Susceptibility/resistance status of Spodoptera frugiperda to Bt and chemical insecticides in the Southern U.S.

The fall armyworm threatens southern U.S. agriculture due to its broad host range, mobility, and resistance to control methods. Rising infestations have raised concerns about the effectiveness of insecticides and Bt crops. To support sustainable management, three complementary studies were conducted to assess the resistance status of S. frugiperda populations to Bt toxins and chemical insecticides in the region. An earlier study documented practical resistance to Cry1F corn in the southeast coastal area of the U.S.: S. frugiperda populations with >85.4-fold resistance, reduced field efficacy, and a high resistance allele frequency (0.293) in Florida and North Carolina. A follow-up study (2021–2022) assessed 23 populations from seven southern states for susceptibility to Cry1F, Cry1A.105, Cry2Ab2, and Vip3Aa using diet-overlay and leaf tissue bioassays. All the populations collected during 2021-2022 were susceptible to Cry1F, representing the first documented reversion from practical resistance to susceptibility in a Bt crop-pest system. Susceptibility was consistent across regions and host plants. A third study evaluated 24 populations for susceptibility to four common insecticides. Compared to a susceptible strain, 82.6%, 60.8%, 41.7%, and 16.7% of populations showed >10-fold resistance to λ-cyhalothrin (up to 2,290-fold), diflubenzuron (672.6-fold), chlorantraniliprole ( >67.7-fold), and methoxyfenozide (23.9-fold), respectively. No correlations in the susceptibility were observed among insecticides, and resistance did not vary by region, host plant, or insect strain. These studies reveal widespread insecticide resistance and absence of practical resistance to Bt crops, offering critical insights for managing S. frugiperda in the southern U.S. and supporting the continued efficacy of Bt technologies.