Monitoring the Acaricide Resistance Status and Baseline Susceptibility of Field‐Collected Populations of <i>Tetranychus urticae</i> Koch in Korea

The two-spotted spider mite, Tetranychus urticae Koch, a major pest in Korean strawberry production, is known to rapidly develop resistance to acaricides. To establish T. urticae's baseline susceptibility and assess its current resistance status in Korea, we tested 15 populations collected from major strawberry-growing regions across the country in 2022 and 2023. Eight acaricides representing distinct IRAC mode-of-action (MoA) groups, including fenpropathrin, abamectin, etoxazole, chlorfenapyr, flufenoxuron, acequinocyl, pyridaben, and cyflumetofen, were evaluated using a standardized direct-spray bioassay. Lethal concentration (LC₅₀) values were compared to those of a laboratory strain maintained without acaricide exposure for more than 10 years to determine resistance ratios (RRs). Most field populations exhibited susceptibility (RR < 1) or low resistance (RR < 5) to fenpropathrin, etoxazole, flufenoxuron, pyridaben, and cyflumetofen. However, several populations have developed moderate resistance (RR = 5-10) to abamectin, chlorfenapyr, and acequinocyl, with one population from Jangseong exhibiting high resistance to abamectin (RR = 31.23). The geographic pattern of resistance, with regional differences, highlights localized selection pressures. These findings provide baseline susceptibility data for Korean strawberry-infesting populations of T. urticae and support the design of region-specific acaricide rotation strategies for use in integrated pest management programs.