1. What are winter bees?

Winter bees are workers produced as the colony gradually enters its overwintering state.

They do not constitute a distinct caste in the same sense as the queen, but rather a specific seasonal state of the worker bee, adapted to a period in which the colony must endure unfavourable temperature and food availability conditions. Knoll et al. (2020) explicitly describe the formation of winter bees as an adaptive mechanism enabling the colony to bridge these difficult periods.

In temperate regions, the distinction between winter bees — described as long-lived — and summer bees — described as short-lived — remains partly theoretical. The transition from one state to the other is gradual and depends on geographic and seasonal context. Knoll et al. (2020) note that September and April may be considered transitional months, emphasising that these are not two entirely separate categories but rather a progressive transformation of the worker population.

The function of these bees is central to colony survival. During the unfavourable season, they remain in the hive, participate in the formation of the thermoregulating winter cluster, and generate heat by activating their thoracic muscles. Their role is not limited, however, to merely enduring the winter. Knoll et al. (2020) go so far as to speak of a "nutritive storage caste" in the functional sense: winter bees accumulate large reserves of lipids and proteins, retained throughout the cold period and subsequently mobilised to restart brood rearing once favourable conditions return. They are thus a generation of survival, but also a generation of restart. In the scientific literature, these bees are referred to by the technical term diutinus (from the Latin "of long duration"), a term introduced by Maurizio (1950) to designate this long-lived phenotype distinct from summer workers. The terms "diutinus bees" and "winter bees (diutinus)" are used interchangeably in the specialised literature.

One frequently underestimated aspect concerns their flight activity. Contrary to the image of confined and passive bees, Minaud et al. (2025) show that winter bees perform, when conditions permit, surprisingly high levels of flight activity compared to summer bees — without any apparent compromise in performance. This behavioural plasticity is consistent with their transitional role: they are not merely nutrient reserves, but active workers capable of progressively resuming the functions of nurse bees and foragers in spring.

2. How do they differ from summer bees?

The most obvious difference between winter bees and summer bees is their longevity.

Summer bees are described as workers living on average 25 to 40 days, whereas winter bees can live up to 250 days or more (Knoll et al., 2020). This difference in lifespan reflects a distinct physiological state, shaped for colony survival during the unfavourable season.

At the core of this physiology is vitellogenin (Vg), a multifunctional protein that acts as the central molecule of the winter phenotype (Knoll et al., 2020). Winter bees accumulate large quantities of it in their haemolymph and fat body, where it accounts for 30 to 50% of circulating proteins in certain individuals (Erban et al., 2013). This accumulation is not trivial: Vg acts simultaneously as a nutritive reserve, antioxidant, immune modulator, and brake on behavioural ageing. By maintaining high Vg levels, the winter bee remains physiologically in a state close to that of a nurse bee — with delayed foraging activity — which is precisely the condition enabling extended longevity.

This dynamic rests on a hormonal feedback loop: high Vg and low juvenile hormone (JH) mutually inhibit one another. In active summer bees, JH rises with age, driving the worker towards foraging — a costly function that shortens life. In winter bees, this transition is delayed: JH remains low, Vg remains high, and progression towards the short-lived stages is deferred (Knoll et al., 2020; Schilcher and Scheiner, 2023).

At the metabolic level, the differences are equally pronounced. Winter bees display reorganised mitochondrial activity (Cormier et al., 2022), trehalose levels approximately twice as high as those of summer bees (Lee et al., 2022), and a distinct antioxidant profile — notably more active catalase and reduced lipid peroxidation (Orčić et al., 2017). These adaptations appear to reduce free radical production and thereby limit age-related oxidative damage.

At the immune level, the picture is more nuanced. Winter bees generally show enhanced humoral immunity — notably increased expression of antimicrobial genes — but sometimes reduced cellular responses (Hurychová et al., 2024). At the same time, overwintering is associated with increased susceptibility to certain viruses, particularly Deformed Wing Virus (DWV), underscoring that winter immune robustness has its own limits (Steinmann et al., 2015).

The most striking insight comes from Bresnahan et al. (2021). Their transcriptomic analysis reveals that winter bees display a "mix and match" profile: their fat body resembles that of summer nurse bees — oriented towards storage and immunity — while their flight muscle resembles that of foragers — primed for thermogenesis. This combination, unique to winter bees, is not observed in any other summer stage. It precisely reflects their dual mission: maintaining the group's warmth while keeping internal reserves intact.

This portrait should not, however, be fixed into an overly rigid model. Quinlan and Grozinger (2024) point out that the markers classically associated with the winter phenotype do not necessarily all vary in the same direction at the same time. In their study of ten-day-old nurse bees, autumn bees had larger fat bodies but relatively lower Vg expression than their summer counterparts. Season predicted these differences better than brood area alone — which anticipates the next chapter.

Sources: Knoll et al. (2020); Erban et al. (2013); Bresnahan et al. (2021); Lee et al. (2022); Hurychová et al. (2024).

3. When does the colony begin producing its winter bees?

The colony does not begin producing its winter bees on a fixed, universal date.

In temperate climates, it is a gradual transition that unfolds from late summer into autumn as brood rearing slows and eventually ceases. Döke et al. (2015) describe this cycle clearly: after the spring peak and summer colony development, brood declines at the end of summer, stops in autumn, and it is during this declining phase that the long-lived workers destined to overwinter are produced.

A formulation as rigid as "winter bees emerge in September" should therefore be avoided. Mattila et al. (2001) show that the proportion of winter bees increases cohort by cohort through autumn: in the control colonies of their study, the first winter bees appeared in the cohort introduced on 31 August, while in requeened colonies their appearance was delayed to 12 September. This shift illustrates an important point: the transition also depends on signals internal to the colony. Requeening delayed the appearance of winter bees, suggesting that the laying dynamic and colony structure influence the timing of the transition. In other words, the production schedule of winter bees depends not only on the season but also on the colony's internal dynamics, of which the progressive reduction in egg laying is very likely a part.

The work of Mattila and Otis (2007) confirms that this schedule is not simply a matter of the season "viewed from outside". By manipulating pollen supply in autumn, they show that prolonging its availability delayed the decline of brood and shifted the appearance of the winter bee population to a later date; conversely, an early reduction in pollen supply accelerated the transition. Pollen availability thus acts as a powerful signal conditioning the pace of the shift towards the overwintering population.

What matters, therefore, is not a specific date but the moment at which the colony gradually ceases to prioritise expansion and begins to invest in survival. The exact date varies with climate, floral environment, laying dynamic, and the internal state of the colony — which Döke et al. (2015) summarise by presenting the entry into the overwintering state as a process regulated by multiple environmental and social factors, rather than as a simple calendrical switch.

4. What factors drive this transition?

The transition to winter bees does not rest on a single trigger.

The most robust reading of the literature points to a multifactorial process in which several signals converge to shift the colony from an expansion logic to a survival logic. Knoll et al. (2020) summarise this idea by linking the winter transition to the mechanisms that normally regulate division of labour — via nutritional reserves, JH, Vg, colony demography, and social pheromones.

The role of the brood and its chemical signals is the most firmly established. As brood rearing declines in autumn, the nursing load borne by young workers decreases, and the pressure exerted by brood pheromone on worker physiology attenuates. Smedal et al. (2009) demonstrated in a factorial design that synthetic brood pheromone alone was sufficient to reduce the amount of Vg stored in the workers' fat body, and that exposure to this pheromone reduced the long-term survival of colonies. The brood therefore acts not only through the nursing load it imposes but also through its social signal.

Reducing the transition to brood quantity alone would, however, be inaccurate. Quinlan and Grozinger (2024) compared ten-day-old nurse bees in summer and autumn across colonies with varying brood areas and conclude that season predicts worker physiology better than brood area alone. Additional seasonal factors — notably pollen availability and colony demographic structure — appear necessary to fully induce the winter phenotype.

Pollen occupies a particular place here, as it operates on two levels. On the one hand, it directly conditions the colony's capacity to maintain brood: without a sufficient supply, rearing collapses. On the other hand, its seasonal decline provides an environmental signal consistent with the approach of winter. Mattila and Otis (2007) demonstrated experimentally that the reduction of pollen resources constitutes a powerful signal that initiates the shift towards a broodless overwintering population — acting largely via the brood as an intermediary between external conditions and the colony's internal dynamics.

This reading allows an important point to be clarified: the available studies document primarily the moment at which the colony reduces and then stops brood rearing, rather than the cessation of the queen's egg laying in the strict sense. That is, they clearly show when and under what conditions the colony progressively becomes broodless, but do not directly measure the queen's laying rate. This dynamic may nonetheless be regarded as a good indirect indicator of the slowdown and subsequent cessation of laying, since the decline in brood very likely reflects a coordinated change in the colony's reproductive state.

The work of Mattila et al. (2001) points in the same direction by showing that this schedule also depends on internal factors. In their control colonies, the first winter bees appeared earlier than in requeened colonies, and the end of the brood was also shifted to a later date after requeening. Requeening thus did not merely alter a minor detail of the calendar: it markedly shifted the moment at which the colony began to produce the bulk of its overwintering population. This reinforces the idea that the queen does not stop laying according to a simple seasonal date, but within the context of a global change in the colony's state.

Temperature and photoperiod are part of the seasonal context in which this transition takes place, but the available data suggest that they do not constitute, in themselves, its primary trigger. Experimental studies show above all that temperature plays an important role in the resumption of brood rearing at the end of winter, while its role in the autumnal brood cessation appears more indirect and intertwined with pollen supply, brood dynamics, and the overall state of the colony (Knoll et al., 2020; DeGrandi-Hoffman et al., 2025).

Finally, colony demography contributes to this picture. A colony entering autumn changes in age structure, in the proportion of young and older workers, and in its overall social context. Quinlan and Grozinger (2024) emphasise that the autumnal physiology of workers cannot be fully explained by brood area alone. The winter state emerges from an overall context, not from an isolated variable.

The transition to winter bees therefore results from a global change in the colony's state. The decline of the brood and its pheromonal signal is the best-supported element; the reduction of pollen supply appears to play a major role in driving this decline; the season as a whole — temperature, photoperiod, demography — forms the context in which these signals operate. The most accurate formulation is therefore not to seek "the" trigger, but to recognise that winter bees appear when several signals converge and the colony progressively ceases to prioritise expansion in order to enter a survival logic.

5. Why does this biology matter so much at the apiary?

The practical relevance of this biology is straightforward:

at the end of summer, the challenge is not simply to have large numbers of bees in the hive, but to produce an autumn generation capable of living a long time, maintaining the winter cluster, and restarting the colony in spring. Döke et al. (2015) recall that overwintering success depends on colony strength, available food stores, queen quality, and the pressure exerted by varroa, viruses, and other stressors.

This is where the work of Dainat et al. (2012) delivers a particularly important message. In their monitoring of colonies in Switzerland, workers from colonies that would die during winter already showed shorter life expectancy in late autumn, higher infestations with Varroa destructor, and higher DWV loads. In their multivariate analysis, only DWV and V. destructor were significantly associated with a reduction in the lifespan of winter bees. Varroa and DWV do not merely reduce colony size — they degrade the biological quality of the overwintering generation itself.

The mechanism is now better understood. Kunc et al. (2022) show, through a multi-omics analysis, that workers parasitised by V. destructor during the winter bee production period display extensive metabolic disturbances and depleted reserves — precisely the traits on which winter longevity depends. Parasitisation during the winter bee production phase is therefore doubly detrimental: it weakens individuals at the very moment the colony needs robust ones.

Late-summer and autumn nutrition also matters, not only for the hive's food stores but for the physiology of the population entering overwintering. The most recent studies show that greater pollen diversity in autumn is directly associated with better body reserves and superior winter survival (Mainardi et al., 2025). The question is therefore not only "is the hive heavy enough?", but "in what physiological state are the autumn workers entering winter?"

Ricigliano et al. (2018) also recall that overwintering does not proceed in the same way everywhere. In their southern climate context, surviving colonies were not completely broodless and exterior activity continued during winter, with a potentially higher nutritional and immune cost. This study serves as a reminder that overwintering can take different forms depending on climatic conditions, and that the model must be adapted to local circumstances.

Understanding winter bees changes the way the period from July to October is read. At this time, the beekeeper is not only preparing for overwintering in the material sense; he or she is acting on the quality of the generation of bees that must survive several months, keep the colony alive, and enable the spring restart. This is why the pressure of V. destructor and viruses, the nutritional quality of the environment, the state of food stores, and queen quality must be considered together, not separately.

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See also:

• Winter survival of honey bee colonies
• Vitellogenin and the keys to the colony
• Preserving the bees' vitality capital
• Practical Guide 1.1: Varroa management concept
• August at the apiary
• A very social immunity

 

References

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