Dimethoate

NOMENCLATURE
Common name: dimethoate; fosfamid
IUPAC name: O,O-dimethyl S-methylcarbamoylmethyl phosphorodithioate; 2-dimethoxyphosphinothioylthio-N-methylacetamide
Chemical Abstracts name: O,O-dimethyl S-[2-(methylamino)-2-oxoethyl] phosphorodithioate
CAS RN: [60-51-5]

PHYSICAL CHEMISTRY
Composition: Tech. grade is 98% pure. Mol. wt.: 229.3; M.f.: C₅H₁₂NO₃PS₂; Form: Colourless crystals; (tech., white to greyish crystals). M.p.: 49 ^oC; (tech., 43-45 ^oC). B.p.: 117 ^oC/0.1 mmHg V.p.: 0.25 mPa (25 ^oC). KOW: logP = 0.704; Henry: 1.2x10^-6 Pa m³ mol^-1 S.g./density: 1.277 at 65 ^oC. Solubility: In water 23.3 (pH 5), 23.8 (pH 7), 25.0 (pH 9) (all in g/l, 20 ^oC). Readily soluble in most organic solvents, e.g. in alcohols, ketones, benzene, toluene, chloroform, dichloromethane >300, carbon tetrachloride, saturated hydrocarbons, n-octanol >50 (all in g/kg, 20 ^oC). Stability: Relatively stable in aqueous media at pH 2-7. Hydrolysed in alkaline solutions; DT₅₀ 12 d (pH 9). Decomposes on heating, forming the O,S-dimethyl analogue.

APPLICATIONS
Biochemistry: Cholinesterase inhibitor.
Mode of action: Systemic insecticide and acaricide with contact and stomach action.
Uses: Control of a wide range of Acari, Aphididae, Aleyrodidae, Coccidae, Coleoptera, Collembola, Diptera, Lepidoptera, Pseudococcidae and Thysanoptera in cereals, citrus, coffee, cotton, fruit, grapes, olives, pastures, beetroot, potatoes, pulses, tea, tobacco, and vegetables. Also used for control of flies in animal houses.
Phytotoxicity: Non-phytotoxic when used as directed, except to some varieties of lemon, peach, fig, olive, walnut, hop, tomato, bean, cotton, and pine. Russetting is possible with Red Delicious and Golden Delicious apples, and with some ornamentals.
Formulation types: EC; GR; UL; WP; Aerosol.
Compatibility: Incompatible with alkaline materials and with sulfur-based formulations.

MAMMALIAN TOXICOLOGY
Oral: Acute oral LD₅₀ for rats 387, mice 160, rabbits 300, guinea pigs 350 mg/kg.
Skin and eye: Acute percutaneous LD₅₀ for rats >2000 mg/kg. Non-irritating to skin (rabbits).
Inhalation: LC₅₀ (4 h) for rats >1.6 mg/l air.
NOEL: (2 y) for rats 5.0 mg/kg diet (0.2 mg/kg daily).
ADI: 0.002 mg/kg b.w. (sum of dimethoate and omethoate, expressed as dimethoate).
Toxicity class: WHO (a.i.) II; EPA (formulation) II
EC hazard: Xn; R21/22

ECOTOXICOLOGY
Birds: Acute oral LD₅₀ for male pheasants 15, quail 84, chickens 108, female mallard ducks 40 mg/kg.
Fish: LC₅₀ (96 h) for mosquito fish 40-60, rainbow trout 6.2, bluegill sunfish 6 mg/l.
Daphnia: LC₅₀ (24 h) 4.7 mg/l; NOEC (24 h) 1 mg/l.
Algae: EC₅₀ (72 h) for Selenastrum capricornutum 282.3 mg/l; NOEC (72 h) 30.5 mg/l.
Bees: Toxic to bees. LD₅₀ (oral and topical) 0.1-0.2 ug/bee.

ENVIRONMENTAL FATE
EHC: Dimethoate degrades with short half-life in soil, water and plants. When used correctly, exposure of the population through air, food or water is negligible.
Animals: In mammals, metabolism follows the same pattern as in plants.
Plants: In plants, oxidised to O,O-dimethyl-phosphorothioate and hydrolysed to O,O-dimethyl-phosphorodithioate, -phosphorothioate, and -phosphate. The ester group is demethylated and the methylamino group is hydrolytically cleaved. Oxidation of dimethoate gives the corresponding oxone (omethoate q.v.), which is classified as toxic and a strong cholinesterase inhibitor, and which appears to show similar rapid degradation in environmental compartments as dimethoate.
Soil/Environment: Adsorption and desorption constants have been shown to be a linear function of soil silt content. Koc ranges from 16.25 (sandy loam) to 51.88 (sand/loamy sand). Aerobic DT₅₀ 2-4.1 d. Photolytic DT₅₀ on soil surface 7-16 d. Low potential for leaching to groundwater because of rapid degradation in soil.