There were several reports of lightning bugs (fireflies) throughout Iowa last week. Some people correlate fireflies with corn rootworm larval hatch in the Midwest. Based on conversations with Marlin Rice, former Iowa State University entomologist, and Mike Gray, entomologist from University of Illinois, they believe these events are unrelated. Instead, corn rootworm hatch predictions are more accurately based on temperature accumulations.
Research shows about 50% of corn rootworm larvae will hatch from 684 to 767 accumulated growing degree days (base 52° F). Corn rootworm hatch should be happening in the southeast part of Iowa. Other parts of the state should experience larval hatch in the next seven days.
Ideally, every cornfield should be inspected for corn rootworm larvae after reaching 50% hatch. Non-Bt fields are most susceptible to larval damage and should be considered a priority. Continuous Bt cornfields with previous damage should also be scouted. Sample for larvae by digging up corn plants and washing the roots in a bucket; larvae should float to the top of the water. Sample corn plants in different areas of the field to estimate infestation levels.