First Flight of European Corn Borer Persists at Low Levels

June 16, 2000
Reports of the first flight of European corn borer moths continued across central and northern Illinois counties. By now, this flight is mostly a memory for folks in southern Illinois. In issue no. 10 of the Bulletin, we presented a table that highlighted the occurrence of different corn borer events or life stages according to the number of accumulated degree days from the initial spring capture of moths. Based on the first reported captures of moths in southern Illinois (May 9), we project that for the southern one-third of the state, many corn borer larvae are already fourth-larval instars. Consequently, these borers are tunneling within stalks and no longer susceptible to rescue treatments. From May 9 through June 12, Bob Scott, Illinois State Water Survey, reports that 635 heat units (base 50°F) have accumulated (average across seven southern reporting stations). As Table 2 in issue no. 10 of the Bulletin indicates, when 567 heat units have accumulated beyond an initial capture of moths, stalk boring has begun. In central Illinois, an average (nine reporting stations) of 476 heat units had accumulated from May 15 through June 12. The first general observance of moths across central Illinois counties occurred on May 15. For central Illinois, mid-rib tunneling and stalk boring should be under way by third-instar larvae. We estimate the first flight of moths began across northern Illinois on May 22. As of June 12, an average of 352 heat units had accumulated (average of three stations) in northern Illinois counties since May 22. This suggests that most second instars will very soon be completing their development, and decisions regarding a rescue treatment should not be delayed.

As we've indicated several times this spring, the first flight has been very weak. However, don't neglect to monitor fields for whorl-feeding injury. Also, we welcome reports of your scouting efforts.--Mike Gray

Author: Mike Gray