Bt resistance management of fall armyworm (Lepidoptera: Noctuidae) host strains

Mason, Charles E.Flexner, Lindsey, J.The fall armyworm, Spodoptera frugiperda (Smith), is a highly damaging, multivoltine pest of commercial crops including corn (Zea mays L.), cotton (Gossypium Spp. L.), sorghum (Sorghum bicolor L.), rice (Oryza sativa L.), and pasture grasses. Fall armyworm have become a growing concern in agricultural communities across the Americas as field populations in many locales have evolved resistance to several Cry1 toxins derived from the bacterium Bacillus thuringiensis Berliner (Bt). An often overlooked aspect of fall armyworm biology is the existence of two host strains, the “rice” and “corn” strains. Despite knowledge of fall armyworm host strains for thirty years, there has been little research devoted to their characterization. Here the Bt tolerances, oviposition preferences, and host plant tissue feeding of one rice strain, two corn strain, and one rice-corn hybrid population fall armyworm were compared. Bt tolerance was assessed using diet-based bioassays were conducted for each population to Cry1Ab, Cry1Ac, and Cry1F, and Cry1F again using tissue-based bioassays. Oviposition preferences of one rice strain and one corn strain population were compared using no-choice and two-choice greenhouse experiments, and host plant tissue feeding was compared between one rice strain, two corn strains, and one rice-corn hybrid population. Results from diet-based bioassays indicated that the corn strain and hybrid populations were more tolerant to the Bt toxins, especially to Cry1F, than the rice strain population. This was corroborated by the tissue-based bioassays, though the results were narrowly non-significant. The greenhouse oviposition experiments indicated that corn was a preferred oviposition host for both rice and corn strain fall armyworm; this was supported by the host plant tissue feeding experiments that indicated that corn is suitable larval host plant for each of the four fall armyworm populations tested. The overall results of this study indicate that the host strain should be taken into account when preparing insect resistance management plans for fall armyworm. Regions whose fall armyworm populations have larger proportions of corn strain or hybrid insects may be identified using pheromone traps and established methods of host strain identification. These identified regions should be monitored more closely for the evolution of Bt resistance.University of Delaware, Department of Entomology and Wildlife EcologyPh.D