Absence of Synergism of DDT by Piperonyl Butoxide and DMC in Larvae of the Diamondback Moth (Lepidoptera: Yponomeutidae)

Susceptible and multiple-resistant (resistance ratio for DDT = 200) strains of diamondback moth, Plutella xylostella (L.), were used to determine the effect of a microsomaloxidase inhibitor, piperonyl butoxide (pb) and a DDT-dehydrochlorinase inhibitor, DMC, on DDT toxicity. For both strains, DMC did not increase, but as a mailer of fact, decreased DDT toxicity somewhat. Although pb synergized DDT by twofold against the susceptible strain, it produced very little effect in the resistant strain. The absence of synergisym by pb and DMC raised the possibility of existence of a nonmetabolic mechanism of DDT resistance in the diamondback moth. This mechanism might also play an important role in diamondback moth resistance to synthetic pyrethroids and may be similar to previously described nonmetabolic mechanism for DDT-pyrethroid resistance in houseflies and mosquitoes.