The farmers came from Windsor and Cornwall and all points in between to crowd into a Toronto courtroom. They came by the busload – five in all – to the Ontario Court of Appeal on Wednesday morning to listen to lawyers and judges spar over a new provincial law that restricts their ability to use a controversial pesticide, neonicotinoids.

The pesticide is believed to play a role in the widespread deaths of bees and other pollinators in Ontario and elsewhere. But farmers see neonics, as they are called, as a vital tool that protects their crops from insects, and their lawyer is asking the three appeal court judges to order a new trial, after the first one didn't go their way.

So more than 150 growers took a day off from their chores to board Toronto-bound buses chartered by the Grain Farmers of Ontario. They sat shoulder to shoulder and stood in the aisles of the wood-panelled courtroom, which was not nearly big enough to hold them all.

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"We're just here to support [Grain Farmers of Ontario chairman Mark Brock] with the turnout," said Allan Carruthers, who grows grain east of Peterborough and boarded a bus in Campbellford well before the sun came up.

Mr. Brock, who farms 1,500 acres in Southern Ontario, said the fact that so many farmers made the trip to the provincial capital shows how strongly they feel about the matter.

"We'd all be getting ready for spring [otherwise]," he said. "It's a beautiful day. There's always something on the to-do list."

"The court will see this and will understand it's important to people," Eric Gillespie, lawyer for the grain farmers, told the group outside the courtroom.

Mr. Gillespie was their advocate and de facto tour guide of the justice system, explaining the appeal court procedures to a group that is, like most Canadians, unfamiliar with the courts. "We're not going to be arguing the facts. We're going to be arguing the law, so it may be a little difficult to follow," he said before the start of the hearing, which was over in less than four hours.

Ontario last year became the first jurisdiction in North America to restrict the agricultural use of neonics. Farmers cannot use the seed treatment on half their land in 2016, and on any of it in 2017, unless they submit crop and soil assessments that show they have infestations of crop-eating worms and insects.

The Ontario government took the step after beekeepers lost more than half their hives in the winter of 2013-14. Experts say the neonics sicken the bees and make them more vulnerable to mites, viruses and winter starvation. Early in the growing season, the neonic-laced dust generated by planting treated seeds has also been linked to bee deaths. "It happens right from spring till fall," beekeeper Tibor Szabo said.

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But farmers say the neonic insecticides, banned in Europe and facing restrictions in Quebec, are needed to protect their crops.

Mr. Gillespie told the court the farmers are not opposed to pesticide regulations, but he said the new set of rules is unworkable and infringed farmers' right to use their land as they want.

Justice John Laskin questioned if anyone had such a right, and told Mr. Gillespie farmers are not prevented from using neonics – they just have to show they need them. He asked if the court had the authority to reverse a law, given the farmers were making no constitutional or jurisdictional challenge.

"You're asking us to rewrite the regulations," said Justice Laskin, who noted that the court would issue a ruling at a later date.

Crown attorney Sandra Nishikawa, who is representing Ontario's Ministry of the Environment and Climate Change, said the new law simply limits the use of neonics to where they are needed, and that other insecticides have always been heavily regulated.

"There is no right to use whichever pesticide one chooses," Ms. Nishikawa told the court.