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.

草地贪夜蛾 (Spodoptera frugiperda) 是美洲的一种多食性害虫，也是 Bt 作物的目标。 2011-2013 年的一项研究表明，美国东南沿海地区的草地贪夜蛾对 Cry1F 玉米具有实际抗性。在这项研究中，进行了饮食覆盖和叶组织生物测定，以确定对玉米中四种常见 Bt 蛋白（Cry1F）的敏感性。 、Cry1A.105、Cry2Ab2 和 Vip3Aa）存在于 2021-2022 年从美国南部七个州收集的 23 个草地贪夜蛾种群中，其中包括来自东南沿海地区的 9 个种群。在使用 Cry1F 的饮食叠加生物测定中，22 个群体与易感参考人群同等或更易感，其中一个群体表现出更高的易感性比率（野外群体的 LC ₅₀ /LC ₅₀ 敏感菌株）为 1.97。 23 个人群的 Cry1A.105 易感性比率范围为 <0.15 至 4.67，Vip3Aa 易感性比率范围为 <0.12 至 5.04。三个群体的 LC ₅₀ 比 Cry2Ab2 敏感菌株高十倍。总而言之，该研究没有提供草地贪夜蛾对四种 Bt 蛋白的实际抗性的证据。相反，结果表明最近收集的群体对 Cry1F、Cry1A.105 和 Vip3Aa 敏感。不同地理位置和寄主植物的 Bt 敏感性是一致的。叶组织测定的结果证实了饮食覆盖生物测定的结果。本研究中发现的草地贪夜蛾中 Cry1F 易感性逆转代表了 Bt 作物昆虫系统中第一个有记录的实际抗性恢复到易感状态的案例，因此对抗性管理具有重要意义。