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.

壁蜂属（Osmia spp.）昆虫是高效的果树传粉者，可被引导在人工筑巢材料中栖息与繁殖。在甜樱桃果园中，它们有时会作为替代或补充西方蜜蜂（Apis mellifera）的人工管理传粉昆虫被使用。然而，目前缺乏针对壁蜂筑巢材料与西方蜜蜂管理实践的实用指南，例如最优投放密度，这可能会损害传粉服务的供给效果。本研究以德国中部17处甜樱桃（Prunus avium）果园为研究对象，评估了投放密度（西方蜜蜂蜂箱与壁蜂筑巢材料）与两类蜜蜂种群丰度之间的关联。此外，本研究开展了传粉实验，探究壁蜂与西方蜜蜂的交互作用对甜樱桃坐果率的影响。试验果园中，西方蜜蜂种群丰度随蜂箱投放密度升高呈线性增长；壁蜂种群丰度则随筑巢材料投放密度升高而增加，并在每公顷2-3个筑巢箱时达到渐近饱和，此后增加筑巢箱数量对访花率的提升效果微乎其微。传粉实验结果显示，试验果园存在花粉限制现象：仅28%经昆虫传粉的花朵成功坐果，而最优人工授粉组的坐果率可达39%。西方蜜蜂与壁蜂均可提升甜樱桃坐果率，但仅当两类蜂同时存在于果园时才可实现该效果，单一蜂种单独存在时则无法达到此效果。本研究结果表明，为壁蜂提供筑巢材料并投放西方蜜蜂蜂箱，可提升甜樱桃果园内的蜜蜂种群丰度。通过同时提升西方蜜蜂与壁蜂的种群丰度，种植者可显著提高坐果率，进而潜在提升甜樱桃产量。为优化传粉服务，种植者应考虑提升传粉者生物多样性的即时效益，以此改善作物产量。