Mature Sycamore Wet Spots: Flatheaded Borer Treatment

Diagnosing Trunk Weeping

A 45-year-old California sycamore with numerous 2-foot wet spots on the trunk (no odor) indicates flatheaded borer (Chrysobothris spp.) activity. The tree exudes sap to drown larvae boring in the cambium.

Targeted Treatment

1. Spray affected trunk thoroughly with Onyx (bifenthrin) or Merit (imidacloprid) until runoff—absorbs into bark to kill larvae.
2. Available at local nurseries; follow label for trunk application.

Addressing Underlying Stress

Borers attack stressed trees. Fertilize with 10-10-10 at 1 lb/inch trunk diameter now and again in fall. Water deeply during drought. This 45-year veteran needs vigor support to compartmentalize damage.

Why This Works

Trunk sprays directly target borers where systemics may not reach in a large, stressed tree. Combined with fertilization, this gives the sycamore its best chance.