MECHANISM OF SYNERGISM BETWEEN THE PYRETHROID INSECTICIDE LAMBDA-CYHALOTHRIN AND THE IMIDAZOLE FUNGICIDE PROCHLORAZ, IN THE HONEYBEE (APIS-MELLIFERA L)

Ergosterol biosynthesis-inhibiting fungicides have been found to syner gize the toxicity of pyrethroid insecticides to the honeybee (Apis mel lifera L.). The mechanism by which the fungicide prochloraz enhances t he toxicity of the pyrethroid insecticide lambda-cyhalothrin was inves tigated. In vitro incubations with honeybee midguts were used to study the metabolism of [C-14]lambda-cyhalothrin. The principal metabolite was identified as 4-hydroxy-3-phenoxybenzyl alcohol (4'-OH-3-PBAlc) wi th small amounts of 3-phenoxybenzoic acid (3-PBAc). Both are products of ester bond cleavage, but microsomal oxidation was implicated in the formation of 4'-OH-3-PBAlc. After treating midguts with prochloraz, m etabolism was predominantly to 3-PBAc, with little formation of 4'-OH- 3-PBAlc, strongly indicating an inhibition of microsomal monooxygenase activity. In vivo investigations showed the major metabolic products of [C-14]lambda-cyhalothrin extracted from frass of treated honeybees were 4'-OH-3-PBAlc, 2'-hydroxy-3-phenoxybenzyl alcohol, and 4'-hydroxy -3-phenoxybenzoic acid. However, when bees were simultaneously dosed w ith prochloraz, there was an absence of metabolites detected in the fr ass of bees for 16 hr post-treatment Thus, prochloraz delayed the meta bolism, detoxication, and excretion of lambda-cyhalothrin by inhibitio n of microsomal oxidation, effectively enhancing the toxicity of the p yrethroid to the honeybee, (C) 1995 Academic Press, Inc.