A review of bioassay techniques for the quantitative determination of pesticide residues has been made. Several organisms are in use. Selection depends on the pesticide used, reproducibility, indication of the true quantity of toxicant present, cost, speed of assay, and other practical limitations. The types of exposure, responses measured, sensitivity, and complexity vary with different methods using the same organism and with different organisms. Though possibly less specific than chemical methods, bioassay offers the opportunity to determine the presence of toxic metabolites which may be bypassed by more specific methods. Bioassay, in its broadest sense, may refer to any method in which some property of a material is measured in terms of a biological response. In the agricultural pesticides field, practically all chemical or biological control programs involving response of an organism to a chemical may be called bioassays. Hoskins (57) has divided bioassay into three groups: screening and formulation, resistance, and residue analysis. Bioassay will be defined here as the quantitative determination of a toxicant using biological material to measure
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 thought to be associated with the significantly higher re-coveries obtained throughout the laboratory study when phorate-soil mixes were held in a 3-in. layer. These phorate soil mixes gradually increased in toxicity lor 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.
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