The ants are taking advantage of the soft, decayed wood that is already present in the tree. A previous injury to the bark, trunk, branches or roots has allowed disease and decay to exist in the tree. Control of the ants will not do anything to improve the health of the tree. You may wish to consult with a Certified Arborist who has completed training and certification in the care of trees. You can find one in your area by clicking on the link above.
For more information: Iowa State University Extension and Outreach.
Other Resources:
Household and Structural Damage Caused by Ants
Purdue Extension – Entomology
Purdue Extension – First Report of the Invasive Asian Needle Ant in Indiana
Purdue Extension – FNR