Bees are vanishing across the United States, leaving empty colonies behind and putting honey production in jeopardy -- and nobody knows why.

California beekeeper David Bradshaw said he's trying not to dwell on the fact that half his bees are gone.

"I'd be an emotional mess if I just kept thinking about the bees dying," he told CTV News.

Experts gathered in Washington Thursday at a House Agriculture Subcommittee, describing the mysterious threat as "Colony Collapse Disorder" (CCD).

The remains of dead bees usually remain inside a hive, unless worker bees carry their bodies them out. But colonies affected by CCD show no signs of the ailment, aside from a notable absence of mature bees.

It's possible the affected bees abandon the hive before dying, but scientists have yet to understand why or how.

In the past six months, U.S. beekeepers estimate they have lost between 50 and 90 per cent of their honeybees. One colony can have 60,000 bees in the summer, and that number drops to about 20,000 in the winter.

The condition of Canada's bees is not fully known, but the U.S. Congress was told it's likely Canadian hives likely share a similar fate.

"Recently, we have reports out of Canada that they have the exact same symptoms and collapses ongoing there," said Diana Cox-Foster, a professor of entomology with the Pennsylvania State University.

Scientists, beekeepers and officials started a CCD group in December 2006 to examine the cause of the disorder, and hopefully find a cure.

Not only are bees crucial to the agriculture industry in the production of honey, they also work as pollinators. Roughly 75 per cent of flowering plants require pollinators to bear fruit, including crops that produce the resources needed for drugs and fuel.

Researchers at the U.S. Department of Agriculture, University of Montana and Penn State are leading the study of CCD.

So far, they have noticed that affected colonies are still active, with remaining bees looking after developing bees. But when a colony is weakened, it's usually taken over by rival bees and other insects looking for honey.

And when scientists examined individual bees in affected colonies, they showed weakened immune systems and an increase in bacteria and foreign fungi.

Caird E. Rexroad, from the Agricultural Research Service, echoed that fact when he testified in front of the committee Thursday.

"We believe that some form of stress may be suppressing immune systems of bees, ultimately contributing to CCD," CNN quoted Rexroad as saying.

U.S. beekeepers had already taken a huge hit from varroa mite, a parasite that killed more than half of some colonies and also affected wild honeybee hives.

With a report by CTV's Tom Walters and files from The Associated Press