Susceptibility and Emerging Resistance of Culex Mosquitoes to Lambda-Cyhalothrin and Dichlorvos in Minna, Niger State, Nigeria

The persistent challenge of controlling Culex mosquitoes, which are critical vectors for debilitating diseases such as lymphatic filariasis and various arboviruses, continues to hinge on the application of chemical insecticides. A formidable obstacle to these control efforts on a global scale is the escalating development of resistance to these chemicals within mosquito populations. This investigation was therefore designed to evaluate the current susceptibility profile of wild Culex mosquitoes sourced from Minna, Nigeria, when exposed to two widely utilized insecticides: lambda-cyhalothrin and dichlorvos. Field-collected immature stages from breeding sites in the Bosso Local Government Area were reared under controlled laboratory conditions to adulthood. Subsequent susceptibility testing was performed using the standard World Health Organization (WHO) tube bioassay protocol. Adult mosquitoes were exposed to filter papers treated with two distinct concentrations of each insecticide: a lower concentration (0.4% for lambda-cyhalothrin and 3.0% for dichlorvos) and a higher, diagnostic concentration (0.7% for lambda-cyhalothrin and 7.0% for dichlorvos). Knockdown was monitored at 10-minute intervals for one hour, with final mortality assessed after a 24-hour recovery period. The results revealed a stark, concentration-dependent response. The lower concentrations induced mortalities of only 52.63% ± 5.42 for lambda-cyhalothrin and 66.66% ± 3.83 for dichlorvos after one hour, figures that fall substantially below the WHO's 98% susceptibility benchmark, thereby confirming resistance. Conversely, the higher diagnostic doses achieved mortalities of 98.48% ± 1.51 and 98.67% ± 1.33, technically classifying the population as susceptible. However, the critical finding was that after 24 hours, mortality for the low concentrations remained incomplete at 87.54% and 84.06%, indicating a resilient sub-population. This study concludes that while the high diagnostic doses remain effective for now, the clear evidence of resistance at lower concentrations signals an urgent need for proactive insecticide resistance management strategies in the region to preserve the utility of these vital public health tools.