Evaluation of Foliar Insecticides for the Control of Onion Thrips in Dry-Bulb Onion in Wisconsin, 2019

This trial was conducted as part of the U.S. Department of Agriculture’s Organic Agriculture Research and Extension Initiative (OREI), to evaluate several organic foliar insecticide treatment programs for controlling damaging thrips populations in onion. Plots were established on a community farm 4.5 miles south of Cross Plains, Wisconsin (43.048740°N, −89.654712°W), on a silt loam soil in 2019. Onion, Allium cepa cv. ‘Cortland’ transplants were established on 21 Apr at a 10 in. within-row spacing on a 60 in. bed, comprised of three rows spaced 15 in. apart. Four replicate blocks of 16 plots were arranged in an RCBD. Plots measured 6 ft. wide by 12 ft. long. The entire trial measured 18 ft. wide by 264 ft. long. All foliar treatment mixes included one or two insecticide products and one adjuvant. Initial foliar applications were performed on 11 Jul reapplied 8 days later on 19 Jul. Applications were made using a CO₂-pressurized backpack sprayer operating at 30 psi, equipped with a 6 ft. boom with 4 flat-fan nozzles (Tee Jet XR 8002VS) spaced 18 in. apart, and delivering 20 gal/ac while traveling at 3.5 ft./sec. Onion thrips, Thrips tabaci, adult and immature life stages were counted on all leaves from 10 randomly selected onions per plot. Counts were performed on 17, 23, and 29 Jul (6, 12, and 18 days after initial foliar applications). The average number of leaves per plant for each plot was recorded on each count date and used to compute the average number of adult and immature thrips per leaf per plot. Thrips counts per leaf were square root transformed prior to statistical analysis to satisfy assumptions of normality. Treatment main effects were determined using ANOVA. Means separation letter codes were generated using Tukey’s HSD procedure (α = 0.05).

No significant differences in adult or immature thrips counts were observed between any of the treatment or control plots on any of the three count dates (Table 1). Lack of statistical significance of treatment main effects can be attributed, in part, to high variance between replicate blocks, generally low insect numbers present in the 2019 season, and a significant block effect (P < 0.05) observed for adult thrips counts on 29 Jul and on all three dates for immature thrips counts. The average number of leaves per onion was 8.9 on 17 Jul, 8.8 on 23 Jul, and 8.0 on 29 Jul.¹

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