The effect of mass-flowering orchards and semi-natural habitat on bumblebee colony performance

Abstract

Bumblebees are important pollinators for agricultural crops and wild plants. However, agricultural intensification and loss of semi-natural habitat may have adverse effects on colony performance. While mass-flowering crops may serve as food sources, landscapes dominated by intensive agriculture may be a poor bumblebee habitat compared to landscapes dominated by semi-natural habitat, such as forests and species-rich grasslands. We investigated the effect of different land use classes on bumblebee colony performance at two periods of time during the flying season. We expected mass-flowering crops to have a positive influence on colony performance during flowering. After mass-flowering, presence of semi-natural habitat was hypothesized to be essential for colony development. We placed artificial colonies of Bombus terrestris in 28 apple and pear orchards during and after mass-flowering. We measured colony growth and took pollen samples from foraging workers. Causal relationships between land-use, pollen quality, amount of pollen collected and colony performance were analyzed. Contrary to our expectations, we found a negative correlation between orchard cover and amount of pollen collected in spring, during mass-flowering, resulting in a lower colony performance. During mass-flowering, colony performance and pollen diversity were positively correlated with cover of open semi-natural habitat, while after mass-flowering, colony performance was positively correlated with forest cover. After mass-flowering, pollen sources were more diverse and colony growth was related to land use classes at a larger spatial scale, indicating that bumblebees foraged further. We did not find a correlation between the performances of colonies at one specific site that were placed during and after mass-flowering. Landscapes dominated by mass-flowering orchards represent inferior habitats for bumblebees during flowering. Heterogenous landscapes, containing a mix of various types of semi-natural habitat increase the habitat quality for bumblebees. We advise to maintain these diverse patches of semi-natural habitat, as mass-flowering crops alone are insufficient to support bumblebee colonies.