When fully grown, thrips are about 1-1.5 mm (0.03-0.06 inches) long and have long slender bodies with two pairs of wings that are narrow and fringed with hairs. Unlike most insects, thrips have asymmetrical mouthparts which they use to scrape the surface of the leaf and extract fluids. Both onion thrips and western flower thrips are similar in size and have yellow or brown bodies, therefore identification of thrips requires close examination with magnification. For more information on how to distinguish onion thrips from western flower thrips, consult the page on identifying pestiferous thrips in western Colorado.
- There are hundreds of thrips species in North America, but two species are major pests of vegetables grown in Colorado. They are onion thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis). The onion thrips has also been recorded in indoor-grown hemp in Colorado.
- Collectively, both thrips species have a wide host range that includes winter wheat, alfalfa, legumes, onions, beans, cucumber, peppers, potatoes, garlic, cabbage, leek, asparagus, and cauliflower. Feeding injury tends to be more noticeable on plants grown indoors where there are fewer natural enemies.
- Viruses transmitted by thrips are called tospoviruses and cause plant diseases such as tomato spotted wilt, impatiens necrotic spot, and iris yellow spot virus. Collectively, these viruses can infect a wide range of vegetables including lettuce, tomato, potatoes, peppers, shallots, and garlic.
- Thrips can reproduce asexually and produce multiple generations each growing season.
Adult of western flower thrips. Note the fringed hairs on the wing edges. Two species of thrips, onion thrips and western flower thrips, are of concern in vegetable production in Colorado. Typically, thrips are more problematic indoors where there are fewer natural enemies. Image credit: David Cappaert, Bugwood.org
Larva of onion thrips. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Onion thrips on onion leaf, both adults and immatures are present. Note their small size relative to the leaf. Image credit: Diane Alston, Utah State University, Bugwood.org
Head of cabbage with feeding injury due to thrips. Image credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Tomato injured by ovipositon of western flower thrips. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Cucumber leaf with feeding injury due to thrips. Note the presence of dark spots (excrement) on patches of silver discoloration. Image credit: Bruce Watt, University of Maine, Bugwood.org
Life history and habits
The eggs of both thrips species are deposited in tissues of the host plant and resemble a kidney bean. Onion thrips are parthenogenic, which means that mature females can lay eggs without mating. Larvae resemble wingless adults and tend to be transparent, white, or pale yellow or orange. After hatching from eggs, larvae develop through four instars which are further divided into two active feeding and two inactive non-feeding stages. The first and second instars feed on newly emerged foliage and flower buds, and both adults and feeding larvae will crawl when disturbed. The third and fourth instars are non-feeding and are often referred to as the pre-pupa and pupal stages. All stages tend to seek shelter between adjacent leaves or flower structures.
Western flower thrips is an important vector of the tomato spotted wilt virus and the impatiens necrotic spot virus. Both viruses can infect a wide range of plants and a single plant can be infected with both viruses. The tomato spotted wilt virus can significantly damage tomatoes, potatoes, peppers, eggplant, potato, lettuce, spinach, cucurbits, and beans. In addition to infecting greenhouse ornamentals, the impatiens necrotic spot virus can also infect cucumbers, tomatoes, and peppers. The onion thrips can also vector iris yellow spot virus, resulting in straw or tan lesions on onion leaves and flowers. Typically, the tomato spotted wilt virus and the impatiens necrotic spot virus are not a concern on onions.
Thrips feeding causes leaves to turn silver in patches or streaks. When the thrips density is high, feeding can cause withering of foliage and tiny black spots (thrips excrement) may be visible on leaves. Injury to leaves can reduce plant growth and increase stress. In areas with sufficient moisture, feeding injury can increase the plant’s susceptibility to microbial infection. Thrips can also feed on flower buds, which can result in deformed flowers or buds that do not bloom. Thrips feeding can also lead to deformed fruits such as the “pig-tail” appearance of cucumber, even when thrips densities are relatively low. Scarring of fruits is another symptom of thrips feeding injury in capsicum, cucumber, and beans.
In onions, yields are reduced when intensive thrips feeding occurs during the bulb enlargement phase which reduces onion size. Furthermore, thrips can continue feeding on bulbs after harvest and during storage, which can further scar the bulbs and reduce their marketability.
Yellow or blue sticky traps are an effective tool for monitoring thrips densities. Place the traps above canopy level to capture thrips during flight and inspect the traps regularly. Indicator plants are more attractive to thrips than the cultivated crop and can be used to indicate whether thrips are present early. All plants should be inspected regularly for thrips and feeding injury. Look for silvery or bronze discoloration on leaves and deformed leaves and flowers. Plants can also be tapped to dislodge thrips over a white sheet of paper.
To look for thrips in onions, expose the neck of the plant and quickly count the number of adults and larvae. For more information on scouting thresholds and thrips management recommendations in onions, consult the Utah State University factsheet.
There are several natural enemies of thrips and in many instances the use of chemical controls is not necessary. Predatory mites, minute pirate bugs, and nematodes are commercially available and can provide some protection against thrips. It is important to note that each of these natural enemies has specific temperature, humidity, and food requirements.
Fine mesh netting can physically exclude thrips when placed over air vents, fans, and other possible entry points into the greenhouse. Workers should avoid wearing brightly colored clothing since they are attractive to thrips and increase the likelihood of spreading an infestation as workers move between infested to clean areas.
Thrips are resistant to many commonly used insecticides. To help prevent insecticide resistance in the thrips population, the quantity and frequency of insecticide applications should be limited, and insecticides from different groups should be rotated. It is also important to note that life stages of thrips differ in their susceptibility to chemical controls. Most insecticides are effective at killing early larvae during the first and second instars, while only some are effective against adults. Few insecticides are effective at killing eggs owing to their location within plant tissue. The nonfeeding larvae (third and fourth instars) mature in the soil and are therefore not in contact with insecticides applied to plant structures above ground.
For more information on chemical management of western flower thrips, consult the pacific pests, pathogens, and weeds factsheet. For more information on chemical management of onion thrips, consult the Utah State University factsheet.
Alston, D. & D. Drost (2008). Onion Thrips. Utah State University: Extension – Fact Sheet. Available https://extension.usu.edu/pests/uppdl/files/factsheet/ENT-117-08PR.pdf
Hannon, B. (n.d.). Western Colorado Thrips. Colorado State University, Agricultural Experiment Station. Available http://wci.colostate.edu/Assets/pdf/ot.vs.wft.pdf
Pacific Northwest Extension (n.d.). Potato, Irish – Thrips. Pacific Northwest Pest Management Handbooks. Available https://pnwhandbooks.org/insect/vegetable/irish-potato/potato-irish-thrips
University of California. 2018. Western Flower Thrips. University of California IPM – Agriculture & Natural Resources. Available https://www2.ipm.ucanr.edu/agriculture/strawberry/Western-flower-thrips/
University of Georgia. (n.d.). Thrips. University of Georgia Extension – Landscape Pest Management. Available https://extension.uga.edu/programs-services/landscape-pest-management/Nursery-pests/thrips.html
Jackson, G. (n.d.). Western flower thrips. Pacific Pests, Pathogens & Weeds – Fact Sheets. Available https://apps.lucidcentral.org/ppp_v9/text/web_full/entities/western_flower_thrips_183.htm
Koppert (n.d.). Western flower thrips. Available https://www.koppertus.com/challenges/pest-control/thrips/western-flower-thrips/#:~:text=Life%20cycle%20and%20appearance%20of%20Western%20flower%20thrips&text=Larvae%20are%20nearly%20transparent%20white,yellowish%20orange%20to%20almost%20black.