Synergism of Insecticides by Enzyme Inhibitors in the Resistant Populations of Spodoptera litura (Lepidoptera: Noctuidae)

The effect of enzyme inhibitors piperonyl butoxide (PBO) and tribufos (DEF) vas 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 hut 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 sytiergism of carbamates, pyrethroids, indoxacarb and spiriosad by PBO and DEF in the present study indicates that (letoxification by cytochrome P450 monooxygenases and esterases is at least partially involved in imparting resistance to these insecticides in S. litura. However, a binned synergism of msecticides shown by both the synergists implies that other mechanisms such as target site insensitivity and reduced euticular penetration may be more important mechanisms of resistance in the Pakistani populations of S. litura.