A honey bee colony’s immunity goes beyond combining the immune system of multiple individuals. When a hive exceeds the population threshold of “many bees nesting together” it creates a non-linear effect as the resulting entity is not “XYZ bees working together”, rather a “superorganism”. A worker insect can survive on its own. When joined with other workers, it becomes part of a social group that cooperates for the survival of itself and groupmates. When enough groups get together, they from one larger group (a colony or superorganism), with each individual working towards its betterment.
Welcome to part two of honey bee immunity. Hygienic behaviour, temperature controls, hive products, waste management and queen health are discussed within, amongst other concepts. There are links to further readings on topics that have been discussed in depth elsewhere. Part One here.
Hygienic Behaviour Provides Natural Resistance Against Pests and Pathogens
Hygienic behaviour is when dead or diseased brood are removed and discarded, reducing the chances of diseases spreading onto the healthy brood. Colony-level resistance is inferred when these behaviours are performed frequently and by a sufficient number of workers (who themselves specialise in this via division of labour, either as undertakers, guards, groomers, or cell cleaners).
Here the individual bee is working in the interest of the colony by removing her sullied siblings-to-be, and in doing so contributing immunity at the colony level; preventing the colony’s downfall due to pests or disease increases her own chances of survival, which in a nutshell, is a description of the superorganism. It is in the interest of the individual bee to contribute to the hive as she cannot survive (for long) without a colony. Grooming and Varroa-sensitive hygiene are described next.
Bees Clean Themselves and Their Nestmates via Grooming
Bees use their legs and mandibles to comb through their hairs (“autogrooming”) or a nestmate (“allogrooming”). Generally, autogrooming is how bees keep their bodies clean from dust, pollens, and the parasites that latch on. In the case of allogrooming, the literature states it could have evolved for purposes of removing mites that ride on their bodies (reviewed in Pritchard, 2016). This nestmate-grooming behaviour is most efficient in the Asian honey bee Apis cerana, who has evolved alongside multiple Varroa species, and is one of several methods used in managing mites.
In self-grooming, the mouthparts and used to bite and lick through the bee’s body parts, whilst the legs (whose hairs act as brushes) clean one another. The forelegs generally have “antennal notches” used for -you guessed it- cleaning the antennae. The mid-legs clean the forelegs, who in turn are cleaned by the hindlegs. Who cleans the hindlegs? Themselves! The bee rubs her two hind legs together to transport the dust/pollen into her baskets, the corbiculae, and this is then removed by other bees back at the hive.
The Allogrooming Process Starts with a Grooming Dance
Adapted from Pritchard, 2016
For the grooming of another bee, the parasitised bee triggers a “grooming dance” by rapidly shaking her abdomen to signal the desire to her nestmates. Initially, the bee reacts to the Varroa‘s bodily penetration with a foreleg-swiping motion in the mite’s direction (Varroa usually pierces the posterior regions of the abdomen). If this motion fails, she performs the grooming dance and position herself motionless with wings fully spread and abdomen lifted (“the social grooming display”; see video below).
The bee(s) that respond to this grooming invitation work on parts that are inaccessible during autogrooming. In A. cerana, they search distinct areas in the thorax, propodeum (region where abdomen meets the thorax), and abdomen of the groomed bee. The disturbance either causes the mite to flee her host, or she is captured by the grooming bees and killed – effectively managing the parasitisation. The average allogrooming bout lasts ~ 45 seconds, and this duration increases in colonies who have a high-grooming tendency (Micholson & Currie, 2024)
Varroa-Sensitive Hygiene Centres Around Grooming & Capping-Uncapping-Recapping of Brood Cells
Main article: VSH & The Basis of Natural Resistance
A subset of hygienic behaviour is Varroa sensitive hygienic behaviour, which is the uncapping and removal of infested brood, or uncapping and then recapping of that brood cell. In the latter procedure, a worker partially uncaps the cell’s silk/wax layer and then reseals it (e.g. Hawkins & Martin, 2021 or Oddie et al., 2021). When workers open a cell for inspection, they disturb the internal environment, interrupting the lifecycle of the pathogen inside, whether it’s V. destructor or the chalkbrood-causing spores of Ascosphaera apis.
In the case of Varroa, uncapping disruption causes the daughter mites to leave their cell without completing their reproductive cycle (Martin et al., 2019). In the study of Martin & colleagues, bee populations in Brazil and South Africa would uncap infested brood and also cannibalise the larvae, reducing the spread of the pests and ensuring that only healthy adult bees emerge.
The literature had identified multiple low- & volatile-organic compounds that trigger hygienic behaviour in worker bees. Noel and colleagues expanded on this work and found that five out of the nine tested volatile organic compounds caused a uncapping-recapping response in bees, with n-tetradecane having the fastest response (Noel et al., 2025). In nature, n-tetradecane is emitted by black-eyed pupae1.
Entombed Brood Reduces the Spread of Diseases
Sometimes, dead or diseased brood can be sealed off completely, rather than being uncapped and removed. Other social insects partake in this, for example, Subterranean termites wall off dead colony members within their nest with dirt (Neoh et al., 2012), whereas A. cerana leaves Varroa-infested drone comb unattended, causing the mites to die inside the cells.
A. mellifera also entombs foreign objects (sticks, rocks, the occasional mouse corpse) with propolis as they do with contaminated pollen. The propolis is antimicrobial and will reduce the risk of pathogen (or perceived pathogen) transmission. Spivak & Gillam report that in some colonies, cells containing chalkbrood mummies are sealed with “a thick covering of propolis” (Spivak & Gilliam, 1993).
Temperature Control: Fanning, Heat Shielding & Social Fever
Temperature matters for bee health, and bees have multiple methods of keeping the brood area at a constant 34-36°C. Only social fevers are directly linked to colony level immunity, as this additional heating is generated in response to a pathogen infection. I make brief mentions to heat shielding, fanning & bearding as they are “indirect” regulators of colony immunity.
Social Fevers Aim to Reduce the Severity of Brood Diseases
Raising hive temperature has been suggested to reduce chalkbrood severity as the fungal spores have a narrow temperature tolerance. Alas, Goblirsch & colleagues refuted this suggestion with their experiments, finding that tested hives continued showing signs of chalkbrood infections after the social fever has come and gone (Goblirsch et al., 2020).
If anything, they suggest that the fever is a manipulation by the pathogen to benefit itself rather than the host, citing a study with Nosema cerana whereby infected workers preferred warmer temperatures in the brood nest, despite not having an increased thoracic temperature (meaning the worker is not contributing to social fever).
Later work using thermal cameras found that bees increase the temperatures of brood cells parasitised by Varroa: “Our results confirmed that the mite-parasitized brood has, on average, a 0.79 °C higher temperature over the two-year period, which was statistically demonstrable” (Sipos et al., 2024). This temperature increase could be due to increased metabolic activity from the wounded pupae, or due to “spot heating” by an adult bee (a worker bee transfers heat to the cell by pressing her thorax against it). The latter is akin to a localised social fever, as when workers detect mites in a cell, they try to shift the ambient temperature to impact the parasite’s development and quality of life2.
Social Chilling – Sick Individuals Seek Out Lower Temperatures
In bumble bees infested with the parasitoid Conopid flies, an infected bee may choose to isolate herself and spend nights at low temperatures away from her warm and cozy hive (Muller & Schmid-Hempel, 1993). The lower temperatures would be uncomfortable for the bee, and detrimental for the parasitoid’s larvae; this either prolongs the host bee’s lifespan or retards the adult parasitoid and renders it unable to infest a nestmate.
To confirm that this was not a behavioural oddity, the authors conducted choice experiments whereby infested bumble bees can spend nights in cold climates versus a warm one, and found them to significantly favour cold climates, reinforcing that this as an adaptation to reduce the spread of infestations in a colony.
Heat Shielding is a Mobile Temperature Regulator
Heat shielding works to manage a single area experiencing overheating, such as a patch of brood at the hive wall receiving direct sun. Some middle-aged workers will press their bodies against the hot area of comb to absorb excess heat and then release it elsewhere (Starks & Gilley 1999). Shielding is more prevalent in the brood area than the honey stores, suggesting that the specialist heat shielders are a mobile insulation unit used to manage temperature excursions within the thin cavity walls of their natural, tree-based beehive.
Bearding is When Bees Cluster Outside the Hive Entrance
Bearding is an “external cluster” formed in response to warm temperatures inside the hive. Workers will clump outside the hive to reduce the number of bodies bees inside. This lowers the passive heat generation by their activities (cleaning, feeding, nectar processing, mending etc…). The increased hive space improves ventilation, as more air passes through. Pair this with fanning, and the bees can effectively reduce the humidity and heat to maintaining their ideal internal environment.
Fanning Circulates Fresh Air
Fanning is when honey bees rapidly flutter their wings while remaining stationary at hive entrance, producing a soft buzzing sound. They do this to circulate cool air to the hive, to speed up the evaporation of the water in nectar, lower carbon dioxide levels and disperse pheromones.
Hive Products as Social Immunity
Propolis has various antimicrobial properties and hives containing propolis-coated walls are healthier than those without. Wax comb and its architecture helps bees maintain a mobile storage and broodrearing location – if a comb gets old or damaged, they’ll use another or build a new one.
Propolis Envelopes Reduce the Overall Risk of Infection
Main article: How Bees Use Propolis
In one study, the presence of propolis on hive walls lowered adult bee immune gene expression (Simone et al., 2009), which reduced the energy spent by bees on immune system activation. Maintaining an immune system is metabolically costly, so when less energy is required for normal adult bee function, that remaining energy is diverted elsewhere: nectar processing, brood care, wax comb building, queen bee feeding, water collection… the list goes on.
Hives with propolis envelopes had reduced microbial growth for pathogens that cause American Foulbrood & Chalkbrood due to antimicrobial activity from compounds like flavonoids, esters and large fatty acids. Al-Ani and colleagues showed that propolis works on both Gram-positive and Gram-negative (meaning those bacteria with and without a certain type of cell wall), as well as aerobic and anaerobic (oxygen-using versus non-oxygen using; Al-Ani et al., 2018). The effect is more pronounced in gram-positive bacteria, which makes me think that one antibacterial mechanism involves cell wall disruptions (amongst many others- propolis is a complex compound).
When Varroa are exposed to a 10% propolis extract, all mites were killed (reviewed in Simone-Finstrom & Spivak, 2010). Further experiments found that increasing propolis concentrations within a managed beehive reduced Varroa loads, as well subsequent viral loads (e.g. Sacbrood & DWV; Drescher et al., 2017). Drescher & colleagues reported that these colonies had a stronger immune system, and other studies showed increased brood survival & increased worker life expectancy in high-propolis producing bees (Nicodemo et al., 2013).
Wax Comb – Cell Spacing & Architecture Limit Pathogen Transmission
Wax comb ages gradually. Beekeepers see this physically as it goes from white-yellow to dark brown. The old comb accumulates pathogens, pesticides, heavy metals, microplastics and other undesirable compounds that the bees bring back from environment. Successive generations of brood rearing in the same comb result in smaller adult bees as the space inside cells decreases due to the accumulation of faeces, silk spinning for cocoon formation, and pupal moulting… it also drives the change from old comb to brown or black.
A scientific description of for newly-constructed wax comb cells “hexagonal prisms with a base formed by three rhombic plates” (reviewed in Meng et al., 2025). This structure degrades overtime due to the residues produced from the brood rearing process, which are not fully removed by nurse bees. The debris increases comb weight and thickens the cell walls, reducing its volume and diameter, degenerating it into a cylindrical shape as the years go by. Bees raised in these cells emerge with smaller bodies with reduced foraging and production capabilities.
Bees do have adaptations for managing old comb: Either cleaning and reshaping it with new wax comb, or as in the case of A. cerana, “gnawing” the surface to mark it for rebuilding (A. cerana discussed in Meng et al., 2024).
While not directly contribution to social immunity, a smaller or weaker workforce makes for a weaker superorganism, and a hive may end up may end up being more susceptible to diseases… replace your comb!
Individual Bees Produce Less Antimicrobial Secretions Under High Densities
Glucose oxidase is produced in the worker bees’ hypopharyngeal gland and catalyses the formation of hydrogen peroxide, an antimicrobial compound that inhibits pathogen growth in brood food. Jones and colleagues found glucose oxidase concentrations to be mediated by colony size; the larger the colony, the less enzyme activity per individual bee (Jones et al., 2018). Bees living in higher densities were able to divert energy away from the production of H2O2 and into other body systems. Individual bees no longer needed to maintain constant glucose oxidase activity. In numbers, the colony “picked up the slack” and per capita outputs were reduced at no cost to individual bee health.
In bumble bees, expression of the immune enzyme phenoloxidase varies according to the number of individuals kept in a group, being higher in solitary bees (Richter et al., 2012). They also note that the total antimicrobial peptide concentrations decrease when the bumble bees are kept in groups. The authors suggest that the phenoloxidase pathway may be short lived & fast acting, whereas the antimicrobial peptide response is slow and long lived, requiring more energy to maintain. Notice the pattern yet? More nestmates means less individual immune activation, which means more energy spared for use elsewhere.
Microbiome Sharing & Trophallaxis Provides a Stable Internal Environment
The microbiome of A. mellifera is highly conserved with ~8 core bacterial taxa; it function as a collective immune system, as all healthy bees have these populations intact, whereas it varies in diseased bees. In individual bees, the gut bacteria forms biofilms in the digestive tracts that prevents the colonisation by pathogenic bacteria from the environment or their diet (Reviewed in Mojgani et al., 2025). This is achieved by occupying the adhesion sites, consuming available nutrients, lowering pH via lactic acid and production of antimicrobial peptides.
Other aspects of the microbiota are in immune priming and activation of signalling pathways, detoxification as well as beneficial metabolite production; you can read more about the individual benefits in Part One. The key point is that bees do not obtain their microbiome from the environment as other animals do, rather from trophallaxis with the nestmates (the process by which bees feed one another). Two other routes include oral-faecal contact between nestmates (as in cleaning), or grooming.
The social transmission of a healthy gut microbiome ensures consistency in gut defences and pathogen resistance within a population. As with immune priming and trans-generational immunity from part one, any defence that Her Her Royal Majesty the Queen develops will be passed onto her offspring, who will also have similar resistance. It is from this angle that microbiome sharing matters for both individual and colony-level immunity.
Waste Management & Undertaking in Social Species
Main article: what do bees do with their dead?
Corpse removal is a specialised tasking within division of labour; around 1-2% of the middle-aged bees perform it. This number is fluid: when there is an increase in corpses, there is an increase in undertakers, or when undertakers are removed, others will take up their place (Breed et al., 2002). This reduces the risk of pathogen transmission associated with corpses. Should the bees be unable to remove the carcass, they embalm it with propolis to stop decomposition-associated diseases, (e.g. mouse skeletons).
Normally, bees defecate outside the hive and will “hold in” their faeces during unfavourable conditions until flight resumes. Diseases like Nosema or chronic bee paralysis virus, are associated with dysentery, causing faeces to be released (uncontrollably) inside the hive. When workers attempt to clean up the waste, they may spread the infection as they come into contact with contaminated material (Ribiere et al., 2007).
In the rare event of in-hive defecation (e.g. due to Nosema), workers cleanup by scraping the dried faeces using their mandibles and forelegs, chewing off small flakes from wood or comb, removing contaminated wax or sometimes regurgitate nectar on surfaces to dissolve residues, then licking it up (see Smith, 2012 about this latter point).
Brood Cannibalisation as Hygienic Behaviour
Main article: When Bees Eat Larvae
Posada-Florez and colleagues found that adult bees cannibalised larvae infected with Deformed Wing Virus (DWV; Posada-Florez et al., 2021). In their study, Varroa-sensitive hygienic bees consumed larvae infected with DWV. In one instance, cannibalisation of a single infected pupa led to infection of 2,000 worker bees, as the virus spread to other workers through trophallaxis.
Hygienic bees had a higher sensitivity to detecting the DWV-infected pupa compared to control, resulting in the development and spread of a low-level infection. However, this infection remained asymptomatic in those bees who acquired DWV orally. The authors suggest that this could be due to their sensitivity, as they were able to detect infection in earlier stages and thus cannibalise larvae at low(er) infection levels. That’s not say bees are immune against oral DWV exposures, as higher oral doses led to the emergence of DWV symptoms (Posada-Florez et al., 2021).
Guard Bees Prevent Infected Nestmates from Entering the Hive
The guard bees play a role in colony immunity by preventing diseased bees from entering the hive. Of note are the “bald bees” associated with foragers infected with chronic bee paralysis virus (see observations from Waddington & Rothenbuhler, 1976 or Drum & Rothenbuhler 1985). Guard bees can sense the infected bees (on account of a differing cuticular hydrocarbon profile) and exclude them from entering the hive. The expelled individuals die and this spares them from transmitting this disease:
- “At first the affected bees can fly, but they become almost hairless, appearing dark or almost black, which makes them seem smaller than healthy bees, with a relatively broader abdomen. They are shiny, appearing greasy in bright light, and they suffer nibbling attacks by healthy bees of their colony, which makes them seem like robber bees. In a few days they become flightless, trembling, and soon die.” – Ribiere et al., 2010,
Swarming & Absconding as Pathogen Reset
Main article: Absconding Colonies
Swarming is the natural process for colony reproduction. When the swarm leaves the nest, it can be deemed a “reset” as they leave their wax comb behind, along with any pathogens and diseases (Varroa being the exception as some mites would have hitched a ride on the departing bees). Absconding is the unplanned departure of a colony due to stress and other unfavourable conditions, and like swarming, the old comb and its miseries are left behind.
Both can be deemed as a way for a colony to “reset” its pathogen loads. Any infected comb (and subsequent infected brood) is abandoned, and the colony searches for a new location to start its home. The survival rates are much, much (much!) higher for swarming as the bees are gorged on honey and the seasonal conditions are generally favourable. Due to the sudden nature of absconding, it can occur anytime the stressor load becomes too great, and the departing colony is less likely to survive – as was the case with my three colonies in winter last year.
I know, these are not directly a part of the “immune system”. When you get to the scale of the superorganism, adaptations that may not meet the classical definition of “immunity” still matter. For example, if bees did not evolve the ability to abscond and depended solely on swarming for leaving the nest, then we would have many more colonies collapsing under high stressors… fortunately natural selection thought otherwise.
Queen Supersedure as Pathogen Control
Main article: Factors influencing Queen Bee Health
I end by mentioning that older queens have reduced egg laying capabilities, whether due to lowered sperm stores or ovum count. Her pheromone concentrations may decrease, weakening her control over larger colonies. Supersedure is the planned replacement of a queen due to the aforementioned factors. A new queen is reared and she mates with drones at their congregation areas and the old queen is killed off.
From the superorganism perspective, if the queen is weak and her laying is insufficient, then future generations of workers will end up weak and impact colony survival. Replacing her ensures the colony can continue, after all, the queen, like her daughter workers, are cogs in the superorganism machine, to be used and then replaced for maximum operating efficiency.
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Footnotes
- ”Black eyed pupae” is as the name suggest, black eyes on the pupae and occurs towards the end of pupation. If I had to assign this to a day, I’d say around the day 18-19 mark (day 21 being when the average worker bee emerges from her cell). The literature describes pupation as happening in three stages: “ white, pink, brown and black eyes, and then beginning of tegument darkening [exoskeleton]” (Roat & Landim, 2010). Tying this back to the above, it suggests that bees are best at detecting when mature pupae are stressed from Varroa parasitism. ↩︎
- The temperature increases were greater when there more mites per cell, as workers are better at detecting a more-wounded pupa, whereas brood cells with a low mite count (less than three) were harder to detect… perhaps thermal-camera-heat-detection for Varroa may not be as good in picking up the early stages of a Varroa infestation? ↩︎