First comprehensive report of the resistance of Culex quinquefasciatus Say (Diptera: Culicidae) to commonly used insecticides in Riyadh, Saudi Arabia

The mosquito Culex quinquefasciatus is a vector of various pathogens including West Nile virus, Saint Louis encephalitis virus, and Western equine encephalitis virus. Insecticides are the main tools for Cx. quinquefasciatus control, but this overreliance on chemical tools has led to the development of resistance to many insecticides in this important insect vector. The resistance of eight field populations of Cx. quinquefasciatus to 10 commonly used insecticides was evaluated. Based on the resistance ratios (RRs), the adults of Cx. quinquefasciatus field populations displayed susceptibility to the organophosphates (OPs) except Al-Masanie adults which exhibited low resistance to fenitrothion (RR(50) = 3.62). Conversely, the mosquitoes exhibited susceptibility, low resistance, and moderate resistance to the pyrethroids alpha-cypermethrin (RR = 0.59–2.56), bifenthrin (RR = 0.59–2.19), deltamethrin (RR = 0.60–7.07), cypermethrin (RR = 0.60–2.66), and cyfluthrin (RR = 0.58–2.39). At the larval stage, Cx. quinquefasciatus field populations displayed susceptibility to low resistance to the OPs chlorpyrifos (RR = 0.03–1.75), malathion (RR = 0.19–3.42), fenitrothion (RR = 0.11–2.78), and pirimiphos-methyl (RR = 0.08–1.15). Although these results in Cx. quinquefasciatus field populations indicated that the OPs and pyrethroids maintained high efficacy in controlling this species in the geographical area of this study, these findings should be utilized wisely to avoid any potential negative effects on human health and environmental safety attributable to the application of these broad-spectrum conventional insecticides. However, these findings provide a solid basis for decision-making for Cx. quinquefasciatus integrated vector management programs.