Studies on methyl parathion resistance in Heliothis virescens

Comprehensive studies were conducted to elucidate the resistance mechanisms of a field strain (R) of Heliothis virescens larvae resistant to methyl parathion, when compared with a susceptible laboratory strain (S). In vitro, monooxygenase activity responsible for desulfurating methyl parathion to methyl paraoxon was lower in the R strain while phosphorotriester hydrolase activity, responsible only for methyl paraoxon hydrolysis, was higher in the R strain than the S strain. There was no difference in the total amount of P450, this suggests that the R strain has different isozymes of P450 from those present in the S strain. No differences in glutathione S-transferase activity or in the inhibition of acetylcholinesterase was observed. In vivo, the amount of methyl paraoxon formed increased with time in S, whereas it stayed at a much lower level in R. Following injection, the amount of methyl paraoxon decreased faster in R than in S. The penetration rate of methyl parathion was significantly slower in R than in S for 12 hr after topical application, but was similar after 24 hr. These results indicate that lower monooxygenase activity as well as higher phosphorotriester hydrolase activity are mainly responsible for resistance to methyl parathion in H. virescens. The importance of this combination is discussed in relation to the role each plays in resistance.