I just completed the red peak pass loop in Yosemite, 7000–11,000 feet. Practically no mosquitoes. I did have the misfortune of putting my pack down on a log and either the log or the ground I stepped on had a yellow jacket nest and suddenly dozens materialized, including the one bastard who stung my hand. I grabbed the pack and ran, could've been an old Hannah Barbera cartoon. Anyway, shortly after this, I encountered a crew from the Agriculture department doing a survey of high altitude bees. They stated that yellow jackets were at their peak of reproduction about now, and I think their queen dies after that , and they get a little bit irritable without royalty.?
In reality, the chance of having an experience like mine is very small, and I would not worry about it. It will not keep me from going out there, but I may look more carefully to make sure there are no holes in the dirt where I plant myself. Yellow jackets apparently like to make nests in prior squirrel and Gopher holes.?