Correct answer: 3 — about -7 °C.
Why?
When the temperature drops, the bees do not simply become motionless: they organize themselves into a winter cluster to limit heat losses and maintain a microclimate compatible with the survival of the colony. This cluster forms progressively, moves slowly over the stores, and changes its density according to need.
At around 15 °C, the bees already begin to draw closer together. When the cold intensifies, the cluster contracts further: its exposed surface decreases and the peripheral bees form a more compact layer. At around -7 °C, practically all the bees of the colony are considered to be integrated into this collective organization.
This figure should nevertheless be understood as an indicative benchmark. It may vary according to the strength of the colony, the insulation of the hive, the possible presence of brood, the available stores, and the weather conditions.
What you need to understand
The cluster is not merely a compact mass of bees. The bees located at the centre actively contribute to heat production, in particular through muscle activity. The peripheral bees limit thermal losses, while the cluster as a whole adapts to the outside temperature and to the position of the stores.
Key takeaways
The colder it is, the tighter the cluster draws together. For the beekeeper, the essential thing is not to "heat" the hive at all costs, but to ensure a sufficiently strong colony, accessible stores, good protection against moisture, and as little disturbance as possible.