POTATO BEETLE

The mechanisms of resistance were investigated in a near isogenic perm ethrin-resistant strain of Colorado potato beetle (Leptinotarsa deceml ineata (Say)). The near isogenic strain was bred from a Massachusetts field strain that was multiply resistant to a number of insecticides b y backcrossing to a laboratory susceptible strain. Permethrin resistan ce was principally associated with two contributing factors, an increa sed level of carboxylesterase activity and a site insensitivity associ ated with the nervous system. Pharmacokinetically, there were no diffe rences noted in the rate of permethrin penetration or excretion betwee n strains. An increased level of carboxylesterase activity was associa ted with the permethrin-resistant strain resulting in a 1.5-fold great er hydrolytic rate of a-naphthyl butyrate. Additionally, the permethri n-resistant strain produced more hydrolytic metabolites of C-14-labele d permethrin when incubated under in vivo and in vitro conditions comp ared to the nearly isogenic susceptible strain. The increased carboxyl esterase activity associated with permethrin resistance is largely mem brane-associated when whole-body extracts are examined. Increased carb oxylesterase activity showed a preference of alpha-naphthyl butyrate o ver alpha-naphthyl acetate, suggestive of a more hydrophobic catalytic center associated with the resistant form of enzyme activity. The nea r 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, respe ctively). Recovery from permethrin-induced knockdown was largely attri buted to enhanced oxidative metabolism of permethrin by the resistant strain. Electrophysiological recordings determined the resistant strai n to be recalcitrant to neuronal hyperexcitability caused by permethri n in the susceptible strain. These findings are consistent with a nerv e insensitivity factor associated with kdr and kdr-type resistances in other insects. (C) 1995 Academic Press, Inc.