Yukon in September -- how late is too late for safe travel?

RE: Mosquitoes

If you can stay out of the deep forests so that your campsite will get some air, that will really help.

Also, although you can't really plan it but some summers have less rain than others and therefore less mosquitoes.

On our all-summer trip to Alaska and lots of time spent in the Yukon, we didn't consider mosquitoes to be an issue for us. The weather reports that summer were also of it being a dry summer. Then, too, we had lived in Michigan and we've encountered more mosquitoes there than our Alaskan trip.