Seasonal fluctuation in susceptibility to insecticides within natural populations of Drosophila melanogaster: empirical observations of fitness costs of insecticide resistance.

To investigate genetic variation and seasonal fluctuation in susceptibility to insecticides, natural populations of Drosophila melanogaster were collected from Katsunuma in mid summer and late fall for two consecutive years. After isofemale lines of each population collected in each season had been established in a laboratory, the susceptibility of each line to five insecticides, including permethrin, malathion, prothiophos, fenitrothion, and DDT, was examined. Lines of each population exhibited the broad ranges of variation in susceptibility to all chemicals. Comparison between populations in different seasons indicated that genetic variation in susceptibility to organophosphates fluctuated in consistency with the population size, in which the susceptibility increased in fall. In addition, highly significant correlations were observed among responses to organophosphates, and the correlations also fluctuated with seasons. On the other hand, genetic variation in susceptibility to permethrin and DDT was less fluctuated. These results suggest that not only a common resistance factor for organophosphate resistance but also different resistance factor(s) for each insecticide could be involved within a natural population, and that the fluctuation observed in the susceptibility to organophosphates could be associated with fitness costs of organophosphate resistance factor(s).