Picnickers in East Coast woods may get some hungry visitors this summer. But at least they won't ask for sandwiches.

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. The moths will also munch on 475 types of foliage.

At their largest, the hungry caterpillars can consume 1 square foot of foliage per day, according to University of Maryland entomologist Mike Raupp.

"As soon as the warm weather hits, they eat like dynamos, juggernauts," Raupp said.

In Maryland, 50,000 acres of forests were sprayed last month with a bacterial pesticide, the most acreage sprayed since 1995. Defoliation there appears to be worse than last year, especially near the Pennsylvania line, according to the state agriculture department.

Nearly 700,000 acres of Pennsylvania 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.

Homeowners in the Washington, D.C., area are reporting green trees "disappearing in the blink of an eye," Raupp said.

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

While spraying can kill gypsy moths' larvae at very early stages, not every caterpillar-covered tree can be treated, said Greg Hoover, an 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 Hoover, who has worked at Penn State for nearly two decades.

Most trees in the region are as full and green as they will be all year. Trees that lose leaves now may not immediately die, but re-sprouting leaves in the middle of the season can stress them.

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

After hatching, caterpillars try to feast on foliage, then return to a "resting" stage before transforming into moths.

Female moths lay eggs, primarily on tree trunks, restarting the cycle. Cool, wet springs can lead to the rise of viral and fungal diseases that attack the eggs, keeping populations down.

Homeowners who suspect gypsy moths should contact forest or environmental officials to see if they need spraying, Eggen said.

The critters first appeared in the United States in the 1860s after an amateur entomologist who settled outside Boston brought them 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, according to Christopher Tipping, an assistant professor of biology at Delaware Valley College in Doylestown. They've spread as far west as Oregon and as far south as Virginia.

The U.S. Forest Service has been working with some states bordering those with high gypsy moth populations to try to suppress their advance.

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

But eradicating the gypsy moth is nearly impossible. More gypsy moths this year will likely mean more eggs ready to hatch in 2008.

"We have to gird ourselves for a pretty serious year next year," Raupp said.