Washington area officials have stepped up surveillance of the advance of the voracious gypsy moth, which has devastated more than 21 million acres of trees in the Northeast since 1981. In some jurisdictions acres of trees have been sprayed to control the infestation while officials in other local areas are monitoring the extent of this season's outbreak.

In a joint campaign by state and local officials, 92 acres in Fairfax County were sprayed from the ground, as well as 31 acres at seven other sites around Virginia. Most sites in the county were in Fairfax Station, Annandale, Springfield, Fairfax City and McLean. In Montgomery County, 1,000 acres were sprayed from the air earlier this week.

In Montgomery County, nearly 600 of the 1,000 acres aerially sprayed were in the Bethesda area, said Stanton Gill, a county extension agent. Other areas included Germantown, Lewisdale and Seneca Creek.

"I wouldn't call this an epidemic. It's a growing problem people should be aware of, but they shouldn't panic about losing all their trees," Gill said, adding he expects the county may have to spray as many as 10,000 acres next summer.

The gypsy moth is most dangerous in its caterpillar stage. When the caterpillars hatch in late April and early May, they feed on leaves, usually at night. Repeated defoliation can kill some trees and weaken others, making them more vulnerable to other pests and borers. Complete defoliation takes about three years.

"You can't eliminate them, but you can try to suppress or minimize their impact," said Donald Kludy, supervisor of the Virginia Agriculture Department's plant and pest-control section. "They have not reached the population where defoliation is going to occur this year . . . We're in the lull before the storm breaks out. In two years, we may see substantial defoliation in Northern Virginia."

Fairfax and Montgomery officials stressed that the chemicals used to kill the moth's egg masses was not harmful to people, pets or plants, and said that residents of the area were notified of spraying plans.

Tony Evans, spokesman for Maryland's agriculture department, said 110,000 acres scattered among 11 counties were sprayed this week, many of them bordering Pennsylvania which has been particularly hard hit by the caterpillars.

Officials in Alexandria,, the District of Columbia, the counties of Arlington, Loudoun, Prince George's and Prince William and The National Park Service say that instead of spraying, they intend to continue monitoring the migration of the moth.

They are concentrating their efforts on informing the public of the potential damage to trees and suggesting various monitoring techniques, ranging from lure traps that ensnare the male moth to bands of burlap wrapped around trees where the caterpillars can be detected when they seek sanctuary from daylight. Ther caterpillars should be plucked off with tweezers and destroyed in alcohol.

Gypsy moth egg masses are tan-colored and resemble a small piece of chamois cloth. Egg masses should be scraped off trees and soaked in chlorine bleach, alcohol, ammonia or burned. They should not be dropped to the ground since they are not easily crushed and probably would hatch.

Newly hatched caterpillars are about one-eighth of an inch long. The adult is dark gray, hairy, multilegged and has a double row of five blue dots and six red dots on its back.

The caterpillars are particularly fond of oak, apple, alder, aspen, basswood, hawthorn, willow and gray and river birch trees. They may also attack other birches, beech, cherry, black gum, hemlock, hickory, hornbeam, larch, maple, pine, sassafras and spruce. Besides trees, their egg masses may also be found on wood piles, window ledges, under the eaves of houses, and on recreational vehicles and tents that have been in infested areas.