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

 

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℃)

K_owlogP: -0.8 (20℃)

 

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

 

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

 

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: E_rC₅₀ (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.

 

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.