Pyrethroid synergism by esterase inhibition in Spodoptera littoralis (Boisduval) larvae

The susceptibility of cotton leafworm, Spodoptera littoralis (Boisduval) larvae to poisoning by trans-permethrin and cis-cypermethrin was increased when these pyrethroids were applied topically after the larvae had ingested profenofos, monocrotophos or azinphos-methyl for 24 h. An ingested dose of 4 nmol profenofos per larva gave a synergism factor of about threefold for both trans-permethrin and cis-cypermethrin. These pyrethroids were not synergized by oxidase inhibitors such as piperonyl butoxide, SV-1 and MPP ingested at 80 nmol/larva. Esterase preparations of larval gut hydrolysed trans-permethrin two to three times more rapidly than cis-permethrin, deltamethrin, trans- or cis-cypermethrin. Integument esterase(s) are less active but show a similar preference for trans-permethrin. The gut esterase(s) hydrolysing trans-permethrin are more sensitive in vitro and in vivo to inhibition by profenofos than by azinphos-methyl or monocrotophos. The susceptibility of S. littoralis larvae to pyrethroids appears to be limited by pyrethroid esterases in the gut. Organophosphorous compounds inhibiting these detoxifying enzymes serve as synergists.