The mass deaths of honey-bee colonies in the U.S. may be caused by a lethal combination of fungi and viruses, suggests new research.

Researchers, predominantly from the University of Montana, have identified three viruses — Varroa destructor-1, Kakugo and an invertebrate iridescent virus — in dead honeybees felled by what is known as colony collapse disorder. They also found these bees were infected with two fungi — Nosema apis and Nosema ceranae.

Colony collapse disorder involves the sudden death of a large number of bees that leave the hive and disappear, reducing the colony to a handful of bees or no bees at all. The phenomenon has been occurring with increasing regularity since 2006, particularly in the U.S.

"In 2010, colony collapse disorder again devastated honey bee colonies in the U.S.A., indicating that the problem is neither diminishing nor has it been resolved," write the authors.

They found the viruses and fungi routinely showed up in bees that had succumbed to the disorder while bees from colonies in Montana and Australia that were not affected by CCD were not infected. The presence of both fungi and viruses appeared to be a much more lethal mix than an infection with one virus or fungus.

The fungi appeared to cause tumour-like structures and cell death in the bees.

The research builds on earlier studies that made the connection between fungi and the bees' deaths. In 2007, Ontario Beekeepers' Association experts collected about 446 bee samples from 25 keepers and discovered that each one contained Nosema apis, a single-celled protozoan that affects the bees' digestive systems. More than half of the samples had Nosema ceranae.

For this study, researchers collected between 200 and 500 samples from each of nine colonies, representing the three most populous, three failing and three collapsed colonies. This was done between December 2006 and April 2010.

The study is published in the Oct. 6 issue of the scientific journal Public Library of Science One.