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Issue No. 12, Article 5/June 16, 2006

Corn Borers Deserve Attention in Some Areas of Illinois

I use the phrase "corn borers" in a broader sense for this article. Although most people in Illinois are concerned primarily with European corn borers, corn growers in Illinois are continually reminded of the threat posed by southwestern corn borer, which, insect for insect, actually causes more damage to corn. I’ll try to assess the current situation with each borer.

Mike Roegge, crop systems Extension educator in Adams County, found a fairly healthy infestation of first-generation European corn borers in early-planted sweet corn on June 7, with 35% to 65% of the plants infested. Second instars were feeding in the whorls, resulting in noticeable windowpane feeding injury when the leaves unfurled. Jim Donnelly, crops specialist with Ag View FS in Walnut, found first instars in early June in an early-planted cornfield. About 10% of the plants also had egg masses. Elsewhere, we have information about flights of European corn borer moths, ranging from virtually zero in southern Illinois (refer to the "Hines Report") to "noticeable" in some northern Illinois counties. I enclose the word noticeable in quotation marks because European corn borer moths have been decidedly not noticeable over the past few years.

Although we usually can’t correlate results from our annual fall survey with the occurrence of first-generation European corn borers the following year, we know that large numbers of overwintering larvae increase the potential for infestations of first-generation larvae. Thus far, it’s interesting to note that the reports of noticeable infestations of first-generation European corn borers have come from areas with relatively larger populations of overwintering larvae (primarily western counties). You can refer to the results from our 2005 survey.

Although European corn borers have not garnered much attention in recent years, we don’t want to forget that they still have potential to cause significant yield losses in fields of non-Bt corn and in non-Bt corn refuges. If you have not already begun to look for symptoms of injury caused by first-generation European corn borers, you should start now. If necessary, European corn borers can be controlled effectively with insecticides while borers are still small (first, second, and third instars) and feeding in the whorls. When larger larvae bore into stalks, they cannot be controlled with insecticides.

Whorl-feeding injury caused by first-generation European corn borer larvae.

Early instar European corn borers.

For information about the biology and life cycle of European corn borers, refer to our fact sheet.

The southwestern corn borer situation seems to be more threatening than the European corn borer situation in southern Illinois. Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, reported captures of large numbers of southwestern corn borer moths in his trap in Massac County (577 captured during the week ending June 13). The numbers of moths captured at the Massac County location have increased steadily since the week ending May 30.

Bt corn has been effective for controlling southwestern corn borers in the United States, but non-Bt corn, including non-Bt corn refuges, is at risk. Scouting for first-generation southwestern corn borers should commence immediately in southern counties. By keeping track of when southwestern corn borer moths have been captured in your area, you can begin to estimate important life-cycle events for this insect. After 190 heat units (base 50°F) have accumulated beyond the initial flight of southwestern corn borer moths, first instars can be found; second instars can be found at 361 heat units; third instars at 533; fourth instars at 713; fifth instars at 902; pupation at 1,153; and emergence of adults at 1,321. At the very least, you should be looking for egg masses and early instar larvae before the larger larvae begin to tunnel into stalks.

Each female southwestern corn borer moth deposits eggs in masses on the upper and lower surfaces of corn leaves. Female moths lay approximately 250 eggs over a 5- to 7-day life span. Eggs are yellow-green when first laid, but after 36 hours they become cream colored, with three orange-red lines on each egg. Economic infestations of the first generation of southwestern corn borers are not common, and they most generally occur in corn planted near last season’s infested and undisturbed corn residue. Yield losses caused by subsequent generations of this pest can be significant due to stalk lodging caused by the girdling of plants by larvae.

Southwestern corn borer egg mass (photo courtesy of Marlin Rice, Iowa State University).

Small southwestern corn borer larvae might be confused with European corn borer larvae. However, a southwestern corn borer larva appears to have indistinct bands across its body, from which hairlike setae arise. The bandlike appearance becomes more pronounced as the larvae grow, until the spots formed by the tubercles (small, knotlike bumps) become visibly distinct in the fourth instar. The last (fifth) instar has very distinct, large, dark tubercles. Like European corn borers, the last two instars tunnel in the stalk. If you have a good magnifying glass or microscope and a little patience, another characteristic also distinguishes the two species. On the bottom of the prolegs (the peglike false legs on a caterpillar’s abdomen) of the southwestern corn borer, the tiny hooks (called crochets) form a complete circle. On a European corn borer larva, the crochets on the bottom of the prolegs do not form a complete circle.

Fully grown southwestern corn borer larva (photo courtesy of Tom Riley, formerly with Louisiana State University).

Whorl-feeding injury caused by southwestern corn borer larvae can be confused with injury caused by both European corn borers and fall armyworms. Early instar southwestern corn borer larvae "windowpane" the leaf tissue, much like small fall armyworms; however, the windowpaning usually is more extensive. Injury caused by larger southwestern corn borer larvae resembles whorl-feeding injury caused by European corn borers, except more leaf tissue is removed. The injury is less ragged-looking than injury caused by fall armyworms, and southwestern corn borer larvae do not produce the dark, coarse frass produced by fall armyworms.

Bt corn probably has suppressed corn borer populations in Illinois and elsewhere in the Midwest over the past few years. However, environmental conditions can benefit survival of both European and southwestern corn borers, so we need to keep both species on our scouting checklists.--Kevin Steffey

Kevin Steffey

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