Corn rootworm egg hatch in Iowa can occur from late May to the middle of June, depending on accumulating soil degree days, but the average hatching date is around June 6th. Research suggests about 50 percent of egg hatch occurs between 684-767 degree days (base 52°F, soil). Although Iowa had a warmer than normal March, soil temperatures have been closer to normal in April and May. So the 2012 predicted corn rootworm egg hatch is only slightly ahead of normal (Photo 1), but about two weeks ahead of the 2011 growing season. The southwest region in Iowa is experiencing 50 percent egg hatch now and other parts of the state will approach it within 7-14 days depending on the temperature. Other states have also reported that corn rootworm egg hatch is underway. Illinois is reporting the earliest initial egg hatch in 35 years.
Roger Elmore, ISU Corn Extension Agronomist, recently reported Iowa is 6 percent ahead of a normal corn planting year. So although predicted corn rootworm egg hatch is slightly ahead of normal, the larvae should have sufficient root tissue to feed on because most corn will be germinated by the time the eggs hatch. Saturated soils will diminish overall corn rootworm pressure, and the high adoption of Bt corn should decrease populations in most fields. However, I anticipate a good larval establishment this year. Early egg hatch could mean we see earlier adult corn rootworm emergence, too.
Shortly after egg hatch, young larvae will begin feeding on root hairs and inside roots. As they develop, larvae will begin feeding on root tips. A severe infestation can destroy nodes 4-6 which interferes with water/nutrient uptake and makes the plant unstable. Every field should be scouted for corn rootworm damage regardless of the seed selection (i.e., dig and rate corn roots even if Bt proteins are used). Continuous corn fields and areas with persistent corn rootworm populations are the highest priority for inspection. Assess corn rootworm root injury and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale (Photo 2). Aaron Gassmann (ISU Corn Entomologist) has a Web page for additional corn rootworm management information, including an interactive node-injury scale demonstration and efficacy evaluations.