I bought systemic Baer Advanced Tree & Shrub Insect Control to rid 3 30+ year-old Noble firs of aphids. It's easy to apply, and not so toxic that it requires an expert. I doused the soil around the trees' base last spring and repeated this fall although once a year is supposed to do it. (Fall is the best time to apply.) I won't know if it's effective until next spring. Sadly, the damaged lower branches will never look good again. Keep even well established trees watered during drought and they won't become so susceptible to pests.