Early reproductive benefits of mass-flowering crops to the solitary bee Osmia rufa outbalance post-flowering disadvantages

Providing ample nectar and pollen, mass-flowering crops were suggested to counteract ongoing pollinator declines in modern agro-ecosystems. Lately, however, positive effects were shown to be transient and highly trait-specific within the social bumblebees. Contrary to bumblebees, solitary wild bees may benefit more sustainably from mass-flowering crops due to a better seasonal match of the mass provision of resources and their sexual reproduction. We quantified reproductive activity and reproductive output of the polylectic solitary bee Osmia rufa during and after mass-flowering in landscapes with differing amounts of oilseed rape and semi-natural habitats. Across seasons, the number of produced offspring increased with availability of oilseed rape and semi-natural habitats while brood abortion decreased with the former and parasitation with the latter. Season-specific analyses suggest that increased nest-building during mass-flowering of oilseed rape early in the season outweighs negative effects on the number of cells per nest and the percentage of parasitized cells once the mass-flowering has ceased. No effect on number of cells per nest during mass-flowering and exemplary pollen analysis indicate that oilseed rape benefits solitary bees in the form of abundant nectar for foraging flights rather than pollen for brood provisioning. Besides providing permanent forage and nesting sites, semi-natural elements seem to benefit pollinators also by mitigating negative effects of parasitation, potentially via enhanced hyperparasitism. In conclusion, O. rufa clearly benefits from mass-flowering oilseed rape. Yet, the outweighing of the negative post-flowering effects by the early benefits of oilseed rape is tightly linked to the bee's polylecty and early phenology. Thus, it remains to be tested if species unable to utilize oilseed rape due to mismatched oligolecty or later phenology suffer disproportionally from the post-flowering phase of oilseed rape.