How wild bees find a way in European cities: Pollen metabarcoding unravels multiple feeding strategies and their effects on distribution patterns in four wild bee species

1. Urban ecosystems can sustain populations of wild bees, partly because of theirrich native and exotic floral resources. A better understanding of the urban beediet, particularly at the larval stage, is necessary to understand biotic interactionsand feeding behaviour in urban ecosystems, and to promote bees by improvingthe management of urban floral resources.
2. We investigated the larval diet and distribution patterns of four solitary wild beespecies with different diet specialization (i.e. Chelostoma florisomne, Osmia bi-cornis, Osmia cornuta and Hylaeus communis) along urban intensity gradients in fiveEuropean cities (Antwerp, Paris, Poznan, Tartu and Zurich) using two complemen-tary analyses. Specifically, using trap-nests and pollen metabarcoding techniques,we characterized the species' larval diet, assessed diet consistency across cities andmodelled the distribution of wild bees using species distribution models (SDMs).
3. Our results demonstrate that urban wild bees display different successful strate-gies to exploit existing urban floral resources: not only broad generalism (i.e. H.communis) but also intermediate generalism, with some degree of diet conserva-tism at the plant family or genus level (i.e. O. cornuta and O. bicornis), or evenstrict specialization on widely available urban pollen hosts (i.e. C. florisomne).Furthermore, we detected important diet variation in H. communis, with a switchfrom an herbaceous pollen diet to a tree pollen diet with increasing urban intensity.
4. Species distribution modelling indicated that wild bee distribution ranges insideurban ecosystems ultimately depend on their degree of specialization, and thatbroader diets result in less sensitivity to urban intensity.
5. Policy implications. Satisfying larval dietary requirements is critical to preservingand enhancing wild bee distributions within urban gradients. For high to interme-diate levels of feeding specialization, we found considerable consistency in thepreferred plant families or genera across the studied cities, which could be gener-alized to other cities where these bees occur. Identifying larval floral preferences(e.g. using pollen metabarcoding) could be helpful for identifying key plant taxaand traits for bee survival and for improving strategies to develop bee-friendlycities.