How to beat corn rootworm in 2021

One of the trademark lines of the 1986 supernatural thriller Poltergeist II was youngster Carol Anne Freeling’s haunting voice echoing, “They’re baaaack!” after answering her toy phone infiltrated by a supernatural being. 

Some corn farmers might have felt a sliver of that dread after viewing their 2020 cornfields. The corn exhibited goosenecked and lodged stalks, the telltale signs of corn rootworm damage. 

“Western corn rootworm numbers were higher in 2020 compared with recent years,” says Ron Beyer, a Golden Harvest agronomist who works with farmers in northwest Iowa. “Particularly in corn-on-corn acres, we saw some of these populations just explode. We had very, very heavy hatches in 2020.”

Corn rootworm ravages yields by root feeding and pruning. “In a worst-case scenario, when the plant dies prematurely from a combination of heat, stress, and corn rootworm feeding, test weight can be minimized substantially,” says Beyer. Iowa State University data show that for every node of roots pruned by larvae, expect an average 15% yield loss. 

Traited Corn Impacted Also 

Corn rootworm particularly ravaged conventional cornfields unprotected by traits in 2020, says Beyer. Still, corn protected by traits isn’t immune either. Another reason outbreaks have occurred in heavy corn areas such as northwestern Iowa is that farmers have relied upon Bt hybrids with just one mode of action, says Beyer. 

Entomologists have now confirmed
resistance to or found unexpected root damage to all four types of Bt proteins that include the: 

  • Cry34Ab1/Cry35Ab1 Bt protein present in the Herculex RW trait contained in SmartStax, AcreMax Xtreme, Agrisure 3122 hybrids, and Qrome products.
  • Cry3Bb1 Bt protein found in products such as SmartStax, SmartStax RIB Complete, VT Triple Pro, and VT Triple Pro RIB Complete. 
  • mCry3A Bt protein included in Agrisure Viptera 3111, AcreMax Trisect, AcreMax Xtreme, Agrisure Duracade 5122 E-Z Refuge, Agrisure Duracade 5222 E-Z Refuge, Intrasect Trisect, Intrasect Xtreme, and Qrome products.
  • eCry3.1Ab protein included in Agrisure Duracade 5122 E-Z Refuge and Agrisure Duracade 5222 E-Z Refuge.

Another development that fueled 2020 infestations were “trap crops,” those 2019 cornfields that farmers planted later than others in the area. Corn rootworm beetles would flock to these later-planted lush fields and lay eggs that hatched in corn the following year, says Beyer.

Jon Zuk showing corn plant roots
Jon Zuk
Photo credit: Gil Gullickson
Corn rootworm numbers also rose in states like Illinois. “Along with higher activity, resistance to Bt traits in western and northern corn rootworm remains our top insect management concern in Illinois corn,” says Nick Seiter, a University of Illinois field crop entomologist. 

What to Do? 

Once corn rootworm starts ravaging cornfields, little can be done. However, root digs in August can reveal the extent of rootworm feeding and help form a plan for next year. “It’s the only way you can find out what’s going on with the roots,” says Jon Zuk, a WinField United agronomist. 

Steps to lessen the severity of corn rootworm outbreaks and corresponding root damage in 2021 include:

  • Rotating to soybeans or another nonhost crop. Larvae that hatch in soybeans or another crop will die, says Beyer. 

The hitch in this strategy comes if extended diapause happens. Extended diapause occurs when eggs laid by predominantly northern corn rootworm beetles remain dormant in soil for an extended time and hatch during a corn year.  

Farmers in the central and eastern Corn Belt also wrestle with the western corn rootworm variant. This occurs when females lay a portion of their eggs on neighboring soybean fields. These eggs then may hatch the following year and the resulting larvae can then feast on corn roots. 

  • Planting pyramided trait hybrids with dual modes of action. In many cases, traits are still effective. For added protection, they can be used in combination with full rates of soil-applied insecticides, says Beyer. Foliar insecticides applied to control adult beetles are other tools. These can be a good option for corn-on-corn farmers, he adds. 

Seiter cautions that planting continuous corn with the same trait package after damage occurs the preceding year is the worst thing to do. 

“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 says.

RNAi Tech is coming

Corteva Agriscience, Bayer Crop Science, and Syngenta are developing new RNAi technology to manage corn rootworm. Company officials say this technology – plus best management practices – will help extend effectiveness of current and future products. Bayer Crop Science plans to launch SmartStax PRO early this decade. It will include a third mode of action using this technology plus current Bt ones (Cry3Bb1 and Cry34Ab1/Cry35Ab1) in SmartStax. Corteva Agriscience will launch its next-generation CRW product with three action modes (Cry3Bb1 and Cry34Ab1/Cry35Ab1 plus new RNAi technology) stacked with the Enlist corn trait for weed control, pending regulatory approval.

The MON87411 event – termed CRW III – contains genes for the Cry3Bb1 Bt protein and for the RNAi technology. The gene for the RNAi technology is inserted into the corn cell, which tells the corn plant to make a specific double-stranded RNA (dsRNA). When rootworm larvae ingest the dsRNA, it stops the corn rootworm cells from making a specific protein needed for rootworm larval survival. SmartStax PRO will be sold as a triple pyramid, with the Cry3Bb1 Bt protein, the Cry34Ab1/Cry35Ab1 Bt protein, and the RNAi trait that Bayer has developed.

Corn rootworm beetles in a clear dish
Photo credit: Gil Gullickson

How to evaluate damage

University of Illinois (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, says Nick Seiter, a U of I field entomologist. They rate the interior three nodes of roots for rootworm damage. 

For example, one entire node of roots pruned away would have a root damage rating of 1.00, while pruning three entire nodes would be 3.00. This rating system enables corn rootworm damage comparisons among fields, he says. 

Measuring rootworm damage can also detect rootworm resistance to a particular Bt trait package, Seiter says. 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 indicate resistance. 

Remember to account for blended refuge plants, he adds. If a farmer plants a 5% blended refuge and 5% of the plants or fewer show expected damage, don’t be alarmed, he says.

Cool it

If applying nitrogen (N) this fall, make sure to do so in soils cooler than 50°F., says Greg Schwab, vice president of technology and innovation for Koch Agronomic Services (KAS). Excessively warm soils enable the bacteria to nitrify the N contained in anhydrous ammonia, increasing the risk for leaching and denitrification loss.

Application zones can differ within a state. In Illinois, for example, Illinois Route 16 in the center of the state is the dividing line for fall anhydrous ammonia. 

“North of that line, anhydrous ammonia can remain stable,” says Schwab. “South of there, though, soils are too warm to apply anhydrous ammonia in the fall, even if a stabilizer is used.” 

Schwab adds that KAS trials found an advantage to using its nitrification inhibitor Centuro in the spring, with an average yield increase of 6 bushels per acre compared with when it was not used. 

“In a wet year like 2019, lots of water was moving through the soil profile,” he says. “Years like that increase the chance of a yield response compared with use during a dry spring like in 2012.”

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