DEGRADATION OF FENPYROXIMATE IN UPLAND SOILS UNDER LABORATORY CONDITIONS

Degradation of [pyrazole-3-C-14]- or [benzyl ring-C-14(U)]fenpyroximate (tert-butyl (E)-alpha-[1,3-dimethyl-5-phenoxypyrazol-4-ylmethyleneamino-oxy]-p-toluate) was studied in Ehime (diluvial) and Kanagawa (volcanic ash) soils under upland laboratory conditions. Soils (25 g) fortified with C-14-fenpyroximate (1.30-2.12 ppm) were incubated at 25-degrees-C for 112 days in the dark, The compound degraded with half-lives of 34.3-49.7 days in the Ehime soil and of 26.3-35.6 days in the Kanagawa soil. In the soil extracts, twelve degradation products were identified by TLC-co-chromatography with authentic compounds. Major degradation products were (E)-4-[(1,3-dimethyl-5-phenoxypyrazol-4-yl)-methyleneaminooxymethyl]benzoic acid (5), 1,3-dimethyl-5-phenoxypyrazole-4-carboxylic acid (12) and 1,3-dimethyl-5-phenoxypyrazole-4-carbonitrile (9). In both soils, C-14-carbon dioxide liberated from [pyrazole-C-14]fenpyroximate during the 112 days accounted for 16.9-17.1% of the applied radioactivity, and that from [benzyl-C-14]fenpyroximate for 51.2-64.6%. Fenpyroximate degradation and CO2 evolution were negligible in sterilized soils during the incubation. It was assumed that fenpyroximate degraded through hydrolysis of tert-butyl ester, isomerization or cleavage of the oxime ether structure, N-demethylation, oxidation of the methyl group at the 3-position on the pyrazole ring and hydroxylation of the phenoxy ring, and that it finally mineralized to CO2 and/or bound to soil organic matter.