Just finished a buggy hike. I seldom use DEET, but went through almost 1/4 bottle on this trip. The flies were competent, serious professionals dedicated to their work. People unused/unprepared for this can find it literally maddening. I might not have continued had I not known they'd thin out above 5500'.
I noticed that they couldn't penetrate Gwynnie's coat. She did not seem particularly bothered by them. I think the only place they could get to her was on the thin fur on her snout.
This might be another reason not to dock a corgi -- they might get some insect protection by curling up with their snout under their tail.