F₂ screen for resistance to Bacillus thuringiensis Vip3Aa51 protein in field populations of Spodoptera frugiperda (Lepidoptera: Noctuidae) from Texas, USA

Fall armyworm, Spodoptera frugiperda, is a major target pest of pyramided Bt (Bacillus thuringiensis) corn and cotton in the U.S. In this study, we used a well-established F₂ screening method to detect resistance alleles to Vip3Aa51 protein in field populations of S. frugiperda in Texas. In 2018, a total of 95 F₂ two-parent families of S. frugiperda were established using single-pair mating of field individuals collected from two sorghum fields in two locations in Texas. For each F₂ family, 128 neonates were screened on a diagnostic concentration of 3.0 μg/cm² Vip3Aa51 protein using diet-overlay bioassays. The results showed that two out of 95 two-parent families possessed a major resistance allele against Vip3Aa51 protein. The estimated frequency of major resistance alleles against Vip3Aa51 in S. frugiperda in Texas was 0.0078 with a 95% CI of 0.0016–0.0188. The Vip3Aa51-resistant strain (TX-RR) derived from the F₂ screening showed a high level of resistance to Vip3Aa51 protein, with a resistance ratio of >395.0-fold relative to a susceptible population (SS). Information generated from this study should be useful in studying mechanisms of Vip3Aa resistance in S. frugiperda, resistance monitoring, and resistance management of the Vip3Aa technology.