MISSOULA -- For a guy with no protective gear, Jerry Bromenshenk is remarkably confident as he walks into an area of 3 million honeybees.

As a bee researcher for more than 30 years, Bromenshenk has been stung so many times he figures he's built up a tolerance. On this summer day, the University of Montana entomologist is showing a couple of visitors around one of his Missoula bee yards, and talking about the disappearance of millions of honeybees in the United States last fall and winter.

Bromenshenk is part of a national team investigating what caused the Colony Collapse Disorder, in which ''catastrophic'' losses were reported in some 35 states.

Losses have subsided, and as fall approaches scientists say there is no way to tell whether Colony Collapse will return in a big way. One theory is that the deaths stem from a virus previously unknown in the United States, but researchers do not know for sure.

''It's like trying to do an autopsy without a body,'' Bromenshenk says.

The impact of Colony Collapse Disorder is far-reaching. Bee pollination is credited for $15 billion in added value to U.S. crops of fruit, vegetables and nuts. The California almond crop alone uses half the nation's honeybees, according to the U.S. Department of Agriculture.

Bromenshenk and the other scientists working to solve the problem are headquartered at the U.S. Department of Agriculture's Bee Research Laboratory in Maryland. The team includes researchers from a half dozen universities and the USDA. While Bromenshenk commits his time there and in Missoula, he also has traveled the country meeting with colleagues and beekeepers.

Missoula-based Bee Alert Technology, of which he is president, gathered questionnaires from more than 700 beekeepers and found 40 percent reported losses averaging 75 percent. Beekeepers attributed the losses to Colony Collapse, mites and other causes, Bromenshenk says.

Colony Collapse Disorder is not the ''dwindling'' or slow debilitation that Bromenshenk says he has seen occasionally over the years, sometimes when bees suffer poor nutrition. Colony collapses happen within extremely short periods, from three days to three weeks.

Chemical exposure, parasites, bacteria, viruses -- all drew attention as researchers tried to determine what led to the honeybee die-off. One researcher recently identified Israeli acute paralysis virus as a possible cause, and scientists will try to infect bees with that virus to see if it's the killer. Researchers emphasized, however, that they have not ruled out other possible causes, including the stress of travel as beekeepers move hives around the country to pollinate crops.

''We don't know if it's a single factor, or all the stressors,'' Bromenshenk says. ''One of my colleagues put it aptly. He said, 'Maybe they (bees) are just tired.'''

The die-offs have devastated beekeepers, but not food production; U.S. growers had enough bees to satisfy pollination needs last spring.

What would happen if Colony Collapse Disorder surfaces again this fall and winter is unknown. Beekeepers not affected last year might be able to withstand it, but those who suffered losses might not have had enough time to rebuild their colonies.

Montana beekeeper Wade Anderson saw an inexplicable reduction of 50 percent during the past year. Through natural rebuilding, Anderson says, his bees now number 60 to 70 percent of normal.

Anderson, whose bees were stricken in January, says it may be months before he knows whether he will experience similar losses. He added conservative business practices, including ''not buying something until you have the money to pay for it,'' will likely allow him to stay in business even if the disorder surfaces again.

Colony Collapse aside, the health of bee colonies has dropped since the 1980s. Pathogens, pests and other problems claimed about 30 percent of beekeepers' 2.5 million colonies last year, the USDA found.

Summer reports on bee health were not encouraging, but not dire either, said Jeff Pettis, research leader at the USDA's Maryland lab.

Bromenshenk and his colleagues have been mapping the geographic extent of the die-offs, important information as researchers tried to gauge the scope of Colony Collapse Disorder and looked for links to environmental factors. The Montana researchers also took samples to analyze for possible viruses.

Bromenshenk, who holds a doctorate in entomology from Montana State University, began studying bees after conducting insect research in the 1970s. Big on teamwork, Bromenshenk talks about himself reluctantly and says, ''Complex problems require greater breadth of knowledge than any one individual is likely to have.''

When pressed, he says curiosity drives him.

''My mother would tell you that the first word out of my mouth, and the one that drove her nuts, was 'why,''' he says.

Curiosity has taken him a long way from the eastern Montana farm of his youth.

Work by Bromenshenk and his colleagues includes a project using bees to detect land mines through scent tracking, work he hopes will save lives.

''Every one of us involved looks at that as a humanitarian issue,'' he says, noting mines are blamed for killing or injuring more than 20,000 people annually, about half of them children. The research conducted in collaboration with federal scientists has been tested successfully at the Army's Fort Leonard Wood in Missouri.

Back at the Missoula bee yard into which Bromenshenk ventures with bare arms and no hat, he talks about the colonies' female-dominated social structure. Some of the graduate students working under him call them ''the girls.'' Bromenshenk likes the language of science, but veers from it easily as he gauges his audience.

At a recent talk in Helena, he discusses the Colony Collapse Disorder with an animated slideshow, telling the crowd that in some instances ''the queen has lost her entire work force.''

He emphasizes that solving the Colony Collapse puzzle is vital to the future of the bee industry and ultimately affects what Americans eat, given bees pollinate the country's fruit and vegetable crops.

''You ought to be worried about this,'' he tells the crowd. ''It could affect what's in your food basket.''

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