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written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA

A new worry in the garden… western flower thrips. We’ve talked before about these tiny little insects that damage rose buds with their feeding, but a variety of other garden flowers are also being attacked. Thrips have rasping mouth parts that they use to tear tender flower tissues and then slurp up the fluids that leak out. Their feeding causes streaks and blotches on flower petals. When severe, flowers may even fail to open.

While western flower thrips are commonly found on roses, they can also attack almost any type of flower and are known to attack over 200 species of plants in 62 different families. They favor white to yellow flowers and their preferred hosts in the garden are roses, mums, geraniums, impatiens, fuchsias, marigolds, pansies, petunias, and carnations.

Thrips are minute, just one-fifth of an inch in length, with slender bodies, making it difficult to detect their presence. The easiest way to check is to tap flowers over a white piece of paper and look for their yellow to tan fast moving little bodies. I had been blaming the weather for the failure of my geranium flowers to fully open until I used this method to check for thrips. Aha! Thrips were the culprits. They’re also causing streaks on the petals of my miniature daylilies. I know they must also be feeding in other flowers, but they aren’t causing the severe damage seen on my geraniums, roses, and daylilies.

Before we discuss control, let me point out some things you should know about these flower feeders:

1. Thrips populations can build up quickly. Female thrips don’t need a male to reproduce and each is capable of laying 300 eggs in plant tissues. The eggs which hatch in a few days to a few weeks (in cooler weather) mature in two weeks, allowing for multiple generations during the growing season.

2. Thrips are active flyers and are capable of moving from plant to plant.

3. Because of the damage they cause to numerous agricultural food and fiber crops and because they have multiple generations during the season, thrips have built up a resistance to many insecticides. Scientists have also found that the outer covering on thrips’ bodies blocks the penetration of insecticides. Add to this the fact that thrips are often feeding within buds or at the base of petals where they’re protected from insecticide applications.

Obviously controlling flower thrips isn’t going to be easy. It’s made more complex by the fact that most pesticides that might be effective in controlling thrips are also likely to be toxic to bees visiting the flowers. An integrated approach to managing thrips is advised. This consists of:

-Pruning out infested flowers and buds and removing them from the garden.

-Getting rid of weeds in and around the garden.

-Avoiding lush, vigorous plant growth that results from excessive fertilization or heavy pruning.

-Using blue, white, or yellow sticky traps to scout and trap thrips.

-Wetting plant surfaces with sprinkler irrigation to deter thrips.

-Encouraging beneficial insects, such as lady beetles and lacewings which feed on thrips.

-Using the least toxic insecticides recommended for thrips control and following label directions.

Insecticidal soaps and summer oils can provide a quick knockdown of some of the thrips, but repeat applications will be needed. Apply insecticide materials directly to buds and flowers. Because theses materials may damage flowers, you should test several flowers first. Only treat badly infested plants where the thrips damage is too severe to be tolerated.

Published: 7/6/2012 2:47 PM


written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

They’re a tiny little pest, seldom noticed by most gardeners, but thrips seem to be plaguing a variety of flowers in area gardens. Thrips are quite small, less than one-twentieth of an inch long, and usually go unrecognized as the cause of flower and plant damage. Under magnification, you can see their slender bodies with unique feather-like wings made of bristles. The ones I find are usually yellowish-tan in color, but thrips can vary in color from yellow to dark brown.

Thrips feed on plants with rasping mouthparts. According to Dr. Arthur Antonelli, WSU Extension Entomologist, a thrips’ mouth contain two stylets and one mandible. They use these mouthparts to scrape open plant cells. The thrips then slurp up the liquid contents of the cell. This type of feeding damage causes blotching and streaking of plant tissues.

The thrips plaguing local gardeners are western flower thrips and gladiolus thrips. These thrips feed within the flower buds, causing stippling and scarring of the petals. If it’s a severe infestation, the flower buds will fail to open. Western flower thrips have a broad host range, but in our area they seem to be particularly troublesome on roses, peonies, and hollyhocks. (Gladiolus thrips bother glad flowers and leaves, causing blotching of the flowers and streaking of the leaves.) They also commonly feed on petunia, Impatiens, Gerbera, pansy, begonia, fuchsia, chrysanthemum, ivy geranium, marigold, hibiscus, verbena, and carnation.

Thrips may also feed on plant leaves leaving at first a silvery mottling or blotching. On the undersides of damaged leaves one can note tiny black specks that are the thrips feces. If leaves are damaged before they emerge from a bud, they will usually be deformed and fail to develop properly.

Lacking a good magnifying glass, your first clue that your flowers are infested with thrips is the blotchy petals of opened flowers, especially noticeable on lighter colored blooms. Another clue is the failure of flower buds to open fully or open at all. To check for thrips, tap opened flowers over a piece of white paper and look at the specks that fall off. Thrips will be yellowish to tan elongated minute creatures that move about on the paper. On blossoms that fail to open, they can be found by tearing open the bud and looking for the moving specks at the base of petals.

Recognizing a thrips problem is easier than controlling the pest. They are very difficult to manage because they tend to hide deep within buds where they are protected from most types of insecticide applications and because they have developed resistance to many pesticides.

Non-chemical options for management include not planting susceptible plants next to weedy or grassy areas where thrips can thrive… or do a better job of controlling weeds. Regularly spray your garden plants with water, this will knock thrips off the plant and will also discourage mites. Keep garden plants healthy and vigorous, especially during the hot part of summer. Trim off and destroy infested blooms.

There are a number of natural predators, such as predatory mites, lacewings, and pirate bugs, in the garden that help keep thrips in check so pesticide applications to infested plants should be the last resort for managing a thrips problem. If you do apply a pesticide, consider the least toxic, naturally derived materials, such as those containing spinosad (a fermentation by-product of a soil bacteria) or Beauveria Bassiana (a fungus). If all else fails, less environmentally materials (acephate or imidacloprid) can be used, but care should be taken to protect beneficial insects. For safe and effective use, read and follow all directions on all pesticide products.

Other things you should know about thrips:

– One of these insects is called a thrips and a group of them is also called thrips. There is no such thing as “a thrip.”

– Thrips are not powerful fliers, but they will take advantage of wind currents to move about. In greenhouse situations they will also hitchhike on clothing from one area to another. They are especially attracted to pink, blue, yellow, white or green.

– Along with damage caused by their feeding, thrips also are a problem because they spread at least two plant virus diseases. This is a special concern for greenhouse production operations.

– While it doesn’t hurt much, some thrips will bite people. These bites are insignificant.

Published: 7/29/2007 2:45 PM



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