Oak Borer Infestation: Dark Rotted Spot on Trunk

Dark, Rotted Bark on Oak Trunk Likely Borer Damage

A western Pennsylvania oak has a 3-inch dark, rotted spot on the trunk at 3 feet height. Tree otherwise healthy and blooming well.

Diagnosis

Oak borer (likely flatheaded or roundheaded borer) tunneling in the cambium, causing sap flow and bark darkening/decay.

Treatment Options

Option 1: Trunk Spray (Immediate)

• Spray Merit (imidacloprid) directly on the wound and surrounding bark.
• Penetrates bark, kills larvae inside.

Option 2: Systemic Soil Drench (Easier, Longer Protection)

• Apply Bayer Advanced Tree & Shrub Insect Control (imidacloprid) to soil around tree.
• Roots translocate to canopy; season-long control.
• Better for larger trees or multiple wounds.

Supportive Care

• Fertilize: 10-10-10 at 1 lb per inch trunk diameter, spring and fall.
• Water during drought (1 inch/week).
• Mulch to reduce stress.

Product info: Bayer Advanced Tree & Shrub Insect Control