Scale (cottony maple?) attacking preziosas. How hard can I prune?
I have three beautiful preziosa hydragneas that I love. I believe they were planted eight years ago, and they are quite large, about 5 x 4 feet. They are being attacked by scale: I'm pretty sure it is cottony maple scale. My husband sprayed neem oil a couple of weeks ago, and it seems like only about 10% are left....but 10% of thousands of scale is still quite a bit. The hydrangeas had a tough time recently: I don't know if it was the drought last summer (I tried handwatering, but doubt I kept up) or the tough winter we had, but I have a lot of branches that aren't dead but don't have any foliage either. Also, I cut off some of the branches heavily infested by scale, so now the bushes look terrible.
My essential question, is how hard can I prune the bushes without killing them altogether? They are just too big, and I don't see how I can get all the scale with all the leaf cover they have right now. I can live without flowers next year, but I really really really hate scale. I've thought about ripping these out and replacing them, but that seems a bit aggressive at this point.
Any other suggestions would be helpful! There is a lot of conflicting information on the web about when to treat etc. We don't in general use pesticides (neem oil or horticultural soap is about as aggressive as I'd like to get)