University of Illinois

No. 12/June 13, 1997

Stalk Borer Infestations Are Widespread

Although it's pretty quiet right now in the world of field crop insects, stalk borer infestations are relatively widespread. Many people have reported finding severe infestations either in field edges or in areas of the field that were weedy last year. On June 10, I visited the Northern Illinois Agronomy Research Center near DeKalb and observed a severe infestation of stalk borers in a small no-till corn plot. Probably 80% of the 2-leaf-stage corn plants were infested or had been injured by very small (1/4- to 3/10-inch long) stalk borer larvae. Although the larvae were small, the yellow head, longitudinal white and dark stripes, and the purple "saddle" around the body were evident. The stalk borers had moved from their weed hosts (including giant ragweed, quackgrass, and foxtail) that had been killed with Roundup into the small corn plants. Most of the infested plants were dead or dying because the larvae had tunneled to and fed upon the growing point. Leaf-feeding injury was evident on most plants, and the "dead-heart" symptom was prevalent.

Figure 1 shows the accumulated heat units (base temperature of 41 degrees F) from January 1 to June 9, 1997. These heat units can be used to appraise the activity of stalk borers in your area. The critical heat-unit accumulations have been printed in several previous issues of this Bulletin. Remember, accumulated heat units are guidelines, not numbers carved in stone. Some slight variations may occur from location to location. Also keep in mind that the heat-unit isolines in Figure 1 are regional, based upon several Illinois Climate Network locations. Local accumulations may vary from regional estimates.

Figure 1. Actual heat-unit accumulation (base 41 degrees F) from January 1 to June 9, 1997, for estimating the development and movement of stalk borers.

Insecticides do not control stalk borers inside the stems. However, after a stalk borer kills its host plant, it moves to an adjacent or nearby healthy plant. Thus, if the percent of plants injured or killed is not excessive, an insecticide kills the larvae when they move from dead to healthy hosts. The combinations of leaf-stage and percentage infestation are the economic injury levels: 1-leaf (15%), 2-leaf (18%), 3-leaf (23%), 4-leaf (25%), 5-leaf (25%), and 6-leaf (50%). Refer to last week's Bulletin (issue no. 11, June 6) for a list of insecticides suggested for control of stalk borers.

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