What Are Thrips?

Also known as thunder flies, thrips are tiny, sap-sucking insects in the Thysanoptera order. There are thousands of thrip species, and many types benefit gardens by pollinating plants or preying on pests. However, some thrips, including rose thrips, onion thrips, and western flower thrips, damage plants. They can weaken vegetables and flowers and even spread plant diseases.

What Do Thrips Look Like?

Thrips are most active in summer, showing up in shades of brown, black, and white. They often hide on the undersides of leaves. While some thrips lack wings, most have fringed wings and slender, tapering bodies typically less than 1/16 inch long. Though they are not strong fliers, thrips are carried by the wind and can overwinter in soil, leading to persistent problems if not managed.

Identifying Thrip Damage on Plants

Many types of thrips are specialist feeders that only target specific types of plants. However, some thrips are generalists that are equally attracted to vegetables, ornamentals, herbs, and houseplants. These pests commonly affect food crops like asparagus, tomatoes, and beans, but they also plague most houseplants and ornamentals, such as carnations, chrysanthemums, gladiolus, and pansies.

If you suspect thrips are damaging your plants, place a sheet of white paper beneath the affected plant leaves and give the plant a good shake. This should dislodge any thrips (and any other small pests) onto the paper and make them easy to spot.

Thrip damage is often confused with aphid issues and plant nutrient deficiencies. Looking for these signs and symptoms can help you identify thrip problems before they get out of hand:

• Yellow or brown stippling or streaking on plant leaves
• Plant leaves with a silvery sheen or a bleached look
• Misshapen fruit, flowers, and leaves
• Stunted plant growth
• Black speckles of frass on leaves
• Plants covered in a sticky “honeydew” residue. Dark, sooty mold occasionally develops on top of the sticky honeydew.
• Plant diseases, like wilt. These diseases can be transmitted by thrips when they feed.

Preventing Thrip Infestations

It's easier to prevent thrips in your garden and home than to deal with an infestation. This approach also reduces pesticide use and limits disease spread, such as wilt. Many prevention strategies for thrips can also help keep away other common pests like aphids and spider mites.

1. Inspect New Plants

The number one way to prevent indoor thrips is to inspect and quarantine new plants before you bring them into your home or greenhouse. Thrips can also hitch a ride indoors on houseplants that are kept outside during summer, as well as cut flowers and fresh produce, so you might want to inspect these items too and spray them with an organic soap spray as a preventative.

2. Maintain Healthy Plants

Healthy plants repel pests much better than sick and stressed plants, so make sure to provide plants with the proper amounts of light, water, and fertilizer. Take care not to use too much fertilizer, though, as overfertilizing can cause plants to produce new, tender growth, which attracts thrips.

3. Stay on Top of Weeds

Thrips use certain weeds as host plants for their eggs. Keep weedy areas of your yard cut back to deprive them of a place to call home.

4. Clean Up Your Garden

Keep plant diseases and pests from spreading by cleaning up and destroying infested plant material. It’s usually best to burn infested plant debris or bag it and throw it in the trash because thrips can survive the composting process.

5. Try Companion Planting

Basil and other strongly scented herbs naturally repel thrips, while flowering plants, like yarrow, dill, and parsley, attract ladybugs and other thrip predators. Growing these plant companions near crops that thrips feed on can keep your garden naturally pest-free.

6. Install Row Covers

If you have an ongoing issue with thrips, install finely woven row covers over vulnerable crops at the beginning of the season. This should keep thrips from laying eggs on your plants, and you can remove the covers later to admit pollinators.

7. Prune at the Right Time

Newly pruned plants are vulnerable to thrip damage, but you can avoid thrip issues by pruning plants when thrips aren’t active. Although different plants need to be pruned at different times of the year, winter pruning is usually the best option for avoiding thrips.

8. Choose Resistant Plants

Growing plants that are naturally less appealing to thrips can make your garden easier to keep. For instance, western flower thrips are known to love pale roses with strong fragrances, but they aren’t as interested in scent-free, dark roses that produce tight flower buds.

9. Use Reflective Mulches

Reflective mulches applied around your garden help conceal plants from thrips and other pests. For a budget-friendly alternative, place sheets of aluminum foil beneath your plants to “foil” thrips.

10. Add Sticky Traps

Blue sticky traps placed strategically around houseplants and greenhouses can help you identify thrip problems early on. Keep in mind that these traps work best as a preventative, not a treatment, and they can harm pollinators if you use them outdoors.

How to Get Rid of Thrips

Despite our best efforts, thrips can still sometimes occur on indoor and outdoor plants. When problems arise, don’t stress or reach for synthetic pesticides. Many pesticide products affect pollinators, and they can also make pest problems worse by harming the beneficial bugs that prey on thrips.

Instead of using pesticides:

• Treat affected plants by washing them in a sink or with a garden hose. As you work, rinse the tops and bottoms of infested leaves and along the plant stems where the leaves attach.
• Scrape away the top layer of soil around potted plants and prune away and destroy heavily infested leaves.
• Release predatory insects like lacewings or ladybugs on outdoor or greenhouse plants.
• Spray the plants with organic soap or neem oil. Just remember that sprays can harm pollinators and other beneficial insects, so don’t apply them to the flowers or in conjunction with predatory insects. Sprays should only be used in the evening to avoid issues with sunscald, and they may need to be reapplied several times at 7- to 10-day intervals to treat adult thrips and any nymphs that hatch after the initial treatment.