Growing corn free from insect damage requires precise timing of control measures. Worm pests, which are the larval stage of moths, can severely compromise both yield and quality. Since these pests are often protected inside the plant structure, the window for successful spraying is narrow, demanding careful observation and a quick response. The decision to spray involves identifying the pest, understanding the corn plant’s development, and considering the economics of the intervention.
Identifying Common Corn Pests and Damage
The primary worm pests requiring insecticide application are the corn earworm, the European corn borer, and the fall armyworm. The corn earworm (Helicoverpa zea) is a threat during the reproductive stage, feeding directly on kernels at the ear tip. This feeding leaves behind chewed kernels and wet excrement (frass), which contaminates the ear and allows for fungal entry.
The European corn borer (Ostrinia nubilalis) and the fall armyworm (Spodoptera frugiperda) are threats during the vegetative growth stages. Both pests initially feed in the tightly coiled leaves of the whorl, causing characteristic “shot-hole” damage when the leaves unfurl. Borers eventually tunnel into the stalk, disrupting nutrient flow.
Calculating the Need for Intervention
A spray application is justified only when the pest population reaches the economic action threshold (ET). This is the point where the cost of control is less than the value of the crop damage prevented.
For European corn borer and fall armyworm, scouting begins at the tassel emergence stage. Treatment is warranted if approximately 15% of plants show signs of infestation, such as feeding damage or the presence of larvae. For corn earworm control in sweet corn, the threshold is stricter due to low quality tolerance, often falling to 5% of plants infested with eggs or small larvae during silking. For the true armyworm, the threshold is met when 25% of plants have two or more larvae, or 75% of plants have one larva. Control measures must be applied before larvae exceed three-quarters of an inch, as larger larvae have completed most of their feeding.
Optimal Timing Based on Corn Growth Stage
Timing the application depends on the corn’s growth stage and the pest’s behavior, as worms must be targeted before they become protected inside the plant. For pests that attack the stalk, such as the European corn borer, sprays are most effective just before the tassel emerges. Larvae deep in the whorl are often shielded from insecticides, but as the tassel pushes out, the larvae are briefly exposed and vulnerable to a well-directed spray.
For first-generation European corn borers, the ideal treatment window is generally between 800 and 1,000 accumulated growing degree-days, which correlates with the larvae being exposed on the plant. Second-generation borers are targeted later in the season, typically between 1,500 and 2,100 growing degree-days, when eggs are hatching and before they bore into the stalk.
Control of the corn earworm is most important during the silking stage (R1), as this is when the adult moths prefer to lay their eggs. Larvae hatch on the silk and quickly feed their way down into the ear tip, where they become protected from sprays. Application must begin as soon as the first fresh green silks emerge, targeting the young larvae before they penetrate the husk.
Due to continuous egg-laying activity and rapid insecticide degradation, multiple, closely timed applications are often necessary during the silking period. Under high pest pressure or when temperatures are above 82°F, the spray interval may need to be tightened to every two or three days. Spraying can cease once the silks have dried and turned brown, signaling that pollination is complete and the ear is no longer attractive to the pest.
Choosing and Applying Control Products
Once the need to spray is confirmed and the optimal window is identified, selecting the correct product is the next step. For stalk-boring pests, pyrethroid insecticides are commonly used, but for the corn earworm, newer chemistries are often preferred due to widespread insect resistance to older products. Biological options, such as products containing Bacillus thuringiensis (Bt), are also available for both earworms and borers.
Effective control requires the spray to physically reach the target area of the plant, which often necessitates specialized application methods. For pests deep in the whorl, the spray must be directed downward to penetrate the tight leaf structure. During silking, sprays must be directed at the fresh silks to ensure the insecticide covers the egg-laying and feeding site.
Whether using ground equipment or aerial application, thorough coverage is paramount, which may require increasing water volume or adjusting nozzle selection. Growers must also strictly observe the product label for safety, including the re-entry interval (REI) and the pre-harvest interval (PHI).