New foreign pests invade U.S. trees; devastation feared

By Howard Libit
The Baltimore Sun

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America's trees are under attack.

Species by species, they're being invaded by insects and fungi, native and foreign. Scientists fear their loss will devastate suburban streets and upset the delicate ecological balance of many woodlands.

"Invasive species are a real threat to the nation's forests," said Dale Bosworth, chief of the U.S. Forest Service, at a recent conference in New Orleans. "There are so many things, it just seems too big to talk about. Like a slow-moving fire, they're going everywhere."

In the past 150 years, fungus-based diseases known as chestnut blight and Dutch elm disease have virtually wiped out the American chestnut and American elm.

These days, scientists point to dozens of newer invaders, from the well-known European gypsy moth to the newly discovered emerald ash borer.

"We're seeing plague after plague come in," said Faith Campbell, head of the invasive species program at the American Lands Alliance and co-author of two definitive reports on the loss of the nation's forests.

Native pests that have long been part of the forest ecosystem suddenly are gaining an upper hand in some areas of the country, as expanding development and extreme weather leave trees more vulnerable to attack.

Last month, the wildfires in Southern California were fueled by more than 400,000 acres of dead ponderosa pine trees killed by Western pine beetles native to the forests. Typically, they co-exist with the trees.

"Our native insects are always present, usually in nonoutbreak or nonepidemic levels," said Michael Raupp, chairman of the entomology department at the University of Maryland, College Park. "A tree gets struck by lightning, or one tree falls on another tree, and the insects come in and take over that tree.

"But when we impose a drought over a large geographic area, instead of having just a few trees, you have thousands of trees that are susceptible, and this is the condition that leads to the outbreak of the native pest," Raupp said.

Among forest watchers, perhaps the biggest concern over the past decade has been an influx of new pests. Often arriving from foreign countries, they hitch a ride on live horticultural imports or sneak in through wood packaging in container freight.

"This is probably the most under-reported threat facing America's trees," said Phyllis Windle, an invasive-species expert at the Union of Concerned Scientists. "There's almost something for every kind of tree.

"If we don't get a handle on this problem, we could lose a third or more of the trees in this country over the next several decades."

Expanded trade with various parts of China has brought in a new set of pests that live in areas with climates similar to those of the U.S. — but here they lack the native enemies that usually keep the pests in check overseas. Just-in-time shipping, which speeds up international cargo traffic, makes it easier for pests to survive trans-Atlantic or trans-Pacific journeys.

In some areas of the country, previous battles with invaders have left communities and forests more vulnerable. For example, the elms wiped out by Dutch elm disease frequently were replaced along suburban streets by ash trees — which are now under attack by a new pest.

State and federal officials who want to save their trees must mount a rapid and heavy-handed response to just about any possible infestation.

Some pests aren't fatal by themselves — they just weaken trees and make them more susceptible to disease. Other invaders attack until they cut off the supply of food and water, killing every tree they infest.

Scientists can spray some trees to drive off pests and inject others with chemicals. But those are expensive propositions, viable for individual homeowners but impractical for large parks or woodlands.

Consider the emerald ash borer, a deadly exotic beetle from Asia discovered for the first time in the United States last year. Raupp described the insect as "the biggest threat to our natural forest, in my opinion, since chestnut blight."

"It kills every ash it sees, from the healthiest to the weakest. It has the potential to basically eliminate ash trees as a component of our natural forest stands here," Raupp said.

The emerald ash borer, which most likely arrived through shipping crates from China five to 10 years ago, has infested at least 5 million trees in Michigan and a small portion of Ohio.

Last spring, a Maryland nursery unknowingly received a shipment of ash trees from Michigan. When Maryland officials discovered the infested imports, their reaction was swift and deadly.

All the ash trees left in the nursery were destroyed, as were trees that had been sold and planted. The borer spreads so quickly that it had infested several hundred other ash trees on the nursery property, so those were destroyed, too. State officials fear that the beetle might have spread to ash trees in the neighborhood around the nursery, so those trees are being searched.

"We're hopeful that we're getting the upper hand on it, but something like the ash borer is so deadly that you can't really be confident," said Robert Rabaglia, an entomologist with the Maryland Department of Agriculture's Forest Pest Management Section. "Those are a lot of resources we're devoting to just one infestation, but if we don't do it, we could see ash trees across Maryland threatened in a few years."

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