Field history impacts levels of corn rootworm populations

SYCAMORE, Ill. — There is a lot of field-to-field variability of corn rootworm populations in Illinois fields.

“Most of the variability is based on field history,” said Nick Seiter, assistant professor and faculty extension specialist at the University of Illinois.

“Where we are finding the highest beetle numbers are long-term continuous cornfields, and DeKalb County is a hotspot,” he said during a presentation in Sycamore at the Illinois Crop Management Conference, hosted by U of I Extension.

“Where we have injury to first-year corn generally, it is when the field is in close proximity to long-term continuous corn,” Seiter said.

“The highest numbers of beetles swept from soybean fields are here in DeKalb County,” he said. “That’s because you have a lot more beetles and that’s where they end up because they have to go somewhere.”

There are four Bt proteins that are available for seed companies to incorporate into corn hybrids that utilize two modes of action.

“The Bts act very fast when they are working,” Seiter said. “An insect feeds on the Bt protein and if it is susceptible to that protein, it stops eating and dies within 12 hours.”

The RNAi technology is a different and a slower mode of action.

“It’s more effective at killing the individual corn rootworm than eliminating the pruning,” Seiter said. “It takes a few days to kill the rootworm and in the meantime they are still eating.”

Therefore, Seiter said, farmers that plant a RNAi hybrid in a field that has resistance to Bt traits will see some pruning, even if the RNAi trait does what it suppose to do, which is kill the insects.

“It takes about five days for the RNAi mode of action to take effect,” he said.

For insecticides, the good news is they are generally doing what they have always done, the assistant professor said.

“We haven’t seen a wholesale change in insecticide efficacy,” Seiter said.

“The really good insecticides we have for corn rootworm control are the same ones from 20 years ago — they haven’t changed much,” he said.

“We have some new players, but they are not really changing the game for rootworms. They are materials that are going after wireworms and they have some rootworm activity.”

Insecticide data shows, Seiter said, that when there is a low population of rootworms, the pruning still reaches one half of a node.

“It doesn’t completely eliminate the insects,” he said. “When you put an insecticide down it protects the center of the root very effectively, but not the peripheral roots, so there is some pruning out there.”

When farmers relied only on insecticides for rootworm control, the beetle populations were a lot higher.

“Insecticides are better at protecting the root from injury than they are at killing insects,” Seiter said.

“That is probably the reason why we don’t see resistance because there is a natural refuge on the plant,” he said.

There was a significant outbreak of corn leaf aphids during the 2024 growing season.

“That was unique for us,” the assistant professor said. “For the last five to seven years, I got occasional calls on aphids and almost all of that was bird cherry-oat aphids at R4 to R5 corn.”

The easiest way to identify the two types of aphids is the corn leaf aphid gets inside the whorl and tassel and it’s up and down the ear zone, Seiter said. The bird cherry-oat aphid will be on the underside of the leaves.

“If you get large populations of corn leaf aphids at or just prior to tasseling, they interfere with kernel set and ear development and then you can have pretty serious impacts on yield,” Seiter said.

“In a normal year, the corn leaf aphids arrive here from the southern U.S. in August and we see just a few of them,” he said. “But last year the aphids were pretty impactful in a lot of areas, but hopefully that was a one-off kind of thing.”

A lot of aphid species go through alternating sexual and asexual cycles.

“They have an overwintering host, they produce males, mate, females lay the egg and the egg survives the winter,” Seiter said.

“Corn leaf aphids don’t do that — they just have an asexual summer cycle,” he said. “All the corn leaf aphids we find are females and all the offspring are asexual clones.”

With this type of life cycle, the corn leaf aphids need green tissue to survive.

“So, they have to overwinter in areas where they can survive and that is mostly in the southern U.S.,” Seiter said.

For Illinois farmers, the corn leaf aphid is a migratory pest.

“They get up in the atmosphere and the wind takes them wherever and then they find a host plant,” the assistant professor said. “Their host plants are a lot of grasses so they will feed on wheat, barley or sorghum.”

Most corn hybrids, Seiter said, are fairly tolerant to corn leaf aphids.

“But I think most of the seed companies had one or two hybrids last year that were impacted by aphids,” he said.

“What happened last year has happened before, but because they have been so infrequent, they get assigned to whatever the weather conditions were that year,” Seiter said.

“In reality, it’s more complicated than that and probably the driving factor is the wind pattern and weather conditions in Texas and Oklahoma,” he said.

“Those areas where they are growing a lot of sorghum, producing a lot of aphids, and we happen to get the winds at the right time and they end up here.”

The bad news is these conditions are not very predictable. However, the good news is they don’t happen very often.

“Last year was the first time it happened in decades and it probably won’t happen again next year,” Seiter said.

“But it’s not any more or less likely to happen this year based on what happened last year,” he said. “Or, what’s going on in Illinois because our aphids all die, it’s what happens where they come from.”