Metabolism and interactions of the castes of the imported fire ant, Solenopsis invicta, treated and exposed to methoprene alone and synergized

Abstract

Studies are presented on the metabolism of methoprene, an insect growth regulator, by the castes of the imported fire ant, (IFA), Solenopsis invicta Buren. The relative rates of metabolism of methoprene in the IFA castes were in the following order: adults>larvae>pupae>pharate pupae. Absorption and excretion rates were in the same order. The metabolism of methoprene in the castes carried between the stadia and within instars. Adult and pupal methoprene metabolism were primarily O-demethylation to yield the alcohol-ester. Larvae metabolize methoprene principally by esterase cleavage of the isopropyl ester of methoprene yielding the ether-acid metabolite. In both larvae and pharate pupae there is a rapid increase in methoprene metabolism prior to the pharate pupal transformation and pupal molt. At these times biologically active metabolites accumulate. The relative toxicity of methoprene and three of its metabolites were in the following order: alcohol-ester>methoprene>ether-acid>alcohol-acid. Thus the alcohol-ester was shown to be an activation product of methoprene. When applied to pharate pupae the alcohol-ester reduced pupation, adult pigmentation and adult emergence more effectively than methoprene. Methoprene and its metabolite, alcohol-ester, also appeared to reduce both ovary development and egg production. In adults, piperonyl butoxide (PB), a microsomal enzyme inhibitor, reduced methoprene metabolism by inhibiting alcohol-ester formation. In contrast, PB did not affect this route of metabolism in pharate pupae or pupae. The reason for the difference are not known. The results suggest that synergists which block O-demethylation in adults may be used effectively to prolong the life of methoprene in colonies without affecting the activation process in immatures where biological activity occurs..