Invasive Mediterranean Oak Borer Threatens Oregon’s White Oaks

A concerning threat to Oregon’s native white oak trees has emerged in the form of the Mediterranean oak borer (Xyleborus monographus), an invasive insect native to Europe and the Middle East. The discovery of this destructive pest has raised alarms among local authorities and conservationists due to its potential to wreak havoc on the state’s oak tree population.

The Mediterranean oak borer, often referred to as MOB, is a diminutive woodboring beetle, categorized as an “ambrosia beetle.” What sets this invasive species apart is its peculiar feeding habit – instead of consuming wood, it feeds on fungus that it cultivates within galleries it creates in the wood of oak branches and trunks. This fungal growth disrupts the water supply to the tree’s canopy, endangering the oak’s growth and overall survival.

MOB was first detected in North America in 2017, in California, where it has already caused severe damage and led to the decline and death of numerous native oak trees. Although in Europe, MOB has been documented on other tree types such as elm, maple, and walnut, it hasn’t inflicted the same level of damage as observed on oak trees.

According to Wyatt Williams, Invasive Species Specialist at the Oregon Department of Forestry (ODF), MOB was initially identified in Oregon in 2018, and subsequent findings occurred in Multnomah County, Marion County in 2020, and Clackamas and Washington counties in 2021-2022. Recently, a white oak at Sandy River Delta also tested positive for MOB.

Cody Holthouse, IPPM Program Manager for the Oregon Department of Agriculture, highlighted ongoing efforts to assess MOB’s impact and control measures by collaborating with California counterparts. Scientists from both agencies are actively devising survey and management strategies to tackle this invasive pest, with more information expected in the coming weeks and months.

In the meantime, the Oregon Departments of Forestry and Agriculture urge residents to refrain from moving firewood from oak trees beyond the local area where it’s harvested to prevent the spread of this destructive insect.

Landowners are encouraged to familiarize themselves with the signs and symptoms of MOB infestation, including the beetle’s reddish-brown appearance, the presence of pale boring dust on tree trunks, and the distinct black galleries on the wood’s surface. Furthermore, MOB creates tiny, perfectly round entrance holes. It’s essential to differentiate these symptoms from other issues, such as holes larger than a pencil lead, brown boring dust, discolored leaves without dieback, and wood staining without associated galleries.

To report oak trees exhibiting crown dieback and any of the specified symptoms, residents can use the Oregon Invasives Hotline at

For additional information on dealing with oak pests, please visit this resource.

Source: ODF

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