University of Illinois

No. 20/August 8, 1997

Prepare for Second Generation of Southwestern Corn Borers in Southern Illinois

Although European corn borers have not made headlines thus far this year, we have experienced another "invasion" of southwestern corn borers in southern Illinois. Although this insect usually does not infest cornfields much farther north than I-64, their presence in southern counties bears acknowledgment. Ron Hines, senior research specialist at the Dixon Springs Ag Center, has captured several southwestern corn borer adults in light traps over the past 2 to 3 weeks, and the numbers increased steadily during that period. Corn growers in southern counties need to scout for this insect because the injury it causes can result in significant yield losses.

The second generation of southwestern corn borers causes damage much more severe than that caused by European corn borers. Larvae that will overwinter tunnel very low in the plant, girdling the stalk from within.

A southwestern corn borer larva has girdled a corn stalk and prepares to overwinter.

Girdling greatly weakens the stalks, which usually results in lodged corn.

Although we usually suggest that scouting for second-generation southwestern corn borers begin about 2 weeks before corn tassels and continue for 2 weeks after pollination is complete, recent moth flights suggest that scouting is still worthwhile. Look for egg masses and larvae on the leaves or for larvae behind the leaf sheaths. Treatment may be justified when 25 percent of the plants have eggs, or larvae behind leaf sheaths. Eggs are yellow-green when first deposited, but within 36 hours they become white, with three broken orange-red lines across each egg. Females deposit eggs in masses of two or three on upper and lower surfaces of corn leaves. The eggs slightly overlap, much like fish scales. Larvae are white, with a pattern of large, raised black tubercles on each body segment and are 1 to 1-1/4 inches long when full grown. The head of a first through third instar is black, whereas that of an older larva is brown. Overwintering larvae lose their spots and are light yellow-white.

The spotted "summer phase" (left) and the pale "winter, or overwintering, phase" (right) of the southwestern corn borer.
Kevin Steffey, Extension Entomology, (217)333-6652