Growers Urged to Scout Fields as Rootworm Activity Rises

By Scout Nelson

University of Minnesota researchers Dr. Yucheng Wang and Dr. Fei Yang have reported the early emergence of western corn rootworm (WCR) and northern corn rootworm (NCR) adults at the Rosemount Research and Outreach Center. Only a small number of beetles have been observed so far, and their pale color indicates they recently emerged from the soil.

Soil temperature is the primary factor affecting corn rootworm development. Researchers use soil-based growing degree days (GDDs) with a 52°F base temperature to predict emergence, which typically begins when GDD accumulations reach 1,300 to 1,500.

As of July 9, southern Minnesota had reached the emergence threshold, while central and northern parts of the state were expected to reach it within one to two weeks. Adult emergence generally begins in late June and can continue through mid-August, depending on local weather and field conditions.

Western corn rootworms usually emerge before northern corn rootworms. Within both species, males typically appear several days before females, making the first beetles seen in fields predominantly male.

After emergence, adults feed on corn pollen, silks, leaves, and exposed kernels. Females begin laying eggs about two weeks later, depositing them in cornfield soils where they remain through winter and hatch the following growing season.

Researchers expect beetle populations to increase in the coming weeks as more adults emerge. Growers are encouraged to scout fields regularly, especially those with a history of continuous corn production, rootworm pressure, lodging, or root damage.

Whole-plant counts and yellow sticky traps can help monitor beetle populations and assess the risk of future infestations. Early scouting can provide important information for making timely crop management decisions.

The University of Minnesota is also seeking volunteers with heavily infested cornfields, particularly fields planted with Bt corn traits. Collected insects will be tested for Bt resistance, and participating growers will receive reports on the resistance levels of their rootworm populations.

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