Social Immunity in honeybees by Dr Marla Spivak

We enjoyed an excellent live talk given through Zoom by Prof Marla Spivak (MacArthur Fellow and McKnight Distinguished Professor in Entomology at the University of Minnesota) and arranged by Somerset Beekeepers.

Honey bees use several strategies to protect themselves and their colonies from parasites and pathogens. In addition to individual, physiological immunity, the colony benefits from behaviours now termed: Social Immunity. Bees work specifically to control the spread of parasites and pathogens within the nest. Two examples of social immunity in honey bees are hygienic behaviour (the detection and removal of diseased brood), and the collection and deposition of antimicrobial resins (propolis) on interior nest surfaces.

Propolis
Propolis is the apicultural term for plant resins that many species of honey bee collect and then lay down inside their nest cavities. The prophylactic property of propolis has been recognised and exploited by humans for many centuries. Both fungal and bacterial agents can be inhibited. Several active chemical components have been identified, but these vary according to the season and plant source. There is no clear, single molecular component providing the anti-microbial performance but flavonoids are significant in all types of propolis. Honeybee colonies have been shown to perform better if they are allowed/encouraged to lay down a full propolis envelope on the inner walls of their home in the same manner as wild bees. Roughening the internal walls of a wooden hive will encourage the bees to provide a propolis coating. Whether beekeepers should now refrain from flaming their spare brood boxes to kill pathogens, and so burn off all the protective propolis coating, was still an open question. Besides reducing the opportunity for infections to develop across the colony, it has been discovered that the internal microbiome, contained within the bee's gut, is healthier when a protective coating of propolis has been extended throughout the hive.

Hygienic behaviour
Early detection and removal of infected pupae is essential to avoid spreading spores throughout the hive. Both AFB and chalkbrood have an early "non infectious" phase before spores develop to spread the infection. Immature varroa mites simply need to be released from the pupal cell and exposed to predation. Honeybees use their probosis to detect dead or infected pupae, including those afflicted by mites and microbes, so uncapping and removal can then take place. Oddly, drone pupae seem not to be included in this hygienic behaviour, only worker brood.

The usual method of beekeepers assessing the level of hygienic behaviour expressed by a colony involves sacrificing an area of worker brood by freezing with liquid nitrogen and seeing what percentage of the dead pupae have been removed after a fixed interval. The correspondence between detecting dead pupae and those living but hosting mites is not perfect, but the freeze-killed brood assay has been judged a good way to screen colonies. Alternative experimental assay methods which avoided handling liquid nitrogen were also described. To breed bees to maximize hygienic genes, it is considered necessary to use instrumental insemination techniques. Just relying on wild drone insemination of chosen queens is not sufficiently effective.

Interestingly, Apis cerana, the eastern honeybee, does not collect and use plant resins but demonstrates vigorous hygienic behaviour to control varroa mites. It also has adopted the strategy of sealing in the infected pupae in such a manner as to prevent them emerging.

A very thought provoking and well presented talk, many thanks to Marla Spivak!

https://www.entomology.umn.edu/faculty-staff/marla-spivak