Common Stalk Borer

Papaipema nebris

Brown worms with white longitudinal stripes and a distinctive purple area in the middle of the body are often found boring into the stems of plants, particularly ragweed, oats, and corn. The worms are active and move rapidly when disturbed. When full grown, they are 1 1/2 to 2 inches long. The pupa is brown and spindle-shaped. The moth has a wing expanse of slightly over one inch, and the forewings are brown to gray with small white spots. The eggs are somewhat globular, ridged,and laid in the creases of rolled or folded leaves, weeds, grasses, and garden crops.

Common Stalk Borer Pupa
Figure 2. Common Stalk Borer Pupa

Life History
The borer passes the winter in the egg stage. The eggs hatch in May and the worms tunnel into nearby plants, becoming full grown in July. They then pupate, usually just below the soil surface, and emerge as moths in August. The moths deposit eggs in August in weeds and grassy patches, particularly along fencerows, ditch banks, and grass waterways. There is one generation a year.

This insect tunnels into the stalks of such crops as the small grains, grasses, and corn. The unfolding leaves of corn plants often show irregular holes where the worms damaged them while they were still in the whorl The heads of individual grain plants turn white prematurely in areas in small-grain fields, especially oats. Examination of the stalk may show this pest. Other insects that hollow out the stems of small-grain plants can cause similar symptoms.

Feeding occurs most frequently in rows adjacent to areas that were weedy during the previous August. Damage from this pest is usually of little economic significance.

Common Stalk Borer in Corn Stalk
Figure 4. Common Stalk Borer
in Corn Stalk

Scouting Procedures
Estimate the infestation by checking 20 plants in each of five locations. Check around border rows of conventional tilled fields in May-June. In no-till fields a random check throughout the field is necessary. Chemical control is usually not very effective.

Threshold Guide
None established. A "rescue" insecticide treatment may be effective if it is applied when the larvae are moving from their weed hosts to corn seedlings. Rescue treatments applied after the larvae have entered the plants are not effective.