They're worst at the Sand Wash put-in. Ultimate Evil!!!
Last time we ran it (mid July), we rigged the evening before our launch wearing rainpaints and shell parkas, headnets, shoes, and even gloves. We still got nailed around the ankles and wrists. Next time I might bring gaiters.
I'm not fainthearted, having worked as a backcountry ranger and range rider in some seriously buggy terrain. But the mosquitoes at Sand Wash are a quantum leap from buggy.
Still, the run is worth it. Best strategy is to get there the day before, rig after sunset, launch early, and go like hell the first day. The river's flat and brushy there, and the first night's camp can be a trial.
But it eases up after that. Where the rapids start, the mosquitos end. You might have yellowjackets or scorpions or ants. But at least you can sit on the shore and watch the sunset in peace.
Deso's a favorite: amazing rock walls, fun rapids, good canyon hikes, excellent camping. I've not run in the fall and would like to: mid-October when the cottonwoods are gold.