Everyone who farms in Illinois should be aware of corn rootworms, but no one should panic. If Nick Seiter were giving a “state of the state of corn rootworms” address, that is how it would begin.

“Overall, rootworm populations are low in Illinois,” says Seiter, Extension field crops entomologist with the University of Illinois. “Rotation-resistant western corn rootworm variant pressure, once a major problem, is almost nonexistent now.

“Populations of northern corn rootworms have increased recently. And populations of both northern and western corn rootworms have resistance to all available Bt traits in most areas. However, it is most pronounced in northern counties where continuous corn is prevalent.

“New RNAi corn rootworm traits offer another tool. Soil insecticides are still effective in Illinois where needed. So, you can manage corn rootworm today in those areas where it is still a concern.”

Closer look at rootworm

Seiter explains the details behind his observations:

Rootworm numbers are low overall. For western corn rootworm, the average number of beetles per plant statewide was 0.1 in ’24, Seiter reports. Counts were zero in five of Illinois’ nine crop-reporting districts — primarily those covering southern, central and eastern Illinois. The northwest corner topped counts at 0.49 beetle per plant.

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Western corn rootworm variant pressure is extremely low. Finding corn rootworm damage in first-year corn is rare, Seiter says. This phenomenon surfaced in east-central Illinois over 30 years ago when variant western beetles laid eggs in soybeans. Beetle counts are now super-low in those areas, Seiter says.

Northern corn rootworm beetle numbers are on the rise. Relatively speaking, numbers are still low in most places. Numbers moved up and down over the past few years, topping out at 10.5 beetles per 100 sweeps for a state average in 2021. However, that was only because numbers in northwest Illinois were very high. Expect to see more northern corn rootworm beetles in northern Illinois, especially in continuous corn.

Populations of both species have resistance to all Bt traits. This was news when confirmed in Illinois not that long ago. Originally, it was documented in Iowa in continuous corn areas. “We’re seeing it in Illinois, primarily in northern and northwestern counties where there is more continuous corn,” Seiter says. “We’re not seeing resistance nearly as much where most farmers follow corn-soy rotations.”

No resistance to RNAi traits is confirmed yet. RNA-interference controls corn rootworm differently than Bt traits. Plants express double-stranded RNA, called the DvSnf7 trait. It codes for an insect protein, which results in rootworms dying about five days after ingesting it from roots. Current hybrid packages with this trait include SmartStax Pro and VT4Pro from Bayer and Vorceed Enlist from Corteva Agriscience.

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Soil insecticides still work. Trials by U of I indicate soil insecticides have not lost their efficacy, Seiter says. Find trial reports for recent years rating hybrid traits and soil insecticides online.

Follow management recommendations. Where you expect problems with Bt resistance or an unexpected break happens, the best option is rotating to soybeans, killing larvae at hatch, Seiter says. The next best option is rotating to hybrids with an RNAi trait package or applying a soil insecticide. Expect some root pruning with RNAi hybrids if rootworm populations are high or strongly resistant to Bt traits. Keep tabs on northern corn rootworm populations, especially in northern Illinois.