Farm Progress

As common stalk borer begins moving into corn and other crops, be prepared to manage pest this year and next.

June 6, 2017

5 Min Read
START SCOUTING: Young larvae are brownish-purple and have three prominent longitudinal white stripes at the front and rear ends of the body.Robert Wright

By Robert Wright

By now, common stalk borer eggs have hatched throughout Nebraska and scouting should begin when 1,300 to 1,400 degree days have accumulated. This is the point when larvae start moving into corn and other crops. Stalk borer growth is based on accumulated degree days since Jan. 1, using a base of 41 degrees F.

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, fencerows, 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 eggs hatch in late April or early May. Larvae bore into the stalks of grasses or other hosts 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 isn't available when eggs hatch, stalk borers may begin feeding directly on corn.

Stalk borer identification
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 midbody 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 1.5 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."

Methods for control
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.

Planting date may have some influence on the degree of injury from common stalk borers. Since smaller plants are more heavily damaged, earlier planting may allow corn to outgrow the most severe damage.

Stalk borer hatch and migration to new hosts can be predicted using degree days (DD). Based on research at Iowa State University, stalk borer egg hatch begins at about 575 degree days and ends at 750 degree days. Begin scouting corn when 1,300 to 1,400 degree days have accumulated. This corresponds with the beginning of larvae moving out of grassy hosts. Determine the need for treatment when 1,400 to 1,700 degree days have accumulated.

Check corn plants bordering grassy areas to determine the percentage of plants with stalk borer injury when 1,300 to 1,400 degree days (41 degrees F base) have accumulated since Jan. 1. Examine several sets of 10 plants. Look for feeding damage, and dissect damaged plants to see if live larvae are present. If weedy grasses were common throughout the field in the previous year, the whole field may need to be scouted for common stalk borers.


Timely treatment
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 degrees 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. Check the label for compatibility of different insecticide and herbicide mixtures.

A variety of foliar insecticides are effective against common stalk borers in corn. See the 2017 Guide to Weed Management in Nebraska with Insecticide and Fungicide Information (EC130) or the insecticide label information for labeled insecticides, their rates, and restrictions.

Several available Bt corn hybrids provide control or suppression of common stalk borer larvae. The label term "suppression" indicates that a lower level of mortality is expected than for insects labeled for control. (See Handy Bt Trait Table for a list of available hybrids.)

Wright is a Nebraska Extension entomologist. This report comes from UNL CropWatch.


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