An infestation of forest tent caterpillars is costing the state's timber industry millions of dollars and causing maple syrup producers added worry. The 2-inch-long caterpillars defoliated about 70,000 acres around the state this spring.

The insects munch on the canopies of northern hardwoods, in some cases almost completely consuming the leaves. Without the ability to photosynthesize, the trees can't produce a winter store of carbohydrates or the chemical defenses that protect them against other invaders. Tree growth is stunted.

This is the second year of the infestation, although just 10,000 acres were affected last year. The caterpillars, characterized by a row of white dots on their backs, are native to New England, but there has not been a widespread infestation of them in New Hampshire since the early 1980s.

"They're here, and they're multiplying and they're moving," said Kyle Lombard, forest health specialist with the Division of Forest and Lands.

The caterpillars don't often kill the trees. But the trees become more vulnerable to other damaging effects, such as poor soils or drought. Tapping for syrup and selective harvesting are also stresses when the trees are weak.

State experts are asking syrup producers to follow conservative tapping guidelines - one tap on trees with a diameter of 12 to 18 inches and two taps on anything bigger - and to monitor for declining health. Foresters should avoid cutting in affected areas.

Lombard estimated that New Hampshire's timber harvest will be down between $4 million and $5 million this year because foresters have had to delay cutting plans.

"The economic impact is immense," he said.

Lombard said the infestation was likely carried by wind from Vermont, where 400,000 acres were defoliated this year, up from 100,000 last year. In New Hampshire, the forests around Mount Sunapee have been most affected.

About 5,000 acres in Goshen, Newbury and Washington managed by LandVest Timberland were defoliated. Forester Bob Bradbury estimated that the company will have to harvest 25 percent to 50 percent less timber this year in an area that normally brings in about $500,000.

Bradbury, who has been a forester in New England for 20 years, said this is one of the worst infestations he's seen. He said one parcel in Goshen was about 50 percent defoliated by summer.

"It really looked like kind of a fall day, where the leaves were falling off, and it was the middle of August," he said.

Bradbury said his crews are still working to clean up areas damaged by the 1998 ice storm, but they can't work where the trees have been weakened. He's worried about losing trees in those areas, where the forest is already under stress.

Forester Don Clifford said it was eerie to see trees on his land defoliated in June. On a 20-acre parcel that he manages on Pike Hill in Newport, he said, "there was not one leaf left."

Another 60-acre plot that Clifford owns in Plainfield was defoliated. In both places, the caterpillars had attacked mostly sugar maples and red oaks, which are two of the most valuable trees in New Hampshire. He said they bring in about $400 per 1,000 board feet, or feet of board produced at the mill that is an inch thick and a foot wide. Clifford said he will probably reduce his timber harvest by about 25 percent this year.

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