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Rootworm Eggs Reaching Peak Hatch in Iowa

June 9, 2025
Content Author: Erin Hodgson, Ashley Dean

Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. Development is driven by soil temperature and measured by growing degree days (GDDs). Research suggests about 50% of egg hatch occurs between 684-767 accumulated GDDs (since January 1; base 52°F, soil). Soil GDDs are slightly behind average, and therefore, peak egg hatch is too. The southern third of Iowa has reached the critical benchmark this week (Fig. 1), with central and northern counties expected to peak in the next 7-10 days. 

Accumulated soil degree days (base 52°F) in Iowa as of June 8, 2025. Expect 50% egg hatch of corn rootworm between 684-767 degree days. Data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.
Figure 1. Accumulated soil degree days (base 52°F) in Iowa as of June 8, 2025. Expect 50% egg hatch of corn rootworm between 684-767 degree days. Data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

 

 

 

 

 

 

 

 

 

 

 

Scouting for larvae

The float test is a simple method for confirming the presence of corn rootworm larvae. Farmers can assess larval activity with the “float test” in mid to late June. Take corn plants with surrounding soil from the field and place them in a bucket of water to loosen the soil. If there are larvae in the soil near the roots, they will float to the surface. Be careful to distinguish corn rootworm larvae from other small, white worms or plant roots (Figure 2). Watch this float test video to better understand how to do the float test and what information can be gathered from this method.

corn rootworm
Figure 2. Corn rootworm larvae are slender and white with a “two-headed” appearance (dark head and dark plate at the tip of the abdomen). Photo by Ashley Dean, Iowa State University.

 

 

 

 

 

 

 

 

 

 

Understanding corn rootworm peak egg hatch is useful to forecast when to scout for larvae and assess larval root injury. A general estimate is to scout four weeks after peak egg hatch. Evaluating the performance of corn rootworm management tactics is critical for making future agronomic decisions. Root injury is directly related to yield losses. Learn more about how to rate larval root injury. 

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 9, 2025. The information contained within may not be the most current and accurate depending on when it is accessed.

Authors
Photo of Erin Hodgson
Erin Hodgson
  • Professor
  • Extension Entomologist Specialist
  • ANR Crops Team Leader
Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's current extension and research programs are focused on improving corn and soybean production by using IPM tactics to protect yield and increase overall farmer profits. Among other projects, she oversees insecticide efficacy evaluations for soybean aphids, Japanese beetles, and aphids in corn. Erin also helps manage emerging field crop pests and invasive species.
ashley dean headshot
Ashley Dean
  • Extension Specialist II
Ashley is an extension specialist for field crop entomology at Iowa State University. Ashley coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and aids in the research efforts of the Soybean Research Laboratory.
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