Variations in target site mutation frequencies of diamondback moth ryanodine receptor in Florida and Georgia, USA, over time

The diamondback moth, Plutella xylostella, is the primary pest of brassicas in the Southeastern U.S. Previously, we demonstrated high-level resistance to chlorantraniliprole in populations collected from Florida and Georgia throughout 2018. Resistance ratios for ranged from 109 to 4,298-fold for chlorantraniliprole in comparison to a susceptible lab colony. Interestingly, these populations remained relatively susceptible to cyantraniliprole, with resistance ratios ranging from 50 to 107-fold. Subsequently, we identified a known target site mutation of the ryanodine receptor in these populations, referred to as G4946E, that is associated with chlorantraniliprole resistance but not cyantraniliprole resistance. Recent insecticide surveys have indicated that diamondback moth populations from Florida and Georgia have developed cross-resistance to diamides, specifically to chlorantraniliprole, cyantraniliprole, and cyclaniliprole. Along this same time another, ryanodine receptor mutation referred to as I4790K has been identified and consistently associated with diamide cross-resistance in diamondback moth populations from Asia. Using Illumina sequencing, we compared the frequencies of the I4790K and G4946E mutations in preserved samples from 2018, as well as samples collected from 2021 through 2023 that displayed diamide cross-resistance. Our results indicated that the G4946E mutation frequencies were greater than 75% in all samples collected from 2018, while frequencies were less than 10% in samples from 2021 onward. The estimated I4790K frequencies were less than 5% in all samples from 2018, and ranged from 5% to 84% in samples collected after 2021. These estimates align with the appearance of cross-resistance in the region, suggesting that the I4790K mutation is contributing to diamide control failure.