University of Illinois

No. 17/July 16, 1998

Southwestern Corn Borers Could Become a Problem in Southern Counties

According to Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, the numbers of adult southwestern corn borers captured in blacklight traps have "exploded" in the past 1 to 2 weeks. Traps in Massac, Pulaski, and Pope counties have captured 200 to 800 moths on certain nights, suggesting that densities of larvae of this southern Illinois pest could be impressive this summer. Now is the time to watch for these insects because this generation of southwestern corn borers causes the most economic damage.

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 larva, the crochets on the bottom of the prolegs do not form a complete circle.

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, look for egg masses and larvae on the leaves or 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 or not this new technology continues to perform in the field as it has in numerous research trials.

Kevin Steffey (, Extension Entomology, (217)333-6652