Borers in Locust and Willow: Identification and Treatment

Sawdust at the Base? It's Likely Borers

Finding sawdust-like frass under a locust or willow tree—without seeing insects on the ground—strongly suggests wood-boring beetles or moth larvae tunneling inside the trunk.

Why Ground Sprays Don't Work

Spraying the soil is ineffective because the pests are feeding inside the trunk, not in the soil. The treatment must reach the borers where they live.

Effective Trunk Treatment

Spray the trunk thoroughly with a systemic insecticide labeled for borers, such as Onyx (bifenthrin) or Merit (imidacloprid). These products penetrate the bark and kill larvae feeding in the cambium and sapwood. Apply according to label directions, typically in spring when adults are active and again in mid-summer. Check with a local nursery or cooperative extension for product availability and timing in your area.

Prevention

Keep trees vigorous with proper watering and fertilization. Stressed trees emit chemical signals that attract borers. Mulch to reduce moisture stress and avoid mechanical injuries to the trunk.