Artificial interruption of egg laying and removal of brood during the season aim to disrupt the reproductive cycle of varroa or to remove part of its population while controlling swarming. These approaches are already well integrated into beekeeping practices, but remain relatively little used outside Italy or Germany, particularly for trapping methods intended to completely avoid medicinal treatments. Their adaptation to different beekeeping contexts and conditions of use still raises questions. The experiment coordinated by the Kirchhain Bee Institute (Germany) within the Coloss “Varroa Control” working group was carried out in 2016 and 2017 and aimed to compare different brood interruption methods under various contexts and control procedures. In total, 370 colonies located at 11 sites in 10 European countries were involved.

 

 

1. A common protocol for evaluating the effectiveness of varroa control treatments

Participants could choose the modalities to be tested from a list of treatment procedures established by the Coloss working group, according to their resources and national opportunities for use, while systematically including a common reference modality applied in all experimental apiaries (Table 1). ITSAP’s participation in late summer 2016 consisted of testing two groups of ten colonies located in Avignon:

• I) complete brood removal,
• II) the reference modality: caging the queen combined with trickling a 4.2% oxalic acid solution (Api-bioxal®) upon her release.

The common protocol applied by each European participant consisted of:

• assessing the effectiveness of the tested treatment modalities by counting varroa fall on sticky boards during treatment and during a control treatment (authorised product of choice: Apivar®, Apistan®, Apitraz®, Bayvarol® or CheckMite®),
• monitoring risks induced in the colonies: queen survival until the following spring, visual quantification of colony size before and 70 days after treatment,
• objectifying the practicality of treatment procedures based on the time required for application.

Two types of brood interruption use were tested: queen caging combined with oxalic acid application, or biomechanical methods using trapping in worker brood, generally aiming to avoid the use of medicinal products.

2. Results of the association between queen caging and oxalic acid use

Effectiveness is calculated as the ratio between the number of varroa falls recorded during treatment and the total number of varroa recorded during treatment and the control treatment. The effectiveness of oxalic acid application after caging depends on the method used (dose, quantity and mode of application; see Table 1), the apiary (and its location in Europe), and an interaction between these two factors, but remains consistent across the two experimental years. Following queen caging, the use of 4.2% oxalic acid by trickling or sublimation yielded the best results, with comparable mean efficacies of 89.57% (70.94–99.89%) and 88.25% (72.37–98.78%), respectively. The use of a less concentrated oxalic acid solution (2.5%) significantly reduced average efficacy to 80.60% (67.63–98.29%) and 48.16% (39.74–56.58%) for applied quantities of 8 or 5 ml per interframe space, respectively. Queen caging did not result in increased queen mortality during the two test seasons. Changes in bee populations did not differ significantly according to the brood interruption method used (caging + oxalic acid, brood trapping, or complete brood removal), but varied depending on location and year of experimentation (local summer conditions). Finally, these methods required the least implementation time, ranging from an average of 19 minutes per colony for trickling to 23 minutes when using a sublimator. These data should nevertheless be interpreted cautiously, as the time required for queen caging depends on the operator’s experience and possible facilitation through prior queen marking, factors not taken into account in this study.

3. Results of approaches using trap combs (PCO and complete brood removal)

Since part of the varroa removed through trapping in brood combs is not counted, the effectiveness of these methods is evaluated based on the number of varroa falls following the control treatment. Residual varroa levels of these biomechanical methods do not differ significantly from each other or compared with the reference treatment using a 4.2% oxalic acid trickling. For information, the highest varroa falls were observed following the use of classic trap combs, and the lowest with the complete brood removal method. As these methods do not involve the use of acaricides, their effectiveness still needs to be improved through combined use of oxalic acid. The simplified brood trap method resulted in a greater reduction in bee population, but this difference is not statistically significant. It is noteworthy that other studies on brood removal have shown that total brood removal during the season did not weaken colonies, which were able to compensate for the population depression within two months. Finally, the classic brood trap method requires the most time to implement, averaging 40 minutes per colony, whereas the simplified trap comb technique requires only 14 minutes.

4. Why and how to use summer brood interruption techniques?

The results of this study—the first to test summer brood interruption at a European scale—confirm the effectiveness of these varroa control methods while clarifying key elements (efficacy, impact on colony dynamics, implementation time) necessary for considering their integration into beekeeping practice. The main obstacles to their use are primarily their time-consuming nature and, for some methods, the need to remove a large number of brood combs, which can be problematic in large apiaries. Their implementation during summer contributes to colony multiplication, but then requires a specific varroa treatment once all brood has emerged. Moreover, the risks of robbing should not be underestimated, particularly for the complete brood removal method involving extensive comb manipulation; however, these risks can be reduced by intervening during periods of lower bee activity (early or late in the day, depending on weather conditions). Large-scale application will likely require changes in practice, such as adapting apiary size and reducing risks of reinfestation in autumn.

These results reinforce others demonstrating the absence of impact on colony production capacity when the timing of queen caging is optimised. They allow a broader consideration of brood management techniques (by interruption or redistribution to strengthen other colonies) to steer colony dynamics, for example to regulate swarming, cope with resource fluctuations, or free workers from brood care and focus colony strength on honey production.

Finally, methods using trap combs are of particular interest to organic beekeepers and to all those wishing to reduce or stop the use of acaricidal treatments. In particular, varroa infestation management is more sensitive in drone-rearing colonies, and it has been demonstrated that parasitism of drone pupae affects long-term sperm production, flight capacity and lifespan. Thus, infestation of drone colonies creates a selection pressure favouring colonies with lower sensitivity. This mechanism is counterbalanced by the use of acaricidal substances, including winter treatments, which allow sensitive colonies to survive. Transitioning to control methods based solely on brood interruption during the season, without acaricide use, could contribute to the broader establishment of varroa-tolerant bee populations and to sustainable parasite management.

Table 1: List of tested treatment procedures (Büchler et al., 2020)

 

1) R. Büchler et al. (2020): Summer brood interruption as integrated management strategy for effective Varroa control in Europe, Journal of Apicultural Research, DOI: 10.1080/00218839.2020.179327

2) For more information on the “Varroa control” working group: https://coloss.org/articles/1109/

 

See also:

• Brood stop: varroa control
• Varroa: brood stop method