Carpenter Ants in Fallen Oak Tree: Role in Tree Failure and Tree Health Care

A Northern Virginia homeowner observes carpenter ants in a huge fallen oak (rootball and all) after Hurricane Sandy. Ants entered via an old branch wound. Base and most limbs were solid. They ask if ants caused the fall and how to protect remaining old oaks and poplars.

Expert Answer: Carpenter ants nest in decayed wood, not sound wood. They don't eat wood but exploit fungal decay entering through wounds. The ants didn't cause the fall; decay did. To improve tree health: fertilize with 10-10-10 at 1 lb per inch trunk diameter, scattered from 3 ft out to beyond drip line, water in (or before rain). Repeat in spring. Keep area natural to prevent soil drying. Ground cover not needed for large trees.

Control only if ants enter home from tree colonies. Baits (Terro, boric acid/sugar water) work best. If nest exposed, spray directly with bifenthrin, cyfluthrin, deltamethrin, or permethrin.