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Typical thrips damage to a tomato leaf.

Thrips are tiny insects that are mainly active at night. As a result, you typically see the damage they do well before seeing the insects themselves. They attack flowers and leaves, piercing their cells to suck out the liquid. The cells then fill with air, giving the damaged area a sort of silvery or bleached appearance. Since thrips tend to cluster together you’ll usually find irregular patches of silvery  or beige tissue on broad leaves and flower petals and silvery stripes on lance-shaped leaves. The damaged parts look like they have been rasped. You’ll likely see their excrements before you see the insects themselves: small black deposits are readily visible on the affected parts.

In addition to damaging the plant by sucking its sap, thrips can do even greater damage by transmitting plant viruses.

There are more than 5,000 species of thrips, the most common in homes and gardens being the western flower thrips (Frankliniella occidentalis), the eastern flower thrips (Frankliniella tritici), the onion thrips (Thrips tabaci) and the gladiolus thrips (Taeniothrips simplex). Depending on the species, they may attack either foliage or flowers and flower buds. Some species harvest pollen (the sight of loose pollen on an African violet flower is usually a clear sign of a flower thrips infestation), while others attack roots and bulbs underground. There are also beneficial thrips that are predators on other insects, although plant-eating thrips are far more numerous and better known.

One thrips, Two thrips

There is no such thing as a thrip: thrips, with an an s at the end, is both singular and plural.

Identifying the Culprit

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Onion thrips (Thrips tabaci) and Western flower thips (Frankliniella occidentalis).

Thrips are hard to see unless you have very good eyes, but try blowing on the infested plant part. This usually sends them scurrying about and you should be able to catch the movement. Why they react this way is a mystery: maybe they find humans have bad breath! This is certainly the best method of spotting flower thrips especially, as otherwise they often work sight unseen inside flowers and flower buds.

In spite of their wings, thrips are not great flyers, usually jumping from plant to plant or coasting with outstretched wings. Outdoors, they are carried far and wide by the wind.

Plant Hosts

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Thrips damage on a gladiolus.

Most thrips aren’t picky about what they eat and have a wide host range, although some species do have their favorite plants. Here are some of the host plants they are most likely to infest.

Houseplants: African violet, azalea, begonia, brugmansia, croton, crassula, cyclamen, dieffenbachia, ficus, fuchsia, gerbera, gloxinia, hibiscus, impatiens, orchid, pelargonium, streptocarpus, yucca.

Edible plants: apple, asparagus, basil, bean, blueberry, carrot, cherry, cabbage, corn, cucumber, garlic, grape vine, leek, onion, pea, pear, pepper, potato, raspberry, strawberry, tomato.

Annuals, perennials and bulbs: aster, carnation, chrysanthemum, dahlia, gladiolus, iris, lily, peony, petunia, pelargonium, snapdragon, sweet pea, verbena, zinnia.

Woody plants: birch, hydrangea, linden, maple, privet, rose, willow.

Control

Controlling thrips can be difficult, because most species lay their eggs inside plant stems and later pupate underground, out of the reach of treatments in both cases. So even if you carefully spray an infected plant from top to bottom with an appropriate product, there were always 2 generations that were out of reach. That’s why thrips always seem to pop out of nowhere just when you think you’ve got them licked. You’ll therefore have to repeat your applications (insecticidal soap, neem oil, horticultural oil, pyrethrin, etc.) every 5 to 7 days until you see no more of them… and that can take months!

African violet growers find they can eliminate flower thrips by systematically removing all flowers and flower buds… for a full 3 months.

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Blue sticky trap.

You can also use commercial sticky traps to catch adults. Usually these traps come in yellow and that color will work, but thrips are even more attracted to blue. In better garden centers, you can find blue sticky traps designed specifically for thrips. If not, try www.leevalley.com. Thrips are also attracted to light and enter readily ultraviolet fly traps.

The Enemies of Our Enemies…

In outdoor gardens, beneficial insects often help to control thrips. Earwigs, ladybugs and pirate bugs are efficient thrips predators as are several mites. Certain species of soil nematode are also know to attack thrips while they pupate underground. The presence of predators explains why thrips infestations outdoors sometimes seem to sputter out all on their own.

There are also thrips predators that can be released in a greenhouse environment, including the beneficial mite Amblyseius cucumeris, but they adapt poorly to use on houseplants raised in typical homes, largely because the air there is too dry for their taste.

Finally, gladiolus thrips overwinter on gladiolus corms stored indoors for the winter. Storing the corms in an extra-cool place (between 35 and 40˚F/2 to 4˚C) for at least 6 weeks will eliminate them, as they can only tolerate cold temperatures for short periods.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

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