Start scouting for common stalk borer in corn
Tunneling into the stalk can result in deformed or stunted plants that may not produce an ear. Severely damaged plants can die. Plants attacked at earlier growth stages tend to be more severely injured. A single stalk borer larva may attack more than one plant if the first plant does not support the larva as it increases in size.
Damage caused by feeding in the whorl will first appear as irregular rows of holes in the unfolding leaves. These irregular rows of holes will be much larger and more ragged than those caused by whorl-feeding of first generation European corn borer larvae. In severe cases an infested plant will have a very ragged appearance, with abnormal growth habits such as twisting, bending over, or stunting. If the feeding injury to the central part of the plant is severe enough, the whorl will appear dead while the outer leaves will be green and apparently healthy. This condition is commonly called "dead heart."
Any weed control method that helps eliminate grasses will reduce the number of potential stalk borer egg-laying sites, reducing the probability of stalk borer damage the next year. Control of grassy weeds is important to keep stalk borer problems from increasing year to year.
Planting date may have some influence on the degree of injury from common stalk borers. Since smaller plants are more heavily damaged, earlier planting may allow corn to outgrow the most severe damage.
If most common stalk borer damage is next to grassy field borders and waterways, burning these areas before eggs hatch has been shown to reduce populations. The best time to burn these areas is in late winter, before the grass begins to grow. At this time all eggs will have been laid, and the soil will be bare and subject to erosion for the shortest time. Also, this timing has the least effect on bird populations that have not yet begun to nest.
Sampling and Economic Injury Levels
Check corn plants bordering grassy areas to determine the percentage of plants with stalk borer injury when 1,300-1,400 degree days (41°F base) have accumulated since January 1 (Figure 3). Examine several sets of 10 plants. Look for feeding damage and dissect damaged plants to see if live larvae are present. If weedy grasses were common throughout the field in the previous year, the whole field may need to be scouted for common stalk borers. Use the information in Table 2 to determine the economic injury level.
To be effective, insecticides must be applied before common stalk borer larvae have entered the stalk. In cases where stalk borers begin feeding on grassy weeds or other vegetation in field edges, control is most effective if timed between 1,400 and 1,700 degree days (base 41°F), which corresponds to the first half of the period when stalk borers are migrating from weedy hosts into corn. If the infestation is restricted to the field margin, use a border treatment.
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