Falls Church may be Tree City U.S.A., but a horde of leaf-munching gypsy moths, apparently ignorant of the designation, is gearing up for an assault on the city anyway.

The moths, which in their caterpillar stage destroy trees by eating every leaf and bud, have cropped up across the city, and officials say many residents' beloved trees will fall victim to them within the next two years.

"It is a cruel irony," said Falls Church arborist Shirley Street last week. "We are mapping strategies of things we can do . . . like spraying chemicals . But we will probably lose the war.

"Trees are going to die, others are going to look awful. It is a shame. We are a group of people that really cares about our trees."

Street said this summer is probably the first of a three-year gypsy moth onslaught. The moths will increase in number drastically for the next two years, she predicted, then probably die of a disease that strikes only when their numbers have proliferated.

For the past four years, Falls Church has been one of 291 American cities winning the Tree City U.S.A. designation from the National Arbor Day Foundation in Nebraska.

Falls Church meets such Tree City criteria as spending more than a dollar a year per capita on trees and hiring Street, who is a full-time arborist, said Jackie Lorenzen, spokesman for the Arbor Day Foundation.

The arborist's position was created four years ago, and Street has held it for the past three years. She said she normally spends her days checking trees throughout the two-square-mile city, making house calls to tend sick trees, deciding which municipal trees should be pruned and which diseased trees should be cut down.

The city is full of graceful native elms and oaks, she said, and has some of the most unusual trees "this side of the National Arboretum."

These days she has been tracking down reports of gypsy moths like a dogged detective. She encourages residents to set up cardboard gypsy moth traps and count the number of moths they discover each week. State and Fairfax County agriculture officials use such data to determine where the moths are most abundant and where they will appear next.

The moths are in their damaging caterpillar stage in early summer. By this time of year, when they are flying, they are annoying but are not dangerous to trees, she said.

But she predicted that by early next summer, this year's moths will have spawned enough caterpillars to chomp their way across much of Virginia.

"They've worked their way down from New England," she said. "Most residents here didn't even notice them this year but I guarantee they will notice them next year. They will be hard to miss."

According to Street and others, an army of gypsy moth caterpillars can defoliate a tree in a single night, eating many times their own weight. The noise of their munching can disturb a quiet suburban night and many of them fall to the ground and decay. "I understand they're disgusting to step on," said Street.

She said most trees can grow a second set of leaves in mid-July but the effort weakens the trees and exposes them to infection. Defoliated evergreens die immediately.

"A heavily infested area will lose about 13 percent of its trees," she said.

Because the moths have few natural enemies, Street said, the city is considering spraying an organic chemical called Sevin next spring.

"But the moths proliferate like crazy. Sevin has a 90 percent effectiveness rate, so you think, 'Good, we got nine out of ten.' But that one left will have up to 1,000 offspring. It's mind-boggling."

Kay Bachman, chairman of the Falls Church Arbor Day Committee, said Tree City residents are concerned about the gypsy moth invasion.

"We are setting traps and there is talk of spraying," she said with hope. "We will do what we can. It is a pity because we have a great many trees. We are not a community of large lawns, a house and one little bush out by the street. We are naturally a wooded area and we are proud to have our beautiful trees."

Tree City U.S.A. or not, Street said, Falls Church "is in for it."