Nobody knows for certain whether it was the work of Harvard dons, World War II, stinky shoes or diligent researchers.

Whatever the case, a fungus that officials believe has subdued the gypsy moth is now well established in parts of the Northeast and the mid-Atlantic, granting what one official called a "reprieve" to those regions' trees.

In the early 1980s, gypsy moths appeared likely to infest whole forests and gnaw off their leaves. But this year marks the fourth spring in a row that Virginia has not had major spray programs or any reported defoliation by the moth. The same circumstances are reported in other states.

That is a far cry from the days in the late 1980s and early 1990s when the Warrenton-based state control teams supervised the spraying of Northern Virginia counties in what appeared to be a low-grade war.

The insect still is present in smaller numbers locally and still is spreading to points north, south and west. But its invasions are tamer than they were during the 1980s, when the moth roared down the Blue Ridge Mountains, showing itself in the state in mass numbers for the first time in Loudoun County.

"That fungus really did a number on them," said Frank Fulgham, supervisor of field operations for the Virginia Department of Agriculture and Consumer Services Office of Plant and Pest Services.

The fungus, called Entomophaga maimaiga, takes about a week to kill the moths during their caterpillar stage in the spring, entering through the skin and infesting the guts. Ann E. Hajek, a Cornell University professor and one of the lead researchers on the fungus, has proposed several theories about how it was spread.

Under one theory, a Harvard researcher who isolated the fungus introduced it in parts of the Northeast in 1910 and 1911, the first attempt at biological control of the gypsy moth with the fungus. Previously, scientists had attempted to use a virus.

Under another theory, returning troops from World War II brought back the fungus, thought to be native to parts of Asia. Still other theories have the fungus spores carried into this country on shoes or by air.

In 1985, 1986 and again in 1991, the U.S. Forest Service and the Virginia Department of Agriculture deposited the fungus in parts of Virginia. Some scientists speculated that it didn't do its damage to the gypsy moth population until 1995 because the moth population wasn't large enough until then.

"You have to have a certain number for it to spread," Hajek said.

According to state figures, the first defoliation was reported in 1984, involving 374 acres. That year, 8,000 acres were treated. Five years later, 213,987 acres were reported defoliated and 345,102 treated.

In 1995, 839,854 acres were defoliated and 269,190 treated. The next year presented a startling development when there were no trees that appeared defoliated.

"It was just a tremendous relief," said Tom Finn, Warrenton-based supervisor for the state's Office of Plant and Pest Services. "[The population] just dropped overnight."

That relief has continued until the present day. Just two weeks ago, Finn said, Prince William County sprayed 1,330 acres, the first such treatment in Virginia since 1996.

The dramatic decrease in activity means that communities no longer have mass sprayings and localities have dismantled their gypsy moth control programs. In Fauquier County, for example, the program that once occupied two full-time employees and one part-timer now is assumed as a part-time duty by one employee.

"The county has decided, with some forethought, to maintain the integrity of the program," said Tim Bridges, assistant director of the Fauquier County Department of Environmental Services. "The guard is always there."

Researchers and federal officials say it better be, because the nature of how the fungus spreads and whether it will maintain itself is uncertain.

"It was a surprise to a lot of people that all of a sudden, it just wiped out the gypsy moth populations in the East," said John Ghent, a West Virginia-based entomologist for the U.S. Forest Service.

The current gypsy moth range in North America includes all of the Northeast and parts of the Southeast and Midwest, as well as parts of eastern Canada. The leading edge is in Wisconsin, and that area may need to get on the war footing that localities in Virginia did in the mid-1980s, considering the moth does its worst damage when it first arrives in large numbers. Virginia officials said the damage would have been far worse without the initial spraying.

Finn remembered what that was like. Each May, he and his crew would be up at 4:30 in the morning, waiting at airports for spray planes to take them over tracts of land in Northern Virginia, where 95 percent of all suppression programs were focused.

"It was pretty intense," he said. "Now we have time to work on other things" -- such as inchworms.