Clinical case: Persistent Varroa mite infestation despite rigorous monitoring

par Caroline Lantuejoul
14 March 2025
Varroa
6 minutes of reading

This article presents a case study written by Dr Caroline Lantuejoul, a veterinarian specializing in beekeeping and apicultural pathologies in France. It describes a deteriorating health situation in an apiary, due to poorly controlled varroa mite pressure over an entire season, despite rigorous monitoring and management measures put in place by a hobby beekeeper.

Clinical examination revealed signs of residual mite pressure after the summer treatment, confirmed by further analysis. This case highlights the importance of constant varroa mite control, and of using control strategies adapted to the level of infestation in the colonies. It also highlights the impact of climate change on beekeeping practices, requiring ongoing adaptation to better manage varroa mite infestations and associated viral diseases.

1. Presentation: history and reasons for consultation

She has been a hobby beekeeper for 10 years. At the end of winter 2024, 2 colonies were present in her apiary located in the countryside.

The beekeeper works with wooden Dadant hives and Nicot bottoms. Insulation (partition as roof insulation) is used depending on the time of year. A queen excluder is used systematically. Beekeeping practices are rigorous, with strict hygiene and monitoring measures. Colony monitoring is detailed and recorded in a health register. Bottoms are changed every spring. Equipment is cleaned (scraping or washing) and disinfected (flame or cold soda bath). Waxes are renewed regularly, and the years in which they were applied are noted on the frame heads to facilitate renewal with commercially available waxes (operculum wax).

Colonies are weighed regularly. A loss of almost 2 kg in weight per hive was recorded in early April. Substantial additions (syrup, fondant, protein patty) were made between February and May. These were estimated at between 5 and 6 kg of sugar per hive between February and May 2024. An artificial swarm was created on April 3, 2024 from 3 frames of brood from the R2 colony in spring 2024, which may partly explain the weight loss that was not quickly compensated for. Fertilization failed on the division and the colony became buzzing.

To combat varroa mites, beekeepers generally use two treatments a year (a summer treatment and a winter treatment): a long-acting solution at the end of the season (in summer) and a fast-acting solution in winter (when the brood is not present).

Below is the medication history and follow-up:

From August 23, 2023 to November 14, 2023: a summer treatment (Apivar®) was carried out. The strips were replaced in the brood at each visit.
January 24, 2024: a winter catch-up treatment (oxalic acid-based liquid solution) was carried out by dripping a little late.
July 27, 2024: a summer treatment (Bayvarol® – not registered in the USA – official dosage is 4 strips per brood box) with 4 strips placed in the brood was implemented. The strips were regularly repositioned in the brood. One strip was reported to have fallen off during the ^3rdweek of treatment.

Varroa mite counts are carried out on the beehives on a regular basis. 38 days of counts were recorded in the beekeeper’s rearing register in 2024. A summary of the counts is given below.

Two crossbar frame for drone brood trapping were installed in 2024 with the aim of removing drone brood during the season to control varroa mites. A single removal of drone brood was recorded on March 19.

As a result of the high counts on sticky boards at the end of winter, 2 passages of treatment (liquid solution of oxalic acid) 1 week apart by dribbling were also carried out at the end of March 2024.

A visit was made to the remaining hive on September 24, 2024, following a varroa infestation deemed too high despite the varroa treatment applied during the summer.

2. Clinical examination

During the visit, the hive is well maintained, as is the apiary. A field of mustard flowers is present at the rear of the apiary.

Colony inspection :

A protective tarpaulin is installed in front of the hives to optimize colony observation and apiary maintenance. Before the colonies were opened, 3 bees died on the tarpaulin at the foot of the hive. These bees had atrophied wings.

Upon opening, the colony is populous, with bees spread across 10 frames. Honey and pollen reserves are satisfactory. The beekeeper reports that the queen is from 2023. The colony has 8 frames of brood. The brood pattern is patchy, with some perforated and sunken cells. Bald brood is present, with some pupae appearing atrophied or deformed (atrophied wings). A few emerging bees were found dead with their tongues extended. Sacbrood disease is also noted. The bees, including newly emerged ones, are of normal size. No bees with deformed wings or phoretic varroa mites were observed.

Samples for analysis :

Samples of asymptomatic indoor bees and brood were taken.

3. Additional tests:

Two counts of phoretic varroa mites were carried out with CO₂during the visit. The results were close to the alert threshold for the season:

3 varroa per 37 g of bees (1.6 phoretic mites per 100 bees).
4 varroa per 38 g of bees (2 phoretic mites per 100 bees).

A multivirus PCR analysis was performed:

On indoor bees: viral loads were high for DWV-B (3.9*10¹¹genome copies/bee), BQCV (1.3*10¹⁰genome copies/bee) and SBV (2.3*10⁹genome copies/bee). ABPV, DWV-A and CBPV were not detected.
On brood: viral loads measured were high for DWV-B (1.1*10¹²genome copies/bee) and SBV (2.2*10¹¹genome copies/bee). BQCV was detected with a viral load of 1.4*10⁵genome copies/bee.) ABPV, DWV-A and CBPV were not detected.

—

Bee virus memo :

DWV-A (Deformed Wing Virus type A): This virus is one of the most common and destructive to honey bees. It is often transmitted by the Varroa mite (Varroa destructor) and causes wing deformities, preventing bees from flying and reducing their life expectancy.

DWV-B (Deformed Wing Virus type B): A variant of DWV-A, it is also transmitted by the varroa mite, but seems more virulent in certain contexts. Studies suggest that it may be more lethal to infested colonies.

BQCV (Black Queen Cell Virus): This virus mainly affects queen larvae, leading to their death and the blackening of queen cells. It is often associated with stress and the presence of parasites such as the small hive beetle (Aethina tumida).

SBV (Sacbrood Virus): Attacks honeybee brood and prevents normal larval development. Infected larvae take on a translucent, fluid-filled, sack-like appearance before dying.

ABPV (Acute Bee Paralysis Virus): This virus causes rapid paralysis and death in adult bees, particularly those weakened by varroa mites. It often causes major losses in infested colonies.

CBPV (Chronic Bee Paralysis Virus): CBPV causes hair loss on the abdomen, making affected bees appear shiny and black. Infected bees become unable to fly and eventually die in large numbers.

4. Treatment and action plan

In view of the varroa pressure remaining at the time of the visit, it was decided to carry out an additional one-week flash treatment using a registered formic acid treatment.

Counts were carried out in early November to measure residual varroa pressure once again. The count data were more favorable, although still high (1.5 varroa mites per day). The usual winter treatment should enable us to return to a satisfactory level of pressure in 2025.

5. Discussion & conclusion

This case study illustrates several important points.

The first point to note is the importance of colony monitoring and the notion of indicators needed to estimate varroa pressure on colonies at key times of the season. Monitoring varroa pressure via counts is essential. Clinical (= visual) observation is insufficient to detect excessive varroa pressure in colonies, with the risk of intervening too late

We can also see how difficult it is to control varroa pressure that is too high during the season. A single treatment is never 100% effective, and a single summer medication may not be enough to achieve a sufficient drop in varroa pressure if initial loads are too high. Varroa infestation must be kept under control throughout the season, and it is important to start a new season with a satisfactory initial pressure to limit the exponential growth of the varroa and its deleterious effects on bees.

Several phenomena need to be taken into account today, particularly in connection with climate change:

The atypical period of dearth was compensated for by the beekeeper with substantial supplies during the season. The colony was probably not significantly impacted during the season, which may also have contributed to the multiplication of varroa mites.
Periods of egg-laying disruption seem to be increasingly short or even absent, which is favorable to the continued multiplication of the parasite in the first instance, but it also complicates the application of winter treatments, which will therefore be less or even less effective. In this case, the winter treatment was probably carried out too late and in the presence of brood.
The presence of intermediate crops at the end of the season (at the end of September) created a dynamic for colonies at the end of the season, which were populous, but an anticipated resumption in the multiplication of residual varroa after treatment with a summer treatment carried out in compliance with the labeling over a 6-week period.

It therefore seems essential to adapt control measures according to the level of infestation of colonies as part of an integrated control strategy (medication and biotechnology).

For this reason, working on artificial brood-breaks (brood removal, queen caging, etc.) to optimize the efficacy of the treatments used, and adapting control measures to colony infestation, is becoming unavoidable.

Medication compliance is also an important issue. In the case described above, one of the strips fell off during the treatment period. The varroa falls measured by the beekeeper were divided by 2 over this period. The summer medication used was a contact treatment, and underdosing led to a delay in the effectiveness of the treatment. Colony activity is also essential to the efficacy of these contact medications.

Virus loads were also high, particularly for DWV-B and SBV.

___

Did you like this clinical case? Would you like to read more? Follow this link

Theses articles might interest you

Clinical Case: Honey Bee Brood Disease

by Caroline Lantuejoul
19 November 2024

This case study investigates symptoms of honey bee brood disease, revealing the presence of European foulbrood combined with high viral loads

Clinical case: Impact of Varroa on bee colony performance

by Gérald Therville
25 July 2024

Discover this case study on the long-term impact of Varroa destructor on bee colonies despite effective management observed in France.

Efficiency of varroa monitoring methods

27 August 2019

Poster presented at Apimondia 2019 in Montreal (Canada) to demonstrate the benefit of a standardised method for varroa monitoring.

Véto-pharma is a pharmaceutical lab focused on animal health, with a special emphasis on protecting honey bee colonies. We create, manufacture, and distribute a range of products aimed at tackling the critical diseases and challenges threatening bee populations.

Véto-pharma is a pharmaceutical laboratory dedicated to honey bees health that develops, produces and markets a range of drugs and products designed to combat the key diseases and problems that threaten colonies.