Unwelcome Guests

What Pests are Lurking in Your Soybean Fields?

By Mace Thornton

Neither Nick Seiter nor Kelly Estes has ever come eye-to-antenna with an adult gall midge fly in an Illinois soybean field, nor have they encountered it in its slimier orange larval stage. But if they ever do, the dreaded soybean pest will know it has met a worthy opponent.

The damaging soybean pest isn’t the only potential crop-muncher on the minds of Seiter, a University of Illinois Extension Crop Entomologist, or Estes, the state of Illinois’ Ag Pest Coordinator. The two scientists told soybean farmers gathered for the Soybean Summit that they are also surveilling other members of the soybean-loving insect kingdom, such as the dectes stem borer, the green clover worm and the bean leaf beetle.

Overall, the researchers noted higher pest populations in some areas of the state in 2023, but nothing so major as to present a significant yield challenge. And each of those situations was controllable with timely pesticide applications. But the best pest news of 2023 had to be the fact that the destructive gall midge was not documented as present in Illinois.

Of all potential soybean pests, the gall midge rep - resents one of the biggest potential perils. Although, according to Seiter, the pest has not yet been spotted in Illinois, neighboring states such as Missouri and Iowa – fields as close as 100 miles away — are already midge battlegrounds. Soybean farmers in western Iowa and parts of Nebraska have encountered severe economic losses resulting from early plant death.

“The gall midge is a little fly larva,” said Seiter, who also teaches in the University of Illinois Crop Sciences Department and studies pests of both soybeans and corn. “It gets in behind the epidermis of the soybean stem, and usually there are a lot of them. It's not just one back there feeding; it's maybe dozens of them, and if you have a severe enough infestation, they cause the plants to die early.”

Seiter told the farmers the gall midge invades fields around June or July, and damage progresses through the season. Loss from the pest is devastating and can be confused with the soil-borne fungal disease sudden death syndrome. Although his work also includes research on corn rootworms and alternative methods of pest control, the march of the gall midge across the Midwest has put him on high alert.

The good news is that in the gall midge portion of the 2023 state pest survey, conducted along the border of northwestern Illinois, Seiter did not detect any signs of the pest crossing state lines. The survey covered a significant stretch of Illinois, ranging from the tip of Missouri along the Iowa border and up to Wisconsin with inspections occurring every five to seven miles driven.

An advantage for scientists hunting the gall midge is that it tends to favor habitat along field edges, but as Seiter says, “Borders on state maps do not deter them.” Although Seiter continues to hunt for the gall midge in hopes of not finding it, his effort is just one small component of the larger Illinois Pest Survey coordinated by Estes. An Illinois native, she manages the collaborative effort among the Illinois Department of Agriculture, USDA’s National Institute of Food and Agriculture, which funds the survey, and USDA’s Animal and Plant Health Inspection Service.

The annual pest survey looks at all crop types, including corn, soybeans, specialty crops, and covers and includes a varied landscape of study sites.

“We also look at what we would consider high-risk pathways for how invasive species might be introduced, not; things like rest areas if they are hitching a ride on cars or trucks, rail lines and other modes of transportation,” she said. However, with deep experience in field crop agronomy, entomology and integrated pest management, she finds the field work and collaboration associated with multiple field-crop and pest surveys the most appealing.

Each year, among Illinois’ nine crop reporting districts, Estes visits five to six counties in each district, with five random scouting stops in each county.

“There'll be soybeans across from or adjacent to corn,” she said. “We're doing sweeps in soybeans – literally taking a net and swooshing it back and forth among the plant canopies – and then we’re also doing rootworm counts in corn.”

Like Seiter, Estes is looking along field edges and hoping not to find the soybean gall midge. In addition, she leads invasive insect scouting that uses both sweeping and traps. In all, this year’s survey included 59 counties in Illinois.

Other pests such as the dectes stem borer, the green clover worm and the bean leaf beetle are in Illinois. To soybean growers, they are a bigger challenge, but only because of the fact they are present. Seiter explains that unlike the gall midge, they do not kill the plant and can be controlled before they cause extreme yield damage.

“The bean leaf beetle, at least in our locations, is pretty easy to control,” Seiter said. “And it was a pretty serious year for green clover worm, but most things we applied did a pretty good job of controlling them, though not everything. There were some pyrethroids in particular that I would have expected to provide pretty effective control, but they did not.”

As for controlling field pests, Seiter and Estes recommend a robust scouting program to first determine if additional control efforts are worth the cost and time.

“What we find is that for the pests we have right now, the numbers have to approach pretty incredible levels to warrant additional steps,” Seiter said.

Although the insects might be munching on soybean leaves, they often don’t result in much yield loss. If farmers have concerns, he says, it is best to wait until those pests are spotted before treating because of the short window of effectiveness for most pesticides.

After approval, the final Pest Survey Report will be posted here: https://go.illinois.edu/pestmanagementresearchreport.