University of Illinois

No. 11/June 5, 1998

Another Corn Borer (Southwestern)
To Contend with in Southern Illinois

Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, reports that the flight of southwestern corn borers laying eggs for the first generation in southern Illinois is "intense." Although this insect pest usually is not a problem for most growers in Illinois, corn south of I-64 is at risk, and corn planted between I-64 and I-70 should be monitored.

The southwestern corn borer (a serious pest of corn in the southern Great Plains region of Kansas, Texas, and New Mexico) has been a persistent problem in the bootheel of Missouri. Although the southwestern corn borer always has posed a potential threat in southern Illinois, it's only been in the last 3 years or so that densities have been consistently great enough to attract considerable attention. Infestations of second-generation southwestern corn borers were relatively common, and some were intense in southern counties last year. Also, some folks have verified that larvae successfully overwintered in our fair state. As a result, the moth flight this spring is worthy of note.

We shouldn't have too much to worry about regarding the first generation of southwestern corn borers. According to experts, economic infestations of first-generation southwestern corn borer are uncommon and restricted to corn planted near undisturbed corn-stubble fields where adults emerge in the spring. However, their presence bears watching. Larvae attacking whorl-stage plants feed on developing leaves in the whorl. Injury is evident as leaves unroll from the whorl. Leaf injury ranges from pinhead-sized holes and small circular lesions, or "windows," on leaf surfaces, to large, elongated holes. Third instars leave the whorl, crawl down the stalk, and tunnel into the plant, primarily between nodes. Very large densities may cause economic yield loss.

As southwestern corn borers and European corn borers develop in southern Illinois, keep your eyes peeled for both, and keep your diagnostic skills sharp. European corn borers are discussed in another article, so you should also know how to detect southwestern corn borers.

Female southwestern corn borers deposit eggs in masses of two to three on upper and lower surfaces of corn leaves. Eggs are yellow-green when deposited, but within 36 hours they become white, with three broken orange-red lines across each egg. The eggs slightly overlap, much like fish scales. Southwestern corn borer 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. Heads of first through third instars are black, whereas those of older larvae are brown.

Remember, unless densities are quite high, economic damage by first-generation southwestern corn borers is not common. So, insecticides seldom are justified. However, we are interested in learning more about this pest in Illinois; keep us informed if you encounter infestations. Also, we should see some benefits with Bt-corn against southwestern corn borers. Efficacy tests with Bt-corn in Kansas and Texas have had very positive results.

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