The worst outbreak of forest tent caterpillars in more than 50 years has damaged trees and caused alarm as the white-spotted, worm-like black larvae mass on tree limbs and dangle from silken skeins.

"It's like Alfred Hitchcock's 'The Birds,' except these are caterpillars," said Nelson V. Martin, Fine. "The side of my house is loaded. They're eating everything. I came in from outside the other day and my wife took about eight off me."

The major hot spots in New York of forest tent caterpillars are around Tully and LaFayette, Camden to Mexico, and in more northern areas east and west of Lowville and Croghan and west to Black Lake and Parishville, said Jeffrey T. Duflo, owner of Duflo Spray Chemical, New Bremen.

"You can fly in any direction and you'll see defoliation. It's very widespread," he said. "It's a war out there. People are sick of them."

Stephen F. VanderMark, a horticultural and natural resources educator with Cornell Cooperative Extension, hasn't been able to keep up those who want information about the caterpillars.

"Since last week, for each call in, I've gotten three new calls," he said. "I haven't had time to count how many people have called."

Despite their name, forest tent caterpillars don't make distinctive webbed canopies, as do eastern tent caterpillars. Forest tent caterpillars favor poplar and sugar maple trees, while eastern tent caterpillars - also active this year - prefer black cherry and crabapple trees.

Forest tent caterpillars are native to the north country, and their outbreaks tend to run in cycles that last two to six years between intervals of 10 to 12 years.

"There are historical accounts of trains being stopped by them because they build up on the tracks and there's no friction," said St. Lawrence University biology professor William P. Rivers, who has been studying the effect of the caterpillars on sugarbush production. "What's unknown is exactly what controls the peak."

One factor leading to the latest onslaught may be forest fragmentation, which occurs when a continuous canopy isn't maintained. Logging out 10 or so acres would increase the number of edges, for example, Mr. Rivers said.

Forest managers who thin diseased trees in the middle of an onslaught of caterpillars might actually worsen conditions, he said. Soil compaction from logging may be another factor.

"You really just want to stay out of the forest until it can recover," he said.

The effect on maple sugarbushes has already been seen.

"As one would expect, the spring following a year when there's been heavy caterpillar damage, the sugar content drops significantly," Mr. Rivers said. "On average, there's going to be a drop, but for any individual tree, it's hard to predict because there are so many variables."

The caterpillars have taken a toll, said Hugh L. Newton, Hannawa Falls, president of the St. Lawrence County Maple Producers Association.

"I know some people who didn't tap at all," he said. "The trees, they're stressed."

Drought prior to the immediate explosion of forest tent caterpillars, which were around Sylvia Lake and Lake Bonaparte two years ago, may have hampered natural controls that include the "friendly fly" and a parasitic wasp that eat the developing moths, Mr. VanderMark said.

Caterpillars can also be hit by a virus and a fungus.

Mr. Duflo has sprayed for the last few weeks with Bt, Bacillus thuringiensis, a naturally occurring bacterium that paralyzes the stomach of the caterpillar.

This year, he was hired by the state of Vermont to spray 19 sugarbushes, and by lake associations, villages, camps, hunting clubs and individual property owners.

Bt works as long as the caterpillars are eating, which should continue for another week or so, Mr. Duflo said. Chemical controls are also available until the caterpillars spin cocoons from which the moths will hatch later this summer.

Sugarbush and forest managers can monitor egg masses later this year to determine whether they want to spray with Bt next spring, Mr. VanderMark said.

The egg masses, which can each hold 300 caterpillars, appear as a hard foam in what seems to be an enlarged portion of a twig, Mr. Rivers said.

"They're out there, just waiting," he said. "I'm hoping after this year we'll start to see them decline."

But no one is sure when the end will come.

"Historically, these things will crash after awhile," Mr. Duflo said. "This one seems to be spreading to new areas. I think in a number of areas, it's just starting."