A Piperonyl Butoxide Synergizable Resistance to Permethrin in Helicoverpa armigera Which Is Not Due to Increased Detoxification by Cytochrome P450

Abstract

An in vitro assay has been used to measure the activity of the cytochrome P450 able to metabolize the insecticide permethrin. The levels of the specific P450 in the midgut of the fifth instar stage of a resistant and a susceptible strain of Helicoverpa armigera have been measured. Although the resistant form showed a 20-fold resistance toward pyrethroids and this resistance can be eliminated by piperonyl butoxide, no evidence for increased permethrin detoxification in the resistant strain could be found. Microsomal extracts of adult abdomen tissue gave results which could not be explained simply in terms of increased detoxification. Extracts from the resistant strain gave a lower rate of permethrin metabolism than those from the susceptible strain, and piperonyl butoxide increased the rates of metabolism in both resistant and susceptible extracts. We suggest that synergism of pyrethroid toxicity by piperonyl butoxide is not necessarily an indication of resistance due to increased detoxification by cytochrome P450. We have also suggested that these results may be interpreted in terms of the involvement of a cytochrome P450 in the process of penetration of the insecticide through the insect cuticle.