Fairfax County officials have launched an aggressive monitoring and spraying program to combat a scourge of tiny green worms that infested Mount Vernon and other areas last spring, leaving behind thousands of acres of leafless trees and a foul, brownish slime.

Helicopters will descend on eastern Fairfax in early April to spray the property of as many as 20,000 households with a chemical to kill the inch-long insects, called cankerworms. Residents in affected areas will be notified through mailings and public meetings early next year.

The $334,000 cankerworm program comes in response to a devastating outbreak this year that affected 5,200 acres of maples, oaks and other hardwoods.

In addition to effectively stripping trees of their leaves, the worms left driveways, lawns and roofs covered in droppings and spiderlike webs. At one Hollin Hills home, thousands of the caterpillars invaded the living room by inching down a chimney.

"They have spread, and they have gotten worse every year," said Troy Shaw, who is directing the county's assault on the worms. "It's something that has gotten beyond the ability of homeowners to control on their own."

The caterpillar of the fall cankerworm, often called an inchworm, feeds voraciously in early spring before dropping off trees to form a cocoon underground. The adult moths emerge in fall and early winter, when the wingless females crawl up trees to lay their eggs, beginning the cycle anew.

Cankerworms are native to most of North America and rarely pose a serious problem to trees or humans. But over the last several years, the bugs' population has soared in neighborhoods around Mount Vernon and, to a lesser extent, Franconia. Fairfax officials estimate that 1,700 acres of trees were stripped bare and an additional 3,500 acres were seriously affected.

"Trees can take it one year, but they can't take it as well the next time," said Supervisor Gerald W. Hyland (D-Mount Vernon), who has been deluged with complaints about the outbreaks. "I'm hopeful this will take care of the problem. We don't want it to spread to other parts of the county."

The Board of Supervisors voted 9 to 1 in September to allocate surplus money to the project. In addition, the board will ask legislators in Richmond to allow residents to be taxed if necessary for eradication and suppression of troublesome pests.

Currently, the special taxing districts can raise revenue only for control of gypsy moths, which have diminished as a threat in recent years.

The cankerworm program includes money to set up 60 monitoring stations, which will be used to count the number of female cankerworm moths attempting to lay eggs over the next two months. The results will determine the final map for the spraying of Bacillus thuringiensis, also called Bt, the same chemical used to combat gypsy moths.

Shaw said that the two sprayings, which will be conducted about a week apart, will pose no health dangers but that residents may opt out of the treatment if they wish.

"Our guess is that this will take care of it as a recurring problem and we won't have to spray again," Shaw said. "That's what we hope, at least."