Laboratory Evaluation of Phenthoate against Anopheles Culicifacies, Anopheles stephensi and Culex Quinquefasciatus and Its Cross Resistance to Malathion

The study was undertaken to determine the efficacy of phenthoate, which has not been used in public health so far, against Anopheles culicifacies and Anopheles stephensi vectors of malaria in rural and urban areas in India and Culex quinquefasciatus and to investigate its possible cross resistance to malathion, being used at present for indoor residual spraying. Efficacy of phenthoate was evaluated against larvae and adult mosquitoes of Anopheles and Culex spp. in the laboratory bioassays using standard procedures. To investigate cross-resistance, malathion susceptible and resistant strains of An. stephensi were used for bioassays. Phenthoate was found to be effective against larvae of malathion, susceptible strains of An. culicifacies (LC₆₀ 0.0028 μg mL⁻¹), An. stephensi (LC₅₀0.0018 μg mL⁻¹) and Cx. quinquefasciatus (LC₅₀ 0.0217 μg mL⁻¹). However, phenthoate was not as effective egainst malathion resistant strain of An. stephensi. The LC₅₀ values aaainst larvae of malathion resistant strain were 0.368 μg mL⁻¹ The LC₅₀ values of phenthoate against adult mosquitoes of the three strains, when exposed to impregnated papers of different concentrations were <0.25%, 0.51% and 0.33%, respectively. However, against malathion resistant strain of An stephensii only 6.6% mortality was observed with 4% insecticide (phenthoate) impregnated papers as compared to 100% mortality in susceptible strain with 2% insecticide impregnated papers. Though phenthoate was found effective against malathion susceptible strains of An. Culicifacies, An. stephensi and Cx. quinquefasciatus, results revealea cross-resistance between phenthoate and malathion and therefore phenthoate can not be used as a substitute for malathion.