Climate Change May Be Putting Honey Bee Colonies at Greater Risk of Winter Failure

Introduction

In summer, worker bees typically live for about six weeks. In winter, however, they need to survive for six months or more. A colony therefore depends on producing enough healthy winter bees to carry it through the cold months. Yet winter bees are the daughters of the same queen as summer bees, so what makes them so different?

The transition from summer bees to winter bees is influenced by several internal and external signals, notably :

• shorter periods of daylight;
• cooler temperatures;
• fewer incoming resources, especially pollen;
• reduced brood rearing;
• changes in colony pheromonal signals (Smedal et al. 2009¹).

How Do These Signals Produce Longer-Lived Bees?

Together, these environmental and colony-level signals trigger hormonal changes through the bee’s endocrine system. In the adult honey bee worker, the system consists of three key components:

• In response to external and internal stimuli, the neurosecretory cells in the brain produce signalling molecules, or neurohormones, which are sent to the corpora cardiaca.
• The corpora cardiaca, a pair of glands which lie behind the brain near the aorta, store these neurohormones and release them into the haemolymph.
• The nearby corpora allata synthesise and secrete juvenile hormone (JH), which helps regulate physiology, development and behaviour.

Stimuli → Neurosecretory cells → Neurohormones → Corpora cardiaca → Haemolymph → Corpora allata → Juvenile hormone (JH) | Enlarged fat body → Vitellogenin (Vg) → Haemolymph → Physiology, development and behaviour

Vitellogenin (Vg), a reproductive glycolipoprotein made mainly by the worker’s fat body, works in opposition to JH. Vitellogenin promotes longevity, whereas JH accelerates ageing and the relationship between JH and Vg forms an important feedback loop:

• When JH levels in the haemolymph exceed Vg levels, the worker progresses from nurse bee to forager.
• When Vg levels are higher than JH levels, this progression slows or may stop altogether, and the worker remains in the nurse-bee state for longer (Münch et al. 2015²).

Winter bees contain much higher levels of Vg in their fat bodies than summer bees. The most widely accepted working model is therefore that the winter bee represents an extended nurse state. With little or no brood to feed, workers retain their protein reserves instead of using them to produce brood food, and they do not make the hormonal switch into foraging.

Where Did I Go Wrong?

For years, I believed that feeding pollen supplement from late July onwards was the best way to produce large numbers of winter bees. I assumed that big colonies with plenty of brood would sail through winter and emerge strongly the following spring. When some of the strongest colonies dwindled in November or failed altogether, I struggled to understand why.

I blamed the usual suspects: weather that was too cold or not cold enough; conditions that were too wet or too dry; Nosema; Varroa; and a neighbour’s field of sunflowers that may not have provided enough pollen diversity.

I later discovered that studies published as early as 2007 found that winter bees need a good quality, diverse supply of pollen in spring, and that a dearth of pollen in late summer or early autumn is an important trigger for the transition into winter-bee production (Mattila and Otis 2007³).

Other more recent studies showed that pollen supplements are consumed by older bees rather than by the nurse bees that need protein to feed larvae. As a result, nurse bees have to draw down their own precious reserves and, in some cases, cannibalise larvae to make up for protein deficiency (Kleckner et al. 2026⁴).

In effect, by feeding pollen supplement at that point in the season, I may have stimulated further brood production, but I was feeding the foragers rather than the nurses and stimulated the colony to produce summer bees that died in October or November.

If only I’d read the science much earlier in my beekeeping journey!

Why Climate Change Raises the Stakes

That brings me back to the question of climate change. It is the end of September here on the south coast of England and temperatures are still reaching the mid-twenties. There was a shortage of pollen in August and early September, which may have stimulated the shift towards winter-bee production.

Now, however, nectar and pollen are relatively abundant again. My concern is that warm weather, incoming pollen, and continued brood rearing could cause colonies to switch back towards producing summer bees rather than consolidating into a long-lived winter population.

Shortening days and cooler nights may help, but I am not convinced they are enough. The evidence suggests that a prolonged warm autumn can indirectly alter the timing and physiological development of the winter-bee population by extending brood rearing, nursing and foraging activity, and by delaying the onset of winter brood (Frunze et al. 2024⁵).

Varroa: A Double Whammy

Extended brood rearing also extends the period during which Varroa mites can reproduce. As the bee population begins to decline, Varroa numbers may continue to rise exponentially until the colony is weakened or overwhelmed by mites and the viruses they vector, especially Deformed Wing Virus. This can create a “doom loop” that increases winter losses (Smoliński et al. 2021; Rajagopalan et al. 2024⁶). And the bigger the colony, the greater the risk (Scientific Beekeeping 2017⁷).

Moreover, warmer winters mean there is no natural brood break and varroa continue to multiply in brood cells, damaging the pupae shortening the lives of the bees required to contribute to the spring buildup.

What About the Queen?

As days shorten and temperatures fall, the queen’s ovaries reduce in size. She cuts down laying or even stops during winter and resumes in late winter or early spring. However, this process differs from the seasonal changes in worker physiology.

In queens, JH is associated with reproductive activity rather than age-related division of labour. Egg laying appears to be controlled by nutrition-sensitive checkpoints during egg development, not by pheromones alone. The likely cues are reduced fresh pollen and shorter days.

In practical terms, the queen’s ovaries follow the food the workers provide, so her winter pause is largely driven by her colony (Aamidor et al. 2022⁸).

Conclusion

The key lesson is that successful overwintering depends not simply on the number of bees in the hive, but on the type of bees the colony produces before winter. True winter bees are physiologically distinct: they retain higher protein reserves in enlarged fat bodies, remain in a nurse-like state for longer, and are equipped to survive for months rather than weeks. Anything that prolongs late-season brood rearing may therefore weaken the colony’s preparation for winter, even if the hive appears populous in autumn.

Climate change may make this balance harder to manage. Warmer autumns, renewed pollen flows and extended foraging can blur the seasonal signals that normally help colonies shift into winter mode. At the same time, prolonged brood rearing gives Varroa more opportunities to reproduce, increasing the risk that colonies enter winter with both fewer long-lived bees and a heavier mite burden.

For beekeepers, the practical conclusion is clear: late-season management should aim to support the colony’s natural transition into a durable winter population, rather than simply stimulating more brood. Ensuring adequate spring nutrition, avoiding unnecessary autumn stimulation, and bringing Varroa under control before the bee population contracts may become increasingly important as autumn weather becomes less predictable.

Further reading: The Beelistener – Ann Chilcott’s beekeeping site, with articles and research summaries on bee health, varroa and winter survival.

References:

1. Smedal, B., Brynem, M., Kreibich, C. & Amdam, G.V. (2009).
2. Daniel Münch, Kate E. Ihle, Heli Salmela, 2015
3. Mattila, H.R. & Otis, G.W. (2007). Ecol. Entomol. 32: 496–505.
4. Kaylin Kleckner, Emily R. Noordyke, Cody Prouty and James D. Ellis
5. Olga Frunze, Yumi Yun, Hyunjee Kim, Ravil R. Garafutdinov, Young-Eun Na, Hyung-Wook Kwon. Published: December 9, 2024
6. Smoliński, S., Langowska, A. & Glazaczow, A.. Sci Rep 11, 22256 (2021).
7. scientificbeekeeping.com/the-varroa-problem-part-11/
8. Aamidor et al. (2022). Journal of Insect Physiology 136: 104347,