susieq

You might want to make sure you have gotten rid of any ants that are around the tree too. Ants actually "farm" aphids like cattle. They place them on plants and work the herd, they milk the aphids by stroking the bodies and collecting the sticky substance that the aphids give off. If you don't get rid of the ants, you will never get rid of the aphids. Of course, a good systemic poison might do the trick, making the tree poisonous to anything that feeds on it.
susieq