Naturespeak showing the recent situation on the shores of Lake Erie. I remember being on the south shore of Lake Erie and being swarmed by ladybugs, but ladybugs bite and mayflies don't.
His post reminded me of summer days at Leech Lake in northern Minnesota when the mayflies would emerge en masse and cover the docks, boats, cabin, trees, cars. We didn't even bother to go fishing those days because the fish would be so glutted with mayflies and unlikely to care about minnows.
Photo credit wykesgerald's Flickr photostream. See also this post re mayfly swarms seen on radar.