Barefoot Trees for Bees: Nesting Site Characteristics of the Ground-Nesting Bee Andrena vaga in an Urban Environment

Ground-nesting bees constitute the majority of all wild bee species, but are critically endangered. Despite their crucial role in pollination and soil health, knowledge about the nesting requirements of ground-nesting bees is lacking, with most nesting site descriptions being vague and unsupported by empirical data. In our study, we conducted extensive measurements of relevant soil parameters at 27 nesting sites of the oligolectic mining bee species Andrena vaga within the city of Braunschweig, Germany, and compared them to uncolonized control areas, where no bee nests were found. Andrena vaga nested in (loamy) sand and sandy loam, and nesting sites were preferably located near trees under canopy cover. We identified the proportion of bare ground, soil temperature, and soil hardness, as well as the water content, to be the main factors distinguishing nesting sites from uncolonized areas. Artificially created bare ground plots located within control areas did not provoke colonization of Andrena vaga, assumingly due to social aspects, but were colonized by other bee orders such as Lasioglossum. The causal relationships between bee nest presence and soil characteristics are unclear, but likely bidirectional: ground-nesting bees tend to prefer drier soils with sparse vegetation, while potentially increasing proportions of bare ground and enhancing soil drainage, thus acting as environmental engineers. Our results allow conclusions about how public places like parks, cemeteries, or roadsides can be managed to provide suitable nesting sites for ground-nesting bees.