The insect attack adds to the harmful effects of wood-rotting fungi, both in reducing the physical strength of the tree and in lowering the quality of the wood. The presence of otherwise unexplained coarse sawdust beside a house timber, pole, or tree usually indicates that carpenter ants are at work. The tunnels or galleries are principally gnawed by the workers and gradually enlarged to accommodate a growing ant colony. These galleries usually follow the soft portions of the wood, parallel to the grain. The inner surfaces of the galleries are clean and appear as if they had been coarsely sanded.
Most damage occurs during the warm summer months when carpenter ants are most active.
Their out door activity varies with the weather, but usually they are active between the first of May and the end of September. In heated buildings ant activity may continue later in the fall and begin earlier in the spring than is usual with outdoor colonies. The most common variety is black, although some of the varieties of less importance are partly red. In the early life of the colony, the ants produce wingless workers. After three or more years some of the ants, born with wings as reproductive males and females, swarm and mate.