A Beet Armyworm (Lepidoptera: Noctuidae) Ryanodine Receptor Mutation Associated with Diamide Resistance is Present in a Chlorantraniliprole-Resistant Population from South Georgia, USA

A population of beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), larvae was collected from cowpeas (Vigna unguiculata [L.] Walpers) from Scooterville, GA (Tift Co.) in the summer of 2022, following a report of control failure after chlorantraniliprole application. The collected larvae were transported to Tifton, GA for study at the Coastal Plains Experiment Station. The 72-h maximum dose leaf-dip bioassays of the field-population determined percent mortality to be 26, 49, and 87% for the insecticide active ingredients chlorantraniliprole, cyantraniliprole, and spinetoram, respectively. In comparison, the percent mortality of a susceptible lab strain exposed to the same insecticides was 76, 88, and 98%, respectively. Previous studies of a ryanodine receptor target site mutation, I4743M, identified in Chinese populations of beet armyworm, demonstrated an association with resistance to diamide insecticides. All screened samples from the diamide-resistant field population were shown to possess this mutation, while samples from the diamide-susceptible lab colony were shown to be wild-type. A second mutation, G4900E, associated with diamide resistance in other lepidopteran species, was not identified in any of the sequenced samples from either population. This is the second report of diamide insecticide resistance in beet armyworm populations from the southeastern United States, and the first report of the I4743M target site mutation in Georgia.