POTATO BEETLE

The mechanisms of resistance were investigated in a near isogenic permethrin-resistant strain of Colorado potato beetle (Leptinotarsa decemlineata (Say)). The near isogenic strain was bred from a Massachusetts field strain that was multiply resistant to a number of insecticides by backcrossing to a laboratory susceptible strain. Permethrin resistance was principally associated with two contributing factors, an increased level of carboxylesterase activity and a site insensitivity associated with the nervous system. Pharmacokinetically, there were no differences noted in the rate of permethrin penetration or excretion between strains. An increased level of carboxylesterase activity was associated with the permethrin-resistant strain resulting in a 1.5-fold greater hydrolytic rate of a-naphthyl butyrate. Additionally, the permethrin-resistant strain produced more hydrolytic metabolites of C-14-labeled permethrin when incubated under in vivo and in vitro conditions compared to the nearly isogenic susceptible strain. The increased carboxylesterase activity associated with permethrin resistance is largely membrane-associated when whole-body extracts are examined. Increased carboxylesterase activity showed a preference of alpha-naphthyl butyrate over alpha-naphthyl acetate, suggestive of a more hydrophobic catalytic center associated with the resistant form of enzyme activity. The near isogenic permethrin-resistant strain was also determined to br cross-resistant to DDT, indicating the possibility of site insensitivity as an additional mechanism involved in permethrin resistance. Time to 50% knockdown for the resistant strain was 3.6 times longer than for its nearly isogenic susceptible strain (i.e., 56 min versus 15 min, respectively). Recovery from permethrin-induced knockdown was largely attributed to enhanced oxidative metabolism of permethrin by the resistant strain. Electrophysiological recordings determined the resistant strain to be recalcitrant to neuronal hyperexcitability caused by permethrin in the susceptible strain. These findings are consistent with a nerve insensitivity factor associated with kdr and kdr-type resistances in other insects. (C) 1995 Academic Press, Inc.