Many different insects feed on corn leaves, and a visual inspection of the injury pattern is the most direct way to narrow down the potential culprit. Understanding the signs of feeding allows for a targeted response, which is far more efficient than applying a general treatment.
Identifying the Culprit by Damage Pattern
One of the most common signs is shot-holing, which appears as a series of uniform, horizontal holes across a fully expanded leaf. This pattern occurs because the insect chews through the leaf while it is still tightly rolled in the whorl; the perforations only become visible when the leaf unfurls.
Other pests cause ragged edges or large, irregular notches along the leaf margins. This damage is typical of larger chewing insects that consume tissue from the outside in. Smaller insects may create an effect known as window-paning, where they scrape off only the soft, green surface layer, leaving the transparent lower layer or epidermis intact.
Skeletonization gives the leaf a lacy appearance. This pattern results when the pest consumes all the tissue between the leaf veins, leaving the tougher vascular structure behind. Finally, the severing of young plants at or just below the soil surface is known as clipping, which is a sign of a pest that feeds at the base of the tender seedling.
Pests Targeting Young Growth and the Corn Whorl
The European Corn Borer (Ostrinia nubilalis) is a significant pest in this stage, with its larva being a creamy-white to light-pink caterpillar with dark spots and a brown head. Early-generation larvae feed deep within the whorl. Their chewing results in the classic shot-hole pattern seen when the leaves emerge.
The Fall Armyworm (Spodoptera frugiperda) is another caterpillar that favors the whorl. Its larva is distinguishable by an inverted, pale “Y” shape on its dark head capsule. This pest causes ragged feeding damage and produces a large amount of wet, sawdust-like excrement, called frass, which collects in the whorl.
Cutworms, particularly the Black Cutworm (Agrotis ipsilon), pose a threat to the smallest seedlings. These larvae are greasy-looking, pale gray to black, and hide in the soil during the day, emerging at night to feed. While smaller larvae cause irregular pinholes, larger cutworm larvae are notorious for clipping young corn plants completely at the soil line.
Pests Targeting Mature and Fully Expanded Leaves
The Corn Flea Beetle (Chaetocnema pulicaria) is a tiny, black, jumping beetle that feeds on the soft tissue of the expanded leaf. Their feeding leaves small, circular pinholes and bleached-out streaks on the leaf surface. The Corn Flea Beetle can transmit the bacterium responsible for Stewart’s wilt, making its presence a concern beyond simple defoliation.
The Japanese Beetle (Popillia japonica) is easily identified by its metallic-green body and coppery-bronze wing covers. Adult Japanese beetles cause the characteristic skeletonization damage, selectively consuming the green tissue between the leaf veins. These beetles often aggregate in large numbers, and while leaf feeding is rarely a major economic concern, their presence during pollination can lead to significant silk clipping.
Grasshoppers (Melanoplus spp.) are generalist feeders that move into cornfields, especially during dry conditions or from adjacent grassy areas. Both the nymphs and adults cause large, irregular chewing and notching along the edges of the fully developed leaves.
Immediate and Long-Term Control Strategies
For caterpillars like the armyworm and corn borer, a biological insecticide containing Bacillus thuringiensis (Bt) is effective. Since Bt must be ingested, it needs to be applied in a way that penetrates deep into the whorl for these pests. Smaller infestations of pests like Japanese beetles can sometimes be managed by simple hand-picking and dropping the beetles into soapy water. Chemical treatments, when necessary, are most effective when applied before pests like the European Corn Borer have tunneled into the stalks, as they become shielded from topical sprays at that point.
Long-term management relies on cultural practices that disrupt the pest life cycle and reduce favorable habitat. Crop rotation prevents the buildup of pest populations that overwinter in corn residue or soil, such as the Japanese beetle grubs. Weed management is also important, as many pests, including cutworms and armyworms, use grassy weeds as host plants before moving onto the corn. Ensuring good soil health and proper drainage supports vigorous plant growth, allowing the corn to better withstand minor feeding damage.