Brood interruption: control of Varroa mites

The developmental cycles of Varroa destructor and the honey bee are closely linked. Since the emergence of the varroa mite in Europe in the early 1980s, numerous chemical treatments have been recommended and applied to control varroosis. Residues of synthetic chemicals and persistent compounds have accumulated in beeswax over the years, contributing to the selection of varroa populations with increased resistance. It is therefore possible to manage varroa populations by intervening in the bee’s developmental cycle. Caging the queen to obtain a brood-free colony is an example of an interesting biotechnical method for controlling varroosis.

1. Summary of a European Study

Most colony losses due to Varroa occur in autumn or winter, as a result of high varroa infestation levels and insufficient health status of adult bees. Even with a low initial varroa infestation at the beginning of spring, critical mite infestation levels and viral infections can be reached before winter if colonies continuously rear brood throughout the entire season. To address this challenge, beekeepers can artificially interrupt brood production through appropriate colony management procedures, depending on their beekeeping management plan. In order to assess their efficacy, the associated workload, and the impact on colony development, different brood break methods (queen caging combined with oxalic acid treatment, total brood removal, drone comb trapping method) were tested over two seasons at 11 sites with 370 colonies in 10 European countries. A protocol was developed to standardise application of the methods under different environmental conditions. The efficacy of queen caging depended on the mode of oxalic acid application and ranged from 48.16% to 89.57% mite elimination. The highest efficacy values were achieved by spraying a 4.2% solution (89.57%) and by oxalic acid vaporisation of 2 g oxalic acid (mean 88.25%) during the broodless period. The efficacy of purely biotechnical methods without chemical agents — drone comb trapping and total brood removal — did not differ significantly from that of the queen caging groups. We conclude that correct application of any of the described brood break methods can contribute significantly to effective varroa treatment and to the production of bee products meeting the highest quality and food safety standards (Ralph Bürchler et al., 2020, ?action=get_file&id=106&resource_link_id=275a).

2. Introduction

Varroa destructor is a parasite of the honey bee. Its development cycle is adapted to that of its host. It nevertheless remains dependent on the presence of bee brood for reproduction. In the absence of brood, the varroa mite population cannot develop, and all individuals are then in the phoretic phase. They are accessible and vulnerable to acaricides at this stage. For these reasons, achieving a broodless period is beneficial in order to optimise the efficacy of a medicinal treatment (e.g. with oxalic acid).

To take advantage of the brood break, it is advisable to carry out this procedure in July. Only when the queen resumes full egg laying activity in early August can the colony build up the necessary winter bee population within 6 to 8 weeks. In contrast to the drone comb trapping method, the queen's egg laying is interrupted here.

3. Objectives

The brood break, followed by oxalic acid treatment, replaces the first summer formic acid treatment. The second summer formic acid treatment must be carried out as usual in order to keep the mite infestation level low until the winter treatment and to destroy varroa mites resulting from any re-infestation. Only exceptionally — in cases of low natural mite drop (which requires regular infestation monitoring) and for very isolated apiaries — can the second summer treatment be dispensed with.

4. Basic Principle

The basic principle of this method is queen caging for 24 days in a queen cage that allows worker access.
The queen continues egg laying in the Scalvini cage throughout the 24 days without interruption, but the brood fails to develop, as the cage is not tall enough for the bees to draw out the cells.
After 24 days, the queen is released and an oxalic acid treatment (by trickling method, spray treatment, or oxalic acid vaporisation) is applied. Since no brood is present, all varroa mites are on the bees (phoretic mites), maximising the treatment efficacy.
4 days later, a second oxalic acid treatment is applied with the aim of killing the mites that escaped the first treatment before they take refuge under larvae in the brood food. (The queen has resumed egg laying immediately after release 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 the cage dimensions into the wax).
If a nectar flow is still in progress, sufficient super frames must be available within the colony to prevent brood frames from being filled with honey.
Carry out the last honey harvest no later than after 3 weeks.
After the honey harvest, remove the frame with the caged queen from the colony.

Rearrange the frames, remove old frames, and spray all bees — including those inside the hive — with oxalic acid. The spray treatment has the advantage of allowing all frames to be checked at the same time. The treatment can also be carried out by oxalic acid vaporisation.
Add foundation frames as required.
After 24 hours, return the queen to the colony without spraying her, or introduce a new queen using a slow-release cage with fondant.
Add a feeder and immediately provide the colony with liquid feeding (sugar syrup 1:1 is preferable) to stimulate brood rearing. If new foundation frames are installed, it is very important to ensure a constant supply of feed for 2 weeks so that they are drawn out quickly and completely.
Verify that the queen is laying actively one week after the oxalic acid treatment.
Carry out the 2nd summer formic acid treatment at the beginning of September.

An alternative to the cage is to use a super frame, fit a starter strip, and seal it with queen excluders and elastic 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 Total Brood Removal

Another method consists of removing all brood frames. New nucleus colonies can be formed from the removed frames, and the varroa infestation level of the parent colony is thereby reduced significantly. In cases of heavy infestation, this technique is preferred over caging. It does, however, require sufficient drawn frames to be available to replace the frames removed.

Immediately after the last honey harvest in July, locate the queen in the selected colonies and place her temporarily in a closed introduction cage.
Remove all brood frames and place them in a nucleus box with 6 frames.
Replace the removed frames with drawn frames or foundation frames.
Treat with oxalic acid and release the queen.
Place the nucleus box with the brood frames at a distance of at least 3 kilometres from the production colonies; open the entrance immediately but narrowly.
Introduce a queen or allow emergency queen rearing.
Feed.
10 days after establishing the nucleus, treat with oxalic acid.
After 30 days, check whether the new queen is laying.

6.2 Requeening

At the beginning of July: locate the old queen and remove her.
5 days after rendering the colony queenless, remove all already capped queen cells and retain only two well-formed, still open (uncapped) queen cells.

After 24 days, carry out the last honey harvest.

Then apply oxalic acid treatment (by trickling method, spray treatment, or oxalic acid vaporisation).
The new queen begins egg laying on day 26–27.
Feed immediately with syrup to stimulate brood rearing.
Verify that the queen is laying actively one week after the oxalic acid treatment.
Carry out the 2nd summer formic acid treatment 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 tend and devote almost 100% of their activity to foraging for nectar and pollen in preparation for the new laying period.
A colony development dynamic similar to that observed when hiving a swarm or a package of bees is noted.
Depending on the environment, egg laying resumes actively, producing good-quality winter bees.

6.3 Drone Comb Trapping Method

At the beginning of July (day J): isolate the queen in a queen cage on a drawn, brood-free drone brood trap (1).
J+7: drone brood trap (1) is moved out of the queen cage into the colony, and a new drawn frame (drone brood trap 2) for egg laying is inserted in its place in the queen cage.
J+14: drone brood trap (1), which was moved out of 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 (drone brood trap 3) for egg laying is placed in the queen cage.

J+14: drone brood trap (1), which was moved out of 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 (drone brood trap 3) for egg laying is placed in the queen cage.
J+21: drone brood trap (2), which was moved out of the cage at J+14, is also fully capped and can likewise be removed from the hive and placed in a nucleus box.
J+28: carry out the last honey harvest.
The queen is then gently released into the colony and the queen cage can also be removed from the hive. Drone brood trap (3) from J+21 is removed from the colony and distributed for swarm formation. Oxalic acid treatment.
Feed immediately with syrup to stimulate brood rearing.
Verify that the queen is laying actively one week after (J+7) the oxalic acid treatment.
Carry out the 2nd summer formic acid treatment at the beginning of September.

Observations:

Throughout the entire period, the queen continues egg laying without difficulty inside the queen cage.
Cell capping takes place between the 7th and 9th day of the larval stage.
The brood frame placed for egg laying in the queen cage (7 days) and then transferred to 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 carry large numbers of varroa mites. If used for a swarm, an oxalic acid treatment during a broodless period is essential.

Some beekeepers in Europe practise queen caging during the winter period. This ensures that no unnecessary egg laying occurs during the unfavourable season (when present, such laying is energetically costly for winter bees) and optimises the efficacy of their supplementary winter treatment. The cage models used for this technique differ from standard cages and allow the queen to move with the winter cluster as required.

7. Discussion: Advantages and Disadvantages

Within the context of alternative varroa control, end-of-summer treatment methods are based on formic acid or thymol-based products. In both cases, despite systems for regulating evaporation surfaces, these treatments are dependent 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 can subsequently endanger the colony. With the aim of overcoming this dependence on external conditions, queen caging followed by oxalic acid treatment during the broodless period can represent an interesting alternative. It has already been demonstrated that oxalic acid treatments during the winter period achieve an efficacy of 95–98% (Imdorf et al. 1997, Radetzki 1994; Charrière et al. 2000), and similar results can therefore be expected when applied at the end of summer after the harvest in colonies rendered broodless. This would allow the varroa infestation level in colonies to be reduced rapidly and healthy winter bees to be produced once treatment is complete.

Yves Goic (voir présentation pdf) reported on trials in France since 2012 by a group of beekeepers under the heading "Brood interruption technique". Over 5 years, nearly 6,500 colonies underwent queen caging with Scalvini cages, and nearly 2,700 colonies were subjected to other brood break methods. Upon release, a queen loss rate of approximately 3.1% was observed. Each year, the majority of beekeepers had caged their queens between 7 and 21 July. Overall, this technique makes it possible to reduce the varroa infestation level in colonies to an acceptable level before winter. The technique is frequently combined with requeening. All the conditions are in place, and the new queen can be introduced without difficulty.

According to a study by the Liebefeld bee research centre (► view study), this method has shown satisfactory results in the context of alternative varroa control.

Over the two years studied, results were comparable and no differences were observed regarding colony survival or queen problems. Likewise, no negative effects on the bee population at the onset or end of winter were observed in comparison with standard formic acid treatments. Finally, the low natural mite drop counts during winter indicate that the control strategy combining an oxalic acid treatment following a brood break with a winter treatment is generally sufficient to ensure good conditions for the following year.

However, this treatment approach does not represent a revolution compared to formic acid and is therefore better characterised as an equivalent alternative rather than a replacement method. In particular, some uncertainty remains as to whether its efficacy is sufficiently high under heavy infestation conditions for a single treatment to be adequate. For this reason, ApiService recommends carrying out the 2nd summer formic acid treatment.

This method, if effective, places the following demands on beekeepers:

Queens must be young in order to resume abundant egg laying rapidly after release.
They must be marked to facilitate their rapid location within the hive.
To facilitate locating the queens, hives must be in good condition; this operation can be very difficult in hives where frames are stuck together with wax and propolis.
All hives in the apiary must be treated simultaneously and in the same manner to prevent re-infestation.
Queens must be released gently.
Where necessary, plan a stimulative feeding in the absence of nectar foraging to restart the queen's egg laying (sugar syrup 50/50).
The schedule of operations must be followed precisely for the method to be effective (queen caging, 24 days later release of the queen and oxalic acid treatment, a second formic acid treatment at the beginning of September at the latest).

But it is a good alternative when:

one is dependent on temperature variations and ambient humidity
one is seeking a less invasive treatment method
one wishes to reduce the varroa infestation level rapidly in colonies and to produce healthy winter bees once treatment is complete