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Rogue rootworms: Managing resistance

Rootworm-resistant traits remain a good way to protect corn hybrids from corn rootworm. Still, cracks in the trait armor continue to appear.

In Iowa, Iowa State University (ISU) entomologists have confirmed rootworm resistance to Cry3Bb1 and mCry3A Bt proteins. The Monsanto trait in the YieldGard RW group contains the Cry3Bb1 protein. Syngenta’s Agrisure RW trait in several Agrisure trait products contains the mCry3A protein. So far, entomologists have just confirmed resistance in the western corn rootworm Northern corn rootworm does not yet resist Bt traits.

ISU researchers also confirmed cross-resistance between the two traits. “If you have rootworms that resist the Cry3Bb1 protein and switch to the mCry3a protein, you can still expect root injury,” says Erin Hodgson, ISU Extension entomologist.

In 2012, University of Illinois entomologists confirmed rootworm resistance to the Cry3Bb1 protein in Illinois. Resistance has not been confirmed in the Cry34/35Ab1 protein that is contained in Dow and DuPont Pioneer Herculex RW trait packages.

Even where resistance has not been confirmed, though, performance problems have surfaced to the Cry3Bb1 and mCry3A proteins.

“The problem is growing in areas like southern Minnesota, northern Iowa, eastern South Dakota, and eastern Nebraska,” says Hodgson. “That is not only in range and number of fields with problems with poor performance, but also in different kinds of hybrids. The problem is primarily in areas of continuous corn. It should be high on your radar of things to look for.”

Cases where resistance is not confirmed but performance is subpar is termed unexpected damage.

“In a lot of cases where you see unexpected damage, fields have been corn-on-corn for many years,” says Miloud Araba, Syngenta technical product lead for corn traits. “You can be in corn for just two years and have a high population in that particular field. You can also have high damage in just part of a field.”

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High rootworm populations can pressure all traits, regardless of whether resistance has been confirmed, says Ken Ostlie, University of Minnesota Extension entomologist.

He notes a Springfield, Minnesota, field that was overwhelmed by corn rootworm in 2012. Rootworms resisted the Cry3Bb1 protein contained in the Monsanto Genuity VT Triple Pro hybrid that the farmer had planted for several years.

Cross-resistance to the Agrisure RW hybrid also occurred in the field. Similar results happened in 2013. However, performance of a pyramided SmartStax (two rootworm traits) and Herculex products containing the Cry34/35Ab1 protein also had excessive root damage.

“As resistance developed to the Cry3Bb protein, rootworm numbers built up,” explains Ostlie. “There were huge numbers of beetles in the field. In high-pressure situations, all traits may be at risk. You don’t get high population levels like these unless you have resistance to one or more traits.”

So What Caused it?

Planting traited corn is much simpler and easier to do than applying a soil-applied insecticide or juggling a crop rotation, says Ostlie.

“When we look at resistance in insects, the one common denominator is using the same tactic over and over and over again,” he says. “It comes as no surprise to entomologists that resistance is appearing. Rootworm has adapted to crop rotation, foliar insecticides, and about everything we have thrown at them.”

It’s a lot easier to forestall resistance instead of dealing with it when it surfaces, says Hodgson. That can be accomplished by mixing up control measures including the following.

  • Plant different crops. “Rotating with soybeans is a way to break the rootworm cycle,” says Luke Samuel, Monsanto corn insects trait product development manager. One caution: In Illinois, corn rootworm populations adjacent to cornfields where entomologists confirmed Cry3Bb1 resistance were thriving and laying eggs in nearby soybean fields. “If the bioassays confirm resistance to the Cry3Bb1 protein, producers across a wide swath of Illinois will have a formidable insect foe capable of overcoming both crop rotation and at least one Bt protein (Cry3Bb1),” says Mike Gray, University of Illinois Extension entomologist.

  • Use soil-applied insecticides. Although they protect roots, soil-applied insecticides do not curb rootworm numbers. Since soil insecticides just protect roots in a narrow 7-inch band, rootworms can thrive in remaining areas. On the plus side, though, the increased survivorship is why this is the only control measure that corn rootworm has not resisted.

  • Rotate to a different trait. In cases of cross-resistance to Cry3Bb1 and mCry3a, though, this won’t work.

  • Plant a pyramid. SmartStax by Dow and Monsanto, Syngenta's Agrisure 3122 trait stack, and DuPont Pioneer’s Optimum AcreMax XTreme are examples of pyramids that contain two rootworm-control modes of action.

“They are more effective where there are no resistance problems,” says Hodgson. “If you plant a SmartStax hybrid and rootworms are resistant to the Cry3Bb1 protein, the other trait containing the Cry34/35Ab1 protein is doing the heavy lifting. If this is the case and you plant SmartStax five years in a row, you will develop a problem to both traits.”

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Mix ‘Em Up

Rootworm traits still work most of the time.

“When you look at all corn using the Agrisure RW trait, thepercentageofunexpected damage has been less than .2%,” says Araba. “We have not seen that level increase in the last three years.”

This gives farmers who have not experienced resistance a head start to forestall it.

“The more you can make a longer-term commitment over three to five years of constantly mixing things up, the less rootworm larvae will get used to any certain control method,” says Hodgson.

Pack a Shovel

There are lots of apps and tools you can use while scouting your fields. Don’t forget to pack another time-tested tool – a shovel. If you’re a continuous corn grower, you should be digging up roots in midsummer to assess control measures.

“The first time you’re apt to know how your fields are doing is when wind blows over your corn,” Hodgson says. “Check roots each year in every field to see how the rootworm-resistant traits are working.”

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