First report of Tetranychus urticae resistance to fluxametamide, decreased susceptibility to acynonapyr, and efficacy of some insecticides

Tetranychus urticae Koch (Acari: Tetranychidae), an extremely destructive pest worldwide, has shown high propensity for developing resistance to insecticides. This study assessed the efficacy of 16 insecticides at recommended concentrations, including the new active ingredients fluxametamide (IRAC Group 30) and acynonapyr (Group 33), against 18 populations collected from strawberry, chrysanthemum, and rose fields in Nara Prefecture, Japan. Only bifenazate exhibited high efficacy, with mortality of more than 90% against the entire population. Fluxametamide, acynonapyr, abamectin, and acequinocyl were effective, with mortality of more than 90% against all strawberry populations. However, one to three populations in rose or chrysanthemum fields exhibited less than 90% mortality for these four insecticides. The BK-1 and FH-2 populations showed low mortality for fluxametamide (12.0 and 21.4%, respectively) and acynonapyr (74.7 and 69.6%, respectively). LC ₅₀ assays revealed resistance ratios of 64.0 in FH-2 and 32.6 in BK-1 for fluxametamide, and 6.9 in BK-1 for acynonapyr, relative to a susceptible strain. These findings represent the first documented resistance to fluxametamide and show decreased efficacy of acynonapyr in T. urticae . The results underscore the importance of resistance monitoring and the implementation of integrated pest management strategies, including biological and physical control methods, to sustain insecticide efficacy and to delay resistance development.