Last year we hunted mid Sept Muzzleloader Season. Our camp was at 10,000' and we would hike up from there. We had read that the mosquitos were pretty thick in the area; however, the only time we saw them was right at dusk. It didn't last long bc the temps would drop pretty quickly and they'd be gone. We did bring a small bottle of bug spray. Well worth the 4 oz it added to the pack.
Hot tip: check out Alltrails website. It is a hiking resource. If you're hunting near a trail then there is usually someone on there who has written a review about it. They don't ever talk about seeing a 350" elk; however, they'll give you insight on things like mosquitos, trail conditions, etc. Decent intel and worth the digging.