Description of carpenter ants
Carpenter ants found in trees are usually “large, black ants” that are one-half inch or more in length and shiny black. Carpenter ants have a smoothly rounded thorax when viewed from the side. See the ISU website for an identification guide to common ants of Iowa. https://hortnews.extension.iastate.edu/ants-iowa-common
Carpenter ants in trees are often found after observing a pile of sawdust at the base of the tree below a crack or wound.
Damage caused by carpenter ants
Carpenter ant nests are very common inside trees, especially older trees that are hollow or have a significant amount of dead limbs and branches. The nests are usually in rotted, decayed wood, although some nests may extend into sound heartwood in the center of the tree. Carpenter ants cannot eat wood. Instead they discard sawdust outside of the tree as they chew up the wood to make nest tunnels and chambers inside the tree.
Carpenter ants in trees are not directly harmful to the tree. Therefore, control is not essential for the tree's health, as the ants are only taking advantage of an existing situation of soft, weak wood in which to establish their colony. Stress, mechanical injury, environmental conditions, disease, or other insects are responsible for killing limbs or sections of the trees in which the ants can nest. Once injury has occurred, wood decay can set in if moisture is present; the wood decay allows the carpenter ants the opportunity to colonize the tree. Carpenter ants use knots, cracks, holes, and old insect tunnels to gain access to these areas.
Management of carpenter ants
Control of carpenter ants inside trees is difficult but can be done to potentially reduce the ants' invasion into adjacent structures. It is also possible for ant colonies located inside trees to form satellite colonies inside a nearby home wall. Available controls are not likely to permanently rid a tree of carpenter ants, so retreatment every year or so may be necessary. Dust insecticides containing pyrethroids or carbaryl labeled for use on trees in the landscape are suggested for control. Apply the dust directly into the nest cavity. Residual insecticide sprays can be applied to the tree trunk, the nest opening, and other surfaces where the ants travel. For more information please read this article.
Plugging or sealing tree cavities or treating tree wounds with wound dressings is not advised. Such treatments are unnecessary and will not eliminate nor prevent decay or carpenter ant activity. Also, cutting down otherwise viable trees that happen to be infested with carpenter ants is generally not necessary.