University of Illinois

No. 10/May 29, 1998

Update on Corn Rootworm Egg Hatch

The timing of the 1998 corn rootworm egg hatch should match the descriptions provided in most entomological textbooks--rootworm eggs should hatch in late May and early June. Because the egg hatches in 1996 and 1997 were so late (mid-June), this year's more "typical" hatch seems early by comparison. Soil heat-unit accumulations (4-inch level, base 52 degrees F) provided by Bob Scott, Illinois State Water Survey, reveal a range of approximately 250 to 600 heat units for the state (Figure 2). Laboratory studies suggest that when 380 to 426 heat units have accumulated, as many as 50 percent of rootworm eggs may have hatched. This implies that for the southern two-thirds of Illinois, egg hatch is underway. Donít be surprised to find signs of larval injury much earlier in June this season than in years past. Because of the late planting and replanting of corn in many fields, larvae are likely to put severe pressure on very small root systems. Please pass along your observations when you begin to find this type of injury. In future issues of this Bulletin, we will offer some projections regarding peak larval injury and beetle emergence.

Figure 2. Actual 4-inch soil temperature heat-unit accumulation (base 52 degrees F), January 1 to May 24, 1998.

Mike Gray (, Extension Entomologist, (217)333-6652