Bee foraging strategies are shaped by pollen nutritional quality

Characterizing bee patterns of visitation to host-plants is challenging because of many factors (floral traits, ecological conditions, experience) influencing foraging decision-making. Ultimately, however, bees must forage for floral resources as their sole source of nutrition and may have species-specific nutrient requirements for optimal growth. Therefore it is likely that foraging decisions are driven by how rewarding floral resources are nutritionally. Pollen, bees’ primary source of proteins and lipids, is absolutely necessary for the development of larvae and therefore the most important and limited resource for supporting bee communities. By successively reducing environmental and floral cues, we analyzed the pollen foraging preferences of bumble bees to (1) host-plant species in semi-field conditions (2) host-plant pollen in cages and (3) synthetic diets representing a range of protein and lipid concentrations. In our field experiments, Bombus impatiens foragers exponentially increased their foraging rates to plant species with pollen of higher protein:lipid ratios. These preferences were verified in cage studies where, in the absence of other floral cues, B. impatiens workers preferred the same pollen types. Finally, when fed nutritionally-defined diets, B. terrestris workers selectively consumed a ratio of 10:1-5:1 proteins:lipids. Interestingly, the pollen species most preferred in our experiments contained a ~5:1 ratio, the highest amongst those tested. These results suggest that nutritional requirements are the primary factor driving bee foraging strategies. Furthermore, these studies provide novel information about the specific nutritional requirements of bumble bees and other bee species, thereby improving the selection of host-plant species for conservation protocols.