The first serious large-scale damage by gypsy moths in Virginia was announced yesterday by state pest-control officials, who said the insects have devastated a portion of a mountainside in Loudoun County.

The pests have caused moderate to heavy damage to 374 acres of foliage on Short Hill Mountain west of Leesburg, officials said.

"It's just the beginning of what we're looking toward in Virginia 10 to 15 years down the road," said Robert E. Bailey, a Virginia Department of Agriculture pest-control official.

"We're seeing a continuing spread [of gypsy moths] to the south of the state," said Donald Kludy, an agriculture official.

Kludy said state officials caught 51,000 male gypsy moths in a one-month program that ended last week.

Federal and state authorities designated a quarantine area several years ago covering Loudoun, Fairfax, Prince William, Stafford, Fauquier, Warren, Clarke and Frederick counties. Under the quarantine, state officials have asked residents to submit their property to inspections before moving trees, shrubs, plants, recreational equipment and other possible carriers of gypsy moth eggs into non-quarantined areas.

Kludy said the restricted area probably will be expanded as the moths grow more common in the state.

Gypsy moths were introduced to America in Massachusetts and have spread gradually southward. Maryland, Pennsylvania and New York are widely infested with the pests, Bailey said. Pest-control officials say the gypsy moth moves at an average rate of six to 10 miles a year.

"Virginia is not generally infested like those states," Bailey said. "We're just beginning to feel the impact of the pest."