Garden pest whiteflies gone with heavy frost

By Marianne C. Ophardt, Special to the Herald

During the last few months, a number of local gardeners, including myself, noticed that many of our garden plants were infested with zillions of tiny white flying insects.

The culprit was the greenhouse whitefly (Trialeurodes vaporariorum). It attacks a number of garden veggies and annual ornamentals. The greenhouse whitefly doesn't overwinter outdoors in the garden in this region. This means that the source of infestation most likely was some of the transplants that we purchased at the beginning of the season.

There are more than 200 known species of whiteflies, but only a few of these cause real problems on plants. The adult greenhouse whitefly is about one-tenth to one-sixteenth of an inch in length and looks like a tiny moth. As the name implies, their bodies are white and covered with a powdery wax. Most gardeners notice them only when they're present in large numbers, when they envelop plants in a cloud-like swarm. If you can find one at rest you'll see that they hold their wings in a tent over their body.

With favorable conditions, the adult whitefly has the potential to lay up to 400 eggs, laying approximately 25 a day on the undersides of plant leaves. In about a week, these eggs hatch into nymphs. After transitioning between three distinctly different immature stages the nymphs turn into adults in a month. The adults can live as long as 45 days. Adults and nymphs have sucking mouth parts and feed by sucking sap from the plant.

Heavy infestations of whiteflies can stunt plants and cause their leaves to turn from yellow to brown and dry, but light whitefly infestations usually don't cause serious problems in the garden. However, as they feed the whiteflies excrete sugary "honeydew." This doesn't harm the plant, but it can make plant parts very sticky.

That's what happened to my tomatoes this year. Infested with whitefly, I had to wash the sticky fruit before eating. I didn't worry about the whiteflies on my tomatoes because it was a light infestation that wasn't causing significant damage. I also knew that all the flies would die with frost in the fall.

Next year I plan to inspect my transplants before buying them to make sure they don't have whitefly. It also pays to check them several weeks after planting in the garden to make sure an infestation hasn't started. If an infestation is noted, there are some easy non-chemical approaches that you can take, such as removing infested leaves and using water to wash the flies and dust off the plant.

Insecticides are not routinely recommended for whitefly control for several reasons. One reason is that non-selective insecticides kill the many beneficial insects, such as lacewings, minute pirate bugs and lady beetles, that help keep whitefly populations under control. Because of this, it's best to avoid using carbaryl, pyrethroids or foliar sprays of imidacloprid. Dust protects the whiteflies from some of its enemies. Keeping the dust washed off leaves will encourage the work of beneficial insects.

Another reason not to use most insecticides is that whiteflies have a tendency to build up resistance quickly to different chemicals. Insecticidal soaps can be used to keep populations down. Spray in the early mornings when the temperatures are cool to avoid damaging plants and to have the soap come in contact with the adult whiteflies while they're sluggish and not as likely to be flying about. Be sure to spray the lower surface of the leaves. Soap will only kill the nymphs to which it's directly applied. It does not kill the eggs, so repeat applications are usually needed.

A unique approach to control is using a small handheld cordless vacuum to suck up the adults from the leaves. This also is done in the early morning when the adults are slow and sluggish. When the vacuuming task is completed, be sure to freeze the flies by placing the vacuum bag in the freezer for a day. (You may want to put it in a plastic bag before putting it in your freezer.)

With heavy frost, the whiteflies plaguing local gardens will be gone. Next year avoid reintroducing them into your garden by making sure your transplants are "whitefly-free" when you bring them home.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County. Read more of Ophardt's Garden Tips columns at www.tricityherald.com/ophardt.

tool name

Saturday, Oct. 25, 2008

Garden pest whiteflies gone with heavy frost

By Marianne C. Ophardt, Special to the Herald