Malathion resistance levels in sympatric populations of Drosophila simulans (Diptera: Drosophilidae) and D. melanogaster differ by two orders of magnitude.

Malathion resistance levels were determined in populations of the sympatric species Drosophila melanogaster (Meigen) and Drosophila simulans (Sturtevant) from a collection site in Tampa Bay, FL, with a 20+ yr history of intensive malathion exposure. Bioassays of insecticide resistance were done by using a desiccation technique that reduces both the effects of avoidance behavior on the results and the total time of the assay. Resistance levels in isofemale lines of D. simulans are as much as 2 orders of magnitude higher than those in the D. melanogaster lines and are significantly higher than any previously reported resistance levels for organophosphate exposure in Drosophila sp. Comparison with specimens from additional collection sites in the region indicates that the normal ratio of these sympatric species has been altered at our study site. The use of Drosophila sp. as a model system to study resistance management strategies and differences in the evolution of insecticide resistance among sympatric species is discussed.