How, When, and Why: Utilizing Drone Brood Trapping for Varroa Control

Why isn’t using pesticides enough to eliminate Varroa mites?

In the natural world, parasites typically do not kill their hosts, as their survival is intricately linked to the life of their host. However, nature also shows us that parasites can switch hosts. In such cases, if the new host isn’t equipped with natural defenses against the parasite, the results can be catastrophic.

This nearly mirrors the scenario when the Varroa mite made its way from its original Asian host to the European honeybee, the latter being the most widespread bee species globally. Beekeepers initially tried a myriad of chemical warfare, but to little avail. The crux of Varroa’s resilience lies in its breeding within the protected environment of the honeybee, specifically within the sealed brood cells.

This environment, naturally engineered to safeguard bee larvae, also significantly protects Varroa mites from environmental threats, including chemical pesticides. Furthermore, these parasites have developed a degree of resistance to these chemicals, providing them with limited protection even during the phoretic stage when they attach to bees outside the cells.

Nevertheless, there are natural pesticides, such as thymol, against which Varroa has not developed resistance. Yet, their efficacy varies significantly from one hive to another.

Adding to this, Varroa mites benefit from an annual reprieve during the peak nectar collection and honey production period. During this time, beekeepers halt pesticide use to prevent honey and wax contamination.

The European honeybee, Apis mellifera, lacks the robust resistance seen in its Asian counterparts and some North African bee species. Consequently, it heavily relies on beekeepers’ interventions to combat Varroa mites.

Beekeepers have devised numerous strategies to tackle this parasite. Today, we highlight a key method known as the drone brood trap, usable throughout the year as long as the queen is laying eggs. This method is especially beneficial in the period following the initial pesticide application and before the subsequent application prior to wintering. This timing is preferred for treatments that do not compromise the purity of honey or wax.

Why target drone brood as a Varroa trap?

The rationale behind targeting drone brood to combat Varroa stems from a fundamental observation well-known to beekeepers: Varroa mites are more prevalent in drone brood than in worker brood. While worker brood is vital for the colony’s survival and cannot be compromised, beekeepers can afford to temporarily do without drone brood without adversely affecting the colony.

This ingenious strategy involves periodically removing drone brood two to four times a year to reduce the Varroa population within the colony.

It’s worth noting that a queen bee mates only once in her lifetime. Hence, after mating, drones become redundant, their primary role inadvertently becoming a support for Varroa spread within the apiary.

Of course, this places an unfair bias against the drones. Bees are incredibly efficient in their life processes, and if they continue to produce drones, there’s undoubtedly a significant reason for it, likely linked to maintaining genetic diversity within the colony. The potential negative effects of temporarily depriving the colony of drones are not fully understood, though it’s known not to negatively impact honey production or the queen’s fertility. This strategy has proven effective in reducing the Varroa count within a colony.

If the idea of removing drone brood concerns you, rest assured that even when a frame of drone brood is removed, hundreds of drones remain scattered throughout the hive and on the edges of other frames. Bees build their cells in various places, ensuring the continued presence of drones.

Some beekeepers believe that Varroa mites are drawn to drone brood because of the scent of the drone larvae or due to a specific pheromone that attracts the mites. However, there’s no concrete scientific evidence to support this theory. In reality, we don’t even need this explanation to understand why Varroa mites are more commonly found in drone brood than in worker brood. Two practical realities suffice to explain the abundance of Varroa in drone brood:

1- The behavior of worker bees in caring for the brood generally leads to the spread of Varroa among the drones. Nurse bees spend more time caring for and feeding the drones, almost three times as much as they spend on worker larvae (Calderone & Kuenen, 2003). Additionally, the time window that allows Varroa to move from an infected worker bee to a drone larva before it is sealed is approximately 40 to 50 hours for drone larvae, compared to no more than 20 hours for worker larvae (Boot, Calis, & Beetsma, 1992).

I want to elaborate on this point, as the concept of the time window before sealing might be confusing for some beginner beekeepers.

You should know that Varroa mites only enter the wax cells a few hours before they are sealed, and a cell is sealed nine days after the queen lays the egg. During these nine days, the egg, which turns into a larva after three days, is not suitable for hosting Varroa or aiding its reproduction.

Before sealing, about 20 hours in advance, Varroa can sneak into a worker larva’s cell.

However, it has a longer time, up to 50 hours, to infiltrate the cells of drone brood.

This makes Varroa mites associated with an infected worker bee more likely to contact and enter drone brood, providing them with a longer timeframe to move to their cells.

2-Drones require a longer period to hatch and emerge from their sealed wax cells, which provides Varroa mites with additional time not only to lay eggs but also for the mite offspring to develop. Consequently, the number of Varroa eggs found within drone brood cells is significantly higher. The term ‘founding Varroa’ refers to the mites that lay eggs inside the hexagonal cells. These mites begin laying their eggs 60 hours after the cell is sealed, with an additional egg laid approximately every thirty hours, potentially laying up to five or six eggs. It takes about eight days for Varroa to develop from an egg into a sexually mature mite capable of mating, meaning not all Varroa larvae will reach maturity before the wax cell is opened. The pupation stage for worker bees lasts 12 days inside the sealed hexagonal cell, while for drones, it extends to 14 or 15 days. This longer development period for drones suggests that Varroa mites have enhanced opportunities for reproduction and mating within drone cells.

After the wax cells are opened, the mature bees emerge, accompanied by the mature mites that have attached to them. Left behind in the now-abandoned cell are any immature mites and male mites. It’s important to note that male Varroa mites, and any females that have not reached mating maturity, do not survive, implying a natural selection process within the cell.

How the Drone Trap Works

The frames used by beekeepers are designed to fit the size of worker bee brood. However, drone brood is constructed by the bees themselves in the gaps between frames or on their edges, or in any vacant space within the hive.

It’s possible to monitor natural drone brood and remove it to reduce Varroa mite infestation, but this is practically impossible. Constantly inspecting every frame and knowing exactly which cells have been sealed and which have not is unfeasible.

Therefore, it’s easier to introduce a special frame for drone brood into the brood chamber. Practically, this is not difficult because it simply involves placing a frame with slightly larger wax cells that match the size of the drones.

Using a Green Plastic Frame for Drone Brood

Plastic frames designed for this purpose are available online. They usually come in green to distinguish them from other frames and cost around five dollars or less per frame.

When the frame is filled and most of its cells are sealed, it’s removed from the hive and placed in a freezer for twenty-four hours to kill the Varroa mites (and unfortunately, the drones too). Then, it’s returned to the hive after spending some hours at room temperature, and the worker bees are expected to open those cells and remove the dead drones and Varroa mites.

As you can see, this method is not easy at all.

If you’re managing two hundred bee hives, imagine the effort required to freeze all the frames, especially if you plan to repeat the drone trapping process more than twice a year. The task becomes even more challenging.

Moreover, relying on the bees to clean the frames adds a new burden to the bee colony.

Some beekeepers prefer to dispose of the drone brood along with its wax before returning the green frame to the hive. However, even this method requires cleaning the frame of wax residues and drying it after washing with water.

In my view, all these methods are complicated, especially the freezing technique, which also requires special care in storage to avoid moisture. It’s important to recognize that Varroa mites are not the only threat to bees, and some types of infections, such as Nosema ceranae, can withstand even high freezing temperatures (Macías-Macías et al., 2020). If a beekeeper manages dozens of hives, it’s easy to mistakenly mix and transfer frames from one hive to another without proper monitoring or disinfection.

Using a Divided Frame for Drone Brood

Bees like to add honeycombs to the brood frames and may also add pollen combs. Usually, you find drone brood at the bottom of the frame and some honeycombs on top. Bringing the food source closer makes it easier for the nurse bees to care for and feed the young.

Following this natural design, some beekeepers divide a frame into two sections: the top third remains normal, like other frames dedicated to honey collection, and the bottom two-thirds, which is left empty without wax or wires.

Why without wax?

Because the standard wax sizes used by beekeepers are tailored for rearing female brood. Drone sizes are slightly larger, so the bees build their own in the bottom section.

Why not use wires?

The brood mass is removed with a knife or a beekeeping tool, and it’s easier to remove one large wax mass than three or more when the bottom frame is divided by wires. If you use the drone brood trap technique in all your hives and choose to apply it several times a year, you’ll add a significant burden to your schedule, especially if you have dozens of hives. Therefore, it’s always advisable to find the best methods to facilitate the work.

After the drone cells are sealed, you can pull out the frame and dispose of the brood, then immediately return the frame to the hive.

When to Use the Drone Brood Trap Technique?

This method is most effective during the hive’s active periods when it’s ready to rear drone brood, typically from April to July. However, as Varroa activity gradually increases during these months in correlation with the increase in brood, the number of Varroa mites caught in the early months might be minimal but gradually increases, with the largest percentage caught in July.

When to Pull the Sealed Frame The success of the drone brood trap technique lies in choosing the optimal time to pull the frame.

If you pull the frame before most cells are sealed, the results will be poor.

If you delay pulling the frame and the eggs hatch inside the box, the results will be detrimental and counterproductive, especially during peak Varroa infestation periods, as you essentially turn this frame into a breeding ground for the parasite.

Since the queen doesn’t lay all the drone eggs in one day, estimating the appropriate time to pull the frame becomes more challenging, especially for beekeepers who can’t monitor their hives daily due to distance from the apiary.

The queen’s egg-laying activity is not uniform across all hives, and while she can theoretically lay up to 3000 eggs a day, this maximum limit remains theoretical and is only achieved under ideal environmental conditions, in a large hive with a perfectly healthy queen.

Under normal conditions, a queen lays less than two thousand eggs a day, and possibly much less if the number of worker bees in the hive is low because the queen only lays the number of eggs the workers can care for. In all cases, nothing guarantees that she will focus solely on drone eggs and ignore her other duties.

Some beekeepers adopt harsh and complicated methods to control the queen’s behavior, such as confining her next to the drone frame or artificially dividing the colony to create a situation where the hive is broodless except for drone brood. These methods, based on a deep understanding of bee behavior and knowledge that the biggest competitor to drone brood is worker brood—which, despite the drones’ attractiveness to Varroa, remains the largest bastion of infection—assume that focusing all efforts on drones will ensure much higher Varroa capture. However, these aggressive techniques remain experimental and impractical for commercial apiaries due to the significant effort and risk involved.

Returning to our original method, with a bit of luck, the queen can fill a bee frame in two or three days, depending on many variables including the size of the colony, weather conditions, the queen’s age, and the bee species. Workers might take two days to build the combs. All this means that filling the frame with eggs should not be delayed too much, allowing for the frame to be pulled before twenty-four days from its placement.

According to the French beekeeper Gwenaël DELAMARCHE, who conducted a comparative field experiment involving 483 Dadant hives, including 222 hives where the drone brood removal technique was applied, a period of twenty to twenty-one days is most suitable for pulling the drone frame.

Beekeepers from different regions use varying timeframes, understandable due to the diversity of apiaries and individual hive characteristics, leading to discrepancies in determining a uniform timeframe.

However, generally, this period should not be less than 18 days and not exceed 24 days, as we risk entering the phase where drones emerge from the sealed cells, which is undesirable under any circumstances.

Is This Method Truly Effective?

The drone brood trap technique is not intended to replace traditional Varroa mite control methods but remains an effective tool within an Integrated Pest Management (IPM) strategy.

It proves particularly useful when the use of insecticides is impractical during peak nectar collection and honey gathering seasons, or when the goal is to reduce Varroa mite presence in the hive in preparation for decisive elimination using pesticides.

When precisely implemented according to scientific principles, this method can significantly reduce Varroa mite numbers. A study conducted by Charrière and others in 2003 showed that hives treated throughout the year had 3.5 times fewer Varroa mites compared to untreated hives. This difference was especially pronounced towards the end of the year, particularly in August and September, when formic acid was used as a supplementary treatment. This effect was achieved without any negative impact on honey production or hive growth.

A 2019 study by Mustafa Gunesdogdu and his team in Turkey found that using drone brood traps reduced Varroa spread by 43% without harming the bees.

A study conducted in a commercial apiary in Buenos Aires, Argentina, in 2004 found that this technique’s effectiveness against Varroa, when three sequential trap frames were used within the hive, reached 84.72% – a level that some pesticides might not achieve. (Damiani, N., & Marcangeli, J., 2006)

However, the same study observed that honey production was lower in hives that applied this technique.

In my opinion, the conclusions of this study cannot be generalized to the practical reality of beekeeping, neither in terms of the very high effectiveness rate nor regarding the negative impact on honey production. This is because the study followed a specific laboratory method in using drone brood traps, which involved confining the queen next to a drone brood frame for periods ranging from eight to twenty-four days, depending on the test duration. Confining the queen for weeks will inevitably affect the hive’s growth and honey production level. This decline should not be attributed to the drone trap method but rather to confining the queen.

Conversely, an experiment by the professional French beekeeper Dolamarche, one of the few documented trials of the drone trap technique in large commercial apiaries, yielded more positive results regarding honey yield.

Using this technique in his large apiary required hiring labor specifically for replacing frames. Despite the labor costs (14 €/hour, totaling 174 hours of work, including 6 hours for wax extraction), the apiary recorded an increase in profits of about 6 euros per hive that followed the protocol.

In his 2017 published study, the beekeeper estimated an annual profit increase of five thousand euros if the drone trap approach were applied to all 800 hives in his apiary.

But where did this profit increase come from?

The primary reason is the increase in honey and wax production in hives that implemented the drone trap technique. In 2016, which was a challenging year for beekeepers overall, applying the drone trap protocol as a supplementary treatment during spring led to an increase in honey production by 1.6 kilograms per hive, amounting to a summer production increase of between 15 and 20 percent.

Sources:

• Calderone, N. W., & Kuenen, L.P.S. (2003). Differential tending of worker and drone larvae of the honey bee, Apis mellifera, during the 60 hours prior to cell capping. Department of Entomology, Cornell University.
• Boot, W.J., Calis, J.N.M. & Beetsma, J. Differential periods ofVarroa mite invasion into worker and drone cells of honey bees. Exp Appl Acarol 16, 295–301 (1992). https://doi.org/10.1007/BF01218571
• Macías-Macías, J.O., Tapia-Rivera, J.C., De la Mora, A. et al. Nosema ceranae causes cellular immunosuppression and interacts with thiamethoxam to increase mortality in the stingless bee Melipona colimana. Sci Rep 10, 17021 (2020). https://doi.org/10.1038/s41598-020-74209-3
• Güneşdoğdu, M., Şekeroğlu, A., & Tainika, B. (2021). Effect of Using Drone Brood Cells as Traps Against Varroa destructor (Varroa Mite). Turkish Journal of Agriculture – Food Science and Technology, 9(6), 1226–1231. https://doi.org/10.24925/turjaf.v9i6.1226-1231.4374
• Charrière, Jean-Daniel & Imdorf, Anton & Bachofen, Boris & Tschan, Anna. (2003). The removal of capped drone brood: An effective means of reducing the infestation of varroa in honey bee colonies. Bee World. 84. 117-124. 10.1080/0005772X.2003.11099587. 
• Damiani, N., & Marcangeli, J. (2006). Control of the parasite Varroa destructor (Acari: Varroidae) in honeybee colonies of Apis mellifera (Hymenoptera: Apidae) applying brood trap combs. Revista de la Sociedad Entomológica Argentina, 65(1-2), 33-42.