University of Illinois

No. 13/June 19, 1998

European Corn Borers Are in Full Swing

Heavy infestations of European corn borers are scattered throughout the state, just as early planted corn is scattered here and there. Because corn heights vary so much in many areas of Illinois, as a result of delayed planting and replanting, corn borer infestations are prevalent in early planted fields. We have received reports of infestations of 80 to 90 percent of the plants in fields in which corn is thigh- to waist-high. However, because so much of the corn is too short to encourage development of corn borers, infestations of first-generation corn borers are not widespread.

Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, has found corn borer larvae ranging in age from second to fifth (fully grown) instars, so stalk boring is under way in southern Illinois. Ron believes that several fields in southern counties might have benefitted from insecticide applications to control first-generation borers. Unfortunately, fields have been too wet to scout, so many folks may be unaware of economic infestations of corn borers. Kevin Black with Cargill reported a field with 90 percent infestation west of Peoria. Don Rhoads with Burrus Power Hybrids and Mike Roegge, Extension crop systems educator in Quincy, both reported that some fields in western Illinois have been treated for control of first-generation corn borers. In all of these instances, the infested fields were the earliest planted in the area.

With the initiation of stalk-boring activity in southern Illinois, treatment for economic infestations of first-generation corn borers would probably be futile. As soon as the borers tunnel into stalks, they are protected from contact insecticides. However, in central and northern counties, people are finding first, second, and third instars still feeding in whorl leaves. Figure 1, provided by Bob Scott with the Illinois State Water Survey, shows accumulated heat units above a base temperature of 50 degrees F from January 1 to June 15. By comparing these accumulated heat units with the accumulated heat units on May 18 (Figure 2, issue no. 9 of this Bulletin, May 22, 1998), we can estimate accumulated heat units from the time of initial moth flight (using May 18 as an "average" date of initial moth flight). In southern Illinois, we had accumulated about 550 to 600 heat units between May 18 and June 15, indicating that stalk boring is under way (refer to Table 2 in issue no. 11 of this Bulletin, June 5, 1998). In northern counties, we had accumulated about 400 to 450 heat units between May 18 and June 15, indicating that development from second to third instars is under way. Observations from the field indicate that corn borer development is right on track.

Figure 1. Actual heat-unit accumulations (base 50 degrees F), January 1 to June 15, 1998.

If economic infestations of first- generation corn borers are observed in central and northern counties, fields may still benefit from treatment with an insecticide. Refer to issue no. 11 (June 5, 1998) of this Bulletin for the management worksheet for first- generation corn borers and for a list of suggested insecticides.

Several people who have been able to scout fields, slogging through mud after heavy rainfall, have reported finding a lot of dead larvae or a lot of whorl feeding injury and no larvae. This drives home a point we have made many times. Although whorl feeding may be excessive, the absence of live larvae suggests that a treatment is not needed. This year, with all of the rainfall we have experienced, many young larvae have perished in whorls that filled up with water. Some research conducted a few years ago by W. B. Showers, formerly (now retired) with the USDA lab in Ames, Iowa, indicated that inundation of whorls with water could cause very high mortality. He inundated larvae with 1-1/8 inches of rain 0 to 2 days after egg hatch and 7/8 inch of rain 4 to 6 days after egg hatch. The level of mortality reached 87.5 percent by the fourth day after egg hatch and eventually climbed to almost 98 percent by 24 days after egg hatch (Table 13). Although the excessive rainfall has not been welcomed by many growers. Mother Nature has helped reduce the number of corn borers in many fields.

As indicated in a previous article in this Bulletin (issue no. 11, June 5, 1998), growers who planted Bt-corn should be monitoring for corn borer larvae. We have initiated our statewide survey of Bt-corn fields in central and northern Illinois. We will be looking for corn borer larvae that have survived on corn plants expressing the Bt endotoxin. Like you, we will also find corn borers surviving on plants that are not expressing the Bt endotoxin. However, we are interested in the former, not the latter. We will report our findings as soon as all of our data have been gathered.

Table 13. Percentage mortality of first-generation European corn borer larvae caused by inundation with water

Days after
egg hatch
Percentage larval
mortality per plant

With all of the late-planted corn in the state, we should prepare ourselves for second-generation corn borers within a month. However, if the first generation does not survive well, the second generation may not amount to much. Nevertheless, when the time comes, we'll have to stay alert.

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