Thrips are one of the more troublesome pests for gardeners because they are hard to spot and the damage they cause often looks more like a nutritional or disease problem, not insect damage. Thrips can affect hundreds of different ornamental and edible plants, and they are extremely resistant to eradication. Thrips are tiny, slender-bodies insects usually about 1/25 inch in length, although some species can be as much as 1/2 inch. They are piercing, sucking insects that inflict damage by feeding on the juices of plants.
Thrips include more than 6,000 species in the Thysanoptera order, of which more than 200 have been identified as problem pests for both indoor and outdoor plants. Most species have fringed wings, and though they move quickly, thrips are poor fliers that are more likely to travel on the wind than by use of their wings. Under a magnifying glass, thrips have a distinctive cigar-shaped body that looks a bit like a worm with legs. They are social insects that are usually found in clusters. Thrips are usually black or yellow-brown, but may have red, black, or white markings.
Because thrips are so tiny, they can be difficult to see until infestations become large. So, one way to identify thrips is to put a blank sheet of white paper beneath the flowers or leaves of the plant and shake the plant. If thrips are present, some will fall off and their darker bodies will be easily seen on the white paper. A 10- to 15-power magnifying glass will help identify the pest.
You also can use sticky traps to capture thrips for monitoring and identification. This will not provide control of the thrips, but it will let you know if a plant is becoming infested. It's best to use specially made blue traps rather than standard yellow traps. Blue traps seem to be more effective for trapping thrips, and they are more easily seen against blue than against yellow.
7 Ways to Get Rid of Thrips
Early detection and integrated pest management (IPM) are the best options for preventing a wide-ranging infestation. This involves some tolerance for minor plant damage, kept in check by selectively pruning and destroying affected plant parts, or by regularly washing plants with blasts of water to dislodge thrips. Thrips are so prevalent that attempting to control them through the use of chemical pesticides is often counterproductive since it kills a wide variety of helpful insects and may cause local thrip populations to develop chemical resistance.
Integrated pest management is defined by the EPA as “...the coordinated use of pest and environmental information with available pest control methods to prevent unacceptable levels of pest damage by the most economical means and with the least possible hazard to people, property, and the environment.” Thrip control, in other words, is an ongoing activity for most gardeners that requires some tolerance for the presence of these stubborn creatures and the damage they cause—the price for an overall balanced and healthy garden ecosystem.
Prune Damage Plants
When you see the telltale stippling or wrinkling of leaves, with the concurrent presence of black fecal spots, it's likely you have identified thrip damage. Immediately prune and destroy the affected leaves. A plant that is badly infected should be removed entirely to prevent thrips from spreading to other plants.
Routine inspection and pruning of thrip-damaged plants are sometimes enough to keep thrip infestations at manageable levels.
Wash Affected Plants
Thrips are tiny insects that are easily dislodged by hard blasts of water. Outdoor plants can be hosed down with water spray to remove thrips. Make sure to focus on the bottoms of leaves, where the insects often cluster. For indoor houseplants, spray or wash the plant with a soap and water solution (about 2 teaspoons detergent in a gallon of water), making sure you get the solution on all the leaves and other areas of the plant.
Washing plants is one method among several to keep thrips under control. Washing alone usually isn't sufficient to control thrips.
Encourage Natural Predators
A variety of natural predators will devour thrips and their eggs—in fact, these predatory insects are so efficient that they are often purchased and introduced deliberately by commercial growers and greenhouse operators. Predatory mites, minute pirate bugs, and lacewings all are well-known predators, and you may be able to purchase these to introduce into your home garden. It may take repeated monthly applications to fully control thrips.
There may also be certain nematode species that will control thrips. Check with your local extension office for information about what species of these microscopic soil worms are effective against thrips in your area.
Another way to encourage natural predators is through the careful, restricted use of pesticides. Rather than doing any widespread spraying of your plants, use chemical pesticides very selectively, or not at all, since they usually kill helpful predatory insects as well as your targeted pests.
Dust Bulbs With Pesticide Powder When Storing
While spraying actively growing plants with chemical pesticides is usually discouraged, it is a very good idea to dust any bulbs that you dig up and store for the winter with insecticidal dust, which will kill any overwintering pests. This is especially helpful for gladiola bulbs, which are notorious for harboring thrips.
Spray With Neem Oil
Neem oil is a natural substance derived from the neem tree. It is an effective natural insecticide against many pests, but unlike synthetic chemical pesticides, it is not devastating to bees and other pollinators. When used for thrips, a good method is to combine four teaspoons neem oil, two teaspoons of white dishwashing liquid, and one gallon of water. Spray directly on affected areas of the plants. Neem oil works by disrupting the feeding and reproduction cycles of the thrips.
Spray With Pyrethrin
Pyrethrin is a natural pesticide derived from the flowers of certain types of chrysanthemum flowers. Spraying plants with two applications of pyrethrin applied three to four days apart will treat serious infestations of thrips.
Make sure to use a pure pyrethrin pesticide, which is considered safe and organic, rather than a pyrethroid pesticide, which includes synthetic chemicals added to increase the effectiveness of the pyrethrin. Pyrethroids are not considered organic pesticides, although they are admittedly safer than other traditional chemical pesticides, such as malathion.
Spray With Chemical Pesticide
A variety of synthetic chemical pesticides are effective against thrips, though some experts do not advise using them on thrips, especially by homeowners. These pesticides are usually not very effective, because of the way thrips feed and reproduce. And thrips will continue to invade throughout the growing season, so a one-time use is unlikely to have long-term effectiveness.
Even commercial food growers recognize the limitations of using strong synthetic chemical pesticides on thrips. These tiny insects often hide in curled leaves and flower petals, where chemicals can't reach them. And chemical pesticides are very likely to kill the predatory insects that are most helpful for controlling thrips. Even more important, thrips quickly develop resistance to chemicals. Homeowners who insist on using chemical pesticides should seek the advice of the local Extension office on recommendations for what pesticides to use and how to rotate their use.
The Insecticide Resistance Action Committee (IRAC) assigns a letter coding system to identify the mode of action of chemical pesticides, and growers are advised to never use pesticides with the same letter code in back-to-back treatments for thrips. For example, if you spray with a pesticide carrying the 3A IRAC code (which identifies it as a pyrethroid pesticide), you should shift to a different code for the next spraying.
But for homeowners, the better advice is to avoid synthetic chemicals altogether and rely on the other methods of integrated best management.
What Causes Thrips?
There are different species of thrips that thrive in almost any environment, evolved to feed on almost all different types of plants. In general, though, thrips are more likely to be a problem in gardens where there are lots of surrounding weeds, which often serve as the host plants from whence garden infestations begin. And gardens with a lot of ground debris are likely to be thrip havens, as the eggs deposited in plant tissues can readily overwinter to hatch as nymphs the following spring.
How to Prevent Thrips
Preventing thrips completely is very difficult, but you can minimize populations by cleaning up plant litter rather than allowing pruned leaves, stems, and deadheaded flowers from lying on the ground over winter. As winter approaches, remove and destroy dead plant stalks to prevent eggs from overwintering.
Where possible, keep your own garden and nearby areas as free of weeds as possible. Many weed species serve as hosts for thrips, which can easily drift into your garden on breezes.
Covering important plants with fine mesh screening may prevent infestations. Thrips travel primarily by drifting on the wind, and covering vegetables, cane fruits, and other vulnerable plants with fine mesh cloth can prevent the insects from gaining a foothold.
How Do I Identify Thrip Damage?
It's important to identify thrips and the damage they cause in order to properly treat the problem. Many species feed within the plant buds or curled leaves, so they can be very difficult to detect. The first sign of thrips is usually yellow or bleached spots on leaves, deformed leaves, or dead blotches on flower petals. Next, the leaves are likely to take on a silvery varnish-like look and black spots from the thrips' excrement. Eventually, the leaves and petals will become deformed, or become thin and wilted, then die and drop off.
These symptoms are similar to many fungal or viral diseases, and in fact, thrips are often implicated in the spread of plant viruses and other plant diseases. But if the plant damage is due to thrips, the leaves will nearly always show the black spots created by the pest's excrement.
What Plants Are Most Vulnerable to Thrips?
Because there are so many different types of thrips, most indoor and outdoor plants, both woody and herbaceous, can be affected by them. Indoors, thrips generally prefer plants with broad thin leaves. Some of the species most likely to be affected are palm, dracaena, dieffenbachia, peace lily, and ZZ plant.
Outdoors, thrips are a common problem for many fruit and vegetable crops, including asparagus, cabbage, lettuce, onions, and peas. They generally prefer to feed on new plant tissues, primarily young leaves. Damage is usually fairly moderate, though large numbers of thrips can kill a plant. Among the flowering plants, gladiolas, carnations, chrysanthemums, gerberas, marigolds, pansies, hibiscus, and any of the plants from the rose family, are common targets. However, almost no plant is immune to attack from some species of thrips.
Thrips also can affect a variety of woody plants, including azalea, Ardisia, dogwood, gardenia, hibiscus, magnolia, maple, palm, and viburnum.
What Is the Lifecycle of Thrips?
Females thrips, which are larger than the males, can reproduce without male fertilization. The females make slits in the leaf tissue to lay their eggs—25 to 50 eggs at a time—which hatch in one to two weeks. Hatching babies (nymphs) look a lot like the adults, except they are a translucent light-yellow color, and have no wings. They have short antennae and short legs. These immature thrips are very active, feeding heartily on the plants and growing quickly through a series of metamorphic molts. It is during the early nymph stages that most of the plant damage occurs.
Soon after reaching full size, the nymphs pupate by forming cocoons, either on the plant or in the ground. After several days, winged adults emerge from the cocoons to begin the cycle again. A full generation may repeat every two to three weeks, but populations will be largest from late spring to midsummer. Thrips can survive the winter in the egg stage, which is why removing plant debris is important for thrip control.
Are There Plants That Repel Thrips?
There are so many different types of thrips that no single plant works as a blanket repellant for all species of thrips. However, some plants—such as basil, chives, catnip, and garlic—seem to repel most thrip species. Planting these herbs among your vulnerable edibles and ornamentals may work as one method in an overall integrated pest management practice.
Do Thrips Bite?
While thrips have mouthparts adapted for piercing and sucking plant juices, they are too small to pierce the skin and are not inclined to bite humans, as they are not blood feeders. They can, however, spread some plant viruses and bacterial diseases from plant to plant.
Thrips vs. Aphids
Some of the plant damage caused by aphids is quite similar to that of thrips. Both insects can cause deformities in leaves, and the residue left on leaves by both insects can encourage the growth of black mold. However, under magnification, aphids will appear as distinctly oval-shaped, compared to the long, narrow profile of thrips.
Thrips often leave white or silver patches on leaves, and there will usually be small dark spots of excrement, signs that are not present in aphid damage. Aphids are more likely to leave a noticeably sticky residue on leaves, known as honeydew, which often draws ants (a plant teeming with ants is likely to also have aphids). Thrips normally feed on new leaf growth, while aphids may attack stem and other plant parts, as well as leaves.
Of course, it is entirely possible that a plant may experience both thrips and aphids simultaneously. Fortunately, many of the solutions for thrips also work to control aphids.