No. 10/May 29, 1998
Stalk Borers Are Active
Howard Brown with Pioneer Hi-Bred International, Inc., visited a field on May 26 in Kankakee County with a severe infestation of small stalk borer larvae. The field was infested with giant ragweed last year, a classic set-up for stalk borers. The symptoms of injury in the field were a mixed bag of ragged defoliation of whorl leaves and plants with "deadheart" injury. We'll describe more about stalk borer management in this article.
Because stalk borer adults lay their eggs on weed hosts in late summer and the eggs overwinter, stalk borer larvae begin feeding on young weed hosts early in the spring when they hatch. When the larvae grow too large for weed hosts, they move to larger hosts such as corn. In most fields, injury is confined to the first 4 to 6 rows of corn. However, in fields that were infested with weeds last summer, stalk borers could be more widespread.
If corn has emerged in your area, it's time to look for stalk borers. Start looking for symptoms of stalk borer injury and the larvae first in field borders along roadsides and waterways. In northern Illinois where strip planting corn and alfalfa is more common, stalk borers can be particularly bothersome. Ragged leaf feeding, usually accompanied by a lot of frass (insect excrement), is the first sign of the presence of stalk borer larvae. However, the goal of stalk borer larvae is to enter the stalk, as the name implies, and they do so in one of two ways. They may enter from the top after feeding on whorl leaves, or they may drill in from the bottom of the stalk near the ground. In either situation, if larvae severely injure or kill the growing plant, the "deadheart" symptom appears--that is, the center leaves wilt and die. Plants injured in such a way either die or produce many suckers and yield poorly.
Stalk borer larvae are 1-1/2 to 1-3/4 inches long, depending upon instar, and dark purple to black with 5 longitudinal white stripes (1 on top, 2 on each side) broken by a dark purple band, or "saddle," encircling the body just behind the legs. This band fades as larvae mature. The head capsule is yellow and has a dark stripe on each side.
Management of stalk borers, if control is justified, requires timing an insecticide application to kill larvae before they bore into stalks. Application of an insecticide can be timed to coincide with movement of the larvae from weed hosts to corn. Stalk borers first begin to move into corn when about 1,100 heat units have accumulated above a base temperature of 41 degrees F 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 weeds (dead stems, larvae inside) or border rows of corn. According to Figure 3, stalk borer larvae should have begun their movement into border rows of corn throughout most of the state. By now, 50 percent movement could have occurred anywhere south of St. Louis, and scouting for stalk borers should be underway throughout the southern half of the state.
Figure 3. Actual heat-unit accumulation (base 41 degrees F), January 1 to May 24, 1998.
Economic injury levels based upon different leaf stages of corn growth and different prices for corn have been published by Iowa State University. We offered them in table form in issue no. 8 of this Bulletin (see Table 2 of the May 15 issue). If treatment is warranted, based upon these thresholds, the following insecticides are suggested for control of stalk borers:
- 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 3.2EC* at 4 to 8 oz per acre
- Warrior 1EC* at 2.56 to 3.84 oz per acre
Some insecticides can be tank-mixed with fast-acting "burn-down" herbicides to control the borers that are forced from the dying weeds into seedling corn. Check the labels to determine compatibility of insecticides with herbicides. If a slow-acting herbicide is used, an insecticide can be applied 7 to 10 days later. Products marked with an asterisk (*) are restricted for use only by certified applicators.Mike Gray (firstname.lastname@example.org) and Kevin Steffey (email@example.com), Extension Entomology, (217)333-6652