Honey Bee Medicine for the Veterinary Practitioner. Группа авторов
Source: Photo by Robin Radcliffe.
Propolis Envelope
Propolis (“bee glue”) is a resinous substance collected by honey bees from the buds and wounds of trees. When combined with beeswax, it makes a cement that bees use to fill the crevices and coat the walls of their nest cavities, often completely enshrouding their nests. This coating of the walls, floor, and ceiling of the nests of wild colonies with tree resins makes a “propolis envelope” that can be 2–3 mm thick (Seeley and Morse 1976). The propolis lining of the nest cavity probably serves several functions: creating a solid surface for comb attachment, reducing cavity draftiness, enhancing nest defense, waterproofing, and bolstering a colony's defense against microbial infections.
Ancient Greeks used propolis to treat abscesses, Assyrians put it on their wounds, and Egyptians used it for embalming their dead. Although humans have long recognized the health benefits of propolis for its antiseptic, anti‐inflammatory, antibiotic, antifungal, anesthetic, and healing properties, only in the last century have humans discovered the specific compounds that give propolis its medicinal value – of the more than 180 compounds identified in propolis to date, one group (a class of plant‐based polyphenols known as flavonoids) are of particular interest for their protective antioxidant properties. These same compounds that mankind values in propolis also confer health benefits to the honey bee colony through social immunity – a collective behavioral defense that produces colony‐wide immunity that in turn reduces the expression of immune genes in individual bees (Borba et al. 2015).
Figure 1.6 A month‐long comparison of temperatures (°C) inside a thin‐walled nest cavity made of 1.9‐cm‐thick lumber (a) and inside a thick‐walled cavity made in a living sugar maple tree (Acer saccharum) having a wall thickness of 20–30 cm (b). Each cavity had two temperature probes, located c. 10 cm from either the floor or the ceiling of the cavity. In both figures the green line represents the ambient environmental temperature, while the orange and blue lines are the probes located within the respective cavities.
Curiously, the use of propolis for colony defense is limited to the temperate regions of the world. Neither the tropical honey bees in Asia (A. cerana, Apis florea, and Apis dorsata) nor those in Africa (the African subspecies of A. mellifera) make use of propolis other than for structural purposes (Simone et al. 2009; Kuropatnicki et al. 2013). It is the European honey bees living in nature for which the collection and use of propolis for its colony‐level immunoprotective effects has reached its highest expression. Yet, rather than being viewed as a specific compound to be cultivated, propolis is more often than not regarded as an annoyance by modern beekeepers. Beekeepers are constantly scraping off propolis as they remove frames to manipulate their colonies. And the Langstroth hive bodies used by the vast majority of beekeepers today lack the rough inner surfaces of a bee tree or other natural cavity that stimulate propolis deposition by foragers. Colonies managed by beekeepers are not strongly stimulated to collect and use propolis. Indeed, it is the complex surface of the natural cavity that provides the tactile stimuli necessary for the deposition of propolis as a hive barrier by worker bees, something almost entirely lacking in modern hives made from smooth planed lumber (Hodges et al. 2018). Hodges and colleagues investigated three methods to increase the textural complexity of the interior surface of a standard hive body; these methods included using plastic propolis traps stapled to the inside wall surfaces, cutting horizontal parallel saw kerfs that were 7 cm apart and 0.3 cm deep, and roughening of the interior wall surface using a mechanical wire brush. The three interior hive wall types were compared to an unmodified, smooth‐walled hive by measuring the bees' propolis application. Although the colonies were not challenged with specific pathogens, all three texturing methods induced significantly more propolis deposition compared to controls. The authors concluded that using unplaned, rough lumber for the interior hive surfaces would increase propolis deposition over standard hives built using lumber that is planed smooth on both sides.
A curious observation arising out of the mapping of the honey bee genome was the discovery that honey bees possess just one‐third of the genes coding for immune function typically found in solitary insects (Evans et al. 2006; Honey Bee Gene Sequencing Consortium 2006). It was hypothesized that the weak capacity for an immune response in individual honey bees might be compensated by behavioral or colony‐level defenses, or a form of social immunity. Indeed, as social insects, honey bees are steadfastly hygienic by removing alien organisms that gain entry to the nest, by feeding young bees antimicrobial products, by creating compounds that offer barriers to infection, and evolving complex interaction networks that serve to compartmentalize infections. The first indication that the bees' nest environment could influence immune expression in honey bees was discovered by Simone et al. (2009). Honey bees living in hives whose inner walls were coated with propolis extracts (derived from resins found in Minnesota and Brazil) invested less energy on immune function compared to bees living in hives without such coating. The colonies living in the propolis enriched hives also had lower bacterial loads. Scientists believe that individual bees are not immunocompromised, but rather that they conserve energy by not upregulating their immune genes except when a pathogen is encountered. This means that the defenses provided by social immunity (e.g. the collection of tree resins for propolis) allows individual bees to divert energy resources from immune function to other hive activities such as nursing, wax building, and foraging. This strategy likely maximizes the health and fitness of the entire colony.
Bee Microbiome
An oft‐overlooked aspect of the bee environment that is essential to the good lifestyle of honey bees is their microbiome, that is, the community of specialized microbes (bacteria and yeasts) that have coevolved to live inside the bees and in their nests (e.g. in their pollen stores). We again return to the tenet of our chapter: the need to learn about the honey bee's natural biome to understand its biology, including its relationships with its pathogens. The honey bee microbiome is remarkable in that it is nearly consistent across thousands of individuals from hive to hive and even across continents. The honey bee's microbiome is similar to that of humans in that both feature specialized bacteria that have coevolved with their host and are socially transmitted (Engel et al. 2012; Zheng et al. 2018). Honey bees are first inoculated with bacteria in the larval stage, presumably through the food provided by nurse bees. However, during pupation, when bees undergo the final phase of metamorphosis, a bee's exoskeleton (including the gut lining and any associated bacteria) is shed in a process known as ecdysis. Therefore, honey bees emerge as young adults without a gut flora, except for those microorganisms they pick up when chewing through the wax cappings of their cells. The characteristic microflora of a worker bee is, therefore, developed mainly following emergence and through direct social interactions with conspecific worker bees. By four to six days of age, the population of a worker bee's gut flora stabilizes at 108–109 bacterial cells.
Although both wild honey bees and those living in apiaries possess complex microbiomes, some beekeeping practices – such as feeding pollen substitutes and treating with antibiotics – can alter the microflora of honey bees (Fleming et al. 2015; Maes et al. 2016). Dysbiosis, or unhealthy shifts in gut microflora, was observed in bees consuming aged pollen or pollen substitutes and was linked to impaired larval development, increased bee mortality and infection with pathogens such as Nosema and Frischella. Raymann et al. (2017) observed considerable changes in the gut microbial community composition and size following treatment with tetracycline, the most commonly used antibiotic in beekeeping operations globally. The authors concluded that decreased survival in honey bees was directly attributed to increased susceptibility to infection by opportunistic pathogens that colonized the gut after antibiotic use. The honey bee microbiome is thought to promote bee health and development in several ways.