BAKERSFIELD, Calif. -- A mysterious malady that has been killing honeybees en masse for several years appears to have expanded drastically in the last year, commercial beekeepers say, wiping out 40 percent or even 50 percent of the hives needed to pollinate many of the nation's fruits and vegetables.
A conclusive explanation so far has escaped scientists studying the ailment, colony collapse disorder, since it first surfaced around 2005. But beekeepers and some researchers say there is growing evidence that a powerful new class of pesticides known as neonicotinoids, incorporated into the plants themselves, could be an important factor.
The pesticide industry disputes that. But its representatives also say they are open to further studies to clarify what, if anything, is happening.
"They looked so healthy last spring," said Bill Dahle, 50, who owns Big Sky Honey in Fairview, Mont. "We were so proud of them. Then, about the first of September, they started to fall on their face, to die like crazy. We've been doing this 30 years, and we've never experienced this kind of loss before."
In a show of concern, the Environmental Protection Agency recently sent its acting assistant administrator for chemical safety and two top chemical experts to the San Joaquin Valley of California for discussions.
In the valley, where 1.6 million hives of bees just finished pollinating an endless expanse of almond groves, commercial beekeepers who only recently were losing a third of their bees to the disorder say the past year brought far greater losses.
The U.S. Agriculture Department is to issue its own assessment in May. But in an interview, the research leader at its Beltsville, Md., bee research laboratory, Jeff Pettis, said he was confident that the death rate would be "much higher than it's ever been."
Following a now-familiar pattern, bee deaths rose swiftly in autumn and dwindled as operators moved colonies to faraway farms for the pollination season.
Beekeepers say the latest string of deaths has dealt them a heavy blow.
Bret Adee, who is an owner, with his father and brother, of Adee Honey Farms of South Dakota, the nation's largest beekeeper, described mounting losses.
"We lost 42 percent over the winter. But by the time we came around to pollinate almonds, it was a 55 percent loss," he said in an interview in Bakersfield this week.
"They looked beautiful in October," Mr. Adee said, "and in December, they started falling apart, when it got cold."
Mr. Dahle said he had planned to bring 13,000 beehives from Montana -- 31 tractor-trailers full -- to work the California almond groves. But by the start of pollination last month, only 3,000 healthy hives remained.
Annual bee losses of 5 percent to 10 percent once were the norm for beekeepers. But after colony collapse disorder surfaced around 2005, the losses approached one-third of all bees despite beekeepers' best efforts to ensure the insects' health.
Nor is the effect limited to beekeepers. The Agriculture Department says a quarter of the U.S. diet, from apples to cherries to watermelons to onions, depends on pollination by honeybees. Fewer bees means smaller harvests and higher food prices.
Almonds are a bellwether. Eighty percent of the nation's almonds grow in the San Joaquin Valley, and 80 percent of those are exported, a multibillion-dollar crop crucial to California agriculture.
Pollinating up to 800,000 acres, with at least two hives per acre, takes as many as two-thirds of all commercial hives. This past winter's die-off sent growers scrambling for enough hives to guarantee a harvest.
Bee shortages pushed the cost to farmers of renting bees to $200 per hive at times, 20 percent above normal. That, too, might translate into higher food prices.
Precisely why last year's deaths were so great is unclear. Some blame drought in the Midwest, although Mr. Dahle lost nearly 80 percent of his bees despite excellent summer conditions. Others cite bee mites that have become increasingly resistant to pesticides. Still others blame viruses.
But many beekeepers suspect the biggest culprit is the growing soup of pesticides, fungicides and herbicides used to control pests.
While each substance has been certified, there has been less study of their combined effects. Nor, many critics say, have scientists sufficiently studied the effect of neonicotinoids, the nicotine-derived pesticide that European regulators implicate in bee deaths.
The explosive growth of neonicotinoids since 2005 has roughly tracked rising bee deaths.
Neonics, as farmers call them, are applied in smaller doses than older pesticides. They are systemic pesticides, often embedded in seeds so that the plant itself carries the chemical that kills insects that feed on it.
Older pesticides could kill bees and other beneficial insects. But while they quickly degraded -- often in a matter of days -- neonicotinoids persist for weeks and even months. Beekeepers worry that bees carry a summer's worth of contaminated pollen to hives, where ensuing generations dine on a steady dose of pesticide that, eaten once or twice, might not be dangerous.
Research to date on neonicotinoids "supports the notion that the products are safe and are not contributing in any measurable way to pollinator health concerns," the president of CropLife America, Jay Vroom, said Wednesday. The group represents more than 90 pesticide producers.
He said the group nevertheless supported further research.
"We stand with science and will let science take the regulation of our products in whatever direction science will guide it," he said.
A coalition of beekeepers and environmental and consumer groups sued the EPA last week, saying it exceeded its authority by conditionally approving some neonicotinoids. The agency has begun an accelerated review of their effect on bees and other wildlife.
The European Union has proposed barring their use on crops frequented by bees. Some researchers have concluded that neonicotinoids caused extensive die-offs in Germany and France.