Nov 1, 2022
Onion thrips, disease management strategies

Onion thrips are a major insect pest in onions. They feed on onion leaves, resulting in silvery scarring, reducing the amount of green leaf tissue (Fig. 1A).

A reduction in green tissue negatively affects the onion’s ability to photosynthesize, and results in smaller bulbs and decreased yield. Additionally, when onion thrips feed, they leave open wounds on the onion tissue, creating an entryway for plant pathogens.

Onion thrips spread many kinds of plant pathogens, including viral, bacterial and fungal pathogens. While onion thrips are primarily known to spread tospovirus, such as Tomato spotted wilt virus, they also contribute to the spread of bacterial pathogens such as center rot, and fungal pathogens such as Stemphylium leaf blight. In Michigan, Stemphylium leaf blight is the main pathogen in onion fields and presents many challenges to growers.

Recently, onion thrips have been shown to spread Stemphylium leaf blight by picking up fungal spores on the outside of the body and moving the spores between onion plants. Symptoms of Stemphylium leaf blight include tan water-soaked lesions on the leaves, and premature leaf die back, resulting in severe yield losses (Fig. 1B).

Once an onion is infected with Stemphylium leaf blight, it is also vulnerable to infection by secondary pathogens, which can cause storage rot.Growers manage onion thrips and plant pathogens in different ways. Onion thrips are managed through scouting programs prior to insecticide application.

Scouting for onion thrips is critical to successful control because they are resistant to multiple insecticide classes. To prevent further insecticide resistance, growers must rotate insecticide classes and implement action thresholds. The action threshold indicates the pest population level a grower should start treatment to prevent significant crop loss and is necessary for reducing insecticide resistance.

Additionally, using an action threshold also reduces the amount of insecticide applied throughout the growing season and may save the grower time and money.

In contrast, plant pathogens are controlled on a calendar schedule and are advised to apply fungicides early in the season. After the first fungicide application, treatments are applied at regular intervals as a preventative measure. Increasingly, research provides evidence that fields infested with onion thrips experience higher disease pressure, indicating that the management of both pests should be considered simultaneously. However, because of the differences in spray recommendations for onion thrips and plant pathogens, it can be difficult for growers to implement a spray program that effectively combines both management approaches.

 In the 2022 growing season, researchers at Michigan State University tested the combination of synthetic conventional insecticides and fungicides at varying action thresholds to evaluate their ability to simultaneously manage onion thrips and Stemphylium leaf blight. The spray trial was conducted in a commercial onion field in Michigan and consisted of 8 different spray programs, with varying action thresholds. The treatments were: 1) Control – no spray, 2) Fungicide only applied weekly, 3) Low threshold + Fungicide, 4) Medium threshold + Fungicide, 5) High threshold + Fungicide, 6) Low threshold, 7) Medium threshold without fungicide and 8) High threshold without fungicide.

All the treatments with fungicides began with Miravis Prime and were alternated with Bravo Weather Stik weekly. For the low threshold treatments, either with or without a fungicide, treatments began when the action threshold reached 0.5 thrips/leaf and consisted of Movento, Minecto Pro, and Radiant each applied twice 7 days apart before rotating to the next insecticide. The medium threshold treatments began at an action threshold of 0.6 thrips/leaf and consisted of Minecto Pro and Radiant, applied twice 7 days apart before rotating to the next insecticide. The high threshold was set at 1.0 thrips/leaf and consisted of 2 applications of Radiant applied 7 days apart. The date of insecticide applications began varied depending on the thresholds, with low threshold application starting on July 5, the medium on July 26 and the high threshold starting on August 9. For each application, the pesticides were tank mixed with Syl-Tac 1 % v/v and applied with a CO2 pressurized backpack sprayer equipped with a fan nozzle.

Results

The fungicide only treatment did not effectively control onion thrips populations, the thrips pressure in this plot was similar to the no-spray control plots and the high threshold + fungicide. All treatments with insecticides controlled onion thrips populations below the economic threshold, and we did not see a difference among those treatments. Treatments with insecticides reduced onion thrips populations by 50% when compared to the no-spray treatments, but only reduced onion thrips by 25% compared to the fungicide only treatment (Fig. 2). At the end of the season, there was a significant decrease in green tissue across all treatments. On August 29th (final disease rating date), the no-spray plots had 40% green tissue remaining, which was similar to the medium threshold and high threshold without fungicides. The combination of insecticides and fungicides increased green tissue to 55%, and the fungicide only treatment also significantly increased green tissue when compared to the no-spray plots.

Interestingly, there was no statistical difference in the onion thrips numbers among the treatments with varying action thresholds. The high threshold treatments with 2 applications of Radiant was as effective as the low threshold treatments with Movento, Minecto Pro, and Radiant applied twice. Overall, there was low onion thrips pressure throughout the season, and when pressure is low, growers may be able to effectively control onion thrips with Radiant only. While there was no difference in onion thrips numbers between treatments that contained insecticide, there was a significant difference in green tissue between treatments that contained both insecticide and fungicide, compared to treatments with insecticide only. These differences in treatment effectiveness for onion thrips control compared to disease control indicate that the ideal spray program for onion thrips (high threshold, without fungicide) did not effectively control disease. This highlights the need for an effective multiple pest management strategy that focuses on both insect and disease pests simultaneously.

Acknowledgements: We thank the cooperating onion grower who allowed us access to their field for the spray trial. Funding for this research was provided by the USDA’s Specialty Crop Block Grant Program.

— Natalie Constancio and Zsofia Szendrei, Michigan State University

Photos: Figure 1: (A) An onion infested with onion thrips larvae. Onion thrips feed on plant tissue which results in silvery leaf scarring and reduces the plants’ ability to photosynthesize. Severe onion thrips infestations result in smaller bulbs and decreased yield. Onion thrips also contribute to disease spread in onion fields. 

(B) An onion leaf infected with a plant pathogen, Stemphylium leaf blight. Stemphylium leaf blight symptoms include tan, water-soaked lesions on the onion foliage. In Michigan, Stemphylium is the main pathogen found in onion fields, and can result in severe yield loss. 




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