Slightly old post and I was so close to also going the pesticide route out of concern for some of the trees affected that i wanted to share my learning for others.
I'm in the UK so recipes may change but our standard dish washing soap has taken care of some heavy infestations, the aphids can't breath if they have soap on their skin. I was considering insecticide sprays but this worked well and doesn't harm other beneficial organisms.
Mostly to experiment other organic alternatives to pesticide sprays I also sent out some ladybug larvae on a couple of fruit trees, an acer palmatum and a year old seedling that was particularly badly affected and was impressed by the results as well as some extra entertainment while I'm out staring at trees as you can actually fairly easily spot them rapidly binging on eggs and young aphids like an all you can eat buffet with amazing speed. Would highly recommend these and I'm keen to experiment with more biological controls in future.
I am also researching 'why' the aphids were here in the first place, I heard a horticultural theory about excess N which I can admit my circumstance may correlate towards as I knew I used high N chem fert really early on the acer which was worst hit, in turn it kicked out noticeably more than usual (even noticed by my dad who gave me the tree)
I'm wondering if you would perhaps consider did this tree also have extra N? really strong growth maybe more than usual?