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 Dro sophila spp. at a frequency of approximately 1% (0.01), despite the wi thdrawal of most cyclodiene type insecticides except endosulfan. Howev er, we have previously documented that resistance-associated amino aci d replacements in the gene Rdl, a gamma-aminobutyric acid receptor, ca n significantly affect several channel functions of the integral chlor ide ionophore. We were therefore interested in investigating if differ ent resistance-associated replacements confer significant fitness disa dvantages and whether the use of endosulfan could be maintaining selec tion for cyclodiene resistance in the field. Using PCR amplification o f specific alleles (PASA) within 3000 individual flies, we report that neither the alanine302 > serine (allele I) replacement in Drosophila melanogaster Meigen nor the alanine302 > serine (allele I) or alanine3 02 > glycine (allele 2) replacements in D. simulans Sturtevant showed any reduction in frequency in cage experiments nm 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 c ontinued use of endosulfan, may maintain cyclodiene resistance at this relatively high frequency in field populations.