Finding western corn rootworm damage in a Bt cornfield is upsetting. Finding corn rootworm damage in a Bt cornfield following soybeans is even more unnerving.

Nick Tinsley, University of Illinois Department of Crop Sciences, says there’s a, “troublesome population of rootworms that will lay their eggs not only in corn, but also in soybeans.” Resistance to crop rotation and reduced susceptibility to Bt corn has been observed in Champaign, Ford, Livingston and Kankakee counties.

BATTLE AHEAD: Corn rootworm resistance is expanding, but IPM tactics can make a difference. New biotech tools help too.

“This was really frustrating when it first happened because one of the best management practices that technology providers recommend when you start to have performance problems is to rotate away from corn,” says Tinsley. “If rotating to soybeans is not a way to correct this issue, it becomes a problem of ‘what do we do?’”

Knowing how to manage Bt resistant corn rootworm starts with understanding why it’s occurring. “Much of this problem stems from using the same Bt product year in and year out, even in areas where rotation is common,” says Tinsley. “Another issue is that many of the assumptions of how the refuge works to delay resistance may be incorrect for rootworms.”

Related: 5 steps to head off corn rootworm resistance

Additionally, there is confirmed cross resistance between Cry3Bb1 and mCry3A in a number of rootworm populations. That means rotation between the two Bt proteins is not effective for managing resistance.

That all adds up to a potentially serious problem in Illinois. So what can farmers do?

Integrated pest management strategies have to be a priority, Tinsley says, despite historically low rootworm populations in 2015.

Rotation still matters

Tinsley says crop rotation is still one of the best IPM tactics available. “The insects are hedging their bet by laying eggs in both corn and soybeans,” says Tinsley. “Killing off a large portion of the population with rotation is a valuable tool.” He also recommends using a pyramided Bt hybrid.

“Pyramided hybrids have more than one toxin targeting rootworms and may be more effective for delaying resistance,” he says.

You can also suppress adults in cornfields with high rootworm densities. “This requires a commitment to careful scouting,” says Tinsley. He also cautions adult suppression in soybeans to prevent damage to next year’s corn is likely not effective.

What about using soil insecticide with Bt hybrids? You may reduce injury with soil insecticides, but yield benefits vary with pressure levels and must be weighed against application costs. “In the case of pyramided Bt hybrids, the potential root injury has to be near maximum. It’s not to say that can’t happen, but it’s probably very uncommon.” Tinsley also notes there is some evidence this combination approach may actually speed up resistance development based on the way these insects emerge and mate.

Overall, it’s both good news and bad news for Illinois corn farmers. The good news is corn rootworm densities are down. The bad news is Bt resistant corn rootworms are evolving and becoming trickier to manage. Farmers may be winning on one front, while another battle forms right behind it.