Insecticidal Resistance of Culex tritaeniorhynchus (Diptera: Culicidae) in Japan: Genetics and Mechanisms of Resistance to Organophosphorus Insecticides

The Toyama strain of Culex tritaeniorhynchus Giles collected and colonized in Japan was highly resistant to organophosphorus insecticides. Resistance ratios were more than 125,000 for temephos, 6,000 for malathion, 26,500 for fenitrothion, and 22,929 for fenthion. Resistance was incompletely dominant over susceptibility. Insensitivity of acetylcholinesterase to the compounds was the major factor of resistance. The insensitivity index for paraoxon was 734 in comparison with a standard susceptible (Taiwan) strain. There was also evidence for detoxification by carboxylesterase. Two carboxylesterase genes and one gene coding a major molecular species of acetylcholinesterase were all located on chromosome II. These multiple enzymatic factors acted together, indicating a single gene for resistance on chromosome II, when the Toyama strain was crossed with susceptible marker mutants without recombination of the chromosome.