Thrips tabaci (Thysanoptera: Thripidae) Control in the Columbia Basin of Oregon

The objective of this study was to evaluate the efficacy of insecticides to control onion thrips in onions grown under central pivot irrigation in the Columbia Basin of Oregon.1 Field plots were established at the Hermiston Agricultural Research and Extension Center (HAREC) (45.8411° N, 119.2917° W). Experimental plots were laid out under center pivot irrigation system; commercial practices were followed. Soil was classified as an Adkins fine sandy loam (coarse-loamy, mixed mesic xerollic camborthid); soil was fumigated with sodium methyl-dithio-carbamate (Sectagon, Novasource, Phoenix, AZ) applied at 374.2 lph (40 gpa) in the fall. Preplant soil test results indicated that the soil pH was 6.8, organic matter 0.7 ppm, nitrate, P, K, Ca, and Mg was 40, 22, 263, 5, 2.1, respectively (meq/100 g); N was 230.9 k/ha (205 lb/acre). ‘Vision’ onions were seeded with a Monosem vacuum precision planter the first week of Apr. Onions were planted in 4-m (13 ft) beds with four rows per bed, and 0.86 m (2.8 ft) between beds. A 2.1-m (7 ft) buffer was left between beds. Wheat was used as buffer crop between plots. During the second week of Apr, dimethyl tetrachloroterephthalate (Dacthal, DCPA, AMVAC) was broadcast at 6.7 kg/ha (6 lb/acre) for weed control, and fertilizer (10-35-0; N-P₂O₅-K₂O) was banded over the seeded rows. During planting, seed corn maggot, Delia platura Meigen (Diptera: Anthomyiidae), was controlled by banding chlorpyrifos (Lorsban 4EC, Dow AgroSciences) over the seeded rows at 1.42 L/acre since poor stand development as a result of seed corn maggots, resulted in plots being rototilled, and replanted during the first week of May. On early May, Dacthal was applied at 5.4 kg/ha (4.9 lb/acre). Plots were laid out as a randomized complete block design with four replications per insecticide rotation treatment. Insecticides, active ingredients, class, IRAC mode of action number, rate, and source information are listed in Table 1. Season-long rotation consisted of 2–3 insecticides sprayed 10–14 days apart (Table 2). Insecticides were applied with 3-XR8002 nozzles spaced at 50.8 cm (1.7 ft) at 206.84 kpa, and 4.02 km/h (2.5 mph) resulting in a 185.3 liter/ha (75 gpa). Eight insecticide rotations and an untreated control (UTC) were tested (Table 3); weekly applications started when 1–2 thrips per plant were present (Table 2); plots were taken to yield when 80% of the onion leaves lost their color and/or leaves were visibly weak and tips flopped over; at this point, onions were lifted above ground, and left in the ground for 2–4 days for the ‘curing’ or drying process; then, onions were harvested; yield was determined based on the weight and quality of onions. On a weekly basis, five entire plants were randomly removed from each plot, and thrips (adults and larvae) were counted under a compound microscope (Leica S8) a day before and 3 DAT following application. Data were log transformed (mean + 1) and were arcsine transformed prior to the ANOVA and F-protected LSD (P ≤ 0.05) to distinguish treatment mean differences.

Thrips tabaci population pressure was moderate during the trial. At the end of the season, the mean number of T. tabaci varied irrespective of rotation treatment. Only on 13 Jul, there were significant differences. Despite the use of insecticides, we found only modest numerical differences in yield among the insecticide programs. Rotations on treatments 3, 7, and 8 were numerically the highest (Table 3).

Disclosure of Potential Conflicts of Interest

This research was supported in part by industry in-kind products.