Whiteflies have become serious pests of gardens, greenhouses and houseplants in this area. If you need to ask what whiteflies are, you probably don't have them. They are sap-sucking tiny insects that multiply rapidly, and they can kill small ornamentals such as fuchsias and vegetables such as tomatoes and squash in a short time.

They look like tiny white moths as they sit on the underside of leaves. When the plant is jarred, they fly about erratically for a few seconds and then settle down again to feed. They're tropical insects that can't survive the winter outdoors in this climate, and spend the winter on plants in greenhouses and homes. They're taken inside on plants that were outdoors for the summer.

Actually not a fly, but related instead to aphids, mealy bugs and scale insects, the whitefly is one of the most difficult of agricultural pests to control, according to specialists. The only effective insecticides against them, they say, are pyrethroids -- high priced synthetic substitutes for a natural insecticide extracted from pyrethrum plants.There is no insecticide labeled for use against them on food plants.

Entomologists Ralph E. Webb and Floyd F. Smith, of the Department of Agriculture Science and Education Administration in Beltsville, have been working on the problem for years and have discovered that homeowners and small-scale growers of greenhouse crops can use sticky yellow boards to control the whiteflies.

In one experiment in a greenhouse, Webb and Smith placed foot-square yellow boards between tomato plants infested with whiteflies. Each day thereafter the boards trapped 25 percent of the adult whiteflies until the infestation was under control.

In a third experiment, the researchers demonstrated the use of sticky yellow boards with another non-chemical control method -- a parasitic wasp called Encarsia Formosa, which is a natural enemy of the whitefly. The combined methods provided virtually complete control of whiteflies in the greenhouse. While the parasites attacked and killed immature whiteflies, the boards trapped adults. E. Formosa is sold as a control agent for whiteflies in Canada, England, Holland and Norway, but is not yet commercially available in the United States.

Smith attributes the success of the yellor boards to the restless nature of whiteflies. As they fly from leaf to leaf, they are distracted by yellow and become stuck on the surface of the boards.

The yellow boards can be U.S.D.A. experiments or painted with Rust-Oleum 659 Yellow, but other deep orange-yellow paints also would be effective. Of the many sticky substances tested, Webb found Tack Trap, a commercial insect-trapping compound, best.

Smith found that heavy motor oil (SAE 90) is also an effective trapping material, and that it's easier to wash off the boards than Tack Trap. The oil was used in most of the experiments, he says.