University of Illinois

No. 14/June 25, 1998

European Corn Borers: End of a Generation

The first generation of European corn borers is drawing to a close in most areas of Illinois. Emergence of moths that will lay eggs to begin the second generation will begin very shortly in southern Illinois, and stalk-boring activity is under way throughout central Illinois. Figure 1 shows the accumulated heat units above base 50 degrees F from January 1through June 23, 1998. Using May 18 as an "average" date of emergence of adult corn borers this spring, we estimate that we have accumulated about 800 heat units in southern Illinois and abouty 600 heat units in central and northern counties. Pupation begins when about 1,000 heat units accumulate from the time of moth emergence in the spring.

Figure 1. Actual heat-unit accumulations (base 50 degrees F), January 1 to June 23, 1998.

During a recent trip into DeKalb, LaSalle, and Kendal counties in northern Illinois, I scouted a few fields of Bt-corn and non-Bt-corn to look for surviving corn borer larvae. The few surviving corn borers I found were second and third instars, and midrib tunneling was the order of the day. Stalk-boring activity is occurring right now.

Most people did not encounter economic infestations of first-generation corn borers this year. The worst infestation I encountered in a field of non-Bt-corn was 3 percent of the whorls with feeding injury. The rainy, stormy weather we've experienced this year reduced the numbers of corn borers significantly. Adults might have been killed by storms, and young larvae might have expired in whorls filled with water. Several of the larvae I found were dead, apparently from excess water.

Late-planted and replanted corn will be attractive for moths laying eggs for the second generation. However, if few first-generation corn borers survived the deluge of rain this spring and summer, the second generation may not amount to much. Time will tell, and we'll try to keep you posted.

Kevin Steffey (, Extension Entomology, (217)333-6652