Carpenter Ants in an Oak Tree: To Treat or Remove?

A 60–70 ft oak in upstate New York has a large cavity at 30 ft (old limb site) and a 6-inch wide, 3-ft tall split at 10 ft filled with sawdust and black ooze—classic carpenter ant signs. Two arborists disagreed: one recommended removal, the other suggested killing ants and plugging holes.

Expert Assessment

Jack DeAngelis explains carpenter ants nest in already decayed wood; they don't cause the decay. The tree's hollow areas result from long-term fungal decay following injury. The living sapwood cylinder outside the decay provides structural strength. Plugging cavities traps moisture, worsening decay, and does not eliminate ants.

Recommended Approach

1. Do not plug holes. It accelerates decay.
2. Treat the colony directly: Apply insecticidal dust (e.g., carbaryl, boric acid) into the nest cavity at the split. This reduces ant numbers and satellite colony risk to nearby structures.
3. Baiting: If the nest is inaccessible, use carpenter ant baits (e.g., Terro, or 1% boric acid in 10% sugar water) placed along foraging trails. Baits are slow but can eliminate the colony.
4. Fertilize: Apply 10-10-10 fertilizer at 1 lb per inch of trunk diameter in spring and fall to boost tree vigor.
5. Monitor structure: If large limbs begin breaking (hollow inside), the tree may become hazardous and require removal by a professional.

The tree can likely be preserved for years with treatment and care. Annual retreatment may be needed as new colonies can establish.