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Searching for Overwintering Corn Borers

April 6, 2001
Everyone who is interested in corn production wonders about corn borers at some time during the growing season. In southern Illinois, corn growers have been dealing with relatively heavy infestations of southwestern corn borers for at least the past 3 years. Throughout all of Illinois, corn growers are wondering what happened to European corn borers. And at this time of year, a lot of people wonder how well these borers survived the winter. In issue no. 1 of the Bulletin (March 16, 2001), we offered an article, "Will Another Mild Winter Promote Insect Problems?" in which we cursorily addressed the effects of winter weather on European and southwestern corn borers. This time we offer visual evidence.

Ron Hines, senior research specialist at the Dixon Springs Agricultural Center in Pope County, embarked on a survey of southwestern corn borers in Massac County in late March. He took 27 samples (10 corn stalks per sample) and examined the stalks for borer larvae and the cavities they excavated last fall. He found that 87% of the no-till corn stalks had cavities, and 63% of the damaged stalks contained living or dead southwestern corn borer larvae. He found only two living European corn borer larvae in the 270 stalks he examined. Steve Ebelhar, agronomist at the Dixon Springs Agricultural Center, also found some overwintering European corn borer larvae in cornstalks.

Overwintering southwestern corn borer larvae in corn stalks. (Photo courtesy of Ron Hines.)

Overwintering European corn borer larva. (Photo courtesy of Ron Hines.)

Overwintering European corn borer larva in stalk. (Photo courtesy of Steve Ebelhar.)

Distinguishing between apparently healthy and apparently unhealthy or obviously dead southwestern corn borer larvae is relatively easy. As Ron found, the live larvae were white and plump, whereas the unhealthy or dead larvae appeared at least slightly discolored or completely dark. The cause (or causes) of the larval deaths is unknown, but it's obvious that many of them did not survive the winter very well.

Mixture of healthy, unhealthy, and dead southwestern corn borer larvae. (Photo courtesy of Ron Hines.)

Dead southwestern corn borer larvae. (Photo courtesy of Ron Hines.)

The information Ron gathered from his survey reflects the findings by entomologists from the University of Kentucky. They offered the following commentary in the March 12, 2001, issue of Kentucky Pest News ( "This is the third year that we have conducted such a survey. Unlike the previous two winters, we had a severe cold period in December and January. Survival this winter is less than we estimated last year. Last year we estimated SWCB survival to be between 20 and 24% in those counties. In Daviess and Henderson counties the rate of survival is about half of what we saw the last two years. In Caldwell County it is considerably less. The average survival rate for Caldwell, Henderson and Daviess counties is 5.8%, 12.2%, and 9.2%, respectively. Why the difference among counties, it may be due to snow cover during the cold period. There was more snow cover in the Daviess and Henderson counties than in Caldwell County."

If you decide to look for corn borer larvae on your own, a simple characteristic that will distinguish between southwestern and European corn borer larvae might help. The prolegs (nonsegmented, peg-like legs on the abdomen) of most caterpillars have hooks, called crochets, on the bottom. On southwestern corn borer larvae, the crochets form an O, whereas the crochets on European corn borer larvae form a C. This characteristic and some color differences should be sufficient for distinguishing between the two species.

Crochets on the underside of a southwestern corn borer larva. (Photo courtesy of Ron Hines.)

As the season progresses, we'll try to keep you informed about survival and emergence of southwestern and European corn borers throughout the state. Remember, all reports are helpful, so don't hesitate to contact us.--Kevin Steffey

Author: Kevin Steffey

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

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