Borers in Weeping Willow: Signs and Management

A weeping willow with shotgun-like holes in the trunk, darkening bark, and progressive dieback is showing classic symptoms of wood-boring insects. Borer larvae tunnel beneath the bark, disrupting vascular tissue and causing sap flow interruption, which leads to canopy thinning and death.

Common Borers in Willow

• Carpenterworm (Prionoxystus robiniae): Large larvae bore in trunk and major limbs; push out sawdust-like frass.
• Willow borer (Cryptorhynchus lapathi): Weevil larvae tunnel in branches and trunk.
• Flatheaded borers (Chrysobothris spp.): Larvae feed in cambium, creating winding galleries.

Diagnosis

• Small round or oval exit holes in bark (1/8–1/4 inch).
• Sawdust or frass at base or in bark crevices.
• Loose bark revealing larval galleries.
• Progressive crown dieback, often starting at top.

Treatment Options

Homeowner option: Imidacloprid (Bayer Advanced Tree & Shrub Insect Control) as a soil drench. Systemic uptake kills larvae feeding in vascular tissue. Best applied early spring or fall. Most effective on flatheaded borers; less consistent on roundheaded borers.

Professional treatment: Certified arborists can inject insecticides (e.g., emamectin benzoate) directly into the trunk for faster, more reliable control. They can also assess structural risk from internal damage.

Cultural Care to Reduce Stress

• Water deeply during drought (willows need consistent moisture).
• Mulch 2-3 inches out to drip line; keep off trunk.
• Avoid wounding trunk with mowers/string trimmers.
• Fertilize moderately in spring if growth is poor.

Borers attack stressed trees. If the willow is valuable, combine systemic treatment with professional assessment. If decline is advanced, removal may be necessary for safety.