Spatio-temporal Variation of Spinosad Susceptibility in Drosophila suzukii (Diptera: Drosophilidae), a Three-year Study in California's Monterey Bay Region.

Spinosyn insecticides are widely used in conventional berry production, and spinosad is regarded as the most effective insecticide for managing Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), spotted-wing drosophila, in organic berry crops. Following the 2017 identification of spinosad resistance in caneberry fields in the Watsonville area, Santa Cruz Co., California, we conducted a study to examine the seasonal and annual susceptibility of D. suzukii over a three-year period. Adult flies were collected from two conventional and two organic caneberry fields in the Monterey Bay region, California, at 'early', 'middle', and 'late' time points during the 2018-2020 growing seasons, and their susceptibility to spinosad was assessed. Results demonstrated that spinosad susceptibility in the D. suzukii field populations generally decreased during the fruit production season (from June through November), and over consecutive seasons. LC50 values of adults from the conventional sites were determined to be as high as 228.7 mg l-1 in 2018, 665.6 mg l-1 in 2019, and 2700.8 mg l-1 in 2020. For the organically managed fields, LC50s of adults were as great as 300.0 mg l-1 in 2018, 1291.5 mg l-1 in 2019, and 2547.1 mg l-1 in 2020. Resistance ratios based on the LC50 values were as high as 10.7-, 13.2-, and 16.9-fold in 2018, 2019, and 2020, respectively. These results should serve as a caution for growers in other production areas, facilitate informed choice of insecticides used in D. suzukii management, and emphasize the need to develop effective insecticide resistance management strategies for this insect.

Timing Spring Insecticide Applications to Target both Amyelois transitella (Lepidoptera: Pyralidae) and Anarsia lineatella (Lepidoptera: Gelechiidae) in Almond Orchards.

Amyelois transitella (Walker) (Lepidoptera: Pyralidae) and Anarsia lineatella Zeller (Lepidoptera: Gelechiidae) are key Lepidoptera pests of almonds in California. Spring insecticide applications (early to mid-May) targeting either insect were not usually recommended because of the potential to disrupt natural enemies when broad-spectrum organophosphates and pyrethroids were applied. The registration of reduced risk compounds such as chlorantraniliprole, methoxyfenozide, and spinetoram, which have a higher margin of safety for natural enemies, makes spring (early to mid-May) application an acceptable control approach. We examined the efficacy of methoxyfenozide, spinetoram, and chlorantraniliprole at three spring application timings including the optimum spring timing for both A. lineatella and A. transitella in California almonds. Our study also examined the possibility of reducing larval populations of A. lineatella and A. transitella simultaneously with a single spring insecticide application. There were no significant differences in the field efficacy of insecticides targeting either A. lineatella or A. transitella, depending on application timing for the three spring timings examined in this study. In most years (2009-2011), all three timings for each compound resulted in significantly less A. transitella and A. lineatella damage when compared with an untreated control, though there was some variation in efficacy between the two species. Early to mid-May applications of the reduced-risk insecticides chlorantraniliprole and spinetoram can be used to simultaneously target A. transitella and A. lineatella with similar results across the potential timings.