European Corn Borer

Problem type: Insect

Name of problem:
 European corn borer

Plant name(s): Corn

Symptoms / Characteristics:
Beginning in mid-July, symptoms of early larval feeding appear as rows of tiny pinholes in the whorled and newly unrolled leaves. Expansion of damaged leaves may result in midrib breakage. Almost all above-ground parts of the corn are eventual targets, including the tassel, stalk and developing ear. Damage can cause weakening, poor development or even breakage of these plant parts. Feeding at the base of the stalks often results in lodging.

The European corn borer (Ostrinia nubilalis) larva is a greyish-light brown caterpillar with a dark brown head. Along the back are rows of brown spots and pale, reddish stripes running lengthwise. At emergence (mid-July to early August) the larva is only a few millimeters long but soon reaches a mature length of about 2.5 cm. Mature larvae overwinter inside the corn stalk. The adult moths emerge around mid-June. They range in color from yellow to brown to grey. Dark, wavy bands run across the 2.5 cm-span wings. Females are typically larger and lighter in color than males. In early July, female moths begin depositing hundreds of creamy white eggs on the corn leaf undersides, near the midrib. The eggs are laid in groups of about 20 and have a collective diameter of about 0.5 cm. The eggs darken one day prior to hatching due as the heads of the developing larvae darken. This is often referred to as the blackhead stage.

Although on the prairies corn is the primary target of the European corn borer, neighboring provinces have reported severe damage to other vegetable crops, including snap bean, peppers, potatoes and tomatoes. The larvae feed inside the fruit and penetrate the stems, causing wilting, girdling and breakage. Leaf damage is minimal. It has been reported that there are over 200 hosts for the European corn borer.

Control / Preventions:
Mature larvae overwinter by boring into the corn stalks. In the fall, after harvest, remove all remaining corn stalks in order to eradicate any larvae that may be overwintering inside the stalks. Begin monitoring corn plants in early July. Check leaf undersides for creamy white egg masses. Manual removal is effective but may not be feasible for larger plantings. Also inspect the plants for larvae and feeding injury. If large populations of the corn borer are detected, an application of insecticide may be warranted. There are several biological and chemical insecticides available to the home gardener that may effectively control the European corn borer. A corn variety with genetic resistance to corn borer (Bt corn) has been developed for commercial use. Consult your local government agency for information regarding commercial corn production and pest control.