Decline of Phorate and Dimethoate Residues in Treated Soils Based on Toxicity to Drosophila melanogaster

Dimethoate and phorate were studied as soil treatments in the field and laboratory, using toxicity to Drosophila melanogaster (Meigen) to determine the rate of breakdown. Initial recoveries of surface treatments with both insecticides in the field were significantly greater than the amount applied. The high dimethoate recoveries were confirmed in laboratory surface treatments of moistened soil. Surface treatment of air dry soils and soil insecticide mixtures produced recoveries lower than applied. The high recoveries of phorate field surface treatments were not confirmed by laboratory surface treatments, but are obtained throughout the laboratory study when phorate-soil mixes were held in a 3-in. layer. These phorate soil mixes gradually increased in toxicity for 28 days, when they were equivalent to 4 times the phorate applied. Following the high initial recoveries, the surface treatments of both insecticides declined in a conventional manner. Phorate breakdown was greatest during the first week and almost complete at the end of 1 month. Dimethoate declined more slowly, with about 30% remaining at the end of 1 month.