Metabolic resistance to pyrethroids (Py) and organophosphates (Op) in Callosobruchus maculatus (fab.) (Coleoptera: Chrysomelidae: Bruchinae) a major pest of stored cowpeas in West Africa

Callosobruchus maculatus is the main pest of stored cowpeas in West Africa. To control this pest, chemicals are frequently used during post-harvest storage. In this study we assessed insecticide resistance in C. maculatus populations collected from Burkina Faso, Senegal and Niger. Enzyme analysis was also performed to detect the basic mechanisms underlying insecticide resistance. Burkina Faso western strains proved to be more resistant to cypermethrin (RR50 = 25–26.6) and to have moderate resistance to permethrin (RR50 = 10.5-13) and Pirimiphos-methyl (RR50 = 8–11). The enzymatic activities confirmed insecticide resistance in these strains (NA esterase: 0.2- 0.12 µM/mg/min; PNPA esterase: 1.15–1.96 µM/mg/min; GST: 1.01–1.32 µM/mg/min; oxidase: 0.11–0.12 µM/mg/min). Dakar and Niamey strains were less resistant to cypermethrin, permethrin, Chlorpyrifos-methyl and Pirimiphos-methyl. Resistance to OP and Py is likely due to the selection pressure imposed by their systematic use in fields and storage systems. Alternative pest control methods should be considered for resistance management or for delaying it as much as possible taking into account insect origin and behavior.