SIBLEY -- Honey bees are dying in Iowa and throughout the nation, and no one understands why.

Researchers are scrambling to understand the mystery but no solution seems imminent, leaving fruit growers and beekeepers unsure what to do.

For people like Pat Randol, an independent honey producer from Winterset, it's all very frustrating. She was surprised to find half of her 45-colony operation dead.

"Our inspector came out, and we found most of them dead," Randol said. "I was shocked. They were healthy, they were bringing in pollen, they were bringing in nectar. I don't know, there's just dead bees up there."

Randol's experience is being repeated at the 2.5 million hives across the country as bee populations inexplicably plunge.

Iowa's State University apiarist Andrew Joseph said one cause for the decline is a hive condition called colony collapse disorder, which causes older worker bees to flee the colony.

"We're not exactly sure what the mechanism is causing that," Joseph said. "Whether they go off to die or whether they're foraging and never return is pretty much unknown. So you lose that adult worker bee population, and without them, the colony can't function and eventually dies."

But Randol and others said additional factors also are likely playing a role in the die-offs, including tough winters in parts of the Midwest and pests such as varroa mites.

Jeff Pettis, a researcher with the U.S. Department of Agriculture's Bee Research Lab in Beltsville, Md., believes that, colony collapse disorder aside, multiple, unknown factors are weakening colonies and adding to the dropping bee population.

"There's a 'one, two, punch' going on," Pettis said. "Something's weakening them and setting them up and then there's a final thing that kills them, but that thing acting alone would never do it."

The questions surrounding the die-off leave beekeepers and others unsure whether to invest in new bees or hold off.

"They're a little afraid to rebuild into the same hives -- the physical wooden structure with the honeycomb in it," said Bill Huser, vice president of research at Sue Bee Honey in Sioux City. "They're afraid to go right back into that in case there's something there that made these bees leave."

Those keepers who want to rebuild their operations are largely on their own.

Aside from loan programs, the government doesn't offer aid to bee producers who have experienced losses, said Donna Brahms, president of the Iowa Honey Producers Association.

She called for spending more on research to understand the problem.

The issue prompted U.S. Sen. Chuck Grassley of Iowa and other senators to express concern to the USDA and urge the agency to "identify the cause and remedy for Colony Collapse Disorder."

Kevin Hackett, with the USDA's Department of Agriculture/Agricultural Research Service, said the federal government has spent plenty on the disorder. He noted that in 2008, the service spent $7.8 million on honeybee research, of which 80 percent was focused on colony collapse disorder.

It's not only beekeepers who are worried about the die-off. Bill Eggers, owner of Burr Oak Farms in Winterset, lives near Randol and worried about the lack of the neighboring bees.

"I grow cherries, peaches, plumbs," Eggers said. "None of those will have anything on them if you don't have pollination."

Eggers' crops are just a few of the many fruits and vegetables that require the work of honeybees. Blueberries, cranberries, tomatoes, apples, pears, apricots and almonds also require the work of pollination.

Soaring fuel prices are primarily to blame for rising food costs, but Hackett said bee declines could lead to additional price increases.

"Because of the current situation, prices have doubled or tripled over the past three or four years for renting bee hives for pollination of crops," Hackett said. "Either the grower or the consumer is going to absorb those costs."