Bt corn was primarily developed to control the European corn borer and prevent losses in yield. However, many insects feed on corn but they may not be affected by Bt corn. The feeding injury caused by other insects may be evident to the farmer or field scout. This injury may be incorrectly identified as European corn borer feeding, thereby creating doubt about the value and performance of Bt corn. This article will highlight leaf feeding insects and their differences in injury to corn leaves. The European corn borer does not feed on seedling corn plants, therefore many insects that injure very young plants are not included here.
European corn borer
|||European corn borer larvae have a black head and "neck," with a lighter-colored body.|
|||Corn borer shotholes in corn leaves.|
Very small holes in whorl leaves. Newly-hatched larvae often feed deep inside the whorl and do not chew completely through the leaf but only remove a shallow layer of plant cells. This early feeding creates an opaque "window" of leaf tissue. As larvae grow larger, they then can chew through the leaf. Leaves emerged from the whorl will have small holes either randomly scattered or arranged in a repeating pattern across the leaf. Holes are not large enough to sever the leaf from the plant. Injured leaves are not wrinkled. Holes are not bordered by wide halos of brown or yellow. European corn borers seldom feed on plants shorter than 17-21 inches (extended leaf height).
|||European corn borer shotholes and tunnel in the midrib.|
Tunnels in leaf midrib. Mid-sized larvae may tunnel into midribs of the most recently developed leaves. Tunnels range in length from 1/4-inch to 3 inches. Tunnels occur mostly near the middle section of a leaf, never near the tip, and occasionally at the base of a leaf. Wind or rain may cause the leaf to break downward where the tunnel occurs.
|||European corn borer frass at leaf collar indicates a tunneling insect.|
Tunnels in leaf collar. Mid-sized and large larvae will tunnel in the leaf collar (leaf base) where it wraps around the stalk.
|||Early stage stalk borer has a purple midsection and an orange head with a black stripe.|
|||Stalk borer injury to whorl-stage corn.|
Large, ragged holes in whorl leaves; leaves may be severed from plant. Larvae feed inside the whorl, eating large, irregular holes in leaves. Large holes, 2-3 inches long, will cause leaves to break over or to be cut completely from the plant. Holes are much larger than those of the European corn borer. Infested young plants are stunted and may die. Injury is most common along field margins or in areas with giant ragweed or weedy grasses.
|||Armyworm injury to whorl-stage corn.|
|||Armyworm injury to whorl-stage corn.|
Tissue removed from the leaf edges; often only the midrib remains. Larvae begin eating near the edges of the leaf, consuming all of the leaf except for the tougher midrib. Feeding starts on the lower leaves and progresses up the plant. The whorl leaves are eaten last. Armyworms are exposed when feeding and rarely hide in the whorl like European corn borers. Up to a dozen larvae may feed on the same plant.
|||Corn earworms are striped with an orange-freckled head.|
|||Corn earworms are multi-colored and may be green, red, or brown with shades of lighter stripes.|
|||Corn earworm whorl-feeding injury.|
Large holes in whorl leaves. Larvae feed deep inside the whorls, especially on very late-planted corn in the upper Midwest. Holes may be 1-2 inches across and often mirror each other on opposite leaves that have emerged from the whorl. Injury resembles fall armyworm injury.
|||Fall armyworm has four black spots on the top of each body segment; spots are largest near the tail.|
|||Fall armyworm injury to whorl-stage corn.|
Large holes in whorl leaves. Larvae feed deep inside the whorls, especially on very late-planted corn in the upper Midwest. Holes may be 1-2 inches across and often mirror each other on opposite leaves that have emerged from the whorl. Injury resembles corn earworm injury.
|||Stink bugs are triangular-shaped.|
|||Stink bug injury on corn leaf.|
Plant stunted and may grow lateral shoots; leaves wrinkled; holes of various sizes scattered randomly or in repeating patterns often with a yellow halo; whorl leaves may be wrapped tight and fail to expand. Injury is most common on newly-emerged plants through the fourth true-leaf stage, but also may occur on midwhorl-stage plants. Stink bugs pierce the side of the stalk with their beak. Saliva injected into the leaf during feeding creates holes--pinhole-sized up to 1-inch diameter on expanded leaves. Holes often are surrounded by dead, brown tissue and a yellow halo. Feeding patterns often are repeated across the leaf, and injured leaves often are twisted. Heavily injured plants that are not killed by the feeding will grow new lateral shoots (tillers).
|||Differential grasshopper is a common corn pest.|
|||Grasshopper injury to corn leaves.|
|||Western corn rootworm beetle.|
|||Western corn rootworm beetle injury on corn leaf.|
Large sections of the leaf edges eaten; often only the midrib remains. Feeding may begin anywhere on the plant, but rarely on the bottom leaves. Injury may start at the leaf edge or in the center of the leaf adjacent to the midrib. There is no pattern to the feeding. Very large populations will consume all the leaf except for the tougher leaf midrib. Injury is similar to armyworms, but armyworms start feeding on the bottom leaves and progress up the plant.
Western corn rootworm
Long, narrow, light-gray strips eaten in leaves. Beetles eat long, narrow strips in leaves if tassels have not emerged and pollen is not available as food. Leaf feeding turns a light gray. Heavily eaten leaves may split and fray in the injured area.
This article originally appeared on pages 118-121 of the IC-478(15) -- June 30, 1997 issue.