Soybean aphids are spreading in central Minnesota and farmers are in a pickle because one of the pesticides used to kill the bugs isn’t working and the other is harmful to the environment.

The little bugs, born pregnant, reproduce more quickly in mild, wet weather. The University of Minnesota Extension has put out several warnings in recent weeks that farmers should check their soybean fields for the bugs, which look like sesame seeds.

“Right now it’s kind of central Minnesota, maybe St. Cloud going out toward Willmar and Morris, where aphid numbers are building up the quickest and some fields are being treated,” Bob Koch, an entomologist with University of Minnesota Extension, said.

In the rest of the state, aphid numbers are still pretty low, but “that can change quickly,” Koch said, and conditions lately have been perfect for them to spread.

The U Extension office advises that farmers shouldn’t spray until they see a lot of aphids in their fields, around 250 aphids per plant on more than 80 percent of plants.

Farmers have in recent years used two types of insecticide to kill aphids — pyrethroid and chlorpyrifos — and both have major drawbacks. Since 2015, aphids have been developing resistance to pyrethroids, while studies have shown chlorpyrifos may cause developmental and physical delays and attention-deficit disorder in children.

The Environmental Protection Agency banned chlorpyrifos in household insecticides in 2000 and has since restricted its use on certain vegetables.

The Obama administration was ready to take the chemical off the market completely in 2016, but that decision was reversed by the Trump administration in 2017, which was a relief to soybean farmers, who said they need the chemical.

But chlorpyrifos has been designated as a “surface water pesticide of concern” by the Minnesota Department of Agriculture and even soybean pest-control experts are uneasy about its widespread use.

“We’ve got a lot of concerns in Minnesota about pesticide contamination in surface waters and groundwaters,” Koch said. “Some of those contaminations are being linked to insecticide applications for soybeans, especially chlorpyrifos.”

The dilemma is sharpened by the fact that pyrethroids aren’t always working on aphids.

Not all aphids are immune to pyrethroids, and Koch and other researchers at the U Extension are rushing to study bugs from different parts of the state to see which ones can be killed with the less harmful pesticide.

But for soybean farmers, there’s no time to dillydally. They don’t want to spend time and money to spray a pesticide that might not work, and any delay in addressing the fast-multiplying bugs could cause them to lose part of their crop.

“We’re really hesitant to recommend that people use the pyrethroid insecticides alone, just because there’s a pretty good chance that they won’t work,” Koch said.

Other options, such as mixing a pyrethroid pesticide with a neonicotinoid, are available and working. But that, too, has drawbacks, since neonicotinoids have been identified as a serious danger to the state’s bees.

Last year, aphids from southern Manitoba to western Iowa were resistant to pyrethroids.

It’s still early, but when researchers tested aphids from near Willmar last week, they showed resistance to the pesticide.

Several other types of pesticides have been banned already, said Kris Folland, a farmer in northwest Minnesota who directs the Minnesota Soybean Research and Promotion Council. When farmers use fewer types of pesticides, the bugs are more likely to develop resistance to them.

“Less tools in the toolbox is a problem, because then they’re going to get overused,” Folland said.

Folland said his fields in Kittson County are pretty much aphid-free and with only six weeks left in the growing cycle he doesn’t expect them to be a problem this year. But for farmers in central Minnesota, pyrethroid insecticide is preferable. It’s about half as expensive as the harsher chemical, and farmers prefer to use the less harmful insecticide if they can.

“We don’t blanket-spray anything,” Folland said. “We’re not out here doing things that we don’t have to do, because we have to live with the consequences too.”