The huge die off of bees worldwide, a major threat to crops depending on the honey-making insects for pollination, is not due to any one single factor, the World Organisation for Animal Health (OIE) said Wednesday.

Parasites, viral and bacterial infections, pesticides, and poor nutrition resulting from the impact of human activities on the environment have all played a role in the decline, the OIE said.

At normal times, bee communities naturally lose around five percent of their numbers.

But with the syndrome known as colony collapse disorder (CDD), a third, half - sometimes even 90 percent - of the insects can be wiped out.

In the United States, government figures released last month showed a 29 percent drop in beehives in 2009, coming on the heels of declines of 36 and 32 percent in 2008 and 2007.

Disturbing evidence that honeybees are in terminal decline has emerged from the United States where, for the fourth year in a row, more than a third of colonies have failed to survive the winter.

The decline of the country's estimated 2.4 million beehives began in 2006, when a phenomenon dubbed colony collapse disorder (CCD) led to the disappearance of hundreds of thousands of colonies. Since then more than three million colonies in the US and billions of honeybees worldwide have died and scientists are no nearer to knowing what is causing the catastrophic fall in numbers.

The number of managed honeybee colonies in the US fell by 33.8% last winter, according to the annual survey by the Apiary Inspectors of America and the US government's Agricultural Research Service (ARS).

The collapse in the global honeybee population is a major threat to crops. It is estimated that a third of everything we eat depends upon honeybee pollination, which means that bees contribute some £26bn to the global economy.

Parasites, viral and bacterial infections, pesticides, and poor nutrition resulting from the impact of human activities on the environment have all played a role in the decline, the OIE said