Corn Rootworm Beetle Counts Forecast Problems For 2023
Farmers in parts of the Midwest face a high potential for significant corn rootworm pressure in their fields this season.
That’s according to work conducted by the Corn Rootworm IPM Regional Working Group, which monitored beetle traps last summer to develop the “forecast” for 2023. The Group is made up of university, industry and government personnel from at least 12 U.S. states and five Canadian provinces.
“Some of the worst rootworm problems in the U.S. over the last several years have been in parts of Iowa, northern Illinois, southern Wisconsin and Minnesota, and Nebraska that have a lot of corn-on-corn production—especially in those areas where long-term continuous corn is produced,” Nick Seiter, research entomologist at the University of Illinois, told Farm Journal. “Not surprisingly, resistance is most intense in those same areas."
Seiter and Joe Spencer, principal research scientist, Illinois Natural History Survey, provided an outlook for Illinois last week in a CRW webinar.
“The most pressing concern is the ongoing development of resistance in both western and northern corn rootworm to pyramided Bt trait packages,” Seiter says. "And presumably, the recent increase in the population of these species in these areas is due to their increasing ability to overcome Bt corn.”
The EPA says farmers are likely in the “red zone” for corn rootworm problems this season, if they have these conditions or scenarios on their farm: corn-on-corn for multiple years, heavy use of Bt corn, use of the same Bt trait year-after-year, regular corn rootworm infestations and spotty compliance with current refuge requirements.
Location, Location, Location....
For Illinois, farmers in the northern third of the state are seeing more of the pest show up in fields, while in other parts of the state farmers are seeing fewer CRW.
“Right now in east-central Illinois where we're giving this webinar, our population's pretty low, so the information can seem a little bit schizophrenic sometimes depending on where we’re talking about,” Seiter says. “If you’re in DeKalb County you're going to agree with us (that CRW is a significant problem), but if you’re in Champaign County you might say, ‘What are you talking about, man? I haven't worried about corn rootworm in years.’”
2022 corn rootworm beetle counts conducted in 562 corn and soybean field locations by Bayer Crop Science paint a concerning picture for the 2023 season in much of Iowa, southern Minnesota, southern Wisconsin and parts of Nebraska and eastern South Dakota (see charts above).
Bt Traits Struggle To Keep CRW In Check
Over time, the Bt traits designed to keep CRW at bay have started to lose their efficacy, allowing resistance to build in many areas and complicate farmers' management decisions.
Still, Bt traits are a valuable tool to use in the ongoing battle to keep CRW contained.
Chris DiFonzo, Michigan State University field crops entomologist, updates the list of traits available each year in a tool called the Handy Bt Trait Table for Field Corn, which was updated in March.
In addition, DiFonzo offers directions on how to use the trait table here.
Joe Spencer, with the Illinois Natural History Survey, says farmers often struggle to identify whether they have resistance issues.
"Unless growers are leaving non-Bt check strips in or near their Bt cornfields and actually comparing root injury, they are likely unable to judge whether their chosen Bt hybrid is providing the efficacy they expect," he says.
"Planting a check strip is a great way for growers to see the difference between their chosen management tactics and the effect of field-level pest pressure on unprotected corn," he adds.
New Trait Technology Will Help Address CRW
New technology from Bayer and Corteva, Ribonucleic acid interference (RNAi) technology, works in CRW as it feeds on corn roots by turning off functions the pest needs for development and survival.
“They're silencing translation of a protein that the beetle needs to survive and develop. And because that protein is essential for 
rootworm growth, the insect is going to die in around five days,” Seiter explains.
That timing is why RNAi technology is paired with Bt traits. The combination gives CRW a one-two punch of control.
“You get that rapid cessation of feeding (from Bt), and also limited survival of that insect to adulthood, because of the RNAi,” he adds.
Don't Let Your Guard Down This Season
Seiter says the potential exists for CRW to develop resistance to RNAi, so stewardship of the technology as it comes to the marketplace is critical.
Along with that, scouting for corn rootworm is the best way to determine 
if other management actions are needed, say Erin Hodgson and Ashley Dean, Iowa State University.
They report that once scouting reveals corn rootworm is present and causing injury to a cornfield, choosing the best management strategy depends on field history, resistance issues (insecticide, traits and/or crop rotation), financial constraints, and agronomic practices in the field.
Unspoken Truths About Pests: Corn Rootworm
