These are properbly Carpenter ants. 9 {+ o2 {* B( A) j* }" HCarpenter ants nest in rotted, decayed wood or in the center heartwood section of the tree. They are harmful to the tree. Control is unnecessary for the tree's health, as the ants are taking advantage of preexisting soft, weak wood to establish their colony.
If you really want to get rid of them, to go the hardware store and buy a small bottle of liquid ant-killer or ant-B -gone. Follow the instruction to set the bait and let them carry the poison home to feed their queen.