It seems almost silly to talk about stalk borers under the circumstances, but many farmers in northern counties, where quite a bit of corn has been planted, have had enough experience with stalk borers to know that watchfulness is worthwhile. Although stalk borers can be very difficult to manage, timely field scouting can forestall significant problems.|
Life history of stalk borers. Stalk borers usually have a close relationship with weeds, in and around cornfields. In late summer, female adults deposit eggs on suitable grasses, such as smooth brome grass, quackgrass, orchardgrass, woolly cupgrass, and wirestem muhly, and on some broadleaf weeds such as giant ragweed. The eggs overwinter on weed hosts. Fencerows infested with these weeds are excellent sites from which stalk borer larvae can move into adjacent rows of corn.
Larvae hatch from overwintering eggs in late April and early May. Newly hatched larvae tunnel into above-ground stems, usually of grasses. Older larvae outgrow the grass stems and crawl to nearby larger-stemmed host plants such as corn.
Stalk borer larvae develop through 7 to 10 instars and actively feed for 8 to 10 weeks. Older larvae feed on many secondary hosts (an estimated 176 plant species), including many weeds. Weeds with larger stems, such as giant ragweed, cocklebur, giant burr-elder, docks, and burdock, can support fully grown stalk borer larvae. After the larvae finish feeding, they pupate within host stems or in cells in the soil. Adults begin emerging in late August; peak emergence occurs during the first 2 weeks of September. The adults mate, shortly after which females seek hosts for oviposition. Stalk borers complete one generation each year.
Description. Larvae are 1 1/2 to 1 3/4 inches long, depending upon instar. (Small instars are very small, rendering them difficult to find.) Although newly hatched larvae may appear to be brown, they are purple to black 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 last instar is dirty gray or whitish and may be more difficult to identify as a stalk borer.
Young instar stalk borer larva.
Close-up of young stalk borer larva.
Injury to corn. Stalk borers injure corn plants in two ways. If a larva enters the plant through the lower portion of the stalk and tunnels upward, its feeding injures the growing point, often resulting in dead heart--the center leaves discolor, wilt, and die. A larva that enters through the top of the plant tunnels downward into the stalk. Injury first appears on newly emerging leaves, which may be cut off or have ragged holes that increase in size as the leaves expand. Later, the growth of the tassel may be affected, and the upper part of the plant may be deformed.
Corn plant injured by stalk borer larva.
Scouting for stalk borers. Start watching for stalk borers when the larvae outgrow their initial host plants (typically weeds along field margins or in waterways) and begin to crawl to nearby corn. This movement takes place over several weeks. We can estimate the movement of stalk borer larvae by accumulating degree-days above a base temperature of 41°F. Stalk borers first begin to move into corn when about 1,100 degree-days have accumulated from January 1; 50% movement occurs when about 1,400 to 1,700 degree-days have accumulated. We recommend scouting when 1,300 to 1,400 degree-days have accumulated, and a decision to treat with an insecticide should be made when 1,400 to 1,700 degree-days have accumulated.
Figure 2 shows actual heat-unit accumulations (base 41 deg F), from January 1 through May 20, 2002. Initial movement of stalk borers may be under way in the southern one-third of the state. Fifty percent movement may have just begun in the extreme southern tip of Illinois.
Management of stalk borers. Probably the best way to manage stalk borers is to eliminate their oviposition sites before late summer. Obviously good weed control within a field reduces the potential for stalk borers to become established within the field. Burning or mowing grassy field edges also may reduce egg-laying sites, but the drawbacks (e.g., potential erosion, destruction of wildlife habitat) may outweigh the benefits. Consequently, many farmers rely on insecticides applied when they begin to notice stalk borers feeding on corn plants along the edges of fields. (Refer to degree-day information in the preceding paragraphs.)
I will provide economic thresholds and insecticide recommendations in next week's issue of the Bulletin. In the meantime, keep your eye on field edges that may begin to show the first signs of the presence of stalk borers.--Kevin Steffey