To provide corn producers with a method to estimate peak corn rootworm egg hatch and when to best scout for root feeding damage, the Illinois State Water Survey's Illinois Climate Network calculates accumulated pest degree days. They use daily maximum and minimum bare soil temperatures (52 degree minimum) at a 4 inch depth to calculate the total pest degree day units that have accumulated since January 1st (Allen, 1976).
According to historical research of an Illinois population of western corn rootworm (Diabrotica virgifera virgifera), it takes anywhere from 684 and 767 accumulated pest degree days for approximately 50 percent of eggs to hatch (Levine et al., 1992).
Scouting for root injury is an important way to gauge the effectiveness of your current corn rootworm management strategy. According to Iowa State University Extension Entomology Specialist Dr. Erin Hodgson in a recent Integrated Crop Management article,
"Regardless of agronomic practices to suppress corn rootworm (e.g., crop rotation, Bt rootworm corn, or soil-applied insecticides), every field should be scouted for corn rootworm root injury. Continuous cornfields and areas with Bt performance issues are the highest priority for inspection. Looking at corn roots 10-14 days after peak egg hatch is encouraged because the feeding injury will be fresh."
At the Northwestern Illinois Agricultural Research and Demonstration Center in Monmouth, corn rootworms would have likely reach peak hatch sometime between June 8 and June 11, meaning that the best time to scout for root injury would be sometime between June 18 and June 25 (Table).
Table. Date on which each corn rootworm pest degree day milestone was reached (50% hatch is reached between 684 and 767 PDD) at each Illinois location in 2016 and when root injury symptoms would be the most fresh for scouting purposes (WARM, ICN, ISWS, 2016).
| Date on which each Corn Rootworm pest degree day was reached | Peak injury scouting | ||
| Location | 684 PDD | 767 PDD | 10-14 days after PDD |
| Monmouth | June 8 | June 11 | June 18 – June 25 |
| Big Bend | June 6 | June 9 | June 16 – June 23 |
| DeKalb | June 16 | Not reached | June 26 – |
| Freeport | June 10 | June 13 | June 20 – June 27 |
| Peoria | June 5 | June 9 | June 15 – June 23 |
| St Charles | June 6 | June 11 | June 16 – June 25 |
Dr. Hodgson also suggests that corn producers,
"Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. 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."
Literature Cited
Levine, E., Oloumi-Sadeghi, H. Ellis, C.R. 1992. Thermal requirements, hatching patterns, and prolonged diapause in western corn rootworm (Coleoptera: Chrysomelidae) eggs. J. Econ. Entomol. 85:2425-2432.
Allen, J.C. 1976. A Modified Sine Wave Method for Calculating Degree Days. Environ. Entomol. 5:388-396.
Pest Degree Day Calculator. Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2014). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495.