AFTER a decade in which Washington was relatively free of the dangerous pests, wax scale insects have returned as a serious threat to several kinds of woody plants. Gardens in parts of Georgetown and Cleveland Park have already been harmed.

The insects can be present in a garden without the owner's knowledge. They don't look like insects, particularly destructive ones, and that may be why they have been able to spread unnoticed. How they were introduced to this area is unknown, but a bad infestation here about 10 years ago was brought under control by a concentrated effort by gardeners and garden clubs.

The scale resemble white (or gray) drops of wax, are attached along the stems, seldom on the leaves, are about one-fourth inch long, circular and sticky to the touch. Under the wax coating is the pink insect.

The waxy coating protects the insect from sprays. Anything strong enough to penetrate the wax would probably kill the plant also.

They feed by sucking juice from the plant cells. An infestation may kill a plant in about three years. There are few woody ornamentals this pest will not attack. Preferred food plants appear to be Burford, Chinese, Japanese and English hollies, pyracantha, Japanese quince, spirea, euonymus and hemlock.

In May the female (all overwintering wax scales are females) lays its eggs, up to 2,000 of them, depositing them in a hollow space just under its body. Soon afterwards it dies, but the dead body remains and protects the eggs until they hatch. It drops from the plant later on.

The eggs hatch about mid-June. They do not all hatch at once. Hatching may continue for two weeks or longer.

The young are called crawlers because during the first few hours of their life they can move about. It is during this brief period they are spread to other plants by wind or birds.

They then settle down in one place and remain there the rest of their one-year life span (unless destroyed by the branch being cut off and burned) and begin to acquire a waxy coating.

The pinkish-red crawler is no larger than a speck of pepper and may escape detection unless one looks very closely, preferably with a magnifying glass.

Control of this pest depends entirely on timing of insecticide treatments, according to specialists. Unless sprays are applied before the crawlers acquire a protective waxy coating, they won't work.

About June 15 spray infested plants with Spectracide or Malathion. Directions on the label should be followed closely. Spray two more times at weekly intervals.

If an infested plant, such as Burford holly, is badly infested and is too thick with branches for good spray penetration, it should be thinned by pruning before spraying.

It is difficult to kill all the crawlers on a plant when there may be thousands of them. That is why it is important to spray three times. If only 50 to 100 survive, that would be a bad infestation.

By all means check your shrubbery this weekend and find out if it is infested. This is very important because you don't want it seriously weakened and subject to damage by diseases. Also alert your neighbors.

The time of hatching may vary from year to year. The eggs of some may hatch days or weeks later than those of others. In 1969, crawlers were first seen in Alexandria on June 10, in Arlington on June 11, in the District on June 13 and at College Park on June 14. Scale on stems in the shade do not produce crawlers as early as those in full sun.

Studies on the control of wax scale were initiated at Beltsville in 1966 under the direction of entomologist Floyd F. Smith.

It was found that initial infestations reduce the vigor of plants, and branches or the entire plant will die during the second and third year.

There is one annual generation in the Virginia-Maryland-District area. In the greenhouse with night temperatures near 65 degrees, a generation developed in about seven to eight months, but egg laying by females and hatching of young occurred with less regularity than out-of-doors.