Favourite plants of wild bees

Flower strips are a commonly used tool for improving agricultural landscapes for pollinators. The plant composition of the seed mixtures employed is based on logistic decisions, e.g. costs or seed availability, the literature and expert opinion, and only rarely on direct information about bee-plant interactions. We combined two datasets on bee-plant interactions involving 460 bee species. The first consisted of 23,864 bee-plant interactions, recorded in twenty locations in Germany from 2018 and 2019, whereas the second consisted of 86,509 bee-plant interactions sampled across multiple sites in southwest Germany over more than 30 years. We explored three objectives. (1) Which plant species attract the greatest number of wild bee species and individuals across plant communities within and across seasons of bee activity? (2) Do the most attractive plant species also support threatened and specialized wild bees? (3) Do seed mixtures contain attractive plant species? High attractiveness was defined as a consistently high interaction frequency and number of visiting bee species to a plant species across the sampled communities. Our results identified 34 herbaceous key plant species that were highly attractive for wild bees independently of plant abundance and across locations. Further, we identified a large number of plant species for which the attractiveness for wild bees depended on the sampled community. The identified key plant species attracted between 2% and 32% of oligolectic or red-listed bees present in our datasets. Although some of these plant species, such as Centaurea cyanus or C. jacea, are commonly found in seed mixtures, others, such as Stachys recta or Carduus nutans, are not. These large-scale and robust interaction data highlight key plant species that provide an attractive core for herbaceous floral strips aimed at supporting high numbers of wild bee species.