Near-natural habitats near almond orchards with presence of empty gastropod shells are important for solitary shell-nesting bees and wasps

Solitary bees serve as important native pollinators of wild plants and crops. Several species of bees, which form strong populations in open habitats of European landscapes, nest in empty snail shells. In the present study, we focused on shell-nesting bees in near-natural habitats neighbouring almond orchards near the town Lleida in north-east Spain. We performed a series of manipulative experiments, which aimed to elucidate the importance of empty shells of four common large- and medium-sized gastropod species for nesting bees in near-natural and ruderal habitats surrounding the almond orchards. We also matched these data with findings of bees in naturally occurring shells at each examined site. All study sites consisted of habitats with naturally present shells and evidence of the natural presence of the study species of bees. In total, we recorded 15 nesting bee and wasp species and eight species of hymenopteran parasites in collected shells, one of which, the spider wasp Priocnemis propinqua, was recorded nesting in empty gastropod shells for the first time. In total, the deployed shells hosted eight nesting species and two species of parasitic Hymenoptera. The prevalence of nests in experimentally deployed shells was low, with only a single nest present near irrigated orchards. The abundance of nests was also low at sites where the naturally present shells were highly abundant. Combined, available evidence suggests the importance of near-natural habitats for the presence of shell-nesting bees and wasps in South-European landscapes. The abundance of shell nesting bees and wasps does not increase proportionally with the increase in empty snail shell abundance and other factors should be considered limiting at such sites.