Some species of two tribes (Anthidiini and Osmiini) of the bee family Megachilidae utilize empty gastropod shells as nesting cavities. While snail-nesting Osmiini have been more frequently studied and the nesting biology of several species is well-known, much less is known about the habits of snail-nesting Anthidiini. We collected nests of four species of the genusRhodanthidium(R. septemdentatum,R. sticticum,R. siculumandR. infuscatum) in the Czech Republic, Slovakia, Catalonia (Spain) and Sicily (Italy). We dissected these nests in the laboratory and documented their structure, pollen sources and nest associates. The four species usually choose large snail shells. All four species close their nests with a plug made of resin, sand and fragments of snail shells. However, nests of the four species can be distinguished based on the presence (R. septemdentatum,R. sticticum) or absence (R. siculum,R. infuscatum) of mineral and plant debris in the vestibular space, and the presence (R. septemdentatum,R. infuscatum) or absence (R. sticticum,R. siculum) of a resin partition between the vestibular space and the brood cell.Rhodanthidium septemdentatum,R. sticticumandR. siculumusually build a single brood cell per nest, but allR. infuscatumnests studied contained two or more cells. For three of the species (R. siculum,R. septemdentatumandR. sticticum) we confirmed overwintering in the adult stage. Contrary toR. siculum,R. septemdentatumandR. sticticumdo not hide their nest shells and usually use shells under the stones or hidden in crevices within stone walls. Nest associates were very infrequent. We only found twoR. sticticumnests parasitized by the chrysidid waspChrysura refulgensand seven nests infested with pollen mites Chaetodactylus cf. anthidii. Our pollen analyses confirm thatRhodanthidiumare polylectic but show a preference for Fabaceae byR. sticticum.
