Saving an Elm Tree from Borer Infestation

Elm Tree

A 30-year-old Japanese or Chinese elm in Southern California shows borer damage after carpenter ants were eliminated. Despite trunk treatments and pruning sealer, borers returned and moved higher, leaving all leaves dead.

Treatment Plan

1. Spray trunk with Onyx insecticide as high as reachable to kill active borers.
2. Fertilize with 10-10-10 at 1 lb per inch of trunk diameter (64 lbs total), scattered around the tree and watered in deeply. Repeat in spring.
3. Address stress factors—borers target weakened trees, so improving vigor is essential.

While recovery isn't guaranteed with complete defoliation, this combined approach offers the best chance.