One of the better‐known peptides produced from the cells of both vertebrates and invertebrates is defensin. A role in innate immunity has been suggested for defensins given their diverse activity against bacterial, fungal, and viral pathogens (Raj and Dentino 2002). One such defensin compound is royalisin isolated from royal jelly – the nurse bee secretion fed to young worker larvae and developing queen larvae. Royalisin has broad antibacterial and antifungal properties, even possessing inhibitory growth against Paenibacillus larvae, the causative agent of American foulbrood (Bíliková et al. 2001). It is likely that these protective antimicrobial peptides can boost immunity at the level of the colony since the compounds are transferred widely during trophyllaxis.
Figure 2.3 Allogrooming, or the grooming of one bee by another nestmate, is a form of social immunity that helps remove potential pathogens from the hive. Through the deliberate spread of bee venom, this regular grooming may also play a central protective role in biosecurity by acting as a cuticular form of antisepsis.
The transmission of microorganisms from one bee to another does not always result in the spread of pathogens and disease. On the contrary, many beneficial bacteria are transferred throughout the colony during the complex interactions of honey bee society. Socially transmitted gut microbiota helps protect honey bees from infection by hive pathogens. Worker honey bees lack a bacterial microflora at the time of emergence and the future microbiome community of an individual bee is dictated by contact with nurse bees, the hive environment, and through trophyllaxis (Powell et al. 2014). In an experimental model of bumble bees (Bombus terrestris), contact with nestmate fecal material upon pupal emergence was required for protection against a virulent trypanosome gut parasite Crithidia bombi (Koch and Schmid‐Hempel 2011). The community of microorganisms in colonies of honey bees and bumble bees is distinctive from that found in solitary bees and its role in providing a first line of defense against potential pathogens is an area that deserves more thorough investigation.
Figure 2.4 Tree saps or resins are collected from leaf buds and packed onto the worker bee's corbiculae for transfer back to the hive where other bees offload the resin, mix it with beeswax and enzymes, to make propolis or “bee glue.”
Resin Collection, Propolis, and Immune Modulation
Only a very small group of honey bee foragers (5–15 per day) in any given colony devote their time to the collection of tree and plant resins while 10 times as many foragers are off collecting nectar and pollen (Namakura and Seeley 2006). The bees do this laborious job (it may take 30 minutes to several hours to offload the sticky resins from a bee's pollen basket) without any apparent benefit to the individual bee (Figure 2.4). As in other forms of social immunity, the collective health benefits of resin collection to the colony may be significant by limiting the entrance of pathogens into the nest and reducing the cost for maintaining expensive immune functions for every single colony member (Simone et al. 2009). The latter function of modulating costly immune activity may represent the most important benefit to the superorganism – conserved energy at the level of the individual bee can be directed toward important colony functions of brood rearing and foraging that builds strong bees having adequate vitellogenin storage for overwintering and spring emergence. The collection of plant resins likely evolved as a colony‐level adaptation for relieving workers of the need for sustaining an energetically costly immune response, especially when the colony is not being challenged by pathogens (Simone et al. 2017; Borba et al. 2015).
Trees and plants synthesize resins (flavonoids, monoterpenes, and many other biologically active compounds) to protect young leaf buds and injured tissues from infection with pathogens and to deter feeding by browsing herbivores. Honey bees and some other social insects utilize tree resins for their antimicrobial, antifungal, and antiviral properties. It is unknown whether bees select tree species for their resins based on simple availability or more purposely for (as yet unknown) pharmacologic actions (Simone‐Finstrom and Spivak 2010). Bees are not known to ingest these compounds directly. Rather, the tree resins collected by honey bees are mixed with wax to make a sticky glue‐like substance called “propolis” that is used to secure combs to the roof and walls of the bees' nest cavity as a kind of cement and to seal holes or spaces in the nest architecture. In their detailed portrayal of the nests of wild honey bees, Seeley and Morse (1976) describe a complete propolis envelope surrounding the bee's wild home, essentially sealing off the inner cavity from invading parasites and pathogens. The propolis barrier is incomplete inside the smooth‐walled hives of modern managed apiaries, but the barrier can be augmented with commercial propolis traps or by roughening the inner hive wall surface to stimulate propolis deposition (Hodges et al. 2018; Simone‐Finstrom et al. 2017).
Propolis production is a heritable trait and varies considerably among lines of honey bees. In Africanized bees, more eggs were produced and more brood survived from larva through pupal stages in colonies having queen‐drone crosses with high‐propolis production; likewise, the adult bees from such colonies lived longer than bees from crosses from low‐propolis colonies (Nicodemo et al. 2014). Bees from high propolis colonies also collected more nectar and pollen than bees from low propolis colonies and showed greater hygienic behavior than low propolis colonies (Nicodemo et al. 2013; Borba et al. 2015). Although other factors may impact resin collection and signs of fitness, the health benefits of propolis may be far‐reaching and offer the colony the critical advantage it needs to survive across seasons and reproduce in an environment increasingly dominated by harmful pests and pathogens.
In vertebrates, propolis enhances cellular immune function by increasing the cytotoxic effects of macrophages and the lytic activity of lymphocytes against invading microbes, but few studies have been done on honey bee immune responses with and without propolis. In their study of the health benefits of a complete propolis barrier, Borba et al. (2015) did not see a reduction in bacterial or viral loads in colonies having a natural propolis barrier (by use of propolis traps in Langstroth hives) compared to colonies not having such a barrier, although their methods did not distinguish between pathogenic and commensal organisms. The authors concluded that the reduced investment in immune expression at the level of the individual bee during periods of low pathogen challenge suggests a direct effect of propolis on the bee immune system. And these immune sparing effects were sustained over the entire summer and fall foraging season, only diminishing over the winter when the bees no longer collected resin until such effects were negligible by spring (Simone‐Finstrom et al. 2017).
More is known about the benefits of propolis for the health of humans than for the honey bee with reported antiseptic, anti‐inflammatory, antioxidant, antibacterial, antifungal, antiulcer, anticancer, and immunomodulatory properties (Pasupuleti et al. 2017). Propolis consists of resin (50%), wax (30%), essential oils (10%), pollen (5%), and other organic compounds (5%). The organic compounds from the plant resins are responsible for the health benefits and include primarily phenols, esters, flavonoids, and terpenes. Propolis also contains both vitamins (B1, B2, B6, C, and E) and minerals (Mg, Ca, K, Mn, and Fe) and a few enzymes from the bee saliva. The health benefits of propolis for honey bees has focused on the important bee pathogens causing American foulbrood (P. larvae) and the fungal agent of chalkbrood disease (Ascosphaera apis). Historical studies showed a positive correlation with feeding of propolis to bees in sugar solution, but since bees are not thought to ingest propolis, such models
