Bored to Death

As invasive emerald ash borers kill trees, scientists race to reduce damage. By Susan Cosier

In the forests of the Midwest, ash trees are dying because of a metallic green invader: the emerald ash borer. So far, the winged beetle has killed between 20 and 25 million ash trees and it is poised to attack billions more.

“Everything it has encountered so far in the Great Lakes region it has been able to successfully attack and kill,” says Robert Haack, a Michigan-based researcher for the U.S. Forest Service.

The invasive species is on the move. People inadvertently help it spread by transporting firewood from infested trees, and so far the bugs have been spotted in Michigan, Indiana, Illinois, Ohio, and Maryland. To combat the invasion, the U.S. Forest Service plans to release other insects—natural beetle parasites—into the forests the borers currently inhabit.

A native of Asia, the emerald ash borer probably traveled to North America in wood products in the 1990s before infesting American trees. In 2002, scientists found that ash trees in Michigan and Canada were suffering because the insect larvae were growing underneath the bark of the trees, preventing nutrients from reaching the leaves. As the eggs change into larvae, they burrow into the bark to eat and grow. Over the course of a few years, several generations of larvae eventually kill the tree. When scientists remove the bark of an infested tree, they see the path of the larvae—zigzag patterns that look like scribbles on a page.

After Dutch elm disease killed countless millions of residential elm trees in the 1950s, many communities replaced them with ash trees. Now, ash comprises two percent of the tree canopy in North America, according to Robin Usborne, a spokesperson from Michigan State University. That may not sound like a lot, but if the pest kills the trees, more sunlight could reach the ground and heat up the surrounding area around, including houses nearby. People who once relied on shade from ash trees might have to crank up their air conditioning during the summer, consuming more energy. 

The cost of cutting down the trees infested by the borer is also troublesome. The Ohio Department of Natural Resources estimates that taking infested trees out of residential neighborhoods in the state could add up to $1 billion over the next 10 years. Add to that $2 billion for removing infested ash trees from managed forests, and the total price tag jumps to $3 billion—and that’s all from an insect that’s one-half-inch long.

“It has the ability to kill them all and it probably will unless we can find some methods to stop it,” says Usborne.

One approach is developing insecticides and microbial fungicides that kill the critters. Some have proven effective, but large-scale application could be difficult and expensive. Eventually, these chemicals might be used with other control measures, such as introducing natural predators to kill the borers.

In the United States, less than one percent of borers are killed by natural enemies, whereas in China parasitic predators kill up to 60 percent of the larva, says Haack.

Federal scientists in Michigan plan to introduce parasitic wasps and egg parasites this summer to kill borers. If the project is approved, they will keep a close eye on the area where they release the parasites to see how other species are affected, says Therese Poland, a research entomologist with the Forest Service.

“It may not be the best answer, but it would certainly bring about a greater natural balance,” says Haack.

If the pest can’t be stopped and it kills all of the ash trees here, the Natural Resources Conservation Service and the U.S. Department of Agriculture have a back up plan: The agencies entered into an agreement with the National Center for Genetic Resources to preserve ash tree seeds. In a worst-case scenario, scientists could use the seeds to grow new ash trees in the future. Hopefully it won’t come down to that.

“It could potentially be very devastating,” Haack agrees, but “it probably won’t ever kill all the ash trees.”


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