Stalk borers are an occasional pest of corn in Nebraska. Stalk borer damage in corn commonly is confined to plants in the first few rows near field margins, fence rows, grass terraces, and waterways. In addition to attacking corn, stalk borers attack over 100 other species of plants, including ornamentals, garden vegetables, broadleaf weeds, and grasses. They may feed on soybeans as well, but they are not an economically important pest of soybeans.

Understanding the common stalk borer life cycle and behavior is critical to selecting management practices to reduce damage in corn.

Stalk Borer Life Cycle

Female stalk borer moths lay their eggs primarily on grasses such as smooth brome or ragweed in late summer and early fall. Egg-laying sites usually are in fence rows, terraces, and waterways, but can be found throughout a field if preferred hosts are available. Eggs overwinter and hatch in late April or early May. Larvae bore into the stalks of grasses or other hosts such as ragweed, and begin feeding. As they become larger or if the plants are mowed or burned down with herbicides, the stalk borers migrate into adjacent corn plants to complete their development. In some cases, if an appropriate weed host is not available when eggs hatch, stalk borers may begin feeding directly on corn.

Corn between the two- and eight-leaf stages can be attacked by the migrating stalk borer larvae. Larvae develop through seven to 10 instars, or stages, in about 10 weeks. Pupation occurs in the soil and moths emerge in August, September, and early October. There is a single generation each year.

Stalk Borer Identification

Common stalk borer larvae are distinctive in appearance. Young larvae (Figure 1a) are brownish-purple and have three prominent longitudinal white stripes at the front and rear ends of the body. The stripes are interrupted at mid-body by a solid dark purple to black area on the third thoracic segment and first three abdominal segments. Fully grown larvae do not have these characteristic markings and are uniformly dirty gray (Figure 1b). Fully grown larvae can be 1 1/2 to 2 inches long.

Stalk Borer Damage

Stalk borer larvae injure corn plants in June and early July. They feed on leaves in the whorl and then tunnel into the stalk, or they burrow into the base of the plant and tunnel up through the center of the stalk. Leaf feeding alone does not cause economic damage.

Tunneling into the stalk can result in deformed or stunted plants that may not produce an ear. Severely damaged plants can die. Plants attacked at earlier growth stages tend to be more severely injured. A single stalk borer larva may attack more than one plant if the first plant does not support the larva as it increases in size.