The losses threaten beekeepers' livelihoods

The cause of the losses, which range from 30% to more than 70%, is a mystery, but experts are investigating several theories.

American bee colonies have been hit by regional crises before, but keepers say this is the first national crisis.

Bees pollinate more than $14bn (£7bn) worth of US seeds and crops each year, mostly fruits, vegetables and nuts.

Box after box after box are just empty. There's nobody home

David Bradshaw, Beekeeper

The mysterious disappearances highlight the important link that honeybees play in the chain that brings fruit and vegetables to supermarkets and dinner tables.

The crisis threatens numerous crops, from avocados to kiwis and California almonds - one of the most profitable in the US.

"I have never seen anything like it," California beekeeper David Bradshaw, 50, told the New York Times.

"Box after box after box are just empty. There's nobody home."

Under pressure

With an industry increasingly under consolidation, some fear the disorder could prove the breaking point for even large beekeepers.

The bee losses range from 30 to 60% on the West Coast, with some beekeepers on the East Coast and in Texas reporting losses of more than 70%.

Beekeepers consider a loss of up to 20% in the offseason to be normal.

Researchers say the bees are presumably dying in the fields, perhaps becoming exhausted or simply disoriented and eventually falling victim to the cold.

"The real question is why they leave," Jerry Hayes, a bee expert for the Florida Department of Agriculture told the Orlando Sentinel newspaper.

"Bees are highly social insects. They don't leave their babies and the queen."

The investigators are exploring a range of possibilities to explain the losses, which they are calling "colony collapse disorder". These include viruses, a fungus and poor bee nutrition.

They are also studying pesticides banned in some European countries to see if they are affecting the bees' innate ability to navigate their way back to their hives.

In some cases, bees are being raised to survive a shorter offseason, to be ready to pollinate once the almond bloom begins in February. This could have lowered their immunity to viruses.

Mites have also damaged bee colonies, and the insecticides used to try to kill them are harming the ability of queen bees to spawn as many workers.

Urban sprawl

Once the domain of hobbyists, beekeeping has become increasingly commercial and consolidated in the US.

During the last two decades, the number of beehives has dropped by a quarter, and is now estimated at 2.4 million. The number of beekeepers has fallen by half during the same period.

Bees are vital for the pollination of important US crops

In addition, urban growth means that the areas where the insects can forage for nectar to stay healthy and strong during the pollination season are being squeezed.

"There are less beekeepers, less bees, yet more crops to pollinate," said Zac Browning, vice-president of the American Beekeeping Federation.

"With so much added loss and expense due to disease, pests and higher equipment costs, profitability is actually falling."