Origin, composition, and role of antimicrobial plant resins collected by honey bees, Apis mellifera

Abstract

The use of plant resins in nest building by honey bees, <italic>Apis mellifera<italic>, is an oft ignored, but critical behavior to bee health. Termed `propolis' by beekeepers, deposited resins in the nest have many positive physiological effects on the colony. Honey bees are feral and abundantly managed in many regions of the United States, from the Sonoran Desert to icy Minnesota, yet a diversity of very different resinous plants exist in every environment they call home. We know very little about what resinous plants bees utilize in these different regions, or what benefits bees might derive from specific plants. It is thought that the antimicrobial properties inherent in resins, which are complex mixtures of phenolic and isoprenoid compounds, are important drivers of their derived benefits to bees. The research herein focuses on creating better methods to track resin forager behavior, and then using those methods to discover the botanical sources of bee-foraged resins, while also exploring how resins from different plants directly affect the growth of two bee pathogens, the gram-positive bacteria <italic>Paenibacillus larvae<italic> and the fungus <italic>Ascophaera apis<italic>. I found that individual resin foragers can be chemically tracked to their resinous plant targets using metabolomic methods that hold great advantages over traditional chemical analyses, and that there is much diversity in the ability of resins from different <italic>Populus spp.<italic> to inhibit the <italic>in vitro<italic> growth of <italic>P. larvae<italic> and <italic>A. apis<italic>. I go on to further explore the benefits of different resins and find that propolis from Fallon, NV was particularly active against <italic>P. larvae<italic> and <italic>A. apis<italic> out of samples from 12 different regions in the U.S. Finally, I used bioassay-guided fractionation against <italic>P. larvae<italic> to isolated several flavanone-3-alkyl esters from NV propolis that displayed very high activity (IC50 = 17 µM to 68 µM) against <italic>P. larvae<italic> and <italic>A. apis<italic>. Re-examination of data from my previous studies indicated that these compounds were strong contributors to overall anti-<italic>P. larvae<italic> activity in regional propolis samples, and that <italic>Populus spp<italic>. are likely the botanical sources of these compounds.