A comparison of onion leaf tissue remaining at the end of the growing season. Photo courtesy of MSU.

Sep 21, 2024
Advancing onion thrips management: The innovations and latest research insights

The Vegetable Entomology Laboratory at Michigan State University has been conducting research on onion thrips management in onions for 15 years. During this time, we have initially focused on developing insecticide programs to prevent thrips from becoming resistant to insecticides.

Updated insecticide program recommendation

The most up-to-date recommended insecticide program starts with two applications of Movento (Bayer, spirotetramat), seven days apart, applied at 0.5-1 thrips per leaf threshold. This is followed by various other insecticides with different modes of action, applied at a threshold.

Figure 1: A) Larval onion thrips in the neck of an onion plant. B) Symptomsof multiple plant pathogens on onion foliage. Photos courtesy of MSU.
Figure 1: A) Larval onion thrips in the neck of an onion plant. B) Symptoms of multiple plant pathogens on onion foliage. Photos courtesy of MSU.

The threshold to apply insecticides is 1 thrips/leaf for most insecticides, except Radiant (Corteva, spinetoram) which is 3 thrips/leaf. The most effective insecticide currently available for thrips control is Radiant, it is recommended that this product is used when thrips populations are the highest, usually in late July in the Midwest.

In our research, there have been many years when we only needed two Movento and two Radiant applications for the entire growing season to keep thrips below the economic threshold, thus the use of thresholds can be crucial for minimizing insecticide applications.

Onion thrips and fungal pathogens

More recently, our lab has been interested in how onion thrips and fungal pathogens of onion interact with each other (Figure 1). We found that onion thrips preferred to reproduce and spend time on plants that were infected with a fungal pathogen over healthy plants. Our next question was: “How are thrips contributing to onion pathogen spread?”

We determined that the more thrips there are on plants, the more the pathogen spreads from one plant to the next; thus thrips suppression can be important for reducing onion fungal diseases. We also learned that at high temperatures, thrips reproduced at a higher rate and moved more between plants leading to increasing pathogen spread.

While thrips contribute to spreading onion fungal pathogens, they are not the sole spreaders of pathogens in the field, many of them are spread mechanically. Therefore, fungicides are still necessary for managing pathogens in onion fields. However, our research suggests that by reducing thrips, especially in those years when pathogens are problematic, there may be additional benefits of reducing onion thrips for keeping onion foliage healthy longer (Figure 2).

Figure 2: A comparison of onion leaf tissue remainingat the end of the growing season. On the left, plants did not receive insecticides or fungicides. On the right, plants received an insecticide (four applications over the season) and fungicide program.
Figure 2: A comparison of onion leaf tissue remaining at the end of the growing season. On the left, plants did not receive insecticides or fungicides. On the right, plants received an insecticide (four applications over the season) and fungicide program.

Non-chemical methods for managing thrips

We explored non-chemical methods for onion thrips management using plastic mulches, also called plastic films. Various colored plastic films were tested, including black, silver, red and white. The entire test plot was covered with the mulches and then we applied an insecticide program to half of the plots, leaving the other half unsprayed.

This allowed us to compare mulches to each other without an insecticide program and then determine if the addition of an insecticide program can provide further benefits for thrips suppression. We found that the silver and red mulch, without insecticides, were as effective as an insecticide program without plastic (grower standard).

Using an insecticide program together with mulches generally improved thrips control. Both black and white mulch increased thrips number compared to the no- plastic treatment; therefore we recommend not using white mulches where thrips are a problem.

These results highlight that silver and red plastic mulches can be a powerful insecticide-free solution to managing thrips. It is important to note that not all silver mulches were effective. We used a textured silver mulch called “Diamond Reflective Film” from Viagrow that increased light reflectance compared to non-textured silver reflective mulches.

Written by Natalie Constancio and Zsofia Szendrei

Natalie Constancio is currently a Ph.D. student in the Vegetable Entomology lab. Her research focuses on the interactions between plants, insects and plant pathogens in the context of developing an integrated pest management plan for Michigan based growers. Her email is [email protected].

Zsofia Szendrei is a professor in the Entomology Department at Michigan State University with a 50% extension, 40% research and 10% teaching appointment.
She has developed a research program focusing on arthropod pests in order to provide solutions to the vegetable industry. Her email is [email protected].




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