Studies on the contrasting susceptibilities of the larvae of two Hide beetles ( Dermestes spp.; Coleoptera, Dermestidae) to malathion

Abstract

The ld 50 of larvae of Dermestes maculatus Deg. to topically applied malathion was about ten times that of larvae of D. lardarius L.; D. maculatus was also slightly more tolerant of malaoxon, but the two species are known to have similar susceptibilities to several other organophosphorus insecticides. Triphenyl phosphate was a very effective synergist for malathion against D. maculatus but slightly reduced the toxicity of malathion to D. lardarius, so that the latter species was the more tolerant to the mixture. Triphenyl phosphate synergized coumaphos in both species, but in D. maculatus it did not affect the toxicity of malaoxon, diethyl malathion, fenthion or morphothion and antagonized dicapthon and fenitrothion. Histochemical studies of the cholinesterase inhibition in the nerve cords of larvae of both species treated with malathion, malaoxon, diethyl malathion, coumaphos or fenitrothion, both with and without triphenyl phosphate, revealed a pattern of susceptibility similar to that found in the toxicological studies. These results are discussed in relation to malathion-specific insecticide resistance in Dipter. D. maculatus appears to degrade malathion more effectively than D. lardarius and the mechanism of this degradation is probably one of carboxyester hydrolysis.