Soybean Aphids Have Arrived. Send in the Scouts

Sometimes a quick check on Twitter can tell you whether an insect pest is showing up in farmers' fields, ready to inflict damage on crops. In the last few days, that’s been true regarding mentions of soybean aphids.

“Have you forgotten about soybean aphids? They’re back,” Stephanie Porter, outreach agronomist for the Illinois Soybean Agronomist, announced via Twitter on Thursday. Read her article here.

It’s a sentiment other agronomists and entomologists across the upper Midwest have also been sharing in recent days, as scouting gets underway.

What’s driving the concerns about soybean aphids this early in the 2023 season? 

Chris DiFonzo, Extension field crops entomologist for Michigan State University, attributes the infestations at least in part to the increase in early-season soybean crops.

“We’re pushing those beans, planting earlier, so we’re seeing more aphids in late May to early June here when those soybeans are coming out of the ground. They catch that first flight of aphids coming out of overwintering areas,” she tells Farm Journal.

In fields DiFonzo has checked to date, she has seen as high as 70% of the plants infested, but only in early-planted fields.

“There were a lot of beneficial insects out there, too, going at the aphids. There were also some fields, later-planted soybeans, that were clean” she adds.

DiFonzo says this the time of the growing season is when farmers can get a sense of how good a job their beneficial insects – Asian lady beetle, minute pirate beetle, parasitic wasps and green lacewing – are able to do. 

“This is a key battle time,” she says. “The good insects are trying to control the aphids. By the Fourth of July here in Michigan, we can often get some sense of what’s going to happen.” 

Spraying Could Create A Bigger Problem
The ability beneficial insects have to suppress soybean aphids is a key reason why Extension specialists urge farmers to not treat soybean aphids too soon.

“When you remove the predators too quickly you can encourage the problem of resistance, so save your bullets,” advises Bruce Potter, University of Minnesota Extension IPM specialist. “Treating soybeans too soon is the best way to have a bad aphid problem, because you’re also taking out all the beneficials in that field.”

Aphids that move into those fields that were treated too early often enjoy a soybean buffet, because no predatory beneficial insects are present to stop them.

The value of beneficial insects to hold aphids in check is a recent development and a big reason why aphids have done less damage to soybeans in recent years, DiFonzo notes. 

“That parasitoid piece was missing when aphids first showed up here in the U.S.,” she says.

Native to Eastern Asia, the soybean aphid was detected for the first time in the U.S. in Wisconsin in 2000 and quickly became a costly pest.

Depending on the severity of the soybean aphid population, historical yield losses have ranged from 10% to 15%. However, severe cases can cause losses of up to 40%, according to Erin Hodgson, Iowa State University Extension entomologist.  See her soybean aphid video here.

Scouting Can Save Money And Your Beneficials

This time of year, Extension recommends scouting soybeans every seven to 10 days for aphids. 

When scouting, Hodgson recommends these tips:
•    Focus on the new growth for signs of aphids. 
•    Look on the undersides of leaves.
•    Ants or ladybeetles can be signs soybean aphids are present.

“Use a threshold of 250 aphids per plant through the R5 growth stage, with most of the plants infested, and aphid populations increasing,” Potter says.

“Remember, this threshold is not the point at which yield loss starts to occur,” adds Potter’s colleague, Bob Koch, Extension entomologist, University of Minnesota. “It is instead the trigger point to start lining up an insecticide application to prevent the infestation from increasing to higher levels that will cause economically significant yield loss.” Learn more here.

Economic loss in soybeans occurs when there are more than 600 aphids present per plant. Applications in the early reproductive stages are usually of most economic value.

If you choose to spray, strive for 100% kill with applications, Hodgson encourages. 

“The aphids are all females giving births to asexual clones,” she says. “So, you want to eliminate as many as you can. You can hopefully get to that with uniform coverage, high volume and pressure when spraying. To know if it was a profitable application, leave an untreated test strip to evaluate.”

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