Damaged kernels at the tip of a corn ear, often accompanied by dark excrement, are a common problem for gardeners and farmers alike. This damage is primarily caused by the larva of the corn earworm, Helicoverpa zea, one of the most destructive pests of sweet corn. Preventing this damage requires a strategic, multi-layered approach that targets the pest before it can enter the protective husk. This guide outlines the steps for effective prevention, focusing on precise timing and application methods.
Identifying the Pest and Its Vulnerable Stage
The insect commonly called the “corn worm” is the larval stage of the corn earworm moth, also known as the tomato fruitworm or cotton bollworm depending on its host plant. The buff-colored adult moths migrate northward each season, or emerge from pupae overwintering in the soil, before laying eggs on fresh corn silks and foliage. A single female moth can deposit an average of 1,000 eggs, often up to 3,000, which are laid individually and appear as tiny, dome-shaped, greenish-white specks.
The most susceptible stage for prevention is the brief period between egg-laying and the larva’s entry into the ear. Eggs can hatch rapidly, sometimes within 24 hours during hot summer conditions, with the newly emerged larva immediately crawling down the silk channel toward the developing kernels. Once the caterpillar has entered the ear and is protected by the tight husk, controls are largely ineffective. Preventative measures must be timed to eliminate the egg or the newly hatched larva while it is still exposed on the silk.
Non-Chemical and Cultural Prevention Techniques
Cultural controls offer a foundational layer of defense by manipulating the environment and the plant itself to reduce the pest’s impact. Adjusting the planting date is an effective strategy, as corn that silks early in the season often escapes the heaviest infestations. Earworm populations typically build throughout the summer, making late-silking corn far more vulnerable to high moth activity in August and September.
Selecting corn varieties with specific physical traits can provide natural resistance. Hybrids that develop a long, tight-fitting husk offer better protection because the snug fit impedes the movement of the newly hatched larvae and holds applied treatments more effectively. Some older varieties, like ‘Silvergent’ and ‘Staygold,’ are known for this trait, which forces the larvae to feed only on the tip of the ear.
Physical intervention is successful on a small garden scale, particularly applying a barrier to the silk channel. Once the silks have begun to wilt and turn brown, indicating successful pollination, a simple clothespin or rubber band can be placed tightly across the silk at the tip of the ear. This action creates a physical blockade, preventing the larvae from crawling past that point and into the kernels. Another manual method is to clip the silk off about a half-inch above the ear tip a few days after pollination, which removes the eggs and the newly hatched larvae.
Applying Targeted Pest Control Treatments
For a high level of control, especially for sweet corn where minor damage is unacceptable, direct treatments to the silk are necessary. These treatments must be applied precisely to the silk channel, as the goal is to create a lethal barrier that the larva encounters immediately upon hatching. Timing is paramount, with the initial application ideally beginning when silks first emerge, or when about 10% of the ears are silking.
A classic, non-toxic treatment involves applying a small amount of mineral oil or vegetable oil directly to the silk channel. Approximately five drops, or 0.5 milliliters, should be placed on the silk where it enters the ear, about five to six days after the silk has fully emerged. The oil suffocates the larvae as they attempt to move down the silk, but applying it too early can interfere with pollination, resulting in poorly filled ears.
The effectiveness of oil can be enhanced by mixing it with a biological insecticide like Bacillus thuringiensis subspecies kurstaki (Bt). This naturally occurring soil bacterium, which is safe for humans and beneficial insects, produces a protein that is toxic when ingested by the corn earworm larva. A common formulation is to mix one part Bt concentrate or neem product with about 20 parts oil, applying the mixture with an eyedropper directly to the silk tip.
For commercial growers or situations with high pest pressure, synthetic insecticides, such as those containing spinosad or pyrethroids, are applied directly to the silks. These treatments must be repeated every three to five days until the silks begin to dry out, ensuring a continuous protective residue on the fresh silk as it grows up to 1.5 inches daily. Regardless of the product used, the focus remains on thorough coverage of the silk to intercept the pest before it enters the ear.