By Erin Hodgson, Department of Entomology
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average hatching date of June 6. Development is driven by soil temperature and measured by growing degree days. Research suggests about 50 percent of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Cooler spring temperatures mean slower egg maturation in 2014. But a few areas of Iowa are approaching 50 percent corn rootworm egg hatch now (Fig. 1), particularly around Muscatine. Many other regions will be reaching 50 percent egg hatch within two weeks.
Figure 1. Accumulated soil degree days in Iowa as of June 4, 2014. Expect 50 percent corn rootworm hatch between 684-767 degree days. Map data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.
To generate up-to-date information for your area, use this website for current degree day accumulation for corn rootworm eggs. To create an accurate map, make sure to set the start date to January 1 of the current year and the end date to today, and set the plot parameter to "soil growing degree days (base = 52)."
A severe corn rootworm larval infestation can destroy nodes 4-6; each node has approximately 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable (Photo 1). A recent meta-analysis showed a 15 percent yield loss for every node pruned.
Photo 1. Severe root pruning by corn rootworm larvae can dramatically impact yield. Photo by Erin Hodgson, Iowa State University.
Several environmental factors influence the survivorship of corn rootworm eggs. Crop rotation or the use of Bt corn should decrease populations in most fields. Regardless of the agronomic practices, every field should be scouted for corn rootworm injury (i.e., dig and rate corn roots even if Bt proteins are used). Continuous cornfields and areas with Bt performance issues are the highest priority for inspection. Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Aaron Gassmann, Iowa State University corn entomologist, has a webpage for additional corn rootworm management information, including an interactive node-injury scale demonstration and efficacy evaluations.
Erin Hodgson is an associate professor of entomology with extension and research responsibilities; contact at email@example.com or by calling 515-294-2847.