Efficacy of erythritol solutions as a field treatment against spotted wing drosophila infestation and oviposition, 2022–2023

The performance of 2 erythritol formulations as spray treatments against spotted-wing drosophila (SWD) infestation and oviposition was evaluated in the field. Erythritol solutions were composed of 1.5 M meso-erythritol (>99%) (Oakwood Chemical, Estill, SC) and a phagostimulant: either 0.5 M sucrose (Kroger, Cincinnati, OH) [183.18 g erythritol + 171.15 g sucrose] or 0.1 M sucralose (Bulk Supplements, Henderson, NV) [183.18 g erythritol + 39.76 g sucralose]. Compounds were dissolved in dH₂O over a stirring hot plate and vacuum filtered to remove impurities prior to application. Erythritol solutions and a dH₂O check were applied to leaves and fruit clusters with a CO₂ pressurized backpack sprayer at 262,000 Pa (XR 8002 VS TeeJet Technologies, Glendale Heights, IL) on the trial start dates. Field trials were conducted in blueberries at Oregon State University’s Lewis Brown Farm, and wild Himalayan blackberries (WHB) at Lewis Brown Farm, a small mixed farm, and a large mixed berry farm in the Willamette Valley of Oregon. Blueberry trials were in mature ‘Reka’ blueberries in 2022 and mature ‘Elliot’ blueberries in 2023 in the same 0.64-acre mixed-cultivar field (Table 1). Reka plots in 2022 were located on the east and west perimeter rows of the field and divided into 5 blocks, each consisting of 2 erythritol-treated plots and 1 dH₂O untreated check. Elliot plots in 2023 were randomly dispersed throughout the interior of the mixed-cultivar field and treatments followed a completely randomized design with 5 replicates of the 2 erythritol treatments and a dH₂O check. Blackberry trials were conducted with a randomized complete block design in strips of WHB by managed fields of other crops in 2022 (Table 1). Two blocks consisting of 2 erythritol-treated plots and 1 dH₂O untreated check were located at both of the local mixed farms, and 1 block was located at OSU’s Lewis Brown Farm. In blueberry trials, SWD infestation was assessed with weekly collections of ~300 g of ripe fruit from each plot. The fruit was incubated in a clamshell with a mesh lid at 22°C and 40% RH for 2 weeks in the laboratory, then the number of adult spotted-wing drosophila in the sample was counted and analyzed as a function of flies per gram of fruit. In total, 101 samples were collected across 7 weeks in 2022 and 103 samples were collected across 7 weeks in 2023 due to the limited availability of ripe fruit at the trial ends. In WHB, infestation was assessed by collecting ~250 g of ripe blackberry fruit per plot per week. Fruit was transported to the laboratory to incubate in a plastic clamshell with a mesh lid for 2–3 days at 22°C and 40% RH. One cup (273 g) of table salt was dissolved in 1 gal (3.79 L) of dH₂O, then added to gently mashed blackberry samples to float and irritate any SWD larvae in the sample. Larvae per sample were counted and analyzed as a function of larvae per gram of fruit. To assess oviposition in blueberries, 6 ripening clusters of 10–30 blueberries per plot were enclosed in mesh bags directly after erythritol solutions were applied. Once a week, 1 ripe fruit cluster per plot was selected, re-sprayed with the corresponding solution, and 10 adult female 7-day-aged SWD were introduced to the bag, along with a water wick. The flies used were the F₁ progeny of wild collected flies from unsprayed raspberries at the OSU Lewis Brown Farm and were reared according to Woltz et al. (2017). After 24 h, mesh bags containing flies and fruit were clipped off the plant and transported to the laboratory. The number of SWD eggs in each fruit sample was counted. Eggs in mesh bags were counted across 5 weeks in 2022 and 2023, for a total of 75 bags in each year. Data from all infestation trials were analyzed with repeated measures generalized linear mixed model (GLMM) with treatment, week, and their interaction as fixed effects, block (if relevant) as a random effect, and treatment*replicate as a random subject effect with repeated sampling. Data from oviposition trials were analyzed with a similar GLMM without repeated measures. A Tukey HSD compared treatments if the treatment effect was significant, or compared treatments by week if the interaction was significant.

SWD infestation in Reka and Elliot blueberries was not significantly affected by treatment (Table 2). Treatment*week effects were significant in the Elliot trial, and the erythritol:sucralose treatment had significantly fewer SWD than controls. Oviposition in Reka blueberries in 2022 was not affected by treatment, but in week 4, the erythritol:sucrose treatment had 89% less eggs than the control. In 2023, oviposition in Elliot blueberries was significantly higher in the untreated check plots than in either erythritol-treated plot over the season. In WHB in 2022, infestation was not affected by treatment. Across all trials, the week had significant effects on infestation and oviposition outcomes, likely due to variations in weather and fruit ripeness.¹

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