Managed crop pollination strongly relies on the honeybee Apis mellifera and the bumblebee Bombus terrestris, which is risky, may impact wild pollinator communities and does not always give the best pollination outcomes. The mason bees Osmia cornuta and Osmia bicornis are increasingly used as alternative crop pollinators, but it is not clear how suitable these species are for fruit pollination. Here we assessed the suitability of O. cornuta and O. bicornis as managed pollinators in two cherry and four apple orchards. O. cornuta and O. bicornis were introduced in the orchards at the start of blooming. We assessed their time till emergence, nesting rate, abundance in orchards via transect walks, flower visitation rate in 20-min time counts and pollen composition on their bodies. O. cornuta emerged 4 ± 2 days earlier and had a minimum of 5-fold higher nesting rate than O. bicornis in cherry and apple orchards. Transect walks revealed that O. cornuta was more abundant in orchards than O. bicornis (21.2% ± 10.1% vs. 1.3% ± 1.8% of all observed pollinators) and more frequently observed visiting flowers (53 vs. 5 visits for O. cornuta and O. bicornis, respectively). Pollen loads of O. cornuta contained 95% of Rosacea plants (likely apple and cherry), whereas this was 30% for O. bicornis, indicating that O. bicornis mainly foraged outside the orchards. These findings indicate that O. cornuta is a better pollinator than O. bicornis for early blooming crops, such as cherry and apple, in the relatively cool and rainy weather conditions during our study.
