Abstract
The soluble esterases from six strains of the housefly have been separated by electrophoresis in partially hydrolysed starch gel, according to the technique of Smithies. The separated enzymes were then classified as cholinesterases, aliesterases, and aromatic esterases by use of a series of specific inhibitors. A detailed study with a susceptible and a malathion-resistant strain showed marked differences in amount and migratory behaviour in the various stages of the life cycle but no pattern characteristic of resistance. A semi-quantitative correlation was found between the synergistic effects of a group of non-insecticidal organic phosphates and their inhibitory effects upon the cholinesterases and aliesterases of the two strains. The enzymes hydrolysing naphthyl acid phosphate are more numerous and more active in the malathion-resistant strain. The results lend support to the theory that resistance to organic phosphate insecticides such as malathion is related to higher phosphatase activity.
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