Western flower thrips (Frankliniella occidentalis) field resistance to spinetoram in North Carolina

The western flower thrips (Frankliniella occidentalis (Pergande), "WFT") is an invasive pest of fruit and vegetable crops throughout the southeastern United States. The spinosyn insecticides spinetoram (Radiant (R)) and spinosad (SpintTor (R)) have been used to manage WFT in NC tomatoes for >20 years. Control failures with spinetoram were reported by growers in the Piedmont of NC beginning in the mid-2010s. The goal of this study was to investigate the development of WFT resistance to Radiant in tomato fields in Piedmont and mountain production regions of NC. In 2019 and 2020, WFT populations were collected from staked-tomato fields and subjected to dose-response bioassays to estimate lethal concentration values (LC50) for comparison with a susceptible laboratory colony. In 2021, additional populations were collected from each region and evaluated using diagnostic doses of Radiant (100 and 1000 PPM v/v). Results from dose-response bioassays revealed resistance ratios (RR50) as high as 1487.7 in populations from Rowan County in the Piedmont. Additional diagnostic dose bioassays conducted in 2021 further supported reduced susceptibility to Radiant in all five tested populations. Populations from Henderson County in the mountains were much more susceptible than those in the Piedmont, with resistance rations of only 1.18 and 26.15. Bioassays with a single highly resistant colony over 17 generations found survival to remain mostly constant. These results provide evidence supporting the development of practical, field-evolved resistance to spinetoram in WFT in NC, and the need for improved insecticide resistance management strategies.