There are carpenter ants in my tree trunk — what should I do?

Large black or black-and-red ants, a quarter inch or bigger, trailing steadily up and down the trunk, most visible at dusk. Small piles of coarse sawdust at the base or caught in bark crevices, mixed with insect body parts. A hollow sound when you knock on the lower trunk. Sometimes winged ants swarming from the trunk in spring — pinched waist and elbowed antennae, which is how you separate them from termites. Carpenter ants do not eat wood and cannot chew sound heartwood. They hollow galleries in wood already softened by moisture and decay fungi. Their presence tells you a decay pocket exists inside the trunk, usually larger than it appears from outside, and that it stays wet. The ants are tenants, not the cause of the damage — but they are a dependable indicator that the interior is not solid.