Wildflower plantings and honeybee competition impact nutritional quality of wild bee diets

Wildflower habitats planted along field borders are a widely promoted
strategy for supporting bees in agricultural landscapes. However,
honeybees (Apis mellifera), which are often stocked at high densities in
crop lands, can compete with wild bees for pollen and nectar, potentially
limiting the successfulness of wildflower plantings in supporting diverse
bee communities. Using weekly samples of five study sites in Northern
California we assessed how plants in pollinator-friendly seed mixes varied
in their ability to provide bees with abundant and nutritious pollen under
intense honeybee competition. We quantified pollen production, protein and
lipid content, and end-of-day pollen availability for different plant
species. We also sampled bee visits to flowers and assessed the
composition of pollen on bee bodies. Using these data, we investigate how
the nutritional quality of pollen in wildflower plantings and honeybee
abundance impacted native bee pollen nutrition. Bees collected more
nutritious pollen (i.e., pollen with more protein) from plantings with
more nutritious plant species (i.e., sites with more high-protein plants).
However, as honeybee abundance increased, the nutritional quality of
native bee diets declined. We also detected important interactions between
honeybee abundance and the nutritional quality of flowers in plantings,
such that, for some bee taxa, there was no impact of competition on pollen
diet quality in high-nutrition plantings.