University of Illinois

No. 11/June 5, 1998

Corn Rootworm Egg Hatch Update

The common observance of fireflies and rapid build up of heat units indicate that the corn rootworm egg hatch has been under way for some time. Within a single week, from May 24 to June 1, about 150 to 200 heat units (4-inch soil temperatures, base 52 degrees F) have accumulated for northern and southern Illinois, respectively! Compare Figure 4 of this week's edition with Figure 2 of issue no. 10 of the Bulletin. Predictive models using soil heat units (base 52 degrees F) suggest that about 50 percent of the larval population will have reached third instars when 418 heat units have accumulated. Figure 1 indicates that most of the state has already eclipsed this total. These predictions of rootworm phenology should be viewed with some caution; however, beetle emergence will be early this season. How early? Using air temperature data from Urbana (base 52 degrees F), we should expect to see as much as 10 percent of the western corn rootworm population emerge as adults when 897 heat units have accumulated from January 1. Don't be surprised in roughly 2 weeks to begin finding the first emerging western corn rootworm adults! Cooler temperatures throughout the first 3 weeks of June may push emergence to the last week of June. Bottom line--expect an early flush of western corn rootworm beetles this season.

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

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