The **Corn Earworm** (*Helicoverpa zea*), also known as the tomato fruitworm or cotton bollworm, is one of the most economically destructive insect pests in North America, with a huge host range covering corn, cotton, tomatoes, and numerous vegetable crops. Damage is caused by the larval stage (the caterpillar) feeding on the silks and kernels of corn ears, burrowing into tomato fruit, or destroying cotton bolls, resulting in reduced yield and contamination.
Taxonomy and Classification
The Corn Earworm is a member of the family Noctuidae (owlet moths) in the order Lepidoptera. It undergoes complete metamorphosis. It is a polyphagous species, meaning it feeds on a vast array of plant families, making it extremely adaptable. It is classified as an obligate annual migrant in temperate regions, unable to overwinter except in the southernmost climates.
Physical Description
The adult moth is medium-sized, measuring about $1 \frac{1}{2}$ inches across the wingspan. It is typically buff-colored or light yellowish-brown, often with a faint dark spot near the center of each forewing and a dark band on the edge of the hindwings. They are strong, nocturnal flyers.
The **larva** (caterpillar) is highly variable in color, ranging from pale green, pink, or light yellow to dark brown, often with alternating light and dark stripes running lengthwise. It reaches $1 \frac{1}{2}$ inches long when mature. Larvae are known to be **cannibalistic**, especially on corn ears, which limits the population to one or two per ear.
Distribution and Habitat
The Corn Earworm is found throughout the Western Hemisphere. In the U.S. and Canada, populations migrate north annually from the Gulf Coast states, infesting fields as they move. Its habitat is any field or garden growing susceptible host plants, with a strong preference for corn during the silking stage.
Behavior and Life Cycle
The life cycle involves multiple generations per year, driven by the moth’s continuous migration northward. Females lay tiny, dome-shaped, pale eggs singly on the host plant. On corn, the eggs are laid almost exclusively on the fresh **silks**. This is the key window for control.
The larva hatches and feeds its way down the silk channel into the developing ear. It feeds on the kernels at the tip of the ear for several weeks, then bores out of the ear, drops to the ground, and pupates in the soil. Adults emerge and continue the generational cycle until cold weather forces the last generation to attempt (often unsuccessfully) to overwinter.
Feeding and Damage
On corn, the damage is concentrated at the **tip of the ear**, where the larva feeds on kernels, leaving behind messy frass and often facilitating the entry of fungal pathogens, particularly *Aspergillus flavus*, which produces highly toxic **aflatoxin** (a critical concern for stored grain). Damage to the ear tip is aesthetically unpleasing and lowers the market value of sweet corn.
On tomatoes and cotton, the larva bores into the fruit or boll, making the product unusable. Due to its wide host range and rapid life cycle, the Corn Earworm poses continuous risk throughout the growing season.
Management and Prevention
Management requires precise timing, focused on the brief window between egg hatch and larval entry into the ear.
- Monitoring: Pheromone traps are used to monitor moth migration and peak egg-laying activity, establishing the optimal time for intervention.
- **Chemical Control:** Insecticides are applied to the silks of sweet corn immediately upon silking, and re-applied every few days until silks dry, to kill the newly hatched larvae before they burrow.
- **Genetic Resistance:** The widespread use of **Bt corn** (genetically engineered to express a protein toxic to the larvae) provides the most effective commercial control.
- **Cultural/Organic Control:** For small gardens, dabbing a small amount of **mineral oil** on the silks after pollination is complete can suffocate the small larvae before they enter the ear.
Conservation and Research
The Corn Earworm is a persistent agricultural pest. Research focuses on refining resistance management strategies for Bt corn, improving monitoring of migratory populations, and investigating host plant resistance genes that could be utilized in conventional breeding.