Date Published: May 31, 2019
Publisher: Public Library of Science
Author(s): Rodney N. Nagoshi, Isabel Dhanani, R. Asokan, H. M. Mahadevaswamy, Chicknayakanahalli M. Kalleshwaraswamy, Robert L. Meagher, Maohua Chen.
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.
The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), has long been a significant economic pest of maize and other crops in the Western Hemisphere . It is a tropical species incapable of diapause and so has limited capacity to survive freezing winter temperatures, yet still has an infestation range that includes most of the maize-producing regions in North America . This was shown to be due to long-distance migration behavior that allows northward dissemination over thousands of kilometers from wintering areas in southern Texas and Florida over the course of a single growing season [3, 4]. These long-distance movements are associated with, and likely dependent upon, air transport systems that show a strong and consistent northerly air flow across North America as well as a progressively northward expansion of maize agriculture during the spring-to-fall growing season that makes available a continuous food supply . Therefore, while fall armyworm has the capacity for extensive migration, the extent and consistency of the behavior is dependent on environmental factors such that the behavior in North America may not be the norm.
There are multiple insect species where recurring seasonal migration over thousands of kilometers have been documented [34–36]. Most relevant is evidence that the Globe Skimmer dragonfly undergoes seasonal migrations between India and eastern Africa, which includes a trans-oceanic crossing of 3500 km that most likely uses the high-altitude winds associated with the Inter-tropical Convergence Zone . This provides a mechanism for how fall armyworm might have arrived in India as well as the possibility of regular interactions between the Indian and African fall armyworm populations through natural migration. However, while fall armyworm is documented to undergo annual long-distance migration in North America , the plausibility of an analogous journey between Africa and India is uncertain. Fall armyworm has only been observed to undergo continuous flight from dusk to dawn (6–12 hours) using regional air transport systems . This typically limits a single flight to a few hundred kilometers, making traversing the 3500 km crossing of the Arabian Sea seemingly problematic given known fall armyworm behavior.