Crop rotation important for reducing damage from corn rootworm

SYCAMORE, Ill. — Farmers in northern Illinois are in the epicenter for dealing with corn rootworms in their fields.

“When you go south of I-80 to areas that are heavily one-to-one corn-soybean rotation, the rootworm populations are tiny,” said Nick Seiter, assistant professor at the University of Illinois. “But up here in DeKalb, Ogle, Lee and into Stephenson counties, we’ve got plenty of beetles in areas where we have a lot of continuous corn.”

The western and northern corn rootworms are the two species that are important for Illinois farmers, said Seiter during a presentation at the Illinois Crop Management Conference hosted by U of I Extension at the DeKalb County Farm Bureau building in Sycamore.

“For many years, the western has dominated, but over the last five years, we’ve started to see more northerns back in the system,” the university professor said.

“What is likely happening is the second protein in most pyramids, Cry 34/35, is very good against the western corn rootworm and in general it’s a little more iffy against the northern corn rootworm,” he explained. “But the western is the species that’s more important to control and that toxin does a better job on that insect.”

The two species of corn rootworm have biological differences.

“As an adult, the northern corn rootworm gets up and travels a little further than the western,” Seiter said. “You probably have a little better genetic mixing of those populations as a result of that dispersal.”

Also, the northern corn rootworm will go through an extended diapause.

“It will have a multiyear life cycle,” Seiter said.

The development of resistance is measured by the generations of an insect.

“Part of the northern corn rootworm population is going through a multiyear generation, which means it takes two, three or sometimes four years for that insect to go through a generation,” the university professor said. “So, it makes sense it would take longer for resistance to Bt proteins to occur.”

When there is injury in first-year corn, Seiter said, it is likely due to extended diapause in northern corn rootworm rather than egg lay in soybeans by western corn rootworm.

Although there is a lot of field to field variability, the U of I survey data shows the northern corn rootworms have been increasing over the last few years.

“Historically, our overall corn rootworm population in the state is still a lot lower than it once was because the pyramid Bt hybrids have reduced numbers considerably,” Seiter said. “The control you get with an effective trait package is much greater than we see with a soil insecticide, so we have a lot of vested interest in maintaining the traits.”

Jamie Herring planted SmartStax hybrids in 2023 on his farm near Esmond.

“I haven’t had any challenges with corn rootworm,” he said. “Going into this year, I won’t be using SmartStax because of my rotation.”

Herring decides on the number of corn and soybean acres he plants based on the markets.

“I’m growing more soybeans this year,” he said. “I’m growing 60% soybeans and 40% corn, which will all be first-year corn.”

The single rootworm trait introductions occurred in 2003 and 2005, followed by the pyramid trait introductions in 2008 and 2009.

“That’s when we really started to see populations overall decline,” Seiter said.

“But we have field evolved resistance in western corn rootworm to all four proteins and we’ve got resistance to all four in northern corn rootworm that hasn’t been confirmed yet,” the university professor said.

“The major reason we’ve lost susceptibility to these traits relatively rapid for corn rootworm is we failed to achieve a high dose so we’re killing insects that have just one resistant allele,” he said.

“Ideally, we want resistance traits to be recessive so if they have one allele and they are exposed then they die, but that hasn’t been the case with corn rootworm as it has been with corn borers.”

The first commercial release of SmartStax PRO occurred last year.

“There are a number of other RNA interference or RNAi traits coming down the pipeline,” Seiter said.

“RNAi is going to be deployed with Bt proteins and there is a good reason for that,” he said. “With pyramid trait packages we’re trying to accomplish redundant killing, which means if you have a rare insect that’s resistant to one of those components, hopefully it’s susceptible to one of the others and it will kill that insect.”

Bt and RNAi have different modes of action.

“The RNAi mode of action has nothing in common with the Bt mode of action other than the insects have to ingest the material,” Seiter said. “So, the probability of cross resistance between RNAi and Bt is extremely low.”

One of the disadvantages of the RNAi mode of action is that it is relatively slow to act, the university professor said.

“When you have an effective Bt protein, the insect feeds on the Bt protein which binds in the gut of the insect so the gut is no longer able to absorb nutrients,” Seiter said. “In relatively short order the cells of the gut start to burst open and within a few hours after feeding on the material the insect stops feeding.”

With RNAi, the university professor said, it shuts down the production of the protein that the rootworm needs to survive so the rootworm beetle dies.

“That takes about five days for a dead insect so the insects will be feeding for those five days,” Seiter said.

“When you put RNAi into a Bt resistant situation, you should get a dramatic reduction in adult survival because that’s one of the real strengths of this mode of action,” he said. “But you’re not going to reduce pruning to zero if you have a Bt-resistant population.”

For farmers with a corn rootworm problem, the best thing they can do is rotate to a nonhost crop.

“We don’t have options for rotating modes of action because even the new trait packages still include Bt,” Seiter said.

“RNAi is going to be pretty good for killing beetles, but if we take it into a problem and keep throwing it at that problem, we’re going to lose it in short order in these fields,” he predicted.

“A lot of the resistance stays somewhat local so it takes a little bit to move through the population,” he said. “So, your field history matters quite a bit in your resistance development.”