Minnesota's soybean growers will need to find a new weapon against pests.

Starting in February, federal regulators will ban most uses of the state's most common and widely used insecticide — chlorpyrifos. The powerful poison has long been one of the most effective ways to kill crop-destroying bugs, but a growing number of scientific studies have found it to be harmful to children.

"We're glad to see it go," said Joshua Stamper, director of the fertilizer and pesticide management division of the Minnesota Department of Agriculture. "It's really the last of this nasty line of organophosphate pesticides. It's one of those technologies that was useful and important, but as time progressed society has come to the fact that there are better solutions."

Chlorpyrifos has been around since the 1960s. But over the past 20 years, Minnesota farmers have come to rely on it more than in anywhere else in the nation. That's because it's particularly effective against soybean aphids, invasive pests that have built up some immunity to other common pesticides.

Nobody can say exactly why, but soybean aphids have thrived in Minnesota even as their populations have fallen or crashed in other parts of the Midwest. They were first detected in the state in the early 2000s, said Robert Koch, entomologist with the University of Minnesota.

Over the years, they largely died off in places like Ohio, Indiana, Illinois and Michigan, Koch said. But in Minnesota the tiny green pests kept coming back, and as farmers sprayed for them, they started building up resistance to certain poisons.

Chlorpyrifos was one of the few tools that would consistently kill them, Koch said.

"There are still some chemicals that work, but that box is getting smaller," he said.

Pressure to ban the insecticide has been growing for more than a decade.

The Pesticide Action Network North America and the Natural Resources Defense Council asked the Environmental Protection Agency to prohibit its use in growing food in 2007.

After nearly a decade of reviews, studies and court hearings, the Environmental Protection Agency found in 2016 that children exposed to the chemical were more likely to be developmentally delayed and have attention deficit disorders. Other studies have found that fetuses exposed to the chemical are more likely to grow up with autism and reduced intelligence and memory.

The Obama administration started the process of taking chlorpyrifos off the market, citing risks to children, drinking water and wildlife. But the EPA reversed that decision shortly after President Donald Trump took office in 2017.

That reversal was challenged, and in April, a federal appeals court ordered the government to determine whether the pesticide is safe. In August, the EPA ruled that the pesticide did, in fact, pose a risk to children and farmworkers and announced the ban, which will go into effect Feb. 22.

Shortly after it was announced, Rep. Rick Hansen, DFL-South St. Paul, and several state lawmakers called on the Minnesota Department of Agriculture to immediately suspend sales of chlorpyrifos to close the window during which it could still be applied.

"If we know it's a bad chemical, why continue to sell it?" Hansen said.

On Monday, state Agriculture Commissioner Thom Petersen wrote in a letter to lawmakers that the agency will not allow the chemical to be used on Minnesota crops starting Jan. 1.

It would be rare and unlikely for any growers to apply chlorpyrifos this late in the season, Petersen said.

"We'd like the ban to happen sooner, but we're glad the agency has taken these concerns into account and is standing up to the agrochemical interests that are used to getting their way," Hansen said.

The chemical will still be allowed for a handful of uses, including at tree nurseries, golf courses and sod farms.

It's unlikely the ban will cause Minnesota soybean fields to go out of production or force farmers to change their crops, said Bill Gordon, a farmer in Worthington and chair of the American Soybean Association.

But it may actually increase the amount of insecticide used in Minnesota, he said, because growers will likely have to reapply less effective insecticides multiple times each season since soybean aphids have grown resistant to them.

"So instead of having just one pass that kills all the butterflies, bees, pollinators and beneficial insects in the field, you have to come back in two weeks and do it again," Gordon said. "So now you don't really have a chance to build that beneficial population back up. It just negates some of the things they're trying to do with this ban."

More applications also mean more costs borne by the farmers.

Zoe Hollomon, organizing co-director of the Pesticide Action Network, said she doesn't doubt that farmers will turn to other insecticides.

"We understand there are economic concerns, but this pesticide is very toxic to human life," she said. "If a ban isn't in place anybody on neighboring farms or properties, schools and community centers is unprotected from it. They're unprotected from its drift and they're left with dealing with whatever a farmer wants to spray."

Greg Stanley • 612-673-4882