Nutrition of honeybees is constrained by the ratios of essential amino acids in pollen protein

Bees and flowering plants have co-evolved as mutualists. In this relationship, bees facilitate genetic outcrossing for plants, and in return, they collect and eat floral nectar and pollen. Nectar is produced as a reward for pollinators, but pollen is not: it is the plant's male gamete. While we expect bees to have adapted to use pollen as their main source of essential amino acids (EAAs), we know relatively little about nutritional constraints imposed on them by pollinivory. Here, we measured the EAA profiles of pollen, bee bread, honeybees, and royal jelly to understand how natural variation in pollen protein quality impacted bee feeding behavior and performance. Specifically, we tested how mismatches in EAAs relative to bee tissues impacted protein-to-carbohydrate regulation in adult workers. Bees fed diets with an EAA profile that matched their own tissues consumed more food, gained more weight, and ate proportionally more protein relative to carbohydrates, while those fed with pollen sources, including bee bread, ate proportionally less protein and less food overall. Deficiencies found in pollen led us to discover that nutrient balancing for protein and carbohydrate in bees was driven by the inverse relationship between quantities of the branched-chain amino acids relative to histidine in dietary protein. We predict that bees create bee bread, a mixture of pollen, as an adaptation to pollen feeding that reduces the impact of imbalances in the EAA profile of pollen protein.