Southwestern Corn Borers Are at It Again in Southern Illinois

June 25, 1999

· Injury by first-generation southwestern corn borers is evident in southern counties.

· Some stalk-boring activity has occurred.

· Economic damage caused by first-generation southwestern corn borers does not occur commonly.

Apparently southwestern corn borers have decided to establish residence in southern Illinois. It's not that we haven't experienced problems caused by this important pest of corn in the past; it's just that southwestern corn borers have become much more obvious in recent years. Maybe our string of mild winters has enhanced their survival. Whatever the case may be, corn growers and other agriculturists in southern counties need to be on the lookout for this insect.

Ron Hines, Senior Research Specialist at the Dixon Springs Agricultural Center, continues to be our sentinel for southwestern corn borers. According to cone-trap captures in Massac County, peak moth flight occurred during the first week of June. Both Ron and Dennis Epplin, Extension Educator, Crop Systems at the Mt. Vernon Extension Center, observed first-generation southwestern corn borers feeding in 28-inch-tall corn at the Ewing field in Franklin County on June 21. Ron reported that about 10 to 15 percent of the non-Bt corn plants were infested, some with as many as five to six larvae per plant.

According to most experts on the subject, economic infestations of first-generation southwestern corn borer are uncommon and are restricted to corn planted near undisturbed corn stubble fields where adults emerge in the spring. Nevertheless, some entomologists have established 35 percent whorl-feeding injury as a static economic threshold, so the presence of first-generation borers 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. Ron Hines found some third and fourth instars bored into corn stalks at the Ewing field. Very large densities may cause economic yield loss.

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 first 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 fully grown. The head of first through third instars is black, whereas the head of older larvae is brown.

Remember, unless densities are quite large, economic damage by first-generation southwestern corn borers is not common. Insecticides seldom are justified. However, we are interested in learning more about this pest in Illinois, so keep us informed if you encounter infestations.--Kevin Steffey

Author: Kevin Steffey