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

To investigate genetic variation and seasonal fluctuation in susceptibility to insecticides, natural populations of Drosophila melanogaster were colle cted 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 insect icides, including permethrin, malathion, prothiophos, fenitrothion, and DDT , was examined. Lines of each population exhibited the broad ranges of vari ation in susceptibility to all chemicals. Comparison between populations in different seasons indicated that genetic variation in susceptibility to or ganophosphates fluctuated in consistency with the population size, in which the susceptibility increased in fall. In addition, highly significant corr elations were observed among responses to organophosphates, and the correla tions also fluctuated with seasons. On the other hand, genetic variation in susceptibility to permethrin and DDT was less fluctuated. These results su ggest that not only a common resistance factor for organophosphate resistan ce but also different resistance factor(s) for each insecticide could be in volved within a natural population, and that the fluctuation observed in th e susceptibility to organophosphates could be associated with fitness costs of organophosphate resistance factor(s).