Examining the Impact of Pollen Diet Composition on Bee Development and Lifespan
Creators: name: McAulay, Megan
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2018
Subjects: Bee Ecology
Host Specialization
Plant-Insect Interactions
Insect Nutrition
Pollen
Osmia
Bombus
Rocky Mountain Biological Laboratory

description: Pollen is the sole protein source for most bees and the largest component of their larval diets. The high pollen requirements of bee larvae may select for a generalist foraging strategy that reduces dependency on individual pollen sources. Alternatively, specialization on low-quality pollens could be favoured by selection if it increases efficiency, reduces competition for floral resources, or lowers the risk of larval predation. Some pollens, including those from the plant family Asteraceae, appear to be poor diets for generalist bees, possibly owing to nutrient deficiencies or the presence of secondary metabolites or digestive barriers that prevent nutrient assimilation. The high pollen and nectar yield of Asteraceae flowers combined with the long blooming period of these plants should otherwise make them an important pollen source for both specialist and generalist visitors. However, physiological adaptation—and an accompanying loss of ability to thrive on other diets—may be necessary to tolerate the unfavourable qualities of Asteraceae pollen. This could be the reason so many specialist bees are associated with this plant family while many generalist bees appear to avoid it. In my thesis, I conducted feeding trials on larval and adult bees to examine how specialist and generalist foraging strategies affect performance on different pollen diets. First, I compared larval survival and development of both specialist and generalist Osmia (Megachilidae) bees on host and nonhost pollen diets. I then investigated whether the observed differences in survival and development on Aster provisions could be explained by nutritional deficiencies or low digestibility by analyzing the amino acid profiles of the different pollen diets and comparing the digestive efficiencies of bees reared on different pollens. I found that Aster specialists could develop on both host and nonhost provisions. However, Aster provisions were unsuitable larval diets for bee species that do not collect Asteraceae pollen (i.e., Osmia iridis and O. tristella). These findings suggest that Aster specialists are physiologically adapted to the chemistry of their host pollen, but have not lost the ability to tolerate other pollens. The Aster provisions were not deficient in amino acids, and the Aster specialists could digest their host pollen as efficiently as the generalist species, O. tristella. Deficiencies in other essential nutrients or the presence of toxic secondary metabolites could be the reason O. iridis and O. tristella failed to develop on Aster provisions. Mixing pollens might be another strategy for bees to exploit unfavourable pollens while still stabilising larval diet quality, if the different pollen sources help to complement nutrient deficiencies and alleviate the effects of toxic secondary metabolites. In my second study, I examined how the proportion of sunflower (Helianthus annuus, Asteraceae) pollen in the diet of captive-reared bumblebees (Bombus impatiens) affects the survival of queenless microcolonies. Bees fed sunflower pollen had significantly shorter lifespans than bees fed broad bean, rapeseed, or Cucurbitaceae pollen. However, mixed pollen diets containing 50% sunflower pollen did not reduce survival, which suggests that the other pollens were able to compensate for the low nutritive quality of the sunflower pollen. Due to agricultural intensification and a loss of wildflowers, farmland monocultures (e.g., sunflower crops) can be important floral resources for bumblebees. My study suggests that providing alternative floral resources of high nutritive quality could help mitigate the potential harmful effects of a monofloral sunflower diet on bumblebees. In my thesis, I demonstrate that pollen specialization and pollen mixing can be alternative strategies to exploit low-quality pollen resources. Since the foraging decisions of mother bees ultimately determine the pollen composition and nutritional quality of offspring provisions, feeding studies should be paired with host-selection experiments to assess the willingness of adult bees to exploit different plant species. Understanding both the host range and the nutritional requirements of different bee species is important for pollinator management since changes in the availability of host plants could lead to pollen shortages, potentially forcing bees to visit nonhost or less-preferred plants to feed their offspring.