Minnesota Soybean Business

The resistance: a guide to managing aphids

May-June 2019

If past is prologue, soybean farmers somewhere in Minnesota will deal with insecticide resistant soybean aphids at some point in 2019; it’s just a matter of where.

University of Minnesota Extension Entomologist Robert Koch says that for the past four years, farmers in the state have experienced instances of soybean aphids that were resistance to pyrethroids, the most common insecticide group used to treat aphids. Koch says resistance has been documented in all Minnesota soybean-growing regions.

“Resistance has been evident in all areas of the state, but not in all areas every year,” Koch says. “Resistance is likely not going away.”

Koch says farmers need to be aware that resistance to pyrethroids is becoming widespread and they’ll need to manage accordingly to treat soybean aphids without contributing to further resistance development. That could be a challenge because there aren’t many options for treating aphids. Pyrethroids such as bifenthrin and lambda-cyhalothrin had been effective and the most widely used.

“I hesitate to recommend insecticides with just pyrethroids,” Koch says. “You have to look at other groups. Pyrethroids have been used a lot. They’ve been the gold standard. Now that we have insecticides that aren’t working, we have to look at non pyrethroids.”

Other pesticide groups, including organophosphates and neonicotinoids work, but Koch says they come with added regulatory and potential human health impacts.

The Minnesota Soybean Research & Promotion Council (MSR&PC) recognizes the challenge presented by pesticide resistant aphids and has supported several research efforts in recent years to help mitigate the problem. Checkoff-funded projects include a successful field evaluation to determine if aphids are pyrethroid resistant, screening of multiple aphid insecticides, and mapping resistant outbreaks.

Not Weeds
Unlike treating herbicide resistance in weeds where herbicide mixes with different modes of action are frequently prescribed, for aphid treatment Koch recommends rotating modes of action in singular applications.

“Mixes can work,” Koch says, “but when you are managing insecticides to reduce the risk of developing resistance, it’s a finer line.”

Farmers are limited in their insecticide options, which is why proper chemical rotation and careful management of the available tools is paramount.

“If a field has been treated and farmers need a second application because the insecticide didn’t work or there’s been a reinfestation, be sure to alternate groups,” Koch says. “If a pesticide didn’t work the first time, why would it work the second time? Using the same group back to back is when problems arise.”

MSR&PC Director Kris Folland, who also chairs the Council’s production action team, says it’s become increasingly important that farmers have a range of choices to combat aphids.

“It’s a pretty simple, narrow issue,” Folland says. “We need more options.”

Koch says the unfortunate reality for farmers is they simply don’t have many treatment options beyond insecticides.

Buckthorn, which serves as a winter host for aphid eggs, is also the focus of several checkoff supported research projects. MSR&PC has supported a project to explore the relationship between buckthorn density and soybean aphid populations. The long-term goal is to explore treatment methods for controlling buckthorn, decreasing soybean aphid populations while increasing quality and yields for soybean growers.

Farmer Options
Scouting is a farmer’s best practice for determining if soybean aphid populations warrant treatment. Koch says 250 aphids per plant is the trigger mechanism to prepare for spraying. He says actual economic injury doesn’t occur until populations are much higher. Preemptive spraying is not recommended.

“Farmers must scout and be diligent to spray only when the economic threshold is met,” says Folland, who farms in Halma in northwest Minnesota.

Koch agrees.

“We have so few bullets, don’t use them unless necessary,” Koch adds.

Another emerging option is planting aphid resistant soybean varieties. Koch says they are effective, but not widely available for Minnesota growing conditions. Checkoff funds are supporting University of Minnesota soybean breeder Aaron Lorenz’s efforts to incorporate resistance genes into more varieties that are suitable for Minnesota.

“I encourage farmers to try resistant varieties if they are suitable based on the other agronomic factors farmers need,” Koch says.

Whichever options work best for farmers, Koch says it’s important that growers know pesticide resistant aphids are likely here to stay and must be properly managed.

“Aphids are almost always an issue, but not always in the same area,” Koch explains. “We have a persistent aphid problem, but it’s hard to predict where those problems are going to occur.”


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