Varroa, an overview

Varroa destructor is an external parasite of honey bees. It is non-native to the UK and Europe and was accidently introduced in the early 1990s via the movement of infested honeybees from Asia. It has now spread worldwide.

Varroa lives harmoniously with the Asian honeybee (Apis cerana) which has adapted to it. Elsewhere, Varroa has caused untold damage to worldwide populations of honey bees including the European honeybee (Apis mellifera).

Asian worker bees have developed highly effective grooming techniques to detect and remove mites from each other.

Worker bees also learnt to detect infested younger drone brood and remove it.

The cappings of drone brood in the Asian Honeybee contain a central pore to allow gaseous exchange. When worker bees detect disease or Varroa mites in a cell, they plug the pore with wax. This suffocates and entombs the developing drone and inhibits the spread of the mite.

Apis cerana and varroa now co-exist quite happily.

The life cycle of the Varroa mite consists of two phases – phoretic (feeding and travelling on the back of a mature bee) and reproductive. In the phoretic stage, a mated female mite detects the capping pheromone produced by the brood when it’s time for workers to cap them over ready for pupation. At this point, this ‘mother’ mite enters the cell and hides in the pool of brood food underneath the larva.The cell is then capped.  As the larva gradually consumes the food, the ‘mother’ mite is revealed and attaches herself to the larva and starts to feed on it.   The larva is weakened but continues to pupate. The ‘mother’ mite produces 5 – 6 eggs.The first mite to hatch is always a male followed by four or five females. The male mates with the females and then dies within the cell. The ‘mother’ mite and any other mated females leaves the cell on the emerging worker or drone.  

The mite can spread between different colonies as it hitches a ride on drifting bees. Perhaps we should be re-thinking keeping hives in straight lines so close to each other? Beekeepers can easily spread Varroa during manipulations.  

Varroa weakens the honeybee and makes it more susceptible to other illnesses

Worker bees store food in their bodies and the Varroa feeds it depleting and weakening the worker.

‘Winter’ bees have highly developed fat bodies which enables them to live through the winter.

It is therefore vital to ensure Varroa numbers are low going into winter to limit damage.

Deformed Wing Virus

The parasitic Varroa mite weakens bees but it also vectors viruses which are what really causes the damage.

Deformed Wing Virus results in crumpled and deformed wings and stunts bees’ bodies. Infected bees are useless to the colony.  Eventually, there won’t be a sufficient number of foragers and the colony will starve. 

Factors controlling mite populations in honey bee colonies

(Those which we can affect are shown in bold)

Male Varroa have a high mortality rate so mating may not always occur.

Worker larvae have a shorter pupation period than drones resulting in an average of only two mated daughters leaving with the mother.

The reproductive rate of the mother Varroa varies and she may lay non-viable eggs.     

The beekeeper can limit Varroa numbers by maintaining good apiary hygiene and regularly changing comb.

A good apiary layout with hives well-spaced and not laid out in straight lines can help by preventing drifting.

The beekeeper can limit Varroa populations by sourcing local bees from reputable suppliers and raising her or his own queens.

Beekeepers can limit the movement of bee colonies to prevent Varroa spreading.

Swarming creates a broodless period which halt the reproductive cycle of the Varroa.

Artificial swarms or other swarm control manipulations also breaks the reproductive cycle of the Varroa mite.

The importation of bees, is it worth it?

Since January 2021 it illegal to import colonies and packages of honey bees into Great Britain. Importation of queen bees is still allowed but is strongly discouraged by the British Beekeepers Association, the National Bee Unit (DEFRA), Bees for Development and other organisations.

It may sometimes seem like a quick fix to import bees but it does pose an avoidable risk to the health of our bee population.

The Varroa mite was accidentally introduced by importation and look at the trouble that caused!  Small Hive Beetle is now confirmed in Italy but Italian bees are still regularly imported by some UK beekeepers.

Locally reared bees are best adapted to local forage and weather conditions.

Current best practice is to avoid the importation of queens or colonies in order to best assure the long-term sustainability of beekeeping in Britain.

Hygienic genes – increased grooming/mite detection and elimination.

Sugar dusting – see below

Drone trapping – see below

Queen trapping – see below

Varroa monitoring

The National Bee Unit cautiously considers a population of 1,000 Varroa mites to be a critical level for a colony of honey bees. There are various methods of counting the Varroa population in your hives and these have varying degrees of accuracy and complexity. All these methods are described in detail on BeeBase. Beebase, varroa

The simplest method is to use your Varroa boards. Click here to see a short video demonstrating this method Counting varroa. Insert a clean board under your hive and leave it for a number of days; it’s important to remember how many days! Remove the board and study it carefully. You will find fragments of wax and propolis, bits of dead bee and pollen and with a bit of practice and patience, you’ll be able to spot dead Varroa mites. Count them up and then visit the Beebase website where there’s a very useful calculator which estimates the population of mites in your colony. Varroa calculator.

The graph shows how keeping mite numbers low at the start of the season slows down the reproductive rate ensuring the colony is kept out of the danger zone for longer.

The graph also shows how Varroa numbers are increasing in August/September whilst bee numbers are decreasing. This means the colony could be in danger of being overwhelmed by Varroa if left unchecked.

Before the mite population of your bees reaches the critical level, you can consider a combination of tried and tested methods of control.

Described below are number of non-chemical treatments which can be used during the honey collecting season.

Most beekeepers use an ‘Integrated Pest Management’ system (IPM) and apply a chemical treatment after the honey harvest and non-chemical methods during the nectar flow. Other beekeepers do not use chemicals.

Treatments vary in their efficacy and so using the most effective treatment at the right time of year will help to ensure numbers take longer to increase again.

It is also best practice to treat all the colonies in the apiary at the same time to limit the opportunity of re- infestation via drifting.

There are several biotechnical (non-chemical) methods available to keep Varroa populations down and these may be used at different times of the year.

Spring

Queen trapping

This can be done at the beginning of the season after the queen has started laying. 

The National Bee Unit states that the technique has an efficacy of up to 95%. It works by restricting the queen to a single frame for nine days. A special queen excluder is placed on either side of a frame of drawn comb. Varroa attempting to reproduce enter the open brood cells within the caged comb. When the cells are capped over, the queen is released and the frame is removed along with any mites inside the cells.

The process interrupts the reproductive cycle of the Varroa mite. Full instructions on how to undertake the process are available from the National Bee Unit website: Queen trapping. If you search the site, there’s some other useful resources too and some photographs.

Culling drone brood

Varroa prefer to lay their eggs in drone brood because its capped for longer. It’s possible to use this information to your advantage in your efforts to reduce the mite population.

Simply replace a couple of frames at the edge of your brood box with super frames. The bees will typically draw-down wild comb below the super frame and raise drone brood within it.

Once the drone larvae is capped, you can cut it away from the frame.  Destroying these cells removes a large proportion of the total mite population.

Spring and summer

Sugar dusting

Sugar dusting encourages the bees to groom the mites from reach other. Sugar (icing sugar is best) is readily available, inexpensive, and harmless to your bees.

One cup of icing sugar per brood box is ideal and there are various suggested methods of application.

The sugar method only affects the phoretic mites which means that the sugar must be applied regularly in order to remove mites as they hatch.

Some experiments have shown the best coverage is achieved by removing every frame from the hive, treating both sides, and then replacing the frame. This is quite a lot of work but can be done during hive inspections. Other experiments have shown that dusting the top bars is more effective, as long as at least some of the sugar immediately falls through to the bottom of the hive.

Sugar dusting should only be done in dry weather and low-humidity conditions otherwise the sugar will clump on the bees instead of dusting them reducing the efficacy of the treatment.

Shook Swarm

The shook swarm or shakedown method is a simple and effective process. Although it may seem drastic, when it is carried out in the spring it usually has the benefit of invigorating the bees.

Shook Swarm can be used for a number of reasons, including combs change or to separate the bees from pathogens or disease spores. It is an accepted method of treating a colony that has been infected with European Foulbrood. Shook swarm has become a useful manipulation where a colony is heavily infected with Varroa. Beebase has some excellent information and photographs. Shook swarm

In the same way as the Asian honey bee developed tolerance to Varroa, there is growing evidence that leaving your colonies without chemical treatment allows them to build-up resistance or tolerance to the Varroa mite.

More beekeepers are going chemical free and there are encouraging studies showing that over a number of years, bees may be able to adapt and survive without chemical treatments. Where beekeepers have gone chemical free, they have often seen that they encounter initially high winter losses before the bees eventually reach an equilibrium.

For some small-scale beekeepers, these initial colony losses can be unacceptable and they therefore opt to control Varroa with chemical treatments.

European honeybees (Apis mellifera) have not yet fully developed a comprehensive armoury of defence against the mite. Some scientists argue however that chemical controls are hindering the development of their natural defences.

Chemical treatments, autumn post honey harvest

Currently, there are 15 authorised chemical treatments. As new products gain authorisation, they are added to the list found on the Veterinary Medicines Directorate website.

Each of these products has been rigorously tested to ensure efficacy of treatment against Varroa and safety for the bees and beekeeper although they are only safe if used correctly.

The product information leaflet about each treatment including season of application and duration of treatment can be found on the Veterinary Medicines Directorate or from suppliers such as Thorne Beekeeping or the Beebase website.

It is vital to use any treatment at the right time and to follow the instructions carefully to ensure that it is applied at the correct concentration for the size of colony and that it is removed when completed and any residue disposed of safely.

Currently, there are 15 authorised chemical treatments. As new products gain authorisation, they are added to the list found on the Veterinary Medicines Directorate website.

Each of these products has been rigorously tested to ensure efficacy of treatment against Varroa and safety for the bees and beekeeper although they are only safe if used correctly.

The product information leaflet about each treatment including season of application and duration of treatment can be found on the Veterinary Medicines Directorate or from suppliers such as Thorne Beekeeping or the Beebase website.

It is vital to use any treatment at the right time and to follow the instructions carefully to ensure that it is applied at the correct concentration for the size of colony and that it is removed when completed and any residue disposed of safely.

API-Bioxal, 886 mg/g Powder                                            Oxalic acid

Apguard Gel                                                                          25% Thymol

Apilife Var Bee-Hive Strip                                                  Camphor, Eucalypus oil, Menthol Levo, Thymol

Apistan 10.3% w/w                                                             Tau Fluvalinate, Mite resistance in UK

Apitraz 500 mg Bee-hive Strips                                          Amitraz

Apivar 500 mg Bee-hive Strips                                           Amitraz

Bayvarol 3.6 mg Bee-hive Strips                                        Fluethrin, Mite resistance in UK

Dany’s BienenWohl Powder and Solution                        Oxalic acid dihydrate

MAQS Formic Acid 68.2g Beehive Strips                         Formic acid

Oxuvar 5.7%, 41.0 mg/ml Concentrate for Solution         Oxalic acid

Oxybee Powder and Solution                                             Oxalic acid, didydrate

PolyVar Yellow 275 mg Bee-hive Strip                              Flumethrin

Thymovar 15 g Bee-hive Strips                                           Thymol

VarroMed 5 mg/ml + 44 mg/ml dispersion Formic acid, Oxalic acid dihydrate VarroMed 75 mg + 660 mg dispersion Formic acid, Oxalic acid dihydrate

  Beekeepers are required by law to keep proof of purchase and a record of purchase, administration and disposal of all veterinary medicines for a minimum of 5 years under the Veterinary Medicines Regulations 2011SI 2159 Forms can be downloaded from the National Bee Base website.  
  The overriding caveat is to only treat when necessary, use authorised products, alternate treatments to limit resistance, follow the instructions carefully and keep appropriate records of applications.  

There are a number of simple methods to assess the level of a Varroa population within a colony and treat accordingly.

Bee Base offers an excellent advisory leaflet which can be downloaded free-of-charge from its website. The leaflet describes in details everything that has been summarised here. In addition, Beebase has a whole range of advisory leaflets and booklets that you can download covering all aspects of beekeeping which are available free of charge here: Beebase advisory leaflets

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