How to Spot, Stop, and Prevent Spider Mites in Your Greenhouse

Since greenhouses remove the mortality factors that would suppress spider mites, growers are especially vulnerable to an infestation. These factors include predators, extreme temperatures, and volatile weather conditions.

“Greenhouses have suitable temperatures year-round and often ideal temperatures for the growth rate of insects,” says Laura Ingwell, Ph.D., associate professor of entomology at Purdue University. “Growers create a perfect mesocosm of a well-fed, ideal temperature food source that’s never being interrupted by predators or rain—spider mites flourish in that environment.”

Because growers control their environment so precisely, they often underestimate the conditions they create. There aren’t predators, temperature swings, or rain to keep spider mite populations in check.

Every grower could experience an infestation if they don’t take precautionary measures. Even if they do, they must know how to respond.

Considering a Greenhouse’s Vulnerability

The types of plants grown in a greenhouse have a direct correlation to spider mite infestations. This is true for both crop type and how long it’s been in the greenhouse.

“The longer a plant is in the greenhouse, the more likely it is to accumulate pests,” Dr. Ingwell says. “So your highest area of risk in the greenhouse is the older plants that are harboring mites.”

To reduce the risk of harbored mites spreading, Dr. Ingwell suggests considering the spatial organization of plants and the movement between older plants and plant starts. Movement especially, she says, is how pests tend to spread.

“Points of entry are usually going to be the origin of an infestation,” she explains. “So greenhouse growers should think strategically about the organization of their plants.”

Dr. Ingwell offers three specific recommendations for greenhouse growers:

1. Have protocols in place at the point of entry. This helps reduce the risk of workers bringing in a spider mite on their clothing, which is also known as a hitchhiker.
2. Consider how workers are moving through the greenhouse. Ensure your workers are going to the seedlings and young plants first, then go to the older plants last.
3. Be mindful of the crops you’re growing. While spider mites feed on thousands of host plants, there’s evidence that some crops are preferred and more likely to be colonized than others.

Growers cultivating susceptible crops should prioritize early detection and proactive prevention. That starts with knowing which crops are most at risk. To see how you can control spider mites check out this helpful guide.

Crops to Monitor Closely

Food crops are much more likely to attract spider mites, and it’s difficult to prevent them from entering the greenhouse altogether.

“Food crops have to be grown for a long time in a greenhouse for flowering and fruiting,” she explains. “I think cucumbers are at the top of the list for susceptible greenhouse crops. If you’re growing cucumbers in a greenhouse, you’re going to have spider mites.”

Cucumbers and tomatoes are especially vulnerable to mites because they’re vining crops that have thick leaves.

“They penetrate into the leaves of the plant, pulling the juices, photosynthesis, and sugars out of the plant material,” Dr. Ingwell says. “They don’t damage plant cells and they don’t leave holes in the leaf. Their mouth parts are very exact, and they can go in between cell layers to get the exact tissue they want to feed on.”

As time progresses and the spider mites continue to feed on the plant, chlorotic spots will start to appear due to the photosynthate loss in those cells. Experienced growers will notice those spots early on—but newer growers may not notice them until more severe warning signs emerge.
Knowing what to look for and when can be the difference between early intervention and a full infestation.

Spotting Signs of a Spider Mite Infestation

One of Dr. Ingwell’s biggest goals is teaching growers how to identify warning signs sooner. Here are the different infestation signs growers will encounter ranging from the least to most severe.

1. Spotted leaves. Distinct bleach spots on the leaves of the plant.
2. Mottled leaves. Larger bleach spots on the leaves that start to blend together—not distinct individual spots.
3. Webbing on leaves. When spider mite populations are high, webbing will be visible on the leaves of the plant. “Eventually, you will start to see spider mites crawling around on that plant material,” she adds.

To make sure they’re dealing with the two-spotted spider mite, she encourages growers to perform the “white paper test.” This test makes it easy to spot an infestation before the other signs start to appear.

“You hold a white piece of paper under the plant and tap it, which will dislodge the mites that are harbored in the plant,” she says. “Spider mites are almost translucent with two black dots on them. If you have mites, you’ll start to see little black dots crawling around on that sheet of paper.”

The faster a grower acts, the better their chance of containing it.

The Importance of Sanitation

Sanitation is the most effective weapon against a spider mite infestation.

“It’s really about starting with a clean environment,” she says. “That means having no weeds and clean benches in the greenhouse.”

This applies to plant material as well. Dr. Ingwell suggests finding a healthy seed source and ensuring the growing media entering the greenhouse is sterile and clean.

“That could mean physically inspecting all the plants that enter your facility,” she says. “It could also mean proactively releasing a predator to take care of anything that might be residing on those plants before you plant them in the greenhouse.”

But prevention doesn’t stop at planting in a clean greenhouse environment. Growers must maintain clean, sterile surroundings during and between growing cycles. This means removing all plant material from the greenhouse and cleaning as well as disinfecting all structures.

“If you can’t get a sanitizer on the surface, do a heavy hosing down,” Dr. Ingwell explains. “Dislodge them to the ground and wait as long as you can until planting again. This gives them time to starve.”

For Dr. Ingwell, preparedness is the bottom line.

“Growers have a balancing act of all the different things they have to manage in the greenhouse,” she explains. “They need to understand the biology of each potential pest they could encounter on their crops.”

In a greenhouse, the conditions are always right for spider mites. The question is whether a grower is ready for them.

Tags: biosolutions, greenhouse, spider mites