In issue no. 23 (October 4, 2002) of the Bulletin, we provided a thumbnail sketch of our annual fall survey for second-generation European corn borers. At that time, extension entomologists and extension educators were in the midst of sampling cornfields in all crop-reporting districts in the state. We have completed our task, analyzed the data, and tabulated our results for your reference. We sampled 10 fields in each of 40 counties in 2002.|
Table 1 shows the percentages of plants infested and average numbers of European corn borers per plant for all counties sampled in 2001 and 2002. Table 2 shows the percentages of plants infested and average numbers of European corn borers per plant for all crop-reporting districts sampled in 2001 and 2002. The data from 2001 are provided for comparison with results from the 2002 survey. However, because some counties within certain crop-reporting districts were combined for regional averages in 2001, the averages are not exactly comparable.
A glance at the county data (Table 1) reveals that densities of European corn borers were largest in a band from southwestern Illinois (Monroe County, 3.01 borers per plant) to central Illinois (McLean County, 1.43 borers per plant). Other counties with an average of more than 1 European corn borer per plant were Adams (1.19 borers per plant), Christian (1.03 borers per plant), Effingham (2.29 borers per plant), Logan (2.74 borers per plant), Madison (2.6 borers per plant), Morgan (1.41 borers per plant), and Washington (1.62 borers per plant). In general, the counties in eastern Illinois had very low densities of European corn borers.
The most dramatic decrease of average numbers of European corn borers per plant from 2001 to 2002 occurred in the Northwest and Northeast crop-reporting districts (Table 2). The most dramatic increase in average numbers of corn borers per plant from 2001 to 2002 occurred in the West-Southwest and Southwest crop-reporting districts.
A review of the raw data strongly suggests that we should present the data from our surveys in additional ways. Most surveyors indicated that infestations of European corn borers were either absent or relatively heavy in many areas of the state. It is not unusual for data from a county to include a range of 0 to 100% infested plants with 0 to 3 borers per plant. Although we do not know what type of corn hybrid we are sampling during the survey, it is apparent that we are sampling both Bt cornfields and non-Bt cornfields. Consequently, the averages for a given county or crop-reporting district may not tell the whole story.
For example, in Bureau County in 2002, the percentages of plants infested in 10 fields were 0 to 4% (four fields), 24 to 36% (two fields), and >75% (four fields). Densities of corn borers ranged from 0 to 2.88 borers per plant. The average percentage of plants infested and average number of corn borers per plant were 88.6% and 0.89, respectively. Because averages do not tell the whole story for a county or crop-reporting district, we plan to develop frequency distributions for both percentage of plants infested and average number of corn borers per plant to present the data in a different light.
In the near future, watch for the availability of results from annual fall surveys of European corn borers in Illinois from 1943 through the present year. The Web site will be ready soon.--Kevin Steffey and Mike Gray