How to get rid of aphids that are on the top branches of our trees?

What kind of trees are they?

Purchase many lady bugs...they eat aphids. Ladies can be purchased in bulk on line.

I agree with the ladybug purchase. As long as the aphids are there, so will be the ladybugs!

I agree about the Lady bugs, we call them Lady Birds in UK, it was the first thing which came into my head before I read the comments. I once was on holiday in a seaside town in Devon UK when we were completely inundated with the bugs. It was unbelievable they were everywhere, unfortunately, they were lying dead in the gutters etc too but I was told that they had come for the greenfly, (aphids), which had attacked the roses etc in the town, little sap suckers. The Lady Birds were still feeding when we left 2 weeks later. I'm sure they will solve your problem. Good luck