IDENTIFICATION
Common name: Methamidophos
Other name: methamidophos (BSI, E-ISO, (m) F-ISO, ANSI)
acephate-met
Iupac name: O,S-dimethyl phosphoramidothioate
Chemical abstracts name: O,S-dimethyl phosphoramidothioate
Type: Insecticide for Agriculture
CAS RN: [10265-92-6]
M.F.: C2H8NO2PS
Mol Wt: 141.1
PHYSICAL CHEMISTRY
Form (appearance): Colourless crystals, with a mercaptan-like odour.
M.P.: 44.9℃; (tech., 20-25℃)
V.P.: 2.3 mPa (20℃); 4.7 mPa (25℃)
F.P.: 66℃ (EU A.9/ASTN-D56)
S.G.: 1.27 (20℃)
Solubility.: In water >200 g/l (20℃). In isopropanol >200, dichloromethane >200, hexane 0.1-1, toluene 2-5 (all in g/l, 20℃).
Stability: Stable at ambient temperature, but decomposes on heating without boiling. Stable at pH 3-8. Hydrolysed in acids and alkalis, DT50 (22℃) 1.8 y (pH 4), 120 h (pH 7), 70 h (pH 9). Photodegradation is of minor importance
Henry: <1.6 × 10-6 Pa m3 mol-1 (calc., 20℃)
KowlogP: -0.8 (20℃)
APPLICATION
Formulation types: SL
Biochemistry: Cholinesterase inhibitor
Mode of action: Systemic insecticide and acaricide with contact and stomach action. Absorbed by the roots and leaves.
Uses: Control of chewing and sucking insects, and spider mites on ornamentals, potatoes, pome fruit, stone fruit, citrus fruit, vines, hops, brassicas, beet, cotton, maize, tobacco, and other crops. Applied at 0.35-1.2 kg/ha.
Compatibility: Incompatible with alkaline materials
MAMMALIAN TOXICOLOGY
Reviews: FAO/WHO 59, 61 (see part 2 of the Bibliography).
Oral: Acute oral LD50 for male rats 15.6, female rats 13.0 mg/kg.
Skin and eye: Acute percutaneous LD50 for male rabbits 122, female rabbits 69 mg/kg. Not irritating to skin; slightly irritating to eyes (rabbits). Not a skin sensitiser. Inhalation LC50 (4 h) for male and female rats (nose only) 213 mg/m3.
Noel: (1 y) for dogs 0.06 mg/kg b.w. daily; (2 y) for rats 0.1 mg/kg b.w. daily; (2y) for mice 0.7-0.8 mg/kg b.w. daily.
ADI: (JMPR) 0.004 mg/kg b.w. [1990].
Toxicity: WHO (a.i.) lb; EPA (formulation) I
EC hazard: T+; R28| T; R24| Xi; R36| N; R50;
PIC SL formulations of the substance which exceed 600 g a.i./l are included
ECOTOXICOLOGY
Birds: Acute oral LD50 for bobwhite quail 10 mg/kg. Dietary LC50 (5 d) for mallard ducks 1302 ppm.
Fish: LC50 (96 h) for rainbow trout (static) 25 ppm, bluegill sunfish (static) 34 ppm, sheepshead minnow 5.6 mg/l.
Daphnia: EC50 (48 h) 0.27 mg/l.
Algae: ErC50 (96 h, static) for Scenedesmus subspicatus >178 mg/l.
Worms: LC50 (14 d) for Eisenia foetida 34 mg/kg dry soil.
Bees: Toxic to bees.
ENVIRONMENTAL FATE
Plant: EHC 63 (WHO, 1986; a general review of organophosphorus insecticides).
After treatment with methamidophos via the roots, the a.i. was taken up rapidly and translocated into the leaves with the transpiration flow. However, a more prolonged uptake via the roots cannot be expected because of rapid degradation in soil. After foliar treatment, methamidophos was taken up rapidly; however, there was little translocation into untreated parts of the plant.
Animals: In rats and farm animals, radiolabelled methamidophos was absorbed rapidly and distributed uniformly among all organs and tissues. More than half of the radioactivity was rapidly eliminated from the body, mainly via urine and respiratory air. Radioactivity remaining in the animal was incorporated into endogenous compounds (carbon-1 pool) and eliminated with the natural turnover of these compounds. Metabolism in the rat was by deamination and demethylation.
Soil/Environment: Rapidly degraded in soil; field DT50 c, <2 d. In water. DT50 5-27 d (pH 7); photolysis may contribute to degradation. Rapidly degraded in air, DT50 0.578 d.
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