Evolution of rootworm, corn borer protection

BLOOMINGTON, Ill. — The first genetically-engineered Bt corn to combat western corn rootworm was introduced over 20 years ago; however, resistance was documented in less than a decade.

The story is different for European corn borer since the high-dose Bt corn rolled out in 1996, and the trait continues to provide control with some confirmed resistance, but not to the extent of corn rootworm.

Nick Seiter, University of Illinois Extension entomologist, said populations of both northern and western corn rootworms have developed resistance to all available Bt traits.

“I would say the one insect pest that like continuously drives some yield loss, at least in parts of the state, not so much around here, is rootworm,” Seiter said at U of I Extension’s recent Farm Assets Conference.

Discussion turned to the historical perspective of controlling corn rootworm versus corn borer with Bt products — one being successful; the other, not.

The high-dose Bt products were able to control European corn borer and maintain its efficacy for the most part since its release in 1996. The western corn rootworm Bt was a low-dose product.

Seiter was asked why the efficacy of the Bt corn impacted the two insects differently.

“What we’re trying to achieve with that high dose is to kill what we call heterozygous. They have the resistant allele on one hand, the susceptible on the other. In terms of practical, it means with European corn borer on day one of those Bt traits being introduced, about one in 10,000 of those corn borers, something like that, survived on it,” he said.

In comparison, when Bt was introduced for corn rootworm, about 93% of the insects were killed.

“So, if you take that out of 10,000, that’s 700 corn rootworms that survived. You have a lot more available on day one to start passing those resistance alleles on,” Seiter said.

“That, in a nutshell, is what we mean by the dose. The reason that dose is so important when a trait is rare in the populations, it’s almost always a heterozygous. Almost always, it’s paired with the much more common susceptible. If you kill those heterozygous, you maintain really low frequencies of that trait.

“The other thing that happens, with European corn borer they mate randomly and they disperse a lot when they mate it. They shuffle their population really well.

“With corn rootworm, that little female beetle climbs up the cornstalk, and she begins releasing a sex pheromone before she leaves that plant, and about nine times out of 10, she’s mated before she ever leaves that plant. They don’t shuffle very well. They don’t mix. You get these kind of isolated pockets of corn rootworm naturally.”

It’s been slower in developing compared to rootworm, but Bt-resistant European corn borer was first documented in about 2018.

“It’s not so much in the U.S. We’ve seen one little bit in Connecticut. It’s been in Canada in Nova Scotia, Quebec, Manitoba, in areas where they grow just a little bit of corn, and it’s surrounded by millions of non-corn acres, whether it’s forest, prairie, or whatever it might be. So, you get this isolation effect, even though those disperse very well, because that corn is so isolated,” Seiter said.

“Rootworm almost does that on its own, with its mating behavior. Your field is different from my trial field in Urbana as far as the rootworm population because they don’t shuffle around as much. They’re more kind of insulated, and that contributes to the problem, as well.”

The third element, some insects are really good at developing resistance.

“Western corn rootworm is one of them. As far as I’m aware, there’s no insecticide-resistant European corn borer populations anywhere. I’ve never heard of such a thing. With corn rootworm, they’ve developed resistance to multiple insecticide modes of action. They’ve developed resistance to crop rotation. They’re quite good at overcoming these things in a way that European corn borer isn’t,” Seiter said.

Crop Stress, Insects

Crops experience stress throughout the growing season. Seiter was asked if high-stress environments hinder the production of material to kill rootworms and corn borers.

“Crop stress probably reduces it a little bit, but not enough that we detected difference the field. The Bt proteins are proteins like any other, anything that could impact protein production in that plant could theoretically impact it,” he said.

“They’ve looked at things that really influence protein production in a big way, like nitrogen rate, for instance, comparing very low nitrogen rates, very high nitrogen rates. They haven’t been able to take something like that and make it impact trait performance in a way we can measure in the field.

“Presumably, other stresses are going to have less of an impact on it than that. It’s not none, but the odds of seeing like a practical impact from that in the field are apparently fairly well.”

Cutworms, Weeds

An audience member at the conference noted that he has not heard much mention of black cutworm issues in recent years.

“Cutworm, that’s an interesting one, and there’s a few things going on with mainly black cutworm that we’re mostly talking about. One, they really thrive on fields that have a high winter annual weed population. A black colored moth doesn’t want to lay eggs on clean corn. It’s not a very attractive egg laying site for her,” Seiter said.

“Generally speaking, I think we do a pretty good job of controlling winter annuals. So, that’s part of it. It’s those years where we don’t, where things go wrong, that we can get into problems with that.

“We have a lot of seed treatments now. We have a lot of traits now and very few of those are spectacular controls for black cutworm. The Viptera trait, for instance, is really, really good for black cutworm. I think some of the diamide seed treatments that we don’t use as much of are pretty good on black cutworm.”

In recent years, even with notably strong moth flights, Seiter said it didn’t translate into cutworm injury.

“I think our weed management is a lot of it. We do a pretty good job, especially in this part of the state, of keeping pretty clean fields. And with black cutworm, in particular, it likes winter annual broadleafs like henbit, chickweed, sheperd’s purse, stuff like that really gets them to lay their eggs,” he said.