Dave Hackenberg's bees have been on the road for four days. To reach the almond orchards of California's Central Valley, they undertake a 2,600-mile journey from Florida. Each truck carries close to 500 hives, tethered with strong harnesses and covered with black netting to prevent the millions of passengers from escaping. When the drivers pull over to sleep, the bees have a break from the constant movement and wind speed, but there's no opportunity to look around and stretch their wings.

Their final destination is some two hours north of Los Angeles. Every February, the valley plays host to billions of honeybees as trees burst into blossom. In the cool hours after sunset and before sunrise, more than one million hives are unloaded by commercial beekeepers such as Dave Hackenberg, who have travelled from the far corners of the US to take part in the world's largest managed pollination event. The mammoth orchards of Central Valley stretch the distance from London to Aberdeen, and the 60 million almond trees planted with monotonous uniformity along the 400-mile route require half of all the honeybees in the US to pollinate them - a staggering 40 billion.

By February 16, National Almond Day in the US, the trees are usually covered in flowers and humming with the sound of busy bees. Attracted by the sweet nectar that each flower offers, the bees crawl around on the petals to find the perfect sucking position. As they do so, their furry bodies are dusted with beads of pollen. As they fly from blossom to blossom in search of more of the sweet energy drink, they transfer pollen from the male part of the flower to the female part, and so fertilise it. Not long afterwards, the plant's ovaries swell into fruit, which by late August turn into precious, oval-shaped nuts.

Without this army of migrant pollinators paying a visit for three weeks every year, the trees would fail to bear the almonds that are California's most valuable horticultural export. But the supply of almonds in confectionery, cakes and packets of nuts is now threatened by a mysterious malady that is causing honeybees to disappear.

Hackenberg was the first beekeeper to report that his bees had vanished. A few months ago he checked the hives in his Florida bee yard to find they were empty. "They weren't dead, they were just gone," he recalls.
Since then, close on two million colonies of honeybees across the US have been wiped out. The strange phenomenon, dubbed colony collapse disorder (CCD), is also thought to have claimed the lives of billions of honeybees around the world. In Taiwan, 10 million honeybees were reported to have disappeared in just two weeks, and throughout Europe honeybees are in peril.

In Britain, John Chapple was the first to raise the alarm. In January 2007, he lost all of the 14 colonies in his garden in west London.

Chapple, who chairs the London Beekeepers' Association, says, "I could attribute some losses to a failing queen bee or wax moths, but there were a few I could find no reason for." His inquiries as to whether the parks where he kept some of his hives had sprayed new pesticides also drew a blank.

He was not alone. Beekeepers in north-west London also reported strange losses. Chapple calls the disappearance the "Mary Celeste syndrome". A year later, a survey of hives by government bee inspectors across Britain has found that one in five colonies has perished.

According to Albert Einstein, our very existence is inextricably linked to bees. He is reputed to have said: "If the bee disappears off the surface of the globe, then man would only have four years of life left."

Bees are a barometer of what man is doing to the environment, say beekeepers. Empty hives are a harbinger of a looming ecological crisis. But what is causing them to vanish - pesticides, parasites, pests, viruses? No one knows for sure.

Scientists around the world are trying to pinpoint the culprit, but it is proving elusive. They have even set up an international network to monitor honeybee losses, which is operating out of Switzerland. In the US, where the genetic code of the honeybee was unravelled by scientists two years ago, they have been employing advanced technology to discover if a new virus is responsible for killing the bees.

The answer is probably overwork, coupled with various environmental factors. After Hackenberg's bees have pollinated the almonds in California, they head north to the apple orchards of Washington State, then east for the cranberries and pumpkins, before reaching Maine in May to pollinate blueberries. In a year, they can cover 11,000 miles.

In France, beekeepers have for more than a decade waged a war against the chemical giant Bayer CropScience. They hold responsible the company's bestselling pesticide, imidacloprid, for killing a third of the country's 1.5 million colonies. Bayer is also being blamed by German beekeepers for the eerie silence along the Rhine valley, where the buzzing of bees is a common sound at this time of year. They say two-thirds of honeybees have been killed this month by the pesticide clothianidin, which has been widely applied on sweet corn. As a result of the bee deaths, eight pesticides, including clothianidin, have been temporarily suspended in Germany. Anecdotal evidence of pesticide-related bee deaths in Italy and Holland is also piling up.

European beekeepers accuse scientists and government agencies of being in the pocket of the chemical companies. It's a similar story in the US, where scientists maintain that there is no correlation between the bees' disappearance and pesticide use.

Jeff Pettis, senior manager at the federal bee laboratory in Maryland, advises some beekeepers may do well to forgo the almond pollination and rest their bees.

 "You are getting them ready for February when the sunlight hours and the temperature are telling them it's too early in the year to be foraging at full strength," he says.

Deceiving bees is an essential part of the business. Beekeepers dupe them into thinking it's already summer by moving them to warm locations in winter and feeding them an array of protein and energy supplements. The more food that comes into the hive, the more eggs the queen lays, to create more of the worker bees to go out and pollinate.

Environmentalists argue for conservation measures on land planted with single crops that will both improve honeybee nutrition and attract wild pollinators that could shoulder some of the honeybees' workload.

Planting hedgerows of wild flowers would give honeybees a more varied menu. While this has happened in Europe, US almond growers have proved resistant to the idea, concerned that the bees would make fewer visits to the almond blossom if they had a choice.

This is not the first time that honeybees have disappeared. The first recorded unexplained loss was in the US 150 years ago and ever since large numbers have vanished at intervals throughout North America, Europe and Australia. Then, as now, the main suspects were deficiencies in the bees' diet, pollution in the environment, pests and parasites and mismanagement by beekeepers, but the killer was never identified.

The Guardian