Cry1 Bt Susceptibilities of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains

The fall armyworm, Spodoptera frugiperda (Smith; Lepidoptera: Noctuidae), is a highly polyphagous, multivoltine pest of commercial crops including corn (Zea mays L.), cotton (Gossypium spp. L.), rice (Oryza sativa L.), and pasture grasses. Fall armyworm has 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. There has been little research devoted to the characterization of fall armyworm host strains, although there is evidence that the rice and corn-strains may differ in their tolerances to Bt toxins expressed by transgenic plants. In this study, diet-based bioassays were conducted to compare the susceptibilities of one rice-strain, two corn-strains, and one rice-corn hybrid population to Cry1Ab, Cry1Ac, and Cry1F protein. Results indicate that the corn-strains and hybrid populations are more tolerant to the Bt toxins, especially to Cry1F, than the rice-strain population. Results from this study, when combined with existing techniques for host strain identification, may aid in the development of regional insect resistance management programs for fall armyworm.