Corn growers should start scouting for common stalk borer, say experts at Iowa State University and University of Nebraska-Lincoln. Scouting for migrating larvae in corn should begin now to make timely treatment decisions, says Erin Hodgson, Iowa State University associate professor of entomology.

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 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.

Damage caused by feeding in the whorl will first appear as irregular rows of holes in the unfolding leaves. These irregular rows of holes will be much larger and more ragged than those caused by whorl-feeding of first generation European corn borer larvae. In severe cases an infested plant will have a very ragged appearance, with abnormal growth habits such as twisting, bending over, or stunting. If the feeding injury to the central part of the plant is severe enough, the whorl will appear dead while the outer leaves will be green and apparently healthy. This condition is commonly called "dead heart."