Development of organophosphorus resistance in Indian strains of Aedes aegypti (L.).

Populations of the yellow-fever mosquito, Aedes aegypti, have developed resistance to cholorinated hydrocarbons in many parts of the world, but not to organophosphorus insecticides. Seven Indian strains of Ae. aegypti were found to be tolerant to DDT and highly susceptible to certain organophosphorus compounds such as Abate, Dursban, fenthion and fenitrothion. Hence selection studies were started with these organophosphorus compounds. Laboratory selections on these strains for 20 generations with Abate, Dursban, malathion, fenthion and fenitrothion increased the tolerance of the F(20) larvae to these insecticides by 2.4 times, 3.7 times, 3 times, 5.6 times and 2 times, respectively. The dosage-mortality lines of the successive generations were steep and parallel, suggesting these were instances of tolerance and not of resistance. In contrast, DDT selection showed rapid changes in dosage-mortality lines, indicating the development of resistance. The organophosphorus selected strains generally showed only a 2-3-fold increase in cross-tolerance to other organophosphorus compounds.