Drought events reduce reproductive success of a long-tongued bumblebee species

Climate change challenges pollinators through rising temperatures, altered precipitation regimes and an increased frequency of extreme weather events. Droughts are becoming more frequent and severe, with largely unknown effects on pollinator fitness and associated ecosystem services. In a large-scale field experiment, we compared colony performance of a long-tongued non-commercially bred bumblebee species between a drought year and a year with average climatic conditions. Colonies established from wild queens were placed in 25 high-quality semi-natural grasslands and either received sugar-water supplementation or did not to evaluate resource limitation. Colony lifespan was shorter in the drought year, and both colony weight and reproductive output (male and queen numbers) were strongly reduced. Sugar-water supplementation enhanced male production and colony weight only during drought, suggesting that nectar scarcity limits colony performance to some extent during dry periods. However, queen production was unaffected, and when comparing fed colonies between years, those in the drought year performed much worse, implying crucial pollen limitation. We conclude that increasingly frequent droughts pose a major threat to bumblebee populations and the pollination services they provide, even in flower-rich protected areas. Adaptive conservation management could enhance tree shade in open grasslands and promote wetlands for extended water retention during droughts.