Biochemical and Toxicological Studies with Laboratory and Field Populations of Heliothis virescens (F.)

Abstract

Mixed-function oxidase (MFO), glutathione S-transferase (GST), and esterase (EST) activities toward spectrophotometric substrates were measured in tissue homogenates of larval and adult tobacco budworms, Heliothis virescens (F.), from colonies collected throughout the 1993 cotton growing season and were compared with activities in pyrethroid-susceptible and -resistant laboratory strains. Levels of enzyme activities from both larvae and adults were variable among strains and did not correlate with susceptibility to cypermethrin. This finding, together with previous results showing expression of enhanced levels of cypermethrin metabolism in some of these strains, suggests that activities measured with these model substrates provide a poor indication for expression of metabolic resistance to pyrethroids. However, larval EST and GST activities were moderately con-slated ( r 2 > 0.60) with resistance to profenofos, and the toxicological significance of activities measured with these substrates bears further examination. In addition, small (but significant) differences in sensitivities of acetylcholinesterase to inhibition by paraoxon were measured, indicating a potential contribution of reduced target site sensitivity as a mechanism of organophosphate resistance in these strains. Finally, levels of MFO, GST, and EST activities from field-collected insects were higher in larval than adult homogenates and, except for MFO activities, were not correlated between life stages from the same strain. These results provide further evidence that expression of enzyme activities differs between developmental stages of this insect.