Divergent Rules for Pollen and Nectar Foraging Bumblebees – A Laboratory Study with Artificial Flowers Offering Diluted Nectar Substitute and Pollen Surrogate

Abstract

Almost all bees collect nectar and pollen from flowers. Female bees collect pollen to provision their nest cells, whereas they use nectar for individual energy supply and nest cell provisioning. Bees fine-tune nectar foraging to the amount and to the concentration of nectar, but the individual bees' response to variability of amount and concentration of pollen reward has not yet been studied thoroughly in laboratory settings. We developed an experimental set-up in which bumblebees simultaneously collected sugar solution and pollen from artificial flowers; natural pollen was mixed with cellulose powder or glass powder as a pollen surrogate. Here we show that bumblebee (Bombus terrestris) workers do not specialise in nectar or pollen collection, but regularly collect both rewards on the same day. When offered a fixed pollen reward and varied amounts and concentrations of sugar solution, the bumblebees fine-tuned sugar solution foraging dependent on both the volume and concentration, with strong preferences for the highest concentration and the greatest volume. In the reciprocal tests, when offered a fixed sugar reward and varied amounts and concentrations of pollen mixed with a nutrient-free pollen surrogate, the bumblebees follow more an all-or-none rule for pollen, accepting all amounts and concentrations except pure surrogate. It is discussed how the bumblebees' ability to sense sugar, and their apparent inability to sense the pollen protein content, shaped their foraging behaviour. It is argued that the rarity of nectar mimicry and the frequency of pollen mimicry in natural flowers might be interpreted in the context of divergent abilities of nectar and pollen recognition in bees.

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

“…Interestingly, higher protein concentrations in modified diets led to fewer feeding events and reduced pollen consumption. It is important to note that in the previous experiments that found increasing protein concentration increased attraction (27,28), the protein composition of the pollen was modified by diluting it with cellulose powder, thereby reducing all nutrient concentrations and sensory cues. Furthermore, our studies demonstrate that B. impatiens does not simply avoid pollen (or diets) with high lipid content: One of their most preferred plant species, T. ohiensis, had relatively high lipid concentrations (Table 1), and they preferentially consumed modified diets with moderate lipid concentrations.…”

“…While foraging in the same landscape, bumble bees foraged preferentially on plant species with higher protein content than did honey bees (25), suggesting species-specific differences in protein acquisition. Bumble bee workers can taste and discriminate among diets with different protein or pollen concentrations (26), and their foraging activity has been positively correlated with pollen protein content using modified (diluted with cellulose powder) single-source pollen diets (27,28) or a single plant species in which pollen protein content varied with soil conditions (20). [Note that in field studies honey bees do not appear to forage preferentially on pollen with higher protein concentrations (29,30)].…”

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

“…Evolutionarily, this preference aligns with a goal of providing optimal resources for their brood, because suboptimal pollen quality can lead to reproductive deficit, egg cannibalism, and larval ejection (Génissel, Aupinel, Bressac, Tasei, & Chevrier, 2002; Tasei & Aupinel, 2008). In the laboratory, bumble bees prefer pollen diets with higher protein concentrations (Kitaoka & Nieh, 2008; Konzmann & Lunau, 2014; Ruedenauer, Spaethe, & Leonhardt, 2015; Ruedenauer et al., 2016), and these preferences extend to the field among plant species or within the same species (Cardoza et al., 2012; Hanley et al., 2008). 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).…”

How to know which food is good for you: bumblebees use taste to discriminate between different concentrations of food differing in nutrient content

“…For instance, as they can detect the sugar content of nectar via gustatory receptors on their antennae, mouthparts and tarsi (de Brito Sanchez, 2011), they preferentially collect nectar of high sugar concentrations (Hagler, 1990). Several studies have shown that bees also preferentially collect pollen that is rich in protein (Regali and Rasmont, 1995;Cook et al, 2005;Kitaoka and Nieh, 2009;Leonhardt and Blüthgen, 2012;Konzmann and Lunau, 2014; but see Pernal and Currie, 2002), suggesting that bees are further able to assess pollen quality (e.g. protein content).…”

“…Alaux et al, 2010;Konzmann and Lunau, 2014). Detailed studies on the bees' ability to assess the nutrient concentration of food are, however, still missing.…”