Bumblebee colony density on farmland is influenced by late-summer nectar supply and garden cover

1. Floral resources are important in limiting pollinator populations, but
they are often highly variable across time and space and the effect of
this variation on pollinator population dynamics is not well understood.
The phenology (timing) of floral resources is thought to be important in
structuring pollinator populations, but few studies have directly
investigated this. 2. Our study quantifies the landscape composition,
seasonal nectar and pollen supply, and Bombus terrestris colony density of
12 farms in southwest UK to investigate how landscape composition
influences the phenology of floral resources and how both these factors
affect colony density. We use this information in a spatially explicit
predictive model to estimate the effect of different farmland management
scenarios on seasonal resource supplies and colony density. 3. We find
that farmland nectar supply during September is a strong predictor of B.
terrestris colony density in the following year, explaining over half of
all the variation in colony density; no other period of resource
availability showed a significant association. Semi-natural habitat cover
was not a good proxy for nectar or pollen supply and showed no significant
association with colony density. However, the proportional cover of
gardens in the landscape was significantly associated with colony density.
4. The predictive model results suggest that increasing the area of
semi-natural flowering habitat has limited effect on bumblebee
populations. However, improving the quality of these habitats through
Environmental Stewardship and other management options is predicted to
reduce the late-summer resource bottleneck and increase colony density. 5.
Synthesis and Applications: Our results demonstrate the importance of
considering the phenology of resources, rather than just total resource
availability, when designing measures to support pollinators. Late-summer
appears to be a resource bottleneck for bumblebees in UK farmland, and
consequently management strategies which increase late-summer nectar
availability may be the most effective. These include mowing regimes to
delay flowering of field margins until September, planting late-flowering
cover crops such as red clover, and supporting late-flowering wild plant
species such as Hedera helix. Our results also suggest that rural gardens
may play an important role in supporting farmland bumblebee populations.