Sweet corn is vulnerable to insect pests that feed directly on the developing ears. Protecting the harvest requires a strategic approach because the larvae of the primary pests quickly burrow into the ear, where they are safe from sprays. Successful pest management relies on the precise timing of insecticide applications, targeting the brief period when the pests are exposed. This strategic timing is the most important factor determining whether your corn will be clean at harvest.
Identifying the Primary Sweet Corn Pests
Successful spraying begins with knowing which pests are present, as three main caterpillar species threaten sweet corn. The Corn Earworm (Helicoverpa zea) is often the most damaging. It primarily targets developing kernels by entering through the silks at the ear tip, causing visible kernel damage that ruins the top portion of the ear.
The European Corn Borer (Ostrinia nubilalis) is also a major concern, but its damage pattern differs. Early generations feed on the leaves and tunnel into the stalk, which can weaken the plant and cause tassels to break. Later generations may bore into the ear shank, causing ears to drop or remain small.
The Fall Armyworm (Spodoptera frugiperda) is the third major pest. It initially feeds deep within the corn whorl, creating ragged, shot-hole damage as the leaves unfurl. As the plant matures, the larvae move to the ears and bore through the husk at any point, not just the tip. Understanding these distinct feeding behaviors helps determine where to focus scouting efforts.
Scouting Methods and Action Thresholds
Before any application, growers must confirm that pest populations have reached an Action Threshold. This is the point at which the cost of control is justified by the prevention of economic loss. Scouting should begin once the tassels appear and continue frequently, ideally every one to two days, as the corn approaches the silking stage. This regular inspection is necessary because larvae are vulnerable for only a short time before they burrow into protected areas.
Visual inspection involves looking for signs of active infestation rather than old damage. For borers and armyworms in the whorl stage, check for fresh feeding damage and “frass,” which is a sawdust-like excrement left by the feeding caterpillars. The established action threshold during the pre-tassel stage is reached when 15% or more of the plants show signs of infestation with live larvae. Once the corn begins silking, the threshold drops significantly to as low as 5% infested plants, or is determined by the number of moths caught in pheromone traps.
The Critical Window for Spray Application
The critical time for applying insecticide begins the moment silks emerge and lasts until they turn brown and dry. This period is the only opportunity to intercept the Corn Earworm and later-generation Fall Armyworm larvae before they enter the developing ear. Adult moths prefer to lay eggs directly onto the fresh silks, and newly hatched larvae immediately crawl down the silk channel to feed on the kernels.
Spraying must commence immediately after the first silks appear, ensuring the silks are coated with the control product to kill the larvae upon entry. Because new silks continuously emerge and the insecticide residue breaks down, repeat applications are necessary throughout the entire silking period. The reapplication frequency is generally every two to five days, depending on the pest pressure and the temperature. Spraying before the silks emerge or after they have dried is ineffective, as the target pests are either not present or are already protected deep inside the ear.
Choosing the Appropriate Control Product
Control products fall into two categories: synthetic insecticides (such as pyrethroids) and biological controls. Synthetic options often offer longer residual activity, which can extend the time between re-applications. However, Corn Earworm populations in many regions have developed resistance to pyrethroids, making them less reliable.
Biological options, like the active ingredient Spinosad, are highly effective against caterpillars and often preferred by organic growers. Bacillus thuringiensis (Bt) is also used, but foliar applications have limited effectiveness against Corn Earworm. This is because the larvae enter the ear quickly and may not ingest a lethal dose. Regardless of the product chosen, the application technique is paramount: the spray must be directed specifically at the fresh silks at the tip of the ear.