By Dr. Yucheng Wang
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.
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.
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.
Source : umn.edu