European Corn Borer
Ostrinia nubilalis (Hubner), Pyralidae, LEPIDOPTERA


DESCRIPTION

Adult - The female moth has a robust body and a wingspread of about 25 mm. It is colored pale yellow to light brown. The outer third of the wings is usually crossed by dark zigzag lines. The male moth is smaller, more slender, and darker than the female. The outer third of its wings is usually crossed by two zigzag streaks of pale yellow, and often there are pale yellow areas on the forewings.

Egg - Each white egg is about half the size of a pinhead. The eggs change to pale yellow and darken just before hatching as the brown head of the borer inside becomes visible. Within the egg mass, the eggs overlap each other like fish scales. The masses of 20 to 30 eggs are covered with a shining waxy substance.

Larva - The newly hatched larva, about 1.5 mm long, has a black head, five pairs of prolegs, and a pale yellow body bearing several rows of small black or brown spots. It develops through 5 or 6 instars to become a fully grown larva about 25 mm long.

Pupa - The brown pupa is 13 to 15 mm long with a smooth capsule-like body.


BIOLOGY

Distribution - Introduced into the U.S. from Europea in 1909, the European corn borer has spread throughout the contiguous states and into Canada. In North Carolina, the largest populations of this pest occur in the Coastal Plain where 75% of the stalks in some fields have been attacked. However, 40% lodging of stalks due to borers has been observed in the Piedmont when harvesting has been delayed.

Host Plants - The European corn borer infests over 200 plants, but corn is a preferred host. Other vegetable crops likely to be injured include bean, beet, celery, potato, pepper, and tomato.

Damage - On most crops, borers begin feeding on the leaf surface. In corn, feeding first occurs in the whorl. Later the larvae bore down midribs of leaves into the stalk. Frass and silk near entrance holes are evidence of their presence. Borers weaken stalks, or stems, and interfere with the movement of plant nutrients. Consequently, yields are reduced. On infested corn plants, tassels and stalks may break, ears drop, or only small ears develop.

Potatoes and peppers are affected more adversely than corn by borer injuries which allow the entrance of bacterial and fungal pathogens. Reduced yields occur when disease organisms cause stems to collapse and tubers to rot. Potatoes that are produced on infested plants are often of poor quality due to the development of diseases such as black leg. Pepper fruit infested by European corn borers are commercially unacceptable.

Life History - Mature larvae overwinter inside tunnels in stubble, stalks, ears, or other protective plant material. They pupate in spring. During April and May, adult moths emerge and mate. Each female lays 500 to 600 eggs in small masses of 20 to 30 on the undersides of leaves. Eggs hatch in 3 to 12 days, depending upon temperature. Young larvae usually begin feeding on leaf surfaces and, as they mature, begin boring in the midribs of the leaves. Two to 3 days after eggs hatch, stalk or ear boring commences and continues until pupation. In Florence, South Carolina, the European corn borer completes four generations per year and may do so in parts of North Carolina too. If this is the case, eggs of the second generation are laid in mid- to late June, those of the third generation in late July, and those of the fourth generation in September. This last generation is not a threat to corn. First generation European corn borers are a threat to potatoes, and the second and third generations are pests of other vegetables.


CONTROL

Many natural parasites of this corn borer, mainly flies and wasps which have been introduced from Europe, exist in corn-growing areas. Other biological control agents such as ladybird beetles, predaceous mites, and the downy woodpeckers have also been responsible for some borer reduction. The bacterial insecticide, Bacillus thuringiensis however, shows some promise for borer control.

Chemical control for the European corn borer is difficult because sprays are effective only during the 2- to 3-day period after eggs hatch and before larvae bore into stems. Therefore, close attention must be paid to the presence of moths and eggs. The emergence of the first moths can be determined by using either light traps or screened cages. Treatments should begin 7 to 10 days after a moth flight or about 5 days after eggs are found. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.

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