Scientists say they have another culprit to add to the list of causes for the collapse of honeybee colonies: cold weather, and particularly its tendency to magnify the effect of insecticides linked to the problem.

A very cold winter and exposure to the insecticides may be combining in a one-two punch the bees have trouble surviving, they say.

Researchers at the Harvard School of Public Health report they've found two widely utilized insecticides, of a class known as neonicotinoids, apparently cause significant harm to the colonies during the winter, and the colder the winter the greater the harm.

The phenomenon of dying bees in their hives has been given the name CCD, for Colony Collapse Disorder

The insecticides can cause bees to forsake their hives in winter, leading to their eventual death, the researchers said in their study published in the Bulletin of Insectology.

The researchers focused on honeybee colonies at locations in Massachusetts in the winter of 2012-13, monitoring 18 colonies for exposure to the insecticides and the eventual fate of each.

"We demonstrated again in this study that neonicotinoids are highly likely to be responsible for triggering CCD in honeybee hives that were healthy prior to the arrival of winter," said lead author Chensheng (Alex) Lu, an expert on environmental exposure effects in biology.

Bees are the prime pollinators for around a third of most crops around the world, with some estimates putting that figure at 80 percent for crops in the United States, making CCD and its resulting significant depletion of honeybees a concern, the researchers say.

Several recent studies have focused on neonicotinoids, which are suspected of impairing the neurological functions of bees.

The insecticides might also be lowering the bees' resistance to parasite or mites, several studies have suggested.

Although a link between the chemicals and CCD seems strong, exactly what it is about neonicotinoids that makes the bees abandon their hives to die from exposure to the elements remains unknown, the researchers admit.

"Future research could help elucidate the biological mechanism that is responsible for linking sub-lethal neonicotinoid exposures to CCD," says Lu. "Hopefully we can reverse the continuing trend of honeybee loss."

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