Mosquitoes? In a word: horrid.
Just got back from a multi-day Desolation trip and it's still spring in the high basins and north-facing slopes. Runoff everywhere, still lots of snow, beautiful wildflowers and vast quantities of mosquitoes. Our destination and route were still snowed in, so it was ad hoc camping for us.
Depending on one's tolerance, it's either a great time to have large swatches of usually busy wilderness to yourself or better to wait a week or two. It's probably possible to pick a dry destination based on good southern exposure. I took a dayhike up Mt. Ralston two weeks ago and the trail was dry as a bone, which lulled me into my plan for last week. Hah!
Cheers,
Rick