Biochemistry of Malathion Resistance in Culex tarsalis

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Abstract
Larvae of the resistant strain of Culex tarsalis Coquillett, on exposure to malathion, come to contain one-third as much malaoxon as normal larvae. This is partly due to a higher phosphatase activity hydrolyzing malaoxon. But the major difference is in the higher carboxyesterase activity hydrolyzing malathion; this interstrain difference was demonstrated in vivo in larvae cleared of gut contents, and in vitro in homogenates and particularly the mitochondrial fraction. EPN, a carboxyesterase inhibitor, proved synergistic for malathion against the resistant strain. Increased carboxyesterase content was inseparable genetically from malathion resistance in hybridization experiments, which also indicated the malathion resistance to be due to a single partially dominant gene allele.

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