No. 11/June 6, 1997
Keeping Track of Stalk Borers
We have received several reports of infestations of stalk borers along the margins of cornfields or in areas of fields that were infested with weeds last year. As we have stated in previous issues of this Bulletin, stalk borers are closely associated with certain weeds because the moths lay eggs on these weeds in late summer and young larvae feed within the weed stems in the spring. When the stems of the original weed hosts become too small for the growing larvae, they move to a larger host, often an adjacent field of corn. If the larvae are moving from grassy field borders or grass waterways, they usually don't move much more than four rows into the field. If the stalk borers are in the field where weeds were present last year, the small larvae usually attack corn almost immediately because the weed hosts have been killed with herbicides. In either situation, if you can delineate the area of infestation, spot treatments usually are effective.
Economic injury levels for stalk borers were developed by entomologists at Iowa State University. Depending upon the leaf stage of the corn plants, an insecticide application might be warranted if 15 to 50% 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%), 2-leaf (18%), 3-leaf (23%), 4-leaf (25%), 5-leaf (25%), and 6-leaf (50%).
We repeat the list of insecticides suggested for control of stalk borers in corn: *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. Please follow all label directions and precautions, and avoid pesticide drift. If you plan to tank-mix an insecticide with a burndown herbicide, read the label and follow directions related to compatibility.
If you are still keeping track of stalk borer development, the map in Figure 1 should help you assess the biological progress of this bothersome pest. Remember the following biological events and critical heat-unit accumulations (in parentheses): egg hatch (575 to 750 heat units above a base temperature of 41 degrees F since January 1), first movement into corn (1,100 heat units), and 50% movement (1,400 to 1,700 heat units).
Kevin Steffey, Extension Entomology, (217)333-6652
Figure 1. Actual heat-unit accumulation (base 41 degrees F) from January 1 to June 2, 1997, for estimating the development of stalk borers.