Characterization of Vip3Aa resistance in two different Spodoptera frugiperda strains from USA

Transgenic cotton and corn producing Cry and Vip3Aa insecticidal proteins from Bacillus thuringiensis (Bt) represent the cornerstone of fall armyworm (FAW, Spodoptera frugiperda) control in the Western Hemisphere. However, the evolution of resistance threatens the sustainability of this technology for FAW control. Given the prevalence of resistance to the Cry1F protein, Vip3Aa remains the sole Bt toxin with high efficacy against this highly polyphagous insect. My work investigates the resistance mechanism to the Vip3Aa protein in the two strains of FAW (LA-RR and TX-RR) selected from field populations in Louisiana and Texas. Biochemical, molecular, and bioassay experiments demonstrated distinct resistance mechanisms in each strain. In LA-RR, reduced proteolytic processing of the Vip3Aa protoxin linked to reduced serine protease activity emerged as a significant resistance factor, in addition to reduced Vip3Aa binding to midgut receptors. In contrast, TX-RR showed no impairment in protoxin activation and displayed reduced Vip3Aa toxin binding to midgut receptors. The genetic loci associated with the resistance phenotype in each strain also differed. This research offers the first mechanistic details for field-derived Vip3Aa resistance in FAW from the United States. The results from the study underscore the importance of simultaneously monitoring multiple resistance routes to detect early shifts in insect susceptibility. Ultimately, this work advances our understanding of Vip3Aa resistance in lepidopteran pests and supports the development of durable Bt crops.