Originally reported on the South Shore and Cape Cod, the new insect outbreak was thought to be an infestation of native fall cankerworms, which were also in the area. These usually crash from natural causes after about three years, so specialists didn't think there was a problem.

But the South Shore defoliation has continued and spread for at least a decade now. Horticulturalist Deborah C. Swanson of the Plymouth County Cooperative Extension/UMass Extension, raised the alarm (and kept raising it) and in 2003, and scientists at the University of Connecticut and Cornell finally identified the pest as a new arrival from Europe, Operophtera brumata.

''This was a tricky little critter that snuck in under radar because it shares many of the same hosts as the cankerworm caterpillar and looks very similar to it," said Childs. ''We didn't notice it until Deborah started notifying us seven or eight years ago and the first sample she sent were fall cankerworms. There were a couple of winter moth caterpillars in the second sample, but I thought maybe they were just spring cankerworms. I asked for a third sample in 2003, and we sent that to Dave Wagner in Connecticut," who first identified it as winter moth.

Meanwhile, the new moth multiplied because it has no natural enemies here. And even though the females are flightless, it managed to spread. ''We know it's in the coastal towns from Gloucester to Boston, in and around Boston out as far as Newton-Wellesley, and pretty much throughout southeastern Massachusetts, and most of the Cape out as far as Eastham. There's going to be more research this year to locate it," said Childs.

Now the numbers at the center of the outbreak are astounding. ''We've got some crazy, incredible densities," said the Brenda Whited of the UMass-Amherst survey team. Individual banded maples and oaks in already hard-hit Hanson and West Bridgewater last winter yielded up to 1,600 females laying eggs on each tree monitored. Since winter moths can lay 150 eggs, that adds up to almost a quarter of a million caterpillars this spring per tree. Many of these inchworms may starve to death, especially if they hatch before the tree buds swell, and numbers won't be that large inside Route 128, as the moths just reached here about three years ago. ''But it's going to get worse for the next five years, at least," said Joseph Elkinton, forest entomologist and ecologist at UMass-Amherst.

Then, if all goes according to plan, a counter measure will start kicking in. Elkinton collected a predator of the winter moth called Cyzenis albicans, in Nova Scotia, where they were released to control a winter moth outbreak there 50 years ago. He is now raising these caterpillar-killing flies in the quarantine facility at the USDA lab Otis Air National Guard Base in Falmouth. ''This is important because we need to make sure that only Cyzenis is released and not some other organism brought in accidentally with Cyzenis, which is very specific to winter moths," Elkinton added.

But the moths have a big head start. ''It took five years for them to multiply enough to catch up with the winter moth population when they were released in Nova Scotia," Elkinton said. After that the winter moth population declined dramatically there over a couple of years, and has been kept at a low level ever since.

Until then, however, you're on your own, and trees that have been defoliated for several years are very unhappy. They face decline and even death without extra help. (Canadian research found that many trees die after four consecutive years of complete defoliation, though this will vary with the tree condition and species.)

The tiny green inchworms start with oaks, maples, fruit trees, ash, rose-of-sharon and blueberry bushes, but almost any leaves or flowers are fair game after that. One way to tell if you're going to be hit hard is to check your tree trunks for the tiny orange eggs the moths randomly scatter in bark crevices. Just before they hatch, the eggs turn bright red.

The inchworms usually hatch around April 20 and immediately weasel their way inside swelling leaf buds, where they cannot be reached by sprays. The leaves open in tatters. After that, the caterpillars are ''free feeders"; they spread by swinging tree to tree like Tarzans on their own silken ropes when looking for a new food source.

In the past, the state took the ''atom bomb" approach to caterpillar outbreaks with massive and largely ineffective spraying of chemicals such as lead arsenate and DDT. Now state specialists have learned that introducing diseases and predatory insects that kill only that pest is much safer, and works better, too. A fungus from Japan called Entomophaga maimaiga has all but finished the gypsy moth as a serious pest since 1989, though it can recur when springs are dry enough to inhibit the fungus, which needs wet conditions to thrive.

There are outbreaks of other caterpillar species as well, for even the natives are restless. For some unknown reason, when one type of caterpillar overpopulates, several other kinds often follow suit. Perhaps the weather conditions suit them all. Or the birds and other predators have been just too busy keeping up with winter moths to get to them. Or perhaps when there were more gypsy moths around, they spread their diseases and pests to the other caterpillars, Childs speculated.

Winter moth caterpillars are active for only about three or four weeks. Around May 20, they will dangle down to the ground on skeins of silk and burrow into the top layer of soil, where they will become dormant pupae until emerging as adult moths between Thanksgiving and New Year's for their nighttime mating ritual. They don't eat anything then. The males just flutter around and mate with the flightless females, who clamber up trees, lay eggs, and die. Then next April those eggs will hatch . . . and on and on it goes.