An invasive beetle that attacks oak trees was recently found in several Oregon white oaks in Wilsonville.

The Mediterranean oak borer can cause a disease called oak wilt, which can kill trees in as little as two to three years.

Wyatt Williams, Oregon Department of Forestry invasive species specialist, said the insect has been responsible for the death of hundreds if not thousands of native oak trees in California.

So far, the tiny reddish-brown beetle appears to prefer large diameter trees, said Josh Vlach, Oregon Department of Agriculture entomologist.

“There’s not been an indication that it likes to attack smaller diameter stuff like nursery stock,” Vlach added.

Williams said similar beetles have turned out to be problems for nurseries, however.

Greenhouse and nursery plants are Oregon’s most valuable agricultural commodity, earning $1.3 billion in 2021.

Oregon officials are assessing the insect’s spread and damage and are meeting with California counterparts to determine survey and management strategies.

Washington scientists also are putting out traps for the pest, Vlach said.

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The Mediterranean oak borer seems to prefer the white oak family, Williams said.

The Oregon white oak is an ecologically important native species and there have been widespread efforts to restore savannas in the Willamette Valley.

According to Oregon State University, before European settlement, the Willamette basin had roughly 1 million acres of white oak habitat.

That shrank by more than 95% due to factors including fire suppression — large oaks survive blazes that kill other trees.

Oregon white oaks can live 500 years and their native habitat stretches from Vancouver Island to Southern California, according to OSU.

The new pest’s arrival comes as Oregon oaks are stressed from heat, droughts and other invasives, Vlach said.

Oak lace bug arrived in Oregon in 2015 and European oak phylloxera followed in 2018. Both attack leaves.

“If you look at a lot of oak trees, they’re already damaged,” he said.

Mediterranean oak borer was first found in North America in 2017 in California. The oak trees that have been damaged or died in California are not native to Oregon.

Since 2018, Oregon agencies have been setting out traps for the bug along waterways and trade sites, including wineries that import oak barrels from France.

The bug was first found in Oregon in 2018, in a single trap in Multnomah County. The beetle also was found in traps in Marion County in 2020, and in Clackamas and Washington counties in 2021 and 2022.

This spring, a tree infestation was first discovered in Oregon at the Sandy River Delta, a park east of Portland.

The Mediterranean oak borer is an ambrosia beetle — instead of eating wood, it transmits and consumes fungus grown in galleries created in the wood of branches and trunks.

As the fungus grows, it robs trees of water needed for growth and survival.

In Europe, the insect has also infected elm, maple and walnut trees, though it hasn’t damaged those.

To avoid spreading the beetle, people should not move firewood from oak trees beyond the area where it is cut.

Williams said residents also should chip as small as possible and cover the remaining wood in thick sturdy plastic for a year to provide a barrier.

Signs of Mediterranean oak borer infestation include: crown dieback; pale boring dust found on the exterior trunk of the tree; and galleries that look like tiny, perfectly round black holes smaller than the diameter of a pencil lead on the exterior of the wood and black, branched trellises on the cut face.

The state is asking landowners to report oaks with both crown dieback and other symptoms to the Oregon Invasives Hotline.