Seasonal Benefits of Propolis for Honey Bee Health and Immunity

The 30% average winter loss of honey bee colonies in the U.S is not sustainable, making it critical to research ways to improve bees’ natural defenses. Honey bees, and other social insects, rely on collective behavioral defense mechanisms called social immunity to augment individual bee health. One of these behavioral defenses is the collection and deposition of antimicrobial plant resins in the nest, called propolis. When honey bee colonies nest in tree cavities, they construct a contiguous propolis layer within the rough inner walls surrounding the nest area which acts as an antimicrobial layer that enshrouds the colony. Colonies nesting in standard beekeeping equipment with smooth inner walls deposit propolis only in dispersed cracks and crevices. The goal of this experiment was to 1) test “propolis trap” configurations to encourage bees to naturally deposit propolis within commercial beekeeping hives, and 2) quantify the relative benefit of these propolis configurations to the immune system of 7-d old bees. We compared three treatments:  Twelve colonies were provided with propolis traps stapled to the four inner walls of each bee box to encourage the bees to construct a propolis envelope. Another 12 colonies were provided with propolis traps only on top of the frames. An additional 12 colonies served as controls where no propolis trap was provided. Seven day old bees were collected in July 2012, September 2012 and May 2013 to compare the relative immune-gene expression of bees from among the treatments using real-time PCR. Colony strength (disease levels, number of adult bees and brood) was also assayed to analyze additional benefits of propolis to colony health and survivorship. Preliminary findings show that the transcription of the three immune-related genes, Hymenoptaecin, Abaecin, and Defensin-2 from colonies with a propolis envelope were significantly lower compared to control colonies in both July and September of 2012, indicating bees in colonies with additional propolis did not have to up-regulate their immune systems as much as control bees.  Data from May 2013 show that the transcription of Defensin-2 from colonies of either propolis treatment was significantly lower compared to control colonies. However, colonies of either propolis treatment showed higher Hymenoptaecin transcription compared to control colonies, suggesting that different pathogens may be present at higher concentrations in early spring. Additionally, colonies with a propolis envelope had significantly more brood in May 2013 compared to the two other treatments. This study sheds light on the immune benefits of propolis to honey bees, which has high potential to positively impact bee health and consequently, the beekeeping industry.