New pest raises concern for oak trees in Oregon

A tiny invasive insect from Europe and the Middle East that attacks oak trees has been found several times in Oregon, including most recently in an Oregon white oak in Wilsonville, the Oregon Department of Agriculture (ODA) reported.

The Mediterranean oak borer (MOB), Xyleborus monographus, transmits multiple fungal species to the trees it infests, some of which may cause oak wilt.

Officials say the insect is tiny and brown, about 3mm or 0.1 inches long. People are more likely to notice its damage than the insect itself. A factsheet on the pest can be downloaded here.

ODF and ODA are asking people not to move firewood from oak trees beyond the local area where it is cut, to avoid spreading the insect. Landowners are asked to report oaks with crown dieback to the Oregon Invasive Species Hotline.

This insect was first found in North America when it turned up in 2017 in California, and then in Oregon a few years later.

“[The oak borer] was found in a single trap set by ODF in Multnomah County, Oregon in 2018,” said Wyatt Williams, an invasive pest manager with the Oregon Department of Forestry. “It was then found in traps in 2020 in Marion County, and in 2021-2022 in Clackamas and Washington counties. This spring it was found in a single Oregon white oak at Sandy River delta.”

Cody Holthouse, a pest management official with the Oregon Department of Agriculture, said his agency and ODF have been in touch with their California counterparts to assess the impact of MOB on native and introduced oaks, while also exploring what control measures they recommend.