Greenhouse Pest Problem Solved with Donna Balzer

Posted on August 9, 2021 | Categories: Donna Balzer

Like something out of a horror film, the army of purchased insects sense the trouble-makers and bite their head off as they emerge…. And just like that, my greenhouse problem is solved.

In the Beginning

Speckled leaves on my strawberries make me think of spider mites, but I am wrong. When I tap a leaf with tell-tale speckled leaves, I catch three white immature thrips on a piece of paper.

Leaf Stippling

Strawberry leaf stippling due to thrips

Young thrips are whitish, with thread-like wings. I identify these hard-to-see insects with my iPhone close-up lens. It’s a bad photo but good enough to see the troublemakers.

Thrips move slowly, crawling or sometimes hopping but never really flying. They fall easily on the piece of paper I hold below the leaf. Whiteflies are a brighter white and they fly quickly as soon as a leaf is touched. I do not have white flies.

My immature thrips are the size of a 12-point Helvetica lower case “e” on the recycled paper I use to catch them and they are a serious bother on my strawberry plants. (see this video to help learn the life-cycle of this pest: https://www.youtube.com/watch?v=Yp2zXV0f-cQ)

Size of a thrip

Size of a thrip

As well as speckled leaves, thrips cause distorted fruits on strawberries and weirdness on blooms. I have all the signs but have not considered thrips, thinking my problem is spider mites.

Strawberries affected by thrips

Strawberries affected by thrips

As thrips pierce leaves, flowers and fruit, they cause problems. They take nutrients from leaves, causing unsightly speckling damage. Their eating distorts flowers and fruits of strawberries. While thrips nibble cucumber flowers and pollen, they cause curling in the baby fruits and the fruits stay curled as they mature.

According to Applied Bio-nomics, a biological predator, A. cucumeris, controls thrips because “they can sense the [immature] thrips emerging from the leaf tissue and bite their heads off.”

Cucumber affected by thrips

Cucumber affected by thrips

Why I thought I had spider mites:

As the greenhouse heats in summer the air dries out and populations of almost invisible pests like two-spotted spider mites explode. The first sign of mites is stippled leaves.* Once you see webs on the back of leaves, it is only possible to treat the problem with relatively large biological controls. So as soon as I see tell-tale leaf-stippling, I assume I have spider mites and I order P. persimilis .

Low humidity in the August attracts spider mites. They are also carried into the greenhouse on a breeze or dragged around the garden and greenhouse on my jeans or the fur of my dog, Corle. When I see speckles on my strawberry leaves the first thing I do is boost humidity by watering the paths and walls of the greenhouse at least once a day. Next, I order Persimilis spider mite predator. But the speckling persists.

P. Persimilis - Spider mite eliminator

P. Persimilis – Spider mite eliminator

A. fallacis are another predator option for two-spotted spider mites. The “trouble” is they can’t work around webbing so must also be brought in early if there is a problem. Luckily, I don’t have webbing yet, only speckled leaves. Again, after the introduction of A. fallacies the speckling persists. Finally, I placed the piece of paper beneath a speckled leaf and three thrips fall on the paper. Too bad I didn’t think of this first.

Because thrips have developed resistance to most registered pesticides, and because most gardeners want to stay away from sprays, biological controls are the best way to control these pests in the greenhouse. But it helps to know which pest you have before you buy bugs and the biological control agent for thrips is another predatory mite, A. cucumeris.

The advantage of a larger greenhouse:

In large commercial greenhouses systems humidity is higher by design and soil is changed out with every crop because most commercial systems are hydroponic.

In our smaller home greenhouses, it is the tougher to control humidity automatically and, unless you change the soil in your pots every time you change a crop there is a chance insects will overwinter in soil. The good news is that good bugs like A. Fallacis and Stratiolaelaps scimitus overwinter in the soil in the greenhouse and appear as soon as spider mites appear, making it a no-brainer to include the addition of these beneficial insects in your program.


In large commercial greenhouses, where humidity is high, predatory mites are able to outcompete spider mites. And insects like thrips, are easy to control once you know what you have, and you buy the right predator.

When I finally discover I have thrips- caused spots on my strawberry leaves, I am relieved. And in good news, A. cucumeris is easy to use. Just open the package and sprinkle the vermiculite on affected leaves.

I know it is impossible and impractical to eliminate or prevent all insects from entering the home greenhouse. Cleaning up old leaves as they drop on the ground is one way to remove thrips in their later stages. “Scouting” for problems using blue and yellow sticky traps is another easy step.

Yellow sticky trap with fungus gnats. A blue sticky trap is more likely to catch thrips

Yellow sticky trap with fungus gnats. A blue sticky trap is more likely to catch thrips

I aim for general control, and never look to completely eliminate all insect life because in nature everything plays a role and everything is connected.  And ironically, sprays are more likely to kill off beneficial insects than insects pests so I never resort to sprays like soap or commercial chemicals. The biologicals are just too sensitive and I hate to risk killing my allies.

The funny thing is that after 40 years of gardening I am still learning new things. I hope you are too.

*“Spider mites lack chewing or piercing-sucking mouthparts. They use a pair of needle-like stylets to rupture leaf cells and then push their mouth into the torn tissue to drink the cell sap. Small groups of cells are killed, which results in a stippling or speckling on the upper leaf surface.”

For more great tips from Donna, visit www.donnabalzer.com.

You can also read Donna’s gardening books: No Guff Vegetable Gardening with Steven Biggs and her just released Gardener’s Gratitude Journal:  Part Diary, Part Personal Growing Guide.

donna balzer