Genetic structure of the polyphagous pest Helicoverpa armigera (Lepidoptera: Noctuidae) across the Sub-Saharan cotton belt.

The cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a key pest of various cropping systems in West and Central Africa, and developed insecticide resistance recently. To understand how such insecticide resistance expands across the region, the genetic structure of bollworm populations was studied using microsatellite markers. At first, the study was performed within several populations from Northern Cameroon: during one year, 19 populations (504 larvae) were sampled in different locations, dates and host plants (6 villages, 6 dates, 5 host plants). Their genetic relationship was analysed using 10 polymorphic microsatellite markers. Despite the high polymorphism (5 to 50 alleles per locus), results reveal low level of genetic distances among locations, collection dates and host plants. The estimated values of F(ST) were very low across all populations and reveal a high level of gene flow. Moreover, all the loci presented heterozygote deficiency. This may arise either from inbreeding (sampling methodology) or from the presence of null alleles. Subsequently, larval sampling was performed at a higher scale, in five locations from Africa (Senegal, Mali, Burkina-Faso, Togo and Cameroon), to detect population differentiations according to geographic distance/isolation. Two other samples, from Madagascar and from Australia, were added to this experiment. F(ST) values and heterozygote frequency data were identical to the first one, indicating a high level of gene flow between these locations and the high migration capacity of the pest. Samples from Thailand, China, Pakistan and France were added to this study but it has been impossible to infer the presence of distinct populations. The opportunity to use neutral markers as microsatellites to understand population dynamics of H. armigera is discussed.