by Dr Carlos Marín This case study, authored by Carlos Marín, a veterinary graduate who began specializing in bee health and production in 1999, explores the challenges of managing Varroa destructor infestations in Iberian black bee colonies at a northern Spanish commercial apiary.
Since starting his specialization, Carlos Marín has worked as a field technical advisor for various beekeeping associations. He has also conducted beekeeper training programs throughout Spain, focusing on the professional development of the sector and improving the profitability of beekeeping operations. Currently, he continues his advisory work, overseeing technical management and practices for a beekeeping company.
The study highlights the importance of regular monitoring and the complexities of interpreting infestation data due to the dynamic interplay between bee colony demographics and mite populations.
Two apiaries were monitored during August and September 2024, revealing a dramatic increase in Varroa infestation levels over a short period. The Corrected Bee Parasitization Index (CBPI) surged nearly tenfold within two months, escalating from initially acceptable levels to severe infestation.
This rapid change was attributed to shifts in colony conditions, particularly the bee-to-brood ratio, rather than a simple increase in mite populations. The findings emphasize the importance of frequent, context-aware monitoring and timely interventions in Varroa management. The case also highlights the limitations of current predictive models and underscores the need to consider both mite levels and colony conditions when deciding on treatment strategies.
In recent years, apiaries have faced significant declines in honey yields, posing severe economic challenges for beekeepers. These issues arise from multiple environmental factors (e.g., climate change, habitat reduction, and phenological mismatches) and health challenges, with varroosis being the primary concern in apiary management.
The optimal strategy for managing Varroa destructor hinges on proactive and timely intervention. However, in practice, many beekeepers face challenges such as:
Delays in treatment often lead to chronic health problems that compromise colony productivity and economic viability, especially in large-scale or migratory beekeeping operations.
To address these challenges, efficient and feasible sampling protocols must be established. Once data is collected, the analysis must account for the dynamic nature of both mite populations and bee colony demographics, including factors like:
Therefore, when conducting sampling to evaluate these indices, it’s crucial to assess the current adult and brood populations, as well as their projected growth. This comprehensive approach is essential for accurately interpreting the obtained sample values.
This clinical case comes from a northern Spanish commercial apiary where periodic colony sampling is conducted to monitor parasitic infestation levels and determine if immediate treatment is necessary.
In this apiary, hives are clustered in groups of 40 colonies. The beekeeper in Asturias, where this farm is situated, oversees more than 600 hives (40 per site) out of 5000-7000 total hives across various locations. Regular inspections involve examining 20% of each apiary (8 hives), checking their general appearance, looking for clinical signs of diseases. The following parameters are recorded:
The parasitization index in adult bees (VAR in the tables below), measured using the CO₂ test on a volumetric sample of 300 bees.
The parasitization index is later adjusted, acknowledging that approximately 20% of Varroa mites are not detached by this method. The Corrected Bee Parasitization Index (CBPI) is thus determined using the following formula:
Data is consistently collected from the same hives (which are duly identified) to allow for the evaluation of their progression over time.
On August 9th, 2024, after the honey flow period and in preparation for autumn, the following data were collected from two of the farm’s apiaries:
Varroa infestation levels appeared optimal for the season, with colonies showing robust vitality and no observable pathological signs in either the brood or adult bee populations. The colonies had reached full strength, and only moderate growth was expected during the upcoming ivy bloom period between September and October. Given this stable condition, the beekeeper opted to delay acaricide treatment until a significant brood reduction occurred, anticipated around mid-October. Over the following month, field activity remained limited, and the colonies showed reduced brood rearing.
On September 29th, the colonies were re-checked. While the general condition and bee density remained good, the colonies did not exhibit a strong brood-rearing response despite the ivy bloom. Some colonies showed symptoms of viral infection in individual bees, specifically Deformed Wing Virus (DWV). The following findings were recorded:
The CBPI index experienced a dramatic surge, escalating up to tenfold within a compressed timeframe of less than two months. While the total Varroa population could not realistically have increased tenfold in this time, the changes in colony conditions—particularly the bee-to-brood ratio—caused the apparent parasitic levels to become alarmingly high in a very short period.
Diagnosis:
The colonies are diagnosed with severe varroosis, as evidenced by the significant increase in the Varroa parasitization index (CBPI), accompanied by emerging symptoms of viral infections such as Deformed Wing Virus (DWV). The rapid escalation in infestation levels, influenced by changes in colony conditions, underscores the urgent need for intervention.
Treatment strategy:
At this stage, heavily infested brood is unable to produce a sufficient number of healthy bees to sustain the colony through overwintering. The priority now is to reduce the Varroa load before severe brood reduction occurs in late fall and early winter. Delaying treatment until brood reduction, although improving treatment efficacy, risks further compromising colony health and exacerbating the damage caused by Varroa destructor and associated viral infections (e.g., Deformed Wing Virus – DWV). This would significantly reduce the colonies’ chances of successfully overwintering.
To address the current situation, the following strategy is recommended:
– Current treatments typically achieve 80–90% efficiency when applied in the presence of brood, as is the case here. However, a single treatment will likely not reduce infestation to acceptable levels for the upcoming season.
-A second treatment is recommended when brood levels decrease to residual amounts.
-The goal of the second treatment is to reduce Varroainfestation to levels below 1% before the start of the new beekeeping season. To achieve this, systems aimed at minimizing brood production during the second treatment phase are advised.
The explosive breeding patterns of the Iberian black bee, particularly in situations where colonies have a large amount of brood but relatively few adult bees, can obscure the true Varroa infestation levels. This phenomenon makes it challenging to accurately detect the mite or can create a false sense of control during routine samplings.
Conversely, abrupt interruptions in brood production due to adverse weather conditions—common in the environments where these bees thrive—can trigger a massive reappearance of Varroa as mites emerge from the brood cells and concentrate on the adult bee population.
This underscores the importance of carefully considering:
Furthermore, there is a need to develop new predictive models to better understand and anticipate the evolution of Varroa parasitization within colonies. Until such tools are available, the most effective approach is to establish regular periodic sampling—for example, monthly inspections—to provide a more accurate and consistent assessment of infestation levels.
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