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

Abstract 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. Synthesis and applications : Our study reveals that honeybee competition can reduce the nutritional quality of native bee diets. From an applied conservation perspective, we therefore recommend that honeybee introductions in natural areas be approached with extreme caution. However, our results also suggest that high‐protein flower plantings could mitigate negative effects of honeybee competition in managed landscapes. Where simultaneous support of managed and wild bees is a key management objective, we recommend including high‐protein plant species in plant mixes to support diverse bee populations.