The Benefits of Propolis to Bee Health
A summary of a presentation at the State Conference last November.
The hemolymph of the honey bee does not produce antibodies, so how do bees defend themselves against pests and pathogens? Certainly there are levels of individual immunity (mechanical barriers, innate and inducible defenses, and proteins which are antimicrobial peptides) but collectively the bees show grooming and hygienic behaviors, and an antimicrobial defense expressed in the collection of resin, which was the focus of this talk.
Resin is a sticky exudite produced typically by pine and spruce trees, but also cottonwoods, as a defense against herbivores, pathogen and UV. It contains a complex and diverse mixture of antimicrobial compounds and is scraped off of the trees by the bees with their mandibles before moving it to their hind legs. In a feral hive the bees combine resin with wax to make propolis with which to surround the hive entrance as well as varnish the walls. In a managed hive any gap of 1/16” is filled with propolis, (eg. where the walls of a Langstroth hive join, or under and around the ends of a frame as it sits in the hive.) Gaps of 1/4” of more are filled with wax.
Marla posed a number of questions. If resin foraging requires considerable energy from the bees and offers no direct individual reward, why do they do it? Do honey bees with a propolis envelope inside the nest have less disease? Are some resins more antimicrobial than others, and do the bees self-medicate with chosen resins?
She described experiments in which propolis was either painted on the inside of boxes or propolis traps were taped on the inside of boxes for bees to deposit a natural envelope. Bees in these hives had a significantly lower immune gene expression throughout the summer, thus did not have to invest as much energy in baseline immune function, and thus the effort involved in resin collection was justified. It was noted that propolis loses some of its antimicrobial activity over the winter and the bees will cover the envelope with new propolis collected in the spring.
What about as a defense against pathogens? Certainly colonies with a propolis envelope showed fewer signs of chalk brood, which is fungal, but what about a bacterial pathogen like AFB? One to two day old larvae are susceptible to AFB which they contract orally via brood food contaminated with AFB spores, and one mechanism of resistance is antimicrobial activity in that food. Experiments showed that seven day old nurse bees that had been exposed to both propolis and AFB spores had higher immune gene level expressions, brood food from these colones had a higher inhibitory activity against AFB and that levels of the disease were lower in colonies with a propolis envelope.
The answer to the question as to whether bees increase resin foraging after the colony is challenged with fungal or bacterial pathogens, is that the number of resin foragers does significantly increase for the former, but not the latter, for reasons that are presently unclear. And after the chalk brood (fungal) challenge, bees increased resin collection from trees they were already visiting (eg cottonwood) rather than visiting new sources, even though, for example, the highest antimicrobial resin comes from the white spruce, which the bees do not collect. Perhaps it is too toxic?
The practical implications for beekeepers include roughening the interior wooden sites of hives bodies, taping propolis traps to the inside walls of hive bodies, or adding 1/16” grooves to follower boards, all to encourage the bees to build a significant propolis envelope around there colony.