The phenomenon was first noticed late last year in the United States, where honeybees are used to pollinate US$15 billion worth of fruits, nuts and other crops annually. Disappearing bees also have been reported in Europe and Brazil.

Commercial beekeepers would set their bees near a crop field as usual and come back in two or three weeks to find the hives bereft of foraging worker bees, with only the queen and the immature insects remaining. Whatever worker bees survived were often too weak to perform their tasks.

If the bees were dying of pesticide poisoning or freezing, their bodies would be expected to lie around the hive. And if they were absconding because of some threat -- which they have been known to do -- they wouldn't leave without the queen.

Since about one-third of the US diet depends on pollination and most of that is performed by honeybees, this constitutes a serious problem, according to Jeff Pettis of the US Agricultural Research Service.

"They're the heavy lifters of agriculture," Pettis said of honeybees. "And the reason they are is they're so mobile and we can rear them in large numbers and move them to a crop when it's blooming."

Honeybees are used to pollinate some of the tastiest parts of the American diet, Pettis said, including cherries, blueberries, apples, almonds, asparagus and macadamia nuts.

"It's not the staples," he said. "If you can imagine eating a bowl of oatmeal every day with no fruit on it, that's what it would be like" without honeybee pollination.

Pettis and other experts are gathering outside Washington for a two-day workshop starting on Monday to pool their knowledge and come up with a plan of action to combat what they call colony collapse disorder.

"What we're describing as colony collapse disorder is the rapid loss of adult worker bees from the colony over a very short period of time, at a time in the season when we wouldn't expect a rapid die-off of workers: late fall and early spring," Pettis said.

SMALL WORKERS IN A SUPERSIZE SOCIETY

The problem has prompted a congressional hearing, a report by the National Research Council and a National Pollinator Week set for June 24-30 in Washington, but so far no clear idea of what is causing it.

"The main hypotheses are based on the interpretation that the disappearances represent disruptions in orientation behavior and navigation," said May Berenbaum, an insect ecologist at the University of Illinois, Urbana-Champaign.

There have been other fluctuations in the number of honeybees, going back to the 1880s, where there were "mysterious disappearances without bodies just as we're seeing now, but never at this magnitude," Berenbaum said in a telephone interview.

In some cases, beekeepers are losing 50 percent of their bees to the disorder, with some suffering even higher losses. One beekeeper alone lost 40,000 bees, Pettis said. Nationally, some 27 states have reported the disorder, with billions of bees simply gone.

Some beekeepers supplement their stocks with bees imported from Australia, said beekeeper Jeff Anderson, whose business keeps him and his bees traveling between Minnesota and California. Honeybee hives are rented out to growers to pollinate their crops, and beekeepers move around as the growing seasons change.

Honeybees are not the only pollinators whose numbers are dropping. Other animals that do this essential job -- non-honeybees, wasps, flies, beetles, birds and bats -- have decreasing populations as well. But honeybees are the big actors in commercial pollination efforts.

"One reason we're in this situation is this is a supersize society -- we tend to equate small with insignificant," Berenbaum said. "I'm sorry but that's not true in biology. You have to be small to get into the flower and deliver the pollen.