BELTSVILLE, Md. — What is happening to the bees?

More than a quarter of the country's 2.4 million bee colonies have been lost — tens of billions of bees, according to an estimate from the Apiary Inspectors of America, a national group that tracks beekeeping. So far, no one can say what is causing the bees to become disoriented and fail to return to their hives.

As with any great mystery, a number of theories have been posed, and many seem to researchers to be more science fiction than science. People have blamed genetically modified crops, cellular phone towers and high-voltage transmission lines for the disappearances. Or was it a secret plot by Russia or Osama bin Laden to bring down American agriculture? Or, as some blogs have asserted, the rapture of the bees, in which God recalled them to heaven? Researchers have heard it all.

The volume of theories "is totally mind-boggling," said Diana Cox-Foster, an entomologist at Pennsylvania State University. With Jeffrey S. Pettis, an entomologist from the U.S. Department of Agriculture, Cox-Foster is leading a team of researchers that is trying to find answers to explain "colony collapse disorder," the name given for the disappearing bee syndrome.

"Clearly there is an urgency to solve this," Cox-Foster said. "We are trying to move as quickly as we can."

Cox-Foster and fellow scientists who are here at a two-day meeting to discuss early findings and future plans with government officials have been focusing on the most likely suspects: a virus, a fungus or a pesticide.

About 60 researchers from North America sifted the possibilities at the meeting Monday. Some expressed concern about the speed at which adult bees are disappearing from their hives; some colonies have collapsed in as little as two days. Others noted that countries in Europe, as well as Guatemala and parts of Brazil, are also struggling for answers.

"There are losses around the world that may or may not be linked," Pettis said.

The investigation is now entering a critical phase. The researchers have collected samples in several states and have begun doing bee autopsies and genetic analysis.

So far, known enemies of the bee world, like the varroa mite, on their own at least, do not appear to be responsible for the unusually high losses.

Genetic testing at Columbia University has revealed the presence of multiple micro-organisms in bees from hives or colonies that are in decline, suggesting that something is weakening their immune system. The researchers have found some fungi in the affected bees that are found in humans whose immune systems have been suppressed by AIDS or cancer.