Interruption of chemical mosquito control and evolution of insecticide resistance genes in Culex pipiens (Diptera : Culicidae)

Within the Llobregat Delta (Barcelona, Spain), Culex pipiens L. has been th e target of organophosphate insecticide (OP) control for 10 yr (1982-1992). Between 1991 and 1992, OPs were replaced by Bacillus-based toxins in all t he mosquito control programs within >150 km of this area. The distribution of several OP-resistance genes was surveyed within the Llobregat Delta and neighboring populations (<25 km) during the 2 yr following this regional pe sticide change to investigate how the change in selection pressure affected the dynamics of OF-resistance genes. The immigration failure of the A2-B2 resistant esterases and the observed difference in OF-resistance dynamics b etween isolated and nonisolated populations may indicate fitness disadvanta ges associated with OF-resistance Genes, hence a tendency for a decrease in OF-resistance. In contrast, one OF-resistance gene further increased in fr equency, whereas the frequencies of some others were maintained. These unex pected results question the importance of pesticides from sources other tha n mosquito control, and the variability of pleiotropic fitness costs among pesticide resistance genes.