UC Riverside

Bumble bees (Bombus spp.) are essential pollinators in agricultural and natural landscapes. Recently, bumble bee populations across North America have declined due to climate change, habitat loss, pathogens, and environmental toxins. Microbial symbioses, both mutualistic and parasitic, contribute to how bumble bees interact with the environment. Mutualistic microbes residing in the bumble bee gut protect against toxins and parasites. Parasitic microbes are detrimental to bee health and can act synergistically with additional stressors. For my dissertation research, I examined how beneficial gut bacteria contribute to Bombus impatiens learning and memory. I also examined how a common bumble bee gut parasite, Crithidia bombi, interacts with a heavy metal stressor (cadmium) outside of and within Bombus impatiens workers. In Chapter 1, I trained microbe-inoculated and microbe-depleted bumble bees to associate a sucrose reward with a color. I found that microbe-inoculated bees and microbe-depleted bees performed similarly in the behavioral assay. The results of Chapter 1 indicate that the absence of a healthy microbiome does not affect short term memory or learning in bumble bees. In Chapter 2, I performed in vitro assays to investigate the interactions between C. bombi and cadmium. I found that C. bombi strains vary in resistance to cadmium, but that all strains’ growth rates were significantly reduced at field realistic concentrations of cadmium. Further, I found that all strains were able to remove significant amounts of cadmium from their growth medium. Finally, in my Chapter 3, I experimentally infected bumble bees with C. bombi and fed them cadmium chronically over 7 days. I found no significant differences in mortality or C. bombi infection intensity between treatments. Gene expression assays demonstrated that one detoxification gene’s expression was significantly lower in cadmium treated bees, while the other was unaffected. I found no significant effects of treatment on immune gene expression in my experimental bees. Altogether, my dissertation results indicated that the gut microbiome does not play a role in bumble bee learning. Further, cadmium and C. bombi do not act synergistically in induvial bees and C. bombi is robust to cadmium exposure within the bee gut, but not in vitro.