Emerald Ash Borer May Have Arrived in Early 1990s

  • Emerald Ash Borer. Photo: U.S. Dept of Agriculture

June 6, 2014

A version of this story originally aired on Michigan Radio.

The emerald ash borer is a little shiny green beetle that loves to feast on ash trees. The adult beetles only nibble on the leaves. It's the larvae you've got to watch out for. They munch on the inner bark of the ash tree, and mess with the tree's ability to transport water and nutrients.

The pest has killed tens of millions of ash trees in the states and provinces around the Great Lakes.

Now, researchers know a little bit more about how the emerald ash borer ate its way around the region.  

The beetle is believed to have gotten into the U.S. by catching a ride in wooden crating or pallets in imported cargo.

Reading the tree rings

Deb McCullough is a professor of forest entomology at Michigan State University. She and her team took cores from more than a thousand trees and examined the rings. They could see different environmental stresses in the rings, things like the 1998 drought. Then they used that as a marker year, to identify when ash borers had infested and killed the trees.

“And because those trees had already been killed by that point, we know ash borer must’ve been there for at least a few years before that, because it takes anywhere from four to five to six years before a population gets high enough to kill trees.”

McCullough says they found that the ash borer was already feasting on trees in Michigan by the early 1990s. Her study is published in the journal Diversity and Distributions. Michigan was the epicenter for the ash borer invasion. Since then, the pest has spread to Pennsylvania and 22 other states, along with two Canadian provinces.

How do we stop more pests from getting into the U.S.?

“I don’t think anybody believes you can stop emerald ash borer any more. There’s too many populations, the densities are so high in a lot of areas. But we’ve made some progress, and in terms of protecting landscape trees, we’ve made a lot of progress.”

Invasive species are so hard to fight once they’re in that experts say it’s much better to try to keep them out in the first place.

But McCullough says only one percent of the cargo coming into the U.S. is inspected for pests.

“Our imports are increasing at roughly an exponential rate and that doesn’t show any sign of tapering off. The problem is certainly well-recognized, though. And especially when it comes to wood crating and wood pallet material, the federal government in the U.S. is aware of it.”

McCullough says the U.S. now regulates how the material used in wood crating and pallets has to be treated before it can be used in international trade.

She says the regulations have helped, but there's a lot more work to do to keep new pests out of our trees.