Stalk borers are notorious for killing or stunting corn rows next to fences, grassed waterways, and conservation terraces. This insect can be most troublesome in nonBt corn, which of course could include the Bt refuge acres. To stop this damage, fields must be scouted and an insecticide applied on a timely basis before the larvae have an opportunity to tunnel into the growing point of the young plants.

Stalk borer injury to border rows of corn. (Marlin E. Rice)

Stalk borers in grass

There is one generation of stalk borers each year, and the eggs were laid late last summer and early fall. By early June, stalk borer larvae will have hatched in Iowa and most of the larvae will be inside brome grass, other grasses, or giant ragweed. However, a few stalk borers may already be in corn because they moved there first instead of to grass. Stalk borers in brome grass eventually kill the grass seed head, causing "dead heads." The larvae eventually grow too large for the grass stems, then they crawl out in search of larger diameter plants, including corn.

Stalk borer destroying the growing point inside a corn plant. (Marlin E. Rice)

Degree days and migration

Early June is the predicted time for southern Iowa when stalk borers will move out of grass and into corn. Approximately 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 (Table 1), scout to determine whether the larvae are moving into corn. These dates predict when 10 and 50 percent of the larvae will move into corn.

Scouting border rows

Scout corn adjacent to grass terraces, waterways, ditches, and fencerows, and especially those areas where stand loss has occurred in previous years. Stalk borers don't crawl very far from grass, so only the first four rows of corn next to grass would need to be sprayed. Look for small larvae resting inside the whorls or for new leaves with feeding holes. Larvae that are still feeding in the whorl, but that haven't tunneled into the plant, can be killed with an over-the-top insecticide. The smaller the corn, the more likely it is to be killed by stalk borers. Once corn reaches the 7-leaf stage (V7 stage), stalk borers are unlikely to kill the plants.

Early-stage stalk borer showing the purple "saddle bags" and black stripe on the head. (Marlin E. Rice)

Fields with weeds

An exception to the border row problem is when weedy grasses or giant ragweed are growing throughout a corn field. If these weeds are killed with herbicides, the stalk borers move out of the weeds and into the corn. Stalk borers can destroy a corn stand under these circumstances. To prevent this destruction, an insecticide (Table 2) should be tank mixed with the herbicide (if it is a fast burndown herbicide) or the field should be sprayed with the insecticide approximately 7 days after the herbicide (if it is a slow burndown herbicide). Be sure to read the insecticide label before mixing pesticides.

Bt corn

In some of our experiments, we have found that Bt corn (YieldGard® Corn Borer hybrids) suppresses or slows down stalk borer injury. Hybrids with this technology do not have the same toxic effect on stalk borers as on European corn borers, so don't expect complete control of this pest in Bt corn. You may want to scout your Bt corn acres as well if you have historically had a problem with stalk borers. We have not evaluated Herculex® I or Herculex® Xtra hybrids, so we are unfamiliar with the performance of this technology against stalk borers.

Economic thresholds

Economic thresholds can help in deciding whether to apply an insecticide (Table 3).

The original thresholds were developed at Iowa State University by Larry Pedigo and his students many years ago and were based on $2.00 corn.

We have estimated new thresholds based on $3.00 and $4.00 corn. The thresholds are based on the percentage of infested plants and assume $13 per acre control costs and 80 percent control with an insecticide. If the number of infested plants in the first four rows of corn (adjacent to grass, terraces, fencerows) exceeds the percentage given for the plant stage, an insecticide application can be economically justified. Young plants have a lower threshold because they are more easily killed than older plants. Scouting is not necessary beyond the V7 developmental stage in corn.

Table 1. 2007 projected dates for migration of stalk borer (base 41 °F).

Table 2. Commonly available insecticides labeled for stalk borer control in corn.

Read and follow all label directions.

Table 3. Stalk borer economic injury levels for corn at three values.

Marlin E. Rice is a professor of entomology with extension and research responsibilities. Rich Pope is an extension program specialist working with the Corn and Soybean Initiative.