STATE COLLEGE, Pa. (AP) – These creepy critters enjoy nice long picnics in the woods.

The leaf-eating gypsy moth caterpillars are out in force in parts of the mid-Atlantic following a warm, dry spring – just the kind of weather that can make the insects thrive.

Experts are predicting an especially bad year for trees, primarily oaks, which are the caterpillars’ favorite snack.

Nearly 700,000 acres of forests – primarily in the Poconos and central Pennsylvania – were defoliated last year because of the persistent bugs, and the infestation could be worse in 2007, said Donald Eggen, director of the Office of Forest Pest Management for the Pennsylvania Bureau of Forestry.

“This is a bad outbreak. Trees are completely defoliated in some areas,” Mr. Eggen said. “Literally trees are inundated with them.”

In New Jersey, this year’s infestation has been called one of the largest in recent years. That state’s agriculture department expects more than 200,000 acres to be defoliated this year, up from 125,000 last year.

In Maryland, 50,000 acres of forests were sprayed last month, the most acreage sprayed since 1995. Defoliation there appears to be worse than last year, especially near the Pennsylvania line, said state agriculture spokeswoman Kate Wagner.

While spraying can kill gypsy moths larvae at very early stages, not every caterpillar-covered tree can get treated, said Greg Hoover, an ornamental entomologist at Penn State University.

“In all my years as a forest entomologist, I have never witnessed the quick development of larvae as I have in 2007,” said Mr. Hoover, who has worked at Penn State for nearly two decades.

Most trees in the region are just hitting peak, at their fullest and greenest. Trees that lose leaves now may not necessarily immediately die, but can be stressed from having to re-sprout leaves in the middle of the season.

Trees that grow new leaves may not be as full as healthy ones, and may be more susceptible to other ailments after using up energy reserves, Mr. Eggen said. Oak trees can die after three straight years of infestation, while conifers can succumb after just a year.

Spraying typically occurs in the early- to mid-spring in areas where scientists think there might be high infestations. The spraying is timed to try to catch the caterpillars just after they hatch from eggs laid the previous summer.

Those caterpillars are now feasting on leaves before returning to a “resting” stage, in which they transform into moths.

The female moths lay eggs, primarily on tree trunks, starting the cycle over again. Cool, wet springs can lead to the rise of a fungus that affects the eggs, and in turn keep populations down.

Homeowners who suspect gypsy moths should contact their forest or environmental officials now to determine if spraying should occur there next spring, Mr. Eggen said.

The critters first made their appearance in the United States in the 1860s after an amateur entomologist who settled outside Boston brought them over from France. He was conducting an experiment on trees near his house when some of the larvae escaped.

Since then, gypsy moths have expanded about five miles a year, with the largest populations having spread as far west as the Great Lakes and as far south as Virginia, according to Christopher Tipping, an assistant professor of biology at Delaware Valley College in Doylestown.

Gypsy moths don’t seem to be as serious a problem in the Midwest this year though. For instance, Michigan didn’t have a statewide spraying program for the bugs for the first time in 20 years, said Michael Philip of the state’s agriculture department.

The U.S. Forest Service has also been working with some states bordering those with high gypsy moth population on programs to try to suppress the advancement of the insects.

But eradicating the gypsy moth is nearly impossible.

“There’s not really much we can do to stop this,” Mr. Tipping said. “This is part of the natural environment right now.”

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On the Net:

U.S. Forest Service gypsy moth information: http://www.fs.fed.us/ne/morgantown/4557/gmoth/

State Forest Pest Management office: http://www.dcnr.state.pa.us/FORESTRY/fpm.aspx