Varroa mite

Varroa destructor

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The Varroa mite(Varroa destructor) originates from Asia and parasitizes bees and bee brood. It reproduces in the bee brood. In addition, it is a vector of various bee viruses. Both the parasitization and the transmitted viruses damage bees and brood stages. It has been present in Austria since about 1980 and is now found in every bee colony. It is an important trigger of colony collapses and is associated with high winter losses.

Occurrence

In Asia, the varroa mite lives in colonies of the Eastern honey bee(Apis cerana). In the course of evolution, these bees have developed effective defense mechanisms to keep varroa infestation low. Therefore, infested colonies of this bee species do not perish from varroa mite. In the last century, Western honey bees(Apis mellifera) were brought to Asia by humans and thus came into contact with the varroa mite. As a result, the mite was able to use the Western honey bee as a host as well, and spread almost all over the world. Currently, the varroa mite is found in Asia, Europe, Africa, North and South America.

Host animals

Varroa mites are generally present in every bee colony. During the breeding season of the bees, the varroa mites multiply in the colony and thus become a serious problem. For reproduction, the Varroa females migrate into brood cells that are ready for mating. The complete development from egg laying to mating of the young females then takes place in the closed brood cell. During this time, both the mother mite and the offspring feed on the bodily fluids of the bee larva. With the hatching young bee, the mature, already mated Varroa females also leave the brood cell and transfer to other bees. Thus, a new cycle begins again.

The number of female offspring per reproductive cycle is different in worker and drone brood. In worker brood, one to two adult daughter mites are produced per mother mite, while in drone brood two to four daughter mites are produced due to the longer mating period. This can lead to high infestation numbers and the development of varroosis symptoms in the course of the season.

Infestation of bee brood has the greatest impact on the health of the colony, as the brood is damaged by the mites in the course of its development (withdrawal of nutrients from the bee larvae, transmission of viruses). Infected bee larvae give rise to damaged young bees with shortened lifespan, crippling and reduced performance.

Infection route

In Austria, all bee colonies show Varroa mite infestation and are thus infected. Local spread of the mite and thus new infection or reinfection of neighboring colonies occurs especially when colonies collapse unnoticed in summer or fall due to varroosis (e.g. wild colonies, insufficient care or experience of the beekeeper). These collapsing colonies run the risk of being robbed by bees from other colonies. In the process, many of the mites are taken by the robbing bees into their own colonies, thus dangerously increasing the mite population in the colonies (=reinfection). The flight of mite-infested bees between different hives or apiaries, as well as the international trade in bees, contribute significantly to the spread of the Varroa mite on a local and global level.

Inkubationszeit

Symptomatology

Varroosis is the symptom pattern that occurs in colonies of the Western honey bee in the course of a massive varroa infestation - usually in combination with virus infestation. The symptoms affect both the adult bees and the brood. Only when there is already severe damage to bees and brood, the symptoms of varroasis become obvious. However, even if the colony looks strong and healthy at first glance, a dangerously high number of varroa mites may already be present.

Typical signs of a colony with varroosis are bees with shortened, crippled wings, with shortened abdomens, varroa mites visible on bees and combs, and a patchy brood nest. Colonies dead from varroosis were typically strong until recently and were producing abundant honey. Now, either the hive is largely empty of bees or there are still large quantities of dead bees in the bottom board. On the combs are capped, not hatched brood cells, which usually contain large amounts of mites. Often there are several mites in one brood cell. Usually there are also abundant honey and pollen stores. Such colony collapses usually occur in late summer, fall and winter.

Therapy

To get the varroa mite under control, an integrated concept of varroa control is required. This must be planned and coordinated with the farm. It consists of a combination of successive measures to reduce infestation over the course of the year. The measures have to be adapted to the seasons, the period of the grape harvest, the state of the colony and the type of operation (conventional or organic beekeeping). For details on the integrated concept of varroa control, please refer to the technical information below and the PDFs in the download area.

Prevention

All methods of varroa control that stop or slow down the growth of the varroa population in the colony serve to prevent varroasis. Particularly worthy of mention here are the biotechnical methods that do not involve the use of veterinary medicines. These methods can also be used during the breeding period, when veterinary medicines may not yet be used. Their effectiveness is based on the fact that in colonies with brood, the majority of varroa mites are found in the capped brood cells. If this brood is partially or completely removed from the colony, the mites in it are also removed. This delays the development of a critical varroa infestation. For details on biotechnical methods, please refer to the technical information below and the PDFs in the download section.

Another prevention strategy is breeding varroa tolerant bees. Breeding aims to select colonies that are themselves capable of limiting Varroa mite infestation to such an extent that no, or reduced, additional control measures are required. In the existing bee population, those colonies are identified that are more resistant to the mite due to their genetic characteristics. These colonies have the ability to remain healthy and productive despite Varroa mite infestation. Information on this can be found in the folder "Varroa mite - selection for varroa tolerance" in the download area.

Situation in Austria

The Varroa mite has been present in Austria since about 1980 and can now be found in every bee colony. According to the new EU Animal Health Law (AHL), infestation with Varroa spp. (varroosis) is notifiable. Analogous to the previous regulation, it is planned that notification will only be mandatory in the case of epidemic occurrence of varroosis. Details are currently being worked out (as of January 2021).

Specialized information

Varroa control - Use of veterinary medicinal products (TAMs)

TAMs used to control varroa mites must be approved in Austria. All approved TAMs are listed in the Pharmaceutical Specialties Register. By selecting the target species "honey bee", you can retrieve all information on medicinal products that are approved for use on honey bees in Austria. Appropriate records must be kept on the use and type of preparations used (template see download area). These records have to be kept for five years and have to be presented during controls.

Currently, TAMs with the active ingredients formic acid, thymol, amitraz, flumethrin and oxalic acid are approved in Austria (as of January 2021). All available TAMs have advantages and disadvantages. For example, formic acid is effective against mites on bees and in capped brood, but can cause damage to brood and queen if temperatures are too high. Oxalic acid acts against mites on bees but not in the capped brood cells, so it should only be used when brood is free. Thymol preparations develop their effect slowly, therefore they are not recommended in case of already heavy varroa infestation. So-called "strip preparations" with long-term application over several weeks are easy to use, but are only effective if there is not yet resistance of the varroa mite to the active substances used. For details on the advantages and disadvantages of the various TAMs, please refer to the folder "Varroa mite - Application of veterinary medicinal products" or the instructions for use of the approved preparations. Please note that effective control of varroa mite is only guaranteed with correct use of the selected TAM. It is therefore necessary to carefully read and follow the instructions for use of the selected product. This is the only way to ensure that you achieve good efficacy against varroa mites and at the same time protect your bees from undesirable side effects of the product.

Formic acid

Formic acid is applied for the main summer mitigation and is introduced into the colony by means of evaporators. The main principles of formic acid treatment are summarized in the video below.

Thymol

TAMs with the ingredient thymol are introduced into the bee colonies in ready-to-use form as strips, platelets or gel. They are used after the end of the honey season for the main summer mitigation.

Contact strip preparations

The active ingredients amitraz and flumethrin are introduced into bee colonies in Austria by means of contact strip preparations. Depending on the preparation, they are available only on prescription and/or at pharmacies. Contact strip preparations are used after the close of the honey bee season for the main summer mitigation.

Oxalic acid

Oxalic acid is used in brood-free colonies and is introduced into the colony either by trickling or by evaporation. It is used as standard for residual mitigation when brood is not present in winter. It can also be used in the summer after the end of the season, if the colonies have been made brood-free beforehand by appropriate measures. The main principles of treatment with oxalic acid using the drip method are summarized in the video below.

Varroa control with biotechnical methods

Biotechnical methods include all measures to control varroa mites that do not involve the use of veterinary medicines. Their effectiveness is based on the fact that in colonies with brood, the majority of varroa mites are found in covered brood cells. If this brood is partially or completely removed, the mites in it are also removed. At the same time, the brood stages affected by viruses are also eliminated from the colony. In case of heat treatment of bee-free brood combs, the mites in the brood are killed.

The advantage of biotechnical methods is that they are applicable when veterinary medicines cannot be used (e.g., during the tracht or due to the mode of operation). In addition, some biotechnical methods have multiple benefits: For example, making scions with capped brood combs serves both to increase colonies and to reduce the number of varroa mites in the colonies of origin. Many biotechnical methods can be combined well with a renewal of the comb material. In this way, pathogens and permanent stages are also removed from the colony along with the old comb material, thus reducing the risk of future diseases at the same time.

A disadvantage of biotechnical methods is the increased labor and material costs. This results from the measure itself and from the associated accompanying measures, such as the melting down of combs and the renewal of the comb construction (frames, middle walls, etc.). For details on the advantages and disadvantages as well as the implementation of various other biotechnical methods, please refer to the folder "Varroa mite - use of biotechnical methods".

Typical biotechnical methods are:

  • Removal of capped drone brood
  • Removal of capped worker brood
  • Brood interruption
  • Honeycomb method
  • Total brood removal
  • Heat treatment

In the following video we explain how to perform the total brood removal, which is used for the main spleen removal:

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DI Hemma Köglberger

Last updated: 02.02.2024

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