While we are still in the throes of concern about the early-season insects in corn, it's time to start thinking about the insects that will "step up to the plate" when flea beetles, southern corn leaf beetles, white grubs, and wireworms are nothing more than memories. Stalk borers may threaten corn in some areas of Illinois quite soon, so keep your eyes peeled. |
Small stalk borer larva.
Overwintering stalk borer eggs hatch in late April and early May. Newly hatched larvae tunnel into aboveground stems, usually of grasses. Older larvae outgrow the grass stems and crawl to nearby larger-stemmed plants such as corn. Larvae are 1/12 to 1-3/4 inches long, depending upon instar. Larvae usually are about 1/2 to 3/4 inch long when we first find them in corn. The purple-to-black caterpillars are distinctively marked with five longitudinal white stripes (one on top, two on each side) broken by a purple band encircling the body just behind the legs. The head is yellow with a purple stripe on each side.
You need to begin watching for stalk borers when the larvae outgrow their initial host plants (typically weeds along field margins or in waterways) and crawl to nearby corn. This movement takes place over several weeks, and we can estimate it by accumulating heat units above a base temperature of 41°F. Stalk borers first begin to move into corn when about 1,100 heat units have accumulated from January 1; 50% movement occurs when about 1,400 to 1,700 heat units have accumulated. We recommend scouting when 1,300 to 1,400 heat units have accumulated, and a decision to treat with an insecticide should be made between 1,400 and 1,700 heat units.
Figure 1 shows actual heat-unit accumulations (base 41°F) from January 1 through May 8, 2000. Bob Scott with the Illinois State Water Survey supplied the map. Based on this information, initial movement of stalk borers has begun in the southern one-third of the state, and 50% movement was under way as of May 8 in a small area in southeastern Illinois. We will continue to supply these maps in the next few issues of the Bulletin.
Corn plants injured by stalk borers.
When you begin to scout for stalk borers, focus on field edges and look for the following symptoms of injury after stalk borer larvae tunnel into corn stems above the soil surface or climb into the whorl: Young plants are cut off or wilt rapidly because of the tunneling. The center leaves of older plants usually discolor, wilt, and die, a symptom commonly referred to as dead heart. Plants that do not die may be stunted, develop slowly, and produce tillers.
Fields that are most susceptible to attack by stalk borers are those infested with or adjacent to preferred weed hosts such as smooth brome grass, quackgrass, orchard grass, woolly cupgrass, and some broadleaf weeds such as giant ragweed. Start inspecting these fields first.
Insecticides suggested for control of stalk borers are *Ambush at 6.4 to 12.8 oz per acre; *Asana XL at 5.8 to 9.6 oz per acre; *Capture 2EC at 2.1 to 6.4 oz per acre; Lorsban 4E at 2 to 3 pt per acre; *Pounce 3.2EC at 4 to 8 oz per acre; and *Warrior T at 2.56 to 3.84 oz per acre. (Use of products preceded by an asterisk is restricted to certified applicators.) Some Bt-corn hybrids also provide come control of stalk borers.
We have received no reports of stalk borer activity yet, but we will share that information with you as soon as we do.--Kevin Steffey