University of Illinois

No. 9/May 22, 1998

An Update on Soil Heat-Unit Accumulations and Corn Rootworm Egg Hatch

Will corn rootworm eggs begin to hatch soon? Based upon an examination of heat-unit accumulation data (base 52 degrees F) at the 4-inch level, egg hatch appears likely sometime during the last week of May. This time frame is considered to be more "normal" than the previous 2 years, in which egg hatch was occurring in mid-June. As indicated in last week's Bulletin, laboratory studies have shown that when 380 to 426 heat units have accumulated, as many as 50 percent of the eggs may have hatched already.

Figure 1 reveals that for the northern half of Illinois about 200 to 300 heat units have accumulated from January 1 to May 18. More record-breaking or above-average temperatures in late May will soon bring a more typical beginning of the corn rootworm "season."

Figure 1. Actual 4-inch soil temperature heat-unit accumulation (base 52 degrees F), January 1 to May 18, 1998.

A 1996 paper written by Paula Davis and others (Environmental Entomology 25: 767-775) describes several heat unit-driven models that can be used to predict corn rootworm phenology. The authors point out in their article that "rootworm development is not strictly a function of temperature. Adult emergence seemingly is synchronized with time of corn flowering, although the mechanism for this synchronization is not clear." In future issues of this Bulletin, we will offer some projections for when to expect the various life stages of corn rootworms to occur throughout the upcoming season.

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