Answer: The problem with your tree is not with ants, it's with an insect population (probably aphids) feeding on your tree's leaves and exuding a sticky substance called honeydew. Ants are attracted to this substance and will actually herd or nurse the aphids, protecting them from predators. As long as the aphids are feeding, the ants have a ready source of sugary food. Usually when populations of aphids get large, parasitizing wasps come to the rescue, followed by hungry lady bugs. The ants will leave the tree at dusk and return in the morning. You can limit the number that go back up into the tree by wrapping a few inches of the trunk with strips of old sheet or other material that you can tie tightly, then spreading something sticky like Tanglefoot (available in nursery/garden depts.). The Tanglefoot will stop the ants in their tracks. Without ants to protect them, the aphids should attract beneficial insects to lower the populations. Monitor the Tanglefoot - ants have been known to use other dead ants as a bridge over the material. Replace the Tangelfoot when it becomes full of ants.
Your alternative is to have the tree professionally sprayed with insecticidal soap to kill the aphids. Insecticidal soap kills soft-bodied insects such as aphids but won't harm people, pets or other, more beneficial, insects.
Best wishes with your landscape!
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