Question
What can I do to stop ants from attacking my crabapple tree. Will they eventually kill the tree

Answer
Joseph,

Ants won't damage a tree. If they are small ants they may be feeding on aphid honeydew on leaves. Aphids can damage trees. If they are large ants (>3/8") they could be carpenter ants which would mean the tree is partially hollow and the ants are nesting in it. Either way the ants are not causing any damage.

Post a follow up if you have questions.

Jack DeAngelis
http://www.livingwithbugs.com