Growers know full well the management challenges of herbicide-resistant weeds.

Add insecticide-resistant pests to the agronomic management list.

For the last two year, growers in southwest, central and northwest Minnesota reported soybean aphid resistance issues with pyrethroid insecticides. Not good.

According to Bruce Potter, a University of Minnesota integrated pest management specialist based in Lamberton, in 2015 there were farmers in nine counties who reported issues with pyrethroid performance.

“In fields I visited, there were pockets of poor control, indicative of insecticide-resistant female clones, scattered through the field surrounded by good control,” he says. “Detective work indicated that it was unlikely to be poor coverage or other rate issues or recolonization of the field.”

Soybean aphid pressure in 2015 was heavy in parts of central, south-central and southwest Minnesota later in the season.

In 2016, farmers in six counties reported insecticide resistance, including Polk County in northwestern Minnesota. How did it show up there?

“Compared to weeds, the difference with aphids — and many other insects — is their mobility,” Potter says. “During late July to early August in particular, winged soybean aphids tend to disperse long-range.”

The other possibility is what immigrant and native aphid populations were exposed to previously.

“There was a field in Ottertail County, as well as in fields in Stearns and Redwood counties, which had reduced susceptibility to lambda-cyhalothrin [e.g., Warrior],” he says. “The field in northwestern Minnesota could have been populated with immigrant aphids coming from those affected fields, or the 2016 resistant population could have been selected locally. This uncertainty of what an aphid population has been exposed to is why resistance management for aphids takes some thought. With weeds, soybean cyst nematode and western corn rootworms, the pesticide and trait exposure for the pest population in the field is mostly known.”

“Soybean aphid pressure was much higher than in 2014 or 2016,” Potter says. “As a result, the fields with high populations are most likely to see visible effects of poor control. Low pressure in much of Minnesota may be why reports are fewer in 2016."

Insecticide update
To broaden understanding about insecticides approved for use to combat soybean aphids, growers currently have products available in four chemistry subgroups — carbamates (Group 1A), organophosphates (Group 1B), synthetic pyrethroids (3A) and neonicotinoids (4A) (see table). Historically, most Minnesota growers have used synthetic pyrethroids and the organophosphate chlorpyrifos.

Source: Bruce Potter, University of Minnesota Extension

Chlorpyrifos, also known as Lorsband, and used in mixes and generics, has been used as a pesticide since 1965 in both agricultural and nonagricultural sectors. The U.S. EPA estimates that more than 40,000 crop-producing farms currently use chlorpyrifos to control a wide range of insect pests.

“Chlorpyrifos currently has regulatory issues,” says Bob Koch, U-M entomologist.

Chlorpyrifos remains registered as it undergoes routine EPA registration review, a process that all pesticides are legally required to go through once every 15 years. Since 2000, EPA has either limited or discontinued chlorpyrifos’ use in food production. The agency has proposed to revoke all chlorpyrifos tolerances due to concerns that residues on food crops exceed safety standards. Officials also are concerned about workers who mix, load and apply chlorpyrifos, and workers re-entering treated areas after application.

If the agency follows through on its proposal, all agricultural uses of chlorpyrifos would cease. EPA plans to issue a final rule on chlorpyrifos tolerances by March 31.

Sulfoxaflor, a Group 4 insecticide, does not currently have a label for use in Minnesota soybean fields. Efforts have been going on for a couple years to get it labeled. Koch and Potter submitted an application for a Section 18 emergency exception to the Minnesota Department of Agriculture to regain use of this product for soybean aphid management in the state. The Minnesota Soybean Growers Association and Minnesota Soybean Research and Promotion Council provided letters of support for this application.

“This would allow another type of chemistry for effective aphid control,” Potter says. “Chlorpyrifos works well, but may not be available long-term. Plus, it would be good to have another insecticide class available for insecticide resistance management.”

If you have any questions, contact Potter at [email protected] or 507-276-1184; or Bob Koch, U-M Extension entomologist, St. Paul, at [email protected].

The website for reporting performance issues is z.umn.edu/aphidinsecticidefailure.