University of Illinois

No. 16/July 11, 1997

Southwestern Corn Borers in Southern Illinois

Ron Hines also reported some information about southwestern corn borers, a pest of corn only in the southern counties of the state. During his stalk-splitting evaluations at the Dixon Springs Agricultural Center, he found nearly full-grown larvae in the stalks. However, the numbers present this year are not nearly as large as they were last year, when several fields in southern Illinois had economic infestations. Ron anticipates the flight of adults to begin in a couple of weeks, marking the beginning of the second generation of this pest.

Because southwestern corn borer larvae can be confused with European corn borer larvae, a reminder of distinguishing characteristics and differences in biology is in order. This is most important for the second generation because southwestern corn borers cause distinctly different damage to stalks than European corn borers.

Southwestern corn borer females may deposit eggs singly or in small masses of up to five eggs on either the upper or lower surface of a corn leaf. The eggs are oval, flattened, and cream-colored when first deposited, and, if deposited in masses, overlap like the scales of a fish, much like the eggs of European corn borers. However, within 48 hours after the eggs are deposited, three pink to red transverse bars appear on the eggs.

Small southwestern corn borer larvae might be confused with European corn borer larvae. However, a southwestern corn borer larva appears to have indistinct bands across its body, from which hairlike setae arise. The bandlike appearance becomes more pronounced as the larvae grow, until the spots formed by the tubercles (small, knotlike bumps) become visibly distinct in the fourth instar. The last (fifth) instar has very distinct, large, dark tubercles. Like European corn borers, the last two instars tunnel in the stalk. If you have a good magnifying glass or microscope and a little patience, another characteristic also distinguishes the two species. On the bottom of the prolegs (the peglike false legs on a caterpillar's abdomen) of the southwestern corn borer, the tiny hooks (called crochets) form a complete circle. On a European corn borer larvae, the crochets on the bottom of the prolegs do not form a complete circle.

Whorl-feeding injury caused by southwestern corn borer larvae can be confused with injury caused by both European corn borers and fall armyworms. Early instar southwestern corn borer larvae "windowpane" the leaf tissue, much like small fall armyworms; however, the windowpaning usually is more extensive. Injury caused by larger southwestern corn borer larvae resembles whorl-feeding injury caused by European corn borers, except more leaf tissue is removed. The injury is less ragged looking than injury caused by fall armyworms, and southwestern corn borer larvae do not produce the dark, coarse frass produced by fall armyworms.

Southwestern corn borers have two generations per year in southern Illinois. The second generation of southwestern corn borers causes damage that is much more severe than damage caused by European corn borers. The larvae that will overwinter tunnel very low in the plant, girdling the stalk from within. Girdling greatly weakens the stalks, inevitably resulting in lodged corn.

For the second generation of southwestern corn borers, begin scouting about 2 weeks before corn tassels and continue for 2 weeks after pollination is complete. Look for egg masses and larvae on the leaves, or for larvae behind the leaf sheaths. A treatment may be justified when 25 percent of the plants have eggs, or larvae in the whorls or behind leaf sheaths.

Results from the western Corn Belt and from the Dixon Springs Agricultural Center indicate that Bt-corn provides effective control of southwestern corn borers. However, just as you should be observing the absence (or presence) of European corn borers in Bt-corn, keep your eyes peeled for signs of southwestern corn borers, too. We need to monitor whether this new technology continues to perform in the field as it has in numerous research trials.
Kevin Steffey, Extension Entomology, (217)333-6652