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Corn rootworm: ‘They’re back.’

High corn rootworm populations are surfacing in parts of Iowa and Illinois.

Corn rootworm isn’t called the billion-dollar bug for nothing. Corn rootworm – pegged as causing $1 billion in lost yields for corn growers year in, year out – is once again on the move.

Besides the derecho in Iowa, farmers in that state have also wrestled with corn rootworm this year.

“Western corn rootworm numbers are rising,” says Ron Beyer, a Golden Harvest agronomist who works with farmers in northwestern Iowa. “Particularly in in corn-on-corn acres, we’re seeing some of these populations just explode.” 

Beyer discussed corn rootworm issues earlier this month on a Golden Harvest webinar. The bad news is that once corn rootworm starts ravaging cornfields, little can be done. However, scouting cornfields now can help farmers make plans for the coming year.

Although western corn rootworm numbers rule this year, northern corn rootworm numbers were high in a number of northwestern Iowa fields in 2019 due to extended diapause, says Beyer. Extended diapause occurs when eggs laid by predominant northern corn rootworm beetles remain dormant in soil through the next year when soybeans are planted. 

Corn rootworm has particularly ravaged conventional cornfields. “We have had some very heavy, heavy hatches,” Beyer says.

Corn rootworm outbreaks in northwestern Iowa have been fueled by corn farmers relying on Bt hybrids with just one mode of action. Foregoing a soil-applied insecticide can also put these fields at risk. 

Another development fueling infestations are “trap crops,” those cornfields that were planted later this spring in the area. Exploding populations will result from corn rootworm flocking to these lush fields, says Beyer. 

Corn rootworm activity generally has also been higher in much of Illinois than it has been for the last several years, writes Nick Seiter, a University of Illinois field crop entomologist in this week’s U of I Crop Central Update

Root pruning is the way that corn rootworm curbs yield potential. “In a worst-case scenario, if the plant actually dies from a combination of heat, stress, and corn root worm issue, the test weight can be minimized substantially,” says Beyer. 

Corn rootworm can be managed, Beyer says, through pyramided traited hybrids with dual modes of action. That’s not even foolproof, though, for corn rootworm now resists all four types of Bt proteins in traited corn hybrids. Soil-applied insecticides and crop rotation are other control measures that can be incorporated into an integrated system. 

How to Evaluate Damage

Seiter writes that U of I entomologists evaluate rootworm damage by measuring the proportion of the root mass that has been pruned to within 1.5 inches of the base of the root. They rate the interior three nodes of roots for rootworm damage. 

For example, one entire node of roots pruned away would be a root damage rating of 1.00, three entire nodes pruned would be 3.00, a quarter of one node pruned would be a 0.25, and so on. This rating system allows you to quantify and compare rootworm damage among fields, he writes. 

Measuring rootworm damage in a field is critical to assess whether or not a resident rootworm population might resist a particular Bt trait package, Seiter writes. For reference, a rating of 0.5 (one half of one node pruned) is considered unexpected damage to a pyramided Bt corn plant, and could be evidence of resistance. (Just remember to account for the blended refuge plants; if you have a 5% blended refuge and 5% of the plants or fewer are showing “unexpected damage,” that is no cause for alarm). These ratings can also be used to compare the performance of different control tactics over time on your farm.

The rating scale described above is the same rating scale used for annual rootworm control evaluations, such as from 2019. 

Seiter writes that unexpected damage in a pyramided trait package should be reported to a seed company representative (and also Seiter, if you’d like, at 

Seiter writes the best thing to do with a cornfield that had elevated 2020 corn rootworm damage is to rotate to soybeans in 2021. While crop rotation will not eliminate the potentially resistant beetles that emerged from the field (especially in areas where rotation resistance is an issue), any larvae that do hatch into a soybean field next spring will die (except in cases of extended diapause). 

The worst thing you could do from a resistance-management standpoint would be to plant continuous corn with the same trait package after observing unexpected damage in a field the previous year, he writes. 

“As we have seen with herbicide resistance over the last several years, overreliance on the same tools in the same fields will yield a predictable outcome,” he writes. 

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