Abstract: The effect of enzyme inhibitors piperonyl butoxide (PBO) and tribufos (DEF) was studied in combination with insecticides profenofos, methomyl, thiodicarb, cypermethrin, λ-cyhalothrin, bifenthrin, indoxacarb, and spinosad in the resistant Pakistani populations of Spodoptera litura using a leaf-dip bioassay. Both the inhibitors synergised carbamates methomyl and thiodicarb but showed no synergistic effect on an organophosphate profenofos. These inhibitors produced a synergism with cypermethrin but had no synergism with bifenthrin. PBO and DEF enhanced the toxicity of λ-cyhalothrin and indoxacarb in one population but not in the other. Spinosad was synergised by DEF but not by PBO. The potent synergism of carbamates, pyrethroids, indoxacarb and spinosad by PBO and DEF in the present study indicates that detoxification by cytochrome P450 monooxygenases and esterases is at least partially involved in imparting resistance to these insecticides in S. litura. However, a limited synergism of insecticides shown by both the synergists implies that other mechanisms such as target site insensitivity and reduced cuticular penetration may be more important mechanisms of resistance in the Pakistani populations of S. litura.

Key words: Spodoptera litura, insecticides, resistance, synergism, piperonyl butoxide, DEF