Introduction: Methomyl was introduced in 1966 as a broad spectrum insecticide. It is also used as an acaricide to control ticks and spiders. It is used for foliar treatment of vegetable, fruit and field crops, cotton, commercial ornamentals, and in and around poultry houses and dairies. It is also used as a fly bait. Methomyl is effective in two ways: (a) as a "contact insecticide," because it kills target insects upon direct contact, and (b) as a "systemic insecticide" because of its capability to cause overall "systemic" poisoning in target insects, after it is absorbed and transported throughout the pests that feed on treated plants. It is capable of being absorbed by plants without being "phytotoxic" or harmful, to the plant.
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Common name: Methomyl
Empirical formula: C5H10N2O2S
Another name: BSI, E-ISO, (m) F-ISO, ANSI, ESA, JMAF
Chemical Name: S-methyl N-(methylcarbamoyloxy)thioacetimidate
Mol. Weight: 162.2
CAS No.: 16752-77-5
Symptoms of Acute Exposure: An extremely severe irritant to eyes. May cause severe skin irritation and contact dermatitis. Allergic skin reactions are possible.
Hazardous Decomposition Products: Can decompose at high temperatures forming toxic gases.
Unusual Fire, Explosion and Reactivity Hazards: During a fire, irritating and possibly toxic gases may be generated by thermal decomposition or combustion.
Product analysis by hplc (CIPAC Handbook, 1998, H, 199; AOAC Methods, 17th Ed., 995.14, J. E. Thean et al., J. Assoc. Off. Anal. Chem., 1978, 61, 15; R. E. Leitch & H. L. Pease, Anal. Methods Pestic. Plant Growth Regul., 1973, 7, 331). Residues determined by glc with FPD (idem, ibid.; R. T. Krause, J. Assoc. Off. Anal. Chem., 1980, 63, 1114; M. A. Luke et al., ibid., 1981, 64, 1187; A. Ambrus et al., ibid., p. 733) or by rplc (AOAC Methods, 17th Ed., 985.23). Details available from DuPont. In drinking water, by rplc and fluorimetry of liberated methylamine (ibid., 991.06).
Reviews FAO/WHO 92, 94 (see part 2 of Bibliography). Oral Acute oral LD50 for male rats 34, female rats 30 mg/kg. Skin and eye Acute percutaneous LD50 for male and female rabbits >2000 mg/kg. Mild eye irritant (rabbits). Not a skin irritant (guinea pigs). Inhalation LC50 (4 h) for rats 0.3 mg/l air (aerosol). NOEL (2 y) for rats 100, mice 50, dogs 100 mg/kg diet. ADI (JMPR) 0.02 mg/kg b.w. . Toxicity class WHO (a.i.) Ib; EPA (formulation) I, IV EC classification T+; R28| N; R50, R53
Birds Acute oral LD50 for mallard ducks 15.9, pheasants 15.4 mg/kg. Dietary LC50 (8 d) for Pekin ducks 1890, bobwhite quail 3680 mg/kg diet. Fish LC50 (96 h) for rainbow trout 3.4, bluegill sunfish 0.9 mg/l. Daphnia LC50 (48 h) 31.7 mg/l. Algae EC50 (72 h) 60 mg/l. Bees Toxic to bees, contact LD50 0.1 mg/bee, but not hazardous when the spray has dried. Worms LC50 (14 d) 23 mg/kg dry soil.
EHC 178 (WHO, 1996). Methomyl Health & Safety Guide: 97 (WHO 1995). Animals In rats, methomyl was rapidly converted to methomyl methylol, oxime, sulfoxide and sulfoxide oxime; these unstable intermediates were converted to acetonitrile and CO2, which were eliminated primarily via respiration and in the urine. Metabolism of carbamate insecticides is reviewed (M. Cool & C. K. Jankowski in "Insecticides"). Plants DT50 following leaf application c. 3-5 d. Rapidly degraded to CO2 and acetonitrile, with incorporation into natural plant components (J. Harvey & R. W. Reiser, Agric. Food Chem., 1973, 21, 775). Soil/Environment Rapidly degraded in soil. DT50 4-8 d at 20°C and soil moisture pF 2-2.5 in soils with pH from 5.1 to 7.8 and 1.2 to 3.6 % o.m. DT50 in groundwater samples <0.2 d (J. H. Smelt, Pestic. Sci., 1983, 14, 173-181). Koc 72.
Usage: Control of all stages of major Lepidoptera and Coleoptera pests and some Hemiptera and Diptera on cotton, soya beans, maize, vines, fruit, vegetables, and many other crops; seed treatment. Also used as a molluscicide for control of slugs in cereals and oilseed rape.
Application: Biochemistry Cholinesterase inhibitor. Mode of action Systemic insecticide and acaricide with contact and stomach action. Uses Control of a wide range of insects (particularly Lepidoptera, Hemiptera, Homoptera, Diptera and Coleoptera) and spider mites in fruit, vines, olives, hops, vegetables, ornamentals, field crops, cucurbits, flax, cotton, tobacco, soya beans, etc. Also used for control of flies in animal and poultry houses and dairies. Phytotoxicity Non-phytotoxic when used as recommended, except to some varieties of apple.