Corn Rootworm damage

Western corn rootworm damage was more prevalent in parts of the Midwest this year.

Western corn rootworm populations were up this year in Illinois, even showing up in some places they are rarely seen in large numbers.

“We saw some elevated populations throughout the state, more than in the past few years,” said University of Illinois entomologist Nick Seiter. “We’ve always had corn rootworm in southern Illinois, it just tends not to be at levels where we see major pest problems like we do in central and northern Illinois.”

Infestations rarely rise above economic thresholds.

They could have been more prevalent this year due to a combination of factors. Bob Lawless said some farmers across the state have gotten a bit too comfortable with farming practices that can usher in the beetles.

“Some is due to cultural practices,” said Lawless, an agronomist with Syngenta who works in east central Illinois. “Guys have gotten away from using pyramid-stacked rootworm-treated corn. Plus a lot of guys have planters now with no insecticide boxes on them. Some have gotten lackadaisical.”

Continuous corn is one cultural practice that can result in higher populations of Western and Northern corn rootworm.

“Perhaps there is more continuous corn than in the past in southern and central Illinois,” Seiter said. “And more continuous corn with no control applied. Environmental conditions may have been more than in recent years. It’s most likely a combination of those factors.”

Seiter said that farmers in southern Illinois are less likely to grow continuous corn than their northern neighbors.

“Where I heard reports of issues in southern Illinois it was in fields where you had multiple years of corn on corn,” he said. “In one case it was 10 years. That insect is very capable of building up populations of corn on corn anywhere in Illinois.

“That’s the reason we see it more commonly in central and northern Illinois — especially when we get up in dairy country, where we have black soils. In southern Illinois the landscape is a little more variable. They grow other crops, and there’s more forested area, more pasture area and so on. So you don’t see the same issues with rotation resistance in southern Illinois.”

Conversely, there is more uniformity among fields in other parts of the state, and continuous corn is much more common.

“If you’re growing corn in southern Illinois it’s something you need to look out for, especially if you’re growing continuous corn,” Seiter said. “It’s not a huge risk, but something you have to be on the lookout. The longer you have continuous corn the more risk it is.”

Agronomists with Wyffels Hybrids monitored 877 Illinois fields this year for adult rootworm activity, and found much higher incidence in continuous corn. In soybeans, 91% of the fields had little to no beetle activity. The same was found in first-year corn plots. But 46% of fields of continuous corn had populations at threshold or near-threshold levels.

“The last several years the epicenter has been along the I-88 corridor,” Lawless said. “But we’re seeing more beetles this year. They’re very adaptive insects, so they always worry me. There’s nothing more adaptive in corn than a corn rootworm, whether Western or Northern.”

Farmers who encountered problems this year may want to consider protection measures for 2021, especially if they grow continuous corn.

“If you don’t control something for several years, your populations are going to grow on you, so it’s no surprise,” Lawless said.

Seiter agrees.

“In southern Illinois, rotation has always been effective for us,” he said. “I would throw on top of that closely monitoring your continuous corn acres in particular for beetle activity.”

Nat Williams is Southern Illinois field editor, writing for Illinois Farmer Today, Iowa Farmer Today and Missouri Farmer Today.