thanks for the replies - I think I found the problem - fungus gnats. I've got the little suckers everywhere in my willows - and after reading up on them, they sound like the culprit. They do minimal damage to trees - unless the trees are very young and unestablished, such as my trees. The leaves start turning black on the tips all over the tree, and then the trunk starts to die eventually. Some of them I have been able to at least slow down the progression, and possibly save them - but at this point I have lost about 6-8 trees. Since they don't do the cold weather well when they are young, they have been in my front bedroom to get a good supply of Eastern sun, and a few are doing well. I have had some luck with a Pyrethrin spray, but the gnats have an 8 day life cycle, and manage to do nothing but breed in that time, so it's difficult. Basically, I juts have to limp through the rest of Winter and hope for the best I guess.