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, says Erin Hodgson, Iowa State University Extension entomologist. In 2016, the average hatching date was slightly behind normal and approximately the same time as in 2014 and 2015.
Development is driven by soil temperature and measured by growing degree days. Research suggests about 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52 degrees F, soil). Most areas in Iowa reached peak corn rootworm egg hatch or did eventually reach it within a few days of June 14 this year (see Figure 1).
Accumulated soil degree days in Iowa as of June 14, 2016. Expect 50% corn rootworm egg hatch between 684 and 767 degree days. Map data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy
To generate degree day accumulation on corn rootworm egg hatch for your area, Hodgson says you should use this website. To create an accurate map, make sure to set the start date to Jan. 1 of the current year and the end date to today, and set the plot parameter to “soil growing degree days (base equals 52).” Be aware the website may be having some technical difficulties with the soil temperature probes this year (Muscatine and West Point locations are not accurate).
A severe corn rootworm larval infestation can destroy the root nodes 4 to 6 on a corn plant; each node has approximately 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable (See photo 1). A recent research analysis showed a 15% yield loss for every node of roots that is pruned.
Severe root pruning by corn rootworm larvae can dramatically impact yield. Photo by Erin Hodgson, Iowa State University.
Regardless of agronomic practices you use to suppress corn rootworm (practices such as crop rotation, planting Bt rootworm corn, or applying soil-applied insecticides), every field should be scouted for corn rootworm root injury, says Hodgson. Continuous cornfields and areas with Bt performance issues are the highest priority for inspection.
Looking at corn roots 10 to 14 days after peak egg hatch is encouraged because the feeding injury will be fresh. You should assess corn rootworm feeding in your fields and adjust management strategies if the average injury is above 0.5 on a 0 to 3 rating scale. Also, consider monitoring for adult corn rootworm to supplement root injury assessments. Aaron Gassmann, Iowa State University corn research entomologist, has a webpage for additional corn rootworm management information. His page includes an interactive node-injury scale demonstration and efficacy evaluations.