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How to identify it
- Summer larvae are white to off-white with conspicuous black or dark spots. Newly hatched larvae are reddish and take on the spotted look by the second or third stage. Full-grown larvae reach 1 to 1 1/4 inches long.[1][2][3]
- Larvae that settle in the stalk base for the winter lose their spots and look pale yellow to white.[1][2][3]
- The adult is a dull white moth with prominent, snoutlike mouthparts that rests on corn plants or nearby vegetation during the day.[1]
- Eggs are flattened and creamy or greenish white when laid, and within 24 to 36 hours they develop three red bars across them.[1][3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| European corn borer | Another stalk-boring caterpillar of corn with similar biology. Missouri Extension notes that the European corn borer has many more alternate host plants, while the southwestern corn borer is limited mostly to corn and a few grasses. Kentucky Extension adds that southwestern corn borer larvae do not wander through the stalk the way European corn borer larvae do, but tunnel in a straight line through the middle.[1][3] |
Where it lives
Extension services in Kansas, Missouri and Kentucky cover it as a corn pest. In Kansas the first generation is most active in the sandy regions of the south. In Kentucky it reappeared after more than ten years without reports and, since 1992, has spread across the western half of the state.[2][1][3]
Biology and life cycle
It spends the winter as a full-grown larva in a cell it builds near the base of the corn stalk, within about 6 inches of the soil surface, and pupates in the stalk.[1][2]
In Missouri, moths emerge and mate in mid to late May. The moths do not feed and live about five days, during which a female lays 100 to 400 eggs at night, singly or in small groups, mostly on the upper surface of leaves. Eggs hatch in about five days, and development from egg to adult takes four to six weeks.[1]
There are two or three generations a year in Missouri, and the second usually causes the greatest yield loss. In Kansas the first generation lays eggs in late June, and second-generation egg laying peaks in the first half of August, mostly on leaves around the ear.[1][2]
Corn is the preferred host. Milo (Sorghum bicolor) and johnsongrass are also attacked in Missouri, but they matter mostly as places where the borer builds up and overwinters.[1]
Damage and signs
- First-generation larvae feed inside the whorl for up to about two weeks, leaving shot holes in the leaves as they unfurl. If they kill the growing point, the plant becomes stunted and bushy, a condition called dead heart, though this is unusual once plants are more than 30 inches tall.[1][2]
- Second-generation larvae feed on leaves, ear shoots, husks, silks and kernels before tunneling into the stalk, usually below the ear. Stalk tunneling can stunt plants and reduce the nutrients reaching the ears.[1][2]
- From about September 1 into early October, many full-grown larvae cut a girdle around the inside of the stalk near the base, leaving only a thin outer shell. Girdled plants break in the wind and lodge, leading to large harvest losses.[2][1]
Health risk
It feeds only on plants. The Missouri, Kansas State and Kentucky fact sheets describe it as a crop pest and mention no risk to people or animals.[1][2][3]
Control
Prevention
Avoid planting late. Missouri Extension reports that corn planted before May 1 usually escapes serious girdling damage and generally needs no insecticide, while very late corn can be heavily infested by the second generation.[1][2]
Harvest early, before girdling begins; girdling usually climbs quickly after mid-September in Kansas. Planting a short-season hybrid early, or growing corn for silage, makes early harvest easier.[2][1]
Corn hybrids carrying Bt traits for European corn borer also control the southwestern corn borer, so scouting can focus on the non-Bt refuge rows.[2][3]
Sanitation
Fall tillage breaks up root stubble and exposes overwintering borers to natural enemies and winter cold. On hard land, plowing stubble under at least 5 inches deep buries larvae and pupae so most moths cannot emerge in spring.[2]
Pesticides
After moths show up in light or pheromone traps, scout weekly. Missouri's threshold is 25 percent of plants with egg masses, or larvae with whorl damage, with the spray timed about a week before larvae bore into stalks; Kansas State suggests treating susceptible hybrids when 20 to 25 percent of plants carry eggs or newly hatched larvae. Missouri stresses early detection, because a larger plant canopy gets in the way of insecticide control.[1][2]
Common questions
- Why are my corn plants breaking off at the base in late summer?
- One cause is the southwestern corn borer. Its larvae girdle the inside of the stalk near the base as they settle in for winter, usually from early September into October. The weakened stalks snap in the wind and the plants lodge.[2][1]
- Does Bt corn control the southwestern corn borer?
- Yes. Hybrids with Bt traits aimed at the European corn borer also control the southwestern corn borer, which leaves the refuge rows as the main place to scout.[2][3]
- When should I look for southwestern corn borer in my corn?
- Start once moths are caught in traps, usually late May or early June in Missouri, and check weekly while plants are in the whorl stage and again from tasseling on. Larvae feed in the whorl for about 11 to 14 days before boring into the stalk.[1]
Sources
- University of Missouri Extension, “Southwestern Corn Borer Management in Missouri, G7111 (M. L. Boyd and W. C. Bailey; revised by K. Rice, September 2017).” extension.missouri.edu/publications/g7111 (accessed 2026-11-08)
- Kansas State University Department of Entomology, “Southwestern Corn Borer (Facts and Information on Crop Pests).” entomology.k-state.edu/extension/crop-protection/corn/swcb.html (accessed 2026-11-08)
- University of Kentucky Entomology, “Southwestern Corn Borer, ENTFACT-108 (R. Bessin).” entomology.ca.uky.edu/files/ef108.pdf (accessed 2026-11-08)
- iNaturalist, “Diatraea grandiosella taxon page (order and family).” www.inaturalist.org/taxa/554220 (accessed 2026-11-08)