Answer: I don't think the ants are nesting in your tree (unless the tree is dead and they are carpenter ants). It's more likely the ants are finding a food source in your tree - perhaps an insect infestation. Insecting with piercing mouthparts suck the fluids in the leaves and excrete a sticky substance called honeydew. Ants find this a delightful substance and will climb up a 40 tree for a treat. So, inspect your tree to try and find out what they are feeding on. Once you eliminate the insect pests and there's no more honeydew, the ants will find another place. I can't really diagnose the problem that may have caused the hollow part in your tree because I can't see it. If the tree is a valuable one, you may want to hire a professional to examine it and relate the findings so you can make an educated decision on whether or not you can save the tree. I hope so!
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