Weeping Willow Borer Damage: Treatment and Prevention

Identifying the Problem

Loose bark with reddish, dime-sized knots on a 5-year-old weeping willow indicates borer infestation—likely carpenterworms or willow borers. The knots are callus tissue responding to larval tunneling.

Immediate Treatment

• Spray lower trunk with Merit (imidacloprid) or Onyx (bifenthrin) to kill larvae under bark.
• Apply as soil drench for systemic uptake—easier and longer-lasting.

Addressing Underlying Stress

Borers attack stressed trees. Fertilize with 10-10-10 at 1 lb per inch of trunk diameter to improve vigor. Water deeply during dry periods. Avoid wounding the thin bark.

Prevention

• Mulch to protect trunk base.
• Monitor for new exit holes or frass each spring.
• Maintain consistent soil moisture—willows love water but need drainage.

Prognosis

Early treatment with systemic insecticide and improved care often saves the tree. Severe structural damage may require removal.