How do aphids get into a tree?
Do the ants bring them onto the tree or is it that the ants are just opportunists protecting the aphids after they are already on the leaves?
Ants are just taking advantage of the sugary exudates, aphids get to a new host initially by flying -- one of the forms is winged specifically for this purpose.
Many people assume that ants bring the aphids, or make an aphid infestation worse, but the only mechanism for this might be that the ants provide some protection to the aphids against certain other predatory insects.
For small enough plants the old jet of water method works very well at controlling aphids.
I used insecticide "bayer rose and insect killer" because water doesn't works for me. Use it every two to three months and your trees will be aphid-free.
Aphids have unusual reproductive cycles. Actually winged aphids will start colonies and the babies it produces will not be winged.