Evaluation of foliar insecticide treatments for thrips control in onion, 2022

In 2022, an insecticide efficacy trial was conducted to evaluate various chemicals for the control of thrips in onions at the Vidalia Onion and Vegetable Research Center, Tattnall County, Georgia. Onions, hyb. ‘Macon’, were transplanted on 7 December 2021 into 4 rows per bed at approximately 4–6 inches between plants and maintained with standard cultural practices. A total of 950 lbs/acre of 5-10-15 fertilizer was applied to Tifton loamy sand soil field plots with a follow-up application of 215 lbs/acre of calcium nitrate. In the absence of rainfall, irrigation was applied at about 0.5 inches per week with an overhead sprinkler system. Three applications of insecticide were made on 2 and 22 March and 14 April. Foliar insecticide treatments were applied with a CO₂ hand sprayer delivering 32 gallons/acre at 30 psi with 4 TX-18 hollow cone tips per bed.

Products used included: Spinetoram (Radiant SC, Corteva), a nicotinic acetylcholine receptor (nAchR) allosteric modulator (IRAC Group 5), with broad-spectrum activity; Bifenthrin (Brigade 2EC, FMC), a pyrethroid (IRAC Group 3A), ISM-555 (Plinazolin, Syngenta), a gamma-aminobutyric (GABA) receptor antagonist (IRAC Group 30); and an unsprayed check. All spray treatments included the adjuvant MSO 0.5% v/v at 10 ml per 2-L spray. Each treatment was replicated 4 times, in a completely randomized block design. For each plot, the total numbers of thrips per 10 plants were counted and preserved in 70% ethanol on 3/2, 3/22, 4/6, and 4/21. The preserved samples were later identified as species using a stereoscopic microscope (Olympus SZH10, max. magnification: 140×). Most of the thrips were collected from the plant after bulb formation in April. Onions were harvested from the center of each plot and the harvested area spanned 15 ft of each bed.

Tobacco thrips (F. fusca) were the most prevalent species in this test and were consistently higher in the untreated check than in most treatments, with the exception of Bifenthrin (Tables 1 and 2). ISM-555 was better than Radiant for thrips control on certain dates (Table 1), and marginally better in cases where sufficient numbers could be found, e.g., for F. fusca and overall thrips count (Table 2). However, all 3 rates of ISM-555 did produce significantly heavier yields of jumbo-sized onions than the Radiant treatment, while only the highest rate of ISM-555 produced weights significantly greater than Brigade, and check plots (Table 3). There was no significant rate response in insect control for ISM-555 in this test.¹

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