Vegetable Growers News September 2024

Advancing onion thrips management: innovations and latest research insights

MSU researchers share insecticide and non-chemical strategies for onion thrips management. Discover how to protect onion crops effectively.

3 minute read

The Vegetable Entomology Laboratory at Michigan State University has been conducting research on onion thrips management 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 – one thrips per leaf threshold. This is followed by various other insecticides with different modes of action, applied at a threshold.

Two images, with photo A on the left showing larval onion thrips on an onion plant and photo B on the left showing damage from plant pathogens

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 one thrips/leaf for most insecticides, except Radiant (Corteva, spinetoram), which is three thrips/leaf. As the most effective insecticide currently available for thrips control, Radiant should be 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. Therefore, 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; therefore, 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 pathogens spread mechanically. Therefore, fungicides are still necessary for managing pathogens in onion fields.

That said, our research suggests that by reducing thrips, especially in those years when pathogens are problematic, there may be the additional benefit of reducing onion thrips for keeping onion foliage healthy longer (Figure 2).

Comparison images showing onion leaf tissue at the end of growing season, the photo on the left without pesticides and the photo on the right with insecticide and fungicide

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 onion thrips management

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. Then, we applied an insecticide program to half of the plots, leaving the other half unsprayed.

This allowed us to compare the mulches without an insecticide program to determine if the addition of an insecticide program could 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 numbers 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.

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. She is working toward developing an integrated pest management plan for Michigan based growers. Her email is consta80@msu.edu.

Zsofia Szendrei is a professor in the Entomology Department at Michigan State University. She has 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 szendrei@msu.edu.