By Erin Marissa Russell
Thrips are tiny insects that vary in color from yellow through brown and darkening all the way to black. They are about the same size, shape, and color as a splinter of wood. Adults are incredibly small, most no larger than one sixteenth of an inch, with hairy wings and asymmetrical mouthparts for slashing and sucking plant tissue. In all their stages, thrips have six legs, and the immature versions resemble smaller adults that are paler in hue.
(And just so you know, there’s no such thing as a “thrip.” In both the singular and plural forms, the insects are referred to as “thrips.” The insects are sometimes also called “thunder flies.”)
Thrips can carry diseases, such as tomato spotted wilt virus, impatiens necrotic spot virus, and potentially other viruses as well. Symptoms of these viruses include wilted foliage, streaks or chevron shapes of black on plant foliage, necrotic areas, and alternating rings of dark and light discoloration that expand outward.
There are some varieties of thrips that prey on mites and other small insects like aphids, whiteflies , and other types of thrips, but they will not harm your plants. It’s important not to roll out an anti-thrips campaign simply because you’ve spotted thrips in your garden. Many thrips feed only on other insects, fungi, leaf litter, and garden debris. If you don’t see the matching damage that goes along with the thrips that are a garden nuisance, you likely have the predatory thrips, which you’ll want to keep around so they can help with pest control.
Because many thrips tend to hide in the densest parts of foliage or the deep areas inside blossoms, to be sure your problem is thrips you will need to inspect your plants with a flashlight and magnifying glass. Look inside of flowers and in the joints between leaves (like the base of onion greens, where the foliage emerges from the bulb). In addition to identifying thrips by visually inspecting the damage on the plant and any bugs you can find, also check foliage for the frass (droppings) that thrips leave behind.
You may be able to incite the thrips to motion, making them visible, if you gently flick the flower stamens with your fingers. Choose a few sample flowers, and pull them open to see all the way inside if the thrips may still be hiding. Thrips tend to bury themselves in the base of flowers, making it impossible to find them unless you tip open a bloom or two.
If you still aren’t sure whether your problem is thrips, check some of the buds that have not yet opened. If the pollen sacs inside the buds have been damaged before the buds unfurl, thrips are almost certainly the culprit. You can also shake one of your plant’s branches over a clean white sheet of paper. If dotted lines appear on the paper, you may be dealing with thrips.
Yet another identification method is clipping flowers, buds, and other areas of the plant where thrips are likely to feed and hide. Place the plant material in a clear glass container full of 70% rubbing alcohol, and shake the plant material so any clinging thrips will be shaken free. You will see the thrips float to the surface if they are present.
- Banded palm thrips (Parthenothrips dracaenae): These thrips can strike at any time of year, and they’re especially prevalent on houseplants and tropical plants. Their favorite host plants are Dracaena, Citrus, Ficus, Monstera, Palms, and Schefflera.
- Cuban laurel thrips (Gynaikothrips ficorum): Measuring around an eighth of an inch long, Cuban laurel thrips are slightly larger than other thrips. They are also comparatively dark in color, ranging from brown to black. They feed on the new growth at the top section of leaves and tend to infest Indian laurel, fig trees of several types, India rubber plant, and several types of shrubs, herbs, and orchids. Where Cuban laurel thrips have fed, leaves will roll or form pockets and be pitted with purple markings. They can also be spotted by their dark-colored droppings and the brittle, bleached-looking plant material they leave behind when they feed. Cuban laurel thrips are one of the varieties of thrips that can bite humans, which causes minor discomfort.
- Gladiolus thrips (Thrips simplex): These thrips are most prevalent on gladiolus flowers in the months between June and September. However, they can also be found on freesia flowers. Affected plants will show signs of flowers flecked with white and, when infestation is severe, browned petals and buds that fail to open. Store your gladiolus and freesia plants somewhere not too warm for the winter (but still frost free so they’re protected). A bit of chill can help prevent the gladiolus thrips from spending their winter with your plant in storage to reappear next year.
- Glasshouse thrips (Heliothrips haemorrhoidalis): These thrips can be found all year long in the tropical environment of greenhouses, where winter never comes. Affected plants have silver discoloration on the top side of their leaves and small brown markings nearby from the thrips’ frass (droppings). Outside the greenhouse, these bugs can still survive throughout the year, hosted by shrubs such as Virbinium tinus.
- Honeysuckle thrips (Thrips flavus): Honeysuckle thrips may be hosted by a range of plants in the garden, but they are notorious for infesting honeysuckle plants in the period between May and October. Leaves of infested plants will change color to a marked shade of silvery brown, especially when honeysuckle plants are in a warm spot with a shelter against the wind, such as when they grow against a wall.
- Onion thrips (Thrips tabaci): These well-known pests have favorites in the garden—bean, cabbage, leek, and onion crops— but they can be hosted by a variety of garden plants, flowers, and invasive weeds. Some other favorite hosts include begonia, carnation, cyclamen, dahlia, cucumber, and tomato. Look for silver coloring on foliage and, for some plants, distorted leaves and stunted growth after the thrips feed on developing shoots.
- Pea thrips (Kakothrips pisivorus): You can find pea thrips on pea plants of all kinds in June and August, stunting the growth of plants they infest and causing silver-brown discolored foliage. Pea pods may also show this discoloration, or they may stay flat and only generate a couple of peas near the stalk.
- Privet thrips (Dendothrips ornatus): From May to October, be on watch for privet thrips on privet and lilac plants. Symptoms include bare branches that have dropped their leaves and discoloration of foliage to a silver-brown color as summer draws to a close.
- Western flower thrips (Frankliniella occidentalis): Present in Europe, Japan, New Zealand, and North America as well as certain areas in Africa and Asia. Western flower thrips cannot be differentiated from other species of thrips with human sight. However, when viewed at 50 times magnification through a microscope, the spiny black shoulders become visible. Tend to tuck themselves into the hidden interior of plant foliage where they are harder to see. Western flower thrips are hosted on a broad range of garden plants, fruit and vegetable plants, and flowering plants, including achimenes, African violets, apple trees, Cape primrose (streptocarpus), chrysanthemums, cucumbers, fuchsia, geranium, German primrose (primula obconica), gloxinia (Sinningia speciosa), grape vine plants, impatiens, nectarine trees, peach trees, Persian cyclamen, pistachio trees, plum trees, strawberry plants, tomato plants, verbena, and also in many field crops, including alfalfa and cotton plants. They can also be hosted by invasive weed plants. Check the flowers of houseplants you’re adding to your collection carefully to prevent introducing Western flower thrips, and avoid purchasing those with pale spots on their petals or any other symptoms of thrips.
Life Cycle of Thrips
After eggs hatch, thrips in the first two nymph stages feed on plant tissue for about two weeks. The nymphs are then dormant for two more weeks as they pass through the third and fourth nymph stages before emerging as an adult. (Some varieties of thrips spend their dormant stages buried in the soil.)
Mature thrips have wings that fold onto their backs when they aren’t flying. The winged adult stage is the stage when thrips lay their small white kidney-shaped eggs on flowers, stems, and fruit or partially embedded into these plant tissues.
Eggs hatch in about one week, and the time from hatching to laying their own eggs is about one month, making the average life cycle 24 to 35 days long. However, the lifespan of a thrips is variable and can be affected by the temperature. While each year brings two or three generations of thrips, the thrips may have more generations in a year when the summer weather is especially warm. In greenhouses or other especially temperate locations with mild winters, thrips may breed year-round. During an average winter, the thrips spend the season in the soil or hidden on their hosting plants, either in their nymph form or their adult form.
Symptoms of Thrips
When they consume plant tissue, thrips have a taste for the succulent parts of the plant, so they tend to munch on fully unfurled leaves, opened flowers, buds, and grains of pollen. Flowers that are light in color are preferred feeding as compared to darker blooms. There are many different types of thrips, but onion thrips and Western flower thrips are the most common varieties.
Symptoms of Thrips in Flowering and Ornamental Plants
- Foliage where thrips have fed first loses its sheen and looks dull. Eventually, affected foliage dries out and develops a flecked, washed-out, bleached appearance. Thrips damage to foliage can easily be mistaken for the feeding of other insects.
- Areas of thrips damage are often accompanied by small, dark bits of frass (thrips droppings) and bits of bleached-looking, dried plant material left behind from their feeding.
- Buds of plants affected by thrips tend to stay immature and may never open into blossoms if an infestation is severe—while those blossoms whose buds do open that do open are likely to lack their stamens. When thrips feed on buds or the new growth where plants are sprouting from their tips, foliage that comes from these areas may be distorted in shape.
- Flowers often fail to grow to maximum size. Blooms may drop early or die before their time, or they may display rapid withering that leads to a desiccated, papery texture, making petals easy to tear. Alternatively, blossoms sometimes develop a soft or squishy texture.
- Petals can become discolored and turn brown on sides and edges. They may also have pale spotted areas where the pigment of the blossom has been lost to thrips damage. Pollen shakes all over the blossoms of plants infested with thrips, causing them to appear dusty.
Symptoms of Thrips on Fruit
On all kinds of fruit, tissue where thrips have eaten will appear sanded down. The flesh of the fruit underneath the marred areas may have a hard or dry texture, or its taste may suffer.
- Apples show thrips damage as central russet-colored spots that sometimes spread into a halo effect or widen into a shape similar to a pansy.
- Grapefruits and plums affected by thrips will develop scarring on the skins of fruits.
- Nectarines from trees infested by thrips will show silvering or white markings on fruit skin that resembles a net.
- Strawberry plants that are hosting thrips tend to produce deformed berries.
How to Control Thrips
Once you’re certain that the problem in your garden is thrips, it’s time to take quick action against them and evict them from their host plants. Adult thrips are the first ones to consume flowers and the most damaging to plants, so you must go after the adults (not only the larvae), and you must do so quickly if you want to stop the damage to your plants.
- Keep thrips away by placing a camphor tablet or bathroom deodorizer every so often in your rows of plants and flower beds.
- Call in reinforcements by attracting insects that feed on thrips or purchasing and deploying them. Tiny pirate bugs are predators of the Western flower thrip and will consume both mature and immature thrips. Orius nymphs, syrphid fly larvae, lacewings (both larvae and adults), and banded wing thrips are also thrip hunters, and predaceous mites will kill immature thrips but will not prey on adults, making them less effective than some other options. For glasshouse thrips, you can unleash predatory mites (species include Amblyseius, Hypoaspis, and Macrocheles robustulus, which is sold under the name Mighty Mite). A mix of nematodes that will target thrips among other pests is sold under the brand name Nemasys Fruit and Vegetable Protection. For supplier contact information and other details about biological controls for garden pests, refer to this PDF list of predators/prey and sources where you can purchase the biological controls from Colorado State University’s College of Agricultural Sciences.
- Especially when thrips have infested your houseplants, spraying the affected plants with insecticidal soap every couple of days is an effective way to solve your thrips problem.
- Neem oil is an effective measure against thrips, and you can make a neem oil treatment in your kitchen. Mix two ounces of neem oil with four liters of room temperature water, and spray this concoction on your plants every week or so. Make sure to wait until dusk to use neem oil, as it is also poisonous to bees, which will be in their hives once darkness falls. You should test your neem oil spray on a small area of one plant before starting to use it on large swaths of your garden.
After you’ve addressed the pressing, immediate problem of an active thrips infestation, you need to roll out some preventive measures to make sure the thrips stay away for good. Choose a couple of the techniques listed here to make a part of your routine so your garden will stay free of thrips.
- Any time you add a new plant to your collection, inspect it carefully for thrips and other pests or diseases it could pass along to the rest of your garden. You may wish to establish a mandatory quarantine period for new incoming plants to ensure they don’t introduce disease or insects.
- Thrips favor dry weather, so discourage them from staying in your garden by watering and misting plants frequently.
- Thrips often find their way to a garden via the weeds that grow alongside it, so reduce your risk of a thrips infestation by keeping the area that surrounds your garden weed-free and neatly mowed.
- If your gardening is done inside a greenhouse, check the walls (especially in the joints) and the areas around door and window openings for places where thrips could be entering. You can block any openings you find by covering them with fine wire mesh. Blue sticky traps (available at nurseries and garden centers) can also be used in greenhouses to catch thrips, but these traps should not be used in outdoor gardens because they will trap as many beneficial garden insects and pollinators as they will pest bugs like thrips.
- Foster general good health in your plants, and thrips will be less likely to set up shop. Native plant species thrive the best, followed by plants that receive their preferred amount of sunlight, are growing in the type of soil they like best, and have been planted in a zone with their preferred climate. Make sure all your plants get the amount of water they need to flourish and are nourished with fertilizer as necessary. Pruning your plants when called for will also help them maintain a high baseline of health, which helps the plants stave off infestations such as thrips.
- Reflective mulch is recommended by experts as a way to confuse thrips and keep them from finding their favorite host plants in your garden.
- Make it a habit to visually inspect your plants for signs of thrips and other pests or diseases on a regular basis year-round. If you know certain plants are particularly susceptible to an insect or illness that is worst at certain times of year, you may need to check those plants more frequently when they’re most likely to be infested or infected. When you know the symptoms of various plant problems and you check your plants regularly, you’re able to take action to stop plant diseases and pest invasions before they can spiral out of control.
Congratulations—now that you’ve read this article, you should be able to identify thrips visually both by seeing the insect or inspecting plants for thrips damage and the frass and food scraps they leave behind. You know what to do if your garden is suffering from a current thrips infestation, and you know how to prevent thrips from taking hold. Now you’re ready to win the battle against thrips once and for all.