CYTOCHROME P450-ASSOCIATED INSECTICIDE RESISTANCE AND THE DEVELOPMENT OF BIOCHEMICAL DIAGNOSTIC ASSAYS IN HELIOTHIS-VIRESCENS

The metabolic enzymes of a multiresistant (i.e., carbamate, organophosphate, pyrethroid) population of tobacco budworm, Heliothis virescens (F.), were compared with those of a susceptible population. The resistant Hebert population, collected from areas where control failures with cypermethrin and thiodicarb had been reported, was selected in the laboratory with thiodicarb for seven generations. Resistance ratios as determined by topical bioassays were 91- and >150-fold to cypermethrin and thiodicarb, respectively. Resistance-associated increases in metabolism were found in fifth instar larvae for monooxygenases, carboxylester hydrolases, and glutathione transferases. Cytochrome P450 content was elevated in microsomes from gut (3.7-fold), fat body (4.4-fold), and carcass (4-fold). Monooxygenase activity varied substantially among the four substrates and three tissue sources examined. For three monooxygenase substrates (p-nitroanisole, benzo(a)pyrene, and benzphetamine) increases in metabolism varied from 3- to 33-fold. The greatest increases in metabolism were observed with methoxyresorufin, in which increases were observed in all three tissues (23-, 29-, and 58-fold in fat body, midgut, and carcass microsomes, respectively). Significant increases ranging from 3- to 5-fold were observed for two esterase substrates, while smaller differences of up to 2-fold were observed for a glutathione transferase substrate. The high metabolic activity of the resistant population suggested that homogenates of individual larvae might be used in resistance monitoring. The development of a monooxygenase-based microtiter plate assay using p-nitroanisole as substrate clearly discriminates between resistant and susceptible individuals in the third instar, suggesting that biochemical resistance monitoring can be carried out with field-collected individuals. The esterase substrate p-nitrophenyl acetate also may be a useful tool for resistance monitoring. (C) 1995 Academic Press, Inc.