UC Santa Cruz

Bee populations are declining but bees are critically important for pollination. Through resource provisioning, landscape context impacts bees: bees with access to food and habitat are healthier. But landscape context also has epidemiological importance for bees. As a bee forages across landscapes for food, it acquires flower-associated microorganisms. These can be beneficial or pathogenic. Variation in human-managed landscapes may therefore influence bee health. This dissertation addresses whether and how resource availability and landscape composition in urban agricultural systems influence disease dynamics and microbiome composition in three species of domesticated bees with wild counterparts: orchard bees, bumble bees, and honey bees.

The research was conducted in 18-25 urban gardens along the central coast of California. Differences at these gardens in terms of local and landscape features (such crop diversity and natural cover) allowed me to ask how anthropogenic processes such as urbanization impact bee health. In the 1st and 2nd chapter, I examine how garden management influences parasite and pathogen prevalence. In the 3rd chapter, I compare microbiome composition between orchard bees. In the 4th chapter, I use qualitative methods to describe the social factors shaping sustainable beekeeping practices.

I found that floral resources in urban gardens are positively associated with the prevalence of Apicystis and A. borealis in honey bees and with the prevalence of Deformed Wing Virus and Acute Bee Paralysis Virus in Bombus vosnesenskii. While these findings suggest that floral resources in urban contexts may amplify disease risk, I also found that nesting site availability (bare soil) negatively predicts the prevalence of some parasites and pathogens in bumble bees. I suggest more research on the tradeoffs between resource provisioning and parasite and pathogen transmission. Furthermore, while floral resources were associated with disease transmission, they were also associated with the abundance of bacterial groups beneficial to bee health. In Osmia lignaria, floral abundance was correlated with Lactobacillus, which was associated with reduced Crithidia prevalence. These studies highlight complex interactions between environmental context, bee diversity, and bee-associated microbes. I contextualize these findings in a qualitative beekeeping study, suggesting that the unique features of the beekeeping industry can influence the outcomes of resource provisioning on bee health.