This proposal aims to determine if resins, complex plant secretions with diverse antimicrobial properties, provide a colony-level immune defense against pathogens in honey bee colonies. Bees harvest resin (or propolis) for use as caulk within the nest cavity. To test the hypothesis that plant resins play an integral role in honey bee immunity, product levels of seven immune-related genes and the presence of key pathogens will be compared in honey bees collected from resin-rich and resin-poor colonies. It is expected that the immune system in bees from resin-rich colonies will be relatively down-regulated, as indicated by lower immune-related gene transcript levels and lower microbe levels, suggesting that the presence of resin and lack of microbes causes the bees to invest less in the production of immune proteins that are energetically costly. Colonies also will be tested to determine if they collect more resin in response to pathogen challenge, which would indicate that the bees are self-medicating. Additionally, the stimuli that initiate and regulate resin collection at the individual and colony levels will be ascertained by observing the frequency of recruitment signals (bee dances and trembling) by individual resin foragers from high- and low-resin collecting colonies. This research will exploit the recently sequenced honey bee genome, which has provided new gene candidates for immunity and techniques for assessing the activity levels of gene products. Harvesting resin, which ultimately benefits the collective immune system of social individuals, is a fascinating example of environmental effects on innate immunity. An applied goal is to promote the natural defenses of honey bees, the world's most important pollinators of native and agro-ecosystems, and the economic health of the beekeeping industry. The experiments will actively engage graduate and undergraduate students in research that spans disciplines from beekeeping to molecular genetics.
Intellectual Merit. A number of fascinating ecological interactions have evolved between resin producing plants and the wide array of organisms that exploit them. Honey bees, Apis mellifera, collect plant resin and deposit it in the nest where it is called propolis. Our research explored the benefits of propolis to the immune system of honey bees. The harvesting of antimicrobial compounds from the environment and their use in social nest architecture is an exciting but relatively unexplored honey bee colony-level defense We found that a propolis envelope lining the inner walls of the bee nest acts as an external antimicrobial layer surrounding the colony, benefiting bee immune defenses and colony-level social immunity. By measuring levels of immune protein gene transcripts, we found that bees exposed to a resin-rich nest for 7 days reduced stress on individual immune systems, which may indirectly boost colony health and productivity. We also found that colonies increased resin foraging rates after a challenge with a fungal pathogen of immature bees called chalkbrood (caused by Ascosphaera apis). In addition, colonies experimentally made resin-rich had significantly less chalkbrood infection compared to resin-poor colonies, indicating that propolis also combats specific diseases. These exciting results provide a novel example of self-medication in the animal kingdom. Finally, we explored other behavioral mechanisms, in addition to the presence of pathogens, that initiate resin collection. Using tactile response conditioning in the lab (testing the tactile sensitivity of bees’ antennae to a gap between two metal plates and to sandpaper), we found that resin foragers have greater tactile sensitivity to holes and rough surfaces in the nest architecture, which might stimulate them to collect resin to fill in these gaps. Together the findings indicate that bees use plant resins both as mechanical barrier within the nest and as an important social immune defense against microorganisms. Broader Impacts. This research provided training to graduate and undergraduate students, but also has had an important impact on the beekeeping industry that produces honey and provides pollination services to our fruit and vegetable crops. On average since 2006, over 30% of all honey bee colonies across the U.S. die annually. An applied goal of this research was to promote the natural defenses of honey bees and the economic health of the beekeeping industry. Bees hived in man-made bee boxes (e.g., the standard Langstroth hive box) do not build a propolis envelope as they do when they nest in natural tree cavities because smooth inner surface of the box discourages bees from attaching resin. Instead, bees deposit propolis only in dispersed cracks and crevices throughout the hive boxes. We can encourage bees to construct a propolis envelope within their nests, by providing them with a rough inner surface in the nest (e.g., unfinished lumber). We don’t anticipate that the propolis envelope will be the sole solution to decreasing widespread bee mortality. Given the health benefit of propolis to bees and the immune stressors currently facing them, our research opens new avenues of research on novel ways to promote bees’ natural defenses and colony health, and may mitigate some losses.
