It is possible this external rot is an indication of internal rot. The ants may be carpenter ants which are only taking advantage of this rotting wood.  They are not the cause of the trouble.  You can try taking a screwdriver and pushing it into the hole.  If it doesn't meet resistance, your tree may be rotting and hollow inside.  In that case, it could become a hazard.  Just filling the area will not solve the problem.  I suggest you have a certified arborist make a house call to evaluate your tree and follow the advice. You can find one by visiting this site and clicking on Find An Arborist.  Put in your zip code when prompted and a list of certified arborists in your area will appear.  https://www.treesaregood.org/