I wonder where they're nesting - haven't seen any holes in my stairs or railings. (Was experimenting to see how close I could focus with this lens - peaking helped nail focus with these very fast moving subjects.) By the way, the square stems gave it away. The obedient plant is in the mint family. No wonder it spreads so aggressively. I don't mind. It's a great insect attractant.

*If the body is hairy it's a bumble bee. Shiny it's a carpenter bee - but it's hard for me to see it!