Thrips Control Onion, 2013 *

The objective of the study was to evaluate the efficacy of insecticides for control of onion thrips (OT) and western flower thrips on dehydrator onions under desert growing conditions. Dehydrator onion “White Creole” was direct seeded into six row beds on 40-inch centers on 19 October 2012 at the University of California Desert Research and Extension Center, El Centro, CA. Stand establishment was achieved using overhead sprinkler irrigation initiated on 22 October 2012. Sprinklers were used until January 2013, and furrow irrigation was utilized thereafter. Plots were four beds 13.3 ft in width by 50 ft in length and bordered by one untreated bed. The experiment included 11 insecticidal treatments and an untreated check. Four replications of each treatment were arranged in an RCB design. Insecticidal compounds, formulations, and application rates, along with treatment dates, are provided in Table 1 . All insecticide treatments were foliar sprays applied with a four-row sprayer (Lee Spider Spray TracTractor) with three TJ-60 11003VS nozzles per bed, a total of 12 nozzles that delivered 53 gpa at 30 psi. The adjuvant, DyneAmic (Helena Chemical Co.), was added to all insecticidal spray mixtures at 0.25% vol/vol. Thrips were sampled by extracting five random onion plants from each plot on 25 February (pretreatment, PT), 1 March (3 d after treatment 1, DAT1), 5 March (7 DAT1), 11 March (13 DAT1), 15 March (3 DAT2), 19 March (7 DAT2), 25 March (13 DAT2), 28 March (3 DAT3), 2 April (8 DAT3), 8 April (14 DAT3), 15 April (7 DAT4), 18 April (10 DAT4), and 24 April (16 DAT4) 2013. Extracted plants from each replicate were placed in labeled Ziploc bags and taken to the laboratory where a 0.1% silicone surfactant and water solution was added to help dislodge thrips larvae and adults using light agitation followed by washing into a U.S.A. Standard Test Sieve No. 100, 150 UM mesh. The captured thrips were then placed into vials of 70% EtOH, labeled with the sampling date and plot number, for later counting. Adult thrips were examined under a binocular microscope and keyed to species. Because of the difficulty of differentiating thrips species in the larval stage, thrips larvae were counted, and numbers were recorded for each sampling date but not separated by species. Numbers of thrips larvae, WFT adults, and OT adults from five plants per replicate were recorded on each sample date. The posttreatment averages (PTA) were calculated from the accumulative data for each treatment. Only the PT and the PTA for thrips larvae, OT adults, WFT adults, and all thrips larvae plus all thrips adults for each treatment scheme are reported. The adult thrips counts for both OT and WFT, along with the larval thrips counts, were pooled for analysis to determine the insecticidal efficacy of each treatment scheme on the entire thrips population. Iris yellow spot virus (IYSV) can be a serious problem affecting quality and yield quantity of the dehydrator onion crop grown in Imperial Valley, CA. OT thrips is the vector of IYSV and control of OT may have an effect on the incidence and severity of Iris yellow spot (IYS) disease; therefore, on 17 April, 24 April, and 1 May 2013, the percentages of IYS symptomatic plants were measured and recorded in each plot. All onions that were1.25 inches in diameter or greater were harvested on 13.1 row ft from two rows (0.002 acre) on 16 May 2013, and weights were recorded in kilograms. Data were analyzed using analysis of variance. Differences among means on each sampling date and in each experiment were determined using the LSD test ( P  = 0.05).

Thrips population levels were high during this study. There were no differences among the treatments for the PT means for thrips larvae, OT adults, WFT adults, and all thrips stages ( Table 2 ). All insecticide treatment schemes had fewer thrips larvae for their PTA than the untreated check. The untreated check had an OT adults PTA that was lower than all of the insecticide treatments except Movento used alone, Closer SC used alone, Closer used in rotation with (r/w) Radiant, and Torac 22SC used alone. Among the insecticides that were not used in rotation with a different class of insecticide, Lannate LV alone had fewer WFT adults for the PTA compared to the PTA of the untreated check. Among the insecticide treatment rotation schemes, only Torac 15EC followed by (f/b) Radiant, f/b Torac 15EC, and f/b Lannate LV; Exirel 10SE in r/w Radiant SC; Exirel 10SE, f/b Radiant SC, f/b Lannate LV, and f/b Exirel 10SE again; Radiant SC, f/b Movento, f/b Lannate LV, and f/b Radiant SC again had fewer WFT adults for their PTAs compared to the PTA for the untreated check. The PTAs for all insecticide treatment schemes were lower for all thrips than the untreated check PTA for all thrips. There were no differences among the treatment means on any of the sampling dates for percentages of plants expressing IYSV symptoms and no differences among the treatment means for yield of onion bulbs ( Table 3 ). No phytotoxicity symptoms were observed following any of the insecticide treatments.

Author notes