Cost-effective onion thrips control program 2: Apply at right time based on scouting and thresholds

Ben Werling<werlingb@msu.edu>, Michigan State University Extension, and Zsofia Szendrei, MSU Extension, Department of Entomology - January 20, 2015

Timing can make the difference between OK and excellent thrips control in onions.

The Midwest and northeast onion industry now has an excellent toolbox of chemistries with different modes of action that, if used right, could provide durable control of onion thrips into the future. By using the right products with the right surfactants at the right time, you can accomplish this goal while minimizing cost.

The right timing

Applying onion thrips insecticides at the right time is critical to ensure success and delay resistance. Products need to be applied when they are most effective during the roughly eight weeks thrips are active from mid-June to mid-August (see table). Importantly, for all products the same active ingredient should be applied in back to back applications in two consecutive weeks to minimize selection for resistance.

An example of a planned rotation of insecticides for onion thrips control, covering the roughly eight weeks from mid-June to mid-August thrips can be active, and including thresholds for use with each product.

Week	Product	Action threshold	Notes
1	Movento	1 thrips/leaf
2	Movento	1 thrips/leaf
3	Agri-Mek or Exirel	1 thrips/leaf	Only use Exirel if not planned for weeks 7-8
4	Agri-Mek or Exirel	1 thrips/leaf	Only use Exirel if not planned for weeks 7-8
5	Radiant	3 thrips/leaf
6	Radiant	3 thrips/leaf
7	Lannate or Exirel	1 thrips/leaf	Only use Exirel if not applied in weeks 3-4
8	Lannate or Exirel	1 thrips/leaf	Only use Exirel if not applied in weeks 3-4

Movento is more effective at reducing early-season thrips populations than other products, but is not effective later in the season. Agri-Mek is a good product to follow Movento during early to mid-summer. In addition to Agri-Mek, recent research shows that a newly labelled product, Exirel, can provide effective control of thrips when applied at this time. Radiant is a good product to reserve for the next spot in the rotation; it is not as effective as Movento early, but is extremely effective during the hottest part of the growing season for quelling rapid thrips increases. Experience in New York suggests that if the momentum of Movento allows you to skip two to four weeks of sprays, thrips can increase afterwards, and Radiant is a good product to stop them in their tracks.

If thrips are still a problem after Radiant is used, consider using Exirel or Lannate. If you have experienced poor control with Lannate, consider tank-mixing a high rate of this product with a high rate of Warrior. Recent work from Cornell University suggests that Warrior plus Lannate could provide better control than either product alone. Note, in general tank-mixing two modes of action together is not advisable as it could simultaneously select for resistance to two classes of chemical. However, pyrethroids like Warrior are already ineffective on their own for onion thrips control in many areas.

Many of these products are costly, which means you will save significant money by only applying product when thrips damage could reduce yield. This requires using scouting data and thresholds. To make good decisions, onion thrips counts should be in terms of the number of thrips per leaf, not per plant. For Radiant, a threshold of three thrips per leaf can be used, while one thrips per leaf can be used for other products (see table).

Each week of the season, you can decide whether or not to make the next application in your planned rotation; if thrips exceed threshold, apply the product as planned, if not, skip the product and reevaluate the following week. Research at MSU and Cornell University has shown significant cost savings associated with using thresholds with only two early-season applications of Movento providing control comparable to eight weekly sprays in 2014 MSU trials. Overall, data collected over multiple years by Cornell University show that use of thresholds can reduce the number of applications by an average of 50 percent.

A little bit of planning

Many growers are now reaping the benefits of using available products in a diverse chemical rotation for thrips control. Doing so requires a little planning in the off-season. Sketch out a sequence of products to cover the roughly eight weeks thrips might be active, indicating when each product might be used. Make sure your scouts are providing good thrips data in terms of the number of individuals per leaf. Finally, make sure to locate the right surfactant ahead of time. Once the season comes, you will be prepared to make good decisions and use products in a cost-effective manner.

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

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