Bumble bees regulate their intake of the essential protein and lipid pollen macronutrients

Bee population declines are linked to reduction of nutritional resources due to land-use intensification, yet we know little about the specific nutritional needs of many bee species. Pollen provides bees their primary source of protein and lipids, but nutritional quality varies widely among host-plant species. Therefore, bees may be adapted to assess resource quality and adjust their foraging behavior to balance nutrition from multiple food sources. We tested the ability of two bumble bee species, Bombus terrestris and B. impatiens, to regulate protein and lipid intake. We restricted B. terrestris adults to single synthetic diets varying in protein:lipid ratios (P:L). The bees overate protein on low fat diets and overate lipid on high fat diets to reach their targets of lipid and protein respectively. The bees survived best on a 10:1 P:L diet; the risk of dying increased as a function of dietary lipid when bees ate diets with lipid contents greater than 5:1 P:L. Hypothesizing that P:L intake target of adult worker bumble bees was between 25:1-5:1, we presented workers from both species unbalanced but complementary paired diets to determine if they self-select their diet to reach a specific intake target. Bees consumed similar amounts of proteins and lipids in each treatment and averaged a 14:1 P:L for B. terrestris and 12:1 P:L for B. impatiens. These results demonstrate that adult worker bumble bees likely select foods that provide them with a specific ratio of P:L. These P:L intake targets could affect pollen foraging in the field and help explain patterns of host-plant species choice by bumble bees. Jo ur na l o f E xp er im en ta l B io lo gy • A dv an ce a rt ic le