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.
Stalk borer growth is based on accumulated degree days, using a base of 41°F since January 1. With the cooler spring this year, stalk borer growth is delayed compared to last year. Currently stalk borer eggs have hatched across most of the state. Scouting should begin when 1300-1400 degree days have accumulated.
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 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.
Not all of the currently available Bt corn hybrids have activity against common stalk borer. Several Bt corn hybrids list either suppression or control of common stalk borer. The label term "suppression" indicates that a lower level of mortality is expected than for insects labeled for control.
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 first half of the period that stalk borers are migrating from weedy hosts into corn.
Information for this week’s column is from the UNL Crop Watch web site. More information can be found at: www.cropwatch.unl.edu. It is provided by Dr. Bob Wright, UNL Extension Entomologist.
For more information or assistance, please contact Allan Vyhnalek, Extension Educator, University of Nebraska-Lincoln, Extension in Platte County. Phone: 402-563-4901 or e-mail email@example.com