Title: Improvements in the identification of strains facilitate population studies of fall armyworm subgroups Author
Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 19, 2011
Publication Date: March 1, 2012
Citation: Nagoshi, R.N. 2012. Improvements in the identification of strains facilitate population studies of fall armyworm subgroups. Annals of the Entomological Society of America. 105(2):351-358. Interpretive Summary: The fall armyworm is a major agricultural pest of corn, forage grasses,rice, cotton, and peanuts. Two strains of fall armyworm exist that have significant physiological and behavioral differences, including resistance to pesticides and preference for different plant hosts and understanding strain biology will be critical to understanding the population movements. Previously scientists at the USDA-ARS Center for Medical, Agricultural and Veterinary Entomology ,Gainesville, Florida, developed a molecular method that facilitates the identification of strains and potential interstrain hybrids. This study describes modifications of the technique that facilitates its application on the large scales necessary for comprehensive population surveys. The modified method was shown to be more accurate than existing techniques for identifying strains in at least some circumstances and was used to reanalyze earlier studies that suggested aberrations in strain-host plant preferences. The results confirmed that this new technology will significantly enhance future studies on fall armyworm populations.
Technical Abstract: Fall armyworm is a significant agricultural pest in the Western Hemisphere and is becoming animportant system for studying lepidopteran migration and incipient speciation. Critical to these investigations are methods that can differentiate between two morphologically indistinguishable strains that differ physiologically and behaviorally, particularly in their choice of plant host. In a previous study, haplotypes of the fall armyworm Triose phosphate isomerase gene (Tpi; EC 18.104.22.168) were shown to be effective indicators of strain identity. However, the method of analysis had technical complications that made it expensive to apply on a scale large enough for most population studies. The focus of this paper was to develop a more efficient and cost-effective procedure. By combining PCR amplification and restriction enzyme digestion, a strain-specific polymorphism in the fall armyworm Tpi locus can be characterized from single specimens without the need of DNA sequencing. This method is more accurate under some circumstances than COI haplotyping, the current method of choice for population studies. The modified Tpi method was used to expand upon previous indications that interstrain hybridization occurs asymmetrically, to confirm observations of seasonal periodicity in the proportions of the two strains infesting Florida cornfields, and to demonstrate that certain instances where strain specificity to their host plants appeared to be compromised were actually examples of variability in the association of the COI markers to strain identity. The results indicate that the simultaneous application of both Tpi and COI analyses to the study of fall armyworm population distribution and behavior is now practical and should facilitate the understanding of interstrain hybridization and its impact on strain divergence and the distribution of fall armyworm subpopulations in the United States.