Colony collapse disorder first struck honey bees in the United States in late two thousand six. Over the next two years, beekeepers lost more than one-third of their honey bees.

Scientists in the United States and other countries have been working to explain the mysterious disappearances of bees. Now, a new study suggests that several viruses may act together.

A couple in Wilmington, Ohio, look over the honeycombs in their beehives last month

The team compared bees from affected colonies with those from healthy colonies.  They were looking for differences in gene expression in the guts of the bees.

The scientists found that the affected bees had a number of viruses from a group called picorna-like viruses. The infections observed in the bees included Israeli acute paralysis virus and deformed wing virus.

Tiny insects likely play a big part in spreading the viruses. Varroa mites have been causing serious problems in bee colonies in the United States since the late nineteen eighties. These mites carry picorna-like viruses.

The viruses appear to harm the bees' ability to use their genetic material to produce proteins needed to fight infections. Researcher Reed Johnson, now at the University of Nebraska-Lincoln, says the study suggests that the damaged proteins are unable to respond effectively when attacked.

University of Illinois Professor May Berenbaum says it appears that bees could deal with one or two viruses at the same time, but not three or four.

She says the picorna-like viruses "hijack" the ribosome in cells. Ribosomes are structures in which proteins are made. As a result the ribosome produces only viral proteins.

The professor says ribosome is central to the survival of any organism. If it is compromised, then the bees could not defend themselves against pesticides or fungal infections or bacteria or poor nutrition.

These have all been identified as possible causes of the collapse disorder. Spanish researchers, for example, recently said they suspected a parasitic fungus which has been found among affected bees in Spain.

Bees add billions of dollars in value to many crops worldwide. For now, beekeepers have been doing their best to try to protect their colonies.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Faith Lapidus.