California scientists said Tuesday they have identified four new viruses in healthy honey bee colonies, a finding that could help solve the mystery of mass bee die-offs in some parts of the world.

The previously unknown viruses turned up during a 10-month study of a commercial beekeeping operation that included more than 70,000 hives and 20 colonies that were transported across the United States to pollinate crops.

The colonies appeared healthy and did not see any of the mass deaths that have eradicated as much as 30 percent of the US population of honey bees since 2006.

Understanding the 27 unique honey bee viruses -- including four new ones and others possibly involved in colony collapse -- and how they circulate in healthy populations could offer scientists a baseline for further study.

"You can't begin to understand colony die-off without understanding what normal is," said senior author Joe DeRisi, professor of biochemistry and biophysics at the University of California, San Francisco.

Honey bee colony declines in recent years have reached 10 to 30 percent in Europe, 30 percent in the United States, and up to 85 percent in Middle East, according to a UN report on the issue released earlier this year.

Honey bees are critical to global agriculture. They pollinate more than 100 different crops, representing up to $83 billion in crop value world wide each year and roughly one-third of the human diet.

According to co-author Michelle Flenniken of the study published in the online journal PloS One, the patterns of infection show that more than one factor is likely to blame for colony collapse.

"Clearly, there is more than just exposure involved," said Flenniken.

"We noticed that specific viruses dominated in some seasons, but also found that not all of the colonies tested positively for a virus at the same time, even after long-distance transport in close proximity."

The researchers also found six species of bacteria and six fungi, four types of mites and a parasitic fly called a phorid, which had not previously been seen in honey bees outside California.

Among the four newly discovered viruses was one that "turned out to be the primary element of the honey bee biome, or community of bacteria and viruses," said the study, identifying it as a strain of the Lake Sinai virus.

Hundreds of millions of its viral cells were "found in each bee in otherwise healthy colonies at certain times of the year," said the research.

"Here's a virus that's the single most abundant component of the bee biome and no one knew it was there," said DeRisi said.

World health experts believe some combination of parasites, viral and bacterial infections, pesticides, and poor nutrition resulting from the impact of human activities on the environment have all played a role in the bees' decline.

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