The Ontario government plans to restrict the use of a class of agricultural pesticides linked to the widespread declines in honeybees and other pollinators.

The move is opposed by the province's chemical and agriculture industries, which say neonicotinoid pesticides are vital tools that growers of corn and soybeans use to protect their harvests from yield-destroying insects.

The pesticide, which renders plants toxic to pests, has been blamed for the deaths of honeybees and other beneficial insects that are responsible for pollinating one-third of the food we eat.

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The province said on Tuesday it wants to reduce the acreage planted with neonic-treated seeds by 80 per cent within two years.

To do so, the government is proposing to restrict the sale of corn and soybean seeds treated with neonics to farmers who can show their fields are susceptible to pests, verified by a third party. Farmers must also complete pest management training and document their efforts to eliminate pests such as wire worms and grubs.

The proposals will be subject to public consultations and in place by July 1, which is when growers begin buying seed and supplies for the following growing season.

Ontario would become the first province or state in North America to regulate the pesticides, which are halfway through a two-year moratorium in Europe amid concerns over environmental impacts.

"We know, and farmers recognize, there are risks associated with the use of neonicotinoid pesticides. We also know that, in certain circumstances, they are an important tool for farmers and help to increase production and maintain a reliable food supply for our province," said Jeff Leal, Ontario's Minister of Agriculture.

The Grain Farmers of Ontario said the steep target is effectively a ban that disregards efforts its members have taken to reduce risks to honeybees. Farmers changed their planting methods in the past spring to minimize the amount of neonic-laced dust that is kicked up during seeding by fitting their machines with deflectors, at the direction of Health Canada. And neonic-treated seed now comes with a wax-based fluency agent that is intended to reduce the dust that can be immediately fatal to bees.

"A reduction at this level puts our farmers at a competitive disadvantage with the rest of the country and the rest of the North America," said Barry Senft, chief executive officer of the group that represents 28,000 farmers. "It will mean smaller margins for grain farmers and could signal the transition away from family farms to large multinational farming operations that can sustain lower margins."

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More than half of the hives in Ontario did not survive the past winter, losses that beekeepers and some scientists say are attributable to neonic poisoning on top of the other threats to bees, which include virus-carrying mites and starvation over long winters. Losses across Canada averaged 25 per cent, higher than the 15 per cent that is considered normal and sustainable, said the Canadian Association of Professional Apiarists.

Ontario said it wanted to reduce winter bee deaths to 15 per cent by 2020.

In Ontario, all corn, canola and most soybean seeds are coated with neonics. The province has said just 10 to 20 per cent of the five million corn and soybean acreage requires neonics to ward of yield-destroying insects. The pesticide is also used by growers of flowers, fruits, vegetables and sod. The Ontario plan covers only corn and soybean, which are grown for animal feed or biofuels.

Meanwhile, Health Canada is re-evaluating its approval of products containing the three most widely used neonics in partnership with the U.S. regulator.

Beekeepers in Ontario and Quebec have launched a class-action lawsuit again the chemical companies to recover the financial losses they allege they have suffered as a result of bee deaths. These costs include lost honey production and replacing dead bees.

Tibor Szabo, president of the Ontario Beekeepers Association, said a raft of studies has shown "overuse" of neonics is the core problem for honeybee health. The blanket use of the chemicals has discouraged farmers from trying other methods to control pests, Mr. Szabo said.

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"There's a growing preponderance of research showing [neonics] are quite dangerous and they stay in the environment for a long time," said Glen Murray, Ontario's Minister of the Environment. "We're not doing this on an emotional basis. We're doing this on an evidence basis."

The chemical companies that make neonics and are the main sellers of pesticide-coated seeds say the pesticides are safe if used as directed, and are less harmful to people and the environment than older classes of chemicals.

A representative of the chemical and agricultural companies said the plan places a "burden" on vendors to police who can buy which kinds of seed. And the pesticide makers might simply overlook the Canadian market as a place to make investments or introduce new crop technologies, said Pierre Petelle of CropLife Canada, which represents pesticide makers Bayer, Syngenta and others.

"When they look at markets where they should invest, where they should register the newest technologies, Canada hasn't always been at the top of that list, mostly because of market size," Mr. Petelle said in an interview. "But when you add this layer of intereference, if you will, at the provincial level, it's going to make Canada an even less interesting place to invest. The real risk I see is future innovations, the new technologies."