Freezing temperatures, killer parasites, toxic chemicals: The plight of honey bees is getting worse in many parts of the world and no one seems to know precisely why.

This past winter was one of the worst on record for bees. In the U.S., beekeepers lost 31 per cent of their colonies, compared to a loss of 21 per cent the previous winter. In Canada, the Canadian Honey Council reports an annual loss of 35 per cent of honey bee colonies in the last three years. In Britain, the Bee Farmers' Association says its members lost roughly half their colonies over the winter.

"It has been absolutely catastrophic," said Margaret Ginman, who is general secretary of the Bee Farmers' Association. "This has been one of the worst years in living memory."

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"There are some beekeepers that have lost 70 per cent over the last winter, and you can't even make that up in one season," says Dan Davidson, president of the Ontario Beekeepers' Association. "That's a disaster."

Just why so many bees have died is far from clear. In Britain, many blame a wetter than usual fall and winter. Queen bees typically mate while flying and the wet weather kept them from moving around, resulting in lower colony populations.

In the U.S. and Canada, scientists have different theories, with some blaming the Varroa mite, which burrows into bees and feeds on their blood, and others pointing to disease and an increased use of pesticides by farmers. "The decline in honey bee health is a complex problem caused by a combination of stressors," the U.S. Department of Agriculture said in a report released last month.

There's no doubt bees are critical to humans. By some estimates, bees and other pollinating insects, such as butterflies and moths, contribute $200-billion to global agriculture every year. Almost one-third of the food we eat has been pollinated by bees and some crops, such as broccoli and almonds, are entirely dependent on bees for pollination.

David Schuit, a honey producer in Elmwood, Ont., had plans last year to expand his family's honey business when disaster struck. He runs 35 bee yards under the name Saugeen Country Honey with his wife and seven children in Elmwood, Ont. He says he had a healthy, robust crop of bees that had survived the winter, but one spring day in May of 2012, he found his bees in "terrible agony," going around in circles, venom dripping from their back sides. The bees were either staying away from their hives or unable to find their way inside.

"It hurts deep inside when you see your hives dying in this manner," he says.

The family ended up losing 90 per cent of the bees on their home yard alone. All in all, Mr. Schuit said they lost around 37 million bees, and the family produced barely half the honey they make each year. They were forced to give up their 100-acre organic cash crop farm because they could no longer continue making mortgage payments on it.

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The loss of bees was difficult to recover through the following winter, a time when bee populations generally decline because of the freezing temperatures, and this year, Mr. Schuit says bees are continuing to die en masse. "It's hard on us. We need help," he says. "I'm ready to throw the towel in."

The Schuits are among a growing number of beekeepers who are blaming sudden and massive bee deaths on neonicotinoid pesticides. These are nicotine-like substances that attack the nervous system of insects. They are also water soluble, meaning they can be added to the soil and taken in by the entire plant, making every part of it lethal to bugs.

Corn, soybean and canola farmers coat their seeds with the insecticides using a machine that needs to be lubricated with talc to push the seeds out. The talc absorbs some of the pesticide and bees get exposed to this toxic mixture when the machine blows the talc out.

Health Canada's Pest Management Regulatory Agency noticed that honey bee deaths in about 250 bee yards in Southern Ontario and Quebec coincided with the corn-planting season. They found neonicotinoids in 70 per cent of bee samples taken from these regions.

In April, the European Union passed a two-year ban on neonicotinoids because of the risk they pose to bees. Beekeeper associations in Ontario and Quebec are calling for a similar ban in Canada, but they will likely face an uphill battle against grain farmers, who say the pesticide has been crucial to their business since it was approved for use in 2004.

In July, the Grain Farmers of Ontario mailed out 28,000 postcards urging their members to call their local political representatives and oppose any kind of a ban on the neonicotinoids.

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"A knee-jerk reaction shouldn't happen until we find out more about what is actually happening from a bee-health perspective because there are are other issues that affect the bee populations," says Barry Senft, chief executive officer of Grain Farmers of Ontario. "If we start to move away from a science-based approval process, everything then is subjective."

The group says that a ban on neonicotinoids would result in a loss of three to 20 bushels per acre for Ontario farmers. For the average farmer with a 500-acre field, this would translate to a loss of two to 13 per cent of their gross income, making it difficult for them to compete with farmers in Western Canada and the United States, Mr. Senft says.

Health Canada says that more research needs to be done and continues to collect samples of affected bees this year. It says regulatory action to protect bees against neonicotinoids may be taken, if warranted, at any time during this review process.

Ernesto Guzman, head of the Honey Bee Research Centre at Guelph University, says the Health Canada data points to a definitive link between neonicotinoids and bee deaths, but says the pesticides are not the only cause of declining bee populations. It is also not known if the pesticides are the major cause of bee deaths. Mr. Guzman says that, during the winter, Varroa mites are the main cause of bee deaths. They came from Asia about 20 years ago and are found on virtually all bee colonies.

On average, bee keepers expect to lose about 10 to 15 per cent of their numbers in the winter because the cold makes it harder for the bees to survive. Paul Vautour, the Maritimes director for the Canadian Honey Council, says a drought last fall made it even harder for his bees to withstand the prolonged winter, causing him to loss 85 per cent of his bee colonies. He had 238 colonies at the start of winter, and by May, this number was down to 36.

Mr. Vautour says this was a "big loss" as a relatively small commercial beekeeper, and he had to spend $24,000 to buy 100 new colonies.

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Many beekeepers say they know how to manage their Varroa mites, however, and insist that they don't cause as much destruction as the neonicotinoids.

Mr. Guzman says it is important for the government to invest in independent research to help beekeepers facing significant losses.

"The beekeepers need help and they need help immediately," he says. "Research to find answers to the problem and to answer the question of how much pesticides are killing bees relative to other causes might take many years."

HONEY BEES BY THE NUMBERS