Issue No. 24, Article 6/November 7, 2008
Survey for Second-Generation European Corn Borer Larvae, Illinois, 2008 (REVISED)
As most of our readers know, entomologists at the University of Illinois have coordinated a survey for second-generation European corn borers annually (except in 1997 and 1998) since 1943. We decided in 1997 and 1998 not to conduct the survey (now much to our regret) because we were uncertain about how to deal with data from cornfields with hybrids that expressed the Bt trait for control of European corn borers and other caterpillars. Nonetheless, we resumed the survey in 2000 and have conducted it annually since. We question every year whether we should continue because our financial and human resources are stretched very thin, but as the data have arrived in our offices this year, we have been grateful to have continued. It has become quite apparent that the data we have been gathering has become historic, both literally and figuratively.
First, a refresher about how we conduct the annual survey. The survey is a cooperative effort among University of Illinois extension entomologists, extension educators, county directors, professional staff, graduate students, and undergraduate employees. There is no way such a large undertaking could be completed without the voluntary efforts of a lot of people. And for all of their efforts, made in addition to their "day job" responsibilities, we sincerely thank everyone involved for their unselfish cooperation.
The 2008 survey has been completed, all data have been compiled and double-checked, and means have been determined. However, it is always possible that some minor mistakes were made and that data will have to be altered slightly when such mistakes, if they occurred, are discovered. We will provide updates and corrections through "Alerts."
In 2008, 504 cornfields in 51 counties representing all nine Illinois Crop Reporting Districts in Illinois were sampled. Except where noted in Table 1, 10 randomly selected fields were sampled in each county. In each field, 25 consecutive corn plants located a reasonable distance from the field edge were examined for signs of infestation by second-generation European corn borer larvae (e.g., borer entry holes, frass). When possible, two of the infested plants were split and examined for European corn borer larvae, which are recorded by instar and number. From these data, we estimate the percentage of plants infested and the average number of European corn borer larvae per 100 plants and per plant, depending on how we wish to display the data. The numbers of counties and total numbers of fields surveyed have changed over time, but the technique has remained essentially unchanged. The data collection protocol within a given field has been essentially the same for 66 years, allowing us to compare European corn borer population dynamics over time.
Before we present the data and our interpretations, we restate for purposes of comparison a paragraph from "Is the European Corn Borer an Endangered Species?" (tongue in cheek, by the way) published in issue No. 24 of the Bulletin a year ago (November 9, 2007):
"The statewide average density of second-generation European corn borers in Illinois in 2007 was 13.4 larvae per 100 plants, the lowest density we have ever determined from these annual surveys, which began in 1943 (Figure 1). The next-lowest density (about 16 borers per 100 plants) determined from these surveys occurred in 2004. In the 63 years that the survey has been conducted (it was not conducted in 1997 and 1998), the average density of corn borers has been less than 0.35 larva per plant in 9 years . . . six times during the past 9 years, including the past 4 four years. These data suggest that Bt corn has become a major mortality factor in populations of European corn borers."
Now for the 2008 data--and some of the information is a bit startling. The statewide average density of second-generation European corn borers in Illinois in 2008 was 9.61 larvae per 100 plants, or 0.0903 larva per plant, the lowest density we have ever determined from these annual surveys, with the density from 2007 being the second lowest (Figure 1). For the state, only about 13% of the 504 corn plants examined were infested with second-generation European corn borer larvae. However, other numbers are very telling, too. Zero second-generation European corn borer larvae were found in 17 of the 51 counties surveyed, likely unprecedented in Illinois. Even more compelling, in our opinion, are data that are not included in Table 1:
- There were zero European corn borer larvae found in 438 of 504 fields, a shocking 87% of the fields surveyed.
- Only 30 fields (~6% of the fields surveyed) had more than 50% infestation, with only 13 fields with more than 90% infestation (mostly in counties in the West Southwest, Southwest, and Southeast Crop Reporting Districts).
Figure 1. Average number of European corn borer larvae per plant determined from annual fall surveys, 1943-2008, Illinois.
It is apparent from our data that the largest densities of second-generation European corn borers, such as they were, occurred in western and southern Illinois in 2008. Northwestern Illinois, historically a haven for European corn borers, had among the lowest densities.
The impact of transgenic Bt corn on European corn borer populations in Illinois has been dramatic. However, we suspect that in 2008 the weather also played a role in reducing European corn borer densities in Illinois. As you may recall, we reported in previous issues of the Bulletin this year that reports of infestations of first-generation European corn borers were more frequent than they had been in 2007. Shortly thereafter, heavy rainfall events in some areas of Illinois likely caused mortality among the European corn borer moths, contributing to the overall mortality of European corn borer populations caused by Bt toxins.--Kevin Steffey and Mike Gray