An Ontario Superior Court has upheld a provincial regulation to dramatically reduce the number of acres planted with corn and soybean seeds coated with a class of pesticides known as neonicotinoids, which are toxic to bees.

In a decision reached Oct. 23, the court rejected an attempt by the Grain Farmers of Ontario to enact a stay on the seed treatment regulations passed into law in July.

The regulations demand that farmers plant only half their acreage with neonicitinoid-treated seeds in 2016.

Starting in the 2017 planting season, farmers must complete a pest assessment report to prove they need the neonicotinoids before any use will be allowed.

"We are extremely disappointed that the judge did not rule in our favour, leaving the grain industry in a very difficult situation as farmers try to arrange seed orders this fall," said Mark Brock of the Grain Farmers of Ontario.

"We are currently reviewing our legal options and will continue to protect the rights of Ontario's grain farmers."

Farmers are concerned they may face losses to insects or see their crops become less productive if they stop using the pesticides.

Close to 100 per cent of Ontario's corn and canola seeds and about 60 per cent of soybean seeds are treated with neonicotinoids, nicotine-based insecticides that contain neurotoxins that make all parts of the plant harmful to insects feeding on them.

Neonics remain in the soil and can be transported to rivers and lakes by runoff. Research shows the insecticides leave bees disoriented and unable to find their way and play a role in the disappearance of pollinators.

In addition to bees, the insecticides affect birds and bats.

Neonicotinoid pesticides are already banned by the European Union. An outright ban in Canada would have to be issued by Health Canada.