Recognizing and Controlling Thrips - May 16, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Thrips is a small, almost microscopic, slender, insect with rasping mouthparts. Most thrips that we notice feed on plant leaves, buds, and flowers of garden and landscape plants. However, a few species are beneficial predators that feed on other insects and mites. By the way, the word "thrips" signifies a single insect (i.e. there is no such thing as a thrip). This column will describe thrips species, their biology, recognizing their damage, and some control strategies.
Common pest thrips include the western flower thrips which feeds on vegetables, grapes, peaches, nectarines, strawberries, raspberries and herbaceous plants; the onion thrips and the bean thrips which feed on vegetables; the citrus thrips which feeds on citrus and pomegranate; and greenhouse thrips attacks many landscape plants.
You can recognize thrips using a hand lens or magnifying glass. Adult thrips are slender, less than 1/20 inch long, have long fringes on the margins of both sets of their long, narrow wings, and are often yellowish or blackish and shiny. Immature thrips (nymphs) are similarly shaped with a long abdomen but lack fully developed wings and most species are translucent white to yellowish. When disturbed, many species of thrips will curve the tip of their abdomen upwards.
Thrips are tiny and tend to feed in buds, within furled leaves, or in other unexposed areas of the plant; their damage is often observed before the thrips can be seen. Numerous black specks of feces scattered over a stippled leaf surface is a clue that damage is caused by thrips, but look for the insects themselves. Lace bugs and other plant bugs also leave black specks. If you suspect thrips, and need assistance, you may bring a plant sample (several leaves and/or buds in a ziplock bag) to the Cooperative Extension office and ask a Master Gardener volunteer for assistance.
The thrips life cycle includes the egg, two actively feeding nymphal stages, nonfeeding prepupa and pupa stage, and the adult. Thrips metamorphosis occupies one of those biological gray areas, somewhere between gradual and complete. Thrips have several generations (up to eight or more) a year. The life cycle from egg to adult may be completed in as short a time as 2 weeks during warm weather.
Thrips prefer to feed on rapidly growing tissue. Feeding damage caused by thrips adults or nymphs creates tiny scars on leaves and/or fruit, called stippling. This frequently stunts plant growth and causes misshapen buds. Damaged leaves may also become papery and distorted. Thrips can cause dead spots or blotches to appear on flowers. In roses, infested flowers usually discolor, become pinched or deformed, and often fail to open. Black, varnish-like specks of excrement are a distinguishing feature of thrips activity. Feces may remain on leaves or fruit long after thrips have left. Thrips are poor fliers; damage may first appear in one location then slowly spread over the plant.
Thrips infestations reduce the aesthetic quality of landscapes but usually do not seriously harm or kill woody plants. Each gardener must decide when control efforts are warranted and when damage levels warrant using aggressive control methods. If insecticides are used, you may inadvertently kill beneficial insects. Minute pirate bugs, spiders, and predatory mites feed on thrips.
Cultural controls can also be employed. Thrips often migrate in from drying weedy areas or grasslands, so it is wise to avoid planting susceptible plants next to these areas. In small gardens, thrips can be knocked off with a spray of water. Vigorous plants normally outgrow thrips damage, so keeping plants well irrigated and fertilized (do not over fertilize) can help them outgrow damage. Aluminum foil placed under the plants can be used as a disorienting mulch. Prune and destroy injured and infested terminals when managing a few small specimen plants, such as roses and bedding plants, in the landscape. Avoid shearing plants. Shearing--clipping the surface of dense foliage to maintain an even surface on formal hedges--stimulates susceptible new growth. Prune by cutting plants just above growing points, such as branch crotches and nodes, instead of shearing off terminal buds.
Although thrips damage to leaves is unsightly, it normally does not decrease crop yields unless plants are very young. Dormant oils can help control overwintering thrips in deciduous fruit and landscape trees. Several applications of a systemic insecticide such as acephate (on ornamental nonfood plants only) during the growing season may be required to provide control in the landscape. Be aware that spraying not only kills natural enemies, which help control thrips, but can also kill natural enemies and cause spider mite flare-ups. Insecticidal soaps or narrow range oils are effective for temporary reduction of thrips populations when insects and damage first appear and, due to their short persistence, oils or soap sprays are less disruptive of natural enemies. Thanks to the University of California Integrated Pest Management Web Site where much of this information was gathered: http://www.ipm.ucdavis.edu/.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: March 15, 2001
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