Nearly a third of the honey bee colonies in the United States died this past winter, sharply higher proportion than a year ago, according to an official report released Tuesday.

The US population of managed honey bee colonies fell by 31.1 percent in the October 2012-April 2013 period, said the preliminary report by the US Department of Agriculture in collaboration with the Apiary Inspectors of America and The Bee Informed Partnership.

Bees are vital pollinators in fruit and vegetable production and have been dying in significant numbers in recent years, some stricken by Colony Collapse Disorder, the sudden loss of all bees in a colony. The cause remains unknown.

The just-ended winter's losses were 42 percent higher than in the prior winter, when 21.9 percent of the bee colonies died, but were in line with the average loss of 30.5 percent over the past six years.

The latest findings were based on responses of more than 6,000 US beekeepers which represent almost 23 percent of the nation's total estimated 2.62 million colonies.

The beekeepers said that a loss rate of 15 percent was "acceptable" but 70 percent of them had heavier losses than that, the report said.

There were more colonies that dwindled away, rather than suffering from Colony Collapse Disorder, which was not reported as a major cause of colony loss for the second straight year.

One key difference stood out in this year's survey, the researchers said. Beekeepers who took honey bees to California to pollinate almonds reported higher losses than beekeepers who did not take their bees to pollinate almonds.

Almost 20 percent of the beekeepers who pollinated almonds lost at least 50 percent of their colonies, the report said.

The US Department of Agriculture, in a report last week, said an investigation into the decline in honey bee health has found multiple factors, "including parasites and disease, genetics, poor nutrition and pesticide exposure."

The USDA called for further research to determine risks from pesticides.

"Acute and sublethal effects of pesticides on honey bees have been increasingly documented, and are a concern but it is not clear, based on current research, whether a pesticide exposure is a major factor associated with US honey bee health declines," it said.

In a lawsuit in March several beekeepers and environmental groups accused the US Environmental Protection Agency of failing to protect pollinators and challenging practices that speed to market about two-thirds of all pesticides.

The suit seeks to suspend the EPA registrations of pesticides that have been identified as toxic to bees.

Last week the European Commission said it would impose the world's first continent-wide ban on three pesticides which environmentalists say are killing the bees that pollinate Europe's crops.

The insecticides -- imidacloprid and clothianidin produced by Bayer, and thiamethoxam by Syngenta -- are used to treat seeds, and are applied to the soil or sprayed on bee-attractive plants and cereals.