Small-scale evaluation of the efficacy and residual activity of alpha-cypermethrin WG (250 g AI/kg) for indoor spraying in comparison with alpha-cypermethrin WP (50 g AI/kg) in India

BACKGROUND: Indoor residual spraying (IRS) with different formulations of insecticides is being used for the control of mosquito vectors in many countries. In the present study, residual efficacy and duration of effectiveness of IRS with alpha-cypermethrin WG-SB (250 g AI/m(2)) formulation was compared with WP formulation (50 g AI/kg) in a small scale (Phase II) field trial. METHODS: Two dosages, i.e. 20 and 30 mg AI/m(2), were used and the efficacy and duration of effectiveness was assessed on alpha-cypermethrin susceptible population of Anopheles stephensi. Four types of surfaces were selected, namely cement wall with distemper coating, cement wall with lime coating, mud wall with lime coating, and brick wall unpainted. Spraying was carried out with Hudson sprayer fitted with control flow valve. Bioassays were carried out at weekly and then fortnightly intervals. Chemical analysis of filter paper samples collected from the sprayed surfaces was done at Walloon Agricultural Research Institute, Gembloux, Belgium. RESULTS: Alpha-cypermethrin WG-SB showed residual efficacy (>80 % mortality) for 13–15 weeks for the 20 mg/m(2) dosage and 13–16 weeks for the 30 mg/m(2) dosage, whereas alpha-cypermethrin WP showed residual efficacy for 11–15 weeks for the 20 mg/m(2) dosage and 11–14 weeks for the 30 mg/m(2) dosage on the surfaces tested. The average of the applied to target dose ratio ranged from 0.89 to 1.17 for alpha-cypermethrin WG-SB at 20 mg AI/m(2), 0.83–1.80 for the WG-SB at 30 mg AI/m(2), 0.87–1.66 for alpha-cypermethrin WP at 20 mg AI/m(2), and 0.68–1.06 for WP at 30 mg AI/m(2). No adverse events were reported, either by the spray men or the household inhabitants, during and after the spray operations. CONCLUSIONS: The results suggest that the dose of WG 30 mg/m(2) gave slightly longer effective residual action against malaria vector (16 weeks) on most common indoor surfaces and could be used for effective control of Anopheles mosquitoes. The WG formulation was found to be easy to handle, no smell was reported during the spraying and was found to be operationally acceptable for indoor residual spraying.