Pollen mixing in pollen generalist solitary bees: a possible strategy to complement or mitigate unfavourable pollen properties?

Abstract

Summary1. Generalist herbivorous insects, which feed on plant tissue that is nutritionally heterogeneous or varies in its content of secondary metabolites, often benefit from dietary mixing through more balanced nutrient intake or reduced exposure to harmful secondary metabolites. Pollen is similarly heterogeneous as other plant tissue in its content of primary and secondary metabolites, suggesting that providing their offspring with mixed pollen diets might be a promising strategy for pollen generalist bees to complement nutrient imbalances or to mitigate harmful secondary metabolites of unfavourable pollen. 2. In the present study, we compared larval performance of the pollen generalist solitary bee species Osmia cornuta (Megachilidae) on five experimental pollen diets that consisted of different proportions of unfavourable pollen diet of Ranunculus acris (Ranunculaceae) and favourable pollen diet of Sinapis arvensis (Brassicaceae). In addition, we microscopically analysed the pollen contained in the scopal brushes of field-collected females of O. cornuta and three closely related species to elucidate to what degree these pollen generalist bees mix pollen of different hosts in their brood cells. 3. In striking contrast to a pure Ranunculus pollen diet, which had a lethal effect on most developing larvae of O. cornuta, larval survival, larval development time and adult body mass of both males and females remained nearly unaffected by the admixture of up to 50% of Ranunculus pollen diet to the larval food. 4. Between 42% and 66% of all female scopal pollen loads analysed contained mixtures of pollen from two to six plant families, indicating that pollen mixing is a common behaviour in O. cornuta and the three related bee species. 5. The present study provides the first evidence that the larvae of pollen generalist bees can benefit from the nutrient content of unfavourable pollen without being negatively affected by its unfavourable chemical properties if such pollen is mixed with favourable pollen. We conclude that the widespread pollen mixing by females of pollen generalist bees should also be considered as a possible strategy to exploit flowers with unfavourable pollen and to optimize larval food quality.

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

“…Furthermore, as generalist foragers, bumble bees may avoid or dilute the negative effects of toxic phytochemicals by collecting pollen from multiple host-plant species (50,51). It is unclear, however, whether micronutrient variation in pollen or secondary plant metabolites can alter bee foraging for macronutrients or whether the concentrations of these compounds are somehow associated with the macronutrient levels we measured.…”

Different but the same: bumblebee species collect pollen of different plant sources but similar amino acid profiles

“…Mixing pollen may dilute toxic compounds (Eckhardt et al 2014) or compose a diet closest to the bees' required optimum (which enables maximal possible individual or colony fitness), as the optimal ratio of nutrients (this is the stoichiometric relationship among all essential micro-and macronutrients that best supports an animal's metabolic functions and hence fitness) is hardly ever found in resources p r o v i d e d b y p l a n t s ( S i m p s o n a n d Raubenheimer 2012). If mixing was indeed used to balance the chemical composition of pollen loads, mixing at the individual forager level should be more pronounced in bee species which do not store pollen for longer periods but directly feed it to their young.…”

Floral Diversity of Pollen Collected by Honey Bees (Apis mellifera L.) – Validation of the Chromatic Assessment Method

“…A multifloral pollen diet is generally considered better than a monofloral one. In the former the risk of a lack of essential amino acids is lower and the effects of eventual toxic or suboptimal pollens are balanced by the presence of favourable ones (Eckhardt et al, 2014). Moreover, Alaux et al (2010) found that a mixed pollen diet has a positive impact on the colony immunocompetence.…”