Some photographs of Chelostoma philadelphi, the most common guest at my mason bee hotel in past years. Approximately 7 mm in length and all black unless covered in pollen. Seems to prefer nest holes that are 1/8″ in diameter. In looking at photographs online they seem to show up on a variety of flowers (perhaps with preference for asters), but females are reported (Sedivy et al. 2008) to collect only Philadelphus pollen for use in nest provisions for her brood.