Onion thrips are present in Michigan onion fields in 2013

Onion thrips are now active in Michigan onion fields. Start checking your plants for the presence of this insect.

Onion thrips have been commonly found in onion fields throughout Michigan during the first week of June, and although numbers are not alarmingly high at this time, early intervention may prevent populations from becoming abundant early in the season. Onion thrips prefer dry, hot weather, and during these conditions their numbers can rapidly increase on plants.

To scout for onion thrips, visually survey onions and count the number of thrips and the number of leaves per plant. Check 10 plants per location and check at least five different locations spread throughout the onion field to get an average for onion thrips and leaf numbers per plant. Divide the average number of onion thrips per plant with the average number of leaves per plant to calculate the average thrips per leaf. For example, if you find that on average you have three thrips per plant and that plants on average have three leaves, then you have reached the one thrip per leaf threshold, and an application is warranted.

When populations are still low early in the season, a foliar insecticide called Movento may be an option for commercial onion growers to manage onion thrips. A Section 18 exemption for Movento (spirotetramat) is available; this means that onion growers can use this product for onion thrips management in Michigan in the 2013 growing season. The minimum interval between applications is seven days, the pre-harvest interval is also seven days and the maximum amount of Movento that can be applied in a single season is 10 fluid ounces per acre with 5 fluid ounces per acre as the only approved rate of application. This means two applications of Movento can be made to onions within a growing season, and these should be done consecutively, seven to 10 days apart, for insecticide resistance management purposes.

Movento has to be tank-mixed with a non-ionic (penetrating) surfactant (0.25 percent v/v) to maximize leaf uptake since this product moves systemically in the plants. Tank-mixing Movento with fungicides that have spreader-stickers can reduce efficacy of this insecticide. For example, Dithane F45 Rainshield, Rovral 4F, Scala SC and Quadris F tank-mixed with Movento doesn’t impact onion thrips control negatively. Movento is more effective at suppressing larval onion thrips; therefore it is recommended that it is used in the early part of the growing season, before thrips population pressure peaks.

The MSU Vegetable Entomology Lab recommends multiple insecticides be used in rotation in a season for onion thrips management. For resistance management purposes it is best to make two applications one week apart of a particular insecticide and then rotate to the next insecticide. As an example, an eight week program that consists of weekly applications of insecticides would look like this: Movento, Movento, Agri-Mek, Agri-Mek, Radiant, Radiant, Lannate, Lannate. Instead of weekly applications, scouting for thrips can save money if using the one thrip per leaf threshold (except Radiant where three thrips per leaf can be used), and only making insecticide applications once this threshold is surpassed.

A common feature of the most successful onion thrips management programs is that they all have Radiant applied in the hottest part of the summer – typically mid- to late July – so save this product for later in the season. Read more information on onion thrips management.

For more information, visit the Vegetable Entomology website. Use the Michigan State University Extension bulletin E-312, “Nematode, Insect and Disease Control” for guidance on insecticides registered on different vegetable crops.

Michigan State University is not supporting the use of any particular insecticide brand. When using pesticides, always follow the label recommendations.

Dr. Szendrei’s work is funded in part by MSU‘s AgBioResearch.

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