Varroa Mites


Mite on a honey bee

Varroa destructor

Varroa destructor is an external parasitic mite that attacks and feeds on the honey bees. It was originally only found on the Eastern Honey Bee, but has in recent years moved onto the Western Honey Bee.

The adult female mite is reddish-brown in color, while the male is white. Varroa mites are a flat, button shape. They are 1–1.8 mm long and 1.5–2 mm wide, and have eight legs, beng part of the Arachnid family.

An infestation of Varroa mites can damage a colony in two main ways:

  1. Direct weakening of the Bee through feeding - Varroa mites attach to the body of the bee and weakens the bee by feeding on the hemolymph.

  2. Virus transfer - Varroa mites can transfer viruses to the Bee whilst feeding, high Varroa numbers are a major sign of stress within a colony, increasing the susceptibility to viruses.

Varroa mite infestations and the increases in virus load they cause are seen as the most economically damaging issue affecting global beekeeping.

Varroa mites life cycle

Integrated pest management (IPM)

Unfortunately the Varroa mite is now endemic to scottish Bees, so it is something that will be found every year whilst beekeeping. This means that prevention is not a option, we can only manage the mite numbers so that the infestations are small enough that they are of no overall threat to the colony.

Integrated Pest Management is the procedure than is used to control Varroa mite infestation levels.

The key points of IPM are:-

  1. Monitor colonies for infestation.

  2. Treat only when necessary.

  3. Choose treatments appropriately by season, level of infestation and colony strength.

There are two methods commonly used to monitor the level of Varroa mite infestation within a colony:-

  1. Daily Natural Mite Drop.

  2. Sealed Drone brood inspection.

Daily Natural Mite Drop

All modern hives have an open mesh floor below them, not a sealed floor. This allows better ventilation through the colony and promotes good Bee health. It also allows for a passive sampling of the Varroa mite load within the colony.

A sticky board can be placed below the mesh and can be checked weekly for mites that have fallen from the hive and become stuck on the board. If the weekly count of mites is then divided by 7 the average daily mite drop number can be found.

The following table gives a rough guide as to what actions are required, depending on time of year and Varroa Mite count:-

When to Act

Sealed Drone brood inspection

A second method for monitoring Varroa load is by inspecting a sample of sealed Drone brood and counting the number of Varroa mites present. At least 50 Drone cells are uncapped and the pupae within are removed. The red Varroa show up very clearly against the white pupae, allowing them to be easily counted. This can only be done during the summer months when drones are actively being produced within the colony.

The following table gives a rough guide as to what actions are required, depending on time of year and Varroa Mite count:-

Control Measures

Once the level of Varroa infestation has been measured by the above methods, a decision of which control method to use must be taken (if any).

There are three levels of control :-

  1. No action required - Nothing needs done immediately, monitoring continues.

  2. Light Control - Biomechanical methods are used to control Varroa numbers (see below).

  3. Severe Risk Control - The use of acaricides (chemicals which kill ticks and lice) are used to control Varroa numbers (see below).

Light Control

Beekeepers only use chemical controls as a last resort, it is better to not use them for several reasons:-

  1. Safer for the Bees - Use of chemicals can injure bees or make them less productive.

  2. Safer for Humans - The chemicals used to treat Varroa are extremely hazardous, so should only be used when absolutely necessary.

  3. Safer for Honey - The use of chemical treatments can make the honey unusable to humans.

The biomechanical controls that can be used include:-

  1. Artificial Swarming - By creating an artificial swarm and timing the movement to a new hive correctly, a large proportion of the Varroa load can be trapped in sealed cells and disposed of, reducing the overall load.

  2. Drone Comb removal - If a shallow frame is placed into the hive, the bees will create "wild" comb which they will mostly fill with drone cells. As drone cells are favoured by Varroa mites, by removing and destroying these wild comb sections, the overall mite load can be reduced.

Severe Risk Control

The following table gives a guide to some of the options that can be used to reduce Varroa numbers if the load is too high for non-chemical methods:-

The above are classified as Veterinary Medicines and should only be used by someone who is trained to handle them appropriately.