University of Illinois

No. 8/May 15, 1998

Heat-Unit Accumulations
for Corn Rootworm Egg Hatch

Because egg hatch of corn rootworms was very delayed in 1997, there is considerable interest in the projected timing of this event for 1998. Due to the very late hatch that took place in 1997, soil insecticides were put to the test. The late hatch translated into an extended larval feeding period that persisted in some locations until late July and early August.

What do the heat-unit accumulations this season suggest with respect to the egg hatch? Western and northern corn rootworm eggs have a developmental threshold of 52 degrees F. About 380 to 426 degree-day accumulations are required for 50 percent of the eggs to hatch. Figure 4 depicts the soil heat units (base 52 degrees F) that have accumlated at the 4-inch level from January 1 through May 11. For central Illinois, about 200 heat units are still required before 50 percent of the eggs should be expected to have hatched. This translates into about 10 days with average temperatures of 70 degrees F. Bottom line--expect a more typical egg hatch this season, with most of the egg hatch occurring the last week of May, about 2 weeks earlier than last year.

Figure 4. Actual 4-inch soil-temperature heat-unit accumulation (base 52 degrees F), January 1 to May 11, 1998.

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