The corn earworm is one of the most persistent and damaging pests for sweet corn growers and home gardeners. This pest causes significant economic losses because the larvae feed directly on the developing kernels, the marketable part of the crop. The resulting damage, typically concentrated at the tip of the ear, makes the corn visually unappealing and introduces fungal pathogens that can ruin the harvest. Controlling this migratory moth requires a multi-faceted approach, focusing on prevention and precise intervention timed to the pest’s life cycle.
Identifying the Corn Earworm
The corn earworm appears in two main life stages: the adult moth and the destructive larva. The adult moth is a grayish-brown insect with a wingspan of about one and a half inches, often featuring dark, irregular markings. These moths are primarily nocturnal and are responsible for laying the eggs that begin the cycle of damage.
The larval stage, the “corn worm,” varies dramatically in color, ranging from light green and pink to dark brown or nearly black. These caterpillars can grow up to two inches long and are distinguished by light and dark stripes running lengthwise along their bodies. Confirmation of an infestation comes from visible damage, specifically wet, brown excrement, known as frass, near the tip of the corn ear. The larvae bore down the silk channel and typically consume the kernels on the top one to two inches of the ear.
Cultural and Preventative Measures
The first line of defense against the corn earworm involves cultural practices that discourage the pest. One effective strategy is adjusting the planting schedule to avoid the local peak moth generation. Planting corn very early or very late allows the crop to silk and mature between the major egg-laying periods, reducing the infestation risk.
Selecting specific corn varieties can also offer natural resistance. Varieties that develop a long, tight husk at the ear tip create a physical barrier that is more difficult for newly hatched larvae to penetrate. For large-scale operations, utilizing transgenic corn varieties that express the Bacillus thuringiensis (Bt) toxin in the silk can suppress earworm populations. Tilling the soil in the fall after harvest helps by exposing and killing the overwintering pupae, breaking the life cycle before the next season.
Non-Chemical Control Techniques
Once the silk appears, non-chemical methods offer an environmentally conscious way to manage infestations. One precise and effective technique for home gardeners is applying mineral oil directly to the corn silk channel. This treatment suffocates the small larvae as they attempt to crawl down the silk to the kernels.
Timing the application is important to avoid interfering with pollination, which can result in ears with missing kernels. Apply the oil after the silks have fully emerged, wilted, and just begun to turn brown, typically five to six days after the first silks appear.
Mineral Oil Application
- Apply about five drops (roughly 0.5 milliliters) of mineral oil into the silk channel.
A significant boost in efficacy is achieved by combining the oil treatment with a microbial insecticide, Bacillus thuringiensis subspecies kurstaki (Btk). Btk is a naturally occurring bacterium that is harmless to humans but toxic to caterpillars upon ingestion. When Btk is mixed with the oil, the oil acts as a carrier, drawing the insecticide deeper into the silk channel where the larvae feed. For general monitoring, hand-picking or cutting off the damaged, frass-covered ear tips can limit further damage.
Targeted Insecticide Application
In cases of high pest pressure, targeted insecticide application may be required for effective control. The application must be focused exclusively on the corn silks, as the larvae are protected from sprays once they bore into the ear. The goal is to apply a lethal dose to the fresh silk where the adult moth lays eggs and where young larvae begin feeding.
Commonly used chemical controls include pyrethroids, though resistance is an increasing concern. Spinosad, an organic-approved product, is a popular and highly effective alternative. Due to the continuous growth of new silks, repeat applications are necessary every two to five days to maintain a protective residue. Always follow product label instructions, especially regarding the pre-harvest interval and precautions to protect pollinating insects.