Gypsy moth caterpillars chewed their way through more than 1 million acres of the region's forests this year, making it the worst outbreak of the exotic pest in more than a decade.

Fall brought an end to their droppings-feces and leaf debris-that littered many patios and driveways, but experts say the outbreak will affect the forest ecosystem for years to come.

Gypsy moth caterpillars eat vast amounts of tree leaves. The process weakens the trees and makes them more likely to die over time.

They can also add to Bay pollution. Research from the University of Maryland Center for Environmental Science suggests that forests damaged by gypsy moths may release increased pulses of nitrogen into streams and rivers. Nitrogen is a major pollutant in the Chesapeake.

Maryland, Pennsylvania, Virginia and West Virginia felt the impact of the gypsy moth caterpillars this year as they munched their way through the canopy, stripping leaves from drought-stressed trees and marking many valuable hardwoods for an early death.

"It's not as bad as the 1990s, when 4 million acres were stripped. But where it's bad, it's really bad," said Tim Marasco of the Pennsylvania Bureau of Forestry.

When it gets bad, scientists say there is little they can do. Spraying affected forests with insecticide can help reduce their numbers, but officials say that the amount of spraying necessary to contain an outbreak this large isn't economically or environmentally feasible.

"Spraying helps contain the widespread gypsy moth damage we have seen in the past, but the major controlling factor is, and will continue to be, the prevalence of a gypsy moth fungus in our woodlands," said Donald Eggen, forest health manager with the Pennsylvania Bureau of Forestry.

The dry weather played a role in boosting the moth's cyclical return by reducing the presence of a naturally occurring fungus (Entomophaga maimaiga) that kills a large number of gypsy moths in North America. As recently as 2003, only 1,404 acres were defoliated in Pennsylvania.

In contrast, the caterpillars this year defoliated at least 900,000 acres of trees in Pennsylvania; 73,000 acres in Virginia; 68,000 in Maryland; and 78,000 in West Virginia.

The actual numbers may be higher. The damage reports are typically compiled by flying over forested areas and mapping damage that is visible to the eye. Areas with less severe damage may not be spotted.

"There has to be 30 to 40 percent of the leaves gone before it's clear that something's missing," said Bob Tichenor of the Maryland Department of Agriculture.

The amount of trees felled by the gypsy moths won't be known for years, in part because it takes a year or more for weakened trees to die. Most trees survive the first attack. Some even sprout a second round of leaves before fall arrives. But Tichenor said that re-foliating does not guarantee future health.

"Gypsy moths strip the leaves in June," Tichenor said. "The trees re-foliate in July, when it's hot and dry. It's a real disadvantage to the trees. This year, they re-foliated during one of the worst droughts in years. It sets them up for other tree diseases and boring insects, and they can't fight them off as well."

Damage assessments also take time because the outbreak isn't over. Gypsy moth outbreaks last for 3-4 consecutive years, and 2007 was the first year of the latest cycle.

"Populations go up before they go down, and we're on an upswing right now," said Chris Asaro of the Virginia Department of Forestry. "Unless we have a really wet May and June next year, we're probably going to see a lot worse."

After two or more years of an outbreak, trees become increasingly weak and more vulnerable to drought and disease. The West Virginia Department of Agriculture reports that up to 80 percent of hardwood trees can be lost after two consecutive years of defoliation.

Researchers are still struggling to predict how gypsy moths will affect forest vegetation in the long term. One concern is the loss of oak trees, which are valuable to both the economy and the forest ecosystem.

Oak leaves are a favorite food of gypsy moths, so it's likely that oaks may bear the brunt of the damage. Asaro believes that gypsy moths will eventually thin the oak population through the region.

"Some oaks require very exacting conditions to regenerate," Asaro said. "So once some of our mature forest oaks are gone, trees like tulip poplar and red maple will come up in their place. Gypsy moths won't eliminate oak, but they will reduce its prevalence."

That's a concern because oak is already on the decline in the region. In Virginia, they declined from 34 percent of all trees in 1966 to 18 percent in 2001, according to the State of Chesapeake Forests report issued last year. In Maryland, the total volume of oak species declined from 45 percent in 1950 to 28 percent in 1999.

The report said oak were "the most important trees in Chesapeake forests to wildlife and provide numerous other benefits such as soil stabilization and high timber value." After the demise of the chestnut trees to an exotic pest a century ago, the acorns from oaks became a prime source of food for many types of wildlife. When oaks disappear, the trees that replace them, such as red maple, tend to be less economically valuable, thereby reducing the value for harvest.

A team of researchers led by Keith Eshleman at the University of Maryland Center for Environmental Science has found that gypsy moth outbreaks also impact streams and rivers.

Eshleman studies a variety of forest disturbances, including weather, insects, human activities and their effect on water quality. He has found that gypsy moth outbreaks lead to an increased discharge of nitrogen into local waterways that can last for at least six to eight years after the disturbance.

The reasons aren't yet clear.

One theory is that the volume of "frass" material, dropped to the forest floor, changes processes in the soil. Frass includes both the droppings of caterpillars and larvae and the undigested leaf bits that fall from trees. It's a lot like lawn fertilizer-organic but high in nitrogen. Microbes in the soil convert nitrogen into nitrate, which moves through the groundwater and into adjacent streams.

"The amount of frass is very high, in the range of 1 to 2 metric tons per hectare each season," Eshleman said. "It has the potential to drastically affect what's going on in the soil."

In earlier studies, the nitrogen increase was detected during the fall of the first-year outbreak.

"We're anxious to see if it happens that way in western Maryland," Eshleman said. "We expect to see it as water levels rise and start to move through the forested soils, but we can't confirm it yet."

The streams Eshleman has studied have consistently returned to normal conditions after five to eight years. "The pulse decays over time. Forests regenerate," Eshleman said. "But in terms of water and forest management, we have to recognize all of the things that disturb the landscape, not just the ones that are the most obvious."

Moth Invasion Begain in Backyard

Gypsy moths have damaged millions of acres of forest in the United States. Each damaged tree can be traced to a single Massachusetts backyard, where in 1869, a homegrown science experiment suddenly went wrong.

Gypsy moths have existed in Europe and Asia for thousands of years.

They arrived in the United States in the late 1860s in the hands of E. Leopold Trouvelot, an artist living near Boston. Trouvelot collected gypsy moth egg masses while traveling in France. He attempted to cultivate them in his backyard, but some of the larvae escaped.

Ten years later, the first gypsy moth outbreak occurred on his street. By 1889, its spread in the Boston area triggered an eradication effort by the Massachusetts State Board of Agriculture that lasted until 1900. They burned forests, applied insecticides and removed egg masses by hand, but failed to destroy the moths or contain their spread.

"We may be stuck with the gypsy moth," said Tim Marasco of the Pennsylvania Bureau of Forestry, "but it's a good reminder that we have to be more careful with how we move things around this country-like nursery stock and firewood-and with how we handle our imports. I don't know if the message has hit home yet."

Other Insect Invaders

The gypsy moth is one of several invasive or alien species that threaten Chesapeake forests. Other examples include the:

  • Hemlock Woolly Adelgid: This insect feeds on hemlock trees. Found throughout the East Coast, this pest has destroyed most of the hemlocks in Shenandoah National Park and infested 47 counties in the eastern two-thirds of Pennsylvania. Some trees die within four years; others survive in a weakened state.
  • Emerald Ash Borer: This insect feeds on ash trees and has been detected in Pennsylvania, Virginia and Maryland. Six to 6.5 million trees in the Baltimore area alone are ash trees and account for approximately 10 percent of the city's trees. Trees usually die within three years of infestation.
  • Asian Longhorn Beetle: This insect feeds on hardwood species such as maple, box elder, alder, elm, birch, poplar and willow. It has been detected in New York, New Jersey and Illinois.