton sulfone. These two degradates are potent cholinesterase inhibitors and are particularly hazardous to birds. Persistence appears to be greatly increased in anaerobic soils, however, these tests have not been adequately performed and submitted by the registrant. Disulfoton is mobile in soils and poses a high risk for run-off into surface and ground waters. Disulfoton has been detected in groundwater in Virginia and Wisconsin at levels up to 1.0 mg/L. The registrant has not submitted data for degradation in aquatic systems, however, disulfoton is relatively stable in water at neutral and acidic pH's. Disulfoton is resistant to hydrolysis with a half-life of 323 days at pH 7.

• Solubility: Largely insoluble in water, disulfoton is soluble in fatty oils and most organic solvents.

• Bioaccumulation: A bioconcentration factor of 460 for whole fish, 700 for viscera and 460 for fillet indicate a moderate potential for disulfoton to bioaccumulate.

Ecotoxicity 

• Disulfoton is very highly toxic to most species of warm water fish. Cold water species (rainbow trout) appear to be less sensitive. In on study, the measured LC50 for bluegill sunfish was 0.038 mg/L. Disulfoton has been detected in groundwater in random testing surveys in Virginia and Wisconsin at 1.0 mg/L. Massive fish kills have been noted in the past in conjunction with disulfoton use.

• Disulfoton is very highly toxic to mammals through all exposure routes: oral, dermal, and inhalation. The disulfoton LD 50 for rats is 1.9 to 6.2 mg/kg . The LD 50 for the sulfone metabolite in rats is 11.24 mg/kg.

• The metabolites of disulfoton are more toxic to honeybees than the parent compound; the metabolites are more persistent as plant residues than is disulfoton.

• Disulfoton is very highly toxic to birds, particularly to small passerine specie

    Northern bobwhite                     LD 50             28 mg/kg
    Ring-necked pheasant                LD 50             11.9 mg/kg
    Red-winged blackbird                 LD 50             3.2 mg/kg
    Mallard                                    LD 50             6.54 mg/kg

Incidents

• Young County, Texas, 1993. Eighteen Swainson's hawks were found dead and one found severely disabled in a cotton field. The cotton seed had been treated with disulfoton seed treatment prior to planting, about ten days before the birds were discovered. Laboratory analysis revealed the presence of insect material in the gastrointestinal tracts of all the birds. Residue analysis identified disulfoton at a concentration of 7.0 ppm. The systemic action of the pesticide resulted in plant residues which were ingested by insects at high enough levels to cause mortality in the hawks.

• Sussex County, Delaware, 1991. Nine American robins were found dead following the application of disulfoton to a tree nursery.

• Puerto Rico, 1996. Six grackles, one owl, and one heron were found dead in the yard of a private residence. The use aite and method were not reported but birds had depressed cholinesterase levels. Residue analysis on the gut contents revealed disulfoton at 12.37 ppm wet weight.