Any gardener or houseplant enthusiast understands the frustration of plant damage that seems to appear out of nowhere. Spots and leaf damage on plants may be the result of minuscule insects called thrips. These bugs are common pests that affect farms and home gardens. They aren’t harmful to humans, but they can damage plants and reduce fruit and vegetable yields.
What Are Thrips?
Thrips are tiny insects that can damage plants. Most thrips are less than 1/20 of an inch long, so they’re not easy to spot with the naked eye. Instead of spotting the thrips themselves, you’re more likely to notice the damage they cause to both indoor and outdoor plants.
Female thrips can lay eggs with or without mating, a process known as parthenogenesis. Thrips hatch from eggs in a larval form and continue to mature. The reproduction process is very fast, so multiple generations can hatch in a single season.
What Do Thrips Look Like?
The larvae of thrips are long and slender and lack wings, though they can feed as larvae. They graduate to non-feeding prepupae and pupae phases before developing into adults. Adults are long and slim, slender, and have two pairs of wings with long fringes on the edges. The time to develop from egg to adult can be as short as two weeks in ideal conditions.
What Do Thrips Eat?
Thrips feed on plant or fungal tissue by piercing the outer layer of the organism and sucking out the contents of the affected cells. After they finish feeding, the leaves of plants may be marked with stippling, discolored flecks, or silvering on the leaf surface. Thrips damage can result in scars on leaf, flower, and fruit surfaces and distortion of plant parts. They also leave behind traces of their feces, which look like black spots on the leaf. In addition, thrips can spread diseases between plants.
Types of Thrips
There are more than 7,000 varieties of thrips around the world. Not all species are considered pests. Some varieties of thrips feed on fungal materials, leaf litter, debris, or other small arthropods. Other species are considered beneficial predators and feed on other thrips, aphids, mites, and whiteflies.
In the United States, pest thrips and plants they are most likely to damage include:
- Avocado thrips: avocado
- Bean thrips: beans and legumes
- Citrus thrips: citrus and blueberries
- Cuban laurel thrips: laurel fig and Indian laurel
- Greenhouse thrips: avocado, azalea, hypericum, laurel (English and Grecian), photinia, and rhododendron
- Onion thrips: garlic, onion, and pepper
- Western flower thrips: herbaceous ornamentals such as impatiens and petunia; vegetables such as cucumbers, pumpkins, and peppers; fruits such as grapes and strawberries; and shrubs and trees such as roses and stone fruits.
Where Do Thrips Live?
Thrips are found all over the world and in every region of the United States. Thrips can move around on plants as they’re being shipped. If they find a suitable host in their new location, they can easily reproduce and populate the area.
Signs You Have Thrips
If you have thrips in your garden or houseplants, you’re most likely to notice damage to foliage, flowers, or fruit. You may see discoloration, withering, or pitting on the plants. If you look closely, you may be able to see the insects themselves. Thrips are not good at flying, so their spread between plants may be slow.
Do Thrips Fly?
Thrips don't fly, but they can move from place to place. Your plants may be exposed to thrips before you even bring them home. Thrips can transfer between plants during shipping or in a garden center. In addition, thrips can move from the outdoors to the indoors on plants or in soil that gets tracked indoors.
Do Thrips Bite?
Thrips don’t pose any health risks to people or animals. They don’t bite or sting. The damage thrips do to plants doesn’t affect the safety of the fruit or vegetable for eating. However, thrips can transfer plant-specific viruses between plants. In particular, thrips are known to spread Tomato Spotted Wilt Virus and Impatiens Necrotic Spot Virus, both of which kill plants and reduce crop yields.
How to Get Rid of Thrips
If you have thrips in your garden or houseplants, you will need to use multiple methods to control them. Thrips cluster in large numbers and reproduce quickly, so they can be a persistent problem. You can try a combination of chemical and natural treatments to remove and prevent infections of thrips.
- Spray thrips away: You can remove thrips from garden plants with a forceful spray of cold water. Adjust a hose nozzle to a fine spray of water, and wash off the undersides of affected leaves. Repeat the process three times, ideally for three days in a row.
- Insecticidal soaps: You can use insecticidal soap sprays to wash away thrips. Follow the label instructions for the soap. You may need multiple applications to control thrips.
- Water plants regularly: Thrips prefer a dry environment, so adequate watering can discourage them. Keep outdoor plants watered and keep indoor plants adequately misted.
- Trim infested plants: Prune away any plants infested with thrips. Destroy the clippings, and do not use them for compost.
- Reflective surfaces: Thrips are easily disoriented, so surrounding plants with aluminum foil and other disorienting mulches can reduce infestations.
- Horticultural oils: Horticultural oil sprays suffocate the insects. Make sure you are buying oils that won’t damage your plants as well.
- Chemical pesticides: You can use chemical pesticides to control thrips. Insect control sprays that contain pyrethrins, acephate, bifenthrin, imidacloprid, or spinosad can be effective against thrips. You will need at least two applications, seven to 10 days apart. Follow the safety guidelines on the label of any pesticide. You may need to keep children or pets away from the affected area immediately after applying chemicals.