Onion Thrips

Names can be misleading. While it’s true that onion thrips feed on onions, they also eat asparagus, beets, broccoli, cabbage, cauliflower, cucumbers, garlic, leeks, melons, potatoes, pumpkins, strawberries, and zucchini, along with several other fruiting and ornamental plants. If that weren’t trouble enough, they can carry serious plant diseases, too.

Also known as potato thrips, tobacco thrips, and cotton seedling thrips, onion thrips (Thrips tabaci) hide in many herbaceous ornamentals, tucked into cramped quarters where they are easy to overlook.

​Onion thrips description

Like other thrips, these garden pests are very small. Maxing out at 0.05 inches (1.3 mm), you could fit 13 or 14 of them end-to-end across an American dime. ​

Onion thrips (Alton N. Sparks, Jr., UGA, Bugwood.org) CC BY 3.0

Adult onion thrips can be yellow to dark brown and sometimes black. Most of them are female. Using a hand lens, you can see that their wings have a single, central vein and long hairs. Eggs are white at first but turn orange a few days later, just before hatching. Larvae are also yellow to brown.

Onion thrips second instar, leaf damage, and frass (Whitney Cranshaw, CSU, Bugwood.org) CC BY 3.0 US

Onion thrips lifecycle

Females use an ovipositor to cut into leaf tissues. Then she tucks her eggs in, under the epidermis, of up to 80 leaves. Less than a week later, those eggs hatch and start sucking the sap from those and neighboring leaves. There are two larval stages during which a lot of sap is eaten. Larvae then stop eating and go through prepupal and pupal stages. Adults generally only live for two or three weeks, but they can overwinter in legumes, grains, weeds, and plant debris.

Damage caused by onion thrips

To get at the sap they love, thrips use their raspy, piercing mouthparts to shred and penetrate the surface of predominantly young leaves and flower petals, breaking open plant cells and drinking their fill.

Onion thrips feeding damage on onion (Scot Nelson) Public Domain

Since they are feeding on young parts that continue to grow, the damaged areas become elongated, silvery streaks. You may also see leaf curling, stippling, scarring, stunting, leaf tip browning, white patches, and smaller crop sizes. On cauliflower, you may see brown streaks on curds. Damaged areas of leaves cannot perform photosynthesis, injured areas lose water faster than normal, and provide points of entry for other pests and diseases, such as bacterial rot. But onion thrips can infect your plants with a variety of diseases simply by feeding.

Onion thrips are vectors (carriers) for Impatiens necrotic spot virus (INSV), Iris yellow spot virus (IYSV), purple blotch, strawberry necrotic shock virus, tobacco streak virus, and tomato spotted wilt virus (TSWV).

Onion thrips management

Onion thrips are easily dislodged with a garden hose, but that won’t fix the problem. There are some cultural practices you can adopt that will help reduce onion thrips problems. First, clean up plant debris at the end of each growing season. When planting, only use certified pest- and disease-free seedlings, seeds, and onion sets. Monitor the areas surrounding your garden for signs of onion thrips and other pests, and remove volunteer plants that may harbor pests and diseases.

Onion thrips nymphs in new growth, between onion leaves (Whitney Cranshaw, CSU, Bugwood.org) CC BY 3.0 US

Beneficial insects, such as lacewing larvae, pirate bugs, and predatory thrips will help in the fight against onion thrips, so avoid using broad-spectrum pesticides and insecticides.

Since healthy plants are better able to protect themselves against onion thrips, water and feed your plants with that goal in mind and get your soil tested by a lab. You’ll be glad you did.