1. Why look for an alternative to formic acid?

Understanding in which situations a brood break followed by oxalic acid treatment can replace the first summer treatment with formic acid.

The aim of summer treatment is to reduce the varroa population early enough so that winter bees can be reared under the best possible conditions. In Switzerland, formic acid traditionally plays an important role in this strategy, particularly because it can also reach some of the mites present in capped brood (Service sanitaire apicole, 2025).

However, its use is not always easy to plan. Evaporation depends in particular on temperature, humidity, the device used, hive volume and colony strength. A period that is too warm can increase the risks to brood and queen, while conditions that are too cool or too humid can reduce the effectiveness of the treatment. These difficulties become particularly acute when the first summer treatment coincides with a heatwave (Droz et al., 2015).

Brood break followed by oxalic acid treatment offers a different approach. It creates a period without capped brood during which varroa mites are found mainly on adult bees and thus become accessible to oxalic acid.

However, this strategy is neither superior nor simpler in every case. It reduces dependence on climatic conditions, in particular the risk of excessive evaporation of formic acid during hot weather (higher than 30 °C). In return, it requires precise planning, an effective absence of capped brood and, depending on the method chosen, several interventions in the colony. It therefore presents a different risk profile rather than a general reduction in risk.

After a brood interruption, however, oxalic acid treatment must be carried out in a colony that is truly broodless. In studies, trickling and sublimation showed similar efficacy after artificial broodlessness; a European field study found an efficacy of 89.57% when trickling a 4.2% solution, and 88.25% when sublimating 2 g of oxalic acid. In direct comparisons between methods, sublimation achieved very high efficacy in broodless colonies while placing less stress on the bees, whereas spraying was also effective but more labour-intensive and more invasive. In practice, however, the decisive factors are always the authorised products, the package leaflet, the brood status, the absence of honey supers and application outside periods of bee flight.

In the Swiss annual varroa control concept, it is above all an alternative to the first summer treatment with formic acid. It replaces neither infestation monitoring nor, automatically, the second summer treatment or the winter treatment (SSA, 2025).

2. The biological principle: making varroa mites accessible

Understanding why an effective absence of capped brood determines the effectiveness of oxalic acid treatment.

The brood break does not directly kill varroa mites. It temporarily interrupts brood production in order to create a period during which the mites can no longer reproduce in capped cells.

A founding female mite enters a cell shortly before it is capped and remains protected there with her offspring until the bee emerges. During this phase, she remains largely out of reach of oxalic acid, which acts mainly on mites present on adult bees.

After the interruption of laying, the brood already present must therefore have fully emerged before treatment. The biological coherence of the method relies on this sequence: brood break, effective absence of capped brood, then oxalic acid treatment (Büchler et al., 2020).

The schedule must take into account the brood stage at the start of the intervention and its development time, around 21 days for workers and 24 days for drones. Before oxalic acid treatment, the absence of capped brood must be verified directly in the colony. After 21 days of caging, the few cells that are still capped, often drone brood, must be uncapped or removed. Applying the treatment too early would leave some mites still protected in cells, whereas applying it too late could coincide with the capping of new brood after the queen's release.

3. What the studies show

Presenting the documented effectiveness of the brood break while specifying the limitations and differences between the protocols studied.

Available studies indicate that summer brood interruption can contribute effectively to varroa control. A large European study involving 370 colonies at eleven sites in ten countries shows that several biotechnical methods significantly reduce infestation when applied according to a rigorous protocol (Büchler et al., 2020).

There is, however, no single efficacy rate for "brood break". In the groups combining queen caging with oxalic acid treatment, efficacy varied from around 48 to 90 % depending on the application method. The best results reached 89.6 % with trickling of a 4.2 % solution and, on average, 88.3 % with vaporisation of 2 g of oxalic acid during the broodless period (Büchler et al., 2020).

Methods based on brood removal or the use of trap combs achieved comparable efficacy. In this European study, their efficacy did not differ significantly from that of the groups combining queen caging with oxalic acid. Several procedures can therefore interrupt the reproductive cycle of varroa, but their practical requirements differ.

The comparison with formic acid should remain cautious. Formic acid has the advantage of being able to reach some varroa mites within capped brood. Brood break followed by oxalic acid forgoes this effect in the brood, but deliberately creates a situation in which mites become accessible on adult bees. The first method depends more on diffusion and temperature conditions, while the second depends on achieving a window genuinely free of capped brood.

The results regarding the subsequent development of colonies are encouraging but vary according to season, climate and protocol. Several summer studies did not observe lasting weakening of correctly managed colonies (Büchler et al., 2020; Kovačić et al., 2023). 

The timing of caging in relation to the main nectar flow is also decisive for the harvest. In a study conducted in six Mediterranean countries, caging did not cause a significant negative effect on the final colony strength, but caging too early relative to the main flow reduced the amount of honey harvested (Kovačić et al., 2023).

Finally, a substantial reduction in the number of varroa mites does not necessarily cause an immediate drop in viral load. In a study conducted in Slovenia and Italy, loads of Deformed Wing Virus and Acute Bee Paralysis Virus had not decreased significantly 25 days after the interventions, despite high acaricidal efficacy (Bubnič et al., 2024). Bees already damaged do not regain their full vitality. The intervention must therefore take place before the main phase of winter bee rearing.

4. Putting it into practice: which method to choose?

Comparing the main methods for creating a broodless period and identifying the one best suited to each operation.

A brood break can be achieved using various methods. In most protocols based on queen caging, it takes about three weeks for the existing brood to fully emerge. The exact duration, however, depends on the procedure used and on what happens to the new brood.

The essential rule is simple: at the time of oxalic acid treatment, the colony must no longer contain any capped brood. This absence must be verified, not merely inferred from the number of days elapsed.

The choice depends on the number of colonies, their strength, the ability to find the queens, the material and time available, and any additional objectives, such as comb renewal, requeening or nucleus formation.

4.1 Caging the queen in a Scalvini-type cage

The Scalvini cage is generally integrated into a frame placed in the centre of the brood nest.

Worker bees remain in contact with the queen. She can lay eggs in the cage's shallow cells, but the brood cannot develop normally there up to capping.

During caging, the brood present in the rest of the colony continues to develop. In the Swiss trials, the queen was released after 21 days. Any drone brood still capped must then be uncapped or removed before oxalic acid treatment (Droz et al., 2015).

 

 

Advantages:

• existing brood remains in the colony;
• the queen remains accessible to worker bees;
• the device takes up little space;
• the schedule can be planned precisely.

Limitations and risks:

• the queen must be found and handled;
• the cage must be correctly fitted and positioned;
• any drone brood must be checked;
• the queen may fly away or be injured during release;
• limited space for the queen compared with caging on a simplified trap comb; the Scalvini cage allows egg-laying but reduces the movement area and contact with the comb;
• her presence and the resumption of laying must then be checked.

The Scalvini cage should not be equated with a small introduction cage: it is designed to allow the queen to lay eggs while preventing the brood from being capped, owing to the shallow depth of the cells. It does, however, restrict movement and contact with the comb more than caging on a simplified trap comb. The spread of queen pheromones depends largely on contact with worker bees and with the wax; more extensive contact could therefore favour better distribution of these signals within the colony. This could be beneficial for tolerance of caging, but this link with queen losses or requeening events remains plausible rather than demonstrated.

Caution: Small introduction cages are not designed for prolonged caging over several weeks. They greatly reduce the available space, do not allow normal egg-laying and can increase stress for the queen. They must therefore not be confused with a Scalvini cage or with caging on a trap comb.

►Practical Guide: 1.6.1 Brood Break

 

4.2 Caging on a simplified trap comb

This variant consists of confining the queen on a frame fitted with a sheet of foundation or a wax starter strip, closed on both sides with queen excluders.

This is a simplified form of the trap comb method.

The frame must not contain any brood at the start of the intervention. The bees must first build out the comb before the queen can lay eggs in it. Already drawn comb would allow immediate egg-laying and reduce the available margin before the first bees emerge.

This form of caging gives the queen more space than a small cage and maintains extensive contact with worker bees. This point may be important, since queen pheromones are transmitted within the colony by attendant bees, through direct contact and also via the wax (Naumann et al., 1991). A study comparing cages with and without a comb section observed better queen survival when the cage contained a section of worker comb; no queens died there, whereas 8 % of queens died in cages without comb (Toomemaa & Kaart, 2022). This study, however, does not demonstrate that the effect is due to pheromones or to egg-laying. It rather suggests that a more natural confinement space can improve tolerance of caging.

While the colony's original brood comes to term, some of the varroa mites enter the new cells of the trap comb. This must be removed before the bees emerge, then destroyed or melted down together with the brood and the mites it contains. Transferring it to another colony would merely relocate the captured mites.

In the protocol studied by Büchler et al. (2020), the queen remained on the frame for 20 days. The capped comb was then removed and destroyed, and oxalic acid treatment took place five days later.

Advantages:

• combination of brood interruption and varroa trapping;
• more space for the queen and more extensive contact with worker bees;
• removal of some mites together with the capped comb;
• only one trap comb to manage
• potentially more favourable confinement conditions for queen survival than in a cage without comb, according to a comparative study.

Limitations and risks:

• schedule to be followed precisely;
• removal and destruction of the comb before any emergence;
• comb-building dependent on colony strength and resources;
• risk of premature emergence with already drawn comb;
• additional delay before oxalic acid treatment.

4.3 Classic trap comb method

In the classic method, the queen is confined successively on several combs.

After the queen is moved, each comb remains temporarily in the colony until the brood is capped.

The varroa mites attracted by the larvae are trapped in the cells, after which the combs are removed and melted down before the bees emerge. Protocols generally use three or four successive combs and extend over about 27 to 28 days (Büchler et al., 2020).

The method can be carried out without an acaricide if all trap combs are produced, removed and disposed of correctly. However, it requires rigorous organisation.

 

 

Advantages:

• fully biotechnical method possible;
• exploitation of varroa's reproductive behaviour;
• possibility of working without veterinary medicine during a late nectar flow;
• progressive removal of a significant proportion of mites.

Limitations and risks:

• several interventions according to a precise schedule;
• repeated handling of the queen;
• high workload;
• need for suitable combs;
• loss of effectiveness if removal occurs at the wrong time.

►Practical Guide: 1.6.2 Trap Comb Method

 

4.4 Total brood removal

Comparing the main methods for creating a broodless period and identifying the one best suited to each operation.

Total brood removal consists of taking out all combs containing brood. The production colony immediately becomes broodless and can be treated with oxalic acid according to an authorised protocol.

A large proportion of varroa mites is removed together with the frames. These must then be subject to a separate strategy: forming brood units, treatment after emergence, or disposal when their health status does not allow for brood utilisation.

A comb with open brood can temporarily be left as a trap comb. It must be removed once capped and before the bees emerge.

 

 

Advantages:

• immediate creation of a broodless situation;
• removal of a large proportion of varroa mites with the frames;
• possible comb renewal;
• possibility of forming nucleus colonies.

Limitations and risks:

• significant intervention in colony organisation;
• high requirements in terms of equipment, food and working time;
• demanding health management of the removed brood;
• risk of transferring varroa mites and possible pathogens to the new units;
• rebuilding must be supported with appropriate feeding.

► Practical Guide: 1.6.4 Total Brood Removal with Brood Utilisation

 

4.5 Requeening

Replacing the old queen can cause a temporary interruption in egg-laying, thereby integrating requeening into the varroa control strategy.

The duration of the break, however, depends on the method, the acceptance of the new queen and the onset of her laying. The broodless window is therefore less predictable than with planned caging and must be established directly within the colony.

This variant calls for particular caution. In a Swiss trial involving eleven colonies, introducing a new queen immediately after the caging period led to a high failure rate. This result does not justify condemning all requeening methods, but it shows that the introduction procedure must be chosen carefully (Droz et al., 2015).

Advantages:

• combination of requeening and varroa control;
• possible introduction of a selected young queen;
• no prolonged caging of the old queen if she is removed before the new queen is introduced.

Limitations and risks:

• broodless window less precisely controllable;
• risk of poor acceptance of the new queen;
• sometimes delayed resumption of laying;
• necessary inspection of queen cells and confirmation of the queen's presence.

4.6 Artificial swarm and nucleus formation

Comparing the main methods for creating a broodless period and identifying the one best suited to each operation.

Forming an artificial swarm separates the adult bees from the brood. The unit created is immediately broodless and can be treated before new cells are capped.

Forming nuclei or other young colonies can likewise create a favourable window after the transferred brood has emerged and before the new queen's first brood is capped.

These methods make it possible to combine varroa control, colony multiplication, queen renewal and comb sanitation. Each unit must, however, be monitored separately, since the varroa mites present in the transferred brood are not eliminated.

Advantages:

• combination of varroa control and colony multiplication;
• substantial comb renewal;
• treatment possible during a clearly broodless period;
• formation of nucleus colonies with reduced infestation.

Limitations and risks:

• additional work, equipment and feed;
• development dependent on the season;
• separate treatment of the brood or the young colonies;
• solution not always directly applicable to the production colony.

►Practical Guide: 1.4.2 Artificial Swarm

 

Practical comparison

Comparison of brood-break methods by workload, queen handling, comb renewal and the critical point to monitor

Method	Workload	Queen handling	Comb renewal	Critical point
Scalvini cage	Medium	Yes	No	Checking drone brood and resumption of laying
Simplified trap comb	Medium	Yes	One comb	Removal and destruction before any emergence
Classic trap combs	Very high	Repeated	Partial	Adherence to the replacement schedule
Total brood removal	High	Not always necessary	Substantial	Health management of the removed brood
Requeening	Variable	Yes	No	Less predictable broodless window
Artificial swarm	High	Yes or check required	Substantial	Feeding and development of the new unit
Nucleus formation	Medium to high	Depends on the method	Partial	Separate treatment of all units

 

5. Points of vigilance: schedule, queen and checks

Identifying the essential checks and precautions needed to protect the queen and make correct use of the broodless period.

In a Swiss trial involving 32 colonies, queen problems occurred in similar proportions after caging followed by oxalic acid and after formic acid treatment. This result, obtained from a limited sample, suggests that correctly performed caging does not appear to carry a higher risk. It also avoids exposing the queen to formic acid, whose excessive evaporation in hot weather can cause losses (Droz et al., 2015).

The risk profile, however, remains different. With caging, it depends mainly on finding and handling the queen, the type of cage, the duration of the intervention and the conditions of her release.

Intervening early enough

The aim is to reduce the varroa population before the main phase of winter bee rearing. In Switzerland, the brood break should generally be initiated in July so that the queen can resume sufficient laying at the beginning of August (SSA, 2025).

A late intervention can still eliminate varroa mites, but it does not repair the damage already suffered by parasitised or infected bees. It also reduces the time available to build up a sufficient winter population.

Checking for the absence of capped brood

Before treatment, the colony must be inspected. Particular attention must be paid to drone brood, whose development takes longer. Depending on the protocol, any brood still present must be uncapped or removed. Trap combs, for their part, must be removed before the bees emerge.

Protecting and checking the queen

After caging, the queen may be lighter and more likely to fly off. She must be released with care and must not be directly exposed to the oxalic acid solution. About a week later, her presence and the regular resumption of laying must be checked.

The type of cage may influence tolerance of caging. Devices allowing extensive contact between the queen, worker bees and wax could be favourable, but the available data do not allow us to claim that this effect is directly due to better pheromone diffusion. It is therefore a practical criterion to consider, without making it a guarantee of better acceptance.

When several colonies are treated, it is advisable to plan for a replacement option, since losses or problems with the resumption of laying cannot be entirely ruled out.

Supporting the resumption of brood rearing

After the queen's release, the colony must have sufficient food stores. In the absence of a nectar flow, suitable liquid feeding can support the resumption of laying and the drawing out of foundation. It does not, however, replace either a functioning queen or an adequate pollen supply.

Checking efficacy and re-infestation

The infestation level must be checked before and after the intervention using a suitable method. The mite drop caused by treatment indicates the number of mites affected, but does not on its own demonstrate that the residual infestation is sufficiently low.

A renewed increase may occur towards the end of summer through reproduction or re-infestation, particularly in regions with a high density of apiaries, or in cases of drifting or robbing. In the Swiss trial, infestation rose again in some colonies after an initial rapid decline, confirming the need for continued monitoring (Droz et al., 2015).

Complying with the regulatory framework and safety requirements

Oxalic acid must be used in the form of an authorised veterinary medicine and in accordance with its package leaflet. The product, the method of application, the dose, the protective equipment and the restrictions concerning honey supers must be checked before each use.

Spraying, trickling and vaporisation are not interchangeable. Their conditions of use and risks differ, particularly for the person applying the treatment. Spraying and vaporisation carry a risk of inhaling aerosols or oxalic acid dust and require suitable protection (Jans, 2023).

A zero withdrawal period for honey does not mean that treatment may be carried out during a nectar flow or with honey supers in place. This remains prohibited unless the package leaflet of the authorised medicine explicitly states otherwise. The supers must be removed and the conditions specific to the product used must be checked (Andermatt BioVet AG, 2020).

---

6. Integrating the brood break into the annual control concept

Placing the brood break within the Swiss annual calendar without confusing it with a replacement for the entire varroa control concept.

Brood break followed by oxalic acid treatment does not constitute a complete annual concept. In the Swiss strategy, it mainly replaces the first summer treatment with formic acid, particularly when heat, a persistent nectar flow, or difficulties controlling evaporation complicate its use (SSA, 2025).

The queen should be released and have resumed regular laying by the beginning of August, so that the colony has enough time to rear its winter population.

After oxalic acid treatment, the presence of the queen, the resumption of laying and the development of infestation must be checked. A high mite drop immediately after treatment indicates that mites have been eliminated, but on its own does not establish whether the residual infestation is sufficiently low.

The second summer treatment with formic acid normally remains planned in the Swiss concept. It targets varroa mites that have survived, resumed reproduction or been introduced through re-infestation. Forgoing it can only be considered in particular situations, for example in a very isolated apiary where regular checks document a persistently low infestation (SSA, 2025).

Monitoring must continue in autumn, since infestation can rise rapidly when neighbouring colonies weaken or are robbed. Finally, winter treatment with oxalic acid remains necessary when colonies are naturally broodless.

Practical schedule

Chronological comparison between the classic concept and the concept with brood break, from the summer harvest to winter treatment

Time	Classic concept	Concept with brood break
After the last summer harvest	First treatment with formic acid	Brood break or other biotechnical method
When broodless	—	Authorised oxalic acid treatment, except for a fully biotechnical method
Early August	Colony inspection	Resumption of laying and appropriate feeding
Late summer	Infestation monitoring	Monitoring of infestation and re-infestation
Second summer treatment	Formic acid	Depending on check results: generally also formic acid
Autumn	Monitoring	Monitoring
Winter, broodless	Oxalic acid treatment	Oxalic acid treatment

Brood break followed by oxalic acid treatment is a serious alternative to the first summer treatment with formic acid, particularly when high temperatures make its evaporation difficult to control.

Its effectiveness rests on one essential condition: applying the treatment during a verified period without capped brood. Success therefore depends on the choice of method, adherence to the schedule and the quality of the checks.

The various techniques meet different needs. Some retain the brood within the colony, others remove some varroa mites with trap combs or allow the brood nest to be renewed. Their workload, material requirements and risks are not equivalent.

This approach also changes the risk profile. It avoids exposing the queen to a first summer application of formic acid, whose excessive evaporation can cause queen losses and damage to the brood. In return, it requires finding, handling and releasing the queen with care, then checking the resumption of laying.

The brood break must remain integrated into an annual strategy that includes infestation monitoring, consideration of re-infestation, evaluation of a second summer treatment and winter treatment.

The choice should therefore not be based on a method assumed to be universally better, but on the one whose requirements and risk profile best match the apiary, the colonies and the skills of the beekeeper.

---

See also:

• Practical Guide: 1.6.1 Brood Break
• Practical Guide: 1.6.2 Trap Comb Method
• Practical Guide: 1.2 Overview of Summer Treatment Methods
• Winter varroa treatment: what to do if colonies are still rearing brood?
• Practical Guide: 1.6.4 Total Brood Removal with Brood Utilisation
• Varroa control: summer brood interruption

---

8. Bibliography

Andermatt BioVet AG. (2020). Notice d'emballage OXUVAR 5,7 % ad us. vet., concentré pour la préparation d'une solution pour abeilles. Institut de pharmacologie et de toxicologie vétérinaires, Université de Zurich. https://www.vetpharm.uzh.ch/TAK/PDFSPC/06000000/06577601-PB-FR.pdf

Bubnič, J., Prešern, J., Pietropaoli, M., Cersini, A., Moškrič, A., Formato, G., Manara, V., & Smodiš Škerl, M. I. (2024). Integrated pest management strategies to control varroa mites and their effect on viral loads in honey bee colonies. Insects, 15(2), 115. https://doi.org/10.3390/insects15020115

Büchler, R., Uzunov, A., Kovačić, M., Prešern, J., Pietropaoli, M., Hatjina, F., Pavlov, B., Charistos, L., Formato, G., Galarza, E., Gerula, D., Gregorc, A., Malagnini, V., Meixner, M., Nedić, N., Puškadija, Z., Rivera-Gomis, J., Rogelj Jenko, M., Smodiš Škerl, M. I., Vallon, J., Vojt, D., Wilde, J., & Nanetti, A. (2020). Summer brood interruption as integrated management strategy for effective Varroa control in Europe. Journal of Apicultural Research, 59(5), 764–773. https://doi.org/10.1080/00218839.2020.1793278

Droz, B., Dietemann, V., Gauthier, L., & Charrière, J.-D. (2015). L'encagement des reines : une méthode pour traiter contre varroa en été ? Revue suisse d'apiculture, 112(8), 20–28. https://ira.agroscope.ch/fr-ch/Cms/Einzelpublikation/Download?einzelpublikationId=35423

Jack, C. J., van Santen, E., & Ellis, J. D. (2020). Evaluating the efficacy of oxalic acid vaporization and brood interruption in controlling the honey bee pest Varroa destructor (Acari: Varroidae). Journal of Economic Entomology, 113(2), 582–588. https://doi.org/10.1093/jee/toz358

Jans, S. (2023). Minimiser les risques pour la santé des apiculteurs lors du traitement hivernal. Revue suisse d'apiculture, 11–12, 490–491.

Kovačić, M., Uzunov, A., Tlak Gajger, I., Pietropaoli, M., Soroker, V., Adjlane, N., Benko, V., Charistos, L., Dall'Olio, R., Formato, G., Hatjina, F., Malagnini, V., Freda, F., Otmi, A., Puškadija, Z., Villar, C., & Büchler, R. (2023). Honey vs. mite—A trade-off strategy by applying summer brood interruption for Varroa destructor control in the Mediterranean region. Insects, 14(9), 751. https://doi.org/10.3390/insects14090751

Naumann, K., Winston, M. L., Slessor, K. N., Prestwich, G. D., & Webster, F. X. (1991). Production and transmission of honey bee queen (Apis mellifera L.) mandibular gland pheromone. Behavioral Ecology and Sociobiology, 29, 321–332.

Service sanitaire apicole. (2025). Übersicht Methoden Sommerbehandlung (Aide-mémoire 1.2, version 2511). apiservice. https://bienen.ch/wp-content/uploads/2025/11/1.2_uebersicht_methoden_sommerbehandlung.pdf

Toomemaa, K., & Kaart, T. (2022). Caging the queen with and without a comb section to achieve high efficacy of Varroa destructor control. Journal of Apicultural Research, 63, 671–679.