European Corn Borer

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 of the forewings.

Egg-

Each white egg is about half the size of a sewing pin head. 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 five or six 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.

Fig. 82: European Corn Borer
Ostrinia nubilalis (Hubner), Pyralidae, LEPIDOPTERA

A, Adult. B, Eggs. C, Larva. D, Pupa.

BIOLOGY

Distribution-

Introduced into the United States from Europe 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 percent of the stalks in some fields have been attacked.

Host Plants-

The European corn borer infests over 200 plants, but corn is a preferred host. Chrysanthemums and Shasta daisies are often infested in the summer and early fall. 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. 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. Infested stems often lodge.

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. Females do not mate the first day, but most mate within the next 48 to 72 hours. Flight activity begins at dusk when moths apparently disperse to drink dew. Most mating takes place about midnight in tall, dense grass such as foxtail as much as 100 m from the nearest host plants. Usually in early morning, 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. European corn borer larvae are cannibalistic and at most only a few survive to emerge as adults. 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. Therefore in much of the Southeast, 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. The third and fourth generations are much more of a threat to garden mums and other ornamentals as corn is not suitable for oviposition late in the season. The shorter days and high temperatures of late summer induce the larvae to develop into a diapausing state from which they emerge the following spring due to longer days and higher temperatures (following low winter temperatures).

CONTROL

Many natural parasites of the European corn borer have been introduced from Europe. Other biological control agents such as ladybird beetles, predaceous mites, and downy woodpeckers also have been responsible for some European corn borer reduction. The bacterial insecticide, Bacillus thuringiensis, also shows some promise for borer control.

Chemical control of the European corn borer is difficult because the caterpillars are exposed only during the 2- to 3-day period after eggs hatch and before larvae bore into stems. Therefore, close attention should be paid to the presence of moths and eggs. The emergence of the first moths can be determined by using either light traps or pheromone traps. Treatments should begin 7 to 10 days after a moth flight or about 5 days after the first egg masses are found. Pyrethroid insecticides have relatively long residual lives and are toxic to caterpillars and other pests of garden mums. Growers should consider pyrethroids because of the extended flight period of the third and fourth generations of European corn borer moths in late summer. For specific chemical control recommendations, consult the current Cooperative Extension Service publications on ornamental plant pest management.