University of Illinois

No. 21/August 15, 1997

Sudden Acceleration of European Corn Borer Activity

After somewhat of a lull in European corn borer activity, things are getting exciting again. Many people have reported significant increases in captures of adults in light traps and pheromone traps, and scouts are finding egg masses with regularity. Following is a compilation that represents the essence of the reports of corn borer activity we have received.

Dave Mowers with Mowers Soil Testing Plus in Toulon (Stark County) told me that in the middle of the week of August 4, their captures of corn borer adults in light traps were at or near zero. By Friday, August 8, the numbers had increased to 1,000 in one night, and they captured as many as 1,500 moths in a trap on Saturday, August 9. Norm Larson with Midwest Consulting Service told me that moth captures in their battery of light traps throughout the northern half of Illinois had climbed to nearly 3,000 in one night at one location. Rick Weinzierl, Extension entomologist on campus, reported that captures of corn borer adults in light traps near Manito (just south of Peoria) exceeded 1,000 on a few nights during the week of August 4. Maria Venditti, my graduate student working in cornfields in Sangamon County, observed a significant increase in the numbers of egg masses during her weekly visit to her fields on August 6. Others have begun to call in and report that they are finding egg masses.

Apparently we can't give up on corn borers yet. The marked increase in corn borer activity during the past couple of weeks suggests that some fields might experience economic infestations of this perennial pest. Scouting must continue. Yes, it's true that the amount of physiological yield loss caused by corn borers decreases as the corn matures, from 4 percent per borer per plant at pollen shed to 3 percent per borer per plant when kernels are initiated. We suggest you use 3 percent per borer per plant if infestation occurs after silks are brown. The potential economic benefits of treatment decline rapidly if infestations occur after corn reaches the blister stage. However, depending upon the hybrid, the standability of corn could be affected by late-season infestations of corn borers if the hybrid is prone to stalk breakage.

Deciding to treat for second-generation corn borers is tough; most folks don't even bother. However, scads of data show that yield benefits can accrue from treating economic infestations of second-generation borers. Monitor the late-planted fields first, but stay vigilant in all fields, including fields of Bt-corn. Use the management worksheet printed in last week's issue of the Bulletin (issue no. 20, August 8, 1997) to determine whether or not treatment would be economically worthwhile, then consult the table of suggested insecticides if treatment is justified. Some of the toughest calls will occur in areas where the corn has suffered from a lack of moisture. Simply adjust yield expectations accordingly to make decisions.
Kevin Steffey, Extension Entomology, (217)333-6652