University of Illinois

No. 8/May 15, 1998

Stalk Borer Heat-Unit Update

Stalk borers represent one of the most challenging insect pests to manage effectively. Stalk borers overwinter as eggs on weed hosts. Fields infested with ragweed, wirestem muhly, quackgrass, and some other weeds are attractive egg-laying sites for stalk borer moths.

Young stalk borer larvae are easy to identify. Stalk borers are marked with white and purplish stripes along the length of the body. In young larvae, these stripes are interrupted by a prominent dark purple "saddle" or band around the middle of their body. This band fades as larvae mature. The head capsule is yellow and has a dark stripe on each side.

Injury to corn plants also is very characteristic. Ragged leaf feeding accompanied by copious amounts of frass (insect excrement) indicates that the larvae have entered the plant from the top and are feeding within the whorl leaves. The "deadheart" symptom (inner leaves wilted and dying, outer leaves healthy) indicates that the stalk borer has drilled into the plant and killed or severely injured the growing point. Plants that do not die from this type of injury often produce suckers and rarely yield very well. Once stalk borer larvae tunnel into the stems of plants, rescue treatments are not effective.

An insecticide application can be timed to coincide with movement of the larvae from weed hosts to corn. Stalk borers first begin to move into corn when about 1,100 heat units have accumulated above a base temperature of 41 degrees F since January 1; 50 percent 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 weeds (dead stems, larvae inside) or border rows of corn. According to Figure 3, as of May 11 stalk borer larvae should have begun their movement into border rows of corn throughout the southern one-third of Illinois. This movement will soon begin across much of central Illinois.

Figure 3. Actual heat-unit accumulation (base 41 degrees F), January 1 to May 11, 1998.

Economic injury levels based upon different leaf stages of corn growth and different prices for corn have been published by Iowa State University. These are provided in Table 2 and are based upon control costs of $13 per acre and 80 percent control.

Leaf stage $2 perbushel $3 perbushel $4 perbushel

Mike Gray ( and Kevin Steffey (, Extension Entomology, (217)333-6652