Poster exhibited at GPSC Student Showcase, February 24th, 2016, University of Arizona. / We investigated whether bees have an innate preference for flowers with saturated pigments and whether experience altered any preference. Preference could be a result of reward quality varying by color morph and/or responses to the petals, anthers, or their combination. Consequently, we gave bees experience on one of four floral configurations created from two color morphs of Solanum tridynanum. We subsequently tested learned preference using an array of all four configurations. Changes in preference as a result of experience were not mediated by anthers, only by petals. Bees that first experienced configurations with purple petals subsequently preferred configurations with purple petals, relative to naïve bees. However, bees that first experienced white petals showed no subsequent change in preference relative to naïve bees. Surprisingly, naïve bees showed no preference for any particular floral configuration. Rather than an innate preference for flowers with more saturated colors, bees are less able to develop a preference for unsaturated types. Because individuals are more able to develop a preference for saturated flowers, these flowers experience greater visitation, and thus greater pollination success, over unsaturated types.

Gut microbial communities exist in the vast majority of animals, and often form complex symbioses with their hosts that affect their host's biology in numerous ways. To date, the majority of studies of these complex interactions have focused on the nutritional benefits provided by the microbiota; however, the natural microbiota can also influence development, immunity, and the metabolism of its host. Apis mellifera, the honey bee, harbors a distinctive bacterial community that is present in individuals from distant locations around the world; however, the basis of the bee-microbiota association is unknown. This dissertation explores properties of the bacterial microbiota within bees, including its persistence of this association, mechanisms of transmission, localization through host ontogeny, and basic metabolic capabilities that define and maintain the symbiotic relationship. Apis and Bombus species (honey and bumble bees) share a distinct bacterial microbiota that is not present in other bees and wasps. Close analysis of the A. mellifera microbiota revealed consistent communities in adult worker gut organs and a general lack of bacteria in larvae. Contact between workers and with hive materials were identified as major routes of transmission for bacterial communities, showing the importance of social behavior in this association. Genomic analysis of a gut bacterium co-sequenced with the Bombus impatiens genome revealed it as a divergent lineage of Gammaproteobacteria, and deletions of conserved metabolic pathways, reduction in genome size, and its low GC content all suggest that the bacterial species has had a long association with its host.

In this thesis, I combined field observations and lab experiments to explore the ecological significance of floral signals in a North American wildflower, Penstemon digitalis. More specifically, to determine the potential mechanisms driving selection on floral scent, I studied how scent mediates interactions with pollinators and antagonists by (1) observing spatiotemporal variation in scent emission (2), floral volatile ability to suppress microbes (3) the honest advertisement of nectar, and (4) if scent could aid pollinator learning by reinforcing visual signals. Scent sampling of flower development, flower tissues, rewards and inflorescence day/night emission, revealed a complexity in floral scent composition and emission that could reflect several ecological functions. The floral bouquet of P. digitalis was strongest when flowers opened, primarily emitted from flower nectaries and was strongest during the day when pollinators are most active, suggesting a role in plant-pollinator interactions. Because linalool was one of the few floral compounds found in nectar where microbe growth can degrade the pollinator reward, I studied its role in plant-microbe interactions. Bacteria strains isolated from floral and vegetative tissues were exposed to varying concentrations of nectar volatiles: linalool and methyl nicotinate. Linalool inhibited bacteria growth rate from all tissue origins whereas methyl nicotinate had little effect, suggesting that microbes could drive selection on linalool emission strength.    To determine the extent that linalool could honestly signal nectar availability, linalool-nectar associations were measured for inflorescences and flowers. Linalool predicted inflorescence nectar availability but not flower, exposing a limit to its honesty. Pollinator Bombus impatiens could use linalool as a foraging signal at varying concentrations, suggesting linalool could be learned and used to choose the most rewarding plants.    Measurement and comparison of signal-reward associations for both olfactory and visual signals/cues of P. digitalis displays found display size and linalool honest indicators of nectar. Lab behaviour experiments showed multiple signals correlated with reward could increase bumblebee foraging efficiency and promote learning, providing an explanation for why floral displays are complex and consist of multiple signals.    Together my results show that an integrated approach is required to understand the mechanisms driving the evolution of the floral phenotype.

Temperature variation affects most biological parameters from the molecular level to community structure and dynamics. Current studies on thermal biology assess how populations vary in response to environmental temperature, which can help determine how populations differentially respond to climate change. To date, temperature fluctuation effects on endothermic poikilotherms such as the common eastern bumblebee (Bombus impatiens) are unknown even though bumblebees are the most important natural pollinators in North America. A cold-acclimation experiment with B. impatiens colonies revealed individuals acclimated to 5°C or 10°C at night did not differ in resting metabolic rate, flight metabolic rate, wingbeat frequency, or morphological measurements, compared to the control group. Moreover, an infrared camera showed that all colonies maintained maximum nest temperature consistently above 36.8°C. A latitudinal sampling of flight metabolic rate and morphological measurements of B. impatiens from four locations spanning Ontario (N 45°; W 75°) to North Carolina (N 34°; W 77°) indicated no latitudinal trend in the measured variables. This study shows that bumblebees are well equipped to face a wide range of environmental temperatures, both in the short term and long term, and can use a combination of behavioural and physiological mechanisms to regulate body and nest temperatures. These results are reassuring on the direct effects of climate change on bumblebee ecology, but further studies on the indirect effect of temperature variation on North American bumblebees are required to predict future ecosystem dynamics.

Social insect colonies are complex systems with emergent properties that arise from the cooperation and interaction amongst individuals within colonies. By dividing reproduction and physical labor amongst them, individuals contribute to the growth and ecological success of their colonies, a success that is greater than individuals could achieve on their own. A key characteristic of social insects is a division of labor amongst workers that is determined primarily either by age, morphology, or dominance. Social insects are considered one of the most ecologically successful groups of organisms on earth. Colony life cycles include: 1) growth, in which workers are produced, 2) reproduction, in which queens and males with reproductive capabilities are produced, and 3) senescence. In life history theory, phenotypic plasticity (i.e. a change in phenotype in response to an environmental change), allows organisms to adjust and optimize fitness in response the change in environments. Central to life history theory is the idea that traits have costs and benefits. Using an energetics framework that considers the costs and benefits of traits contributes to our understanding as to why organisms exhibit the sets of traits that they have within their ecological environments. Using the annual bumble bee Bombus impatiens, my dissertation investigates the effects of resource availability on worker production and on the relative allocation of energy towards growth and reproduction within colonies. Bumble bees have a morphological division of labor and concomitantly, they show large intra-colony size variation amongst workers. Because body size is an important life history trait, I also examined the costs and benefits of producing various sized workers. Lastly, I examined the association among worker body size, metabolic rate (a measure of maintenance costs), and lifespan.

Temperature variation affects most biological parameters from the molecular level to community structure and dynamics. Current studies on thermal biology assess how populations vary in response to environmental temperature, which can help determine how populations differentially respond to climate change. To date, temperature fluctuation effects on endothermic poikilotherms such as the common eastern bumblebee (Bombus impatiens) are unknown even though bumblebees are the most important natural pollinators in North America. A cold-acclimation experiment with B. impatiens colonies revealed individuals acclimated to 5°C or 10°C at night did not differ in resting metabolic rate, flight metabolic rate, wingbeat frequency, or morphological measurements, compared to the control group. Moreover, an infrared camera showed that all colonies maintained maximum nest temperature consistently above 36.8°C. A latitudinal sampling of flight metabolic rate and morphological measurements of B. impatiens from four locations spanning Ontario (N 45°; W 75°) to North Carolina (N 34°; W 77°) indicated no latitudinal trend in the measured variables. This study shows that bumblebees are well equipped to face a wide range of environmental temperatures, both in the short term and long term, and can use a combination of behavioural and physiological mechanisms to regulate body and nest temperatures. These results are reassuring on the direct effects of climate change on bumblebee ecology, but further studies on the indirect effect of temperature variation on North American bumblebees are required to predict future ecosystem dynamics.

Temperature variation affects most biological parameters from the molecular level to community structure and dynamics. Current studies on thermal biology assess how populations vary in response to environmental temperature, which can help determine how populations differentially respond to climate change. To date, temperature fluctuation effects on endothermic poikilotherms such as the common eastern bumblebee (Bombus impatiens) are unknown even though bumblebees are the most important natural pollinators in North America. A cold-acclimation experiment with B. impatiens colonies revealed individuals acclimated to 5°C or 10°C at night did not differ in resting metabolic rate, flight metabolic rate, wingbeat frequency, or morphological measurements, compared to the control group. Moreover, an infrared camera showed that all colonies maintained maximum nest temperature consistently above 36.8°C. A latitudinal sampling of flight metabolic rate and morphological measurements of B. impatiens from four locations spanning Ontario (N 45°; W 75°) to North Carolina (N 34°; W 77°) indicated no latitudinal trend in the measured variables. This study shows that bumblebees are well equipped to face a wide range of environmental temperatures, both in the short term and long term, and can use a combination of behavioural and physiological mechanisms to regulate body and nest temperatures. These results are reassuring on the direct effects of climate change on bumblebee ecology, but further studies on the indirect effect of temperature variation on North American bumblebees are required to predict future ecosystem dynamics.

Bumble bees and other wild pollinators are crucial to the support of both natural and agricultural ecosystems. However, unprecedented declines of pollinator populations have been observed all over the world, raising concerns of a looming threat to both the human food supply, as well as sustainability of the biodiversity in local ecological niches. Though declines are well described, the cause behind these still evades scientists. Proposed contributors include anthropogenic-mediated environmental stress, including application of xenobiotics for pest control, and increase of pathogen diversity and abundance due to the shipment of infection human-managed colonies. This research examined these theories and attempted to quantify the threats they may pose. Through development of a chronic, oral toxicity experiment, susceptibility of all Bombus impatiens castes to clothianidin exposure was examined. This exposed a substantial increase in vulnerability of male bumble bees to realistic concentrations of neonicotinoid pesticides, highlighting the crucial need to examine all members of wild bumble bee life cycles before determining pesticide regulations. Additionally, sublethal effects on fitness-related foraging behaviors in Bombus impatiens were examined through development of a voluntary task switching assay. The results of this experiment suggest humoral immune stimulation, through pathogenic infection, leads to significant impairment of cognitive flexibility. Taken together, this data suggests that pesticides and pathogens are capable of causing severe detrimental effects, both lethally and sublethally, in wild bumble bees. I hope this data will eventually contribute to reassessment of environmental regulations and establishment of effective conservation strategies in order to sustain the critical populations of wild bumble bees.

Plants are under selection to simultaneously attract pollinators while deterring herbivores. This dilemma can lead to tradeoffs in floral traits, which are traditionally thought to be optimized for pollinators. My dissertation addresses the ecological costs and putative functional significance of nectar secondary metabolites, a paradoxical but widespread phenomenon in the angiosperms. I investigate this issue from the pollinator’s perspective using a series of controlled laboratory investigations focused primarily on the bumble bee Bombus impatiens and the nectar alkaloid gelsemine, from Gelsemium sempervirens. I begin by demonstrating that nectar enriched with the alkaloid gelsemine significantly deters visits from bumble bees at a range of natural alkaloid concentrations. However, this aversion can be mitigated by increasing the sucrose concentration such that the alkaloid-rich nectar is more rewarding than its alkaloid-free counterpart. I then demonstrate that the consumption of gelsemine-rich nectar can inhibit oocyte development and protein utilization in bees, but that this effect is limited to bees of suboptimal condition. Continuous consumption of the nectar alkaloid gelsemine also leads to a reduction in the pathogen load of bumble bees infected with Crithidia bombi, but direct interactions between the pathogen and the alkaloid have no impact on infection intensity. Gelsemine also fails to inhibit floral yeast growth, suggesting that nectar alkaloids may not be universally antimicrobial. Finally, I demonstrate that gross nectar cardenolides from the genus Asclepias are strongly correlated with gross leaf cardenolides and that the majority of individual cardenolides found in nectar are a subset of those identified in leaves. This pattern suggests that nectar cardenolides are a consequence of defense for Asclepias; however, they may not be a costly corollary because bumble bees show an overall preference for nectar cardenolides at mean concentrations. Altogether, my dissertation provides a new perspective on the role of chemical defenses against herbivores in plant-pollinator interactions.

Plants are under selection to simultaneously attract pollinators while deterring herbivores. This dilemma can lead to tradeoffs in floral traits, which are traditionally thought to be optimized for pollinators. My dissertation addresses the ecological costs and putative functional significance of nectar secondary metabolites, a paradoxical but widespread phenomenon in the angiosperms. I investigate this issue from the pollinator’s perspective using a series of controlled laboratory investigations focused primarily on the bumble bee Bombus impatiens and the nectar alkaloid gelsemine, from Gelsemium sempervirens. I begin by demonstrating that nectar enriched with the alkaloid gelsemine significantly deters visits from bumble bees at a range of natural alkaloid concentrations. However, this aversion can be mitigated by increasing the sucrose concentration such that the alkaloid-rich nectar is more rewarding than its alkaloid-free counterpart. I then demonstrate that the consumption of gelsemine-rich nectar can inhibit oocyte development and protein utilization in bees, but that this effect is limited to bees of suboptimal condition. Continuous consumption of the nectar alkaloid gelsemine also leads to a reduction in the pathogen load of bumble bees infected with Crithidia bombi, but direct interactions between the pathogen and the alkaloid have no impact on infection intensity. Gelsemine also fails to inhibit floral yeast growth, suggesting that nectar alkaloids may not be universally antimicrobial. Finally, I demonstrate that gross nectar cardenolides from the genus Asclepias are strongly correlated with gross leaf cardenolides and that the majority of individual cardenolides found in nectar are a subset of those identified in leaves. This pattern suggests that nectar cardenolides are a consequence of defense for Asclepias; however, they may not be a costly corollary because bumble bees show an overall preference for nectar cardenolides at mean concentrations. Altogether, my dissertation provides a new perspective on the role of chemical defenses against herbivores in plant-pollinator interactions.