Environmental groups have launched a court challenge to federal permits for two common pesticides that some say are behind large die-offs in bee populations.

The lawsuit, filed in Federal Court in Toronto, takes aim at neonicotinoids, which are among the most widely used pesticides in Canada.

The David Suzuki Foundation, Friends of the Earth Canada, Ontario Nature and the Wilderness Committee say in court documents that Canada's federal pesticide regulator has allowed the chemicals to be used despite being uncertain about their risks.

"The (Pest Management Regulatory Agency) does not have reasonable certainty about the risks of these products, which they're required to do," said Charles Hatt, the lawyer arguing the case.

Hatt said the agency has a history with two so-called neonics, clothianidin and thiamethoxam, dating back about a decade.

"What you see is that for a number of years the (agency) will note that they are concerned about risks to pollinators, to bees, from chronic toxicity and that they need field studies to determine the nature of the risk. Then they will request that information from the proponent and they either get something they deem inadequate, or they get nothing at all.

"But they continue to register and re-register the product.

"We've had a situation where for years, they're continuing these product registrations without the scientific information that the agency itself flags as critical for determining the risks of these pesticides."

The federal government has yet to file a response in court.

Several causes for deaths

Neonics have been implicated in large die-offs of bee populations, which are crucial to agriculture.

Published reports suggest about a third of the crops eaten by humans depend on insect pollination. Bees are responsible for about 80 per cent of that of that figure.

Bees have been in serious decline across North America and Europe since about 2006. In Canada, the Canadian Honey Council reports that in 2013-14, Canadian beekeepers lost an average of about 25 per cent of their colonies. Ontario's losses were 58 per cent.

The average winter loss is about 15 per cent.

Populations of wild bees are also falling rapidly. A recent American study found numbers fell by about 23 per cent between 2008 and 2013.

The University of Guelph's Honeybee Research Centre blames a combination of disease, parasites, pesticides and habitat destruction.

A major U.S. study earlier this year found neonics harmed domestic bees in some crops but not others. Research suggests the pesticides are much harder on wild bees than honeybees.

Europe has imposed a moratorium on neonics. Last year, the U.S. banned new uses for them. Ontario has announced plans to limit their application. Companies such as Ortho and Home Depot have announced they will phase them out.

Still, they remain in wide use.

The Conference Board of Canada, in a 2014 report prepared with support from the Grain Farmers of Ontario and CropLife Canada, estimated banning neonics would cost Ontario farmers $630 million a year.

Hatt points out the public has never been asked for feedback on the use of neonics, despite requirements in legislation.

"You've got an important and environmentally risky pesticide that has never been subject to public consultation in Canada," he said.

A ban on neonics would have to come from the Health Canada.