The gypsy moths have returned to the Washington suburbs, and they are feasting on the leaves of the region's trees with a vengeance.

"These horrible creatures are everywhere," said Kathryn Ruppert, a resident of Arlington's Country Club Hills, where trees as bare as they usually are in January stand alongside $300,000 homes. "You can't walk down the street without stepping on them, or under a tree without having them fall all over you . . . . They're worse than the seven-year locusts we had."

Although the District of Columbia remains relatively unscathed by the moth's voracious leaf-eating caterpillar, Maryland expects to lose 50,000 acres of trees and Virginia anticipates the destruction of about 2,500 acres this year, officials said.

The moth's relentless migration from the Northeast to the South, where there are more of the oaks and other hardwoods the moths prefer, has led to what some local and state officials describe as a crisis situation. Hardest hit locally are Arlington, Montgomery and Loudoun counties.

Nationally, the U.S. Department of Agriculture said, the moth's caterpillars destroyed 29 million acres of trees between 1980 and 1984.

In Maryland, where almost $1.5 million will be spent this year to combat the pest, officials cite a phenomenal increase in the number of trees affected. Only three acres of trees were devoured by the caterpillars in 1980, said Robert Tichenor of Maryland's agriculture department, compared with this year's expected 50,000 acres.

"Right now," he said, "we're at a crisis. We've got hundreds of thousands of acres infested."

The most severely infested areas are in the western part of the state and the upper Eastern Shore, Tichenor said. The only uninfested areas are Calvert, St. Mary's, Somerset and Charles counties. Montgomery and Prince George's counties, he said, have pockets of low-level infestations.

In Virginia, which recorded its first cases of defoliation only last year when the insects ate the leaves from 374 acres of trees, Donald Kludy of the Virginia Extension Service estimates 2,500 acres will lose their leaves this year. The state will spend $250,000 to combat the problem.

"Northern Virginia is our biggest problem," said Kludy. "There's really no way to stop it. If possible, we're trying to control the spread."

The problem has been exacerbated this year, Kludy and Tichenor said, by the warm, dry spring. Rain helps suppress the caterpillar's development.

While the District and federal parkland in the region have very low levels of infestation, officials say, almost all of Maryland and all of Northern Virginia remain under a "quarantine," including Alexandria, where the infestation is described as "extremely light," and Fairfax County, where it is "fairly low."

Officials are bringing out an arsenal of weapons to fight the moth, which destroys trees only in its caterpillar stage, a phase that is now ending. The devices range from nontoxic pesticides to the release of thousands of gnat-sized wasps that destroy the moth's larvae but do not sting humans.

Left alone, the moths can destroy the sturdiest of trees after an infestation cycle of two to four years by eating the tree's life-sustaining leaves. Although the trees will resprout for a few years, their store of energy becomes so depleted the trees eventually die.

Of the close-in suburbs, Arlington and Bethesda have been hardest hit. But neither has suffered the devastation of Loudoun County, where an estimated 1,700 acres have been fully or partially defoliated, most in the unpopulated Short Hill Mountain area north of Hillsboro.

Terry Laycock, Loudoun's gypsy moth coordinator, said 700 acres on the mountain have been completely stripped of leaves, and "pretty much the entire county" has some level of infestation.

Although nearly 4,000 Blue Ridge Mountain acres have been sprayed, Laycock said, Short Hill has not been sprayed because of the sparse human population and lack of public recreational facilities.

In Arlington, nearly 70 acres in the posh County Club Hills neighborhood in the northwestern sector were sprayed this year, said Nancy Gray, the county's gypsy moth coordinator. The county also is battling the moth in an additional 90 acres.

In Montgomery County, state extension agent Stanton Gill said there has been "quite a bit of defoliation" in Bethesda, Chevy Chase and Kensington. He said trees along Wilson Lane in Bethesda are almost 90 percent defoliated.

In Prince George's County, there is a minor problem in the northern area where large government installations are located, and in about 40 acres in West Lanham, extension agents say.

In Alexandria, there is no spraying this year because the infestation is light. However, in Fairfax County, 1,150 acres in northern Springfield, western Annandale, McLean, Vienna and the Clifton area have been sprayed.

In the District, the moths are "not a serious problem," one extension agent said, although there have been spottings of them in the area west of Rock Creek Park and around American University, Sibley Hospital and Chevy Chase.

The large, gray gypsy moth caterpillars are distinctive from others because of their rows of blue and red dots. The moth will emerge as an adult in late June or early July and begin mating.