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Varroa

Introduction
Varroa destructor
(Anderson and Truman) previously described as Varroa jacobsoni (Oud) is a parasitic mite of adult bees and brood. In the past hundred years or so it has become the most serious pest of Western honeybees across the globe, particularly for the European honey bee Apis mellifera which lacks natural defences to be able to deal with the mite by themselves. When populations overwhelm a colony, it leads to a disease called Varroosis and if left untreated an infested colony will usually die within 2-3 years.

Current status of Varroa
Varroa
is present on all continents with the exception of Australia. It was first found in the UK on the 4th April 1992 in Devon, however, its presence is now no longer notifiable in the UK. The Bee Diseases and Pests Control (England) Order 2006 came into force on the 17th March 2006 where Defra formally revoked, under the powers of the Bee Diseases Control Order 1982, the Statutory Infected Area for Varroa in England.



Life cycle
The adult female mites commonly seen within the hive and on the bees have flat, reddish-brown oval bodies, greater in width than length (1.6 x 1.1mm). The female mites enters an open cell, just before the cell is about to be capped where it will hide in the brood food situated under the larva. There it will wait for 2-3 days, breathing from a respiratory organ known as a peritreme. Once the food has been consumed by the larva, the female mite will begin reproduction inside the sealed cell, laying her first egg (generally male) and later, in intervals of around 1-2 days, the female will continue to lay up to seven eggs which are usually female mites. These hatch into immature mites of which only two to three reach adult stages.

While inside the cell, the mites will feed on the developing pupae, transmitting viruses which can shorten the life span and stunt the pupa’s development. After 21 days, the worker bee emerges (24 for male drones) along with any surviving mites which attaching themselves to the bee’s dorsum until they reach a new open cell with a larva in it.

As well as natural reproduction, mite populations are also increased between colonies through robbing, drifting and swarming. A severe build up of mites in a colony will lead to visual symptoms of Varroosis



Symptoms of Varroosis
Depending on climatic conditions, visual symptoms caused by V. destructor are observed from autumn when the amount of brood being reared is reduced and mite numbers are at their highest through to early spring during the overwintering phase. Severe infestations of Varroa may lead to:

Deformed wings which are shrivelled and adopt a ‘spaghetti’ like appearance;
Stunted abdomens;
general weakening of the colony;
Patchy/ pepper pot brood patterns;
High level infestations can be a direct cause of colony loss;
The mite is also a vector of a number of viruses. Although bee viruses usually persist as unapparent infections and cause no overt signs of disease, they can dramatically affect honey bee health and shorten the lives of infected bees under certain conditions.

Varroa Treatments
In order to keep healthy and productive colonies, the Varroa mite must be controlled. This can be done through treatments which fall one of two main categories, each of which have their pros and cons; the beekeeper will have to choose appropriate controls to suit their methods of husbandry.

Biotechnical Controls: These avoid the use of chemicals and utilises methods based on bee husbandry to reduce mite populations by physical means alone. Biotechnical methods exploit the fact that mites reproduce in bee brood and the most common of these include trapping mites in comb (usually drone comb). The comb is then cut out and destroyed, along with the mites contained within the cells. Generally, these methods are only used during the spring and summer months when drone brood is being reared and they can be a great benefit in delaying when treatments are needed. If you would like to find out more information about each individual technique, please consult our ‘Managing Varroa’ advisory leaflet.

Varroacides: Most varroacides are an important feature in beekeepers control plans because they are highly effective at killing and controlling Varroa mite populations. There are two classes of varroacides, those that contain synthesised proprietary chemicals (‘hard’ varroacides) and those that contain synthesised chemicals which are found in nature ( ‘soft’ varroacides) e.g. formic acid or essential oils.

When using any treatments it is important that you read and follow the label instructions and only use medicines that are approved under EU law. Information about products that are authorised for use in the UK, as well as those available under Cascade is available on the Veterinary Medicines Regulations (VMR) 2013 pages of our website.



Further reading

  • More information about Varroa, including monitoring methods, resistance management and treatment information, is available in our advisory leaflet "Managing Varroa";

  • The World Organisation for Animal Health have published a chapter on Varroa destructor which outlines it’s biology and methods for detecting it. This can be viewed here.