Field Evaluation of Insecticide Residual Activity of Indoor Residual Spraying Campaigns in Kwara State

Indoor residual spraying with insecticides provides a critical complementary vector control tool for malaria control, and monitoring residual activity duration is essential for determining re-spraying intervals. This study evaluated the residual insecticidal activity of pirimiphos-methyl CS applied during an indoor residual spraying campaign across four local government areas of Kwara State, North Central Nigeria. Cone bioassay tests were conducted using laboratory-susceptible Anopheles gambiae sensu stricto at monthly intervals post-spraying for a period of seven months. Sprayed surfaces tested included mud walls, cement walls, and wood. Mortality rates were recorded at each assessment point. Effective residual activity (greater than 80 percent mortality in cone bioassays) was maintained for six months on cement and wood surfaces but declined to below 80 percent by month four on mud walls. Community-level spray quality, assessed by independent observers, was inadequate in 28.3 percent of evaluated structures due to operator fatigue and incomplete surface coverage. Spray operator training scores correlated significantly with spray quality ratings. The study recommends surface-specific re-spraying calendars and enhanced quality assurance protocols for indoor residual spraying operations in Kwara State. Keywords: indoor residual spraying, residual activity, cone bioassay, malaria vector control, Kwara State.