steve:

Flies don't damage leaves or stems. Some do lay eggs in fruit but don't suck sap or eat leaves. I suspect they are just using your tree for rest or some other harmless purpose.

Have you looked at the Colorado Extension website? If you can't find it there then maybe send them an email and ask for identification. The only thing that looked slightly similar is the Tachinidae.

http://www.nadsdiptera.org/Tach/Gen/tachintr.htm

Here is a link that might be useful: Western Colorado Insects

I've got the same bug. I just noticed this little wasp/fly on my peach leaves. It was scouring the bottoms of the leaves so my guess is that it's a good bug.

Last year when Colorado was on fire we got a lot of strange bugs in Denver that I had never seen before. This year we're on fire again and the same thing is happening.

My guess is that they've been displaced and are finding a new habitat.

If anyone has any extra rain could you please send it this way. It is just flipping hot and dry here. Colorado is such a drama queen. Two days ago it thundered and lightninged and the wind blew like crazy for a couple of hours. Finally it started to rain and it stopped 10 minutes later. What a let-down.