How Do Worker Bees Know Their Role?

Worker Castes, Task Allocation & Temporal Polyethism

Internal (hormones, genetics) and external (hive environment) influence a worker bee’s role in the colony. After all, how does a nurse bee know when it’s time to stop brood rearing and begin building comb? Bees are not “conscious” in the human sense – they do not suddenly change their mind without a signal or cue.

Temporal polyethism is defined as an “age-based division of labour”. These are patterns of set tasks that adults undergo from the day of eclosion when they first emerge from the cell. Young bees begin as nest cleaners, becoming nurses four days later until around 12 days of age. From here, they are “Middle aged bees” (MABs), who perform a variety of tasks: nectar processing and water distribution to guarding and undertaking until ~21 days of age. They graduate into foragers until death.

How do the bees know when to start a new duty, or continue with the same one? This is a more technical article, drawing on Bryan R. Johnson’s 2010 review “Division of labor in honeybees”.

There are two shirts that go with this article, designed & printed by yours truly:

Four Developmental Phases: “Temporal Castes”

The traditional view is that bees suddenly transition between roles – one day they’re a guard the next day they’re a forager. We now know that these transitions are gradual, with bees taking on multiple roles as they move between tasks (Johnson & Frost, 2012). The literature identifies four phases (“temporal castes”) of a worker bee’s life, plus one:

1. Days 1-4: cell cleaners.
2. Days 4-12: nurses.
3. Days 12-21: middle aged bees.
4. Days 21-death: foragers.
5. The generalist overwintering bee (described in Johnson, 2010).

Variations in this timeline exist, as these ages are dependant on geography and the honey bee subspecies; some report foragers starting at 22 days of age, or nurses from 3 days of age.  The term “caste” is used since the workers focus on their set task and ignore others. For example, nurse bees will either ignore or briefly examine a corpse before returning to their nursing-related task (Visscher, 1983).

Nothing is set in nature, and the ages above are general trends. The worker’s body undergoes physiological changes as she moves between roles to prepare her for the work ahead. For example, immune function decreases as the bee transitions from guarding to foraging. The bee’s body reallocates energy away from immunity and onto task-specific functions like flight muscles.

Cell Cleaners are an Extension of Early Development

I’m sure you’ve seen those smaller, paler, bees- they are the newly-emerged workers who clean wax cells, a phase suggested to be an extension of early development (reviewed in Johnson, 2010). When not cleaning, these bees are grooming themselves and others or remaining idle.

The newly-eclosed bee is frail, lacking the hardened exoskeleton, flight musculature and stinging capability of older age. Perhaps if the worker pupa had continued developing for a further few days inside the cell (the pupal phase is around 12 days), then the bee may emerge “fully developed” and go straight into nursing. Perhaps not, and cell cleaning is a starter task that gets bees used to their adult form and the internal hive environment¹.

Cell cleaning is an unspecialised task, and both nurses and middle-aged bees perform it during times of need, such as surges of brood emergence (Johnson, 2010). Why don’t the foragers assist? I could guess it’s because their role as food suppliers is more important than nest cleaning: if a forager was to assist with cleaning, then it’s one less bee bringing in food for the next generation.

Nurses for Brood Rearing and Queen Care

Between 4-12 days of age, the worker is a nurse, feeding brood, other workers and the queen. Some may attend to Her Majesty’s needs as members of the retinue. Brood care involves feeding larvae, relocating eggs and preparing the cells of older larvae for capping. At this age, they secrete the proteinaceous “jelly” from their hypopharyngeal gland; “royal” jelly to the queen caste, “worker” jelly to young brood and adult bees (Crailsheim, 1991).

Proteinaceous Jelly is Fed to the Entire Colony, not Just Brood and Queen.

Crailsheim showed that nurses feed jelly to the entire colony (drones and other workers), not just the queen and brood as commonly thought (Crailsheim, 1991). Jelly is easier to digest than pollen; the protective shell and composition of pollen grains require multiple digestive enzymes to breakdown. Foragers also receive jelly, which supplements the nectar they consume in the field, providing a rapidly-absorbed source of energy (Crailsheim, 1992).

Young bees (“cell cleaners”) receive the most jelly and are fed more frequently than the rest. Crailsheim supposes this because their developing body requires more protein to reach its final form. Nurse bees are the best equipped to digest pollen, as the derived proteins are synthesised into jelly from their hypopharyngeal glands. However, in times of pollen of shortages, they will consume jelly from other nurses, and this serves as an indicator of pollen availability: when the ratio of jelly to pollen consumption increases, it signals a decrease in pollen foraging.

Foragers Prefer Jelly Because Pollen is Harder to Digest

Pollen is hard to digest. Think of it this way – it’s the plant’s reproductive cell (sperm), and it must have protection to survive in the environment. This same protection is what makes it hard to digest, whether it’s due to the cell wall composition, or the mechanical strength of the shell, intensive energy is required to receive nutrients from a pollen grain (reviewed in Roulston & Cane, 2000). As above, dedicating energy to maintaining enzymes involved in pollen breakdown is not in the forager’s favour, they are there to collect the food, not consume it.

The young bees on the other hand, are just cleaners, so they have the time and resources to spend digesting pollen along with the jelly supplementation.

Middle Aged Bees Work Outside of the Brood Area

Middle aged work outside the brood area. Some 15 activities are identified, ranging from undertaking, comb building and maintenance, to nectar receiving and processing, to guarding the nest entrance (Trumbo et al., 1997 & Johnson, 2010). Within the 9-10 days they spend in this phase (ages ~12-21 days), the younger MABs work inside the hive on the combs, whereas the older MABs are closer towards the hive entrance as guards and nectar receivers.

Foragers Bring Resources

We know plenty about foragers from a scientific and beekeeping perspective. They work outside the hive until their death, bringing nectar, pollen, water and propolis. The former two are the most prevalent. The need for water is determined by colony temperature, whereas propolis is based on colony demands for social immunity. Foragers communicate resource availability via signals and cues. During swarming, some foragers become scout bees who help guide the swarm to its new home.

Older Middle-Aged Bees & Foragers are Co-Dependant

The activity of older MABs is linked to foragers, and vice versa – the activity of foragers is linked to the availability of nectar-receiving MABs. The colony must balance the ratio of MABs to foragers to maximise resource use. Three “issues” are proposed by Johnson:

1. A MAB must be available to receive nectar at the maximum rate brought in by the forager AND a MAB must be available to build comb for these new resources so the colony does not run out of space.
2. Foragers die quickly due to age and a higher metabolic rate, so their population must be continually replaced.
3. During nectar flows, the colony gains weight and more foragers arise to take advantage of it. When the flow is over, the extra foragers are redundant, becoming a drain on the colony’s resources.

The first and second problem are resolved by having a steady rate of comb builders and forager-ready MABs. This could be in the form of a feedback loop, whereby foragers produce an inhibitor that stops MABs from becoming foragers until it dips below a threshold (i.e. due to a shrinking forager population). Leoncini and colleagues suggest that this is ethyl oleate, which is made in the forager’s honey stomach and transferred to MABs via trophallaxis (Leoncini et al., 2004).

The third problem is related to honey bee behaviours: when there are many nectar receivers, a forager does not take long to offload her produce, and vice versa (the receiver takes a long time to find a forager). This is indicative of a good forager:MAB ratio. Now during a nectar flow, there are plenty of waggle-dancing foragers, and not many onlookers or “followers” to their performance– When a lounging MAB observes this, she interprets it as the colony needing more foragers and thus becomes one herself, taking off in the direction of the waggle dancing. Further reading is available in Seely’s 2024 book “Piping Hot Bees and Boisterous Buzz Runners”.

Winter Bees: The Generalist Caste

Physiologically, winter bees have the same hormone profiles as the nurse bees – low juvenile hormone, an active fat body, high vitellogenin, and an active immune system (Huang & Robinson, 1995). This does not mean they a prolonged nurse caste, for they continue in this physiological state even in the presence of the stimuli that signals the end the nursing phase (low brood or forager levels in the hive). The winter bee caste is triggered when a worker emerges to a colony with little or no brood. For already-emerged adults, the decrease in foragers is thought to trigger the winter state (proposed in Johnson, 2010).

Another reason why winter bees are not nurses is that in warmer days, a ~160-day old bee will leave the nest in search of forage and may continue to do so until the new generation takes over. How this happens, and how these ancients can sustain this demanding activity is unknown.

Promotions and Demotions: Non-Standard Task Allocations

“Bees can go forward but not backward” said a member of my beekeeping club. Nurses can become guards and undertakers can become foragers in extreme circumstances. However, once in that role, they do not go back. If a bee became a forager at eight days of age, she stays a forager. She does stop and commence middle-aged duties.

The literature shows otherwise, in that worker bees are flexible and can transition either way in response to the colony’s needs. The two models described below are examples of behavioural changes (push-pull) and physiological modifications (proximate).

Push-Pull Model for Caste Transitions

The first model proposed by Johnson is “push-pull”, in that workers switch castes according to the colony’s needs (Johnson, 2010). This is driven by pheromones from the hive and hivemates- For example, a nurse bee spends her time in the brood nest and is exposed to brood pheromones and (intense) queen pheromones. Whereas in the honey frames she experiences a different set of stimuli (pheromones, temperature) and the absence of the brood scents. These occur in natural, predictable condition as the bee ages and the worker undergoes textbook maturation.

In a crowded colony, a worker may be pushed to experience a new set of pheromones prematurely (for example, leaving the brood zone early), pulling her into the next caste’s workload. By age, she may be a nurse bee, but by occupation, she is a MAB… is it still child labour when they’re all adults?

The combination of environment and internal stimuli drive caste transitions; an adaptation that provides colonies with flexibility to meet the environment’s demands. Faster MAB development in crowded colonies enables them to build more comb and receive more nectar, which also increases the number of potential foragers (older MABs waiting for the “signal”). I wonder, if that fast-tracked nurse bee becomes a forager, will that worker’s lifespan be shorter or longer than a forager who went down the traditional worker transition pathway?

Returning to our crowded brood nest example, should this crowding happen during a nectar flow, the demand for foragers may outweigh that of nectar-receiving MABs. The pushed nurse bee is exposed to forager-associated signals, driving her out of the nursing caste and into foraging. On the other hand, if there are sufficient foragers bringing in nectar (or water on hot days), then the nurse is exposed to MAB-associated signals and transitions into a role of the older MABs – nectar processing.

The Proximate Model of Temporal Polyethism Uses Pheromones & Environmental Stimuli

Johnson’s second model is similar to the above, stating that a “priming mechanism” (signals from the environment, physiology or other bees) triggers changes in the worker bee at certain points in her life, leading to transitions between castes (Johnson, 2010). In this model, a nurse bee is under “reinforcing mechanisms” that assert her role as a nurse. A new stimulus arises, causing a series of hormonal/signalling cascades within the bee, leading her to develop a new reinforcing mechanism and thus enter the next phase. As she performs the MAB duties and the colony needs shift once again, another stimulus causes a new reinforcing mechanism, which shifts her into the foraging caste.

The nurse priming mechanisms are thought to be queen mandibular pheromone and the brood pheromones. A break occurs when:

• The ratio of nurses to brood increases or,
• When the presence of newly-merged workers causes the older nurses to leave the brood nest.
  • These bees need the heat and pollen of the brood nest to finish their development (Stabentheiner et al., 2010).

The displaced nurses are no longer exposed to the same pheromones; under normal conditions, this primes them for the MAB’s reinforcing mechanism. The reinforcing mechanisms that maintain the MAB caste are more complex, since these bees perform many tasks inside the hive. For example, the reinforcement for an undertaker would be different to that of a comb builder (constant dead bees vs a need the fill empty space with comb).

The transition from a MAB to a forager is more straightforward. As mentioned above, it’s just a matter of a jobless, mature MAB receiving the “signal” from a waggle-dancing forager that drives them to forage– after all, they’re not tasked to anything at that moment, and this forager is advertising a patch of flowers outside the hive. A new reinforcing mechanism emerges, and the MAB officially begins foraging. Once a forager, they stay a forager, this is the endpoint of a worker bee’s life, and there is no new priming mechanism to drive them into a new caste.

What about Task Allocation for Drone and Queens?

Studying social insect division of labour is complex due to the internal and external factors that determine a worker’s role inside her hive. Workers grow and maintain the colony, but what about Her Majesty, who lays more workers, and her mates, the drones? I conclude this article with a brief mention of their tasking process. As in the above, a combination of physiological and environmental stimuli drive the transitions between each working phase.

I wager that task allocation for the queen is relatively simple: emerge from the queen cell, kill/fight other queens to assert dominance, spend a few days in the hive eating and maturing, commence mating flight(s), return to hive and learn how to lay eggs, then continue laying eggs according to comb space and colony resource availability.

Drones follow a similar pattern: emerge, spend time inside the hive eating and maturing, when it’s time (i.e. sexual maturity), fly to drone congregation areas and await a virgin queen to mate with. If mated successfully, death follows. If unsuccessful, return to the hive to rest and refresh, then go out again. Rinse and repeat until death after ejaculation, or death from an old age. Go read part 1 on drone biology for more

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Footnotes

1. Stabentheiner and colleagues suggest that the act of cell cleaning by young bees helps with flight muscle development, as spending time in the heated, constant environment of the brood area ensures a constant rate of development; the “older” bees (> ~ 2 days) generate heat to the warm the colony for the benefit of the brood and young adults (Stabentheiner et al., 2010). Further, pollen is in easy reach to these bees, providing the protein needed for development. ↩︎