We describe and figure the previously unknown male of Hypanthidioides (Ctenanthidium) bifasciata (Urban, 1993) based on specimens reared from a trap nest along with a female. This species, originally known only from the type specimen collected in Colonia, Uruguay, is also newly recorded from Buenos Aires, Argentina. We provide for the first time information on the nest structure and, based on the analysis of pollen grains from brood cells, record five host plant species belonging to the families Apiaceae, Asteraceae, Brassicaceae, and Fabaceae. Most pollen grains belong to an undetermined species of the genus Melilotus L. (Fabaceae).