CASSELTON, N.D. - University researchers in Minnesota, North Dakota and South Dakota are collecting aphids they believe may be resistant to existing pesticides and are asking farmers to alert them to fields where they think resistance may be occurring.

Jan Knodel, North Dakota State University Extension entomologist, says so far this year soybean numbers have been insufficient to test.

Still, she says that could change and she urges farmers to contact her if they have enough to spray and get insufficient control. Aphids after the V6 stage of development are large enough that they don't cause economic damage.

Farmers can call 701-541-4094; or e-mail at Under the research plan, Knodel and colleagues who can find enough aphids will test those suspected of being resistant by treating them with various classes of insecticides.

Minnesota started seeing aphid resistance in 2013 in Polk County, Minn. Resistance is now in North Dakota, South Dakota and Iowa. With bifenthrin, a pyrethroid insecticide, the scientists are seeing up to eight-fold resistance compared to a susceptible laboratory colony of aphids.

"With lambda-cyhalothrin (Warrior) we're seeing up to 23 times resistance," Knodel said.

"We're trying to determine how bad the resistance is and to continue to monitor it to see if it's spreading," Knodel said, speaking at a recent NDSU Agronomy Seed Farm field day at Casselton, adding, "right now it's mainly just in the eastern counties of North Dakota. We're going to go back to the same spots we went to last year and see if there's still resistance, if it's still there. If you move away to different insecticides, it can decrease."

In the 2017 crop year in North Dakota, Knodel and associates collected aphids from eight sites and only one did not have resistant aphids. The locations were from Cass County to the north to Pembina County in the Red River Valley. However, aphids are mobile and can migrate in from other states, including hotspots in southern Minnesota. Southern Minnesota has a larger region.

Knodel suggests scouting fields first and spraying only when the aphids are at an economic threshold of 250 aphids per plant on 80 percent of the field.