Techniques for Increasing the Convenience and Sensitivity of Dry-Film Bioassay of Insecticide Residues with Drosophila spp.

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Abstract
A flightless strain of Drosophila melanogaster Meig. was used with a dry-film bioassay technique for the analysis of dieldrin residues in wheat grain. The flightless fruitflies were much more convenient to handle than a wild strain and the use of a slippery plastic barrier on collecting and sorting dishes avoided the need for immobilising flies by cold or CO2. Experimental evidence is given for the adoption of standardised breeding and handling systems for controlling certain intrinsic factors in the fly population. Also, by controlling such extrinsic factors in the bioassay system as the number of flies per test vial, replication, and holding temperature, a satisfactorily reproducible LD50 of 0.015 μg. dieldrin per vial was obtained. When this technique was applied to the analysis of dieldrin residues in wheat grain, 0.01-0.10 ppm. dieldrin could be conveniently assayed after chromatographic cleanup of a solvent extract of the grain. The accuracy of the bioassay estimates has been confirmed by gas-chromatographic analysis.