The thrips battle plan

Have you ever wondered why thrips, an insect so miniscule in size and ungainly in appearance, provoke such teeth-gnashing cries from all of us? Is it because they disfigure our flowers (Figure 1)? Because they always manage to find a way to enter your greenhouses? Or, no matter what you do, you can’t seem to control them? Yes, yes and yes!

Thrips are among the most prevalent and difficult pests to control, and their reputation is well-earned. They have several advantages: They are small and stealthy, reproduce abundantly and feed on almost every plant species. Many are invasive, transmit viruses, evade control during their pupal stage and are highly adept at developing resistance to pesticides.

Thrips are tough to control, but we have learned a few lessons about managing them. Let’s review some of these lessons, so you can develop an effective management program and give your teeth a little rest.

While it may be tempting to jump straight to the paragraph where insecticides are discussed, stay with me, because effective thrips management begins before there’s any crop in a greenhouse. Pre-planting tactics can help ensure a clean start, delay outbreaks and reduce infestation severity.

Clean your greenhouse between crops so that thrips cannot hide on leftover plants, weeds or debris.

Leave the greenhouse empty for a few weeks so that thrips cannot find anything to feed on and eventually die. If that’s not an option, consider placing pots of blooming marigolds or mums as trap plants in the empty greenhouse. Remove the trap plants after two weeks to prevent population growth.

Manage weeds outside of the greenhouse, particularly flowering weeds that serve as waystations for incoming thrips. Pay attention to what your neighbors are doing. Be prepared to deal with dispersing thrips if the neighbors are harvesting or drying crops or hay.

Use screen doors and vents with a mesh size small enough to keep thrips out (a minimum of 150 microns) while not impeding good airflow. Clean and maintain the screens regularly.

Inspect all incoming plants and treat the infested materials before potting. Alternatively, be suspicious of and treat all incoming materials. An easy way to treat cuttings is to dip them in solutions of 0.1% to 0.5% horticultural oil or Beauveria bassiana. Trap plants can also be used in non-flowering crops afterward to prevent population growth. Even with the best preventative measures, some thrips will still make their way in. That’s why scouting, biological control and insecticides should be key components of your IPM program.

In-season management depends on a strong scouting program that tracks thrips population trends at least weekly. Yellow or blue sticky cards placed just over canopies and near vents and doors are easy, effective ways to detect incoming or growing thrips populations. Examining plants for damage and thrips helps find hot spots.

Tap plants over a piece of white paper or a palette (Figure 2), then use a hand lens to identify the dislodged pests and biological control agents. Cup a flower between your palms and blow gently into it; the warmth and carbon dioxide in your breath can drive out thrips hiding in the bloom.

Many insecticides are registered to control thrips. Check out SePRO’s Bug Poster (sepro.com/BugPoster) for all insecticides registered for use against thrips and other pests. Which one should you include in your program? There are several considerations to keep in mind when developing an effective insecticide program.

Drenching systemic insecticides, such as neonicotinoids (IRAC 4A), spirotetramat (23) and diamides (28), can reduce thrips numbers and damage on foliage, but not on flowers. Systemic active ingredients are translocated to petals at such a low concentration that they are ineffective against feeding thrips. If a preventive drench is desirable, make the application before thrips appear to allow time for the insecticides to translocate to the canopy.

Protection of continuously expanding flowers and foliage on a growing crop relies on repeated insecticide sprays, often on a weekly or three-or-four-day basis. While Mesurol (methiocarb; 1A), the “big gun” in thrips management, is no longer available, there are other effective insecticides, such as abamectin (6), acephate (1B), acetamiprid (4A), chlorfenapyr (13), cyantraniliprole (28), dinotefuran (4A), flonicamid (29), novaluron (15), pyridalyl (unknown MOA), spinosad (5), sulfoxaflor + spinetoram (4C + 5), thiamethoxam (4A) and tolfenpyrad (Hachi-Hachi SC; 21A). Hachi-Hachi SC stands out as a broad-spectrum insecticide with rapid knockdown of various thrips species, as well as caterpillars, aphids, leafhoppers, mealybugs, soft scales and whiteflies. Additionally, it is a fungicide that suppresses powdery and downy mildew.

Thorough coverage of canopies and flowers, whether it’s through adding a surfactant, higher spray volume or adjusting the spray nozzles, is critical to achieving good efficacy. Some folks go the extra mile to achieve greater contact between thrips and insecticide. For example, some apply in early morning or late afternoon, when thrips are known to be more active. Others add liquid sugar or other stimulants or irritants to the spray solution. I haven’t seen any evidence supportive of this approach. In fact, research by Dr. Raymond Cloyd of Kansas State University has suggested that adding Mountain Dew or brown sugar does not increase efficacy.

I provided the IRAC, or the Insecticide Resistance Action Committee, numbers of insecticides to help in developing a rotation program. Thrips develop resistance quickly, making rotation essential. Every insecticide program should include different modes of action or IRAC numbers. I recommend that you include products from at least four modes of action (more products if you have an intensive spray program), and products should change to a different IRAC number after each spray or every other spray. I have provided several examples of rotation programs (Table 1).

Most insecticides — even systemics — are ineffective against thrips pupae in the media. To break the life cycle, you will need to release soil-dwelling predatory mites and rove beetles, or drench biopesticides, such as Steinernema feltiae, Beauveria bassiana, Metarhizium brunneum and Isaria fumosorosea, to the media. Efforts to control thrips pupae should be done in conjunction with control of nymphs and adults in the canopy.

Selection of compatible insecticides is an important consideration for growers with active biological control programs. It is mind-blowing how complex the planning process can get once you consider the compatibility profiles of all insecticides with all biological control agents. The major biocontrol agent suppliers maintain their own pesticide side effect or compatibility databases. Discuss with your preferred biocontrol agent and pesticide suppliers and develop an integrated pest management program for your operation.

This article appeared in the April 2025 issue of Greenhouse Management magazine.

JC Chong is technical development manager for ornamentals at SePRO Corporation. sepro.com