Species richness and abundance of wild bees are declining worldwide. Land-use change and the use of pesticides are among the key drivers. Land-use change, such as increased settlement or traffic area, are often considered to be detrimental to wild bees. Soil sealing caused by the increase of these areas reduces the available space for flowering plants or soil-nesting bees. These changes in the environment may filter the wild bee community according to specific ecological traits. In contrast, some studies suggest cities as refuges for wild bees. Flowering plants cultivated in gardens, the diversity of the landscape with different greenspaces and the low pollution with plant protection products compared to agricultural areas is considered to be valuable for wild bees. In order to investigate the effects of an urban area on wild bee diversity, the city of Braunschweig in Lower Saxony, Germany was selected as study area. Bees were sampled with 49 sets of pan traps set up in the administrative area of the city in April, June and August 2019. During this survey, 1589 wild bee individuals from 107 species were caught, representing one third of the wild bees known for Lower Saxony, with almost one quarter on the Red List of Threatened Species. In the first study, the effects of different land-use types and impervious surface on the sampled wild bee diversity was assessed across multiple spatial scales. Wild bees were grouped according to their nesting and flower-visiting behaviour as well as their body size. The proportion of flower-rich anthropogenically managed areas, such as long-term allotments and cemeteries, had a positive effect on most groups, while the decisive spatial scale depended on body size. Unexpectedly, parks negatively affected below-ground nesting bees. Impervious surface affected a few bee groups, mostly in a non-linear manner with a maximum species richness or abundance at 20 – 40% impervious surface within a 100 m radius. This suggests positive edge effects at the transition between sealed and unsealed areas. In the second study, the differences in wild bee community composition were investigated among the sampling points in Braunschweig and potential environmental filters were examined. Wild bee communities varied markedly between sampling points, and only about one third of the overall citywide wild bee diversity was present at an average sampling point. The local community composition of flowering plants and soil texture (proportion of sand, silt, and clay) in the surroundings were identified as important drivers of wild bee community composition. Geographic distance between sampling points, distance to the city centre, and surrounding land use did not influence wild bee community. However, surrounding land use had an effect on the community composition of flowering plants. The third study examined the hypothesis whether the recently observed lower wing asymmetry of Anthophora plumipes in urban areas, compared to agricultural areas, might be caused by the lower use of plant protection products, as previously suggested. Due to the availability of well-established standard protocols for ecotoxicological assessments of larvae and adults, honey bees were used as a model organism. In the laboratory, honey bee larvae were fed with diets spiked with field realistic concentrations of pesticides most commonly recorded in bee products. After emergence, their wing morphology was analysed. While effects of pesticide exposure on wing morphology were rarely observed in this study, the rearing condition itself caused higher proportions of crippled wings and higher wing asymmetry compared to honey bees of parental colonies. Bees fed with the two active substances, chlorpyrifos and fenoxycarb, included in the study as potential positive controls showed a higher proportion of crippled wings. The high share of wild bees of the regional species pool and the high community turnover between sites found in Braunschweig suggests that urban areas play an important role in the conservation of wild bee diversity. Long-term allotments were shown to be particularly valuable and their area should therefore be preserved or increased. High levels of beta diversity between sampling locations showed that differences in the local environment, particularly flower resources and soil characteristics, determined the presence and abundance of species. This highlights the potential for supporting wild bees with these local improvements. Nevertheless, more research is needed to address supporting opportunities for wild bees in parks. Furthermore, the impact of pesticides on bee wing morphology needs further study.
