Monitoring of citrus thrips resistance to spinosyn insecticides and its management

Citrus thrips are a key pest of citrus in the San Joaquin Valley of California, requiring multiple insecticide applications per season to reduce the rind scarring. Repeated use of synthetic insecticide coupled with multiple generations of thrips in a season has led to resistance development to several insecticides, making citrus thrips management challenging for growers. The Munger cell laboratory bioassay technique was adopted to examine resistance to Delegate^® WG (spinetoram) in field populations collected from the Valley from 2017 to 2023. Two concentrations (1 and 10 parts per million [ppm]) of Delegate WG were prepared to dip leaves and measure the resistance. The numbers of alive and dead thrips were counted after 48 hours of insecticide exposure. The overall trend from 2017 to 2022 showed a higher percentage of thrips surviving at the 1 ppm discriminating dose. More importantly, in later years, a higher percentage of the population survived at 10 ppm, indicating field rates of Delegate may not control thrips effectively. Well-timed and appropriate treatment with a rotation of available active ingredients minimizes the likelihood of resistance development and reduces the number of applications. Implications of the results and recommendations for managing citrus thrips resistance and cross resistance to abamectin and cyantraniliprole are discussed.