ID'd by @colinpurrington via Twitter. Reference article: https://thedragonflywoman.com/2011/06/27/mites/
"Hairy-footed pollen mites"? so thick on thorax of Osmia mason bees that they couldn't fly. (My fault for leaving same tubes up for years). Two years later we have no mason bee activity even with 3 new houses & tubes up.