This article was published originally on 6/13/2007
Carpenter ants commonly nest inside trees, especially older trees that are hollow or have a significant amount of dead, decayed wood. Carpenter ants in trees are not directly harmful to the tree.
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 are able to nest. Once injury has occurred, wood decay can set in if moisture is present; it is the wood decay that gives 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.
Control of carpenter ants inside trees is difficult but can be done as a way to reduce invasion of the ants 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 (such as Sevin or permethrin) labeled for use on trees in the landscape are suggested for control. Apply the dust directly into the nest cavity.
An alternative is to treat the trunk and ground around the tree in hopes of reducing the population of foragers (workers). 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, though a careful analysis should be made to determine if hollow, ant-infested trees are at sufficiently defective to be unsafe and potentially hazardous.
Carpenter ants nesting inside trees often expel large quantities of coarse sawdust.
IC-497(14) -- June 13, 2007