Ten-a-day: bumblebee pollen loads reveal high consistency in foraging breadth among species, sites, and seasons

Pollen and nectar are crucial resources for bees, but vary greatly amongst
plant species in their quantity, nutritional quality, and timing of
availability. This makes it challenging to identify an appropriate range
of plants to meet the nutritional needs of pollinators through the year,
though this information is important in the design of pollinator
conservation schemes. Using DNA metabarcoding of pollen loads, we record
the floral resource use of UK farmland bumblebees at different stages of
their colony lifecycle, and compare this with null models of ‘expected’
resource use based on landscape-scale resource availability (pollen and
nectar), to identify foraging priorities and preferences. We use this
approach to ask three main questions: i) what is the foraging breadth of
individual bumblebees?; ii) do bumblebees utilise a greater or lesser
diversity of plant species than expected if they foraged in proportion to
resource availability?; iii) which plant species do bumblebees
preferentially utilise? Individual bumblebees foraged from a highly
consistent number of different plant taxa (mean: 10 ±0.37 SE per bee),
regardless of their species, sampling site, or time of year. This high
consistency in foraging breadth, despite large changes in the quantity,
identity, and diversity of resource availability, implies a strong
behavioural tendency towards a fixed range of foraging resources. This
effect was most striking in April when foraging diversity was maintained
despite very low landscape-level resource diversity. Bumblebees used some
plant taxa significantly more than predicted from their landscape-level
floral abundance, nectar, or pollen supply, implying certain desirable
characteristics beyond the mere quantity of resource. These included
Allium spp. and Vicia spp. in April; Trifolium repens and Lotus
corniculatus in July; and Cynareae spp. (thistles) and Taraxacum
officinale in September. Our results strongly indicate that resource
quantity is not the only factor driving bumblebee foraging patterns, and
that resource diversity and quality are also important factors. Thus, in
addition to providing large quantities of floral resources, we recommend
that pollinator conservation schemes also focus on providing a sufficient
diversity of preferred floral resources, enabling pollinators to
self-select a diverse and nutritious diet.