Effects of crop and non-crop resources and competition : High importance of trees and oilseed rape for solitary bee reproduction

To mitigate wild bee declines, it is important to understand the separate and joint effects of parallel losses of semi-natural habitats containing flower resources and increases in transient flower resources provided by mass- flowering crops. Since mobility may affect how bee species cope with these changes, they may also affect competitive interactions and thus community composition. We focused on how the reproduction of the solitary bee Osmia bicornis is impacted by crop and non-crop forage availability, and if any impact is modified by competition with more mobile bumblebees. We placed trap nests for O. bicornis with and without bumblebee colonies as neighbors at 0, 300, and 1000 m distance from nearest oilseed rape field in 12 agricultural land-scapes. We found that O. bicornis benefitted from proximity to oilseed rape, as well as availability of trees and buttercups. O. bicornis mainly collected oak and maple pollen early in the nesting season, and later switched to buttercup and other grassland species. In contrast to our expectations, we found no competition effects from the more mobile bumblebees. Our study demonstrates that availability of pollen from early-flowering trees and mid- season flowers is important for O. bicornis, but that a mass flowering crop still benefits its reproduction, most likely by supplying nectar. These results underline the importance of a high configurational heterogeneity of agricultural landscapes for bees. Management aiming at safeguarding bee populations in intensified agricultural landscapes should ensure availability of different types of flower resources, including woody species, at appropriate spatial and temporal scales