Macronutrient ratios in pollen shape bumble bee ( Bombus impatiens) foraging strategies and floral preferences

Significance

Bees pollinate the majority of flowering plant species, including agricultural crops. The pollen they obtain is their main protein and lipid source that fuels development and reproduction. Bee populations are declining globally, in large part because of landscape-level loss of host-plant species contributing to a nutritional shortage. To mitigate declines, we must understand how the nutritional requirements of bees influence foraging behavior. We demonstrate that bumble bees selectively collect pollen from host-plant species based on the protein:lipid ratios of pollen. Our research indicates that bees evaluate pollen quality and adjust foraging decisions to meet their nutritional needs. To be effective, conservation initiatives must include host-plant species that provide pollen that satisfies the nutritional demands of bees to support their populations.

Abstract

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 host-plant 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.