Reliance on traditional pollinators like the honeybee Apis mellifera and the bumblebee Bombus terrestris carries inherent risks, can disrupt wild pollinator populations, and does not always produce optimal fruit pollination. Mason bees, particularly Osmia cornuta and Osmia bicornis, are increasingly considered as managed alternatives, yet their effectiveness for fruit trees remains poorly characterized. In this study, we compared the performance of these two species in two cherry orchards and four apple orchards. Bees were introduced at the beginning of the flowering period, and we tracked their emergence timing, nesting success, population abundance through transect surveys, frequency of flower visits over 20-minute intervals, and pollen carried on their bodies. O. cornuta emerged earlier by an average of 4 ± 2 days and demonstrated at least five times higher nesting activity than O. bicornis across all orchards. Transect data revealed that O. cornuta was far more prevalent than O. bicornis (21.2% ± 10.1% versus 1.3% ± 1.8% of observed pollinators) and consistently visited more flowers (53 versus 5 visits). Pollen analysis indicated that O. cornuta predominantly collected Rosaceae pollen (95%), likely from apples and cherries, while O. bicornis carried only 30%, suggesting it foraged largely outside the orchards. These findings highlight that O. cornuta outperforms O. bicornis as a managed pollinator for early-flowering crops like cherry and apple under the cool, rainy conditions characteristic of this study period.
