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

Abstract

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.

Southeastern Asia fall armyworms are closely related to populations in Africa and India, consistent with common origin and recent migration

“…2a) 36 . All COI-CS specimens from Myanmar expressed the h4 combination of G 1164 G 1287 , which is the subgroup most commonly found in Florida and the Caribbean 29 and a result similar to that observed in India and Africa 16,30 . 21 where the C-strain COI-CS and TpiC markers generally predominate in specimens from corn and sorghum but are a minority in R-strain hosts such as pasture grasses (Fig.…”

“…Critical to projections of the range of crops at risk is determining to what extent the R-strain is present in Asia as this subpopulation would be the primary fall armyworm threat to such crops as rice and millet. The available genetic markers are contradictory as the COI-RS haplotype diagnostic of the R-strain and the TpiC marker indicative of the C-strain predominate in the collections from Africa, India, and from this study, Asia 5,6,16,30 . There are two lines of evidence that suggest that a single strain predominates in the Eastern Hemisphere and that it is most likely the C-strain.…”

“…Therefore, fall armyworm identity for all Asian specimens was confirmed by COI sequence analysis. Collections and data from previous studies include larval collections from Florida 21 , Argentina 17 , India 16 , and Africa 5 .…”

“…In this case, the enhanced monitoring occurring as a consequence of its discovery is giving the illusion of rapid migration. However, the limited amount of genetic variation found in the Eastern Hemisphere populations so far tested and the genetic homogeneity between the fall armyworm in India and Africa are not compatible with this proposal 16 . Instead, they suggest a recent and common origin for these geographically distant populations.…”

“…An unusual feature of the collections so far analyzed from Africa and India is that as defined by the COI marker the R-strain is the predominant form in most locations even though all collections tested to date came from C-strain preferred hosts 5,6,16 . However, when the same collections were tested for the Tpi marker, >95% of the specimens were identified as C-strain.…”