Susceptibility of Culex quinquefasciatus to insecticides in the industrial areas of Koumassi, Vridi, and Yopougon in Abidjan, Ivory Coast, during the rainy season

Insecticide resistance in Culex quinquefasciatus is an important concern for urban vector control because it can reduce the effectiveness of chemical interventions and undermine public acceptance of insecticide-based mosquito control. This study assessed the susceptibility of C. quinquefasciatus populations collected during the 2025 rainy season from the industrial zones of Koumassi, Vridi (Port-Bouët), and Yopougon in Abidjan, Côte d’Ivoire. Larvae were collected from residential, non-chemical industry, chemical industry, and chemical and other materials industry sites, reared to adulthood, and 3–5-day-old females were tested using WHO susceptibility assays. Diagnostic, 5×, and 10× concentrations of pyrethroids were evaluated alongside 5% malathion, 0.1% bendiocarb, and 4% DDT, while 4% piperonyl butoxide (PBO) assays were used to investigate metabolic resistance. At diagnostic concentrations, resistance or suspected resistance was observed across the three industrial zones, whereas malathion produced 98–100% mortality at the Vridi residential site and the INC and ICAM sites in Yopougon. Mortality at 10× pyrethroid concentrations remained below 98% across the sites evaluated, indicating high resistance intensity. Malathion produced the shortest Kdt50/Kdt95 values, reaching 36.34/64.24 min in Yopougon. PBO did not consistently restore susceptibility, indicating that monooxygenases were not the sole resistance mechanism. These findings demonstrate substantial insecticide resistance and support continued susceptibility monitoring to guide effective vector-control strategies in Abidjan’s industrial areas.