Southwestern Corn Borer: Small Signs Can Signal Serious Stalk Damage

In hybrids with effective Bt protection against southwestern corn borer (SCB), the first pinholes and elongated holes in the whorl may not develop into serious damage. In corn without that protection, however, those signs can serve as a warning of larger problems later in the season.

As SCB larvae mature, they leave the whorl and tunnel into the stalk, where feeding becomes harder to see and manage. Tunneling can interfere with water and nutrient movement. Larvae preparing to overwinter may also girdle the stalk near its base, increasing the risk of lodging before harvest. 

Knowing how damage changes throughout the season can help in understanding the risk, the importance of timely scouting and good management options.

Know where and when pressure builds 

SCB is primarily a pest of southern and south-central corn-growing areas, with its range extending into parts of the central Plains and lower Midwest. Depending on latitude and elevation, two to three generations may develop in one growing season. Warmer areas may face a longer period of activity.

Dull white adult moths begin appearing in late May or early June in many areas. Females lay eggs singly or in groups of three to five on upper leaf surfaces of young corn plants. Later generations place many eggs near or below the ear zone. The second generation, which may hatch during August, often causes the most serious stalk damage.¹

After eggs hatch, young larvae feed inside the whorl. Their feeding creates pinholes, shot holes or elongated areas of damaged tissue that appear as leaves unfold. Severe feeding may destroy the growing point and cause “dead heart,” leaving plants stunted and bushy.²

The third instars bore into the stalk and become much harder to find. Damage may not even appear until harvest approaches. Once inside the stalk, they tunnel through plant tissue. Later in the season, larvae preparing to overwinter may move toward the base of the plant and girdle the stalk from within. That shift can turn modest-looking leaf injury into weaker stalks and a greater risk of lodging before harvest if left unchecked.  Knowing which generation is active can help growers understand what symptoms to look for and how quickly the management window may be closing.

Damage progression

1. Pinholes and elongated leaf injury 
2. Tunneling inside the stalk 
3. Girdling near the plant base 
4. Lodging and harvest loss 

What growers should look for 

Calendar dates provide a starting point for scouting, but weather and location can shift moth flights. Regional pheromone trap and light trap reports provide a better signal that adults are active nearby. Growers can use those reports to determine when field scouting should begin.

During the whorl stage, scouts should look for eggs, windowpane feeding, rows of holes in unfolded leaves, dead heart and frass near stalk entry points. Finding eggs, fresh injury or live larvae confirms that the management window remains open. 

Scouting should continue after tasseling. Later-generation larvae may feed on leaf sheaths, husks, shanks, silks or kernels before entering the stalk. Inspect leaves around the ear, look beneath lower leaf sheaths and check the lower stalk for frass or fresh entry holes.

Economic thresholds will vary by state and region, so make sure to check with your local extension service for exact thresholds. However, most recommendations focus on the percentage of plants with egg masses, newly hatched larvae or fresh feeding injury while the larvae are still exposed, which is only around 25% of plants inspected for many states.³

Stalk injury drives yield and harvest loss 

In 2025, SCB caused an estimated 2.16 million bushels of corn loss in the United States. It ranked as the second-most-damaging invertebrate pest across the southern production region, from Oklahoma to Virginia.⁴  

The greatest losses occur after larvae enter the stalk. Tunneling may limit water and nutrient movement, reduce ear development and lower yield potential. Girdling near the plant base also makes stalks more likely to break during wind, storms or harvest. Lodged corn can slow harvest, increase mechanical issues and leave ears on the ground.

Use multiple tactics to reduce pressure 

SCB management begins before larvae enter the stalk. Managing johnsongrass, volunteer sorghum and other grassy hosts can reduce reservoirs that support local populations. Planting date can also influence risk. Late-planted corn may remain attractive to later moth flights, although optimum planting recommendations vary across the southern Corn Belt. Corn planted early enough can escape girdling damage, and insecticide applications may not be needed.¹

Residue management can reduce overwintering survival. SCB spends the winter near the base of corn stalks, where weather, predators and disease kill many larvae. Even so, about 2% survival in a heavily infested area may be enough for populations to recover by the second generation.3 Where tillage fits the farm’s soil-conservation plan, shredding, disking or plowing residue can expose or bury larvae. Earlier harvest may also reduce losses in heavily infested fields.

When insecticides are needed, timing is critical. Applications must reach larvae while they are still feeding outside the stalk. Growers should follow local thresholds and product labels rather than rely on a fixed calendar date.

Build protection into the hybrid 

Hybrids with effective Bt traits provide season-long protection against susceptible SCB larvae as they feed on corn tissue. Unlike foliar insecticides, Bt proteins are produced within the plant throughout the growing season. This means susceptible larvae can remain exposed to the proteins even after they begin feeding inside the stalk.⁵

Bt traits should be part of a broader integrated pest management and insect resistance management plan. That plan should include scouting, local recommendations, cultural controls and all required refuge and stewardship practices. 

When discussing hybrid options with customers, licensees can point to PowerCore^® Enlist^® corn, which contains three distinct Bt proteins and provides three modes of action against susceptible SCB. It also provides three modes of action against European corn borer and fall armyworm. Multiple effective modes of action can help preserve trait durability when paired with required stewardship practices. 

Treat early symptoms as a warning 

A few holes in an unfolding leaf may be the first visible sign of SCB, but they don’t tell the whole story. As the season advances, larvae can move into the stalk and weaken the plant near its base. Recognizing that progression, scouting at the right time, and choosing hybrids with multiple modes of protection can help growers minimize the risk of SCB damaging their crops.

¹ Michaud, J. P., “Southwestern Corn Borer,” Kansas State University, 2 April 2024, https://entomology.k-state.edu/extension/crop-protection/corn/swcb.html.
² Michael L. Boyd and Wayne C. Bailey, “Southwestern Corn Borer Management in Missouri,” MU Extension, University of Missouri, March 2001, https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pub/pdf/agguides/pests/g07111.pdf.
³ G. Michael Chippendale and Clyde E. Sorenson, “Biology and Management of the Southwestern Corn Borer,” Radcliffe’s IPM World Textbook, University of Minnesota, May 1997, https://ipmworld.umn.edu/chippendale.
⁴ Dominic Reisig et al., “Corn Invertebrate Loss Estimates from the United States and Ontario, Canada—2025,” Crop Protection Network, March 9, 2026, https://doi.org/10.31274/cpn-20260309-0.
⁵ United States Environmental Protection Agency, “Insect Resistance Management for Bt Plant-Incorporated Protectants,” March 11, 2026, https://www.epa.gov/regulation-biotechnology-under-tsca-and-fifra/insect-resistance-management-bt-plant-incorporated.

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