Propolis: Honeybees Love It for Good Reason

Introduction

A beekeeper with their hive tool in hand will often be wondering what the best way is to separate and lift frames that are not only sticking together but also sticking to their gloves, tools and ruining their clothes.  It’s like a sticky piece of tape that you just can’t seem to get off your fingers as you’re shaking them over the garbage can.  The beekeeper scrapes and scrapes to remove the reddish material that eventually gets everywhere. In the summer it’s like putty but in the cooler temperatures’ cracks like a piece of hard maple sugar candy with a noise disturbing to both the beekeeper and honeybee. What is this stuff?  Why does it seem the honeybees love it–given they put it everywhere? Is it used for something important, and should I be constantly scrapping it off?

Propolis is a resinous substance composed of sugars, wax, bee secretions, pollens, and various resins gathered by resin forager bees from local flora sources. A feral colony nesting in a tree cavity generally applies a thin (0.3 to 0.5 mm) layer of propolis (“propolis” is an apicultural term for the resins when used by bees within a hive) forming what has been termed a “propolis envelope” around the nest interior. Although this natural resin executes an assortment of functions including maintaining a stable internal nest environment and preserving the physical integrity of the nest against cracks, uncontrolled airflow, and moisture, propolis is most highly regarded for its antipathogenic activity. A strong propolis envelope provides seasonal benefits to colony immunity, and colonies with a thick propolis envelope exhibit higher rates of survival than those without propolis. Chemical composition of propolis varies by region, season, and nearby floral diversity, and often contains flavonoids, terpenoids, and phenolics. These highly prevalent compounds demonstrate antifungal, antibacterial, antiviral, and antioxidant activities and has the potential to be a detoxifying agent or primer of detoxification pathways, as well as increasing bee longevity via antioxidant-related pathways.

Honeybees are constantly dealing with threats from pathogens, pests, pesticides, and poor nutrition. It is critically important to understand how honeybees’ natural immune responses (individual immunity) and collective behavioral defenses (social immunity) can improve bee health and productivity. One form of social immunity in honeybee colonies is the collection of antimicrobial plant resins and their use in the nest architecture as propolis.

Social Immunity

Social immunity is how individual behaviors of group members effectively reduce disease and parasite transmission at the colony level. This phenomenon is widespread across the social bees, ants, wasps, and termites. The behaviors range from more common acts like grooming of nestmates and removal of dead material from the main nest area to the detection and removal of pre-infectious diseased or parasitized brood (hygienic behavior). Since honeybees live in large groups of constantly interacting, related individuals, there is an increased risk of disease outbreaks and evolution of specialized parasites. Considering this, the finding that honeybee immune pathways have fewer family members or paralogs as compared to other non-social insects with complete genomes was surprising, as it indicates that honeybees may have reduced individual mechanisms of physiological defense. It is interesting then to consider the suite of behavioral mechanisms or other traits that may have evolved at the individual and colony levels to compensate for this.

The nests of densely populated honeybees provide a favorable habitat for a wide range of parasites and pathogens that have evolved to overwhelm or suppress their hosts’ immune defenses. Honeybees were perhaps not subject to the multitude of pathogens and other stressors that confront them today within the context of modern beekeeping practices (e.g., transportation of hives, density of hives in proximity, and exchange of comb across beekeeping operations), but, as cavity nesters within trees, they likely encountered saprophytic, symbiotic, and parasitic microbes and fungi. Some of these endemic microbes can induce an immune response in individual bees. The evolution of resin uses by honeybees could have been to combat fungal growth and potential fungal pathogens within the nest. Although both bacteria and fungi are common bee immune stressors, the honeybee immune system appears to be more attuned against bacterial pathogens. One hypothesis is that social immune behaviors, such as the collection of plant resins and their deposition in the nest as an antimicrobial propolis envelope, evolved to compensate for deficiencies in innate or physiological immunity. There is some evidence that propolis may reduce impacts of mycotoxins produced by fungi. In this case, feeding propolis extracts, rich in flavonoids and phenolic compounds, to adult honeybees reduced adverse effects of exposure to toxins produced by Aspergillus, a common hive fungus.

The individual’s immune system is the costliest physiological system in insects. An elevated immune response can lead to reduced colony productivity in honeybees and decreased individual survival. Thus, it is not surprising that honeybees have evolved behavioral mechanisms of social immunity to reduce activation of the individual immune system against these microbes to ultimately benefit colony health.

Self-Medication

One obvious question concerns the idea of “self-medication”. Resin collection may be constitutive (i.e., collected regardless of physiological demand or pathogen level) or inducible (i.e., a conditional response to infection). If it is inducible, it might be considered a form of self-medication, defined as the “defense against pathogens and parasites by one species using substances produced by another species”.  For a honeybee colony, the antimicrobial properties of propolis would reduce the general microbial load within the nest cavity, thereby reducing the production of antimicrobial peptides by the innate immune system of individual bees. Studies in in this area have indeed shown that honeybees in colonies with the propolis-extract envelope have an overall reduction in general bacterial loads, and a corresponding lower investment in immune gene expression compared to honeybees in boxes without the propolis envelope.

Do honeybees reapply propolis and continue self-medicating? As a matter of fact, they do.  When the growing season commences, honeybees add new propolis on top of the old and restore the antimicrobial properties of the envelope. Honeybees in colonies with a propolis envelope also had significantly higher levels of the blood storage protein vitellogenin (Vg) compared to control colonies. Higher levels of Vg are an indicator of well-nourished bees. A decrease in energetic costs associated with the maintenance of an efficient immune system during the foraging season might help bees maintain higher storage protein levels (e.g., vitellogenin) required for overwintering success and allocate energy to perform vital tasks the next spring (e.g., foraging, rearing brood) Also, high-propolis collecting colonies experienced significantly higher brood survival rates (survival from egg to adult stage) and individual adult bee longevity.

Pathogens

Propolis is most well-known for its antimicrobial activity against several bacterial, fungal, and viral pathogens. A honeybee colony collects resin from multiple plant sources, though there is likely source fidelity for individual bees. The antimicrobial activity of the propolis depends on the antimicrobial properties of its source plant resins, and the fact that colonies mix sources of resins over the growing season creates an extremely complex mixture against which susceptible parasites and pathogens would have difficulty developing resistance. Although some single-resin sources may be more effective against a single parasite or pathogen, mixed sources are considerably more effective overall due to increased variety of chemical compounds.

Field experiments have also provided evidence of self-medication in honeybees as the rate of resin foraging increased in colonies after challenged with the fungal pathogens. Colonies have been shown to consistently increase the number of resin foragers after a colony-level infection. This case of self-medication is particularly interesting because it occurs at the colony level and does not simply involve individuals ingesting compounds to self-medicate themselves. Because only larvae are infected by some pathogens, the increased resin collection diversity over time is a great example of a social immune response increasing collection of antimicrobial products by adult bees to protect younger nestmates.

Varroa

It has been noted that the proportion of resin foragers in honeybee colonies increases according to Varroa infestation, suggesting a causal link between parasitic challenge and propolis collection, consistent with increased survival of Varroa-infested worker bees reared in presence of propolis. It has also been observed that propolis reduces deformed wing viral (DWV) loads associated with Varroa infestations at colony level. A series of laboratory assays have also shown that directly exposing Varroa mites held in petri dishes to relatively low concentrations of ethanolic propolis extracts caused high mortality (100% due to contact with 10% extract). Furthermore, exposure to extracts at concentrations as low as 0.5% caused narcotic effects leading to reduced heat production and metabolic rates. These effects could influence the ability of mites to cope with other stressors (i.e. temperature changes) or to successfully infest larval cells and maintain the normal course of the parasite’s population growth. Results also suggest that contact with the propolis used to shine up a cell may reduce the number of mature females produced within a single cell. Altering this level of reproductive output would effectively reduce population growth of the parasites within a colony and hopefully reduce the need for the more caustic chemicals currently utilized as treatments. As a single propolis sample can contain up to 300 chemical components, it may be more difficult for the mites to develop resistance against a suite of combined compounds. Propolis can then be regarded as a natural pesticide used by the honeybee to limit a dangerous parasite.

Leaching

It is possible that the antimicrobial properties of materials used and stored in combs (e.g., royal jelly, honey) are enhanced by the addition of propolis. Current work showing the effect of a propolis-enriched environment on the antimicrobial activity of larval food continues to raise this possibility. Additionally, it has been suggested that some of the phenolic compounds present in honey may be derived from propolis itself. Depending on how much propolis is incorporated into the comb through “cell shining” or along the rims of cells, which is seen in feral colonies and in some managed colonies, the potential of propolis-related compounds and volatiles “leaching” into other hive products remains a possibility. Alternatively, honeybees could potentially actively “enhance” the antimicrobial activity of propolis or these other hive products by combining bee-produced compounds with the plant-produced resins, as has recently been documented in wood ants. This potentially adds another dynamic to the fact that glucose oxidase (a compound produced in the mandibular glands) has been found in propolis.

Longevity

Other than the potential of propolis as a detoxifying agent or primer of detoxification pathways, another hypothesized mechanism that could influence honeybee longevity via antioxidant-related pathways regards increased resistance to oxidative stress. The free radical theory of aging bases the aging process on the production of reactive oxygen species (ROS) via typical cellular metabolism. These ROS result in the oxidation of lipids, proteins and even DNA, disrupting cellular membrane stability and eventually causing apoptosis (cell death). Aging, in some ways, is thought to be an accumulation of all this damage due to oxidative stress. Propolis extracts, through various studies, have been shown to widely inhibit the formation of reactive oxygen species, which reduces oxidative damage to proteins, lipids, and DNA.

Honeybee Gut

Propolis appears to maintain a stable microbial community composition and reduce the overall taxonomic diversity of the honeybee microbiome. Several key members of the gut microbiota were significantly altered in the absence of propolis, suggesting that it may play an important role in maintaining favorable abundance and composition of gut symbionts. Overall, findings suggest that propolis may help to maintain honeybee colony microbial health by limiting changes to the overall microbial community. Results also reveal that the mouthparts of worker bees in colonies with a propolis envelope exhibited a significantly lower bacterial diversity and significantly higher bacterial abundance compared to the mouthparts of bees in colonies without a propolis envelope. Based on the taxonomic results, the propolis envelope appeared to reduce pathogenic or opportunistic microbes and promote the proliferation of putatively beneficial microbes on the honeybee mouthparts, thus reinforcing the core microbiome of the mouthpart niche.

Resin Diversity

Honeybee propolis is a complex, resinous mixture created by bees using plant sources such as leaves, flowers, and bud exudates. The bees must extract the resins from the trichomes and ducts by fragmenting these early leaves using their mandibles. Resin-foragers have shown a preference for young leaves and vegetative buds over more expanded leaves. In temperate climates poplar trees appear to be the primary source for resins. However, it is clear that other trees, like pine, birch, elm, alder, beech, and horse-chestnut species, are adequate resin sources for temperate honeybees, particularly when poplar species are unavailable.

Adversely, it’s been demonstrated that agricultural land use surrounding honeybee apiaries decreases the chemical quality and antimicrobial effects of propolis, which may have implications for the impacts of land use on hive immunity to potential pathogens. Intensive agriculture is increasingly being investigated for its negative impacts on bee health. While row crops can provide short term nutrition, they cannot sustain colonies through different seasons, and the lack of biodiversity in intensively farmed areas likely contributes to poor colony nutrition and colony decline.

Hive Material

When feral honeybee colonies nest in hollow tree cavities, they coat the rough cavity walls with a continuous layer of propolis, a substance comprised primarily of plant resins. Studies have shown that the resulting “propolis envelope” leads to both individual- and colony-level health benefits. Unfortunately, the smooth wooden man-made boxes most used in beekeeping do little to stimulate propolis collection. As a result, most managed bees live in hives that are propolis-poor.

Studies show that rough wood boxes are the most effective box type for stimulating propolis deposition. In both stationary and migratory operations, propolis deposition was correlated with a seasonal decrease and/or stabilization in the expression of multiple immune and bacterial genes, suggesting that propolis-rich environments contribute to hive homeostasis. Research points to the roughened interior of the hive walls as the best method for encouraging deposition. In fact, leaving lumber naturally rough, with no planning or sanding, would provide a simple and effective surface for boosting propolis. These findings provide support for the practical implementation of rough box hives to enhance propolis collection and colony health in multiple beekeeping contexts.

A Sticky Situation

Managed honeybee colonies are currently experiencing high rates of annual mortality, largely due to pathogens, parasites, pesticides, and poor nutrition. Perhaps propolis can, at least in part, help mitigate effects from these threats. Understanding the role that propolis plays as a social immune defense directly against parasites and pathogens and through subtle, indirect effects on individual immunity and detoxification enzymes could be a key part of the puzzle to improve bee health.  Sadly, the deposition behavior of propolis into the hive has been negatively selected by beekeepers because this sticky material disturbs the handling of frames. At the same time, studies highlight the importance of this substance for colony health, suggesting that the development of strategies to stimulate resin collection and propolis storage into the hive could have a beneficial effect on honeybee health and should therefore be promoted. How can we move forward? It’s possible that using rough boxes to stimulate the construction of a propolis envelope represents an important opportunity to bolster honeybees’ natural defenses. Compared to other interventions, using rough boxes to boost propolis collection could be considered an “easy win” because their implementation requires minimal disruption to beekeeping operations and offers measurable benefits to honeybee health in a cost-effective manner.

Referenced Materials

• Propolis Counteracts Some Threats to Honey Bee Health
• Propolis and bee health: the natural history and significance of resin use by honey bees
• Land Use Influences the Composition and Antimicrobial Effects of Propolis
• Honeybees use propolis as a natural pesticide against their major ectoparasite
• Antimicrobial, Antibiofilm and Toxicological Assessment of Propolis
• Propolis Power-Up: How Beekeepers Can Encourage Resin Deposits for Better Hive Health
• Textured Hive Interiors Increase Honey Bee (Hymenoptera: Apidae) Propolis-Hoarding Behavior
• Seasonal benefits of a natural propolis envelope to honey bee immunity and colony health
• Thinking inside the box: Restoring the propolis envelope facilitates honey bee social immunity
• Honeybee microbiome is stabilized in the presence of propolis
• Propolis Envelope Promotes Beneficial Bacteria in the Honey Bee (Apis mellifera) Mouthpart Microbiome