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European Corn Borer: Old Pest, New Problems

Wilfrid Calvin, University of Minnesota Entomology, Postdoc Researcher
Tatum Dwyer, University of Minnesota Entomology, Master Student
Fei Yang, University of Minnesota Extension corn entomologist

What is European corn borer?

The European corn borer (ECB), Ostrinia nubilalis (Hubner), is a significant pest of corn and once caused substantial yield losses and economic damage across various corn-growing regions in the U.S., with damage and management costs estimated to exceed one billion U.S. dollars annually. In 2021, ECB alone was responsible for the losses of approximately 338.6 thousand bushels of corn in the U.S.

The presence of ECB in the U.S. dates back to its initial discovery near Boston, Massachusetts, during the summer of 1917. It is widely believed that ECB was introduced into the country from Europe through shipments of broomcorn. Since its introduction, the pest has rapidly spread to all major corn-growing regions in the U.S. and Canada. Evidence suggests that ECB made its way into Minnesota around the 1940s.

How to identify European corn borer

The female ECB moth has approximately 1-inch wingspan slightly longer than their male counterparts. Both female and male ECB moths have triangular wings. Females typically exhibit a color range from yellow to brown marked with wavy patterns on their wings, while males tend to have darker coloration (Figure 1).

Female ECB moths lay eggs in clusters of 20-30 eggs that resemble fish scales in appearance. Initially, the eggs appear whitish but darken to black prior to hatching (Figure 2). 

The larvae of ECB exhibit a range of coloration, spanning from light tan to pink, with each segment of their bodies with several dark brown spots (Figure 3). The head capsule of ECB larvae is dark brown. Upon hatching, ECB neonates typically measure between 1/32 to 1/16 inch in length, while mature larvae can reach lengths of approximately 3/4 to 1 inch. 

Examples of corn stalk tunnel damage and lodging caused by larval feeding are shown in Figures 4-6.

Management of European corn borer with Bt technologies in the U.S.

The use of Bacillus thuringiensis (Bt) proteins has proven to be a highly effective strategy in managing ECB infestations. Bt corn hybrids are the result of genetic modification, where genes from the soil-dwelling bacterium, B. thuringiensis, are incorporated, enabling the plant to produce proteins with insecticidal properties specifically targeting the desired insect pests. 

The adoption of Bt corn technology has resulted in areawide suppression of ECB in the U.S. This adoption has brought about several notable benefits, including a marked reduction in the need for insecticide applications aimed at combating pests, a decrease in the utilization of fossil fuels associated with pesticide application, and an overall increase in corn yields. Moreover, the adoption of Bt corn has not only been financially advantageous for farmers cultivating Bt corn but has also provided economic benefits to non-Bt corn growers due to the areawide suppression of ECB populations facilitated by the adoption of Bt corn hybrids. Currently, only Cry1Ab, Cry1F, Cry1 A.105, and Cry2Ab2 Bt proteins in the market are effective for ECB control. These different Cry Bt proteins are often utilized in combinations as "pyramids" in various corn hybrids to enhance ECB control.

Field-evolved resistance found in European corn borers in Canada: why we should care?

The high reliance on Bt crops has placed strong selection pressure on target pest populations for the evolution of resistance, the main threat to continued public benefits from this technology. In recent years, the effectiveness of Bt proteins targeting ECB has come under threat. In 2018, field-evolved resistance was identified in multiple field populations of ECB in Nova Scotia, Canada, more than two decades after the introduction of Bt technology targeting this pest. 

More recently, it was reported that resistance to Cry Bt proteins has expanded to several other regions in Canada, including Quebec and Manitoba. This situation has raised concerns regarding potential resistance to Cry Bt proteins in ECB in the U.S. Corn Belt. The proximity of Canada to the U.S. Corn Belt heightens the risk of migration of ECB strains resistant to Bt proteins from Canada to the U.S., where they could proliferate throughout the Corn Belt regions. 

Furthermore, weather patterns, such as wind, can serve as significant agents for the dispersal of resistant strains into the U.S. Indeed, instances of field-evolved resistance to Bt proteins have already been detected within the U.S. In 2023, a field of Bt sweet corn producing Cry1A.105 and Cry2Ab2 proteins in Connecticut experienced significant damage from ECB infestation. 

In addition, a conventional field corn in Crookston, Minnesota was found to be heavily infested with ECB larvae in 2023, with around 30-35% of 1000 surveyed plants exhibiting damage, a clear indication of an ECB "hot spot" in the field. Considering these, we are concerned that the emergence of Bt resistance in Canada and Connecticut poses a severe risk for further spread across U.S., potentially reaching Minnesota, and could accelerate the evolution of resistance to other Bt traits.

What can we do?

The high dose/refuge strategy has proven to be highly effective in delaying the development of resistance to Bt proteins in ECB populations in the U.S. Continued adherence to this strategy is crucial in maintaining susceptibility among ECB populations. Additionally, the adoption of multi-Bt proteins (also called Bt pyramids) corn hybrids is crucial for effective resistance management in this pest.

Proactive resistance monitoring is paramount in slowing the evolution of insect resistance, as early detection provides valuable insights for devising appropriate management strategies. Currently, our corn lab at the Department of Entomology, University of Minnesota is conducting surveys to assess susceptibility levels and Bt resistance allele frequencies in ECB populations collected from Minnesota and neighboring states. 

If you encounter ECB infestations in your fields, please reach out to us, and we can assist in determining the Bt resistance levels of these insects. Moreover, implementing integrated pest management (IPM) approaches targeting ECB is crucial. Cultural practices that foster the conservation of beneficial arthropods and enhance their population and diversity should help control ECB infestation. Additionally, practices such as plowing corn plant residue to a depth of at least 8 inches and shredding corn stalks can significantly reduce larval densities within fields, thereby aiding in the prevention of resistant ECB establishment.

Funding support

We appreciate the Minnesota Corn Grower Association, and Minnesota Corn Research and Promotion Council for their generous funding support for this project.

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