massive green fly infestation

I have a very large, mature maple tree that has become infested with green fly which has now spread all over the back end of my garden - they're on my roses, flowering currants, plum tree, flowering cherry, Japanese anemones, strawberries, in my greenhouse - everywhere at the back end of the garden.  Given the height of the maple tree I can't spray that, and I have been spraying with soapy water on all my plants / on the soil in infested areas, but I feel like I'm fighting a losing battle and my garden is under siege!  Sadly, I've only seen one ladybird in the garden...  Please, any advice would be gratefully received!