Laboratory screening of two-spotted spider mite populations from carrot seed production systems of Oregon for resistance to miticides, 2023 Open Access

Two-spotted spider mites are a significant pest in hybrid carrot seed production in central Oregon. Miticide resistance is an emerging concern for carrot seed growers, as limited efficacy of available products has been observed since 2021. This study aimed to determine the susceptibility of the field, which collected 2-spotted spider mite populations from central Oregon carrot seed fields to 2 miticides (Group 10A and Group 10B). Two populations (Table 1) were collected during the summer of 2023 from Jefferson Co., Oregon’s commercial carrot seed fields, and 1 known susceptible population (no prior acaricide exposure) was collected from Oregon State University’s vegetable research farm in Corvallis, Oregon. Infested inflorescence samples were shipped to the entomology laboratory for acaricide resistance testing at Oregon State University. 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. A total of 5–7 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. Eight dilutions between 0 and 2 times the label rates of formulated products, Etoxazole, Zeal Group 10 B (ranging between 0 and 43 ppm AI) and Hexythiazox, Onager Group 10A (ranging between 0 and 345 ppm AI) were tested for ovicidal effects. 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, UK) with 1 ml of acaricide solution or water (for the control group) using an air pressure of 47 KP (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 (Table 1). 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 both populations exposed to etoxazole were 0.43 ppm AI and 0.13 ppm AI for FH2 and SF17, respectively (Table 1), with an average of 85% mortality at the labeled rate (21 ppm AI) and 2 times label rate (43 ppm AI) in both populations (data not shown) indicating susceptibility to etoxazole. The RR₅₀ values (Table 1) were also suggestive of low to no resistance in FH2 (1.26) and SF17 (0.38), respectively. On the other hand, high-resistance levels to hexythiazox were detected in 1 commercial population, FH2 (RR₅₀ = 724.14), with an LC₅₀ value of 304.14 ppm AI (Table 1). An average of 35% and 65% mortality (data not shown) at the labeled rate (173 ppm AI) and 2 times label rate (346 ppm AI) of hexythiazox. Moderate resistance to hexythiazox was also detected in SF17 (Table 1; RR₅₀ = 26.74; LC₅₀ = 11.23).¹

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