Although we have not received many reports of stalk borer activity in corn, it's prime time for stalk borer larvae to be moving from small weed hosts into corn plants along field margins. At this time of year, we cease producing maps of accumulated degree-days for virtually all insects that attack field crops. However, based on last week's accumulated degree-days above a base temperature of 41°F, we anticipate that stalk borers, if present, can be found in corn plants almost anywhere in Illinois right now.
As with most insect pests of field crops, scouting is essential for determining whether management of stalk borers is necessary. As the small larvae move from weed hosts into corn plants along field edges, you'll begin to notice symptoms of their feeding inside the stems--ragged leaves, stunting, and/or wilting. If a stalk borer larva kills the growing point of a plant, the plant may exhibit "dead heart"--the center leaves wilt and die. After the stalk borer kills a small corn plant, it moves into a nearby healthy plant. If you observe plants that have symptoms of stalk borer injury, dissect the stem to find the relatively small borer. The striking striped pattern, the purple "saddle" midway along the body, and the yellow head are identifying characteristics of stalk borer larvae.
Small stalk borer larva in the stem of a corn plant.
Corn plants injured by stalk borers. The injury is noticeable primarily in the three rows adjacent to the grass waterway.
The key to managing stalk borers is to anticipate their movement into the rows bordering grassy areas and to treat before many of the borers have caused enough damage to kill plants. The larvae are difficult to kill after they have bored into corn stalks. Remember that dispersal of stalk borer larvae (generally fifth to seventh instars) from weed hosts begins when approximately 1,100 degree-days (base 41°F) have accumulated since Janauary 1. You should make a decision whether treatment is necessary after approximately 1,400 to 1,700 degree-days have accumulated since January 1.
The article "Update on Stalk Borer Development and Management Tips" in issue no. 8 (May 16, 2003) of the Bulletin includes more details about control of stalk borers, including a list of suggested insecticides. Keep the information handy during the next couple of weeks, when stalk borers probably will cause the most damage.--Kevin Steffey