Pollen and landscape diversity as well as wax moth depredation determine reproductive success of bumblebees in agricultural landscapes

Bumblebees are important pollinators in agricultural landscapes that are facing global declines. Main pressures include food scarcity mainly due to the reduction of semi-natural habitats (SNH) and parasite-induced vulnerability. Even though intensive agricultural landscapes are poor habitats for bumblebees, the cultivation of mass-flowering crops (MFC) can provide a high amount of floral resources and therefore can promote pollinators. In contrary to SNH, which provide a high diversity of floral resources, MFC provide only monofloral, short-term and unbalanced resources for bees. We explored the direct and indirect effects of landscape composition (proportions of MFC, SNH, urban areas), landscape diversity, diversity of pollen diets and wax moth depredation on the growth and reproductive success of bumblebee colonies. We placed 44 experimental Bombus terrestris L. colonies in 22 agricultural landscapes. The study landscapes represented gradients of the cover of SNH, urban areas and oilseed rape (OSR) and differed in the availability of the mid-season flowering MFC faba bean (Vicia faba L.). We recorded colony growth and reproductive success of the bumblebee colonies, diversity of collected pollen types and depredation by the specialized wax moth Aphomia sociella L. We found no effects of landscape composition or landscape diversity on the diversity of pollen collected by the bees. However, we found a positive effect of landscape diversity on the reproductive success of bumblebees. Moreover, pollen diversity and the availability of faba bean interacted and resulted in higher numbers of young queens in landscapes without faba bean. In addition, colonies that collected a higher pollen diversity had a reduced A. sociella depredation. Increased parasitism had a cascading negative effect on the reproductive success of bumblebees by limiting colony growth. Our study showed that high landscape diversity and diverse pollen diets can enhance the reproductive success of bumblebees. A diverse diet even mitigated depredation by wax moths. To sustain vital bumblebee populations and their pollination services, diverse and floral rich habitat types should be conserved or restored in agricultural landscapes.