Corn Rootworm Egg Hatch Begins in Southern Minnesota

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

Based on historical observations and degree-day models, peak corn rootworm egg hatch in southern Minnesota typically occurs in June. This period represents a critical window for growers to begin scouting for newly hatched larvae and assessing early root feeding activity.

GDDs and egg hatch

Because corn rootworm larvae feed below ground, infestations often remain unnoticed until root injury becomes apparent. Soil temperature is the primary factor driving egg development and hatch timing, and egg development can be tracked using soil-based growing degree days (GDDs), calculated from January 1 with a base temperature of 52°F. Previous research suggests that approximately 50% of egg hatch occurs between 684 and 767 accumulated GDDs. As of June 3, southern Minnesota has reached this critical threshold, while central and northern regions expected to reach peak hatch within the next 7-10 days (Figure 1).

Timing of egg hatch and larval feeding

Compared with the same period in 2025, accumulated GDDs are generally higher across southern Minnesota, particularly in the southwestern region, suggesting that egg hatch and larval development may occur slightly earlier than observed last year. In contrast, accumulated GDDs in central Minnesota are similar to those recorded at this time in 2025, indicating a comparable hatch timeline. Northern Minnesota remains somewhat behind last year’s development, and peak hatch may occur later than in 2025. Therefore, growers should consider local field conditions when scheduling scouting activities rather than relying solely on calendar dates.

After hatching, the larvae pass through three instars and feed on corn roots for approximately 4-6 weeks before pupating in the soil (Dennis, 2023). Newly hatched larvae are particularly vulnerable to drowning before locating and entering corn roots, but once established, larval feeding can significantly reduce root function and increase susceptibility to lodging.

Scouting tips

Growers in southern Minnesota should begin scouting for larvae in about 3 weeks, while those in central and northern regions should prepare to begin scouting as hatch progresses. One simple and effective method for detecting larval activity is the ‘float test’. To perform this test, place corn root masses in a bucket of water and gently agitate the roots to loosen surrounding soil. Corn rootworm larvae are slender, white, and characterized by a dark head capsule and a dark tail plate at the tip of the abdomen. Larvae dislodged from the roots will float to the water surface, allowing growers to confirm larval presence and determine the pressure and management for next season.

Detailed information on corn rootworm root injury evaluation and root rating procedures is available at: https://cornrootworm.extension.iastate.edu/basics/scouting-guidelines

Figure 1. Accumulated soil degree days (base 52°F) in Minnesota as of June 3, 2026. Expect 50% egg hatch of corn rootworm between 684-767 degree days. Data courtesy of Midwestern Regional Climate Center.

Thanks to the Minnesota Corn Research and Promotion Council for their support of this program.