Diminished egg size in fenvalerate resistant strains of the diamondback moth Plutella xylostella (Lepidoptera: Yponomeutidae)

Two populations (Replications 1 and 2) were selected for 8 generations with fenvalerate at a LD_<50> dose to compare the biotic performances and fitness of resistant and susceptible strains of the diamondback moth (DBM), Plutella xylostella. The LD_<50> value rose gradually from 1.4×10^<-3>μg/larva to 6.7×10^<-2>μg/larva in Replication 1 and to 4.9×10^<-2>μg/larva in Replication 2. The resistance ratio was thus 67-fold and 49-fold respectively. The eggs of the selected strain were significantly smaller than those of the non-selected strain within 2 or 3 generations, then became stabilized in Replication 1 and fluctuated in Replication 2 at 1.20×10^<-2>mm^3. The size of the non-selected strain fluctuated around 1.35×10^<-2>mm^3 in both replications. To investigate the genetic changes in biotic performances of DBM in the selected strain, offspring of the selected and non-selected strains were individually reared at every other generation. The survival rate of immature stages, developmental period, pupal weight, adult longevity, fecundity of females and size of eggs laid by females were compared between the two strains. The survival rate of the selected strain was significantly lower than that of the non-selected strain in the 2nd generation. The survival rate of the selected strain also tended to be lower in the other generation. The fecundity of females of the selected strain tended to be higher than in the non-selected strain. The eggs of the selected strain were significantly smaller than those of the non-selected strain in all generations. The development period, pupal weight and longevity of adults did not significantly differ between the two strains in any generation. These results suggest that successive selection with fenvalerate at a sublethal LD_<50> dose yielded a fitness disadvantage in the DBM population.