Nebraska farmers should gear up for an increased threat from stalk borers this year. While historically only an occasional pest of corn in Nebraska, stalk borer damage can be quite severe.
Borer damage is commonly confined to plants in the first few rows near field margins, fence rows, grass terraces, and waterways. Understanding the common stalk borer life cycle and behavior is critical to selecting management practices to reduce damage in corn.
Female stalk borer moths lay their eggs primarily on grasses such as smooth brome or ragweed in late summer and early fall. Small grains (rye or wheat) planted as a cover crop may also be a potential egg laying site. 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, terminated mechanically 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.
Common stalk borer larvae are distinctive in appearance. Young larvae 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. Fully grown larvae can be 1 1/2 to 2 inches long.
Stalk borer larvae injure corn plants typically in late May-June. 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.
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.”
Any weed control method that helps eliminate grasses will reduce the number of potential stalk borer egg-laying sites, reducing the probability of stalk borer damage the next year. Control of grassy weeds is important to keep stalk borer problems from increasing year to year.
To be effective, insecticides must be applied before common stalk borer larvae have entered the stalk. In cases where stalk borers begin feeding on grassy weeds or other vegetation in field edges, control is most effective if timed between 1,400 and 1,700 degree days (base 41°F), which corresponds to the first half of the period when stalk borers are migrating from weedy hosts into corn. If the infestation is restricted to the field margin, use a border treatment.
Insecticides may be mixed with fast-acting herbicides being used to burn down early season weeds, or applied several days after use of slower-acting herbicides. Always check the label for compatibility of different insecticide and herbicide mixtures.
Managing stalk borers in Nebraska
