Synergism and stability of acetamiprid resistance in a laboratory colony of Plutella xylostella

The involvement of metabolic enzymes in the resistance of a laboratory colony of diamondback moth, Plutella xylostella (L), to the neonicotinoid insecticide acetamiprid was determined with the synergists piperonyl butoxide (PBO), which suppresses the activity of cytochrome P-450 monooxygenases, and S,S,S-tributyl phosphorotrithioate (DEF), an inhibitor of esterases, using the leaf-dipping method. Both PBO and DEF enhanced the insecticidal activity of acetamiprid significantly in the resistant P. xylostella strain but not in a reference strain, suggesting that cytochrome P-450 monooxygenases and esterases play an important role in the resistance of P. xylostella to acetamiprid. The resistant P. xylostella strain was also reared without further exposure to acetamiprid to determine the stability of resistance. Maintaining the resistant strain for seven generations in the absence of selection pressure resulted in a drop in resistance ratio from 110 to 2.42, indicating that acetamiprid resistance in P. xylostella is not stable.