Demographic changes in multigeneration Plutella xylostella (Lepidoptera: Plutellidae) after exposure to sublethal concentrations of spinosad.

The sublethal effects of the insecticide spinosad were assessed against Plutella xylostella (L.) (Lepidoptera: Plutellidae) by using a demographic approach. The resulting data were analyzed based on the age-stage and two-sex life table model. Bioassay results showed that the susceptibility of P. xylostella to spinosad decreased in Sub strains selected by LC25 spinosad for 10 generations. Although the egg size was significantly smaller in Sub-1 (selected by LC25 spinosad for one generation) compared with spinosad-susceptible strain SS, egg size recovered to normal levels in Sub-5 (selected by LC25 spinosad for five generations) and were even larger in Sub-10 (selected by LC25 spinosad for 10 generations). The life-history parameters changed in the Sub strains of different generations. Sublethal spinosad had a significant effect on Sub-1; however, the sublethal effect on the Sub strains decreased as selection cycles increased. The intrinsic rate of increase (r(m)), finite rate of increase (lambda) and net reproductive rate (R(o)) in Sub-1 were significantly decreased compared with those in the SS strain. No significant differences were found between the Sub-5 or Sub-10 compared with the SS strain. The greatest difference was observed in the total number of eggs laid by each female, i.e., the fecundity. The fecundity in SS, Sub-1, Sub-5, and Sub-10 were 121.92, 81.26, 114.42, and 159.21, respectively. The life expectancy of an SS, Sub-1, Sub-5, and Sub-10 egg was 10.58, 8.71, 10.21, and 7.45 d, respectively.