Reversal of practical resistance in fall armyworm to Cry1F maize: a case report on the resistance to susceptibility in Bt crops from the southeastern USA

The fall armyworm, Spodoptera frugiperda, is a polyphagous pest in the Americas and a target of Bt crops. A study from 2011-2013 demonstrated practical resistance of S. frugiperda to Cry1F maize in the southeastern coastal region of the U.S. In this study, diet-overlay and leaf tissue bioassays were conducted to determine the susceptibility to four common Bt proteins in maize (Cry1F, Cry1A.105, Cry2Ab2, and Vip3Aa) in 23 S. frugiperda populations collected during 2021-2022 from seven southern U.S. states, including nine populations from the southeastern coastal region. In the diet-overlay bioassays with Cry1F, 22 populations were equally or more susceptible than a susceptible reference, with a single population showing an increased susceptibility ratio (LC₅₀ of field population/LC₅₀ of the susceptible strain) of 1.97. Susceptibility ratios of the 23 populations ranged from <0.15 to 4.67 for Cry1A.105 and <0.12 to 5.04 for Vip3Aa. Three populations exhibited an LC₅₀ >tenfold greater than the susceptible strain to Cry2Ab2. Altogether, the study did not provide evidence of practical resistance in S. frugiperda to the four Bt proteins. Instead, the results show that the recently collected populations were susceptible to Cry1F, Cry1A.105, and Vip3Aa. The Bt susceptibility was consistent across geographical locations and host plants. Results from the leaf tissue assays confirmed the findings of the diet-overlay bioassays. The reversed Cry1F susceptibility in S. frugiperda identified in this study represents the first case of documented practical resistance reverting to susceptible status in Bt crop-insect systems and thus has important implications for resistance management.