You say the trees in your back yard still have leaves? And you say you wonder what happened to that pest whose very name makes tree lovers blanch for their beloved branches?

Don't be fooled, officials say. The gypsy moth is still on the munch, its nocturnal leaf grazing threatening our shade-giving woodlands and parks.

The threat has materialized, the experts say, but the ravages of the gypsy moths may not become highly visible until next year--and then only in some isolated areas.

"Next spring is the first year we expect to see any significant defoliation in Northern Virginia, and it is not going to be widespread, just in some counties," said Marshall Trammell, who monitors the southward migration of the insect that has been responsible for defoliating 20 million acres of trees in the Northeast since 1981.

Virginia agricultural experts are counting the fuzzy, brown teardrop-shaped egg masses of the gypsy moths and in several weeks will predict how much defoliation can be expected next year, Trammell said.

"We know we are going to see some defoliation just by looking at the egg masses," in whose numbers there have been "significant increases over last year," he added.

"It appears from the egg mass counts that we are going to see defoliation for the first time next spring in Loudoun County . . . but we have no indication as of this time that there will be any defoliation in Fairfax County" next year, said Donald Kludy, supervisor of plant and pest control for Virginia's Department of Agriculture.

"It's not hitting all areas at the same time, but it is on the buildup in Virginia and Maryland. It's just a matter of time," Kludy said.

When agricultural officers in the area began warning residents three years ago about the approaching menace to their hardwood trees--especially oak and hickory, which the gypsy moth in its caterpillar stage from late April to June finds irresistible--there was a minor panic among tree lovers.

Many rushed to protect their back yard arbors and wrapped burlap around the tree trunks. The insects migrate to the burlap to escape the daylight and are more easily caught. Last year area Boy Scouts launched Operation Gypsy Moth, promoting the sale of $5 moth traps scented with sex lures to attract male moths and interfere with breeding.

The process of defoliation is a gradual one, usually taking up to three years to become visible, Kludy said. "There is some defoliation in the first several years, but it is not noticeable by the average homeowner. Most of the times the homeowner notices it when it's at the stage of 30, 40 or 50 percent defoliation. Three years is the earliest it would be noticeable," he said.

Defoliation, which makes trees more susceptible to disease and wood-boring creatures, weakens and gradually kills them. Gypsy moth egg masses, laid in August, hold several hundred larvae, which emerge in late April. Over the next month they develop into charcoal gray caterpillars that are about two inches long and identifiable by the double row of blue and red dots on their backs.

After feasting on leaves they form cocoons from which the adult moths issue in 10 to 14 days. The black-spotted white female and her smaller dark brown male suitor then mate and, after the eggs have been laid, die.

Moth sightings first occurred in 1981 when infested areas were found near Great Falls, Annandale, McLean and Springfield in Fairfax County. In 1982 the number of infested sites in Fairfax grew to 302, and officials reported significant infestation north of a line at Fredericksburg and west to Shenandoah County. There also were sightings also in Richmond and around Norfolk.

Local officials responded by aerial and ground spraying of bacillus thuringiensis, a pesticide that is not harmful to humans or animals. Last year, Virginia officials began spraying isolated parts of Fairfax and Loudoun counties.

This spring 92 acres in Fairfax County, including Fairfax Station, Fairfax City, Annandale, Springfield and McLean, were sprayed along with 31 acres at seven other Va. sites.

Last May a Maryland state official said 110,000 acres in 11 counties were sprayed. In Montgomery County, 600 of the 1,000 acres sprayed were in the Bethesda area. Other sites there included Germantown, Lewisdale and Seneca Creek. As many as 10,000 acres in Montgomery County may have to be sprayed.