Flower thrips create new concern in garden

By Marianne C. Ophardt, Special to the Tri-City Herald July 6, 2012 

KENNEWICK -- A new worry in the garden is 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 more than 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 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 and roses.

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

* 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) and mature in two weeks, allow for multiple generations during the growing season.

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

* 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.

* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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