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Why Managing Corn Rootworm Just Became More Complicated

Corn rootworm resistance to the Cry34Ab1/Cry35Ab1 Bt protein means rootworm now resists all four traits.

Scientists for Corteva Agriscience, Agriculture Division of DowDuPont, have confirmed corn rootworm resistance to the fourth type of Bt protein – Cry34Ab1/Cry35Ab1 – contained in corn hybrids planted by farmers. 

“This should be no surprise to anyone,” says Erin Hodgson, Iowa State University (ISU) Extension entomologist. “Beginning with the first trait (resistance) in 2009, it has been a slow progression (of resistance). It follows a similar pattern. Rootworms are adaptive to any kind of control program.” 

The Cry34Ab1/Cry35Ab1 Bt protein is present in the Herculex RW trait contained in SmartStax, AcreMax Xtreme, and Agrisure 3122 hybrids. This resistance finding joins those of the three other Bt proteins.

These include the:

  • Cry3Bb1 Bt protein found in products including YieldGard Rootworm, several SmartStax offerings, VT Triple Pro, VT Triple Pro RIB Complete, and YieldGard VT Triple. This was the first trait resistance entomologists discovered. ISU entomologists confirmed bioassay rootworm resistance in 2011 that originated in a 2009 Iowa cornfield.
  • mCry3A Bt protein included in Agrisure Viptera 3111, AcreMax Trisect, AcreMax Xtreme, Agrisure Duracade 5122 EZ Refuge, Agrisure Duracade 5222 EZ Refuge, Intrasect Trisect, Intrasect Xtreme, and Qrome products.
  • eCry3.1Ab protein included in Agrisure Duracade 5222 EZ Refuge and Agrisure Duracade 5222 EZ Refuge.

Where It Originated

The Cry34Ab1/Cry35Ab1 resistance occurred in an AcreMax Xtra product that a farmer planted in the northeastern Iowa county of Delaware in 2017, say Corteva Agriscience officials. AcreMax Xtra products contain two Bt proteins that control above-insects like European corn borer, and the Cry34Ab1/Cry35Ab1 Bt protein to control corn rootworm.

“It was an isolated incident where we conducted an investigation on a service call,” says Mike Smith, Corteva Agriscience U.S. seed stewardship lead. “We went through an investigative process, assessed the field for damage, and made an insect collection. That population of corn rootworm was found to be resistant to the Cry34Ab1/Cry35Ab1 protein.” 

Corteva Agriscience officials say they are working with the farmer to devise a rootworm-management strategy based on crop rotation for 2019. Corteva Agriscience officials notified farmers within one-half mile of the field and advised them to rotate out of corn in 2019, says Ryan Myers, Pioneer corn category manager for Corteva Agriscience.  

“This was a natural process that occurred over time,” says Smith. He adds corn rootworm has thwarted every other control option including other traits and soil-applied insecticides. 

Planting pyramided hybrids (those containing two rootworm Bt proteins) has been a recommended best management practice. The logic is that if rootworms resist one Bt protein, the other Bt protein will control them. 

However, this places selection pressure all on the remaining Bt protein. Over time, this pressure can trigger resistance to the remaining Bt protein. 

Meanwhile, farmers can also expect cross resistance among the Cry3 Bt proteins, according to University of Illinois (U of I) entomologists Nick Seiter and Joe Spencer and Kelly Estes, state survey coordinator for the Illinois Cooperative Agricultural Pest Survey Program.

“We have been concerned (about resistance) for quite a while, especially with cross resistance to the Cry 3Bb1 protein, the original Monsanto trait has, with the other two (Cry3) ones,” adds Lance Meinke, University of Nebraska entomologist. “That left the Cry34Ab1/Cry35Ab1 trait that did not have a lot of issues.” 

Traits still work in many areas. “With the information we have so far, Cry34Ab1/Cry35Ab1 still works pretty well in most areas of Nebraska,” says Meinke. 

Trait failure also doesn’t necessarily mean the trait will fail in an area where resistance has occurred. Since corn rootworm beetles are not highly mobile, they don’t travel far from their original field, says Myers. 

“Generally, what we see with any kind of (trait) resistance is it is restricted to the field,” says Nick Storer, Corteva Agriscience global regulatory advocacy lead. 

Diverse Strategies Needed 

Still, resistance to all four corn rootworm-resistant traits means is a wake-up call to diversify management rootworm strategies, says Hodgson. The good news is that in most areas, rotating corn with soybeans still controls corn rootworm, she says. 

“It is still one of the strongest management tactics we have for corn rootworm,” says Hodgson. 

Exceptions exist. The western corn rootworm variant initially emerged in east-central Illinois and northern Indiana in the mid-1990s. Since then, it’s spread into areas like eastern Iowa. The western corn rootworm variant 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 roots of corn that’s planted in that field. 

The other exception occurs in more northern areas. Extended diapause occurs when eggs laid by predominant northern corn rootworm beetles remain dormant in soil through the next year when soybeans are planted. 

First-year corn in areas of Minnesota, Nebraska, Iowa, South Dakota, and Wisconsin have all been damaged by rootworms exhibiting extended diapause. 

What to Do with Continuous Corn 

“Rooworms love corn,” says Clint Pilcher, global integrated solutions manager for Corteva Agriscience. “The more continuous corn you have, the more the population is favored and the more the likelihood the population will increase over time.”

Coincidentally, many farmers love to grow corn, too. These two factors can come to a loggerhead in states like Nebraska, where livestock farmers need to grow or access corn for feed. 

“Under irrigation, historically, corn is a more competitive crop (economically in Nebraska) for lot of growers, and there is lots of ethanol production in Nebraska, adds Meinke. “They are a bit reluctant to rotate to something else besides corn. But we are convincing more and more of them to think about growing corn for three to four years, and then one year of beans just to break the rootworm (life cycle) and to lower the overall rootworm population density. Resistance and high populations densities can create a lot of problems.”

Farmers in heavy continuous corn areas have managed rootworm population levels with foliar insecticides that kill adult beetles. This strategy, though, is more difficult than in years past. Federal regulators have banned methyl parathion (Penncap-M) for this use. This insecticide had a long residual period that enabled adult beetle control to occur for an extended time, says Meinke. Remaining insecticides have shorter residual periods, which can require multiple applications to be effective. 

Pilcher says late July and early August are often the best times to spray adult beetles, although optimum control may require another spray two to three weeks later.

“That’s a tough sell to farmers,” says Myers. 

It may be tempting to simultaneously apply an insecticide to control adult beetles and a fungicide to control disease. Still, Pilcher says the optimum time to apply a fungicide may not coincide with the optimum time for a foliar insecticide application. 

“You may knock down some beetles, but male beetles emerge before females do,” says Pilcher. “You may just be killing the males and not the females that lay the eggs and emerge a week or two later.”

Soil-Applied Insecticides

Soil-applied insecticides are another management option. However, performance is highly variable, says Hodgson. “For farmers who plant corn from April 11 to April 15, they are expecting control for two months until eggs hatch in June,” she says. 

Doubling up soil insecticide with a trait is an option. However, consistent yield responses to this practice have not occurred according to ISU data, says Hodgson.

Nor is it advisable under Integrated Management Pest management to simultaneously use all control measures in a single year. “We don’t want to throw the whole kitchen sink (simultaneously) at it, with traits and insecticide and (adult) beetle bombing, because then we could get resistance to everything,” adds Meineke. 

Know Field Populations

Given price pressures for 2019, there may be some farmers planting naked, with no protection at all. 

“Some folks are looking hard as to if they can plant Double Pro (an above-ground insect and herbicide-tolerant trait package) where they have planted SmartStax for many years,” says Mitch Heisler, marketing manager for Wyffels Hybrids. 

“There are growers who are planting non-rootworm Bt corn with no soil-applied insecticide and are just taking their chances,” says Ken Ostlie, University of Minnesota Extension entomologist.  “I can understand the economic necessity, but I would not take that chance unless I knew the populations were low in my field.

It’s here where a year out of corn and into a crop like soybeans can work well even in areas of continuous corn like northeastern Iowa. 

“In those geographies, we recommend a rotation,” says Pilcher. “Two years corn and one year soybeans would be ideal. In the first year of corn, you likely would not need a Bt trait.”

In areas plagued by the western corn rootworm variant or extended diapause, a soil-applied insecticide could provide extra protection for a population depressed by the preceding soybean year. Pilcher recommends a pyramid hybrid during second year of corn.

Pheromone sticky traps

Tools exist to assess the next year’s infestations, such as pheromone sticky traps. Economic threshold is 2 rootworm beetles per trap per day in corn following corn. For rotated corn, the economic threshold is 1.5 western corn rootworm beetles per trap per day in soybean. (These thresholds are based on a recent study in Iowa, which recalculated economic thresholds for corn rootworm based on updated crop values and control costs).

Their use, though, is often difficult. Meinke notes that the required high number of sticky traps per field makes it logistically impossible for many farmers and consultants to use. Threshold levels in states like Nebraska are also dated, due to the sheer number of new hybrids with increased yield potential over the past decade has made readings less reliable than in the past. 

Readings also can be made obsolete by weather, too. Nebraska rootworm population numbers in 2018—particularly in northeastern Nebraska—decreased from 2015 to 2017 levels due to soggy conditions during peak egg hatch and larval emergence times, says Meinke. 

Help Is On The Way – But a Ways Off 

Companies like Corteva Agriscience, Bayer Crop Science, and Syngenta and are developing new RNAi technology to manage corn rootworm. Bayer Crop Science and Corteva Agriscience plan to launch its SmartStax Pro early next decade that will include a third mode of action using this technology in addition to the two now present (Cry3Bb1 and Cry34Ab1/Cry35Ab1) in SmartStax.

The MON87411 event—termed CRW III-has a gene inserted into the corn cell that tells the corn plant to make a specific double-stranded RNA (dsRNA). When rootworm larvae ingest the protein, it stops a specific RNA in the corn rootworm cells from making a specific protein the rootworm larvae need to survive. SmartStax Pro will be sold as a triple pyramid, with the Cry3Bb protein, the Cry34Ab1/Cry35Ab1 Bt protein and the RNAi trait that Bayer has developed. Corteva Agriscience officials say it will offer the trait in combination with its insect control and herbicide-tolerant traits. 

“If there is a rootworm population that has not resisted any of those traits, it would have to overcome three different modes of action to have a field failure,” says Meinke. 

For now, though, farmers are left with existing control options. 

“We have to be stewards of what we have,” says Meinke. “We have to start thinking about rootworm management holistically and use more of an IPM (Integrated Pest Management) approach in using these traits as part of the tool box, and not the sole tool,” says Meineke. 

He also advises farmers to separately manage rootworm in each field across a farm, rather than farmwide management strategies. That’s because agronomic challenges like corn rootworm and weed resistance can vary from field to field. 

“It is more labor intensive, but it is something to consider in the future,” Meinke says.  

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