No. 8/May 16, 1997
Planning Ahead for Stalk Borers
In last week's Bulletin (issue no. 7, May 9), I promised more details about stalk borers and information about heat-unit accumulations. Figure 1 shows the accumulated heat units above a base temperature of 41 degrees F from January 1 to May 12. Stalk borer development occurs whenever the temperature exceeds 41 degrees F. The overwintering eggs hatch when approximately 575 to 750 heat units have accumulated since January 1. Larvae first begin to move into corn when about 1,100 heat units have accumulated since January 1; 50 percent movement occurs when about 1,400 to 1,700 degree days have accumulated. When about 1,300 to 1,400 degree days have accumulated, scout corn to verify the presence of stalk borers in the grasses (dead stems, larvae inside) or border rows of corn.
By the time you receive this issue of the Bulletin, stalk borer larvae could have begun moving from their initial weed hosts into the edges of corn fields in the southern three to four tiers of counties. If temperatures begin to warm up, stalk borer injury could be evident in these counties during the week of May 19. Egg hatch will have begun in counties in the central one- third of the state. By examining the Figure 1, you should be able to determine the approximate stage of development of stalk borers in your area.
Stalk borer larvae are 1/12 to 1-3/4 inch long, depending upon stage of development. You likely will not find the smaller larvae, but as they grow larger and begin to feed on corn seedlings, the distinguishing characteristics become evident. Stalk borer larvae are dark purple to black, with five longitudinal white stripes (one on top, two on each side) broken by a dark purple band encircling the body just behind the legs (Figure 2). Later in the season when the larvae bore into the corn stalks, the purple band fades and virtually disappears.
Stalk borers tunnel into corn stems above the soil surface or climb into the whorl. Whorl feeding by stalk borers results in tattered leaves but has no long-term impact on development or yield. However, stalk borers that tunnel into the plant cause more serious problems. Young plants (fewer than three leaves) are cut off or wilt rapidly from the tunneling. The center leaves of older plants (4- to 7-leaf stage) usually discolor, wilt, and die, a symptom called "dead-heart." After plants grow to the 8-leaf stage, corn usually tolerates feeding with no visible injury.
Grassy field edges and grass waterways are attractive, relatively permanent egg-laying sites for the moths in late summer and fall. Consequently, corn next to these areas often becomes infested when the larvae outgrow their grass hosts and move into the field. As stated previously, insecticide application can be timed to coincide with this movement. Depending upon the stage of the corn, an insecticide might be warranted if 15 to 50 percent of the plants in the two outside rows are infested. The combinations of leaf-stage and percent of infestation are the economic injury levels: 1 leaf (15 percent), 2 leaf (18 percent), 3 leaf (23 percent), 4 leaf (25 percent), 5 leaf (25 percent), and 6 leaf (50 percent). This latter information was determined from research at Iowa State University.
Infestations sometimes occur where grassy weed problems were evident last fall. The situation is aggravated in conservation-tillage and no-till cropping systems, which favor proliferation of some weedy grasses, and in continuous corn, where herbicide alternatives are limited. Long-term control depends upon successful weed management and elimination of associated oviposition sites. Early season perennial grasses like quackgrass also are primary hosts. Some insecticides can be tank-mixed with fast-acting "burn-down" herbicides to control the borers forced from the dying weeds into seedling corn. If a slow-acting herbicide is used, an insecticide can be applied 7 to 10 days later.
The list of insecticides suggested for control of stalk borers is relatively short: *Ambush 2E at 6.4 to 12.8 oz per acre; *Asana XL at 5.8 to 9.6 oz per acre; Lorsban 4E at 2 to 3 pt per acre; *Pounce 3.2 EC at 4 to 8 oz per acre; and *Warrior 1EC at 2.56 to 3.84 oz per acre. The use of insecticides preceded by an asterisk is restricted to certified applicators. Follow all label directions and precautions, and avoid pesticide drift. If you plan to tank-mix an insecticide with a burn-down herbicide, read the label and follow directions related to compatibility.
Kevin Steffey, Extension Entomology, (217)333-6652