Some genetical studies on Culex pipiens complex : (I) Larvicidal activity of various insecticides on DDT-resistant and -susceptible colonies of Culex pipiens molestus

Abstract

Colonies resistant as well as susceptible to DDT were selected from a single laboratory colony of Culex pipienns molestus, and have successfully been established as the laboratory strains genetically fixed for generations. Tests were made on the comparative toxicity of various insecticides against the larvae of the three colonies by the dipping method using ethanol solution of the chemicals roughly following WHO Standard Method. LC 50 values estimated for the resistant colony (R), susceptible colony (S) and the parent colony (P) against p, p'-DDT were 82-100ppm, 0.042ppm and 0.0078-0.012ppm respectively, while that against technical-DDT were 12ppm, 0.0032ppm and 0.00037ppm. The latter was found to be higher in toxicity than p, p'-DDT against both S and R, and the ratio in the toxicity was larger in the susceptible colony than in the resistant. Tests were also made on the susceptibility of R and S colonies against dieldrin, lindane, malathion, fenthion, diazinon, ronnel, sumithion and dichlorvos. The values of LC 50 for these insecticides were roughly on the same levels reported by previous authors using unselected colonies of Culex pipiens complex, and no cross-resistance against these insecticides seemed to have developed in our DDT resistant colony. However, our R colonies were usually twice to three times more tolerant to most insecticides than S colonies, presumably due to the developments of a kind of tolerance rather than of the real resistance. In our tests using S colony as the material, the toxicity of insecticides as estimated from LC 50 values was by the order of tech-DDT, fenthion, dieldrin, p, p'-DDT, sumithson, ronnel, diazinon, dichlorvos, malathion and lindane. It should be pointed out here that DDT is one of the most effective insecticides so long as the susceptible colonies are concarned, though it usually shows much lower toxicity against natural or unselected laboratory colonies due to the resistance more or less developed within the populations.