0. Introduction: should a colony really be "reduced"?

Understanding reducing as the management of space rather than as a method simply intended to cut down a volume of air to be heated.

Reducing colonies is one of the classic operations of beekeeping. Depending on the hive and the method, it may consist in removing combs that remain permanently unoccupied, placing a lateral division board, running a young colony on a limited number of combs, or gradually reducing the space after the harvest.

The commonest justification seems obvious: the colony would thereby have "less volume to heat".

This explanation is nevertheless misleading. A colony does not work like a radiator placed in a room. The bees do not seek to bring all the air in the hive to a uniform temperature. They regulate above all the brood, their own body temperature and, in winter, the cluster. A large part of the space outside these zones can remain markedly colder.

The practical question is therefore a different one:

does the available space correspond to what this colony can currently occupy, supply, defend and incorporate into the organisation of its nest?

From this perspective, reducing is first and foremost a tool for managing space. It can be useful when a small colony genuinely uses only part of the hive. It becomes questionable when it is applied automatically to every colony, or maintained while population, brood and incoming forage call for more room.

The scientific literature is solid on the mechanisms of thermoregulation. It is, by contrast, surprisingly thin when it comes to testing directly a practice that is nevertheless very widespread: the lateral division board itself.

1. Bees do not heat the whole hive

Distinguishing the biologically regulated zone from the geometric volume of the hive. Image: Victor Berthelsdorf, thermography.

The brood nest is the most precisely thermoregulated zone of the colony. Workers actively produce heat with their thoracic muscles and can transfer it directly to the combs and the brood. They also alter their position and their density according to need (Stabentheiner et al., 2010, 2021).

In winter the principle remains the same, but the organisation changes. The colony forms a cluster whose periphery becomes denser as the cold intensifies, while bees on the inside actively produce heat (Stabentheiner et al., 2003).

In both situations, the air in the hive is not the variable principally regulated. It takes part in the transfer of heat and water vapour, but it can show substantial differences in temperature between the nest, the outer combs and the walls.

This does not mean that the space surrounding the colony is unimportant. A larger cavity alters the exchange surfaces, the possible paths of air circulation, the position of the stores and the general geometry of the nest. But its thermal cost is not simply proportional to the number of litres of air it contains.

2. Natural cavity and modern hive: volume matters, but there is no ideal volume

Using natural nests as a source of hypotheses without turning an observed preference into a mandatory dimension for hives.

Bees take volume into account when they select a cavity. Seeley's classic experiments showed in particular that swarms preferred a cavity of around 40 litres to experimental cavities of 10 or 100 litres (Seeley, 1977).

Observations of natural nests likewise frequently describe cavities of a few dozen litres, but with considerable variability (Seeley & Morse, 1976).

Very recent German data point in the same direction without defining a universal optimum. Among twenty black woodpecker cavities monitored over several years, those actually chosen by swarms had a median volume of about 41 litres, against about 19 litres for the cavities not chosen (Rutschmann & Kohl, 2026).

It would nevertheless be incorrect to infer from this that "the bee wants a 40-litre hive". Other populations have accepted or preferred different volumes. Rinderer et al. (1982), for example, showed different ranges of choice between European bees in Louisiana and Africanized bees in Venezuela.

The preferred volume therefore depends on the population and the context. It does not constitute a standard dimension of the species.

Above all, a natural cavity and a modern hive are not equivalent.

In a tree, the swarm arrives in an empty space and builds its combs progressively. In a hive, the beekeeper already provides an architecture: frames, drawn comb or starter strips, floor, crown board, entrance and possibly a super or a division board.

Tree cavities can also have much thicker walls and a different geometry from those of a conventional hive. Models show that these characteristics can strongly alter heat transfer and air movements (Mitchell, 2024).

The lesson from natural nests is therefore not "reduce every hive to 40 litres". It is rather:

volume and geometry form part of the colony's environment, but their effect depends on how the bees can actually use that space.

3. What science says — and does not say — about division boards

Separating the biological plausibility of reducing from the experimental evidence concerning an ordinary or strongly insulating division board.

Colony strength, accessible stores and health status are factors far more solidly associated with successful overwintering than the use of any particular division board. Colony size and pre-winter weight are notably important predictors of the outcome of overwintering (Döke et al., 2019).

The experimental literature on the division board itself, by contrast, is very limited.

No robust field study identified to date demonstrates that a conventional lateral division board on its own improves brood temperature, reduces winter consumption or accelerates spring development under conditions comparable to those of Switzerland.

This does not mean that it is useless.

In a small colony, keeping bees, brood and stores within a coherent zone can:

• maintain a good density of bees around the brood;
• shorten the distances between brood and food;
• avoid leaving numerous combs completely unoccupied for long periods;
• make a small nest easier to monitor and to defend.

These mechanisms are biologically plausible. They do not, however, demonstrate that a division board necessarily has a measurable thermal benefit.

And a strongly insulating division board?

A division board with high thermal resistance can physically reduce heat transfer across the lateral boundary of the occupied nest. But its effect depends on its actual construction.

If air circulates freely above, below or around the division board, it does not form a complete thermal boundary. The interior geometry and the convection paths can strongly alter the outcome, as heat transfer models of hives and cavities show (Mitchell, 2024).

To date, no controlled trial identified compares directly, on comparable colonies, a strongly insulating division board, an ordinary division board and the absence of a division board.

The biological superiority of a "Thermoschied" or of a "high-performance" division board therefore remains to be demonstrated.

Insulation on top is not an absolute experimental truth either

The trial by St. Clair et al. (2022) provides an interesting indication: colonies fitted with a winter protection system lost less mass and suffered less mortality over the winter studied.

But the set-up combined several elements: lateral wrapping, insulation on top and an upper opening. It does not allow the conclusion that roof insulation alone was responsible for the result.

A well-protected upper part remains a physically reasonable principle, in particular because a very cold surface above the cluster can increase heat losses and encourage condensation. But the literature does not allow a universal hierarchy to be established between roof, walls, floor and division board.

4. Managing space through the year

Adapting the space to the dynamics of the colony: reducing when the space is genuinely unused, expanding when growth or incoming forage require it.

The same volume can be appropriate in February and become too small a few weeks later. Reducing cannot therefore be separated from the annual cycle.

In spring: support growth, then release it

A small colony resuming development must maintain a warm brood zone with a still limited population. In this situation, removing combs that remain permanently unoccupied or using a division board can help to maintain a compact nest that is easy to monitor.

This plausibility must not, however, be turned into a thermal rule. We have no trials showing that a given number of combs directly accelerates development through a saving in heating.

As soon as the colony strongly occupies its space and the brood expands, enlargement becomes necessary. A division board that improves the organisation of a small colony can become pointless a few weeks later.

A space kept artificially too narrow can complicate the storage of pollen and nectar and the management of the nest. The precise link with swarming has not been isolated experimentally for thermal division boards; it is therefore best to avoid treating this as an automatic relationship.

During the flow: the problem becomes lack of room

A strong colony needs room to receive, distribute and concentrate nectar. The working volume required can increase very rapidly.

Management therefore does not consist in maintaining a small volume permanently, but in adapting the space to the population and to the incoming forage.

A good practical principle is to add space when the colony genuinely needs it, rather than applying a constant volume throughout the season.

In summer: "smaller" is not automatically "cooler"

When heat becomes the main constraint, the bees move part of the population, ventilate, carry water and may form a beard on the outside.

A reduced space is not then necessarily favourable.

Bourrel et al. (2025), in a study conducted in a semi-arid climate, observed that colonies housed in smaller units were more exposed to certain lateral thermal extremes than colonies in standard Langstroth hives.

This study did not test a division board and cannot be transposed quantitatively to the Swiss climate. It is nevertheless enough to serve as a reminder that reducing the volume does not automatically improve the microclimate in every season.

After the harvest: remove what no longer has a function

Once the supers have been taken off, the colony no longer has the same storage requirements. This is a good moment to assess the combs in the brood box.

Combs that are genuinely unused can be removed when they contribute neither necessary stores nor any function in the organisation of the nest. This reduction often makes it easier to monitor stores, to feed and to manage colony health.

Reducing at the expense of food must nevertheless be avoided. The winter stores must remain sufficient and be arranged so that they can be reached by the cluster.

In autumn and winter: prepare beforehand, disturb little afterwards

Going into winter, the priority is a sufficiently strong colony, properly treated against varroa, with sufficient and accessible stores.

The number of combs is only one indicator among others. A small colony does not become safe simply because it has been heavily reduced.

Once the winter cluster is organised, major internal modifications should be avoided when they are not necessary. The colony must be able to move towards its stores without the beekeeper continually rearranging the combs.

The case of young colonies

It is probably with artificial swarms, nuclei and other small units that progressive adaptation of the space is most intuitive.

A young colony generally gains nothing from immediately receiving numerous combs that it can neither cover nor use. It can be run in a volume better matched to its population and then enlarged progressively.

The aim is not to keep the colony as tight as possible, but to support its growth.

5. Five questions to ask before reducing

Deciding from the actual colony rather than from a fixed number of combs.

6. Conclusion

Remembering that reducing is a tool for adapting space, not a universal thermal rule.

The debate about reducing becomes far simpler as soon as one abandons the image of a hive regarded as a room to be heated.

Bees regulate above all the brood, the cluster and their own temperature. The volume around them matters because it alters the geometry of the nest, the exchange surfaces, the position of the stores and the possibilities for organisation — not because every litre of air would have to be kept warm.

Reducing can therefore be pertinent when the colony uses only part of the space offered to it, particularly in a young or small colony. It becomes pointless when the bees already occupy the space effectively, and it can become restrictive if growth or the nectar flow calls for more room.

As for the insulating division board, its physical plausibility is real, but its specific biological advantage over an ordinary division board has not yet been demonstrated by a controlled trial on colonies.

The most defensible approach therefore consists in observing rather than counting:

what space can this colony actually use today — and what space will it need tomorrow?

Reducing is neither a miracle recipe nor an error of principle. It is a tool. Its value depends on the reason for which it is used.

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See also:

• All about thermoregulation
• Hive insulation put to the test of the bees' collective thermoregulation
• The volume of the brood
• The different types of hive
• Successful overwintering
• The winter cluster

Bibliography

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