One of the most important crop pollinators in the world, honey bees in the United States have been decimated in recent months by a mysterious disease.

Credit: Jon Sullivan/Wikipedia

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The almond trees are blooming and the bees are dying, and nobody knows why. All up and down California's vast San Joaquin Valley, nearly 2,500 square kilometres of small nut trees arranged in laser-straight rows are shaking off the cobwebs of winter. They're gearing up once again to produce nearly half a billion kilograms of nuts, worth US$3 billion to the U.S. economy.

The trees cannot produce the bounty on their own, however. They need bees - a million hives worth - trucked in from nearly forty U.S. states to move pollen from one tree to another, fertilising the blooms in the largest managed pollination event on Earth.

But even as the beekeepers reap record fees for renting their hives, their livelihood is now threatened by the largest loss of honey bees in the history of the industry.

Since October 2006, 35 per cent or more of the United States' population of the Western honey bee (Apis mellifera) - billions of individual bees - simply flew from their hive homes and disappeared.

When the almonds were being plucked from the trees late last year, Gene Brandi of Los Banos, California had 2,000 hives, but by late February he had just 1,200 - a loss of 40 per cent.

And Brandi is one of the more fortunate. Across the 24 U.S. states affected by the mysterious phenomenon, losses have ranged up to 90 per cent. "I've had a couple of yards where I've had 200 hives and they're down to 10 hives that are alive," says David Bradshaw of Visalia, about 180 kilometres southeast of Los Banos along California's Route 99.

What's causing the carnage, however, is a total mystery; all that scientists have come up with so far is a new name for the phenomenon - Colony Collapse Disorder (CCD) - and a list of symptoms.

In hives hit by CCD, adult workers simply fly away and disappear, leaving a small cluster of workers and the hive's young to fend for themselves. Adding to the mystery, nearby predators, such as the wax moth, are refraining from moving in to pilfer honey and other hive contents from the abandoned hives; in CCD-affected hives the honey remains untouched.

The symptoms are baffling, but one of the emerging hypotheses is that the scourge is underpinned by a collapse of the bees' immune systems. Stressed out by cross-country truck journeys and drought, attacked by viruses and introduced parasites, or whacked out by harmful new pesticides, some researchers believe the bees' natural defences may have simply given way. This opens the door to a host of problems that the bees can normally suppress.

What's surprising is that mysterious declines are nothing new. As far back as 1896, CCD has popped up again and again, only under the monikers: 'fall dwindle' disease, 'May dwindle', 'spring dwindle', 'disappearing disease', and 'autumn collapse'.

Even the current outbreak has possibly been going on undetected for two years, according to the CCD Working Group - a crack group of U.S. researchers from institutes including the Pennsylvania State University and University of Montana, who are trying to unravel the mystery.

What has made the members of the Working Group - as well as conservationists, beekeepers, and farmers - really sit up and notice is the scale of this year's decimation; something in the environment has allowed CCD to reach an unprecedented scale that threatens the very survival of the pollination industry.

"We have never seen a die-off of this magnitude with this weird symptomology," says Maryann Frazier, a bee researcher at Pennsylvania State University. "We've seen bees disappear over time and dwindle away, but not die-off so quickly."

Asian mites and latent viruses

A problem preventing clear identification of CCD is that honey bees are already under threat from manifold foes.

Even without CCD, the number of managed hives in the U.S. has dwindled by nearly 50 per cent since the industry's peak in the 1970s. The main culprit for the die-offs is a tiny Asian mite. Known as Varroa destructor to scientists and the 'vampire mite' to beekeepers, these tiny parasites - circular, crab-like arachnids about the size of a bee's eyeball - have been quietly parasitising the Asiatic honey bee (Apis cerana) in Southeast Asia for millennia.

Varroa destructor, a tiny tick-like arachnid, has been wreaking havoc on U.S. honey bees since it was inadvertently introduced from Asia in the 1980s. Scott Bauer/Wikipedia

Some time in the early 1980s, though, the mites hitched a ride to America and hopped on new hosts - spreading like wildfire throughout the defenceless Western honey bee population with the help of migratory beekeepers who obligingly trucked them around the country. The mites suck the vital juices out of both developing and adult bees, and left unchecked can kill a hive within 12 months.

In addition to the damage that the mites do themselves, they also spread viruses. Furthermore, the mites appear to assist the viruses by somehow sabotaging the bees' immune system.

"There's something about a mite feeding on a bee that just knocks its immune system out. [Then] the viruses can take over," says Eric Mussen, a bee researcher at the University of California, Davis.

But mites and their viruses have been infecting U.S. honey bees for nearly 30 years. What has experts worried is that CCD kills bees even more efficiently than mites - destroying a healthy colony in a matter of weeks.

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