Mechanism of physiological resurgence affecting recovery of susceptibility in fenvalerate-resistant strains of the diamondback moth Plutella xylostella.

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A diamondback moth (DBM) strain resistant to fenvalerate was established and the effects of sublethal dose of the insect on egg size and fecundity were examined. The eggs of the selected strain were significantly smaller than those of non-selected strain. The fecundity of females of the selected strain tended to be higher than that of the non-selected strain. The survival rate of the selected strain was lower than that of the non-selected strain.To determine mode of inheritance of resistant development and of reduction of egg size in DBM, F_1 offspring of reciprocal crosses were compared with their parental lines for these traits. Several fitness components were also compared between F1 offspring of reciprocal crosses as well as the resistant and susceptible lines to detect maternal effects on the components including egg size. The dose-mortality regression lines of the F_1 hybrid of susceptible females and resistant males (SR) and resistant females and susceptible males (RS) were roug … More hly intermediate between that of parental lines, i.e., SS and RR. This result indicates that the mode of inheritance of fenvalerate resistance was neither complete dominant nor recessive with no sex-linkage. The egg size of RR lines was significantly smaller than that of the SS lines. The RS and SR lines have an egg size close to that of their own matrilineage. It was considered that the mode of inheritance of resistance and that of egg size were different from each other, although the selection experiment showed that there is a negative correlated response between these parameters.The recovery of susceptibility to fenvarelate in resistant strains of DBM was examined through 10 generations after freeing then from insecticide selection under harsh-rearing (Harsh lines) and optimal-rearing conditions (Optimal lines). The LD_<50> values were lower for the Harsh lines than for Optimal lines in all the generations. Egg size of both lines increased gradually through 10Generations. Comparisons of these two lines show that the egg size increased more rapidly in Harsh lines than in Optimal lines in early generations.Comparison of immature survivability of Harsh and Optimal lines reared under those conditions showed that the survivability of Harsh line individuals was significantly lower than for those of Optimal lines. These results suggest that susceptibility might recovery more quickly in Harsh lines than in Optimal lines because resistant insects with small eggs had lower susceptibility than susceptible insects with normal eggs. Less