Puerto Rico fall armyworm has only limited interactions with those from Brazil or Texas, but could have substantial exchanges with Florida populations

Authors: Nagoshi, Rodney; Meagher, Robert - Rob; Jenkins, David

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2010
Publication Date: 4/1/2010
Citation: Nagoshi, R.N., Meagher Jr, R.L., Jenkins, D.A. 2010. Puerto Rico fall armyworm has only limited interactions with those from Brazil or Texas, but could have substantial exchanges with Florida populations. Journal of Economic Entomology. 103(2):360-367.

Interpretive Summary: Fall armyworm, is an important agricultural pest that is endemic to Puerto Rico and the rest of the Caribbean islands. It was reported that what may be the first example of field-evolved resistance to a Bacillus thuringiensis (Bt) toxin recently occurred in Puerto Rico fall armyworm. Given the long-range migratory capabilities of fall armyworm, the rapid spread of this resistance to the rest of the western hemisphere is plausible. Unfortunately, relatively little is known about the population movements of fall armyworm in the Caribbean and the magnitude of genetic interactions, if any, with populations from North, South, and Central America. To address this issue, Scientists at the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville Florida, a novel genetic method, that is being used to study migration of fall armyworm in North America, was applied to populations in Puerto Rico. The results indicate limited interactions between Puerto Rico fall armyworm and those from Brazil or Texas, but the potential for significant exchange is with populations in Florida.

Technical Abstract: Fall armyworm is not known to diapause and does not survive freezing winters. While this limits where it can become permanently established, its capacity for long-range migration allows at least the potential for widespread seasonal infestations, rapid dispersion and colonization, and interactions with even distant populations. The best studied example of this behavior is the annual northward migration of fall armyworm from overwintering areas in southern Texas and Florida that is responsible for infestations in much of the central and eastern United States and Canada. Past studies provided a broad but not always consistent approximation of the annual migration. Even less is known about the pattern of fall armyworm movements in Central America, South America, and the Caribbean, where documented instances of seasonal migrations have yet to be reported. Recent attempts to measure genetic variation between geographically distant sites from this region (including samples from Argentina, Brazil, Mexico, and Puerto Rico) led to the conclusion that fall armyworm populations are genetically heterogeneous with interbreeding occurring generally throughout the western hemisphere. However, a detailed analysis of moth capture and meteorological data found no conclusive evidence of a significant contribution from the Caribbean to fall armyworm populations in the temperate regions of North America. We recently developed a molecular method that can distinguish between corn-strain populations in Florida and Texas, and Florida and Brazil. Sequence analysis of a portion of the mitochondrial COI gene identified two sites that were polymorphic within the corn-strain population and generated four haplotype classes, CS-h1, CS-h2, CS-h3, CS-h4. While each of these were found in fall armyworm collected from Brazil, Texas, and Florida, the haplotype proportions varied in a location specific manner, best seen by calculating the ratio between CS-h4 and CS-h2 frequencies. Sampling of almost 500 specimens from pheromone traps or larval collections in Florida consistently displayed CS-h4/CS-hs2 values >1.5 that were stable over a five year period and independent of location. In comparison, corn-strain populations from Brazil and Texas routinely gave ratios <0.5. The ability to distinguish fall armyworm from Texas and Florida was used to show that corn-strain isolated from Georgia closely resembled those from FL while those in Alabama, Mississippi, and Louisiana were similar to the Texas profile. These results demonstrate the utility of the haplotype ratios to describing the long-range movements of fall armyworm populations. In this paper this strategy was applied to the fall armyworm corn-strain in Puerto Rico to investigate the likelihood of substantial interactions with populations in Brazil to the south, Texas to the west, or Florida to the north. These studies were complemented by analysis of the same collections using a second molecular marker. It was recently shown that polymorphisms in the triose phosphate isomerase gene (Tpi) could be used to discriminate between fall armyworm strains. Tpi lies on the Z-chromosome and therefore displays a different inheritance pattern than that of the mitochondrial COI gene. This difference was exploited to make inferences about the frequency and pattern of interstrain mating behavior. The implications of these results to the likely migration patterns of fall armyworm in the Caribbean are discussed.