Dr. Fei Yang, University of Minnesota, Extension corn entomologist, and Dr. Yucheng Wang, University of Minnesota, Research Specialist
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).
Western corn rootworm typically dominates southern Minnesota's continuous corn fields, where it tends to outcompete its northern corn rootworm counterparts. In 2024, a variant of western corn rootworm that lays eggs in non-corn crops, such as alfalfa and soybeans, was documented for the first time in southeastern Minnesota. This means that even first-year corn in these fields can be at risk of larval injury. Northern corn rootworm eggs are more tolerant to cold temperatures than westerns and typically predominate in northern Minnesota corn fields. A portion of northern has the extended diapause genetic trait, allowing the diapausing (suspended development) eggs to delay hatching over two winters, with some eggs exhibiting a delay hatching of up to four winters. Extended diapause trait allows northerns to be a significant pest in both continuous corn and rotated corn fields.
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).
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| 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. |
Understanding corn rootworm biology
Two species of corn rootworms, western and northern, are significant insect pests of Minnesota corn (Figure 2). Both species complete one generation per growing season. Rootworm eggs overwinter in the soil and hatch in the spring. After hatching, the larvae go through three instars and feed on corn roots for 4-6 weeks before pupating in the soil. The larvae of both species must feed on the roots of corn for optimal survival and development. They cannot survive on the roots of other crops such as soybeans, small grains, or alfalfa. Newly hatched larvae are especially vulnerable to drowning before they locate and tunnel into corn roots. Larvae typically begin to hatch from overwintering eggs in June. The larval stage is the most damaging. Feeding by later instars can severely prune roots, reducing the plant's ability to uptake water and nutrients, which can result in corn lodging (Figure 3). Root injury from larval feeding can also increase the risk of secondary infections, such as crown rot.Western corn rootworm typically dominates southern Minnesota's continuous corn fields, where it tends to outcompete its northern corn rootworm counterparts. In 2024, a variant of western corn rootworm that lays eggs in non-corn crops, such as alfalfa and soybeans, was documented for the first time in southeastern Minnesota. This means that even first-year corn in these fields can be at risk of larval injury. Northern corn rootworm eggs are more tolerant to cold temperatures than westerns and typically predominate in northern Minnesota corn fields. A portion of northern has the extended diapause genetic trait, allowing the diapausing (suspended development) eggs to delay hatching over two winters, with some eggs exhibiting a delay hatching of up to four winters. Extended diapause trait allows northerns to be a significant pest in both continuous corn and rotated corn fields.
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| Figure 2. . A) Western corn rootworm adults, and B) Northern corn rootworm adults |
Scouting reminder for Minnesota growers
Growers in Minnesota should begin preparing for larval scouting within the next 3-4 weeks, especially in southern and central counties. The ‘float test’ is a simple and effective method to detect larval presence in mid to late June: place corn root masses in a bucket of water to loosen the soil. Corn rootworm larvae, slender and white with a dark head and tail plate at the tip of the abdomen (Figure 3), will float to the surface.Scouting for larvae and assessing root injury four weeks after peak hatch is crucial for evaluating the performance of rootworm management tactics. Use Iowa State University’s 0 to 3 Node Injury Scale to assess injury. Look at nodes 4-6 (the three nodes closest to the soil surface). For each node, count the number of roots pruned back to within 1.5 inches of the soil line or stalk and divide by the total number of roots on that node. A rating of 0.50 is considered unacceptable by the Environmental Protection Agency (EPA) for pyramided transgenic hybrids (multiple belowground Bt traits or Bt + RNAi). Root injury ratings are directly tied to potential yield losses and can inform future management decisions.
Thanks to the Minnesota Corn Research and Promotion Council for their support of this program.
References
Erin Hodgson, Ashley Dean. 2025. Rootworm Eggs Reaching Peak Hatch in Iowa. Available at: https://crops.extension.iastate.edu/cropnews/2025/06/rootworm-eggs-reaching-peak-hatch-iowa.Thanks to the Minnesota Corn Research and Promotion Council for their support of this program.
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