Each year during the early part of the growing season, corn rootworm hatch in the soil. The hungry larvae that emerge start their search for corn roots to feed on. All cornfields in Iowa are at risk for infestation by corn rootworm and should be scouted for root damage by rootworm.
“Dig some roots in your cornfields beginning in late June and early July, and check for signs of rootworm injury,” advises Josh Michel, an Iowa State University Extension field agronomist in southeast Iowa.
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.
Accumulated soil degree days in Iowa as of June 17. Expect 50% corn rootworm egg hatch between 684 to 767 degree days. (Map data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy)
“In 2019, the average hatching date will be behind the average, due to cool spring temperatures. Development is driven by soil temperature and measured by growing degree days,” she explains.
Egg hatch in most of Iowa
Research suggests about 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52 degrees F, soil). Most areas in Iowa have reached peak corn rootworm egg hatch.
To generate degree day accumulation on corn rootworm egg hatch for your area, use the ISU Agronomy Mesonet 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 = 52).” Be aware that some locations are having some technical difficulties with the soil temperature probes this year.
A severe corn rootworm larval infestation can destroy nodes 4 to 6; each node has about 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable. A recent meta-analysis showed a 15% yield loss for every node pruned.
Scout corn for rootworm injury
Regardless of agronomic practices farmers can use to suppress corn rootworm (such as rotating corn crops with a non-corn crop such as soybeans, planting corn hybrids that have the Bt rootworm trait or using soil-applied insecticides), every field should be scouted for corn rootworm injury to roots, Hodgson says. Planting fields to continuous corn 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, she says. Assess corn rootworm feeding and adjust your management strategies if the average injury is above 0.5 on a sale of 3.
Also, consider monitoring for adult corn rootworm to supplement root injury assessments. 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.