By Dr. Fei Yang and Dr. Yucheng Wang
Peak rootworm hatch typically occurs in June across the Midwest, making this the critical period for growers to begin scouting for larvae and evaluating early feeding damage. Recent updates from Iowa State University indicate that corn rootworm egg hatch has reached its critical benchmark in southern Iowa this week (Hodgson and Dean 2025). Given the similar climatic patterns across the Upper Midwest, corn rootworm egg hatch in southern Minnesota is likely approaching a similar peak window, slightly behind Iowa peak (Figure 1).
While lightning bug activity is sometimes used as a visual cue for corn rootworm hatch, it is far less reliable than soil temperature-based degree day models. Development of corn rootworm eggs is driven by soil temperature and tracked using soil-based growing degree days (GDDs), calculated from January 1 with a base temperature of 52°F. Research suggests that approximately 50% of egg hatch occurs between 684 and 767 accumulated GDDs. As of this week, the southwest regions of Minnesota have reached this critical benchmark, with south central, southeast, west central, and central regions expected to reach a peak within the next 3-7 days, and the remaining regions are expected to reach peak hatching within approximately two weeks (Figure 1).
Figure 1. Accumulated soil degree days (base 52°F) in Minnesota as of June 13, 2025. Expect 50% egg hatch of corn rootworm between 684-767 degree days. Data courtesy of Midwestern Regional Climate Center.