Mixed function oxidase and glutathione transferase activity in last instar Heliothis virescens larvae

Methyl parathion-resistant and -susceptible strains of Heliothis virescens were assayed for mixed function oxidase and glutathione transferase activity at four time periods within the last instar. No significant differences were found between the mixed function oxidase or glutathione transferase activities of the resistant and susceptible strains. However, significant differences did appear among time periods within the last instar. Larvae assayed during the postfeeding time period (60–80 hr) had the highest mixed function oxidase activity, which was 28 times that of last instar larvae assayed during the first time period (0–19 hr). Glutathione transferase activity appeared to be highest during the 20- to 39-hr and 40- to 59-hr time periods, but the differences among time periods were small. The importance of these findings to studies of insecticide resistance and plant herbivore interactions is discussed.