Variation in the pollen diet of managed bee species across European agroecosystems

Bee-mediated pollination plays a crucial role in sustaining global food production. However, while the demand for these pollination services is increasing, many bee species are in decline. To address this discrepancy, farmers use managed bee species to improve crop pollination. One key factor affecting pollination efficiency is the affinity for the crop of interest (i.e., the extent to which a bee integrates floral resources of a crop into its diet). In this study, we characterised and compared the pollen foraging preferences of three managed bee species: Apis mellifera, Bombus terrestris, and Osmia bicornis, across European agricultural landscapes and across biogeographic regions. Managed populations of each bee species were experimentally established at 128 agricultural sites growing either apple or rapeseed, in landscapes representing gradients in terms of the proportion of cropland, in eight European countries. We conducted pollen store sampling and employed palynological analyses to describe the foraging preferences of these species and to extrapolate their suitability as pollinators for both crops. Our findings reveal that A. mellifera and B. terrestris exhibited a more generalised pollen diet compared to O. bicornis, which showed stronger preference to certain pollen forage plants, but these were mainly non-crop rather than crop plants. These results question the relevance of using O. bicornis in apple orchards and rapeseed crops given their poor affinity with these crops. Overall, A. mellifera collected the highest proportion of rapeseed pollen in its diet, and A. mellifera and B. terrestris collected higher proportions of apple pollen than O. bicornis. Our findings also highlight substantial variation in the percentage of focal crop pollen in the diet across biogeographic regions, while landscape composition had virtually no impact. These results provide valuable insights for selecting the most suitable managed bee species to enhance the pollination of two key crops in Europe.