No. 9 Article 4/May 26, 2006

European Corn Borer: A Secondary Insect Pest for Now?

Within the next few weeks, we will begin to receive reports that the spring flight of European corn borer moths is under way. In fact, Bob Wright, extension entomologist, University of Nebraska, indicated (May 23) that the first flight of this insect pest had begun in southeastern Nebraska. Have we reached a point where this insect has fallen from the list of primary insect pests of corn to being a secondary threat?

For 2006, once again we anticipate a very small flight of moths this spring across much of Illinois. The results of our fall survey revealed that only 24% of the state's corn plants were infested. This compares with an average infestation level of 49.2% from 1943 to 2005. The level of infestation in 2005 was 0.34 borer per plant. This is significantly below the historic average (1943-2005) of 1.15 borers per stalk.

Although we have had very low densities of European corn borers before the introduction of Bt hybrids in 1996, the trend over the past 10 growing seasons has been a steady decline in the severity of infestations. It seems reasonable to give more credit to Bt hybrids as a powerful areawide management tactic than many entomologists, including me, would have predicted a decade ago. In 2005, approximately 28.6 million acres (35% of U.S. corn acres) were planted to Bt hybrids (National Agricultural Statistics Service, USDA). In Illinois, roughly 30% (3.63 million acres) of corn acres were devoted to Bt hybrids last year. With the escalating interest in multi-event stacks (corn rootworm and European corn borer protection along with herbicide tolerance), transgenic hybrids will increasingly dominate the production landscape. To a large extent, western Corn Belt states are ahead of the curve with respect to Bt adoption as compared with eastern corn-producing states.

Thus far, no field-level resistance by European corn borers to Bt corn (Cry1Ab) has developed. In a recent paper (Journal of Economic Entomology 99: 494-501), some researchers at the University of Nebraska and University of Arizona suggested "that major genes for resistance to Cry1Ab were not common in the founding populations of O. nubilalis (European corn borers)." They further added, "A low initial frequency of major genes for Cry1Ab resistance might be an important factor in delaying evolution of resistance to Bt corn in this pest." These are encouraging findings and suggest that we may continue to rely on Bt hybrids for European corn borer control for many years to come. However, if we become complacent and ignore refuge deployment requirements, then resistance is seemingly more likely at some point in the nearer-term future. When selection pressure is increased in laboratory studies, high levels of resistance to the Cry1Ab protoxin have occurred in as few as 7 to 10 generations of this pest.


European corn borer action site, Piatt County, Illinois (May 23, 2006).

By early June, European corn borer moths will become more noticeable on our windshields in the evening, and during the day in grassy areas surrounding cornfields. These grassy areas are used as aggregation sites and serve as mating areas. Tom Sappington, a USDA-ARS entomologist located at Iowa State University, published a paper in Environmental Entomology (2005) that described the distribution of European corn borer moths in roadside vegetation (action sites) as related to cropping patterns and corn development. I believe he made some interesting findings, such as this one: "For the most part, spatial distribution of first-flight moths in the landscape was not related to the presence of corn stubble, as would be expected if newly emerged moths aggregated close to their natal field and remained there over time. Furthermore, the number of moths in ditches adjacent to minimum-tilled corn stubble declined over time in the absence of a new crop of corn. This pattern suggests that at least some moths emerging from a field may aggregate in the adjacent grass but that they redistribute themselves in the landscape over time."

The earliest-planted cornfields, particularly those with tall grassy ditches as borders, are the most likely candidates for egg laying by the first flight of European corn borer moths. Although the moths are not strong migratory fliers, they are capable of considerable dispersal across the landscape (15 to 50 miles) during their lives. If you've not planted a Bt hybrid for European corn borers and you were among the first to plant corn in your area, scouting for the first generation of this insect pest is still a good idea. Don't be lulled into complacency just because the densities have been lower over the past several years. Rescue treatments remain a management option.

References

Alves, A.P., T.A. Spencer, B.E. Tabashnik, and B.D. Siegfried. 2006. Inheritance of resistance to the Cry1Ab Bacillus thuringiensis toxin in Ostrinia nubilalis (Lepidoptera: Crambidae). Journal of Economic Entomology 99(2): 494-501.

Sappington, T.W. 2005. First-flight adult European corn borer (Lepidoptera: Crambidae) distribution in roadside vegetation relative to cropping patterns and corn phenology. Environmental Entomology 34(6): 1541-1548.

--Mike Gray

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