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Varroa mite wreaks havoc in Swiss bee colonies

Bees get vaccinated at the Swiss Bee Research Centre in Bern Tomas Wüthrich

Swiss beekeepers are hoping for a cold autumn and spring after last year's mild temperatures turned honeycombs into honeyed tombs – mostly because of the Varroa mite, which feeds on healthy bees.

The past few years have been tough for bee colonies almost everywhere. For example, colony collapse disorder was a massive problem in the United States in 2007. But this year has proven disastrous for honeybees and their keepers in Switzerland. A study of a thousand Swiss hives shows that  50 per cent of the bees did not survive the winter of 2011-2012.

There are many hypotheses suggesting why so many bees have died – from a build-up of pesticides to a loss of feeding area diversity, as well as climate change, fungal infections or even electromagnetic waves. However, researchers think the Varroa mite is the biggest culprit.

“Several studies show that bee colonies have trouble getting through the winter when there is a large presence of Varroa,” says Jean-Daniel Charrière, a researcher at the Liebefeld-Posieux agronomic research station and a member of an international study group.

Beekeepers also tend to think that the mite is their biggest enemy.

“There are many causes [for the high death rate] but Varroa is definitely at the top of the list. It doesn’t just weaken bees, it infects them with their virus, and makes them more susceptible to other diseases and problems,” reckons Daniel Bettens, a beekeeper in canton Vaud.

Weathering times

Whether or not Varroa mites were the biggest factor in the death of so many Swiss bees last year, the parasites have been found in Europe for the past 30 years. So why was last winter’s death rate so high? For Bettens, the weather is at least partly to blame.

“In winter, the queen bee stops laying eggs and no larvae are maturing. Last year, because of the mild temperatures, the bees continued to care for the young larvae. Varroa need larvae to breed,” as he tells

Bettens also thinks the weather extended the season, to the detriment of the worker bees: “The bees carried on making honey into October instead of resting up, and that tired them out.”

Researcher Charrière agrees: “The longer the honey-making season, the greater the chances for the Varroa to multiply. As the mites breed exponentially, damage is very quickly done.”

No quick fix in the mite fight

Apiarists do have some tools to combat the Varroa mites plaguing their hives. The current most effective one is formic acid treatment. The acid is applied to hives according to the temperature and the colony size. But it is not 100% effective, points out Bettens.

“You can never kill each and every mite. The formic acid treatment will get rid of 95% of Varroa at best. Realistically you can only hope to keep an infestation to a manageable level,” he says.

Applying the acid is far from easy. You have to choose the right period – outside of the harvest – to avoid compromising the quality of the honey.

“You have to get the dose right and apply it at the correct temperature,” warns the beekeeper. “If it’s too cold, the product simply doesn’t diffuse through the hive.”

Formic acid can also be dangerous for the bees themselves. Too high of a concentration can kill the queen bee, which means that the whole colony will die, too.

Search for solutions

Considering the complications involved with harsh chemicals, the solution to the Varroa mite headache could well be an organic one. One idea is to select European bees naturally resistant to Varroa, rather like their Asian cousins.

“Resistance should be a new criteria of selection,” thinks Bettens. “Until now, we’ve basically chosen our queen bees for their reproductive potential and their docile character.”

Easily said, not so easily done. “German researchers have been exploring this [Varroa resistance] for 15 years, but little progress has been made,” Charrière tells “We’re also looking at fungus that would be harmful to the mites but not to the bees, larvae and the hive.”

No beating around the bush

With bees dying at extraordinary rates it is easy to imagine the worst.  No bees means no cross-pollination, which would mean the end of a large number of plants, thus eliminating some of the flora that feed the fauna.

Yet Charrière is not that pessimistic. He points out that the survival of European honeybees is not necessarily doomed. “Experiments in Sweden show there is a small hope. 150 hives were infected with Varroa and left without treatment on a Swedish island. Six hives survived, which shows there may be some genetic resistance.”

The end of the honeybees would chiefly pose a problem for early-flowering plants, such as cherry, apple and pear trees. Other insects, like bumblebees, could spread the pollen for late-flowering plants.

The busy bee has no time for sorrow

With uncertainty weighing heavily on their minds, many beekeepers are finding it difficult to keep going.

“Of course this whole situation puts us in a funk,” says Bettens. “You ask yourself if it’s worth carrying on. Older beekeepers in particular are throwing in the towel. They’ve kept bees for most of their lives. Then they’ve had to carry out this treatment, and still they lose everything.”

“Practically nobody is selling bees anymore, because everyone’s been losing headcount,” continues Bettens. “Bees have become a rarity – a colony will set you back 400 francs. Do amateur beekeepers really want to spend hundreds of francs just to start over?”

Researcher Charrière thinks this dejection is perhaps the most serious consequence of last winter’s deaths. “Beekeepers put in a lot of time and effort to reconstitute their colonies. It’s thanks to them that there is no cross-pollination emergency at the moment,” he emphasises.

Despite the costs, the passion for apiculture still means many, if not most, beekeepers are trying to keep their colonies together. But another winter like last year could be fatal to morale as well as to the bees. Beekeepers will be hoping for a cold winter.

Varroa (varroa destructor) is a parasitical mite, originally from south-east Asia. It looks a little like a small flattened red crab.

The mite lays its eggs in the honeycomb chambers where the bee larvae develop. Varroa larvae feed off the food deposited for the young bees, as well as feeding directly off the bee larvae.

Varroa weakens the honeybees: in the honeycomb, the parasite feeds on the hemolymph (insect blood) of bee larvae. That leads to a weakening of the immune system, malformations and the transmission of disease.

The Asiatic honeybee (apis cerana) has natural resistance to the Varroa parasite. The European honeybee (apis mellifera) has no Varroa resistance.

Varroa mites can travel easily by clinging to worker bees and drones. Buying and selling bees also aids the spread of the mites.

The Varroa parasite is native to Asia, and it slipped out of the continent in the 1950s.  Varroa has been observed in Europe for the past 30 years – it was first noted in Switzerland in 1984. Only Australia and some parts of Africa remain Varroa-free.

There are around 19,000 apiarists in Switzerland, with around 170,000 colonies.

Switzerland has the highest density of colonies in the world, with 4.5 colonies per square kilometre.

Each hive produces around 10 kilos of honey.

Honey farmers and hives tend to remain static, meaning the plants that are cross-pollinated vary according to region. But the most pollinated plants are dandelions, fruit trees, rapeseed, acacia, chestnut and rhododendron.

Source: Agroscope

(Translated from French by Victoria Morgan)

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