1. Key points at a glance

• The winter varroa treatment requires a colony with no capped brood, or nearly none.
• Milder late seasons can make this window less predictable.
• Late brood can shelter part of the varroa mites and weaken the future winter bees when parasite pressure remains high.
• Queen caging is a relevant option when the natural brood break no longer occurs, but it does not thereby become a routine.
• At the apiary, the issue is to check the brood, the varroa level, the colony strength and the Swiss legal framework before acting.

2. What the study shows

This chapter takes the CARI document as a practical signal: if the colony keeps brood through winter, the classical logic of the winter varroa treatment becomes less reliable.

Question. The CARI document raises a now very topical question: what becomes of the winter treatment against Varroa destructor when colonies retain brood late in the season? The text starts from a field observation: milder late seasons can extend laying, maintain open or capped brood, increase the consumption of food stores and offer varroa a longer reproductive period (CARI, 2019).

Method. The document analysed is not an original scientific study with a complete experimental protocol. It is a technical extension article published in ActuAPI. It describes the experience of Italian beekeepers facing colonies that no longer clearly stop laying in autumn, then presents queen caging as a way to artificially induce a broodless period. For this reason, it should be read as a practical starting point, not as sufficient evidence. The present synthesis sets it against peer-reviewed studies on four neighbouring questions: efficacy of oxalic acid depending on brood status, influence of temperatures on laying, consequences of varroa on winter bees, and methods of artificial brood interruption.

Results. The most robust message does not concern the cage itself in the first place, but the presence of capped brood. Oxalic acid acts mainly on varroa mites present on adult bees; in periods of abundant brood, a large share of the mites is in the capped cells, where oxalic acid does not reach them. A recent systematic review places the efficacy of oxalic acid at 24–50 % in the presence of brood, against 95–99 % in broodless colonies (Kosch et al., 2024). Even more telling for the winter context: Toufailia & Ratnieks (2018), in colonies in a temperate climate, show that a patch of capped brood of barely 500–600 cells is enough to harbour about 14 % of the varroa mites and to substantially reduce the duration of protection provided by a winter treatment. This figure should not be turned into a universal threshold, but it makes the problem very concrete: even a limited amount of winter brood weighs on the efficacy of the treatment.

Interpretation. Queen caging is therefore not a varroa treatment in itself. It is a technique for creating a biological window favourable to the treatment, when the colony does not become broodless spontaneously. The main issue is not to introduce a new routine, but to verify whether the biological condition of the winter treatment — the absence of capped brood — is still met. If it is not, queen caging, total brood removal or other forms of brood interruption become relevant options, to be chosen according to the apiary, the season, the colony and the preparations authorised in Switzerland.

3. Critical view

This chapter distinguishes the real problem — the possible loss of the broodless window — from an overly simple response that would consist in systematically caging queens.

Strengths. The CARI document deserves credit for having identified early a difficulty that many beekeepers now observe: some colonies continue to rear brood late in the season when autumn remains mild. This can increase the consumption of food stores, prolong varroa reproduction and complicate the success of the winter treatment. The text also draws attention to an essential point: a bee emerging late is not automatically a good winter bee if it has been reared under high varroa pressure or in a colony still engaged in a prolonged brood-rearing effort.

Limits. The document remains, however, a technical article with a promotional dimension around a particular cage model. It does not provide all the information needed to evaluate the results scientifically: distribution of colonies, initial infestation level, full protocol, success criteria, queen losses, climatic context, detailed statistics. It should therefore not be used alone to recommend a wintering method.

Possible biases and confounders. The risk would be to turn a real problem into a single answer: «there is brood in winter, therefore you must cage». This reasoning would be too quick. In many Swiss apiaries, especially at altitude or with lines that naturally reduce laying, a broodless period still exists. Conversely, in lowlands, in warmer areas, in very dense apiaries or during prolonged late seasons, the situation can be different. The right reasoning therefore starts from the actual apiary: presence or absence of capped brood, infestation level, weather, colony strength, age and quality of the queen, food stores, history of summer treatment and re-infestation pressure.

An additional uncertainty concerns the queen signal. A caged queen remains present in the colony, but her presence may not be strictly equivalent to that of a freely moving queen in a natural brood pause. Because queen pheromones are largely transmitted through contact with workers and combs, the type of cage and the quality of contact with the winter cluster could influence colony cohesion (Naumann et al., 1991; Richardson et al., 2024). The available data do not show systematic long-term colony disorganization after queen caging, but this point remains poorly studied for prolonged autumn or winter caging.

What cannot be concluded. One cannot conclude that winter caging must become a routine. Nor can one conclude that a colony with late laying is necessarily doomed. By contrast, the literature strongly supports three points: oxalic acid is markedly more reliable when varroa mites are no longer protected in the capped brood; mild periods can alter brood dynamics; and varroa, when it reaches the generation of winter bees, directly compromises the survival of the colony.

4. What neighbouring studies show

This chapter situates the question within five neighbouring fields of research: efficacy of oxalic acid, effect of temperatures, varroa and the fat body, quality of winter bees, and artificial brood interruptions.

1. Oxalic acid and broodless period. Several studies converge on the fact that the efficacy of oxalic acid depends strongly on brood status. Gregorc & Planinc (2001) and Gregorc (2005) showed as early as the 2000s that the acaricidal effect is markedly better when varroa mites are accessible on adult bees. Coffey & Breen (2016), in a cool temperate climate comparable to that of Switzerland, confirm the value of the winter treatment in colonies that are broodless or nearly so, with efficacies of 90 to 99 %. The systematic review by Kosch et al. (2024) clarifies the orders of magnitude: 24–50 % in the presence of brood, against 95–99 % in broodless colonies. These figures should be understood as results of precise protocols, not as an automatic guarantee at the apiary, but they confirm that the broodless window is not a detail: it is the determining factor for the efficacy of the treatment.

2. How varroa reaches the bees’ fat body. A recent finding has clarified the pathology of varroa. Contrary to long-held belief, the mite does not feed primarily on haemolymph but on the bees’ fat body (Ramsey et al., 2019). It is precisely in this tissue that vitellogenin is synthesised and stored, a glycoprotein key to longevity, immunity and the production of royal jelly. This mechanistic finding directly links varroa pressure during brood rearing to the physiological quality of the future winter bees.

3. Varroa and the robustness of winter bees. Winter bees are not simply older workers: their longevity depends on body reserves, a particular metabolism, vitellogenin and a functional fat body. Amdam et al. (2004) showed that bees parasitised by varroa at the pupal stage do not fully develop the physiological characteristics expected of winter bees. Dainat et al. (2011) link varroa and the deformed wing virus to a direct reduction in the life span of winter bees. Van Dooremalen et al. (2012) show that the overwintering of colonies depends strongly on the infestation level during the transition to winter bees. Kunc et al. (2022) confirm, through a molecular approach, that parasitism disrupts the bees’ reserves and metabolism before overwintering. A study conducted in Switzerland highlights that compliance with varroa treatment recommendations is associated with better overwintering (Hernandez et al., 2021), which directly anchors these results in the context of Swiss apiaries. The practical conclusion is strong: killing varroa mites in December does not repair winter bees already weakened during their development.

4. Milder temperatures, laying and varroa. Work on colony phenology shows that temperature strongly influences the onset or maintenance of brood. Nürnberger et al. (2018) show that temperature plays a major role in the start of brood rearing at the end of winter. Villagómez et al. (2021) confirm that temperature and photoperiod interact in the seasonal calendar of colonies. Smoliński et al. (2021) link higher autumn temperatures to a reinforced varroa infestation in autumn. Rajagopalan et al. (2024) model, in a context of warming, a shortening of the stable winter phase and increased risks for overwintering. These results do not mean that all colonies will have brood throughout winter, but they make plausible a loss of reliability of the natural broodless window, especially in lowlands and in mild years.

5. Re-infestation and late brood. The presence of late brood does not concern only the varroa mites already present in the colony. Giacobino et al. (2023) show, in a temperate climate, that varroa re-invasion in autumn and winter must be taken seriously. In a dense apiary, with drift, discreet robbing or heavily infested neighbouring colonies, a properly treated colony can take in new varroa mites. If capped brood remains available, these mites then find an opportunity to reproduce again. This dynamic explains why the winter treatment must not be considered in isolation, but as the final stage of a varroa management concept followed throughout the year.

6. Artificial brood interruption: a relevant, context-dependent option. Studies on queen caging, total brood removal or other forms of brood interruption show that these methods can substantially improve varroa control when they are well synchronised with an appropriate treatment: a large European study on 370 colonies in 10 countries reports efficacies after caging ranging from 48 % to nearly 90 % depending on the mode of oxalic acid application (Büchler et al., 2020), and several reviews place the best combinations at 95–97 % (Gregorc et al., 2017; Gregorc & Sampson, 2019). The strongest data, however, concern summer caging; for autumn-winter use, which corresponds more directly to the Swiss case, a recent Polish study with an isolator extended over 5 to 6 months reports encouraging results, with no excess queen mortality and even superior spring development (Gąbka et al., 2025). This avenue still needs to be confirmed in contexts closer to Swiss apiaries, but the biological logic is consistent with what is known elsewhere. Caging is therefore a relevant option when conditions justify it — brood that does not naturally stop, high varroa pressure, at-risk colonies — but not an automatic answer to every colony still rearing late in the season.

5. What to take away at the apiary?

This chapter translates the results into cautious benchmarks for apiaries in Switzerland and temperate Europe, where the situation depends strongly on altitude, weather and the behaviour of each colony.

• Do not treat «blindly». The winter treatment remains relevant, but its efficacy depends on brood status. If capped brood is present, a significant share of varroa mites can escape the treatment.
• Above all, protect the winter bees. A late correction in December does not compensate for excessive varroa pressure during the formation of winter bees in late summer and autumn.
• Identify at-risk situations. Lowlands, mild late seasons, late forage, high apiary density, re-infestation and colonies that keep brood late deserve particular attention.
• Consider caging as a targeted option. It can help to recreate a broodless window, but it requires finding the queen, working on strong and healthy colonies, having quality queens, and accepting a risk of loss or poor recovery.
• Insulate without «heating». Winter insulation can be useful against humidity, draughts and abrupt fluctuations, but it should not aim at permanent warmth. Studies show that temperature influences the brood calendar and that artificial heating can increase brood rearing under certain conditions (Nürnberger et al., 2018; Çakmak et al., 2023). The objective therefore remains to protect the colony without unnecessarily stimulating laying.
• Check the Swiss framework. Oxalic acid preparations, their modes of application, dosages and periods of use must comply with the authorisations and package inserts in force in Switzerland.

Read the original study: Hiverner sans couvain

The source document is a technical article, not an original scientific study: CARI. (2019). Hiverner sans couvain. ActuAPI, 76, 1-8. It is useful as a practical starting point, but the scientific conclusions must be supported by the studies cited below.

Further reading on ApiSavoir

• Practical Guide 1.1: Varroa management concept
• Practical Guide: 1.3.3. Oxalic acid vaporisation
• Practical Guide: 1.6.1 Brood break
• Varroa: brood interruption
• Winter bees: why summer decides spring
• Veterinary medicines / preparations authorised in Switzerland

 

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