Economic motivation for plant species preferences of pollen‐collecting bumble bees

1. Pollen is an essential resource for bees as it provides the proteins needed for larval development and growth. Bumble bees typically collect pollen from a limited subset of the available plant species, indicating that bees perceive qualitative and/or quantitative differences between species. Because bumble bees typically exhibit different foraging behaviours when collecting pollen than when collecting nectar, and often visit different plant species, the economics of pollen collection are subject to unique benefits and costs. 2. The influences of pollen standing crop, grain volume, protein content and foraging costs on the choice of plant species by pollen‐collecting bumble bees were assessed. Pollen was collected from different plant species available to bees and these species were ranked according to eleven currencies that differentially incorporate these influences on foraging economics. 3. Comparison of observed preferences at three sites with species rankings based on foraging currencies revealed that bumble bees do not assess plant species based solely on intrafloral characteristics. Instead, pollen‐foraging bumble bees are sensitive to protein availability at the site as a whole as well as foraging costs, and forage in a manner that maximizes the site‐specific efficiency of protein collection (protein collected/energetic costs). Such behaviour would promote proper larval development while maximizing forager lifespan.