Resistant and normal colonies of the blowfly Chrysomyia putoria (Wied.) were obtained from the Congo and cultured in London. By selection and inbreeding, sub-colonies homogeneous for resistance to (a) the BHC/dieldrin group of chlorinated insecticides and (b) malathion, were obtained. Homogeneous resistant colonies were markedly less fertile and were difficult to perpetuate.
Toxicological investigations of the BHC/dieldrin-resistant colony revealed a characteristic resistance spectrum similar to that of six other insect species. Genetical experiments with the BHC/dieldrin-resistant strain indicated inheritance by a single gene pair, intermediate in dominance, very similar to that reported in nine other insect species.
Toxicological investigations of the malathion-resistant colony, including tests with 19 organophosphorus insecticides, showed a resistance largely specific to malathion and malaoxon. This was rather similar to malathion-resistant strains of two other species, for which data were available. It is known that the toxic action of malathion on mammals can be potentiated by prior treatment with small doses of certain organophosphorus compounds, which inhibit an aliesterase that detoxifies malathion by carboxylic hydrolysis, and furthermore that potentiation has been found in malathion-resistant strains of the house-fly, Musca domestica L. Accordingly, the phenomenon was investigated in C. putoria by adding non-lethal amounts of EPN or tri-o-cresyl phosphate (TOCP) to malathion. It was found that specific malathion-resistance was virtually abolished by this means, a result that suggests that such resistance in this species depends on a carboxyesterase. Genetical experiments with the malathion-resistant strain indicated inheritance by a single gene pair, with nearly complete dominance, similar to that in three other insect species.
It is concluded that both types of resistance in C. putoria have much in common with those occurring in analogous strains of other insect species.