Midwest CRW Potential -- Be Vigilant in 2023

Researchers at the Corn Rootworm IPM Regional Working Group have raised awareness for 2023 based on historical beetle trap beetle counts. The group is made up of university, industry, and government personnel from at least 12 U.S. states and five Canadian provinces. These models were built to track both Western and Northern Corn Rootworms. Current data suggests that 2023 possibly will be a year to pay laser-focused attention in some geographies. Monitor all your corn acres regardless of location and extend serious attention to those continuous corn acres. Implementation of Best Management Practices to conserve the technology we have available to us today is a critical step in extending the utility of these Bt packages.

“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."

Preventing Development of Bt Resistance

If a Bt hybrid is used and unexpected corn rootworm populations or severe root injury occurs in the field, you should contact your seed company and your local extension agent. Some warning signs that a field may have resistant corn rootworm:

Severe root pruning. A node-injury score of >1 is unacceptable for hybrids with a single Bt trait, and >0.5 is unacceptable for pyramided hybrids.
Symptoms of feeding. If a Bt cornfield has significant lodging, goosenecking, leaf-feeding, or silk clipping, there may be resistance developing.
High adult populations. High densities of adults in Bt cornfields are a red flag. If using sticky cards to monitor adults, >2 beetles/trap/day suggests another tactic should be used the following year.

We recommend the following strategies to reduce the risk of rootworm populations developing resistance to Bt:

Crop rotation. If possible, rotate fields annually from corn to a non-host crop. Fields should be rotated away from corn at least once every four years to break up the life cycle and slow resistance development.
Comply with refuge requirements. Make sure to follow instructions on the seed tag.
Weed management. Eliminate volunteer corn or grassy weeds that could serve as hosts.
Scout and assess activity. Assess root injury by larvae when it is fresh, especially in fields with a history of rootworm injury, and scout for adults.
Switch it up. If there is a lot of adult activity or larval feeding in a Bt cornfield, that is a sign to do something different the next year. If crop rotation is not an option, plant a non-rootworm-Bt corn hybrid the following year and use a soil insecticide. Soil-applied insecticides work best when rootworm pressure is low, but they can help reduce selection pressure for Bt resistance.