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Garden Help Desk: What are the best ways to control thrips?

By USU Extension - | Jun 10, 2023

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Thrips can cause damage to flowers and other ornamental plants.

What are some natural (non-toxic) options to get rid of what I believe to be thrips from my green bean plants?

Thrips are an annual problem in many local gardens. They’re attracted to many of our favorite vegetables and flowers and their feeding can reduce vegetable yields and disfigure blossoms and buds. There are several insecticides that are effective for controlling thrips, including a few options that are safer for people, pets, and beneficial insects. We often refer to those insecticides as soft options.

Insecticidal soap is one soft option for thrips and other soft-bodied insects like aphids, mealybug, whitefly, scale crawlers, etc. It’s also effective for spider mites. Insecticidal soap isn’t a poison; it works by disrupting the cuticle, the outer layer on the pest’s body, causing the insect or mite to dehydrate and die. Insecticidal soap works on contact and there isn’t a residual effect on insects or mites that encounter the spray after it has dried. Purchase an insecticidal soap that is made for use on plants; it’s a safer option than using a homemade spray.

A horticultural oil spray would also be effective. Horticultural oil is used as a 2% spray in the early spring to suffocate overwintering insect eggs and mites before trees and shrubs have leafed out. During the growing season while there are leaves on trees, shrubs, flowers, and vegetables the oil needs to be diluted to 1% to avoid damaging the foliage. Horticultural oils work on contact but doesn’t have any residual effect.

Good coverage is important when using insecticidal soap or horticultural oil because these sprays only work on contact. You’ll want to spray the upper and lower surfaces of the leaves on your plants.

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Thrips can damage a wide variety of plants, like these onions with damage on their leaves.

A spray product with the active ingredient Spinosad is another soft option for thrips control. It’s an organic product made from a soil bacterium. Spinosad affects pest nervous systems on contact or after eating sprayed plants.

Not every insecticide is effective against all stages of an insect’s life, so these sprays need to be repeated once every several days or as often as the label recommends for a few weeks to break the cycle between egg, nymph, and adult.

Any spray with an oil or soap component can burn foliage if the spray is wet when temperatures are high. Whatever you choose to use, it’s safest to do your spraying in the evening to give the spray all night to dry. Evening spraying is also a good practice with Spinosad. It’s an organic product, derived from a naturally occurring soil bacteria, but it is toxic to bees while it’s wet. Once Spinosad has dried, it’s safe for bees. Spraying in the evening when bees are done foraging for the day means the Spinosad will be safe for bees by morning.

I have several large metal planters that I’d like to use for vegetables this season. Filling them up with dirt is a bit unreasonable…. they are 30 inches tall. When I use larger planters for flowers, I will put packing peanuts in the bottom as a filler. Is this ok to do for vegetables as well? I feel hesitant because it seems like some chemicals may leech out of the peanuts into the vegetables that I am growing to eat. Any evidence for that?

I couldn’t find any conclusive, research-based information about using packing peanuts in vegetable containers, but there is some concern that certain kinds of packing peanuts can leach chemicals into the soil. There’s no reason why you can’t use packing peanuts in your ornamental container gardens, though. Of course, biodegradable packing peanuts won’t work.

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Tall containers are prone to tipping if they are top heavy from using too much lightweight filler in the bottom.

I think a better option is to use empty plastic water bottles, soda bottles, and juice bottles. With the lids on, of course! Small, empty nursery pots will also work. Place enough bottles or pots in the bottom of your container to fill the bottom 1/3 of the container. Containers that are very tall, or narrower at the bottom may be top heavy if you use too many empty bottles or pots, so check to make sure your container will be stable.

You used the word “dirt” in your message, but I’m sure you were referring to potting mix, not garden soil. Garden soil should stay in the garden where it can drain properly, which it can’t do in a container.

Filling a large or deep container with potting soil can be expensive. Adding something like these empty plastic bottles to the bottom of a container can reduce the amount of soil needed.

Empty inverted nursery pots can be used to fill the bottom of a large container.


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