1. Summary of a European study

Most colony losses due to Varroa occur in autumn or winter, due to a high level of Varroa infestation and insufficient health of adult bees. Even with a low initial Varroa infestation in early spring, critical levels of mite and virus infection can be reached before winter if colonies continuously rear brood throughout the season. To overcome this challenge, beekeepers can artificially interrupt brood production through appropriate management procedures, depending on their type of beekeeping operation. To assess their effectiveness, associated workload, and impact on colony development, different brood interruption methods (queen caging combined with an oxalic acid treatment, total brood removal, trap comb technique) were tested over two seasons at 11 sites on 370 colonies in 10 European countries. A protocol was developed to standardize the application of the methods under different environmental conditions. The effectiveness of queen caging depended on the mode of oxalic acid application and ranged from 48.16 % to 89.57 % mite removal. The highest efficiencies were obtained by spraying a 4.2% solution (89.57 %) and by sublimation of 2 g of oxalic acid (average 88.25 %) during the broodless period. The effectiveness of purely biotechnical methods without chemicals—trap combing and brood removal—did not differ significantly from that of the queen-caging groups. We conclude that correct application of any of the brood interruption methods described can contribute significantly to effective Varroa control and to the production of apicultural products meeting the highest quality and food safety standards (Ralph Bürchler et al., 2020, https://doi.org/10.1080/00218839.2020.1793278).

 

2. Introduction

Varroa destructor is a parasite of the honey bee. Its developmental cycle is adapted to that of its host. However, it remains dependent on the presence of bee brood to reproduce. In the absence of brood, the Varroa population cannot develop and all individuals are then in the phoretic phase. They are then accessible and vulnerable to acaricides. For these reasons, identifying broodless periods is of interest in order to optimize the effectiveness of a medicinal treatment (for example, oxalic acid).

To benefit from the advantages of brood interruption, it is recommended to carry out this operation in July. Only when the queen is again in full egg-laying in early August can the colony create the winter population required within 6 to 8 weeks. In contrast to the trap comb method (► factsheet 1.6.2.), the queen interrupts egg-laying.

 

3. Objectives

Brood interruption, followed by an oxalic acid treatment, replaces the first summer treatment with formic acid. The second summer treatment with formic acid must be carried out as usual in order to keep mite infestation at a low level until winter treatment and to destroy Varroa mites resulting from possible reinvasion. Only exceptionally—when dead mite drop is low (which implies regular monitoring of infestation) and for very isolated apiaries—can the second summer treatment be omitted.

4. Basic principle

• The basic principle of this method is caging the queen for 24 days in an isolation cage that allows access by worker bees.
•  The queen continues laying eggs in the Scalvini cage for the full 24 days without interruption, but the brood aborts because the cage is not high enough for bees to draw out the cells.
• After 24 days, the queen is released and an oxalic acid treatment (by trickling, spraying, or sublimation) is applied. As there is no longer any brood, all Varroa mites are on the bees (phoretic mites) and treatment efficacy is maximal.
• 4 days later, a second oxalic acid treatment is carried out to kill mites that escaped the first treatment, before they take refuge under larvae in the larval food. (The queen resumed egg-laying as soon as she was released and the first larvae have already hatched).

5. Procedure

At the beginning of July: cage the queen (in a queen cage, e.g. of the Scalvini type). The cage is inserted into a drawn comb frame in the middle of the hive, preferably in the upper part of the frame (cut out the cage dimensions in the wax). While a nectar flow is still ongoing, there must be sufficient honey combs available within the colony so that brood combs are not filled with honey. Carry out the last honey harvest no later than after 3 weeks. After the honey harvest, remove from the colony the frame with the caged queen.

 

5. Rearrange frames, remove old frames and spray all bees, including those that are in the hive, with oxalic acid (as mentioned in factsheet 1.3.1.). Spraying has the advantage of simultaneously checking all frames. Treatment can also be carried out using sublimation.
6. If necessary, add foundation frames.
7. After 24 h and without spraying her, return the queen to the colony or introduce a new queen using a cage closed with candy.
8. Add a feeder and immediately provide liquid feed to the colony (ideally sugar water 1:1) to stimulate brood rearing. If new foundation frames are installed, it is very important to ensure a constant food supply for 2 weeks so that they are quickly and properly drawn out.
9. Check that the queen is active one week after the oxalic acid treatment.
10. Carry out the 2nd summer treatment with formic acid at the beginning of September

 

An alternative to the cage is to take a super frame, place a starter strip and close it with queen excluders and rubber bands. After 24 days, the colony is treated with oxalic acid, the queen is released and the frame is destroyed (Varroa trap).

 

6. Alternative methods

6.1 Remove brood frames

Another method consists of removing all frames containing brood. New nuclei can be formed using the removed frames, and the mother colony thus significantly reduces its parasite load. In cases of high infestation, this technique is preferred over caging. It nevertheless requires having enough drawn frames available to replace the removed frames.

• Immediately after the last honey harvest in July, find the queen in the selected colonies and temporarily place her in a closed introduction cage.
• Remove all frames with brood and place them in a 6-frame nuc (nucleus).
• Replace the removed frames with drawn frames or with foundation frames
• Treat with oxalic acid and release the queen
• Place the 6-frame nuc with the brood frames at a distance of at least 3 kilometers from the production colonies, open immediately, but keep the entrances narrow
• Introduce a queen or let the nuc raise a new queen
• Feed
• 10 days after setting up the nuc, treat with oxalic acid
• After 30 days, check whether the new queen is laying

 

6.2 Queen replacement

At the beginning of July: find the old queen and remove her. 5 days after making the colony queenless, remove all queen cells that are already capped and keep only two good queen cells that are still open (uncapped).

After 24 days, carry out the last honey harvest.

3. Then treat with oxalic acid (by trickling, spraying, or sublimation).
4. The new queen starts laying on day 26–27.
5. Feed immediately with syrup to stimulate brood rearing.
6. Check that the queen is active one week after the oxalic acid treatment.
7. Carry out the 2nd summer treatment with formic acid at the beginning of September.

• Pro version: remove all queen cells (see point 3) and introduce a new queen (F0/F1) after the oxalic acid treatment

 

Observations:

• The colony prepares for the new queen’s egg-laying; the bees no longer have brood to care for and devote almost 100% of their effort to collecting nectar and pollen in preparation for renewed laying.
• A development dynamic similar to the installation of a swarm or a package of bees is observed.
• Depending on the environment, egg-laying resumes actively, producing high-quality winter bees.

 

6.3 Trap comb

At the beginning of July (day J): isolate the queen in an isolation cage on a trap frame (1) that is drawn and free of brood. J+7: the trap frame (1) is shifted into the colony outside the isolation cage and a new drawn frame (trap frame 2) to be laid on is inserted in its place in the isolation cage. J+14: the trap frame (1), shifted outside the cage at J+7, is fully capped and can now be removed from the hive. This frame can be used to form new swarms. A new drawn frame (trap frame 3) intended for egg-laying is placed in the isolation cage.

 

4. J+14: the trap frame (1), shifted outside the cage at J+7, is fully capped and can now be removed from the hive. This frame can be used to form new swarms. A new drawn frame (trap frame 3) intended for egg-laying is placed in the isolation cage.
5. J+21: the trap frame (2), shifted outside the cage at J+14, is fully capped and can also be removed from the hive to be placed in a nuc hive.
6. J+28: carry out the last honey harvest.
7. Then, the queen is gently released back into the colony and the isolation cage can also be removed from the hive. The trap frame  (3) from J+21 is removed from the colony and redistributed for making swarms. Oxalic acid treatment.
8. Feed immediately with syrup to stimulate brood rearing.
9. Check that the queen is active one week (J + 7) after the oxalic acid treatment.
10. Carry out the 2nd summer treatment with formic acid at the beginning of September.

Observations:

• Throughout the entire period, the queen continues laying without difficulty inside the isolation cage.
• Cell capping occurs between day 7 and day 9 of larval development.
• The frame laid in the isolation cage (7 days) and then kept in the colony for the capping phase (7 days) remains in the colony for a maximum of 14 days before being removed from the hive.
• Attention: these frames may contain many Varroa mites. If used for a swarm, an oxalic acid treatment outside the brood period is required.

Some beekeepers in Europe practice queen caging during the winter period. This ensures that there is no unnecessary egg-laying during the bad season (when present, such laying is “costly” in energy for winter bees) and optimizes the effectiveness of their complementary winter treatment. The cage models used for this technique are different and allow the queen to move with the winter cluster if necessary.

7. Discussion: Advantages and disadvantages

In alternative Varroa control, late-summer treatment methods are based on formic acid or thymol-based products. In both cases, despite systems for adjusting evaporation surfaces, these treatments depend on ambient temperature and humidity. As a consequence, queen losses are sometimes observed at very high temperatures, or insufficient efficacy under cool and humid conditions, which may subsequently endanger the colony. In order to free oneself from this dependence on external conditions, queen caging followed by a broodless oxalic acid treatment may represent an interesting alternative. It has already been shown that oxalic acid treatments in winter have an efficacy of 95–98% (Imdorf et al. 1997, Radetzki 1994 ; Charrière et al. 2000), and it is therefore possible to expect similar results when it is applied at the end of summer after the harvest in colonies made broodless. This could rapidly reduce the Varroa load in colonies and produce healthy winter bees once treatment is completed.

Yves Goic (see PDF presentation) reported on tests in France since 2012 by a group of beekeepers (“brood interruption technique”). Over 5 years, nearly 6,500 colonies underwent queen caging with Scalvini cages, and nearly 2,700 colonies underwent other brood interruption techniques. Upon release, they observed queen losses of about 3.1%. Each year, the majority of beekeepers caged their queens between 7 and 21 July. Overall, this technique makes it possible to free colonies from an excessively high Varroa load before overwintering. This technique is often associated with queen replacement. All conditions are met and the new queen can be introduced without difficulty.

 

According to a study by the Liebefeld bee research center (► see the study), this method showed satisfactory results as part of alternative Varroa control.

In both years studied, results were similar and no difference was observed regarding colony survival or queen problems. Moreover, no negative effect on colony population at the entrance to or exit from winter was observed compared with the usual formic acid treatments. Finally, the low natural Varroa drops during winter indicate that the control strategy combining an oxalic acid treatment after brood interruption and a winter treatment is generally sufficient to ensure good conditions for the following year.

However, this treatment approach does not represent a revolution compared with formic acid and therefore appears more as an equivalent alternative than as a substitution method. In particular, there remain some doubts as to whether its efficacy is high enough under heavy infestations for a single treatment to be sufficient. This is why ApiService recommends carrying out the 2nd summer treatment with formic acid.

 

If effective, this method entails constraints for beekeepers:

• Queens must be young in order to resume abundant egg-laying quickly after release.
• They must be marked to facilitate rapid identification in the hive.
• To facilitate locating queens, hives must be in good condition; this operation can be very complicated in hives whose frames are glued by wax and propolis.
• All hives in the apiary must be treated in the same way and simultaneously to avoid reinfestation.
• Queens must be released gently.
• If necessary, provide stimulative feeding in the absence of nectar flow to restart the queen’s egg-laying. (syrup supply 50/50).
• The timetable must be followed to the letter if the method is to be effective (queen caging, 24 days later queen release and oxalic acid treatment, a second formic acid treatment by the beginning of September at the latest).

But it is a good alternative when:

• one is dependent on variations in temperature and ambient humidity
• one is seeking a less invasive treatment method
• one wants to rapidly reduce the Varroa load in colonies and produce healthy winter bees once treatment is completed

 

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References:

Charrière J.D. ; Imdorf A. (2000) Recommandations pour l'usage de l'acide oxalique appliqué par dégouttement pour lutter contre le varroa, Revue Suisse d'apiculture, 97(8) : 286-287.

Fluri P. ; Imdorf A., (1989) Le blocage de la ponte aux mois d’août et de septembre et ses effets sur l’hivernage des colonies, Journal Suisse d’apiculture, 86, pp. 273-275.

Gauthier L. ; Droz B. ; Dietemann V. ; Charrière J.-D. (2013) L’encagement de reines : une méthode pour traiter varroa en été, Journal Suisse d’Apiculture, pp. 19-21.

Imdorf A. ; Charrière J.-D. and Bachofen B. (1997) Efficiency checking of the Varroa jacobsoni control methods by means of oxalic acid, Apiacta, 32(3): 89-91.

Imdorf A. ; Bühlmann G ; Gerig L. ; Kilchenmann V. and Wille H. (1987) Überprüfung der Schätzmethode zur Ermittlung der Brutfläche und der Anzahl Arbeiterinnen in freifliegenden Bienenvölkern, Apidologie 18(2): 137- 146.

Kristijan Jusic (2020) Le blocage de ponte programmé des reines. Une technique efficace et une solution d’avenir dans la lutte biomécanique contre le varroa en apiculture biologique.

Mortarino M. ; Nanetti A. ; Corsi N. and Sesso L. (2014) Trattamenti farmacologici per il controllo di Varroa destructor, Quaderni della Ricerca Regione Lombardia, 162, pp. 26-39.

Radetzki T. ; Reiter M. and Von Negelein B. (1994) Oxalsäure zur Varroabekämpfung, Schweizerische Bienen-Zeitung 117 : 263-267.

 

7. Various studies & photos