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It has stronger arms than a sumo wrestler, a better nose than a dog, and its bite is more deadly than that of a cobra. Its victims: millions of bee colonies worldwide.
The varroa mite, Varroa destructor, is the largest catastrophe with which the European honeybee, Apis mellifera, ever had to cope with. It appeared suddenly and unexpectedly and has destroyed millions of hives worldwide over the last fifty years. It is also the main reason for the annual death of bees in Switzerland, in which up to half of all hives are destroyed during the cold winter months.
The varroa mite is native to Asia and has originally only infested the Asian honeybee, Apis cerana. It has lived with the menace for millennia and has in the course of evolution developed a number of defence strategies against it. One of them is a thorough hygiene: Asian honeybees clean each other of mites and bite them to death.
In the first half of the 20th century, humans have brought the European honeybee to Asia. Suddenly, the varroa mite found a new victim, who did not know how to defend itself. As a result, the pest spread from the 1950s onwards like wildfire from Russia, to China, Pakistan, Europe to South and North America. In Switzerland, the first hives were infested in 1984
Varroa is perfectly adapted to the lifestyle of honeybees. It has a number of olfactory organs with which it can smell the odour of the bee larvae. This «baby smell» also adheres to the nurse bees, who are entrusted with the care of the larvae. Female mites, like the
one pictured here, hold on tight with their strong arms to the nurse bees and let themselves be carried to the offspring.
Once there, the mite squeezes between the larva and the wall of the brood cell to the bottom of the honeycomb. There, it lets itself fall into the feeding honey and sinks immediately in it. Like this, the nurse bees cannot discover it. During this time it breathes by a kind of retractable snorkel, which protrudes from the honey. Its ultra-flat body shape helps the mite to find the gap between honeycomb wall and larva. Hundreds of elongated ‘hairs’ and ‘claw-like’ structures on its dorsal shield hold the parasite in place and avoid that it is pulled or pushed backwards, away from the feeding honey and out of the comb.
Soon after the mite has fallen into the feeding honey, the larva reaches its pupal stage, in which it transforms itself into a honeybee. To foster transformation, the nurse bees cap the honeycomb with a lid. In its cavern, the larva eats the last bits of honey and so frees the varroa mite from its liquid prison.
Now the mite can start producing eggs. The first egg is always unfertilized and will develop into a male. All other eggs are fertilized and will become females. Just hours after oviposition the young mites hatch. In order to feed them, the mother bites a hole in the bee larva. From this wound, her offspring feast for a week; then they are full-grown.
The subsequent mating takes place exclusively between siblings. Under the protection of the still sealed brood cell, the brother copulates with his sisters. Each fertilized female can lay eggs two to three times during its life provided it finds a brood cell shortly before cell capping.
The bee larva completes its metamorphosis despite its permanently bleeding. But it hatches from its cell as a weakened bee that already suffers from a significant weight loss. In addition, the varroa mites transmit various viruses, which have so detrimental effects on the colony that it usually dies within two to three years without the help of humans.
Research article: The biology of the varroa mite.
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