As I have stated previously, European corn borers have been conspicuous by their absence this summer. I recently returned from a series of seminars for dealers in Illinois, Ohio, Michigan, Iowa, and Missouri, and the story is the same throughout most of the Corn Belt. However, we recently received a handful of reports of increases in numbers of corn borer moths either captured in traps or observed flying at night. Rick Weinzierl, Extension entomologist in the Department of Crop Sciences, informed us that a light trap in northeastern Mason County captured 123, 342, 191, and 207 corn borer moths on four consecutive nights during the latter part of the week of August 17. John Shaw, senior research specialist with the Illinois Natural History Survey, also reported an increase in moth captures in his light trap here in Champaign County during that same week. Some folks in attendance at the field day held at the Northwestern Agricultural Research and Demonstration Center near Monmouth (Warren County) told me that they have seen "many moths" flying around at night. And Randy McElroy with DeKalb Genetics told me on August 26 that he, too, had been observing large numbers of corn borer moths flying at night in counties south of I-70.
We are not certain what these late moth flights mean, but they are worth noting. In southern Illinois, flights of European corn borer adults at this time of year usually represent the beginning of a third generation. However, third generations of corn borers in central and northwestern Illinois are not common. Are we witnessing a later-than-normal moth flight this year? We can only speculate an answer to that question, and only time will tell whether corn borers complete their development before declining temperatures and light:dark ratios trigger diapause. Remember, corn borer larvae must reach maturity (fifth instar) to enter diapause and survive the winter. If the moths that are flying now lay eggs, some of the larvae in some areas of Illinois likely will not reach maturity and, thus, will not survive the winter.
A comment about Bt-corn would be appropriate at this time. Many growers who purchased and planted Bt-corn this year will wonder whether the investment was worth it. After all, growers get no return on their investment for Bt-corn when densities of corn borers are subeconomic, as they were in 1998. Consequently, some people may decide that Bt-corn for management of corn borers in 1999 is not necessary. However, we try to impress upon growers that using Bt-corn for management of corn borers should be a long-term investment, not a short-sighted tactic used or not used one year based upon what happened or didn’t happen the year before.
The correlation between densities of corn borers one year and densities the next year usually is not good. Figure 1 shows the average number of corn borer larvae per stalk assessed during annual fall surveys of corn fields from 1987 through 1996. During this 10-year period, average densities of second-generation corn borer larvae ranged from 0.3 per stalk (1992) to 3.48 per stalk (1989). On more than one occasion, years when densities of corn borer larvae were large (e.g., 1989, 1991) were followed by years when densities were relatively low (e.g., 1990, 1992). The reverse was also true: some years when densities of corn borer larvae were low (e.g., 1988) were followed by years when densities were very large (e.g., 1989). Considering this information, one must ask the question Will corn borer densities be small or large in 1999? Obviously we will not know the answer to the question for many months.
The point I am making is that the decision to use or not use Bt-corn to manage European corn borers cannot be based upon short-term population fluctuations. In areas where economic infestations of corn borers are relatively frequent (e.g., 7 or 8 years out of 10), Bt-corn is wise investment for management of corn borers. In areas where economic infestations of corn borers are relatively infrequent (e.g., 2 or 3 years out of 10), growers should really question whether purchasing and planting Bt-corn is a wise investment.
A final note: Growers who purchase Bt-corn and will plant it in 1999 should consider how they will implement their resistance management plan, primarily the extent and placement of non-Bt-corn refuge. As Bt-corn gets planted on more and more acres, concern about the development of corn borers resistant to the Bt endo-toxin increases. We’ll offer more details in a future issue of the Bulletin, but we want to plant the seed (pardon the pun) of resistance management now so growers will begin thinking about it seriously.
Figure 1. Densities of European corn borer larvae (ECB/stalk) assessed during fall surveys of corn fields in Illinois, 1987-1996.