This past winter was the coldest Detroit had had in 36 years. Across the upper Midwest, cities shivered, and more than 90 percent of the surface area of the Great Lakes froze solid.
It seemed like ideal weather to kill an unwanted insect. But it did little to stop the emerald ash borer, an invasive Asian beetle that is devastating ash trees from Minnesota to New York.
“We didn’t find a single dead larva,” said Deborah G. McCullough, a professor of entomology and forestry at Michigan State, who led a study of ash trees in Lower Michigan.
Even before the severe winter, McCullough and other scientists had come to the glum conclusion that they were going to lose the decade-long battle against the ash borer. Now they are assessing the cascade of consequences for Midwestern and Northeastern forests, both urban and wild.
The effects will go far beyond what you see on a hike or how you feel about the loss of a tree on your property. They will ripple through forest ecosystems, affecting other plants, animals and water supplies.
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 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 U.S. Forest Service.
“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.
But the traps are not very sensitive, 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 Liebhold predicts. McCullough 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.
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. Knight has found that those gaps provide an opportunity for invasive honeysuckle bushes to grow unchecked.
“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.