Hi Monika_2006,
The ants themselves are not harmful, but they should be removed because they can spread fungal spores and sometimes will bring aphids and other insects to your tree and farm them. Aphids produce a sweet nectar that the ants love so they will actually move them around to different places on the tree to feed, and of course you do not want Aphids or any other insect that feeds on the tree. The ants will also build large nests in your root ball which is not good for the tree either.You also need to check the label carefully on any insecticide to be sure of its safety for your species of tree. I would recomend organic products such as Safer soap or if you have access to it the best is Talstar, but it is expensive and mainly used at nurseries. Hope this helps you and good luck.