Common stalk borer damage in Bt corn fields

Yucheng Wang, University of Minnesota, Entomology, Postdoc Researcher
Tatum Dwyer, University of Minnesota, Entomology, Graduate student
Fei Yang, University of Minnesota, Extension Entomologist

Figure 1. Common stalk borer damage. Photo: Fei Yang,
University of Minnesota

During July-August 2025, we unexpectedly observed significant damage resembling European corn borer (ECB) injury along the edges of several corn fields in Martin and Mower Counties, Minnesota (Figure 1). 

Figure 2. Common stalk borer in corn stalk. Photo: Fei Yang,
University of Minnesota

Affected plants showed rows of holes on the leaves and boring damage in the stalks. However, dissection of the corn stalk revealed that the culprit was common stalk borer (CSB), Papaipema nebris (Guenée) (Lepidoptera: Noctuidae), rather than ECB (Figure 2). 

Figure 3. Bt trait testing of the affected plants. Photo: Fei Yang,
 University of Minnesota

Bt trait testing of the affected plants confirmed the expression of several Bt proteins, including Cry1F, Cry2A, Cry3Bb, and Cry34/35 (Figure 3). Among these, Cry1F and Cry2A target lepidopteran insects and are expected to suppress CSB infestations, whereas Cry3Bb, and Cry34/35 are only effective against corn rootworms.

Damage was concentrated in the first few rows next to grassy areas, while interior rows remained largely undamaged. In one field, approximately 60-70% of the plants along the first few rows, based on ~200 sampled plants, showed visible injury near the ditch. This edge pattern of injury is characteristic of the common stalk borer’s migration behavior from grassy weeds into corn early in the growing season. Further investigation is needed to determine whether the survival of CSB larvae in these Bt corn plants is resulting from the movement of large larvae or potential resistance to Bt proteins.

Basic biology of the common stalk borer

The common stalk borer is native to North American and completes one generation per year. Females lay eggs in late summer and early fall, typically on grasses and weeds along fencerows, waterways, contour strips, or weedy field margins. The insect overwinters in the egg stage on plants such as bromegrass, bluegrass, and ragweed, and eggs hatch in late spring. Young larvae feed inside grass stems and as they grow, or when weeds are mowed or killed by herbicides, they migrate into adjacent corn, typically infesting the first 4-6 rows along field edges. 

Larval feeding causes two main types of injury to corn, leaf feeding and stalk tunneling. Early feeding produces ragged holes in whorl leaves, while tunneling into the stalk can destroy the growing point, causing wilted centers or “deadheart” symptoms. Severely injured plants may become stunted, tiller excessively, or produce small or barren ears. Damage is most severe on young corn (V2-V5), and although tolerance increases after the V6 stage, infestations along grassy borders can still cause localized yield losses if larvae are not identified and managed promptly.

Identification

A key diagnostic feature for identifying the common stalk borer is the oblique black stripe running laterally across the orange head capsule through the ocelli (eyes). This head marking persists through all instars and is unique to P. nebris, making it a reliable characteristic for distinguishing it from other corn-feeding caterpillars such as the ECB or corn earworm. 

Young common stalk borer larvae are cream-colored with dark purple or brown longitudinal stripes on the thorax and abdomen (Figure 4c). A narrow cream dorsal stripe runs from behind the head to the anal plate, and a lateral cream stripe extends from the fourth abdominal segment to the anal plate. The third and fourth pairs of abdominal prolegs are pale or cream, contrasting with the darker body bands. The metathoracic segment and the first four abdominal segments are distinctly dark purple or brown except for the lighter dorsal stripe. 

As larvae mature, their coloration fades to a dirty white or light gray. Fully grown larvae measure 1.5-2 inches (38-50 mm) in length. The dorsal pronotal shield (just behind the head) is dark orange to brown with a narrow black lateral margin, and the body surface appears smooth and slightly glossy (Figure 4).

Figure 4. Life stages of the common stalk borer. a. adults; b. eggs; c. larvae; d. pupa. Photo: Fei Yang, University of Minnesota)

For more detailed information,  please visit Common stalk borer on the VegEdge website. 

References

Bailey, W. C., Buntin, G. D., Pedigo, L. P.. 1985. Phenology of the adult stalk borer, Papaipema nebris (Guenée) in Iowa. Environmental Entomology, 14: 267–271.

Binning, R. R., Rice, M. E. 2002. Effects of transgenic Bt corn on growth and development of the stalk borer Papaipema nebris (Lepidoptera: Noctuidae). Journal of Economic Entomology, 95(3), 622-627.

Dean, A., Hodgson, E. 2022. Stalk Borer. ISU Extension, Iowa State University. https://crops.extension.iastate.edu/encyclopedia/stalk-borer

Rice, M. E., Davis, P. 2010. Stalk borer (Lepidoptera: Noctuidae) ecology and integrated pest management in corn. Journal of Integrated Pest Management, 1(1), C1–C6.

Wright, R. J., Hunt, T. E., Jarvi, K. J. 2012. Common stalk borer in corn. UNL Extension, University of Nebraska. http://www.ianrpubs.unl.edu/pages/publicationD.jsp?publicationId=515