Anthony D. VaudoaHarland M. PatchJohn F. LookerChristina M. Grozinger
Department of Entomology, Center for Pollinator Research, The Pennsylvania State University


David A. Mortensen
Department of Plant Science, Center for Pollinator Research, The Pennsylvania State University


Stamens of a lily flower It is important to understand how bees forage in their environments to meet their nutritional needs in order to improve habitat that supports pollinator health and populations. It is well-known that solitary and social insects forage very differently to meet species-specific nutritional intake. Pollen is the primary source of proteins, lipids, and other nutrients. The researchers studied the foraging patterns of the generalist bumble bee species Bombus impatiens and compared the specific Protein: Lipid ratios of collected pollen from different plant species.

The results in this study demonstrate that the macronutrient ratios in pollen are a key factor determining bee foraging behavior. B. impatiens chose among plant species based on nutritional quality of their pollen. In feeding studies, B. impatiens preferred diets with a 5:1 or 10:1 Protein:Lipid ratio with an optimal amount of other macronutrients. This research indicates that bees evaluate pollen quality and adjust foraging decisions to meet their nutritional needs. To be effective, pollinator protection activities such as planting pollinator gardens must include plants that provide pollen satisfying the nutritional needs of bees.


To fuel their activities and rear their offspring, foraging bees must obtain a sufficient quality and quantity of nutritional resources from a diverse plant community. Pollen is the primary source of proteins and lipids for bees, and the concentrations of these nutrients in pollen can vary widely among host-plant species. Therefore, we hypothesized that foraging decisions of bumble bees are driven by both the protein and lipid content of pollen. By successively reducing environmental and floral cues, we analyzed pollen-foraging preferences of Bombus impatiens in (i) host-plant species, (ii) pollen isolated from these hostplant species, and (iii) nutritionally modified single-source pollen diets encompassing a range of protein and lipid concentrations. In our semifield experiments, B. impatiens foragers exponentially increased their foraging rates of pollen from plant species with high protein: lipid (P:L) ratios; the most preferred plant species had the highest ratio (∼4.6:1). These preferences were confirmed in cage studies where, in pairwise comparisons in the absence of other floral cues, B. impatiens workers still preferred pollen with higher P:L ratios. Finally, when presented with nutritionally modified pollen, workers were most attracted to pollen with P:L ratios of 5:1 and 10:1, but increasing the protein or lipid concentration (while leaving ratios intact) reduced attraction. Thus, macronutritional ratios appear to be a primary factor driving bee pollen foraging behavior and may explain observed patterns of host-plant visitation across the landscape. The nutritional quality of pollen resources should be taken into consideration when designing conservation habitats supporting bee populations.

Download Macronutrient ratios in pollen paper