Stalk borers can kill edge row corn

Have you ever wondered what killed or stunted the corn in the border rows next to fences, grassy waterways, and conservation terraces? The answer is probably stalk borers. Now that corn prices are in the $3-4 range, management of this insect can be worth the time and money to scout and maybe use an insecticide.

Stalk borer damage to corn border rows.

Stalk borer eggs have already hatched and most of the larvae are now in brome or other grasses, and giant ragweed. However, some stalk borers may already be in corn because they moved there first instead of the grass. Eventually the stalk borers in the grass get too big for the grass stems and they migrate in search of larger diameter plants, which often happens to be corn.

We are approaching the dates in June when stalk borers will migrate out of grass and into corn. About 10 percent of the larvae will move out of the grass by 1,400 degree days (base 41° F) and 50 percent of the larvae will have moved by 1,700 degree days. When 1,300 - 1,400 degree days have occurred in your area (see map on next page), scout to determine if the larvae are moving into corn. These dates predict when 10 percent of the larvae will migrate to corn.

Early stage stalk borer.

Begin by scouting corn adjacent to grass terraces, waterways, ditches, and fence rows where stalk borer stand loss has occurred within the last several years. Look for small larvae down inside the whorls or feeding holes in the new leaves. Larvae that are feeding in the whorl, but haven't tunneled into the plant, can be killed with an insecticide. The smaller the corn, the more likely it can be killed by stalk borers. Once corn grows seven leaf collars (V7 stage), stalk borers are unlikely to kill the plants. Stalk borers don't migrate very far from grass, so only the first four rows of corn next to grass would need to be sprayed.

Economic thresholds (see chart) can help in deciding whether or not to apply an insecticide. These economic thresholds are based on the percentage of infested plants, and assume $13 per acre control costs and 80 percent control with an insecticide. In the chart, determine the expected market value of corn (if you can!) and the plant stage. If the number of infested plants exceeds the percent given for the plant stage and market value selected, then an insecticide application can be economically justified. Young plants have a lower threshold because they are more easily killed than older plants. These thresholds were developed by Larry Pedigo, entomologist at Iowa State University.

Stalk borer economic thresholds

Percent infested plants at four corn prices

Leaf stage $2/bu. $3/bu. $4/bu. $5/bu.
1 10% 7% 5% 4%
2 12% 8% 6% 5%
3 15% 10% 7% 6%
4 16% 11% 8% 6%
5 17% 12% 9% 7%
6 34% 23% 17% 14%
7 100% 100% 100% 100%

These economic injury levels are based on $13/acre control costs

and 80 percent control with an insecticide.

Labeled insecticides and rates per acre include Ambush 2E (6.4 to 12.8 ounces), Asana XL (5.8 to 9.6 ounces), Lorsban 4E (2 to 3 pints), Pounce 3.2EC (4 to 8 ounces), or Warrior 1EC (2.56 to 3.84 ounces). Always read and follow label directions.

This article originally appeared on page 83 of the IC-476(12) -- June 3, 1996 issue.

Updated 06/02/1996 - 1:00pm