Abstract: The susceptibility to insecticides in the larval parasitoid Apanteles plutellae Kurdjumov (Hymenoptera: Braconidae) and its host Plutella xylostella (L.) (Lepidoptera: Yponomeutidae)，collected in Fuzhou, China, were detected using residual film and leafdip bioassays, respectively. The results showed that organophosphates, carbamates, pyrethroids, avermectins and fipronil were highly toxic to A. plutellae, but chlorfluazuron and Bt were not. However, A. plutellae could survive from the conventional control doses of fipronil, fenvalerate, cypermethrin and acephate if the parasitoid was left in contact with the insecticides only for short time (1 h). In A. plutellae, there were obvious synergisms of piperonyl butoxide (PB), triphenyl phosphate (TPP) and diethyl maleate (DEM) on methamidophos, carbofuran, fenvalerate, cypermethrin, avermectins and fipronil, but no synergisms on chlorfluazuron were found. The synergism of PB was the highest. Acetylcholinesterase (AChE) activity could not be inhibited by PB, TPP and DEM, but strong inhibition could be found in carboxylesterase (CarE) activity by PB and TPP, and in glutathione-S-transferase (GST) activity by DEM, in vivo. The apparent Michaelis-Menten constant (K_m), the maximal velocity (V_max) of AChE, and the activities of CarE and GST in A. plutellae were 0.22, 2.08, 4.60 and 0.45fold as those in P. xylostella, respectively. The bimolecular rate constants (K_i) of AChE to methamidophos, dichlorvos and carbofuran in A. plutellae were 14.7, 10.5 and 26.0-fold as those in P. xylostella, respectively. High inhibition of AChE was found in both species when being incubated with insecticides at high temperature, especially in A. plutellae. The results indicated that the high susceptibilities to organophosphates and carbametes in A. plutellae were related to its high sensitivity of AChE to the insecticides, and the oxidative metabolism might be more effective in tolerance to insecticides than non-oxidative metabolism in A. plutellae. In addition, the causes of the intrinsic differences In insecticide selectivity in the two species were also discussed.

Key words: Apanteles plutellae, Plutella xylostella, insecticide susceptibility, acetylcholinesterase sensitivity, detoxification enzyme