University of Illinois

No. 9/May 23, 1997

More Information about Stalk Borers

In last week's issue of the Bulletin (no. 8, May 16, 1997), I wrote a rather detailed article about stalk borers and provided some information about timing of insecticide applications based upon accumulated heat units above a base temperature of 41 degrees F. To reiterate, overwintering stalk borer eggs hatch when about 575 to 750 heat units have accumulated since January 1. Larvae first begin to move into corn when about 1,100 heat units have accumulated since January 1; 50% movement occurs when about 1,400 to 1,700 degree days have accumulated. When about 1,300 to 1,400 degree days have accumulated, scout corn to verify the presence of stalk borers in the grasses (dead stems, larvae inside) or border rows of corn. Figure 6 shows an update of accumulated heat units above 41 degrees F from January 1 to May 19, 1997.

The preceding sentences describe the events that occur when young stalk borer larvae move from grass hosts along the edges of cornfields into the first few rows of corn near the edge of the field. However, another scenario is possible. On May 16, I received a report of small stalk borer larvae feeding within a field of corn in Logan County. Although heat-unit accumulations suggested that it was too early in that area to witness movement of stalk borers into corn fields, it wasn't too early to observe stalk borers feeding in corn in this situation. The grower had an infestation of giant ragweed (a host favored by stalk borers) last year, and the moths laid eggs on this host last fall. After the herbicide killed all potential weed hosts this spring, the stalk borer larvae had no alternative but to begin feeding on corn seedlings. In the infested area of the field, the injury was severe enough to warrant an insecticide application

Watch for stalk borers causing injury in areas of cornfields that were weedy last year and along the edges of the fields, including rows along grass waterways. Symptoms of injury, treatment thresholds, and suggested insecticides were published in last week's Bulletin.

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

Figure 6. Actual heat-unit accumulation (base 42 degrees F) from January 1 to May 19, 1997, for estimating the development of stalk borers.