The protozoan infects the midgut of honeybees.

Some beekeepers have noted that treating bee boxes with gamma rays used for food irradiation has allowed healthy hives to return to the boxes, leading to speculation that a pathogen like Nosema could be involved in the some bee deaths this year.

But this is only one of the possible causes. Scientists are working hard to understand the sources of a staggering decline in honeybees in as many as 27 US states and countries in Europe and Asia this winter.

In the past in the US, both tracheal mites (Acarapis woodi) and varroa mites (Varroa destructor) have threatened the bee industry, causing similar catastrophic die-offs to bee populations in the winters of 1995-96 and 2000-01.

The mites feed on honeybees and act as a vector for several bee viruses, though varroa mites are especially deadly. While many bees this year exhibit symptoms of mite damage, about 25 percent of the deaths cannot be attributed to mites or any other known honeybee pest.

Finding the cause of the problem is vital for US agriculture. Many fruit, vegetable and seed crops, worth between $8 billion and $12 billion each year, rely on honeybees for pollination.

Genetically modified foods, mites, pathogens, pesticides and electromagnetic radiation from cell phones have all been proposed as possible causes of the bees' demise. But, so far, the actual causes are unknown at this time.

While the protozoan has been suggested so too has a class of insecticides known as neonicotinoids. These have been widely detected on pollen at low concentrations in other countries experiencing die-offs. At certain levels these insecticides may impair the bees' abilities to learn, leading some scientists to believe exposed bees may leave the hive and get lost.

As yet, there is little evidence that insecticides from genetically modified crops or electromagnetic radiation have led to bee deaths.

Miticides have been used to combat mites, though over time the mites develop resistance, requiring constant development of new chemicals. Also, miticides can only be used at certain times of the year because if used during a nectar flow, they can contaminate the honey crop. There is also evidence that miticides can accumulate in the bees' wax combs to levels that could be harmful to the bees themselves.

At present, research experiment stations at land-grant universities such as Cornell are putting together funding plans for research to investigate the cause of this year's bee declines. Also, the federal government is considering allotting tens of millions of dollars for long-term competitive grants for programs that target honeybees' overall health.

CCD is the latest, and most serious, die-off of honey bee colonies across the US. It is characterized by, sudden colony death with a lack of adult bees in front of the dead-outs. Honey and bee bread are usually present and there is often evidence of recent brood rearing.

A recent analysis of dissected bees turned up an alarmingly high number of foreign fungi, bacteria and other organisms and weakened immune systems.

Among the clues being assembled by researchers:

* Although the bodies of dead bees often are littered around a hive, sometimes carried out of the hive by worker bees, no bee remains are typically found around colonies struck by the mystery ailment. Scientists assume these bees have flown away from the hive before dying.

* From the outside, a stricken colony may appear normal, with bees leaving and entering. But when beekeepers look inside the hive box, they find few mature bees taking care of the younger, developing bees.

* Normally, a weakened bee colony would be immediately overrun by bees from other colonies or by pests going after the hive's honey. That's not the case with the stricken colonies, which might not be touched for at least two weeks.