Science

Deadly pest reaches Oregon, sparking fears for ash trees

Three years ago, forest scientists on the U.S. West Coast launched an effort to gather nearly 1 million seeds of the Oregon ash. The ecologically valuable tree, found from southern California to British Columbia in Canada, often grows along streams and in wetlands, anchoring rich ecosystems.

In part, the collecting effort represented disaster insurance: The emerald ash borer, an invasive, iridescent green beetle that has wiped out ash trees throughout much of the eastern and midwestern United States, was spreading westward, and the saved seeds might one day help restore the species if the pest ever arrived.

Now, it appears that rescue mission began none too soon. Last week, the U.S. Department of Agriculture confirmed that the emerald ash borer (Agrilus planipennis) had reached Oregon—and likely has been there for up to 5 years. The discovery marked the borer’s first appearance west of the Rocky Mountains; previously it had only gotten as far as Boulder, Colorado. Forest managers now fear for the future of the Oregon ash and at least eight other ash species found only in western North America.

“It’s extremely grave and sobering to have the situation upon us,” Karen Ripley, a forest health monitoring coordinator with the U.S. Forest Service (USFS), wrote last week in an email to colleagues.

On 30 June, a biologist with the city of Portland alerted officials to adult beetles he saw emerging from a tree in nearby Forest Grove, Oregon. The next day, Wyatt Williams, an invasive species specialist with the Oregon Department of Forestry, confirmed that an Oregon ash was infested. “My heart just sank,” he says.

 The report opened a new front in the nearly 2-decade-old fight against the borer, an Asian species that was first found in 2002 outside Detroit and has since been documented in 36 states; Washington, D.C.; and parts of Canada.

Once the borer shows up, “You cannot, generally speaking, get rid of [it],” says Leigh Greenwood, a forest specialist at the Nature Conservancy. But by taking actions such as limiting the movement of firewood and other ash material, states can buy themselves—and their neighbors—time to prepare, she says.

The emerald ash borer now has a foothold in the western United States.Macroscopic Solutions/SCIENCE SOURCE

In Oregon, officials will likely try to slow the borer’s spread and reduce its population by removing infested trees, selectively using insecticides, and releasing tiny wasps that parasitize and kill the beetles’ larvae. The wasps have been deployed widely and have been shown to attack the borer, though whether they can actually protect trees remains unclear, scientists say. In the longer term, some researchers hope to breed trees that can resist the beetle. Of the now-imperiled western ash species, Oregon ash (Fraxinus latifolia) is the most immediate concern. In sensitive wetlands where it can form nearly pure stands, no other tree can readily take its place. “In some areas, it’s the only [tree] species there,” says Richard Sniezko, a USFS geneticist and a leader of the seed collecting project.

Sniezko and Williams launched the effort after attending a 2019 conference on the ash borer; it should be completed this fall, Williams says. Some seeds have been sent to a federal seed vault in Fort Collins, Colorado, for long-term preservation. Others are being planted. Sniezko is growing seedlings from a number of Oregon ash populations at a research station in Cottage Grove; colleagues are overseeing a similar set of plantings in Washington and Ohio. Once the ash borer arrives, researchers will observe how the trees fare. Individual trees that hold up better than others might ultimately help scientists breed new, hardier varieties, following a model being used to restore Port Orford cedar and several species of pine trees. Such breeding efforts are already underway for other ash species in Ohio.

Researchers are also beginning to collect seeds from a half-dozen other endemic ash species that only live in the southwestern United States. That effort is challenging because several of the species are rare and grow in remote areas, says Tim Thibault, a curator at the Huntington, a botanical garden in San Marino, California, who is co-leading the project.

The seed collections are only the beginning of a long and expensive process, scientists warn. Rescuing a tree through breeding, Sniezko says, “is not for the faint of heart.”

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