Insecticide Resistance Mechanisms of the Diamondback Moth, Plutella xylostella, in Georgia and Florida populations

The diamondback moth (DBM), Plutella xylostella, a major pest of cruciferous crops, has developed resistance to multiple insecticides. Genetic contributors to insecticide resistance in DBM have been of recent interest in Georgia and Florida populations. Four target site mutations (E1338D, Q4594L, I4790M, and G4946E) of the DBM ryanodine receptor (RyR) have been discovered in recent studies of diamide (IRAC class 28) resistance in Asian populations. DBM from resistant field populations in Colquitt, Crisp, and Tift counties in Georgia and Manatee County in Florida were collected and used to establish lab colonies (NP, CSP, LTF, and MAN respectively). Dose response assays were performed to determine resistance levels to chlorantraniliprole, cyantraniliprole, and spinetoram. High levels of resistance to chlorantraniliprole (2,800 to 4,100-fold) were observed in the LTF, NP, and MAN colonies, while only intermediate levels of resistance to cyantraniliprole (50 to 108-fold) and spinetoram (29 to 216-fold) were observed. The CSP colony had an intermediate (109-fold) level of resistance to chlorantraniliprole. The G4946E mutation, the most prevalent of the four mutations in Asian populations, was present in each of our sampled populations. Allele frequency estimations of the G4946E mutation in the LTF, MAN, NP, and CSP colonies were determined to be 56, 90, 53, and 32%, respectively. The other three mutations were not detected in these samples. Metabolic resistance to insecticides via detoxification enzymes may further contribute to resistance. Quantitative PCR (qPCR) is currently being used to determine the relative fold-changes in detoxification enzyme expression upon sub-lethal insecticide exposure in these colonies.