The mysterious bee die-off hit North Dakota beekeeper James Browning hard. He lost 40 percent of his 20,000 hives last year.

"To see those bees die and the colonies empty, no bees in there," Browning said, "it's a gut wrenching feeling."

CBS News correspondent John Blackstone reports in each of the past four years about one-third of America's 2.5 million honeybee colonies have been wiped out. University of Montana researcher Jerry Bromenshenkhas been searching for the killer. After screening bees for 30,000 disease markers a group of scientists led by Bromenshenk say they have found a probable cause.

"Out of the data, suddenly emerged a parasite and a virus," Bromenshenk said. "A very unique virus indeed."

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The kind of virus they discovered is common in other insects but very rarely seen in bees. The virus seems to kill only when the bees are also infected with a parasite, a type of fungus.

Bromenshenk said, "It looks like is that the bees can tolerate either one alone." But, "When you combine the two - that tends to become lethal in a hurry."

That combination may help beekeepers. While there is no way to treat the unusual, new virus, the parasite can be killed by a fungicide. Bromenshenk said beekeepers can "buy those treatments and apply them."

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In North Dakota, U.S. Department of Agriculture researchers are looking for another way to help beekeepers fight colony collapse disorder, by improving bee nutrition.

On today's huge farms, commercial honeybees spend weeks pollinating a single crop. Scientists are wondering if the bees' limited diet makes them susceptible to the virus and fungus that appear to be killing them.

"Nobody would do good on a single protein, carbohydrate diet and a lot of times that's what we're asking our bees to do," said bee researcher Mathew D. Smart of Washington State University. "We're giving them one type of pollen, one type of nectar."

As the main pollinator for most fruits and vegetables, honeybees play a vital role in producing about 30 percent of our food. So it's important to all of us that scientists are now closing in on both the cause and the cure of the honeybee die-off.

While the researchers in Montana are not yet ready to say they are certain they have solved the mystery of collapse disorder, their findings will bring hope to the nation's beekeepers, who have lost tens of millions of bees over the last four years.

More of John Blackstone's Reporting on the Bee Mystery:
Hunt for the Honey Bee Killer
What's Killing the Honey Bees?
Flight of the Honey Bee
The Case of the Vanishing Bees