PCR based monitoring of specific Drosophila (Diptera: Drosophilidae) cyclodiene resistance alleles in the presence and absence of selection

Cyclodiene insecticide resistance persists in field populations of Drosophila spp. at a frequency of approximately 1% (0.01), despite the withdrawal of most cyclodiene type insecticides except endosulfan. However, we have previously documented that resistance-associated amino acid replacements in the gene Rdl, a γ-aminobutyric acid receptor, can significantly affect several channel functions of the integral chloride ionophore. We were therefore interested in investigating if different resistance-associated replacements confer significant fitness disadvantages and whether the use of endosulfan could be maintaining selection for cyclodiene resistance in the field. Using PCR amplification of specific alleles (PASA) within 3000 individual flies, we report that neither the alanine302 > serine (allele 1) replacement in Drosophila melanogaster Meigen nor the alanine302 > serine (allele 1) or alanine302 > glycine (allele 2) replacements in D. simulans Sturtevant showed any reduction in frequency in cage experiments run for one year in the laboratory in the absence of selection. Further, repeated applications of endosulfan selected significantly for cyclodiene resistance in the field. Thus the apparent absence of fitness cost, combined with the continued use of endosulfan, may maintain cyclodiene resistance at this relatively high frequency in field populations.