emerald ash borer

Emerald ash borers do their damage as larvae, eating into the bark and burrowing deep into the trunk to insulate themselves against the cold. In the process, they cut off access to the nutrients and water that the tree needs to survive; it is like severing a human’s network of veins and arteries.

After surviving the unusually cold winter, the beetles emerged in spring as adults. Now they are mating and laying eggs, leaving the next generation of larvae to tunnel through the trees’ internal organs. They can kill an ash tree in as little as two years.

Back in 2002, when the borers were first discovered in North America — in Windsor, Ontario — experts thought it might be possible to eradicate them. But after about six months, researchers realized that the insects had been here for years, probably decades, and had already started spreading across the upper Midwest.

Despite a few moments of optimism since, hope has faded quickly.

“Ninety-nine percent of the ashes in North America are probably going to die,” said Andrew M. Liebhold, a research entomologist with the United States Forest Service.

Nobody was really studying the ecology of ash forests until the borers began destroying them. But now scientists are beginning to see what that change might look like.

A 2009 study in the journal Biological Invasions listed 43 native insect species that rely on ash trees for food or breeding. Those insects are the food supply for birds, including woodpeckers.

“You end up with a different ecosystem that different species prefer and where the old ones can’t do as well,” said Kathleen Knight, a research ecologist with the Forest Service.

Invasive insects have been eradicated in the past, but the invasion must be detected early, while it is still localized.

In the summer of 1998, when the Asian long-horned beetle was found in Chicago, people were already on the lookout for the bug, which had previously turned up in New York. Easier to spot than the smaller and less flamboyant emerald ash borer, the long-horned beetle quickly became the focus of an eradication effort that combined insecticides, public awareness and the felling of hundreds of trees in infected neighborhoods.

In 2007, the beetle was officially declared eradicated in Chicago, but it has since been found attacking trees in Massachusetts.

To tackle the emerald ash borers, scientists have experimented with chemical traps that attract the insects and can help spot the leading edge of an invasion into new forests when they can still be stopped.

But the traps are not very sensitive, Dr. Liebhold said, and often reveal an invasion only years after the beetles have been established.

The emerald ash borers’ effect may not be as dire as Dr. Liebhold predicts. Dr. McCullough, the entomologist at Michigan State, noted that the bugs’ conquest varied by tree species and location. Of the four major species, black ash and green ash are probably lost, but the beetles kill only 60 percent to 70 percent of blue ash. White ash falls somewhere in between.

And while the eight billion ash trees in wild forests cannot really be protected, ash trees in the city may stand a chance because of thedevelopment of new insecticides.

Still, the losses are bound to have severe consequences. When ash trees die, they leave gaps in the leaf canopy that allow sunlight to reach parts of the forest floor that were previously shaded. Dr. Knight, of the Forest Service, has found that those gaps provide an opportunity for invasive honeysuckle bushes to grow unchecked.

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“In the worst-case scenario, it becomes a dense, impenetrable thicket of shrubs in the understory,” she said.

The thicket prevents native plants from growing and is likely to affect which kinds of animals can thrive there.

While the emerald ash borer is a particularly destructive bug, it’s not the only invasive insect on the march.

Another is the hemlock woolly adelgid, also from Asia. Each the size of a poppy seed, adelgids make fluffy, white egg sacs in which they wrap themselves and which attach to the undersides of hemlock branches. Safely stuck to the tree, the adelgid inserts a feeding tube and proceeds to suck the sap out of the tree like a vampire.

As with the emerald ash borers, the adelgids’ size and life cycle made them hard to notice until it was too late. While not as deadly or as fast as the ash borers, adelgids have worked their way north from Virginia over the last 50 years, and are capable of killing off more than 50 percent of the hemlocks in infected forests.

Though the percentage of dead trees is lower, the devastation can be just as wide, said David Orwig, a senior ecologist in the adelgid-infected Harvard Forest. That’s because, unlike ash, hemlocks often grow in groves of nothing but hemlock. Even if not all are wiped out, the die-off changes the character of the forest — from dark, cool and moist to sunnier, warmer and drier.

These shifts are like a domino that leads to a series of ecological effects. A 2008 study published by Dr. Orwig in The Canadian Journal of Forestry Research showed that the change in the type and quantity of leaves building up on the forest floor as more hemlocks die caused soils to accumulate higher levels of nitrogen. That excess nitrogen can leach into nearby streams, Dr. Orwig said, where it can change what plants and animals grow there.

It’s important to note that a different ecosystem is not the same as no ecosystem. When ash trees and hemlocks die, they are replaced by other kinds of trees. Over time, a new environmental system takes root. Few people living today remember when the Northeast was covered in forests of American chestnut. That species all but died out more than half a century ago from a series of fungal infections. Today, the forests and the life they harbor are very different; in many cases, hemlocks replaced the chestnuts. And now something else will replace the hemlocks.

But we will still have lost something valuable, Dr. Liebhold said.

“The forests and the species that exist there, they’re part of America and what defines America,” he said. “Without being too corny, they’re a symbol of what this country is.”

Correction: July 3, 2014

An article on Tuesday about the destruction of ash forests by emerald ash borers misstated the number of ash trees in wild forests in North America. It is eight billion, not million.