Monitoring two-spotted spider mite populations from commercial hopyards in Oregon for resistance to Zeal, 2022

This study aimed to determine the susceptibility of two-spotted spider mite populations in Oregon hopyards to active chemistries currently available for arthropod pest management in hop. Field failures to the currently registered acaricides in commercial hop yards led to evaluations of two-spotted spider mite populations (WV1-3, Table 1) identified in the Willamette Valley of Oregon, where mite outbreaks occurred during the summer of 2022. One population (WV4, Table 1) collected during the summer of 2022 from the hopyard with no acaricide exposure maintained at the Oregon State University’s Vegetable Research Farm in Corvallis, Oregon, was used as a known susceptible. Infested leaves were collected from the upper, middle, and lower parts of hop plants to ensure that each sample was representative of the mite population across each hop yard and were brought back to the laboratory for acaricide resistance testing. Mites were reared on lima bean plants (Phaseolus lunatus L.) under laboratory conditions, 27.5 ± 2 °C, at 70.0 ± 5% relative humidity, with a 15:9 L:D h cycle. Mites were subjected to leaf disk bioassays by placing a lima bean leaf disk (2 cm in diameter) in a water-saturated cotton reservoir in a 60 × 15 mm petri dish. Five to 7 mature female mites were allowed to lay eggs on leaf disks for at least 24 h. After oviposition of 5 or more eggs per petri dish was observed, the adult female mites were removed. Eggs were counted on each petri dish using a light microscope and recorded before treatment. Formulated product (Etoxazole, Zeal), Group 10B insecticide was tested for egg mortality at 7 dilutions between 0 and 2 times the label rate (0,1.4, 2.8, 5.7,8.6, 11.5, and 23 ppm AI). Each dose was replicated 5 times. Freshly laid eggs on each leaf disk were sprayed using a Potter precision spray tower (Burkard Manufacturing, Rickmansworth, Herts, United Kingdom) with 1 mL of acaricide solution or water (for the control group) using an air pressure of 47KP (6.8 psi) and a spray distance of 22 cm. After 5 d, the number of eggs that successfully hatched into larvae was counted. The dose–mortality response was adjusted to the control treatment using Abbott’s formula (Abbott 1925). Probit analysis was used to estimate the concentration that kills 50% of the population (LC₅₀ value) and the corresponding slope and 95% confidence interval. The resistance ratio (RR₅₀) was calculated by dividing the lethal concentration values (LC₅₀) of each field-collected mite population by the LC₅₀ value of the known susceptible check colony (Table 1). The LC₅₀ values of 2 commercial hopyard mite populations, WV2 and WV3, ranged between 0.23 and 10.56 ppm etoxazole with overlapping confidence intervals, indicating minimal resistance selection to etoxazole (RR₅₀ values of 0.41 and 2.03 for WV2 and WV3, respectively) (Table 1). Meanwhile, the LC50 value of WV1 was 1.51 ppm etoxazole with a 4.44-fold resistance ratio compared to known susceptible control. Therefore, a proactive resistance monitoring approach in the future can help hop growers inform and support insecticide resistance management strategies in the Oregon hopyards (This research was supported by the Oregon Hop Commission).