Measuring only around 1.6 mm, the small Varroa mite (Varroa destructor) is the biggest enemy of the honey bee. Having spread almost all over the world, there is barely a honey bee colony that is not affected by this pest. The only exception so far is Australia, which has one of the healthiest populations of the Western honey bee (Apis mellifera) worldwide. The Varroa mite sucks the blood of honey bees and additionally transmits deadly viral diseases. There is now broad scientific consensus that it represents the greatest threat to the health of bees.
The Varroa mite came originally from Asia. It was first described 100 years ago on the island of Java in Indonesia. The Dutch zoologist Anthonie Cornelis Oudemans called it Varroa jacobsoni. Originally, the mite infested the Asian honey bee (Apis cerana), which over thousands of years successfully adapted to act as a host to the parasite: it fights the mites through strong cleaning practices in the hive and other behaviours, thereby limiting the damage to the colony.
When European settlers brought the Western honey bee (Apis mellifera) to Asia in the early 19th century, the Varroa mite infested these colonies as well. The parasite with the infected bee colonies was then introduced via Russia and Eastern Europe to Germany in 1978.
Initially it was assumed that only the Varroa jacobsoni mite is responsible for the global bee damage, but more recent genetic studies conducted in 2000 have shown that there are 18 genetically distinct types, which today are divided into two main groups: Varroa jacobsoni and Varroa destructor. In the meantime, the newly identified type Varroa destructor has been causing extensive damage in Europe, since the Western honey bee has no adequate defence functions. A stable host-parasite relationship has not yet materialised.
Meanwhile, the mite has spread to almost all regions of the world: For the first time, even New Zealand and Hawaii reported an infestation in 2000. So far, only Australia has been spared.
The Varroa mite gets from one hive to another, riding on the honey bee as a passenger. While searching for food, bees from different colonies repeatedly come in contact with one another, even over distances of several kilometres. Bee colonies that have been treated for Varroa are therefore also exposed to the invasion pressure of untreated colonies, they can get infected again. Furthermore, high bee density in some areas favours the spread of mites. The introduction of new Varroa mites, the so-called mite re-invasion, has been underestimated for a long time.
The Varroa mite infects the adult bees and also their brood. But the parasite reproduces only in the closed brood cells of the bees. Shortly before sealing, the Varroa females enter the brood cells and migrate to the bottom of the cell, they hide among the bee larvae, to protect themselves from the brood-caring bees.
There, they are in the larval food of the bee brood. Once it is used up, the Varroa mite stings the bee larva and starts the haemolymph to suck a blood-like liquid. The immune system of the larva is attacked, their life expectancy is declining. Reproduction of the Varroa mites takes place only in the closed brood cells of the honey bee, and a Varroa population can double every three to four weeks during the season.
Before the bee hatches, all female mites must mate, because after the bee hatches, the males and unmated female mites die. The mated Varroa females can survive outside the brood cells by firmly attaching themselves to adult bees and feed on the haemolymph. The parasite has optimally adapted its habitat and food to its host.
While sucking on the larvae, the parasite also transmits harmful viruses directly into their haemolymph. These viruses can spread and damage the bee during this sensitive development phase. So far, around 20 different viruses have been discovered.
At a glance - The Varroa mite damages the bee at different levels:
For example, the deformed wing virus (DWV) is widely spread, which can occur both in the brood as well as in the adult bees. In the latter case, however, the infection remains asymptomatic. But if the parasite transmits the virus already at a bee pupa, then the young bees develop deformed wings. The bee cannot fly and is often not able to survive.
In addition, the mite can also transmit other viruses such as the acute paralysis virus (APV), which can infect adult bees and larvae. It is mainly found in the fat body and in the salivary glands of the bee, but does not cause typical symptoms then. However, during the transmission through the Varroa mite, the acute paralysis virus enters directly into the haemolymph of the bee. From there, it attacks the vital organs. In the brain, the virus, for example, causes disorders in behaviour, orientation and the development, such consequences are fatal to the bee. Especially in winter bees, the infection with APV is critical because the winter hardiness is greatly impaired.
From an evolutionary point of view, since the Western honey bee did not have to deal with the entrained parasite for too long, it has not developed adequate defence functions yet. Without the help of beekeepers, a bee colony that has been infected by the Varroa mites, generally dies within one to three years.
Therefore, for beekeepers worldwide, the control of the Varroa mite, is one of the main tasks in ensuring the health of bees. Especially in late summer, their most important work is to contain the infestation in the hive. This is the only way that a sufficient number of winter bees survive the cold weather, to be able to become a strong colony the next spring again.
Before the beekeeper can take any treatment measures, he must regularly check whether and to what extent his bee colonies are infested with the Varroa mites. Then he decides what course of action is appropriate. Diagnoses should also be carried out during and after the mite treatment to verify the success of the measurement.
To control the Varroa mite, there are different chemical or biotechnological options currently available.
Important: A single treatment at the end of the bee season is not enough to protect the colony against the parasite. Although the number of mites is reduced late in the year, but bees that have previously evolved during the heavy mite infestations, are greatly weakened. It is uncertain whether the colony will survive until spring. Beekeepers must also combine appropriate methods depending on the location and their own operation. Temperature and environment-related products for the Varroa treatment are only suitable in certain regions of the world.
In many countries, the opportunities available are limited. Nonetheless, the available medicinal products for bees and biotechnical measures, combined with good beekeeping practice, should at least limit the influence of the pest.
The breakthrough in breeding the Varroa tolerant lines of the Western honey bee was not yet successful, so far, there are no treatment options for the numerous bee viruses available.