Managing stalk borers in corn always has been a challenge for farmers. Corn rows that are adjacent to grass terraces, fence lines, or waterways, or cornfields where grass control was poor the previous summer, can be heavily infested. Field-wide stand loss also can occur. This article discusses how to control stalk borers in fields where infestations occurred last year and are expected to occur again this year.

Larvae may attack corn anytime after it emerges. They can produce two kinds of injury; leaf feeding and stalk tunneling. Leaf feeding is probably the most commonly-observed injury. Larvae crawl down inside the whorl and feed on the developing leaves. Feeding causes ragged holes to appear in the leaves as they unfold. Leaf feeding by larvae does not reduce grain yields.

Stalk tunneling, the other injury, is much more severe. It occurs when larvae tunnel through the plant and destroy the growing point, causing what is known as dead heart. The upper leaves are cut off within the heart of the plant, and it wilts and dies. The outer leaves remain green and apparently healthy. Plants that sur-vive often grow tillers, are delayed in development, and do not produce normal-sized ears or are barren.

When the plant begins to elongate (at the 6-leaf stage), the plants ability to tolerate stalk borer injury increases greatly. Larvae also can tunnel into corn stalks below the growing point, but this does not kill the plant.

Corn is infested when newly-hatched larvae tunnel into young corn plants (such as in a no-till field), or when partially-grown larvae crawl from grass in search of larger-diameter plants. Chemical control is difficult to achieve because larvae are exposed for very short periods of time, and they cannot be killed after tunneling into a plant. Time an insecticide with long residual action to coincide either with egg hatch or with movement from grass to corn.

One approach is to target the hatching larvae. This is best suited for fields with a history of field-wide infestations. An application of permethrin (Ambush, Pounce), timed to coincide with egg hatch, has reduced stalk borer populations effectively. Timing is critical. Application should occur between 575-750 F degree days, using a base threshold of 41F. This is when the stalk borers are hatching. Look at the degree day chart on the last page of the newsletter to determine if stalk borers are hatching in your area. The undesirable aspect of this approach, however, is that the size of the stalk borer population is not considered. An insecticide could very well be applied in the absence of an economically-damaging population.

If the insecticide is not targeted against the hatching larvae, a second approach is necessary when a history of field-wide problems exists. Use a burndown herbicide-insecticide combination, either tank mixed or in a split application, instead of using an insecticide alone. The herbicide kills the grass and weeds, forcing the larvae out of the plants to search for another host. Insecticides can be tank mixed with fast-acting herbicides (such as paraquat), or applied 7 to 10 days after a slow-acting herbicide (such as Roundup). Ambush 2E (6.4 to 12.8 ounces per acre), Asana XL (5.8 to 9.6 ounces), Lorsban 4E (2 to 3 pints), or Pounce 3.2EC (4 to 8 ounces) are recommended for stalk borer control. Long-term management of field-wide infestations requires grass control so that eggs are not laid across the field during late summer. If the grass isnt controlled, the problem can repeat itself the following year.