Resumen:
Around thirty species of European solitary bee species in the family Megachilidae nest in empty gastropod shells. We surveyed this group of bees in semi-natural sites adjacent to almond orchards near Lleida (north-eastern Spain) and collected 35 Hoplitis firtoni and 58 Osmia ferruginea nests in shells of six snail species. We describe the nest structure and report the identity of pollens collected by the two bee species. Both species adjust the number of brood cells to the size of the shell and occasionally build intercalary (empty) cells. H. fertoni uses clay and O. ferruginea chewed plant leaves for building cell partitions and nest plugs. Most nests of both species were built in Sphineterochila randidissima shells. Analysis of the pollen of selected nests confirmed that H. fertoni is oligolectic on Boraginaceae (in our study all pollen was from Lithodor a frutieosa) and O. ferruginea is a polylectic species (collecting mostly pollen from Cistaceae, Fabaceae, and Lamiaceae in our study area). Nests of H. fertoni were parasitized by five species, the golden wasp Chysum hyln-ida, the cuckoo bee Dioxys moesta, the velvet ants Stenomutilla collaris and Stenomutilla hotentotta, and the bee-fly Anthrax. aethiops; nests of O. ferruginea were parasitized by the sapygid wasp Sapyga quinquepunetata and A. aethiops. Except for C. hyhrida these are newly recorded host-parasite associations. Our results confirm previous information and bring new findings on the ecology of both species.
