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What Insecticides Could Replace Radiant SC as the Next Thrips Control Hammer in the Great Lakes Region?
By Brian A. Nault, Professor, Department of Entomology, Cornell AgriTech, Cornell University
Onion thrips (Thrips tabaci) management in onion is dependent on insecticide use. If onion thrips infestations are not kept in check, their feeding can severely damage the onion crop (Fig. 1a and 1b). Onion thrips also can transmit serious pathogens, such as iris yellow spot virus, which causes iris yellow spot disease that can destroy onion fields and ultimately result in complete crop loss (Fig. 1c). To add insult to injury, onion thrips populations routinely develop resistance to insecticides.
Insecticide Resistance
Over the past several years, onion thrips populations have been particularly challenging to manage in onion fields throughout the Great Lakes region. One reason contributing to this problem is that one of the best onion thrips control products of all time, spinetoram (Radiant SC), has become less effective. Although not confirmed, we suspect that onion thrips populations have developed resistance to Radiant SC in these areas.
Effective onion thrips management relies on protection of the crop for as long as two months, and this requires multiple insecticide applications. Because the threat of insecticide resistance is high, insecticide use recommendations are designed to minimize the risk of resistance development. A key element of this resistance mitigation strategy is to use a season-long sequence of insecticides belonging to different classes of chemistry and modes of action. Moreover, the use of any single insecticide should be limited. In fact, many insecticides registered on onion for thrips control are restricted to two successive applications per season. Therefore, a total of three to five insecticides are typically used to protect the crop from thrips over the course of a season.
Radiant SC was positioned in a season-long sequence during the latter half of the season when onion thrips populations were at their highest. When an infestation seemed to be on the verge of becoming uncontrollable, Radiant SC was used as “the hammer” to bring it back down. As Radiant SC becomes less reliable to combat high onion thrips infestations, a new hammer must be identified. The frontrunners are cyantraniliprole (Exirel) and the newcomer, isocycloseram (Vertento). Exirel was first registered on onion for thrips management in 2014. Exirel belongs to the anthranilic diamide class (IRAC 28), which includes insecticides that target the insect’s ryanodine receptors, causing muscle paralysis, feeding cessation and death. Vertento was first registered on onion for thrips control in December 2025. Vertento belongs to the isoxazoline class (IRAC 30), which includes insecticidesthat target the insect’s nervous system by binding to GABA receptors, causing hyperexcitation of neurons and eventually death.
Evaluating Alternatives
In western New York’s Elba Muck, we evaluated the ability of Radiant SC, Exirel and Vertento to control a high infestation of onion thrips. Insecticides were initially applied when thrips densities reached 11.6 per leaf. For context, the action threshold for making an insecticide application to manage onion thrips in New York onion fields is one thrips per leaf. A second application was made one week later. The rates of Radiant SC, Exirel and Vertento were 10 fl oz per acre, 20.5 fl oz per acre and 3.1 fl oz per acre, respectively, and all included the surfactant Dyne-Amic at a rate of 0.25% v:v. One week after the second application, numbers of onion thrips larvae were recorded and the mean number of thrips per leaf was compared. Results indicated that Radiant SC failed to significantly reduce the population of thrips, while both Exirel and Vertento brought the infestation down to a non-economically damaging level (Fig. 2). Exirel provided a 93% reduction and Vertento a 97% reduction in thrips densities relative to the untreated control. We have our answer: both Exirel and Vertento could be used as a hammer to manage very high infestations of onion thrips.
Exirel continues to be effective at rates of 13.5 to 16 fl oz per acre under low to moderate thrips pressure (up to two thrips per leaf), whereas a rate of 16 to 20.5 fl oz per acre is better at higher densities. Vertento is highly effective at a rate of 3.1 fl oz per acre, but the maximum labeled rate is 4.1 fl oz per acre. For both products, a non-ionic surfactant is needed to improve efficacy. In a recent study in New York, co-applications of these products with fungicides, including chlorothalonil (Bravo WeatherStik), have not compromised levels of onion thrips control.
Based on experience evaluating many different products for onion thrips control over the years, Exirel and Vertento should be used during the middle of the season when onion thrips infestations are typically most challenging to manage and when the crop is vulnerable to bulb reduction from feeding damage. Because there are many different season-long sequences of insecticides that could be used to manage onion thrips, consult me or your regional extension vegetable entomologist for details. Similar research has evaluated Exirel and Vertento for thrips control in the Pacific Northwest, so contact Stuart Reitz (Oregon State University) and Tim Waters (Washington State University) for the most current guidelines.