Tree Care and Maintenance

Seasonal Tree Care Tips: Fall 2023
Mediterranean Oak Borer

PHOTOGRAPH CREDIT: Female Mediterranean Oak Borer, just 1/10th of an inch long. Photo by Curtis Ewing, CAL Fire. Source: ODF MOB Fact Sheet. Pale colored boring dust along the trunk of an Oregon white oak. Photo by Morgan Holen, 2023.

A new pest is raising concern for oak trees in Oregon. Mediterranean oak borer (Xyleborus monographus), or MOB, is an invasive insect from Europe, the Middle East and northern Africa that primarily infests weak or dying trees of a variety of oak species. This tiny woodboring beetle is a type of ambrosia beetle that eats fungus grown in galleries created in the wood of branches and tree trunks. Some fungal species may cause a disease called oak wilt that can kill trees within as little as two years. While common native ambrosia beetles infest dead or dying trees, MOB attacks live, mature trees.

Our neighbors to the south in Wilsonville witnessed rapid decline of several very large Oregon white oaks, prompting an investigation that led to confirmation of MOB.

MOB was first discovered in the United States in California in 2017. Here in Oregon, MOB was first found in a single trap set by Oregon Department of Forestry (ODF) in Multnomah County in 2018. In 2020, MOB was found in traps in Marion County, then in Clackamas and Washington counties in 2021 and 2022. In the spring of 2023, MOB was found in a single Oregon white oak at Sandy River Delta. Since then, MOB has been confirmed in more than a dozen Oregon white oaks in the City of Wilsonville.

Upon confirmation of MOB in Wilsonville, the City immediately began collaborating with the Oregon Department of Agriculture (ODA), ODF, Metro and the City's consulting arborists to identify and mitigate MOB’s presence, survey and assess symptomatic trees, and develop management strategies to help local jurisdictions.

Early detection is vital to preventing the spread of MOB. Learn how to identify oak species, review the signs and symptoms of MOB, and report oaks with both crown dieback and any of the symptoms below to the Oregon Invasive Species Hotline at:

Signs and Symptoms of MOB Infestation:

  1. Individual branch death in the top third of the tree.
  2. Very small and perfectly round entrance and exit holes about 1/16” (1.3-1.5mm) in diameter.
  3. Fine white or pale colored boring dust, almost sand-like in appearance, on the exterior of the tree trunk or in mounds along branches.
  4. Black branched trellises, tunnels or galleries 1.2-1.5mm in diameter on the interior cut face of wood.
  5. Tiny reddish-brown beetles about 1/10” (2.3-3.1mm) long.

Symptoms Not from MOB include:

  • Discolored leaves but absence of branch dieback;
  • Holes larger than the diameter of a pencil lead;
  • Brown boring dust;
  • Wood staining without associated galleries.

In the meantime, ODA and ODF are asking people not to move firewood from oak trees beyond the local area where it is cut to avoid spreading the insect pest, and, be sure to inspect wood from oak trees that are pruned or removed for signs of MOB before hauling it away. If signs of MOB are suspected, cover the wood with a tarp and contact ODA through the Oregon Invasive Species Hotline. Infested trees and wood should be safely burned, heat treated, finely chipped or deep buried.

Visit these sites for more information:

ODF MOB Fact Sheet 
ODA MOB Fact Sheet 
Common Trees of the Pacific Northwest 
City of Wilsonville MOB Resources 
Don’t Move Firewood


Please check back quarterly for more tree care and maintenance advice!


 Please check back for more seasonal tree care and maintenance advice!

Link to Tree Care and Maintenance Archives


Summer 2023: Supplemental Watering and Mulch to Prevent Drought Stress in Trees

Spring 2023: Aphids

Winter 2023: Pruning: Standards and Best Practices

Summer/Fall 2023: Emerald Ash Borer

Winter/Spring 2022: Nesting Season

Fall 2021: Healthy Trees are Defended Trees

Special Edition: After the Storm

Winter 2021: What is an Arborist?

Fall 2020: Soil Management  

Summer 2020: Retaining and Creating Snags for Wildlife

Spring 2020: Mulch: Numerous Benefits and Easy Application

Winter 2020: Tree Planting: To Stake or Not to Stake?

Fall 2019: Insects and Diseases

​Summer 2019: Drought Stress Revisited

Spring 2019: Trees and Construction

Winter 2019: Topped Tree

Fall 2018: Autumn Leaves

Summer 2018: Emerald Ash Borer

Spring 2018: Pruning Young Trees

Winter 2017: Tree Related Storm Damage

Fall 2017: New Tree Selection and Planting

Summer 2017: Trees and Turfgrasses

Spring 2017: English Ivy Removal

Winter 2017: Recognizing Tree Risk

Summer 2016: Drought Stress

Fall 2016: Preparing Trees for Winter


Additional Tree Care Resources:

National Arbor Day Foundation The Morton Arboretum
Oregon Department of Forestry Tree Care Info
ISA Find an Arborist Alliance for Community Trees
Oregon Department of Forestry  


Right Tree in the Right Place

Use this handy guide for assistance in choosing the right species of tree based on the constraints of a site, such as overhead wires, narrow plant strips, and proximity to structures.

"Right Tree Right Place" - helpful information such as "Use this guide for assistance in choosing the right species of tree based on the constraints of a site, such as overhead wires, narrow plant strips, and proximity to structures."

"Master Plant List" - a guide to the plants that are acceptable for mitigation requirements in Lake Oswego.