Invasive Pest Threatens Future of North American Ash Trees

A new study shows that ash tree populations are not growing fast enough to replace the trees killed by ash borer larvae

In the upper left corner of the image rest an entomologist's index and middle fingers next to two emerald borer beetles. In the upper right corner, to the bugs' right, rests a bottle cap on the tabletop.
The emerald ash borer first appeared in Michigan in 2002. Photo by Carl D. Walsh/Portland Portland Press Herald via Getty Images

For 18 years, North American ash trees have been under attack by a half-inch long, metallic green beetle called the emerald ash borer. It’s a slow-moving battle, and scientists are now beginning to understand the nation-wide effects of the beetles on forest populations.

A study published in the January 2021 issue of Forest Ecology and Management reports that the forests that faced the beetle infestation first have recovered the most new growth, but it may not be enough to replace all of the trees that were lost. This finding may lead to a downward trend and the eventual loss of North American ash trees altogether, Elizabeth Pennisi reports for Science magazine.

It wouldn’t be the first time that North America has lost a tree to an invasive species. Over a century ago, the eastern United States was covered in nearly four billion American Chestnut trees. But the species is now considered functionally extinct because a deadly blight fungus was introduced around the turn of the 20th century.

Native to Asia, the emerald ash borer first appeared in Michigan in 2002. The leading theory is that the beetles arrived in wooden packing material.

Ash borer larvae live in and eat the wood just under a tree’s bark, which damages the tree’s ability to carry water and nutrients from the soil to its branches and leaves. Once the larvae develop into adult beetles, they bore a D-shaped hole through the bark of the tree and emerge to munch on leaves.

When a borer-infested tree loses its bark, you can see the light-colored squiggles left behind by the larvae that lived inside. The larvae can kill a tree in as little as two years, but it often takes longer for a tree to show deterioration, Michael Hill reported for the Associated Press in 2019.

In the new study, the researchers analyzed surveys of public and private forest lots between 2002 and 2018. The records include exact counts of the numbers of ash trees, seedlings and saplings, so the researchers were able to deduce which trees had died and how well the plots were recovering over time.

They found that while ash trees have the potential to recover quickly, and the plots that faced ash borers first were also the first to show more seedlings and saplings. But the beetles can kill young ash saplings before they’re mature enough to spread more seeds.

“Ash recruitment is not keeping up with mortality, and few seedlings appear to reach reproductive age,” ecologist Juliann Aukema of the U.S. Agency for International Development, who was not involved with the research, tells Science magazine.

If the new trees don’t have a chance to spread more seeds, then recovery may not last beyond their generation. Ash trees take at least 16 years to reach maturity, giving ash borers ample opportunity to lay their eggs in the trees’ crowns.

In places where ash borers are still spreading, like northern New York, infested areas grow by about one to two miles each year, Ellis Giacomelli reports for NNY360. New infestations are often seeded by transported firewood, which is why you’re not supposed to bring firewood into an area from far away.

In St. Paul, Minnesota, the recreation department is removing thousands of ash trees to quell the spread of ash borer beetles, Emma Nelson reports for the Star Tribune. Budget constraints necessitate that new planting doesn’t begin until 2022.

And in North Carolina, researchers at North Carolina State University are beginning a project to monitor the spread of ash borer beetles with drone photography. Over time, the researchers expect that resilient trees will stand out in the birds-eye images. Once those trees are identified, researchers can study how they resist the beetles and whether that resistance may be used to protect ash trees more broadly.

University of St. Andrews systems scientist Ian Boyd tells Science magazine that he suspects ash trees may be tougher than the new study suggests. Ash “doesn’t just roll over and succumb,” he tells Science magazine, and scientists will have a better idea of how the trees have been impacted after more time has passed.

“It will likely take decades for the dynamics of the relationship between the borer and ash to settle down,” Boyd, who wasn’t involved in the new study, tells Science magazine. The new study is “the first chapter of a long story of how a new balance will eventually emerge between [the borer] and ash trees.”

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