Abstract: Individuals were selected for increased resistance to fenvalerate in the SRP (moderate resistance) strain and RP (high resistance) strain of Plutella xylostella, and in SRC and RSC strains of the parasitoid Cotesia plutellae. The SRC strain of C. plutellae was reared on a susceptible strain of P. xylostella larvae (SP), and the RSC strain was reared on SRP larvae. Selection for SRP, RP, SRC and RSC was carried out for 13, 14, 14 and 13 generations respectively. The SRP and RP strains of P. xylostella acquired 68.9 and 605.8 fold resistance, while the SRC and RSC strains of C. plutellae acquired 4.3- and 11.0-fold resistance, respectively. These results show that fenvalerate resistance by C. plutellae can be selected by applying the insecticide to host larvae harbouring the immature parasitoids. Moreover, the resistance level of the RSC strain was higher than that of the SRC strain, demonstrating that selection with more resistant hosts can accelerate the development of resistance in the parasitoid. Ratios of MFO activity in the SP, SRP and RP strains of P. xylostella were 1∶1.15∶1.50; ratios of MFO activity in larvae of the SC, SRC and RSC strains of C. plutellae were 1∶1.10∶1.49, and in the adults of the three strains of the parasitoid were 1∶1.18∶1.54, respectively. However, changes in the specific activities of CarE and Es in the different strains of each insect species did not show a similar trend to the resistance levels. These results indicate that fenvalerate resistance in C. plutellae is positively related to MFO activity, but unrelated to CarE and Es activity.

Key words: Plutella xylostella, Cotesia plutellae, fenvalerate, resistance selection, enzyme activity