Effect of Spiracle-Blocking Pesticide on the Frequency Dynamics of the Acaricide Resistance Gene of the Two-Spotted Spider Mite （Trombidiformes: Tetranychidae） in Soil-Grown Strawberry Plants

Alternative control measures such as spiracle blockers can reduce the frequency of pest population exposure to pesticides and delay resistance development; however, population decline may in itself cause resistance allele fixation through genetic drift. We conducted a release experiment with two-spotted spider mites, Tetranychus urticae Koch, in strawberry fields in Shizuoka and Nara prefectures to examine the influence of spiracle blockers （sorbitan fatty acid esters） on the frequency of the etoxazole resistance allele （CHS1; I1017F）. F2 adult females from resistant and susceptible strains crossed at a 1 : 9 ratio were introduced, followed by two consecutive control rounds using either a spiracle blocker or etoxazole at their registered concentrations. Resistance allele frequencies were measured via quantitative PCR. The resistance allele frequency increased in Shizuoka only after using etoxazole but increased to approximately 80% in Nara after the first round regardless of the control measure used. The population density dropped to one mite per compound leaf after the first round, suggesting a genetic bottleneck. The estimated initial allele frequency in the F2 population was 6%, and etoxazole was effective only once, highlighting the need for risk avoidance （e.g., switching to other control measures） when resistance alleles are still rare.