About Thrips

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Thrips go by many other names, including cornlice, stormflies and thunderflies. Whatever you call the tiny flying insects, it's easy to miss them when you're checking your plants for pests. With more than 5,000 species, you can bet that not all thrips are bad. Some species only eat pollen or fungal spores, while others are beneficial insects that prey on other garden pests.

Thrips ID

All thrips species share common characteristics, including long, narrow wings with feathery fringes and long, thin bodies less than 1/20 inch long. Pest species also have long feeding tubes they use to pierce plant tissues and suck out the sap.

Most thrips species range in color from transparent white or pale yellow to deep brown or black. They're difficult to see without a magnifying glass, but if you check closely, they look like tiny, thin threads or small, dark slivers moving about on plant tissue.

Two of the most problematic thrips species include onion thrips (Thrips tabaci) and Western flower thrips (Frankliniella occidentalis). Both species congregate in large numbers to feed on various flowering plants as well as fruits and vegetables.

Beneficial species include banded-wing thrips (Aeolothrips spp.), black hunter thrips (Haplothrips mali) and sixspotted thrips (Scolothrips sexmaculatus). Most beneficial thrips feed on pest thrips and mites, but black hunter thrips also eat scale insects. Beneficial thrips seldom form large groups, which is a good way to tell the good bugs from the bad.

Thrips Life Cycle

Thrips hatch from eggs and progress through four larval stages before maturing into adulthood. Adult females typically slit and insert eggs into the buds or leaves of host plants. Most species fall to the ground to pupate and change into adulthood.

Thrips can have up to 15 generations a year in warmer climates because the pests can complete entire life cycles in as little as 14 days. That's why you must treat the pests as soon as you notice them. If you don't, you'll soon have a thrips infestation.

Thrips Damage

Thrips rarely inflict severe damage to shrubs or trees, but vegetables and flowering plants can suffer serious injury if the pests swarm them by the hundreds. Fortunately, thrips don't fly well, so the damage often shows up on one plant and then slowly spreads to other plants in your garden.

Thrips feed by puncturing tender plant tissue and slurping out the liquid. This activity can cause stunted plant growth. Affected leaves might look stippled, misshapen or papery, and the leaves often fall prematurely. Most thrips-damaged foliage takes on a silvery hue, but some look dull or dusty. The undersides of affected leaves also have shiny specks of black excrement on them.

If thrips feed on unopened buds, the buds might not open or the emerging petals might have darkened, bleached out or silvery spots or brown edges. Affected flowers also tend to wither early. Affected fruit frequently develops pits. Even worse, thrips can spread tomato spotted wilt virus to various crops in your garden.

Managing Thrips

Thrips are hard to control due to their small size and because they hide within tiny plant folds and crevices. Following an integrated pest program that combines mechanical and cultural practices with low-toxicity pesticides often offers the best results.

Mechanical and Cultural Control Methods

Using barriers such as floating row covers excludes thrips from plants while allowing light, rain and air to penetrate.

Physically remove the infested or injured plant parts and toss the debris in a covered garbage can.

Squirt infested foliage with a strong stream of water from a garden hose to rinse thrips off plants.

When using a hose for pests, make sure you get the undersides of the leaves.
When using a hose for pests, make sure you get the undersides of the leaves. (Image: Comstock Images/Stockbyte/Getty Images)

Controlling weeds and tall grasses near gardens and flowerbeds helps reduce the numbers of pesky thrips.

Placing aluminum foil mulches around plants disorients the pests so they can't find plants.

Chemical Control

Dust insecticides are easy-to-use, low-toxicity options. Just make sure the pests are on the plants before you treat them. Thrips damage frequently isn't noticeable until the insects have already fed and left. Make sure you have thrips by shaking flowers or leaves over a clean sheet of white paper and looking for tiny dark specks moving about the paper.

Diatomaceous Earth

Diatomaceous earth is dust made from microscopic diatoms, or one-celled plants fossils. DE works by absorbing the insect's body fluids, which causes it to dry out and die. Although it kills insects, DE won't harm earthworms, beneficial soil microorganisms, people, pets, plants or the environment.

Make barriers by using a duster to sprinkle DE around trees or dust entire plants starting from the ground up. If you prefer spraying over dusting, mix 1/4 cup of DE dust in 1 gallon of water and pour it into a garden sprayer. Use the garden sprayer to coat target plants. Once the solution dries, it has the same dehydrating abilities as dry DE dust.

Tip

    • Dust late in the evening so you won't harm beneficial insects. 
    • Dust with DE labeled for use as an insecticide. Don't dust plants with DE designed to use in pool filters or other uses because those products may contain chemicals.    

Warning

  • DE dust is abrasive and can harm your lungs if you inhale it. Wear a face mask when handling this product.

Sulfur

A natural element and one of the oldest known pesticides, sulfur dust can effectively control thrips. Use a handheld duster or a plastic squeeze bottle to completely coat affected plant surfaces.

If you prefer to make a wet application, follow the instructions on your sulfur product's label. One product recommends making a solution containing 4 tablespoons of sulfur for every 1 gallon of water. Use a small sprayer to completely coat plant surfaces with the solution, including the tops and undersides of leaves.

Tip

  • Most insecticidal sulfur products are labeled for use on fruits and vegetables, but carefully read the label to make sure your plants are listed.

Warning

    • Sulfur dust can harm plants if you use it when it's above 90 degree Fahrenheit.    
    • Avoid using sulfur within 20 to 30 days of an oil spray or you increase the risk of a chemical reaction on the plant leaves. 
    • Sulfur might be nontoxic to people and animals, but it can still irritate your skin and eyes. Wear protective clothing, goggles and waterproof gloves when dusting or spraying sulfur insecticides. 
    • Keep people and pets off treated areas for at least 24 hours after application.

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