I have a very large American Elm (probably 100 years old), which I treat annually for elm scale (and whatever else the tree service I use recommends). I have lived at this location in Old Town Fort Collins for 34 years.  The elm typically drops honeydew from the scale, some years not so much, other years it's worse.  This year we have not only the sticky honeydew, but a dry, white, sand-like substance dropping onto things under the tree (and even blowing onto surfaces 25 feet away from the tree).  It's almost like the consistency of sugar. I have never seen this before, and my tree service came out to look and they had never seen this either.  Any ideas what this is?  I first noticed it towards the end of May and it is now June 8th.