Insecticide susceptibility status of Culex tritaeniorhynchus giles, vector of Japanese encephalitis in Delhi.

Laboratory studies were carried out to ascertain the current susceptibility status of adult and larval stages of the Culex tritaeniorhynchus mosquito, vector of Japanese encephalitis, to various insecticides used under public health programs in India. The present study revealed that exposure of adult mosquitoes to diagnostic concentrations of DDT - 4.0%, malathion - 5.0%, fenitrothion - 1.0%, and propoxur - 0.1% could induce only 50.0, 10. 0, 15.0, and 5.0% mortality, respectively, indicating that the species was resistant to all of these insecticides. The LT50 and LT95 values calculated using diagnostic concentrations of DDT, malathion, fenitrothion, and propoxur were found to be 56.4 and 136, 138 and 272, 185 and 258, and 187 and 249 min, respectively. However, when adult mosquitoes were exposed to the diagnostic concentration of synthetic pyrethroids, viz., deltamethrin - 0.025%, permethrin - 0.25%, and lambdacyhalothrin - 0.1%, 100.0% mortality was observed, indicating that the species was highly susceptible to these adulticides. Larval susceptibility tests carried out using diagnostic dosages of DDT- 0.008, temephos- 0.02, fenthion- 0.008, fenitrothion- 0.125, and malathion- 0.005 mg/l failed to induce any mortality, indicating that larvae were resistant to these larvicides. The LC50 and LC90 values calculated for commonly used larvicides, viz., temephos and fenthion, were 0.1511 and 1.9098, and 0.6151 and 2.395 mg/l, respectively. Increase in tolerance level were estimated at 95.5- and 299.4-fold when these LC90 values were compared with diagnostic dosages of temephos and fenthion, respectively.