The annual fall survey showed the average European corn borer population in the state to be 0.40 borer per plant (40 borers per 100 plants). This compares to 0.10 in 2004 and a 50-year average of 0.49. Increases occurred in every district except the northwest, a probable outcome given last fall’s record-low population. The most substantial increases were noted in the southwest, south central and southeast districts where populations rose from 0.10 in 2004 to 0.49, 0.05 in 2004 to 0.67, and 0.02 in 2004 to 0.35, respectively. Approximately 87% of the corn fields surveyed had larval populations below 1.0 borer per plant (182 of 210 fields), while 13% of the corn fields had high larval populations, ranging from 1.0-3.5 borers per plant (28 of 210). As a reminder, a corn borer population of 1.0 borer per plant is economically important, having been shown to reduce yield by as much as 5% during the first generation, and 2.5% by the second generation.

A statewide average of 0.40 borer per plant suggests a light to moderate first flight of corn borer moths should be anticipated next spring. What follows the first flight, an increase or decrease in corn borer densities, depends on factors such as activity of natural enemies and weather conditions during May and June.