Evidence that a major subpopulation of fall armyworm found in the Western Hemisphere is rare or absent in Africa, which may limit the range of crops at risk of infestation

Authors: Nagoshi, Rodney

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2019
Publication Date: 4/4/2019
Citation: Nagoshi, R.N. 2019. Evidence that a major subpopulation of fall armyworm found in the Western Hemisphere is rare or absent in Africa, which may limit the range of crops at risk of infestation. PLoS One. 14(4):e0208966. https://doi.org/10.1371/journal.pone.0208966.
DOI: https://doi.org/10.1371/journal.pone.0208966

Interpretive Summary: The fall armyworm is the primary pest of corn production in South America and in portions of the southeastern United States. Severe outbreaks of fall armyworm have now been reported throughout sub-Saharan Africa, posing a significant threat to African agriculture, with the potential for rapid dispersion throughout the hemisphere. Fall armyworm is subdivided into two subpopulations called the R-strain and C-strain that differ in their food preferences and are therefore found on different plant hosts. This means that the scope of the economic risk posed by invasive fall armyworm is dependent on whether one or both strains are present. Current risk assessments for Africa assume the existence of both strains in Africa although this was based on limited data. A scientist at the USDA-ARS, Center for Medical, Agricultural and Veterinary Entomology (Gainesville, Florida) used technology developed for studies of fall armyworm in North America to test strain occurrence in African populations. The results indicate that the R-strain is rare or absent in Africa, suggesting that plant hosts associated with the R-strain may not be at high risk on that continent. This suggests that it may be beneficial to focus control measures towards C-strain plant hosts to provide the most expedient control program of this crop pest.

Technical Abstract: The introduction and establishment of fall armyworm (Spodoptera frugiperda) in Africa presents a major threat to agriculture in that continent and potentially to the entire Eastern Hemisphere. The species is subdivided into two subpopulations called the R-strain and C-strain that differ in their distribution on different plant hosts. This means that the scope of the economic risk posed by invasive fall armyworm is influenced by whether one or both strains are present. Multiple studies have found mitochondrial markers diagnostic of the two strains throughout Africa but there is substantial disagreement with a nuclear strain marker that makes conclusions about strain composition uncertain. In this study the issue of whether both strains are present in Africa was tested by an assay that can detect strain-biased mating behaviors. Western Hemisphere fall armyworm consistently showed evidence of strain-specific assortative mating in the field that was not found in surveys from multiple locations in Africa. The absence of strain mating biases and the disagreements between the strain diagnostic genetic markers indicates that the R-strain is rare (<1% of the population) or absent in Africa. Instead, it appears that the African fall armyworm populations are dominated by two groups, the C-strain and the descendants of interstrain hybrids. These results suggest that plant hosts associated with the R-strain may not be at high risk of fall armyworm infestation in Africa.