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

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 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.foraging behavior | nutritional ecology | pollen | pollinator | preferences F oraging animals must obtain appropriate nutrients for growth, development, and reproduction from their environments. Bees forage in a very complex and changing environment, where floral nutritional resources (nectar and pollen) vary widely in quality and quantity among plant species (1). These resources are accompanied by myriad floral cues, including floral odors, color, morphology, and display area, and can vary dramatically in spatiotemporal availability; all these factors may influence and reinforce foraging decisions (2, 3). Worldwide declines in populations of bees and other pollinators have been linked to reduced diversity and abundance of host-plant species, likely placing bees under nutritional stress (4, 5). To develop strategic conservation protocols that preserve or restore foraging habitat that supports healthy pollinator populations, we must understand how bees forage in their environments to meet their nutritional needs. It is well established that solitary and social insects can forage selectively and regulate their intake of synthetic diets spanning a range of macronutrient nutritional qualities to reach their optimal, species-specific nutritional intake (6-8). Here we examine whether the generalist bumble bee species Bombus...

Consistent pollen nutritional intake drives bumble bee (Bombus impatiens) colony growth and reproduction across different habitats

“…Furthermore, bumble bee colonies will increase their foraging efforts to higher quality pollen (or nectar), or reduce foraging efforts to low‐quality pollen, even if no alternative is available (Dornhaus & Chittka, 2001, 2004; Kitaoka & Nieh, 2008). Our previous research revealed that B. impatiens , when collecting pollen for their colony in an enclosed outdoor foraging‐arena, preferred host‐plant species with pollen of high protein:lipid, or P:L ratios (~5:1 P:L, which was the maximum for the plant species in this study; Vaudo, Patch, Mortensen, Tooker, & Grozinger, 2016). Notably, foragers nearly ignored plant species offering the lowest P:L pollen (0.72:1 P:L), even when abundant pollen was available for collection (Vaudo, Patch et al., 2016).…”

“…Our previous research revealed that B. impatiens , when collecting pollen for their colony in an enclosed outdoor foraging‐arena, preferred host‐plant species with pollen of high protein:lipid, or P:L ratios (~5:1 P:L, which was the maximum for the plant species in this study; Vaudo, Patch, Mortensen, Tooker, & Grozinger, 2016). Notably, foragers nearly ignored plant species offering the lowest P:L pollen (0.72:1 P:L), even when abundant pollen was available for collection (Vaudo, Patch et al., 2016). Additionally, in the laboratory in the absence of external floral cues and brood, B. impatiens maintained these P:L preferences among pollen from different species and exhibited preferences of 5:1–10:1 P:L from nutritionally modified pollens (Vaudo, Patch et al., 2016).…”

“…The pollen collected in each habitat averaged a ~4:1 P:L ratio. This is similar finding to our previous research wherein semi‐field and laboratory choice assays B. impatiens preferred pollen with a ratio of 5:1 (Vaudo, Patch et al., 2016). In the current study, the distribution of pollen across the colonies and weeks was both above and below 5:1 (Figure 2a), indicating substantial variation in the P:L ratios of available pollen in the landscape.…”

“…We analyzed each pollen load for its dry mass (mg) and recorded mg pollen collected per forager. We analyzed the pollen load protein, lipid, and sugar concentrations and protein:lipid ratio (P:L; Vaudo, Patch et al., 2016). Briefly, we analyzed the protein concentration of pollen using a modified Bradford assay and lipid and carbohydrate concentrations using an assay modified from Van Handel and Day (Van Handel & Day, 1988; see Vaudo, Patch et al., 2016 for protocol).…”

“…Notably, foragers nearly ignored plant species offering the lowest P:L pollen (0.72:1 P:L), even when abundant pollen was available for collection (Vaudo, Patch et al., 2016). Additionally, in the laboratory in the absence of external floral cues and brood, B. impatiens maintained these P:L preferences among pollen from different species and exhibited preferences of 5:1–10:1 P:L from nutritionally modified pollens (Vaudo, Patch et al., 2016). Insect nutritional preferences exhibited in controlled settings may reflect the optimum that would be collected in the field.…”