Fei Yang, University of Minnesota Extension corn entomologist, Yucheng Wang, University of Minnesota Dept. of Entomology, Postdoc Researcher
The European corn borer (ECB), Ostrinia nubilalis (Hubner), has historically been one of the most economically important insect pests of corn in Minnesota. With the introduction of transgenic Bt corn in 1996, ECB populations have been significantly suppressed across the state for almost three decades. Recently, however, farmers have planted more acres with corn hybrids without Bt traits targeting ECB. In some areas of Minnesota, ECB populations have reached economic threshold levels in a small number of non-Bt corn fields.
During the first week of October 2025, we conducted the ECB statewide field survey and observed some feeding damage in one Bt corn plant. Upon revisiting the field, we identified a second Bt plant with ECB damage. This represents the first report of ECB damage in two Bt corn plants in Minnesota (Picture 1). Both plants tested positive for Cry1A and Cry2A proteins based on the Bt test strips (Picture 1). Plant A with a rounded hole in the stalk with a tunnel approximately 1.5 cm long (Picture 1A). Plant B had a tunnel approximately 4 cm long inside the stalk. Unfortunately, no larvae were recovered from these two Bt plants (Picture 1B). We examined additional plants in the same field and found six more damaged plants with an estimate of >10 cm tunnel damage. Bt expression tests indicated these were refuge non-Bt plants, and we recovered two ECB larvae with one located in the middle of a corn ear (Pictures 2 and 3), while no larvae were recovered from the Bt plants.
Previously, ECB resistance to the Cry1F Bt toxin was confirmed in Nova Scotia, Canada in 2018. Then, field and early-warning resistance to other Cry1 Bt toxins (e.g., Cry1Ab, Cry1A.105, and Cry1F) has been confirmed in several other regions in Canada, including Quebec and Manitoba during 2019-2022. In 2023-2025, ECB damage on Cry1Ab, Cry1A.105 and Cry2Ab2 Bt corn traits were confirmed in Connecticut. These cases suggest ECB are developing resistance in the fields.
Timely resistance monitoring is important for detecting and managing potentially resistant ECB. Early detection provides valuable information that is needed to devise appropriate management strategies for resistant insects. To determine if ECB is developing resistance in your area, we recommend scouting and collecting ECB larvae or eggs for Bt toxin bioassays. If you observe any ECB infestation in your corn fields, please contact Fei Yang (yang8905@umn.edu). We are interested in checking the damage, testing the Bt expression, and collecting insects for testing their survival on Bt proteins.
Potter, B. 2020. European corn borer in Minnesota. University of Minesota Extension publication. https://extension.umn.edu/corn-pest-management/european-corn-borer-minnesota-field-corn
Calvin, W., Dwyer, T., Yang, F. 2024. European corn borer: Old pest, new problems. https://blog-crop-news.extension.umn.edu/2024/02/european-corn-borer-new-pest-old.html
Smith, J. L., and Y. Farhan. 2023. Monitoring resistance of Ostrinia nubilalis (Lepidoptera: Crambidae) in Canada to Cry toxins produced by Bt corn. J. Econom. Entomol. 116: 916-926.
The European corn borer (ECB), Ostrinia nubilalis (Hubner), has historically been one of the most economically important insect pests of corn in Minnesota. With the introduction of transgenic Bt corn in 1996, ECB populations have been significantly suppressed across the state for almost three decades. Recently, however, farmers have planted more acres with corn hybrids without Bt traits targeting ECB. In some areas of Minnesota, ECB populations have reached economic threshold levels in a small number of non-Bt corn fields.
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| Photo 1. Tunnel damage of ECB in two Bt corn plants and Bt expression test |
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| Photo 2. Presence of ECB and ear damage of ECB in non-Bt corn ears. |
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| Photo 3. Stalk damage of ECB in non-Bt corn plant. |
Timely resistance monitoring is important for detecting and managing potentially resistant ECB. Early detection provides valuable information that is needed to devise appropriate management strategies for resistant insects. To determine if ECB is developing resistance in your area, we recommend scouting and collecting ECB larvae or eggs for Bt toxin bioassays. If you observe any ECB infestation in your corn fields, please contact Fei Yang (yang8905@umn.edu). We are interested in checking the damage, testing the Bt expression, and collecting insects for testing their survival on Bt proteins.
Funding support
We appreciate the Minnesota Corn Grower Association, and Minnesota Corn Research and Promotion Council for their generous funding support for this project.References
Hutchinson, W. D., E. C. Burkness, P. D. Mitchell, R. D. Moon, T. W. Leslie, S. J. Fleischer, M. Abrahamson, K. L. Hamilton, K. L. Steffey, M. E. Gray, R. L. Hellmich, L. V. Kaster, T. E. Hunt, R. J. Wright, K. Pecinovsky, T. L. Rabaey, B. R. Flood, and E. S. Raun. 2010. Areawide Suppression of European corn borer with Bt Maize Reaps Savings to Non-Bt Maize Growers. Science. 330: 222-225.Potter, B. 2020. European corn borer in Minnesota. University of Minesota Extension publication. https://extension.umn.edu/corn-pest-management/european-corn-borer-minnesota-field-corn
Calvin, W., Dwyer, T., Yang, F. 2024. European corn borer: Old pest, new problems. https://blog-crop-news.extension.umn.edu/2024/02/european-corn-borer-new-pest-old.html
Smith, J. L., and Y. Farhan. 2023. Monitoring resistance of Ostrinia nubilalis (Lepidoptera: Crambidae) in Canada to Cry toxins produced by Bt corn. J. Econom. Entomol. 116: 916-926.
I will remember the recommendations from academic entomologists at the time (I did regulatory for Sandoz BT11 maize) that a refuge of 40% would be necessary to delay the development of ECB resistance and, with the allowance of only 5% refuge by regulators, that Bt maize resistance to ECB would only last 3 yrs. As you note this was 30 yrs ago.
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