Resistance to fenvalerate was examined in a laboratory-selected population of the diamondback moth, Plutella xylostella, during exposure through 10 generations. Selection results showed that LD 50 values rose from 0.003 mg per larva to 31.1 mg per larva and 36.7 mg per larva respectively, in replications 1 and 2 during those 10 generations. Resistant line moth eggs were significantly smaller than those of moths before selection. Therefore, we selected a subpopulation of resistant lines under harsh environmental conditions of low humidity and high temperature, over 10 generations (Harsh line) without insecticide exposure. We selected another subpopulation of resistant lines under optimal environmental conditions of high humidity and normal temperature through all 10 generations (Optimal line). After 10 generations, the LD 50 of fenvalerate decreased to 0.03 mg per larva and 0.02 mg per larva under harsh conditions and 0.2 mg per larva and 0.29 mg per larva under optimal conditions. The LD 50 values were lower for the Harsh lines than for the Optimal lines in all generations. The egg size of both lines increased gradually through 10 generations. Comparisons of these two lines showed that the egg size increased more rapidly in the Harsh lines than in Optimal lines in early (first-sixth) generations. In subsequent generations, egg sizes were nearly normal in both lines. 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 Optimal lines. Comparison of the two lines' survivability showed an opposite result from the comparison when they were reared under equivalent optimal conditions. These results suggest that susceptibility to insecticides might recover more quickly in Harsh lines than in Optimal lines because resistant insects with small eggs had lower survivability than susceptible insects with normal eggs. They were eliminated more rapidly under harsh environmental conditions.

Reciprocal crosses between susceptible (SS) and resistant (RR) lines to fenvalerate were conducted to determine whether or not there are any genetic relationships between insecticide resistance and reduction in egg size, as well to whether or not they affect the offspring fitness of the diamondback moth, Plutella xylostella. 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 roughly intermediate between those of parental lines, i.e., SS and RR. This result indicates that the mode of inheritance of fenvalerate resistance was neither complete dominance 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. We 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 negatively correlated response between these parameters. The survival rate of the RR lines was significantly lower than that of the SS lines. However, the survival rate of the RS lines did not differ from that of the SR lines. Reproductive fitness is discussed with regard to different egg sizes among parental lines and reciprocal crosses.

The JapanSociety ofMedicalEntomology andZoology [Med. Entomo], Zool,VoL 59No, 2 p. 81-84 2008) The