Experimental hut comparisons of nets treated with carbamate or pyrethroid insecticides, washed or unwashed, against pyrethroid-resistant mosquitoes

Abstract. The efficacy against mosquitoes (Diptera: Culicidae) of a bednet treated with carbamate insecticide carbosulfan capsule suspension (CS) 200 mg/m²] was compared with four types of pyrethroid‐treated nets in veranda‐trap huts at Yaokoffikro near Bouaké, Côte d'Ivoire, where the malaria vector Anopheles gambiae Giles carries the kdr gene (conferring pyrethroid resistance) at high frequency and Culex quinquefasciatus Say is also pyrethroid resistant. Pyrethroids compared were lambdacyhalothrin CS 18 mg/m², alphacypermethrin water dispersible granules (WG) 20 mg/m², deltamethrin 50 mg/m² (Permanet?) and permethrin emulsifiable concentrate (EC) 500 mg/m². Insecticidal power and personal protection from mosquito bites were assessed before and after the nets were used for 8 months and hand washed five times in cold soapy water. Before washing, all treatments except permethrin significantly reduced blood‐feeding and all had significant insecticidal activity against An. gambiae. The carbosulfan net gave significantly higher killing of An. gambiae than all pyrethroid treatments except the Permanet. Against Culex spp., carbosulfan was more insecticidal and gave a significantly better protective effect than any of the pyrethroid treatments. After washing, treated nets retained various degrees of efficacy against both mosquito genera – but least for the carbosulfan net. Washed nets with three types of pyrethroid treatment (alphacypermethrin, lambdacyhalothrin, permethrin) gave significantly higher mortality rates of Culex than in huts with the same pyrethroid‐treated nets before washing. After five washes, the Permanet?, which is sold as a long‐lasting insecticidal product, performed no better than the other nets in our experimental conditions.