Mite Bombs
Mite Bombs
By Nicole McCormick, TTP Lead Technician
Adapted from presentation by Lynae Ovinge
The parasitic mite Varroa destructor poses a major bee health challenge for beekeepers in Alberta and throughout Canada. Varroa mites parasitize honey bees by feeding on both larvae and adult bees, weakening their immune systems. In severe cases, this can lead to complete colony collapse. Beekeepers must continuously manage Varroa levels, even when infestations are not high, to prevent outbreaks. Additionally, they must contend with several related challenges, such as the increased prevalence of Varroa-associated viruses, the development of resistance to commonly used miticides, and the emergence of “mite bombs,” which facilitate the spread of Varroa. As a result, beekeepers must adapt their management practices to mitigate these escalating issues.
This article will focus on mite bomb colonies, defined as colonies with Varroa levels significantly exceeding economic thresholds. The eventual collapse of these colonies can result in a surge of Varroa populations in surrounding colonies, driven primarily by behaviors such as drifting and robbing1. Over the years, the increasing pressure from rising viral loads and decreasing efficacy of miticide treatments have emphasize the critical need for the implementation of comprehensive Integrated Pest Management (IPM) strategies. These strategies are crucial to mitigate the spread and incidence of Varroa mites within beekeeping operations. By understanding the concept of a mite bomb, recognizing its warning signs, and effectively managing highly infested colonies, beekeepers can reduce the spread of Varroa mites, and keep infestation levels more manageable.
What is a Mite Bomb?
As mentioned earlier, a mite bomb refers to a colony with high levels of Varroa, exceeding economic thresholds. Prolonged high levels of Varroa in colonies can lead to the formation of a mite bomb, which will typically begin to collapse and are unlikely to survive the winter. However, the colony collapse period can be more harmful than the loss of the colony itself, as this is when the high population of mites “explodes” and spreads to neighboring colonies. The current economic threshold for Varroa infestation is 1% (1 mite per 100 bees) when brood is present, and 3% (3 mites per 100 bees), during broodless periods2. Colonies with mite levels ≥5% in the spring and ≥10% in the summer and fall have the potential to become mite bombs. These levels, like economic thresholds, are based on the presence of brood due to factors such as season, nectar availability, and overall colony strength. Peck and Seeley (2019) identified worker and drone drifting, as well as robbing, as some of the main mechanisms that spread mites from mite bomb colonies, leading to the “bomb explosion” effect.
Honey bee drifting is common in commercial beekeeping, where apiaries often contain multiple colonies situated in close proximity1. This arrangement can increase the rate of drift, facilitating the transmission of Varroa mites to neighboring colonies. Similarly, robbing behavior enables inter-colony infestation: Varroa-infested robbers can infiltrate healthy colonies, while healthy robbers may enter Varroa-infected colonies, both acting as pathway for mite transfer. While both drifting and robbing contribute to the spread of mites from mite bombs, Peck and Seeley (2019) found that the exponential increase of Varroa levels in neighboring colonies is primarily driven by robbing behavior. During periods of dearth or in the fall season, robbing behavior increases, with robbing bees targeting weaker colonies that lack the strength to defend themselves. These weakened colonies are often diseased and likely to act as mite bombs for Varroa mite infestations. Therefore, to prevent the spread of mites during periods of increased robbing behavior, it is crucial to ensure that mite bomb colonies are not accessible to healthy foraging bees and colonies.
The effects of a mite bomb are most evident in post-treatment apiaries, where most colonies exhibit low Varroa levels, except for one or two with uncontrolled mite populations (Figure 1). Under the right conditions, this mite bomb can proliferate, jeopardizing the entire apiary by raising Varroa levels. This situation is particularly discouraging for beekeepers, as most colonies typically respond well to treatment and effective management practices. However, the presence of a single mite bomb can undermine these efforts.
Figure 1. Mite bomb colony at 9.6% infestation compared to below 1% colonies. Data from select apiary Colony Health Monitoring Fall 2023.
Best Practices to Identify a Mite Bomb
The only effective method for identifying and managing mite bombs is to implement a robust monitoring program. While it may not be feasible to monitor and sample every colony, it is essential for beekeepers to sample a representative portion of colonies within each apiary3. Regular mite shakes are recommended, as they provide a quick and easy assessment of Varroa levels. By implementing and consistently executing a Varroa mite IPM plan, beekeepers can more readily identify mite bombs and take prompt action to mitigate the risk of infecting neighboring colonies. The Alberta Beekeepers Commission’s Tech Transfer Program (TTP), in collaboration with the Bee Health Assurance Team (BHAT), has developed a general Varroa IPM plan that serves as a template for beekeepers to create their own. This resource is valuable for developing an effective monitoring program to identify mite bombs before they become problematic. Find it online by scanning the QR code.
Mite bombs are often characterized by pronounced symptoms and signs, as high Varroa levels are associated with a range of diseases and visual indicators. Common symptoms include parasitic mite syndrome (PMS), deformed wing virus (DWV), while visual signs include the presence of mites on bees (Figure 2). Recognizing these symptoms and signs is a valuable tool for detecting problematic colonies. Our Colony Health Monitoring data from 2022-2023 shows that 94% of apiaries containing mite bombs exhibited 1-3 common symptoms/signs associated with high Varroa mite infestation. Identifying these issues and flagging affected yards during routine management practices can provide a quick and effective approach to pinpointing problematic mite bombs.
Figure 2. Symptoms and signs of mite bombs, from left to right: Parasitic Mite Syndrome, Deformed Wing Virus, and Varroa Mites on Drone.
Mite Bomb Management
Mite bomb management is influenced by several factors, including colony strength, mite population levels, seasonal conditions, timing of treatment, and the presence of other diseases. Analyzing these factors collectively allows beekeepers to make informed decisions about whether to apply a flash treatment at the colony’s original location, relocate the colony to a hospital yard, or euthanize it.
Flash Treatment – Colonies with high mite levels often have strong bee populations, as a greater number of bees can typically support a larger mite population without immediately collapsing4. However, it is recommended to apply flash treatments when infestation levels exceed 10%, as they are particularly effective in controlling mite bombs. Since the strength of the colony can temporarily reduce the likelihood of robbing and drift, thereby mitigating the risk of a mite bomb explosion, it’s advisable to apply the flash treatment to strong mite bomb colonies at their original location, rather than relocating it. This is particularly recommended during the spring and summer, when robbing is less likely to occur. Common flash treatments include formic acid (liquid 65% and formic pro) and oxalic acid (drip and sublimation). By combining a flash treatment with the inherent resilience of a strong colony, along with consistent monitoring, beekeepers can effectively reduce mite levels and promote the recovery of the colony to a healthier state.
Hospital Yards – Utilizing hospital yards is also an effective strategy for managing mite bombs. Colonies that appear to be dwindling due to high Varroa levels and are not improving with treatment should be relocated from their original apiary to prevent the infestation of surrounding healthy colonies. Additionally, the presence of other diseases alongside high Varroa loads can further justify moving a colony to a hospital yard. To reduce the risk of drift from bees left behind, it is crucial to conduct relocations at night or early in the morning when most bees are inside the colony. Establishing hospital yards helps mitigate the risk of infecting healthy colonies and enhances the management of diseased colonies.
Euthanasia – Deciding when to euthanize a colony is a balancing act. It’s crucial to act before transmission of Varroa mites from the mite bomb to surrounding colonies is imminent. Key factors to consider are the strength of the colony and the season. Weak colonies with high Varroa loads should be euthanized immediately, as they are more vulnerable to robbing, which is a major pathway for Varroa dispersal to healthy colonies. This is especially important in the fall when robbing is more likely, and mite levels can spike. In the spring, addressing mite bombs is equally critical. If mites spread from mite bomb colonies early in the season, they can increase Varroa levels in the apiary just as summer approaches—a time when mite populations usually rise. Therefore, any colony that is weak and has high mite levels should be eliminated.
When euthanizing a colony, it’s essential to contain the mites. Simply shaking out the colony isn’t recommended, as it can allow infested bees to drift to other hives. The best method is to shake the entire colony into a bucket of soapy water, ensuring that all bees and mites are disposed of safely.
With Varroa mites remaining one of the most significant disease threats in Alberta, it is crucial to implement effective IPM practices to safeguard the beekeeping industry. Recognizing the signs and symptoms of mite bombs can help beekeepers maintain low Varroa levels, thereby ensuring the production and sustainability of healthy honey bee populations. Training apiary workers to identify mite bombs can enhance operational monitoring, as they are often tending to colonies and can readily perform mite shakes on-site.
Additionally, utilizing different management practices such as flash treatments and hospital yards can improve the overall health of an operations colonies while enhancing the effectiveness and efficiency of treatment and monitoring efforts. Maintaining low mite levels will enable the bees to thrive, ensuring optimal honey production and effective pollination services.
References
1Peck, D. T., & Seeley, T. D. (2019). Mite bombs or robber lures? The roles of drifting and robbing in Varroa destructor transmission from collapsing honey bee colonies to their neighbors. PloS one, 14(6), e0218392. https://doi.org/10.1371/journal.pone.0218392
2Currie, R. (2008). Economic Threshold for Varroa on the Canadian Prairies. University of Manitoba, Dept. of Entomology, Winnipeg Manitoba. https://capabees.com/shared/2013/02/varroathreshold.pdf
3Lee, K. V., Moon, R. D., Burkness, E. C., Hutchison, W. D., & Spivak, M. (2010). Practical sampling plans for Varroa destructor (Acari: Varroidae) in Apis mellifera (Hymenoptera: Apidae) colonies and apiaries. Journal of Economic Entomology, 103(4), 1039-1050. https://doi.org/10.1603/EC10037
4 Borba, R. S. et al. Phenomic analysis of the honey bee pathogen-web and its dynamics on colony productivity, health and social immunity behaviors. PLoS one 17, e0263273 (2022).