Correct answer: 3 — about -7 °C.
Why?
When the temperature drops, the bees do not simply become immobile: they organise into a winter cluster to limit heat losses and maintain a microclimate compatible with the colony's survival. This cluster forms gradually, moves slowly across the stores and changes its density according to need.
At around 15 °C, the bees already begin to draw closer together. As the cold intensifies, the cluster contracts further: its exposed surface decreases and the peripheral bees form a more compact layer. At around -7 °C, it is considered that practically all the bees of the colony are integrated into this collective organisation.
This figure should, however, be understood as an indicative benchmark. It can vary depending on colony strength, hive insulation, the possible presence of brood, the available stores and the weather conditions.
What 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 muscular activity. The peripheral bees limit heat losses, while the cluster as a whole adapts to the outside temperature and the position of the stores.
What to remember
The colder it is, the more tightly the cluster contracts. For the beekeeper, the essential point is not to "heat" the hive at all costs, but to ensure a sufficiently strong colony, accessible stores, good protection against moisture and as few disturbances as possible.