Rootworms Ramp Up

ST. LOUIS (DTN) -- David Brown's cornfields are getting a little crowded this summer. "We're seeing heavy rootworm infestations," the Warrensburg, Ill., farmer told DTN. Brown hopes to head into his fields next week for another round of root digging, but already, the need for changes to his management strategy are clear, he said.

Brown is counting on adding soil insecticides to his single Bt-trait corn hybrids next year and said he would consider moving more acres into pyramided hybrids, which contain more than one Bt-protein to protect against corn rootworm. "We'll do what we need to do to make sure we don't lose this technology," he said.

University of Illinois entomologist Mike Gray has also reported "impressive" western corn rootworm pressure in check strips at various trial plots in the northwestern and central regions of the state. "In spite of significant precipitation throughout much of June and early July, it appears that larvae were able to hatch successfully and establish on root systems quite well," he wrote in a university pest bulletin.

Monsanto Technology Development Representative Dave Shenaut has been digging in trial plots in central and northern Illinois, where he said he and his team have found "good pressure" levels.

As the summer wears on, entomologists and researchers are urging growers to get out and assess their rootworm damage now before they pick corn hybrids for next year and plan other rootworm control strategies.

"All I can say it that growers NEED to be in the fields!" Shenaut told DTN in an e-mail. "To understand what kind of rootworm pressure you might be facing next year, you need to be looking for adults now. Some fields have them thicker than others, but they are out there."

Corn-on-corn growers should be especially vigilant. "Multiple-year corn following corn likely has the highest number of adults and is a great place to start looking, but they can also be in soybean fields where the variant is an issue," Shenaut wrote, referencing the growing populations of rootworms who have developed resistance to rotation by surviving in soybean fields.

Rootworm eggs hatch in the soil in the early weeks of summer, and the larvae immediately go to work munching away on corn roots, which lowers yield potential and weakens the plant, leaving it vulnerable to lodging later in the season. Adult beetles emerged across most of the Corn Belt in July and will be mating and laying eggs in the soil for a few more weeks yet.

Rootworms, which USDA estimates cost growers $800 million in lost yields annually, have become steadily more difficult to manage as the pest has developed resistance to the Bt-corn proteins used to control them. Many growers have resorted to adding soil insecticides while planting and switching to pyramided Bt-hybrids, which contain more than one Bt-protein targeting the rootworm.

As profit margins tighten for corn growers, evaluating how well these rootworm control strategies are working is increasingly important.

Unfortunately, it's hard to evaluate how well any Bt-variety or soil insecticide is working without getting a little dirty, Shenaut noted. "The only way to know if your current-year control tactic worked is to dig roots," he said.

"Impacts of larval feeding will likely be lessened this year due to sufficient rains in a lot of areas," he added. "That also will allow for a great deal of root regeneration on plants that have pruned roots. That regeneration makes it a little harder to clean and inspect roots, but it is still possible and recommended."

Aboveground signs, particularly lodging, are not reliable for diagnosing rootworm pressure. "Lodged corn can be due to a variety of factors," Shenaut pointed out. "High winds and wet soils are a bad combination."

Using yield loss as an indication of rootworm pressure is also an uncertain measure, particularly with the good growing conditions this year, Brown pointed out. "I think there are a lot of people that are going to get fooled this year," he said. "Corn may make 210 bushels per acre in good growing areas, but how do you know it wasn't going to make 250?"

For detailed instructions on how to properly dig, wash, and evaluate root damage from rootworms, see this previous DTN article here: http://goo.gl/…

PICKING THE RIGHT PRODUCT

At this point in the season, farmers have limited options for controlling rootworm populations. Mid-season adult beetle insecticide applications can lower egg densities, but they require careful timing and constant scouting to ensure female beetles are present and pregnant.

The most important rootworm control farmers can focus on now is selecting the right hybrids for next year or deciding whether to rotate to soybeans.

Many growers continue to rely on single Bt-trait hybrids. Although these products still work well to protect against the western corn rootworm in many parts of the Midwest, rootworm populations in Iowa and Illinois have developed resistance to two Bt-proteins, mCry3A and Cry3Bb1.

In areas with heavy rootworm populations and resistance issues, entomologists recommend growers move to pyramided Bt-products, which stack more than one Bt-protein to control rootworms.

Entomologists from Michigan State and the University of Wisconsin have produced a useful table explaining which Bt-traits are in which corn hybrids: http://goo.gl/….

Identifying which varieties have more than one rootworm Bt-trait can be tricky because most varieties hold a number of Bt proteins designed to control other pests such as European corn borers.

Corn hybrids that stack more than one Bt-protein designed to target corn rootworm include Agrisure 3122; Agrisure Duracade; Optimum Intrasect Xtreme; Optimum AcreMax Xtreme; Genuity SmartStax and Refuge Advanced Powered by SmartStax.

Crop rotation continues to be an effective tool for lowering rootworm populations, but entomologists at the University of Illinois have documented field populations of rootworms that can survive in soybean fields and re-emerge in the following year's cornfield. See Gray's notice on these populations here: http://goo.gl/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/AG)

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