Insecticide, acaricide
organophosphorus



NOMENCLATURE
Common name dimethoate (BSI, E-ISO, (m) F-ISO, ANSI, ESA, JMAF); fosfamid (USSR)
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]  EEC no. 200-480-3  Development codes EI 12 880 (Cyanamid); L 395 (Montedison); BAS 152J (BASF)  Official codes OMS 94; OMS 111; ENT 24 650

PHYSICAL CHEMISTRY
Composition Tech. grade is 95% 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 °C)  K_OW logP = 0.704  Henry 1.2 × 10^-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.

ANALYSIS
Product analysis by glc (CIPAC Handbook, 1980, 1A, 1225; CIPAC Proc., 1981, 3, 204, 211; CIPAC Handbook, 1992, E, 69-72). Residues determined by glc (Analyst (London), 1977, 102, 858; 1980, 105, 515; 1985, 110, 765; J. E. Boyd, Anal. Methods Pestic. Plant Growth Regul., 1972, 6, 357; Man. Pestic. Residue Anal., 1987, I, 3, 6, S8, S13, S17, S19; Anal. Methods Residues Pestic., 1988, Part I, M5, M12).

MAMMALIAN TOXICOLOGY
Reviews FAO/WHO 77, 79 (see part 2 of the Bibliography).  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 (JMPR) 0.002 mg/kg b.w. (sum of dimethoate and omethoate, expressed as dimethoate) [1996].  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 mg/bee. 

ENVIRONMENTAL FATE
EHC 90 (WHO, 1989), 63 (WHO, 1986; a general review of organophosphorus insecticides).  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. K_oc 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.