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Wainia, a species-poor genus of osmiine bees, contains two Palaearctic species, whose biologies are unknown. In the present publication, we describe the nesting site and nest architecture of W. sexsignata and analyse the pollen host spectra of W. sexsignata and W. eremoplana by microscopical analysis of larval faeces and female scopal contents. W. sexsignata nests in empty snail shells. It uses large petal pieces of yellow flowered Asteroideae to separate the linearly arranged brood cells and to construct a series of partitions between the outermost cell and the final nest closure, which consists of masticated green leaves. W. sexsignata is recorded as a new host of the meloid beetle Sitarobrachys thoracica. Both Palaearctic Wainia species have a strong preference for Asteraceae as pollen hosts, but occasionally also collect pollen on other plant taxa. A literature survey revealed that nesting in empty snail shells is widespread among osmiine bees: obligate snail shell nesting is assumed to have independently evolved at least twelve times in their evolutionary history and to occur in at least 56 species belonging to 4 genera and 14 subgenera.