Corn Rootworm Adult Emergence Has Begun in the Fields

By:  Dr. Yucheng Wang, University of Minnesota, Postdoc Researcher, and Dr. Fei Yang, University of Minnesota, Extension corn entomologist

Beetles of the western corn rootworm (WCR) and northern corn rootworm (NCR) have recently been observed in corn plots at the University of Minnesota Rosemount Research and Outreach Center (Figure 1). However, only a small number of beetles were found flying in the cornfields, and their pale coloration indicated that they were newly emerged adults.

Figure 1. Newly emerged western corn rootworm adults in the cornfields located at the University of Minnesota Rosemount Research and Outreach Center. (Photo courtesy of Yucheng Wang) 

How do we predict emergence?

Soil temperature is the primary factor driving corn rootworm (CRW) development, which can be predicted using soil-based growing degree days (GDDs) with a base temperature of 52°F. Previous studies suggest that adult corn rootworm emergence typically occurs when accumulated GDDs reach approximately 1,300-1,500. Based on our calculation of GDD accumulations through July 9, 2026, southern Minnesota has reached this threshold, whereas central and northern Minnesota are expected to reach this threshold in 1-2 weeks (Figure 2).

Figure 2. Accumulated soil degree days (base 52°F) in Minnesota as of July 9, 2026. Data courtesy of Midwestern Regional Climate Center.

What happens during early adult emergence?

CRW adult emergence usually begins in late June and can continue into mid-August, although the exact timing varies with temperature, location, and field conditions. In most fields, the emergence period lasts approximately four to eight weeks.

WCR typically emerges earlier than NCR. NCR males often begin appearing about 5-7 days after the first WCR males. Within each species, males generally emerge approximately 2-10 days earlier than females, allowing males to be present when females emerge and become available for mating. On average, male emergence is completed over a shorter period than female emergence. Therefore, the earliest beetles detected in a field are often predominantly males.

After emergence, adults feed primarily on corn pollen, fresh silks, leaves, and exposed kernels. Females generally begin laying eggs about two weeks after emergence, with most eggs deposited in the soil of cornfields. These eggs remain in the soil through the winter and hatch the following growing season.

What should growers watch for?

The current observations at Rosemount represent the beginning of CRW adult emergence. Beetle numbers are expected to increase as additional males and females emerge over the next several weeks. Growers should begin scouting cornfields, especially those with a history of continuous corn or previous corn rootworm injury, as well as fields showing plant lodging or signs of root pruning. Timely scouting could provide critical information on population status, root injury and beetle abundance. You can use whole plant counts and yellow sticky traps for scouting beetle abundance, predicting population density, and risks for the following season. 

Seeking volunteers!

If you observe heavy rootworm infestation in your cornfields, especially with Bt traits, we are seeking volunteers to collect these insects for testing. We will assay the insects and provide you with reports on their Bt resistance levels. To volunteer for the collection, please contact Fei Yang (yang8905@umn.edu).