Management of ovicidal miticides resistance in the two-spotted spider mites Tetranychus urticae Koch on hops

Humulus lupulus (Hop) is a specialty crop, cultivated mainly for its resins and oils in flavoring of craft beer. The US hop industry is estimated to be worth over $200 million each year, with 99% of overall production from the pacific northwestern (PNW). Successful cultivation of hops in the PNW is threatened by a chronic hop pest, two-spotted spider mite Tetranychus urticae Koch (TSSM). Typically, hop growers apply various miticides to suppress TSSM populations. Among them, three ovicidal miticides etoxazole, hexythiazox and clofentazine have been commonly used in the middle of hop growing seasons for decades. Ovicidal miticides acts by inhibiting chitin biogenesis in arthropods, thus making them more attractive than broad-spectrum miticides. Unfortunately TSSM has been documented to quickly develop tolerance and resistance to these miticides, which has been linked to control failures. In order to effectively cope with the ovicide resistance in TSSM populations, we standardized bioassay methods to evaluate the efficacy of these miticides in a susceptible TSSM population. With the most effective bioassay method, we developed baseline dose response curves of TSSM populations susceptible to these three ovicides. Additionally, we screened five field-collected TSSM populations for the presence or absence of a mutation, I1017F on the target site resistance-associated gene-chitin synthesis 1. The optimized bioassay method and molecular diagnostic tool will provide new information to growers through precisely predicting ovicidal miticide resistance in hop fields.