Chopade_1981_Pest.Sci_12_17

The residues and metabolism of methidathion [S-(2, 3-dihydro-5-methoxy-2-oxo-1, 3, 4-thiadiazol-3-ylmethyl) O, O-dimethyl phosphorodithioate] and its secondary metabolites: demethyl-methidathion [S-(2, 3-dihydro-5-methoxy-2-oxo-1, 3, 4-thiadiazol-3-ylmethyl) O-methyl O-hydrogen phosphorodithioate] (IV), the sulphide (2,3-dihydro-5-methoxy-3-methylthiomethyl-1,3,4-thiadiazol-2-one) (I), tsulphoxide(2,3-dihydro-5-methoxy-3- methylsulphinylmethyl-1,3,4-thiadiazol-2-one) (II) and the sulphone (2,3-dihydro-5-methoxy-3-methylsulphonylmethyl-1,3,4-thiadiazol-2-one (III) were studied in laboratory-treated tomato fruit. The metabolites and residues of methidathion were determined for the applied doses of 1, 7 and 14 mg of methidathion kg-1 of fruit. Methidathion was metabolised extensively over a 14-day period. The amount of metabolites formed was a function of both the applied dose as well as the time after application. Major water-soluble metabolites were found to be IV and the cysteine conjugate S-(2,3-dihydro-5-methoxy-2-oxo-1,3,4-thiadiazol-3-ylmethyl)-L-cysteine (VI). The chloroform-soluble metabolites were identified as the oxygen analogue of methidathion [S-(2,3-dihydro-5-methoxy-2-oxo-1,3,4-thiadiazol-3-ylmethyl) O, O-dimethyl phosphorothioate] (V), the sulphoxide II, and the hydroxy compound 2,3-dihydro-3-hydroxymethyl-5-methoxy-1,3,4-thiadiazol-2-one. The oxygen analogue of methidathion (V) was found in small amounts, corresponding to <5% of the added methidathion. Demethyl-methidathion (IV) appeared to be a precursor in the formation of the cysteine conjugate VI. The sulphide I seemed to be more reactive in forming the cysteine conjugate than the sulphoxide II or the sulphone III..