While I’ve spent the last week coating my sore throat with propolis tincture, the apiary bees have been hunkering down waiting for the good weather to return in an environment enriched by propolis. But the wild bees up the road in a tree have coated the inside of their nest cavity and entrance with propolis, and live in a different environment altogether.
We humans have been using hive products for centuries and propolis has always been a go-to antimicrobial agent. Florence Nightingale (1820-1910) had great success reducing wound infections during the Crimean War from 1854, and she increased soldier survival rates considerably by using propolis in wound dressings.
However, until recently we haven’t known a lot about how honey bees, the makers of propolis, use it themselves to boost a colony’s collective immunity and defend against disease. Thanks primarily to the work of Professor Marla Spivak and her colleagues at the University of Minnesota, we know much more about propolis today and it is quite fascinating to delve deeper into the research currently going on.
Plants have always provided the basis for our own medicine cabinets and most people know that Aspirin used to relieve pain, inflammation and fever comes from willow bark. Before salicylic acid was isolated from willow bark in 1860 and synthesised in a laboratory, people used to chew pieces of willow bark to relieve pain.
Plants produce resin to protect young lead buds or the whole plant from disease, attack from herbivores, or from sunlight. Resin is the sticky exudate often seen on pine trees in colourful globules where a branch has broken off. Pine resin was a popular medicine in 1694 and often used for treating lung diseases. Interestingly, its importance lasted a long time and pine resin was still listed in the British Pharmacopoiea of 19321. Some flowers such as helianthus or sunflower produce resin too. Resin is produced by many different trees in the UK including: poplar, chestnut, cherry, witch hazel, pine, spruce, cedar, larch, yew, juniper and cypress.
Western honey bees forage for and collect resin but not to boost food stores; they never eat propolis. Some sub-species, such as the African Apis mellifera scutellata, and Apis mellifera caucasica, collect and use resin more than others, but the Eastern honey bee, Apis cerana doesn’t collect resin at all. Only a small number of resin collectors in a colony engage in this difficult work which involves mandibular dexterity in chewing up leaves to extract resin, or biting off bits of resin and painstakingly transferring them to the back legs and corbiculae. Less than 1% of a colony takes on this work and once back inside the hive they need help unloading which might take up to 30 minutes to achieve. Resin may have beeswax added but no other chemical changes take place though the name changes and we call it propolis when it reaches the hive.
What Colour Do You Want the Walls, Madam?
In the mid-1970’s, when Dr Roger Morse and Dr Tom Seeley2 were investigating the natural nest sites of honey bees, they discovered layers of propolis lining nest cavities like intricate interior decorating going on in the homes of wild bees in upper New York State. The cavity floor was coated with a hard dry layer of propolis up to around 0.10 inches thick and the ceilings and walls were coated in the same propolis of around 0.02 inches thick. You can see this in the picture showing the decayed wood underneath where propolis has been chipped away.
Morse and Seeley suggest that the propolis envelope functions to provide waterproofing and act as a barrier against tree sap and moisture inside the nest, giving attachment for comb construction, acting as a physical barrier against outside invaders, draughts, rain, and protecting against fungi and bacteria in the nest.
I’ve seen for myself how honey bees in the wild coat the nest entrances with propolis and how some of my colonies produce more propolis than other colonies but they do so on a smaller scale than wild colonies in tree cavities. Have we bred this out of them by selecting for colonies that produce less propolis because we beekeepers find it too sticky and messy to work with colonies producing abundant propolis? When I started beekeeping, the mantra was, “rear queens from colonies that don’t produce propolis”
Or, is it because our modern hives of smooth shiny wood / polystyrene don’t have rough surfaces to smooth down with propolis and therefore stimulate the collection of resin? They certainly stick propolis onto top bars and into cracks and crevices in my hives but nothing like constructing a curtain round the entire nest.
In 2009, Simone3 and colleagues painted the inside of managed hives with a solution of propolis extract and exposed bees to a propolis enriched environment for 7 days. On examining the bees aged 7 days exposed to propolis, they found that they had lower immune system activation and lower bacterial loads in their bodies compared with bees of the same age not exposed to propolis.
Honey bees can fight infection at a cellular level whereby some body cells engulf and destroy bacteria in the same way as our cellular defence system works. However, unlike our humoral defence system that produces antibodies in response to a particular infection, honey bees produce something called antimicrobial peptides (AMPs) which help them fight infection. Producing AMPs is costly in energy for honey bees and what the Simone study showed was that when they were in a propolis- rich environment, honey bees didn’t have to use as much energy activating their immune systems to fight off infection.
A later similar study showed how, in hives coated with propolis, colonies infected with Ascospahaera apis, the causative fungal pathogen of chalkbrood, had less chalkbrood infected brood than colonies not coated in propolis. This led on to work on American Foulbrood (AFB), and, although propolis cannot cure AFB, it can reduce the number of infected larvae when there is a propolis envelope surrounding the colony. Larval food doesn’t come into direct contact with propolis but some of the volatile compounds in it diffuse through the hive and may get into larval food. Whatever happens, the larval food in these colonies had higher antimicrobial activity. This work is reassuring as it demonstrates how having a propolis envelope increases the immune response at individual and colony level possibly by increasing production of AMPs in individual bees and increasing the bioactivity of larval food4.
Today we know that honey bees self-medicate with propolis when their colony is infected with the chalkbrood fungus. They do this by sending more resin foragers out into the field to collect it.
What Can We Do?
As beekeepers we can help colonies to collect more resin by providing hives with roughened inside surfaces which stimulate them to lay down layers of antimicrobial propolis. We can be aware of resin producing trees and site our colonies close by. We can be mindful when scraping propolis off hive parts though propolis does lose it strength after some time and the volatile components disperse. This means that colonies probably need to renew propolis on at least an annual basis.
Propolis traps on the tops of frames are not advised for long periods and certainly not over winter. Research by Borba et al4 demonstrated that such colonies had higher viral loads and this was attributed to the water-resistant trap causing a change in the microclimate of the colony leading to increasing good conditions for pathogens and viruses to thrive in. Air may have been prevented from circulating and humidity may have increased. We mustn’t be tempted either to feed propolis to our bees because they don’t naturally ingest it.
Dr Renata Borba gave a fascinating presentation in 2015 on her scientific work researching the benefits of propolis and if you want to study this subject further for yourself I can recommend this video: https://www.youtube.com/watch?v=PXd_8h9U5EE
1 Hatfield, G. 2007. Hatfield’s Herbal: The Curious Stories of Britain’s Wild Plants. Penguin Books, England.
2 Seeley, T.D., 2019. The Lives of Bees. Princeton University Press. New Jersey and Oxford.
3 Simone, M. Evans J.D., Spivak M. 2009. Resin collection and Social Immunity in Honey Bees. Evolution 63:3016-3022.
4 Borba, R.S., Wilson, M.B., Spivak M. 2017. Hidden Benefits of Honeybee propolis in Hives in Beekeeping-From Science to Practice. Springer. Switzerland.