Plenty of Southwestern Corn Borers in Some Counties

June 30, 2000
In southern Illinois, talk of European corn borers has been superseded the past 3 years by discussions of southwestern corn borers. The mild winters have allowed this pest to thrive in our southern counties, and it has caused far more concern than the European corn borer. Numbers of southwestern corn borers this year are fairly large in some areas. Alan Mosler with Twin County Service Company has found 40 to 70% of the plants infested in cornfields throughout Franklin, Jackson, Perry, Saline, and Williamson counties. Ron Hines, research agronomist at the Dixon Springs Ag Center, also has observed heavy infestations of southwestern corn borers. Most of the larvae now are fourth instars and have tunneled into stalks. Therefore, control with an insecticide would not be effective.

Entomologists debate whether first-generation southwestern corn borers cause economic damage. Some publications indicate that the economic threshold for first-generation southwestern corn borers is anywhere between 20 and 35% of plants with leaf feeding and larvae in the whorls. However, many entomologists believe that economic infestations of first-generation southwestern corn borers are uncommon. Regardless of your take on this issue, the second generation of this important insect pest is worth your attention if you live or work south of I-70.

First-generation southwestern corn borer 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. When the larvae finish feeding and complete their development (after about 18 to 20 days), they pupate inside the stalks. Adults begin to emerge a few days later.

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; older larvae have brown heads.

Begin to watch for emergence of southwestern corn borer moths in southern Illinois within 2 to 3 weeks. We will keep you posted, through Ron Hines's observations, as the second generation of this pest begins.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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