Phenotypic and Genotypic Plasticity of Acaricide Resistance in Populations of Tetranychus urticae (Acari: Tetranychidae) on Peppermint and Silage Corn in the Pacific Northwest

Tetranychus urticae Koch is a generalist pest of economic crops and is notorious for its rapid development of acaricide resistance. This poses a significant threat to the sustainability of integrated pest management (IPM) in cropping systems plagued by T. urticae. It is critical to evaluate the resistance status of T. urticae populations on crops and identify any underlying resistance mechanisms. This study investigated the efficacy of five major acaricides on T. urticae populations on peppermint and silage corn in the Pacific Northwestern United States and identified the underlying resistance mechanisms. Significant variations in acaricide resistance status of T. urticae populations were identified to abamectin, bifenthrin, fenpyroximate, hexythiazox, and spirodiclofen. In most cases, T. urticae populations from silage corn exhibited greater levels of acaricide resistance relative to peppermint populations. We detected known target-site mutations: F1534S and F1538I (conferring resistance to bifenthrin), G126S (linked with resistance to bifenazate), and I1017 (conferring resistance to hexythiazox and etoxazole) in 10, 90, and 90% of the populations, respectively, from peppermint fields. These four mutations were identified in all the populations collected from silage corn fields. Significantly higher transcript levels of metabolic genes associated with resistance to abamectin, fenpyroximate, and spirodiclofen were observed in some T. urticae populations collected from both peppermint and silage corn fields. This study provides evidence of multiple resistance to diverse active ingredients in field populations of T. urticae and the reliability of known molecular markers for active acaricide resistance monitoring. The observed resistance pattern will help in designing a sustainable IPM program for T. urticae.