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Experts caution to scout for corn rootworm in high moisture
By Tim Alexander
Illinois Correspondent

WESTFIELD, IN. – AgriGold agronomists are among the crop management experts warning farmers of a possible corn rootworm invasion of their crop fields in 2022. Moist conditions following a mild winter and relatively dry fall are combining to create optimal conditions for northern and western corn rootworm to thrive, according to AgriGold’s Kevin Gale, whose territory covers northern Illinois.
“It’s important to be cognizant of what’s needed rather than using a blanket approach for battling the pest, especially on ground that has been corn-on-corn for several years,” Gale said. “Switching up modes of action or including an insecticide may be warranted in some fields for maximum root protection.”
Western and northern corn rootworm larvae typically begin hatching and feeding on roots in late May or early June. The damage can hinder nutrient and water uptake, restrict yield potential and increase the odds of standability issues. By late June or early July, adult beetles typically start to emerge and begin feeding on pollen and green silks. If silks aren’t available yet, corn rootworm beetles will attack leaves.
A farmer’s defense against corn rootworm begins with diligent scouting, noted Terry Mente, an Agrigold agronomist in eastern Iowa. “The goal is to kill adults before they begin to lay eggs for the coming year,” Mente said. “One of the ways we can go after the adults is with a foliar insecticide, which often lines up with an application of a fungicide. Attacking the adult helps control populations moving forward.”
A well-rounded solution to managing corn rootworm includes thoughtful seed trait selection and crop rotation. Hybrids should incorporate pyramided Bt (Bacillus thuringiensis) traits or RNai (Ribonucleic acid interference) technology that can be introduced into crop rotations, with modes of action rotated, as well. And because corn rootworms are able to adjust to multiple modes of action, rotation to a non-host crop such as soybeans can be the most effective lines of defense against the pest.
“Crop rotation is the most effective management practice to controlling rootworm, however, growers must be aware of the potential for the Western corn rootworm variant laying eggs in soybean fields that can affect first year corn,” Gale said. “Northern corn rootworm numbers have risen as well, whose eggs laid in corn fields can hatch after two winters (extended diapause), causing concern when corn is planted again.”
Scott Rountree, an agronomist with Pioneer in Wisconsin, warned growers of evidence pointing to potential corn rootworm problems in 2022 as far back as November 2021, when he told Brownfield that conditions were aligning – especially for corn-on-corn plantings – for a corn rootworm invasion.
“We saw some of our highest trap counts ever here in the summer of 2021 so that’s just telling us those beetles are going to go down in those fields, lay eggs, and if that field is going back into corn in 2022, we could have some serious pressures,” Rountree said. If possible, growers should consider planting more soybeans in 2022 due to rootworm pressure, an increase in fertilizer costs for corn and higher soybean prices, he added.
In 2021, Bayer introduced (in limited supply) a new line of SmartStax Pro with RNAi technology that offers greater control over corn rootworm pressure. According to Bayer agronomist Jim Donnelly, the company’s product is the first trait to launch that employs a three-stage mode of action to combat corn rootworm. Two Bt proteins within the product act below-ground to destroy beetles instantly after they consume root material. The RNAi technology controls beetles for longer periods of time and is effective for above-ground pest control, including protection from European corn borer, southwestern corn borer, fall armyworm, black cutworm and corn earworm. Because RNAi technology works differently than a soil-applied insecticide or Bt-traits to control corn rootworm, it can increase corn’s ability to defend itself against corn rootworm.
“Some of the initial data we are getting are just as we expected to see – that SmartStax Pro is providing incremental control over SmartStax and other competitive traits that are on the market,” Donnelly said during a Bayer Field Day in central Illinois in September 2021.
AgriGold will be introducing their RNAi technology in corn hybrids in 2023, according to a company news release. Bayer was expected to make their RNAi corn hybrid selections more widely available in 2022.
A growing number of producers are utilizing granular insecticides, which are insecticides that are attached to porous organic material such as ground up clay or corn stover, to combat corn rootworm, according to Kevin “KJ” Johnson, executive director of the Illinois Fertilizer and Chemical Association.
“We are definitely hearing from retailers that there are a whole lot more farmers who are putting granular insecticides back in their planters. I think we will see more of that in the future. If you are in an area with high corn rootworm pressure, you are looking at every option possible,” Johnson said, referring to proven products such as Force and Aztec granulars.
Mente encouraged farmers to head into their fields to scout for corn rootworm larvae before the end of May (providing they have standing corn by then due to the prolonged wet spring of 2022). “We like to use yellow non-baited sticky cards to identify what type and how much rootworm pressure a farmer is facing,” Mente said.