Something is killing the nation's honeybees.

Dave Hackenberg of central Pennsylvania had 3,000 hives and figures he has lost all but about 800 of them.

In labs at Pennsylvania State University, the Pennsylvania Department of Agriculture, and elsewhere in the nation, researchers have been stunned by the number of calls about the mysterious losses.

"Every day, you hear of another operator," said Dennis vanEngelsdorp, acting state apiarist with the Pennsylvania Department of Agriculture. "It's just causing so much death so quickly that it's startling."

At stake is the work the honeybees do, pollinating more than $15 billion worth of U.S. crops, including Pennsylvania's apple harvest, the fourth-largest in the nation, worth $45 million, and New Jersey's cranberries and blueberries.

While a few crops, such as corn and wheat, are pollinated by the wind, most need bees. Without these insects, crop yields would fall dramatically. Agronomists estimate Americans owe one in three bites of food to bees.

The problem caps 20 years of honeybee woes, including two mites that killed the valuable insect and a predatory beetle that attacked the honeycombs of weak or dead colonies.

"This is by far the most alarming," said Maryann Frazier, an apiculture - or beekeeping - expert at Penn State's College of Agricultural Sciences.

One of the first to notice the latest die-off was Hackenberg, who lives in Lewisburg, north of Harrisburg in Union County.

He and his son truck about 3,000 hives up and down the East Coast every year as part of a large but little-known cross-continental migratory bee industry.

Hackenberg's bees pollinate oranges in Florida, apples, cherries and pumpkins in Pennsylvania, and blueberries in Maine. Come summer, they are buzzing along the Canadian border, making honey.

This season, Hackenberg hauled his hives to Florida by Oct. 10, just as he has done for 40 years. By November, some hives were empty; others had just sickly remains.

He made some calls and found out a beekeeper in Georgia had seen the same thing.

Since then, with concern mounting, experts have been investigating. A few months ago, they were referring to the die-off as "fall dwindle disease." Now, they have ratcheted up to "colony collapse disorder."

Last weekend, apiarist vanEngelsdorp and other researchers headed to central California, where hundreds of acres of almond trees - the source of 80 percent of the world's almond harvest - are about to blossom.

Last fall, workers transported managed hives - about 450 per tractor-trailer - to California from colder areas such as the Great Lakes and the Dakotas. Now, hives are coming from Texas, Florida, Maryland and Pennsylvania. In all, about half the country's managed hives are needed for the mass pollination.

As workers openthe hives to check them, "the picture's not so good," said Jeffrey S. Pettis, a leader in bee research at a U.S. Department of Agriculture lab in Beltsville, Md.

Pettis said bees often had some winter loss, but this level of death was unprecedented.

As dead or dying insects are collected, dissected and tested, several possibilities are emerging.

The most recent mite problem - the varroa mite - compromises a bee's immune system, so a virus might be the new culprit, Frazier said. Or it could be a new fungal pathogen.

Frazier said researchers also were looking at a new group of pesticides that might impair the bees' ability to orient to their hives. So maybe they are dying only because they cannot find their way back home.

Honeybees are not natives. The country already had about 3,500 species of pollinating bees before Europeans brought honeybees in the 1600s. But because honeybees produce honey and can be managed so easily, they have become a mainstay of U.S. agriculture.

"Part of the problem is that today we develop these big monocultures of corn or peas or cabbage," Frazier said. "They wipe out the diversity of nectar sources and reduce nesting sites for wild bees. And we use, unfortunately, a lot of pesticides to keep the insects we don't want from eating these crops, which also works to eliminate the pollinators."

So a Pennsylvania orchard manager, say, will bring in bees for the two weeks the apple trees bloom, then take them out so he can apply substances to control other insects.

Neither entomologists nor growers can say what will happen when the 2007 growing season for most of the country's crops starts. "We're coming up onto the season where people are really going to be worried," Frazier said.

Although research suggests the stress of moving bees long distances might be a factor in the die-offs, smaller beekeepers with stationary hives worry the problem will extend to their colonies as well.

Already, Janet Katz, a beekeeper in Chester, N.J., thinks three of her 21 hives are failing.

And the bees are stressed already, she said. "The weather last season was not cooperative," she said. "Over the course of the season it was too wet, too dry, too hot and too cold, all at the wrong times."

Bees store honey every autumn - a hive needs 60 pounds to survive the winter - but with this year's warm weather, they ate a lot, and beekeepers had to supplement with sugar syrup.

Now, the bees have sealed themselves inside the hives to stay warm, and the keepers can't open the structures until spring.

"Are we going to see this same thing, this collapsing disorder, in these bees? We don't know," Frazier said. "It's very possible this may extend to our nonmigratory population. We just won't know until spring."