How to Spot, Stop, and Prevent Thrips in Your Greenhouse

Greenhouses are uniquely vulnerable to thrips. Stable temperatures let them breed year-round, continuous plant material gives them an endless food source, and limited natural enemies mean nothing keeps their numbers in check. With so many variables working against them, growers often underestimate how quickly a single overlooked thrips can spiral into a full-on infestation.

“Thrips’ morphology is very cryptic and difficult to detect,” says Muhammad “Zee” Ahmed, Ph.D., assistant professor of turf and ornamental entomology at Clemson University. “Let’s say thrips enter your greenhouse today. You’re not going to know for another two weeks, until they finish another generation and really start causing damage.”

That’s what makes proactive pest management critical—if growers can prevent one thrips from entering the greenhouse, they can avoid an infestation of 40,000.

Preventative Action Is Key

If growers are working with imported plant material from another greenhouse, they have no way of knowing what their risks are.

“Growers have to have the assumption that anything coming inside the greenhouse already has thrips,” says Samantha Willden, Ph.D., assistant professor of entomology at Cornell University. “That’s the safest way to get in front of prevention—just assuming the plant material isn’t clean.”

An effective scouting program will scout for bigger, more mature insects. But since most thrips species lay their eggs in the plant tissue, larvae are especially difficult to identify. So even if a plant looks clean, it could very well be infected.

For a free guide on successful thrips management, check out the OHP Thrips Cocktail. (pdf)

Here are Dr. Willden’s recommended actions for growers before they plant foreign plant material in their facility:

• Dipping: Submerging plants in insecticidal soap or horticultural oils for unrooted cuttings or biopesticides. “This is a great way to reduce that incoming population and install your beneficial program,” she says.
• Screening: Using mesh netting to block thrips from entering the greenhouse and affecting rooted crops. “Growers either love screening or they hate it because there is a direct tradeoff with airflow,” Dr. Willden explains.
• Compartmentalization: Separating young plants from old plants in the greenhouse. “Your old plants will have thrips on them, and you want to keep the young ones clean,” she says. “Dip your new cuttings into the insecticidal soap and place them in a different area of your greenhouse—this helps prevent thrips movement between the plants.”

These tips can help growers prevent an infestation from happening in the first place. However, a thrips infestation can go undetected for weeks, likely longer if growers don’t know the warning signs to watch for.

Identifying the Warning Signs

Since they’re miniscule in size, identifying thrips is difficult for growers unless they place sticky cards around their facility. However, sticky cards will only catch mature thrips, which means they’ve already laid eggs.

“Sticky cards are useful for detecting the early arrival of thrips,” Dr. Willden says. “The larvae look similar, but it’s easy to identify the species of the adult thrips you’ll catch on sticky cards.”

If growers don’t have sticky cards and use their plants as indicators, the damage to the plant will be clear and specific to thrips.

“Thrips have a very distinct mouth that leaves scratch marks on the leaves of the plant,” Dr. Ahmed says. “They aren’t sucking the sap out of the leaf like other pests do. They’re scraping the plant before eating it.”

These scratch marks are also known as “silvering,” as the action scrapes the top part of the leaf back. When growers catch adult thrips on sticky cards and find silvering on the leaves of their plants, they’re almost certainly dealing with a thrips infestation. But not every infestation carries equal risk. How urgently a grower needs to respond ultimately depends on what they’re growing.

Considering the Susceptibility of Certain Crops

In a broad sense, thrips are polyphagous—meaning they feed on just about anything.

“They are a bigger pest compared to other insect pests because they don’t have a preference,” Dr. Ahmed says. “If there’s a crop, they will eat it, and they usually prefer ornamentals.”

For a free guide on successful thrips management, check out the OHP Thrips Cocktail. (pdf)

While no grower wants to see pests in their greenhouse, they’re an even bigger problem for ornamental plants. Garden center shoppers don’t want to see any visible damage to their crops. “They have a zero pest tolerance,” he adds. “If you’re buying a plant for your grandma or a family member and they see a small scratch on it, they’re probably not going to like it.”

Dr. Ahmed says thrips specifically target roses, hibiscus, and gardenias. While flower crops are susceptible to thrips, food crops aren’t entirely off the hook—their growers just have a little bit more leeway.

Dr. Willden works closely with tomato growers, and having a certain amount of pests is acceptable as long as their yields aren’t reduced and the fruit doesn’t have any major damage.

According to her, the biggest thing tomato growers should be concerned about is gold flecking, or patches of yellow that make the peel look dull, and depressions in the fruit, as thrips like to feed in protected areas.

“If two fruits are touching, thrips are likely to feed there,” she says. “They also like to feed in the depression where the stem meets the top of the fruit.”

If a grower spots the warning signs of thrips even after taking preventative action, they need to scout their greenhouse thoroughly.

Developing a Quality Scouting Program

When Dr. Ahmed’s students scout for thrips in the greenhouse, they follow a consistent three-step process:

• Identify the hot spot. Pinpoint where thrips are entering the greenhouse first, since that’s where infestations typically take hold.
• Increase scouting frequency. This is labor-intensive, but it matters. If growers don’t have the bandwidth for more frequent in-person checks, adding more sticky cards around the facility is a good substitute.
• Determine the most effective pest management plan. Once thrips are identified, growers must decide if pesticides or biological controls are the right solution.

Dr. Willden offers a few additional tips for building a quality scouting program.

• Invest in a microscope. A microscope will tell growers if they have thrips or another pest. It will also help them diagnose what kind of thrips they have so they can design their management plan more effectively.
• Tap your plants. Growers can tap their plant over a white plate and see what comes off of it. This is an effective method for catching mature thrips.
• Wash your plants. Growers take the plant tissue, shake it up in an alcohol solution, then pour it into a strainer with a coffee filter. This is an effective method for identifying thrips at every life stage.

Whether a grower is tending to ornamentals or food crops, the strategy stays the same: assume thrips are already present, scout consistently, and act before the population outpaces a greenhouse’s natural defenses. Dr. Ahmed and Dr. Willden agree that the growers who are best equipped to handle thrips don’t eliminate every risk factor, they catch the problem early enough for it to become manageable.

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Tags: biosolutions, greenhouse, nursery, thrips