The European Corn Borer

Wm. G. Bradley.

A research worker of the Massachusetts Agricultural Experiment Station in 1917 discovered several pinkish-brown worms on sweet corn in market gardens near Boston. Specialists examined the larvae and found them to be a species that was a pest of corn in Europe. A bit of sleuthing disclosed that they had sneaked into this country a few years earlier in broomcorn imported probably from Italy or Hungary for use in broom factories in Medford, Mass.

Sometimes quickly, sometimes more slowly, the insect, the European corn borer, spread outward from its original point of infestation. By 1952 it had been found in 37 States east of the Rocky Mountains, in which are most of our main corn-growing sections. The losses it caused in field corn were estimated at 314 million bushels in 1949.

From studies of the biology and habits of the insect we learned that two strains of the borer now exist in the United States. The single-generation, or univoltine, strain passes through one life cycle a year. The multiple-generation strain has two or more complete cycles every 12 months, depending on environment.

The multiple-generation strain flourishes in nearly all of the infested area, although its proportion to the single-generation strain varies in different localities, reaching its maximum in the southern sections and diminishing toward the north.

Observation of hundreds of species of plants in the field and tests in experimental plots on plants from many parts of the country showed that the borer can live on more than 200 different kinds of wild and cultivated plants. Corn, however, is infested and injured by the larvae, or borers, to a greater extent than any other crop. The borer injures field corn (both dent and flint), sweet corn, popcorn, and corn planted for fodder or silage. In the western part of the Corn Belt, corn is practically the only cultivated plant that is infested or injured to any extent. Broomcorn, soybean, millet, oats, potato, pepper, sorghum, and some large-stemmed flowering plants may be attacked when they are grown near corn or in years when corn matures late.

In the East, where the multiple-generation strain predominates, it commonly infests many other plants, including vegetables, field crops, flowers, and weeds. Many of them serve as shelter for the borers, rather than as food, and are infested sometimes by the borers which "overflow" from corn and other favorite host plants growing nearby.

THE EUROPEAN CORN BORER is essentially a boring insect and its greatest injury results from the tunneling and feeding of the larvae within the stalk, ears, tassel, midrib of the leaf, brace roots practically all parts of the corn plant except the fibrous roots. The larvae also feed to some extent upon the leaf blades in the whorl, tassel buds, husks and silks of the ear, and behind the leaf sheaths.

The character of the injury depends on the stage of development of the corn plant when it is attacked. Soon after hatching, the borers begin migrating to various parts of the same plant or to other plants nearby. The developing whorl is a favorite feeding place for newly hatched larvae. If the attacked plant is just developing a tassel, some of the small borers enter the tassel buds and feed within; others eat the surface of the tassel buds and protect themselves with a slight silken web. If the infestation occurs at the time of pollen shedding, accumulations of the pollen at the ligules supply favorable material on which the larvae feed. Later they tunnel within the tassel stem and its branches, often causing it to break over. These broken tassels, with bunches of sawdust-like borings at the breaks, are the outstanding signs of infestation in fields of growing corn, although many infested plants may not show this particular injury. The borers may continue tunneling downward into the main stalk, or they may leave the upper part of the plant and enter it or neighboring plants at points lower down. Some of the newly hatched borers, instead of feeding upon or within the tassel buds and tassel stalks, enter the stalk directly at some lower point.

The borers usually enter between the leaf sheath and stalk or between the stalk and the base of the partly developed ear if the plant has advanced to that stage of development. As they gradually increase in size, they make larger tunnels and work upward or downward. Small holes in the stalks, with bunches of sawdust-like borings at or below them, indicate the section in which the borer is at work.

At any stage of their development the borers may enter the ear directly at its tip, base, or side; or they may enter it indirectly through the short stem, or shank, by which the ear is attached to the stalk, in which case the shank is often so weakened by the injury that it breaks over. Frequently the ear is entered at its tip by small borers, which feed first upon the silks or the tender portion of the husk and then work their way down into the cob and grain.

The injury to stalks and ears may be increased still further by disease organisms, which often follow the work of the borers and gain entrance through lesions made by borers.

THE EUROPEAN CORN BORER passes the winter as a fully grown borer, or worm, inside its tunnel in the stalk, stubble, or ear of corn, or in some weed or other plant. The presence of the borers may be detected by small holes on the surface of the infested plants which are usually plugged with castings. When you split open the stalks or stubs, you usually find the borers inside. They are then nearly an inch long and one-eighth inch thick. The head is dark brown or black. The upper surface of the body ranges in color from light brown to dark brown, or it may be pink. Each division of the body bears a row of small, dark-brown spots; several narrow dark-brown or pink lines extend lengthwise of the body. The under side of the body is flesh-colored and is without markings.

As soon as warm weather begins, in April or May, the borer may leave its winter shelter and bore into more suitable places to pass the resting stage.

In May or early June it cuts a small circular opening from its tunnel to the surface of the plant to provide an exit for the future moth. It then closes the hole with a thin webbing of silk and retreats into its tunnel to a point near the last feeding or shelter place, where it usually spins a thin cocoon. Inside the cocoon the borer changes into the resting stage, or pupa, which is shuttle-shaped, light brown to dark brown, and one-half to five-eighths inch long. After 10 to 14 days the skin of the pupa case splits, and the moth, or adult, comes forth and (under average weather conditions) is present in the fields from June to September.

The females begin to lay their eggs soon after they emerge. The moths remain quiet during the day, hiding in patches of weeds and grass or underneath the leaves of other plants. Evenings and sometimes through the night when the weather is good, they fly from plant to plant and lay their eggs in flat, irregular masses. A female lays up to 1,900 eggs; the average is about 400. The moths live 10 to 24 days. An egg mass usually contains 15 to 20 eggs; as many as 162 have been found in a single mass, although eggs deposited singly may be observed. The masses are laid mainly on the under surface of the corn leaves, although they are sometimes laid on the upper surface, on the stalk, or on the husk. Each egg is about half the size of an ordinary pinhead; in the masses the eggs overlap like fish scales. The egg is nearly flat and is white when first laid, but later changes to pale yellow and becomes darker just before the young borer comes out.

The eggs hatch in 4 to 9 days, depending on the temperature. The newly hatched borer, about one-sixteenth inch long, has a black head and a pale yellow body, which bears several rows of small black or brown spots. During its growth the borer molts or changes its skin five or six times, gradually increasing in size with each change until it becomes full-grown.

Borers of the single-generation strain become full-grown in August if weather conditions are normal. They continue to feed, or bore, however, at intervals until cold weather stops their activities in October or November. They remain in a dormant condition throughout the winter within their tunnels in the cornstalks, stubble, cobs, or other plant remnants.

SOON AFTER they discovered the borer in the United States, investigators tried to establish biological controls like the ones that exist in its native haunts.

During the investigations, which started in 1919 and have continued since, more than 23 million borer larvae and pupae from Europe and 3 million from the Orient were brought to the United States. From them the natural enemies were reared. Other parasites were collected and forwarded to this country in the cocoon or pupal stages. Of the 24 species included in the importations, 21 were numerous enough to permit colonization over the borer-infested area in this country. The number of parasites available for colonization obtained from host larvae and pupae or parasite cocoons and pupae shipped from Europe or the Orient exceeded 2.5 million; it was increased by breeding in laboratories and by domestic field collections. About 8.5 million adults from all sources were released in fields.

Entomologists set loose adult parasites at selected localities throughout the infested area where the borer was sufficiently abundant to support a parasite, population. The scientists conducted surveys in the vicinity of these localities to determine which species became established and to get information on their biology as an aid in increasing their distribution within previously colonized areas and in colonizing areas newly infested by the natural spread of the borer. The species known to have become established in the United States and the number of adults of each that were released are: Lydella stabulans grisescens (838,966); Horogenes punctorius (198,145); Macrocentrus gifuenis (2,610,654); Sympiesis viridula (394,382); Chelonus annulipes (401,983); and Phaeogenes nigridens (53,234).

European corn borer.

To determine the effects of the parasites, scientists developed special sampling designs and techniques. Among them were polar coordinate designs suitable for studying extent of establishment, rates and direction of dispersion, and other pertinent points. These designs consisted of sections to aid in randomizing samples in concentric rings about a central circle surrounding the release point. The number of sections in each ring and the width of the ring were varied to suit the objectives sought.