Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2021
Publication Date: 6/21/2021
Citation: Nagoshi, R.N., Koffi, D., Agboka, K., Adjevi, A.M., Meagher Jr, R.L., Georgen, G. 2021. The fall armyworm strain associated with most rice, millet, and pasture infestations in the Western Hemisphere is rare or absent in Ghana and Togo. PLoS ONE. 16(6):e0253528. https://doi.org/10.1371/JOURNAL.PONE.0253528.
DOI: https://doi.org/10.1371/JOURNAL.PONE.0253528

Interpretive Summary: The moth pest fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), was detected in western Africa in 2016 and subsequently found throughout much of the rest of that hemisphere where it has caused significant losses in corn yield and poses a threat to a number of other crops. Native to the Western Hemisphere, fall armyworm is one of the primary pests of corn in the Americas and periodically causes significant economic damage to sorghum, millet, cotton, and rice. In addition, fall armyworm infestations in pasture and forage grasses can substantially reduce resources available for grazing animals. This broad host range is in part the result of two subpopulations historically designated as host strains (C-strain and R-strain) that differ in their host plant preferences. Therefore, it is critical to determine whether both strains are present in the Eastern Hemisphere to provide an accurate assessment of what crops are at high risk. Scientists at USDA-ARS, Center for Medical, Agricultural and Veterinary Entomology, Gainesville, Florida, with colleagues from the University of Togo, the University of Ghana, and IITA collected and genetically characterized moth specimens from rice and corn fields in Togo and Ghana. The results indicate that the fall armyworm R-strain is not (yet) present in significant numbers in Africa, reducing the immediate risk of economic damage to rice, millet, pasture, and forage grasses. However, continued surveillance of these hosts for fall armyworm is recommended as the introduction of the R-strain into Africa could greatly exacerbate the economic damage caused by this species.

Technical Abstract: The moth pest fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), was detected in western Africa in 2016 and subsequently found throughout much of the rest of that hemisphere where it has caused significant losses in corn yield and poses a threat to a number of other crops. Native to the Western Hemisphere, fall armyworm is one of the primary pests of corn in the Americas and periodically causes significant economic damage to sorghum, millet, cotton, and rice. In addition, fall armyworm infestations in pasture and forage grasses can substantially reduce resources available for grazing animals. This broad host range is in part the result of two subpopulations historically designated as host strains (C-strain and R-strain) that differ in their host plant preferences. Therefore, it is critical to determine whether both strains are present in the Eastern Hemisphere to provide an accurate assessment of what crops are at high risk. A meta-analysis of studies from South America was performed that confirmed the previously reported preference of the R-strain to rice. Rice fields in Ghana and Togo were surveyed for fall armyworm from October 2018 to January 2019. Larval infestations were only rarely observed in rice, but substantial numbers of fall armyworm from rice growing areas were collected by pheromone trapping. Genetic characterization found no significant differences in the specimens collected from rice dominated habitats and those from nearby corn sites. Comparisons of heterozygosity frequencies of polymorphic strain specific and nonspecific sites in one marker gene was consistent with the presence of only a single strain in the rice collections, with results similar to that found in populations from African corn locations. These results confirm past studies that suggested the R-strain is rare or absent in Africa and that the frequently used marker, the Cytochrome oxidase I (COI) gene, is not an accurate indicator of strain identity for the African fall armyworm populations. These findings indicate that R-strain preferred crops (i.e., rice, millet, and pasture grasses) are at reduced risk of fall armyworm infestation in Africa at this time.