Flower visitors and fruit set of sweet cherry in Germany

Mason bees (Osmia spp.) are efficient fruit tree pollinators that can be
encouraged to occupy and breed in artificial nesting material. In sweet
cherry orchards, they are occasionally used as an alternative managed
pollinator as a replacement for or in addition to honey bees (Apis
mellifera). Yet, the lack of practical guidelines on management practices,
e.g., optimal stocking rates, for both mason bee nesting material and
honey bees might compromise pollination service provision. In this study,
we assessed the relationship between stocking rates (honey bee hives and
mason bee nesting material) and the abundance of honey bees and mason bees
in 17 sweet cherry (Prunus avium) orchards in Central Germany. We
furthermore performed a pollination experiment
to explore the interactive effect of mason bees and honey bees on
sweet cherry fruit set. In the orchards, both honey bee and mason bee
abundance increased with increasing stocking rates of hives or nesting
material, respectively. Honey bee abundance increased linearly with
stocking rates. In contrast, mason bee abundance asymptoted at 2-3 nesting
boxes per ha, beyond which more boxes resulted in little increase
in visitation rate. Our pollination experiment demonstrated that the
orchards were pollen limited, with only 28% of insect-pollinated flowers
setting fruit versus 39% of optimally hand-pollinated flowers. Honey bees
and mason bees enhanced sweet cherry fruit set, but only when both were
present and not when either was present alone in an orchard. Our findings
demonstrate that offering nesting material for mason bees and employing
honey bee hives can enhance bee abundance in sweet cherry orchards. By
increasing honey bee abundance in combination with enhanced mason bee
abundance, farmers can substantially boost fruit set and
potentially sweet cherry yield. To enhance pollination services, farmers
should consider the benefits of increasing pollinator biodiversity as an
immediate benefit to improve crop yields.