Monitoring two-spotted spider mite populations from commercial hopyards in Oregon for resistance to Onager, 2023

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 production. Field failures to the currently registered acaricides in commercial hop yards led to evaluations of 4 populations of two-spotted spider mites (Table 1) identified in the Willamette Valley of Oregon, where mite outbreaks occurred during the summer of 2023. One population (OSU, Table 1) collected during the summer of 2022 and 2023 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 population. 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 the 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 (Hexythiazox, Onager), Group 10A insecticide was tested for egg mortality at 7 dilutions between 0 and 2 times the label rate (ranging between 0 and 344 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 days, 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 population 2 exposed to hexythiazox were 75.56 ppm with 95% confidence intervals ranging between 54.92 and 103.73 ppm and 179.9-fold resistance compared to the susceptible population (Table 1). Moderate resistance levels to hexythiazox were also detected in the other 2 commercial populations, with 73.0- and 12.4-fold resistance in populations 1 and 3, respectively (This research was supported by the Oregon Hop Commission).