European corn borer populations will peak soon. You’ll want to scouting for this serious pest soon, especially if you didn’t plant Bt corn hybrids, says Adam J. Varenhorst, South Dakota State University entomologist. You also might want to check your Bt hybrids to make sure the seed trait is work. Resistant populations can develop.

Corn borer moths are attracted to the tallest corn plants, so begin scouting cornfields that were planted earlier for egg masses, leaf feeding, and caterpillars.

 What to look for

PEAK TIME: Corn in the Dakotas has entered the peak time for European corn borer populations.

Adult moths are approximately a half-inch in length with triangular wings. Female moths have yellow-brown wavy markings on their wings, while males are slightly smaller and darker in color.

European corn borer caterpillars are light tan to pink in color, and are approximately 1 inch in length when fully mature.

Distinguishing characteristics of the caterpillars include dark brown head capsules, dark spots on each body segment, and three pairs of true legs with four pairs of abdominal prolegs

MOTH ID: These are male (right) and female European corn borer moths. (Photo: Adam Sisson)

In South Dakota, European corn borer can have either one or two generations per year, depending on location. In northern South Dakota, European corn borer typically has only one generation per year (univoltine). Moths of univoltine populations begin flying in mid-June, with peak populations occurring in mid-July.

SECOND STAGE: This is a European corn borer caterpillar. (Photo: Frank Peairs)

Eggs are laid on the underside of corn leaves from June to July depending on seasonal temperatures, and hatch within one week. Caterpillars initially feed on leaf tissue, but will eventually tunnel into cornstalks. Late-instar (or late-stage) caterpillars overwinter in stalk residues and pupate the following spring.

In southern South Dakota, European corn borer can have up to two generations per year (bivoltine). Bivoltine moths begin flight in mid-May, and eggs are laid on the underside of V6 to V9 corn leaves. These caterpillars will also feed on corn leaves and tunnel into stalks. Pupation occurs within cornstalks, and emerging adults begin laying eggs on the underside of leaves, leaf collars, and on ear husks during tasseling (VT) and silking (R1). Eggs hatch roughly a week later, and caterpillars tunnel into the stalks and ear shanks, where they feed on developing kernels. Late-instar caterpillars overwinter in stalk residues and pupate the following spring.

BORER SIGNS: Here are windowpane and shot-hole injury caused by early-instar (or early-stage) European corn borer caterpillars. (Photo: Eugene E. Nelson)

Univoltine and bivoltine European corn borer populations can occur within the same field. This phenomenon is typically observed in central South Dakota. For areas with univoltine (one-generation) European corn borer, scout corn from V8 through R1 growth stages. For areas with bivoltine (two generations) European corn borer, scout for the first generation during V8 through V14, and for the second generation between R1 and R2 growth stages.

CATERPILLAR TUNNELING: These European corn borer caterpillars are tunneling into an ear shank. (Photo: Iowa State University Department of Entomology)

Caterpillars damaging

Yield loss associated with European corn borer is due to caterpillar feeding, Varenhorst says. Adult moths do not injure corn. Newly hatched caterpillars feed on leaf collars and sometimes migrate toward the tassels to feed on pollen. Young caterpillars often feed on the leaf surface and midribs, often referred to as “windowpane” injury. Midway through development, caterpillars will feed within the whorl, creating “shot-hole” type injuries that become visible when the leaves unfurl.

INSECT EGGS: Here are newly laid (above) and ready to hatch (below) European corn borer egg masses. (Photo: John Gavloski)

Near the end of development, caterpillars will tunnel into cornstalks, ear shanks and into the ears. Leaf feeding typically does not cause serious injury to corn. However, tunneling into stalks and ear shanks can result in significant yield losses, Varenhorst says.

Tunnels can result in stalk breakage, and reduce water and nutrient transport by corn plants. Tunnel entrances can permit secondary infections of mycotoxin-producing fungi.

Source: SDSU Extension