Carpenter ants are very common inside trees, especially on larger, mature trees that are hollow with cavities. They nest in rotted, decayed wood, although some nests may extend into sound heartwood in the center of the tree. Carpenter ant presence is an indication of rotting wood, and infested trees should be checked to determine whether the rot has weakened the tree enough that it has become a risk of failure.
Carpenter ants in trees are not directly harmful to the tree. Control is not necessary for the tree’s health; their presence indicates decay in the tree, and they only feed on dead wood fiber. 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 and places where they are unwelcome. 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 Abamectin) labeled for use on trees in the landscape are suggested for control. Apply the dust directly into the nest cavity.
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 unless the tree poses a risk.
More information on carpenter ants: Household & Structural, Department of Entomology, Purdue University
Lindsey Purcell, Chapter Administrator & Master Arborist
Indiana Arborist Association