An Ontario researcher has identified a main cause in the decline of the province's honeybee colonies over the last three years -- the varroa mite.

The mysterious decline of honeybee colonies across the northern hemisphere has consumed bee researchers over the last three years as they try to determine what is causing it.

In a study published in last month's issue of the apiculture journal Apidologie, University of Guelph professor Ernesto Guzman found that the majority of honeybee colony deaths in Ontario could be traced to the varroa mite.

Guzman and his research team studied 408 bee colonies across southern Ontario over three seasons, from the fall of 2007 to the early summer of 2008. Twenty-seven per cent of the colonies examined in the fall were dead by the spring. Of the colonies that died, Guzman and his team found that 85 per cent of colony deaths could be attributed to the varroa mite.

"The varroa mite was by far the main culprit of these mortality cases," Guzman told in a telephone interview Wednesday.

The other four factors examined were the colonies' food reserves in the fall, the size of the colonies and the impact of two other parasites on bee populations. The study found that colony size and fall food reserves also had an impact on colony death.

In combination with a small colony size and low fall food reserves, the varroa mite creates "a perfect storm for the colony to die," Guzman said.

Ontario's beekeepers have faced tough winters over the last several years, with 35 per cent of bee colonies dying over the 2006-2007 winter and 32 per cent of colonies dying during the 2007-2008 winter.

The decline in Ontario bee colonies has been accompanied by mysterious bee colony declines in the United States and some countries in Europe.

Roughly three-quarters of flowering plants rely on bees, birds and other pollinators to allow them to reproduce. The recent decline in bee colonies has been blamed for an increase in food prices. Plants such as blueberry bushes rely on pollination from bees to reproduce.

Although the University of Guelph study looked at honeybees in Ontario, Guzman said the varroa mite is a likely contributor to honeybee colony deaths elsewhere.

"I believe it will be among the three main factors associated to colony mortality anywhere else in the world," he said.

The varroa mite has been a pest Ontario's beekeepers have had to consider for about 20 years, but it seems to have developed resistance to the chemicals beekeepers have used to control the problem, Guzman said.

New products to combat the varroa mite will help the problem, Guzman said. He recommended breeding bees with a genetic resistance to the mite and using bio pesticides that will kill the mite but not the bee as potential methods to curb this problem.

A clear picture of how the province's bee colonies are faring during the current winter won't emerge until spring, when beekeepers open their colonies from late March to early April.