How Honey Bees Defend Themselves

Stings, Pheromones, Buzzing and Shaking

Asian honey bees (Apis cerana) bee ball hornets to death. Giant honey bees (Apis dorsata) deter predators by synchronised shimmering atop their open combs, whereas the dwarf honey bees (Apis florea) abandon their nest and reform elsewhere in response to predation. Social insects like honey bees have multiple defences against predators, ranging from the physical (bites, stings), psychological (shimmering, faeces gathering).

This article describes the physical aspects of honey bee defence in response to predators. These behaviours are largely driven by worker bees, who perform many roles from foraging to guarding. Further, it describes the neurology of defensiveness, nestmate recognition and the role of pheromones in defensiveness.

Guard Bees are the Hive Entrance’s Checkpoint

Middle-aged worker bees take on hive guarding duties. Walking around the hive entrance, they inspect incoming bees to determine if they’re a nestmate or foreigner. Workers bearing the colony’s pheromones are allowed in, those without the scent are denied entry, or (rarely) allowed in if they beg hard enough. Drones come and go– drifting in between colonies of A. mellifera without repercussion. Perhaps foreign drones are welcomed as they improve the chances of a daughter virgin queen mating with diverse males?

Cuticular Hydrocarbon Signatures Determine Friend or Foe

Guard bees identify nestmate from non-nestmates by their cuticular hydrocarbons. Nestmates will have the same “smell” or hydrocarbons on exoskeleton as the hive, informing the guards that she is a friendly bee. Signorotti and colleagues describe this as a “label” and “template”– the smell of incoming bee (“label”) is matched against the colony odour (“template”; reviewed in Signorotti et al., 2015). Guards develop their template from the wax comb and compare the incoming workers against it.

Newly-emergent bees produce their own cuticular hydrocarbons based on their genetics (Signorotti et al., 2015). As they age and work in the hive, they pick up the “hive scent” so that by the time of foraging, they can recognise and be recognised by their colony’s guard bees. The low cuticular hydrocarbons levels (a “blank slate”), signals harmlessness to the hive, meaning the newly-emergent bees can be accepted by a foreign colony – perhaps this is why hybrid A. cerana and A. mellifera colonies can be formed, due to the lack a distinctive cuticular hydrocarbon profile of newbees.

Honey bees rely on a combination of esters and polar compounds to classify nestmates (Frohlich et al., 2001). Workers assigned to different tasks inside the hive, whether undertaking or cleaning, had similar but distinct cuticular hydrocarbon signatures, thus identifying their role in the colony (Vernier et al., 2019).

The Reaction of Guard Bees to Other Insects

When guards spot a hawking hornet, they alert the bees inside. In the event they’re outnumbered, the colony remains “turtled up” on the combs. A. cerana performs deterring tactics with shimmering/buzzing moves, as they are frequently attacked by wasps – more on that below.

In A. mellifera, balling occurs on some wasp species, such as the European hornets Vespa crabro. However when A. mellifera is taken out of her natural environment, as in Asia, the Asian hornets overwhelm hives as these bees lack adaptations against those species. Robinson reports that when Asian honey bees spot a hornet, their flight pattern changes and guards begin defensive measures (Robinson, 2013). A. mellifera on the other hand, does not alter her flight, and returning foragers continue business as usual, only to get captured and eaten by a hawking hornet. Eventually, the colony is overrun by a raiding party, who consume the honey and take the larvae to feed their own brood.

The Queen’s role

Ever experienced a queenless beehive? Such colonies are more far aggressive than usual. The lack of queen pheromones turns the hive into a free-for-all; workers compete with one another for reproductive dominance. They have become generalists like their solitary bee counterparts. In one study, Naegar and colleagues studied hopelessly queenless colonies (those who have no chance of making a new queen) and suggested that this state is “quasi-sociality” (Naegar et al., 2013). These bees are from a single generation, with the same mother, sharing a nest and cooperating in (drone) brood rearing, as workers lay unfertilized eggs.

You’re attacked by a queenless hive because the workers bees no longer see themselves as part of the colony, rather 10,000s of individuals defending a common nesting space.

Why are Queenless Hives Aggressive?

Sakagmi’s 1954 paper suggests three factors for aggressiveness in queenless hives:

1. Physiological changes in other workers (activated worker ovaries).
2. The ability of a worker bee to sense the active ovaries of her nestmates.
3. The degree of aggressiveness between the worker with an active ovary, and the one doing the aggression.

He likens this to a dominance/pecking order hierarchy seen in the social Polistes (paper wasps). The queen keeps all her workers sterile and reproductively equal. In a queenless honey bee colony, worker ovary development causes competition with her sisters for egg-laying dominance. This produces a domino effect: as more workers’ ovaries activate, more “false queens” emerge, right until colony collapse.

Later work showed that a mix of reproductive competition and changes in pheromone production (from queen pheromones to a worker/false queen pheromones) resulted in aggression (Blom 1991; Malka et al., 2008). Workers compete with another to reproduce and pass on their genes. The resulting hostility is experienced by the beekeeper and surrounds- These bees sting indiscriminately and unprovoked in order to protect their nest.

Defence in Numbers: Stinging Workers, and Lots of Them

During hive defence, guard bees at the entrance raise their abdomen and expose the sting chamber along with alarm pheromones- fanning their wings to disperse these volatile components. The compounds signal actual or perceived dangers to the colony, and are released not just when exposing the stinger, but also during stinging, and when the barb is left in your (unfortunately-exposed) skin.

Why can’t Bees Remove their Stinger?

Mammalian skin is elastic. When a bee stings, she is unable to remove the stinger, causing it tear away from her body. The sting is barbed, and these barbs prevent the lancet from easy retrieval. Further, the muscles that control the stinger are weak and atrophied, unable to generate enough force to pull it out. As a result, the stinger and associated muscles remain in the victim.

“Sting autonomy” enables the stinger to continuously inject venom after the bee is detached, causing a single sting to be worth multiple injections. Biologically, this adaptations is found among social insects with sterile workers (Herrmann, 1971), as the loss of a hive member does not impact the colony’s reproductive fitness (after all, the purpose of life and evolution is to reproduce). Harvester ants, Brachygaster wasps and the Apis genus of honey bees all have stinger autonomy.

This sting-less bee will soon die, but not immediately. Worker bees may live up to five days after stinging and continue defending the colony- they can still bite, buzz and pursue intruders, showcasing an evolutionary adaption to hive defence (Cunard and Breed, 1998).

Chemical Recruitment: Alarm Pheromones

Alarm pheromones are produced by the Koschevnikov gland and gland-containing areas around the sting sheath. The active component of the sting alarm pheromone is isopentyl acetate (an “IPA” of questionable taste). Its release alerts other workers to commence defensive behaviours. The concentrations of IPA are negligible in queens, and slowly develop in worker bees, peaking at 2-3 weeks of age when they begin guard duties (Allan et al., 1987). IPA is the main active component, but not the only one – others like 2-heptanol or 2-nonnal can trigger defensive behaviour; the alarm pheromone is a blend of over 40 compounds.

Biting Intruders: Worker Mandibular Pheromones & 2-Heptanone

Worker mandibles produce 2-Heptanone, a component of the alarm pheromone. Despite having a lesser potency compared to the sting gland compounds, it could serve as an immobilising agent that accompanies biting behaviour. When Varroa mites or wax moth larvae are bitten, the 2-heptanone paralyses these too-small-to-sting pests (Papachristoforou et al. 2012).

Balling, Shimmering, Buzzing & Poop in Apis cerana

Balling is a behaviour common to all honey bees. The most pronounced examples of this are in the Asian honey bees, A. cerana. The 2021 review by Cappa & colleagues investigated the co-evolutionary history of balling as a predator-prey interaction, and found that in the Asian Giant Hornet, Vespa mandarinia, developed predatory behaviours in response to adaptions by Japanese honey bees, causing an evolutionary arms race:

1. Predator effect: Scout hornets rub pheromones and markers against or near the colony walls to signal its location to other hornets.
   1. Worker bees can detect the marking pheromone and recruit nestmates to guard the entrance, giving the appearance of a strong and well-guarded hive that deters the scouting hornet.
   2. Worker bees gather “filth” (such as animal faeces) and plaster them around the entrance to muddle scents, and perhaps give the impression that the colony is abandoned.
      1. This was documented by Mattila and colleagues in Vietnam against the Southern hornet Vespa sorer. Only A. cerana colonies that were attacked by these hornets performed this activity to repel future attacks from the same hornets. Colonies that were not attacked did not exhibit such behaviour.
      2. For A. cerana japonica, they perform the analogous “plant smearing” rather than faecal matter in response to raids from V. mandarinia (Fujiwara et al., 2016). Interestingly, their plants of choice happen to contain hornet-repelling compounds.
2. A large solitary hornet enters the nest and begins consuming bees.
   1. 400-500 worker bees cluster around the hornet in groups of 30, furiously beating their wings and stinging it. This raises the temperature inside the ball to ~48°C and increasing carbon dioxide concentrations, suffocating the hornet to death.
      1. V. mandarina hunt in groups, however other Vespine wasp species like V. velutina (Asian hornet) and V. crabro (European hornet) are solitary hunters and the danger will pass after it dies from heat balling.
3. A hornet hovers in front of the nest entrance to hawk (catch) returning bees and departing foragers from the air.
   1. Visual sighting of hornets increases guard bees at the nest entrance. It also causes bees to conduct a “shimmering” maneuverer by coordinating a rapid shaking of abdomen and buzzing in synchrony to gesture “(Peekaboo) I see you”.
      1. Well, maybe not the “peekaboo” part.

Other open-nesting honey bee species like A. dorsata and A. florea also exhibit this shimmering behaviour in response to hornet predation. This visual cue signals the strength of the colony to the approaching hornet, in an attempt to confuse and repel it (Kastberger et al., 2014). The intensity and frequency of the shimmering increases as the predator gets closer to the colony, however it is (unsurprisingly) unsuccessful in deterring larger predators like birds (Kastberger & Sharma, 2000).

Absconding, not Swarming

Absconding is when the entire colony abandons the hive due to disturbances (predation, disease, or environmental stress) or seasonal changes (common in Asian honey who follow the floral blooms). Unlike swarming, an absconding colony aims to leave no brood or food stores (Winston et al., 1976). This is more likely in African rather than European bees, and had developed in response to a survival threats, not a reproductive necessity. An example of seasonal absconding is seen in open-nesting species like A. dorsata, who move between lowlands and highlands in pursuit of nectar blooms.

Schneider states that there is a “preparatory phase” to absconding – colonies forage less, rear less brood (despite the ideal conditions), and consume stored honey (Schneider,1989). The bees do not form a temporary cluster while searching for a suitable nest, instead flying directly to a new location. The scouts perform a long waggle dance, suggesting that these workers sampled sites far removed from their typical foraging ranges (here, 6-16 km in in the Okavango Delta region of Botswana)¹… Does this waggle dancing remain informative at such distances, or more of a signal to “fly in this direction for as long as you can?”

Swarming is not a Defensive Measure: Bees Leave Bees Behind

Swarming is not considered a defensive measure, rather a natural reproductive process during spring and summer. The hive splits in two, led by the mother and daughter queen (afterswarms are beyond the scope of this article). Multiple factors trigger swarming: Hive congestion, favourable weather, or abundant resources. If you find queen cells on the bottom frames, then that’s a sign of an impending swarm. Beekeepers like yours truly destroy these cells to prevent swarming – after all, a cloud of bees are a nuisance in urban environments

Hygienic Behaviours & Undertaking

Hygienic behaviour is defined in two parts:

1. Worker bees that detect and uncap diseased brood,
2. Removal of diseased brood in a timely manner by worker bees.

The keyword here is “timely”. If workers were just to uncap brood (for inspection, rather than disease monitoring), and not remove it, then an early stage disease like European Foulbrood progresses and leads to the production of infective bacterial spores. If the brood was removed early (“a timely manner”), then pathogen transmission is limited, maintaining colony health (reviewed in Evans & Spivak, 2009).

The two steps can be performed by separate bees. Scannapieco and colleagues showed that while some workers perform both uncapping and removal, others are solely focused on uncapping, or solely focused on removal (Scannapieco et al., 2016). Rule of thumb: the more bees allocated to these tasks, the more hygienic a colony.

Death Pheromones are the Absence of Cuticular Hydrocarbons

Extracted from “What do bees do with their dead?“

In death, it is the absence of chemical substances that signals a corpse, rather than the production of “death pheromones”. Visscher noted that dead bees are removed from the hive within an hour of detection. Other objects, like wood chips or plant material may remain for extended periods of time (Visscher 1983).

The loss of cuticular hydrocarbons signals a corpse. In life, these carbon-based compounds are constantly produced as part of the chemical signatures used in nestmate recognition. In the Asian honey bee Apis cerana, Wen and colleagues isolated two cuticular hydrocarbons, heptacosane and nonacosane, and found that undertakers interpreted the reduced emissions of these compounds as a corpse (Wen et al., 2023).

To perform undertaking, the bee touches the corpse and walks around it several times, presumably to detect the cues that confirm death. Once confirmed, the undertaker’s mandibles grab onto the bee and flies to dispose the corpse away from the colony.

Neurology of Honey Bee Defensiveness

Non-technical readers are welcome to skip this section

Nouvian and colleagues showed that when bees sense IPA, it increased brain levels of serotonin and dopamine (Nouvian et al., 2018). Along with octopamine, these three chemicals have diverse functions at the central and peripheral nervous systems, from classical neurotransmitters to neuromodulators and neurohormones.

In the presence of IPA, serotonin increased more than dopamine, producing bees that are more likely to sting. Pharmacologically increasing serotonin and dopamine also triggered increased defensiveness, whilst reductions had the opposite effect. IPA increased dopamine to a lesser degree, as dopaminergic neurons make bees more “focused” and likely to respond to stimulation such as electric shocks (Tedjakumala et al., 2014). It’s possible that in the presence of IPA, the levels of both molecules are increased to a physiology that ensures bees are defensive and focused on intruders.

What about octopamine, the other major amine? Octopamine does not play a role in Hymenopteran aggression, being more pronounced in that of fruit flies or termites. In honey bees, this molecule is involved in associative-learning experiences, whereby its presence helps bees associate sounds or smells with rewards or electric shocks (Hammer & Menzel, 1998).

Defensiveness in Africanised bees

Main article: Africanised honey bees

Africanised honey bees are the result of crossbreeding between an African honey bee (A.m. scutellata) and various European subspecies (such as A.m. ligustica and A.m. carnica). Harpur and colleagues found that both Western European and African ancestry contribute to defensive nest behaviours in the Africanised honey bees (Harpur et al., 2020). It is not the African lineage alone, rather a combination of it and the European genes that produce extreme defensive behaviours.

Can Bees Protect Themselves Against Pesticide Exposure?

Extracted From “The Effects Herbicide & Fungicides on Bees”

In short, the answer is “not really”. In some cases, bees may prefer nectar containing pesticides over clean sources. Honey bees are capable of “tasting” different compounds throughout their lives, from salts and organic acids, to lipids and growth regulator factors (de Brito Sanchez, 2011). Perhaps the brain registers novel pesticides as “beneficial”?

This behaviour is concentration-dependent, as foraging bees will disregard a pesticide-containing food source at higher concentrations but continue collecting it at lower concentrations (Liao et al., 2017). Under semi-field conditions and low concentrations, honey bees were found to prefer sugar syrups containing the herbicide glyphosate and the fungicide chlorothalonil.

Chlorothalonil is commonly reported as contaminating wax and stored pollen. The fungicide has been linked to an increased risk of Nosema infections, lower colony biomass, and increased honey bee toxicity due to synergy with the Varroa acaricide fluvalinate.

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Footnotes

1. This extended travel distance makes sense to me, for the bees have deemed their immediate surroundings & foraging range to be inadequate. As such, they fly long distances to reach areas that were previously untouched by their colony’s foragers ↩︎