Genetic characterization of fall armyworm infesting South Africa and India indicate recent introduction from a common source population

The invasion of the Western Hemisphere native fall armyworm (Spodoptera frugiperda; J. E. Smith) (Lepidoptera: Noctuidae) into the Eastern Hemisphere has been notable for the rapidity and geographical breadth of new detections. In the year following the first discovery in western sub-Saharan Africa in 2016, infestations have been documented in most sub-Saharan maize growing regions and has now expanded beyond Africa with populations recently reported in India. These observations could indicate a remarkable capacity for rapid establishment and long-distance dissemination. However, while fall armyworm does exhibit extended migration in North America where it annually traverses thousands of kilometers, this behavior is known to be dependent on highly favorable wind patterns and so can’t be assumed to occur in all locations. An alternative possibility is that the species has long been present in Africa, and perhaps the rest of the hemisphere, but was undetected until the enhanced monitoring that resulted after its initial discovery. Determining whether the fall armyworm in the Eastern Hemisphere is newly arrived or long pre-existing is important for assessing the risks of significant economic impacts, as the former indicates a change in pest composition while the latter does not. This study examined this issue by comparing collections from two geographically distant locations, South Africa and India. Sequence comparisons were used to quantify differences between the South Africa and India collections, assess the likelihood of their sharing a common source population, and their possible relationship with previously characterized fall armyworm from other regions of Africa. The results indicate genetic homogeneity between the South African and Indian fall armyworm populations tested and substantial similarities between these and collections from eastern Africa. The implications of these findings on fall armyworm population behavior and composition are discussed.