Posted: 28/05/2014 at 11:04
You can purchase ladybirds and ladybird larvae http://www.greengardener.co.uk/product.asp?id_pc=34&cat=75
What sort of tree is affected?
If you're getting the heavy rain that we've had over the past couple of days, that'll get rid of a lot of them, and if you have that many aphids the birds will soon find them.
I wouldn't worry about aphids on trees - trees are usually pretty resilient and will outgrow that sort of problem. Where they can cause a problem is on roses, lupins, aquilegia etc, but I still don't use a proprietary spray - if the infestation is heavy I'll spray with the hose, brush them off with my fingers or as a last resort on a fruit tree I'll use a soap spray.
If you use the types of spray that kill off the predators then the problem will get worse year on year.