Baseline susceptibility of Helicoverpa armigera (Hübner) to Bt toxins Cry1Ac and Cry2Ab2 in West Africa

The susceptibility of African populations of the bollworm Helicoverpa armigera (Hübner) to Bt toxins is still unknown, and nothing has been published regarding resistance management measures required as a follow-up to the introduction of second-generation Bt cottons in Africa. CIRAD (Centre de Coopération Internationale en Recherche Agronomique pour le Développement) is currently building a model to help local authorities to develop a strategy for a sustainable use of Bt cotton in small farming systems. A key input factor for the model is the efficacy of Bt toxins against naïve populations of the bollworm. The purpose of the present study was to determine the susceptibility of bollworm strains collected in various cotton growing areas of Western and Central Africa to Cry1Ac and Cry2Ab as well as to their association in a 1/1 ratio. As expected according to their mode of action, Bt toxins had detrimental effects on H. armigera larvae, both in terms of mortality and larval growth. Despite some methodological difficulties in evaluating larval mortality following Bt toxin ingestion by H. armigera larvae, the LC50 as well as the G150 values obtained in West Africa were quite similar to data published elsewhere in the Old World. (Résumé d'auteur