MN Fruit & Vegetable IPM News - Vol. 5, No. 4

Many Midwestern Entomologists and Crop Consultants have noted, particularly in the past 3-4 years, relatively low numbers of European corn borer (ECB), where Bt field corn, expressing Cry proteins toxic to ECB (e.g., MON 810, BT-11 or Cry1F events), has been widely adopted (e.g., Steffey and Gray, 2007). As we anticipate the emergence of 1st generation ECB moths this spring in Minnesota, and fine-tune IPM programs for sweet corn, we have been reviewing the current risk of ECB in light of Bt field corn use. Although our analysis deals primarily with documenting the direct benefits to field corn, it has become clear that similar benefits of an “area-wide suppression effect” on ECB also appear to be reaping benefits to sweet corn growers. In this article, we summarize 13 years of data (1995-2007), for Rosemount, MN (Dakota Co.) where we annually conduct a variety of IPM research trials against ECB.

The annual data for ECB found in untreated, non-Bt ears in various sweet corn plots, from 1995-2007, is summarized in Fig. 1, based on the Bt field corn use rates (mean % state Bt use rates provided by the National Agric. Statistics Service, as of 2007). The ECB data also reflect the late-season 2^nd generation larval populations, as all sweet corn was harvested between August 22^nd and Sept. 26^th (all years combined). Both average densities/ear and percentage of ears infested, at harvest, are shown. Although many factors are known to affect long-term population dynamics of ECB, including several beneficial, natural enemies (e.g., predators, parasitoids), and weather extremes, the results in Fig. 1 strongly suggest that, over time, a high percentage of the variation in ECB infestations (95-97%) can be explained by Bt field corn use. This relationship is much stronger than we had anticipated. Most of the high Bt use years (>40% state avg.), have occurred since 2002 in Minnesota. Once Bt use data are available for the East-central and South-eastern regions of MN, more detailed analyses can be conducted for this region.

What does this mean for Sweet Corn Growers?
Based on the % ear infestations for the past 5 years (2003-2007), we can use the upper limit of 95% Confidence Intervals (CIs), as one measure of risk in 2008 (there are many other methods available as well). For the past 5 years, the 95%CIs ranged from a low of 5 to 14.3%. Given previous insecticide efficacy research in sweet corn, and such low levels of likely infestation, the results indicate that only 1 or 2 insecticide sprays/ac will be necessary to provide 99-100% ECB control (e.g., use of pyrethroids). Based on previous work, this indicates that this level of control provides a high probability of maintaining maximum ear infestations at <5%, the preferred maximum tolerance by fresh-market growers and processors (B. Flood, Del Monte Foods, personal communication). Prior to 2003, most growers applied a minimum of 4 sprays/ac to protect late-season sweet corn from ECB infestations. Given current costs (pyrethroid insecticides, application costs), and assuming a 50% reduction in sprays (2 total), this reflects a savings of ca. $16/ac, or $1.44 million/year for the state of Minnesota, with ca. 90,000 acres at risk each year (2/3 of total acres).

Fig. 1. Relationship between sweet corn ear infestations with European corn borer (ECB), Ostrinia nubilalis, in untreated, non-Bt hybrids, near Rosemount, MN, and percentage of Bt field corn use in MN (state average); (A) mean number of ECB larvae per ear: (B) % ears infested with ECB. Mean ear infestation data were fitted with a logistic dose response model (WDH, unpublished). Adjusted R2 values = 0.97 and 0.95, for (A) and (B), respectively (Bt corn use statistics, NASS, 2007).

One caveat:
The results reported here are most appropriate where the traditional bivoltine ECB strain is active; i.e., with the bivoltine, Bt field corn has a cumulative effect each year on both generations. However, in some areas of southern Minnesota, and historically in northwestern Minnesota, we also have the univoltine strain of ECB, that thus far, appears to be less effected over time, by Bt field corn. This analysis continues. If the cool weather continues this summer, this may also favor the univoltine strain (given our experience with cool years in 1992-1993). Thus, growers in these areas, particularly with high univoltine flights in 2007 should continue to scout sweet corn carefully during the July flight period.

Selected References:
K. Steffey & M. Gray. 2007. Is the European corn borer an endangered species? The Bulletin, University of Illinois. No. 24 Article 3/Nov. 9, 2007. On-line: http://www.ipm.uiuc.edu/bulletin/article.php?id=865

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison (hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (woldx018@umn.edu), Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jeanne Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us , MDA, 625 Robert St. North, St. Paul, MN 55155. You can access the Newsletter at the U of MN web site in htm format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf format at: www.mda.state.mn.us/plants/pestmanagement/ipm/ipmnews.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association (MFVGA) and the United States Department of Agriculture – Risk Management Agency (RMA). These institutions are equal opportunity providers.

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Last Revised May, 2008 by woldx018@umn.edu
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