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Sweet Corn Newsletter European Corn
Borer Description and Life Cycle The European corn borer (ECB) moth is about one inch long with a wingspan of about one inch at rest (see Figure 1). The moth varies in color from yellowish brown to dark tan with two dark, irregular, wavy bands across the front wings. The male moth may be somewhat darker in color and slightly smaller than the female. The tip of the male's tufted abdomen usually protrudes beyond its closed wings.
European corn borer moths emerge in mid to late June. The moths spend the daylight hours hiding in grassy areas. On warm nights, they fly into sweet corn fields to lay their eggs. They may also lay their eggs on other hosts, such as beans, broccoli, peppers, potatoes, dahlias and many weeds. The eggs, in masses of up to 50, resemble overlapping fish scales and are usually found on the underside of the corn leaves (see Figure 2). The eggs hatch in four to nine days. Just before hatching, the eggs turn black because of the darkening of the larval head capsule.
The newly hatched larvae are very small (¼ inch) and translucent yellow with a dark, shiny head (see Figure 3). As they develop, they become larger and darker in color. Fully grown larvae are 3/4 to one inch in length. They are usually flesh colored but may range from light pink to tan or gray, with conspicuous small, dark brown spots on each segment. The larvae feed on the corn leaves, frequently in the whorl, causing a characteristic "shot-hole" injury that becomes most apparent when the whorl leaves unroll (see Figure 4). The larvae will also tunnel into the tassels and stalk, weakening them and sometimes causing them to break (see Figures 5, 6). The tunneling habit makes control at this stage very difficult. On more developed corn, the ECB larvae may enter the ear at the base, side or the tip through the silk channel.
At the end of the season, the full-grown larvae remain in the corn stalks and stubble throughout the winter. In the spring, the larvae pupate and emerge as moths in about two weeks. Plowing corn stalks and stubble under at the end of the season is an effective and important means of controlling the ECB. Scouting and Control Thresholds Whorl Stage (eight leaves): Scout 100 plants per field or block (20 plant samples in five locations). Look for shot-hole injury in the leaves. A treatment is recommended if 30 percent or more of plants scouted show fresh feeding injury. Pretassel stage: Scout plants as above, look for feeding damage on the leaves and in the developing tassels. A treatment is recommended if 15 percent or more of the plants scouted show injury. Tassel-silk stage: Scout plants and, as above, look for fresh feeding injury on leaves, in tassels or on the sides of ears, especially where the ears meet the stalk. Give the stalk a gentle twist near the ear to detect larvae in the stalk. A treatment is recommended if 15 percent or more of the plants scouted show fresh injury. Pheromone traps: Two Scentry Heliothis® traps or traps of similar design should be placed in grassy weeds bordering the field, spaced at least 100 feet apart. One trap should be baited with Scentry® ECB I (NY strain) pheromone (or equivalent). The other trap should be baited with Scentry® ECB II (IA strain) pheromone (or equivalent). In young corn (whorl-tassel), these traps should be used to judge the emergence of ECB, i.e. no spray threshold. In silking corn that has not yet been sprayed for corn earworm, a treatment is recommended to protect the silks from egg-laying ECB moths if five or more moths are caught in the pheromone traps in a week. |
