“That is extremely unusual,” Dr. Cox-Foster said.

Meanwhile, samples were sent to an Agriculture Department laboratory in North Carolina this month to screen for 117 chemicals. Particular suspicion falls on a pesticide that France banned out of concern that it may have been decimating bee colonies. Concern has also mounted among public officials.

“There are so many of our crops that require pollinators,” said Representative Dennis Cardoza, a California Democrat whose district includes that state’s central agricultural valley, and who presided last month at a Congressional hearing on the bee issue. “We need an urgent call to arms to try to ascertain what is really going on here with the bees, and bring as much science as we possibly can to bear on the problem.”

So far, colony collapse disorder has been found in 27 states, according to Bee Alert Technology Inc., a company monitoring the problem. A recent survey of 13 states by the Apiary Inspectors of America showed that 26 percent of beekeepers had lost half of their bee colonies between September and March.

Honeybees are arguably the insects that are most important to the human food chain. They are the principal pollinators of hundreds of fruits, vegetables, flowers and nuts. The number of bee colonies has been declining since the 1940s, even as the crops that rely on them, such as California almonds, have grown. In October, at about the time that beekeepers were experiencing huge bee losses, a study by the National Academy of Sciences questioned whether American agriculture was relying too heavily on one type of pollinator, the honeybee.

Bee colonies have been under stress in recent years as more beekeepers have resorted to crisscrossing the country with 18-wheel trucks full of bees in search of pollination work. These bees may suffer from a diet that includes artificial supplements, concoctions akin to energy drinks and power bars. In several states, suburban sprawl has limited the bees’ natural forage areas.

So far, the researchers have discounted the possibility that poor diet alone could be responsible for the widespread losses. They have also set aside for now the possibility that the cause could be bees feeding from a commonly used genetically modified crop, Bt corn, because the symptoms typically associated with toxins, such as blood poisoning, are not showing up in the affected bees. But researchers emphasized today that feeding supplements produced from genetically modified crops, such as high-fructose corn syrup, need to be studied.