University of Illinois

No. 10/May 30, 1997

More Information on 4-inch Soil Heat-Unit Accumulations, Corn Rootworm Egg Hatch

In last week's Bulletin, we indicated that the corn rootworm egg hatch this season might be very late, as it was in 1996. Recall that last year corn rootworm eggs didn't start to hatch until mid-June, some two weeks later than "normally" expected. By comparing Figures 2 and 3 (actual 4-inch heat-unit accumulations, base 52F) for 1996 (through May 26) and 1997 (through May 19), respectively, it appears that we can expect another delayed corn rootworm egg hatch for the 1997 season.

Figure 2. Actual heat-unit accumulation (4-inch, base 52F) from January 1 to May 26, 1996. Figure 3. Actual heat-unit accumulation (4-inch, base 52F) from January 1 to May 19, 1997.

Although heat-unit accumulation data for 1997 include one less week, soil heat-unit accumulations this season are low, especially for western areas of the state. In addition to delaying corn rootworm egg hatch, many other subterranean insect pests are likely to remain in the upper profiles of the soil for longer periods of time this spring. This lingering increases the chances for seedling injury to those below-ground portions of the stem.

As mentioned last week, soil insecticides will likely be under increased pressure to protect roots against corn rootworm injury this year. This is especially true for early planted fields. Although a very warm June may help us close the gap with regard to heat-unit accumulations, we have some distance to go.

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