European Corn Borer Fact Sheet - Penn State Entomology Department Fact Sheet

EUROPEAN CORN BORER
Ostrinia nubilalis (Hubner)

The European corn borer, Ostrinia nubilalis (Hubner), was first identified from the Boston, Massachusetts area in 1917. It is believed to have been introduced into the United States on broom corn imported from Hungary and Italy. Since its introduction, it has become established in most states east of the Rocky Mountains. The corn borer has been present in Pennsylvania for at least 60 years.

Image of European corn borer on kernels of corn.

European corn borer is known to feed on about 250 kinds of plants. In Pennsylvania, losses due to this insect are very high in sweet corn and peppers, and they are frequent in snap beans. Losses in field corn are relatively low. However, losses from corn borer do fluctuate and occasional moderate damage occurs in some fields in south central Pennsylvania. Over the past 20 years, losses in field corn from the European corn borer have averaged approximately 3.5 percent annually, statewide. This translates into more than $10 million lost on the 1981 corn grain crop grown in Pennsylvania.

Description


European corn borer adult male on corn leaves. Dennis Calvin Collection Penn State University	European corn borer adult female. Image by Faruque-Uz Zaman Entomology Graduate Student Penn State University	European corn borer larva tunneled into corn stalk. © Iowa State University

Description

The moths are yellowish buff to light tan in color. They have several dark zigzag marks across the wings, and their wing spread is 3/4 to 1 inch. The moths hide in grass, weeds, and trash in and around fields during the day, but readily fly short distances when disturbed. They are active during late evening and at night.

The eggs are usually glued to the undersides of leaves, in small irregular-shaped clusters. The egg clusters are white and very flat and they contain about a dozen or more individual eggs. Clusters resemble fish scales overlapping one another. Each egg is about half the size of a pin head. After 3 to 5 days, the eggs change from white to a yellowish color, and then, just before hatching, a dark spot appears in each egg. The time required for the eggs to hatch is dependent upon the temperature. The first generation eggs usually hatch in 7 to 10 days, whereas those of the second generation normally hatch in 5 to 7 days.

The larvae are dirty white, often having a pinkish tinge. The skin is smooth and free of hairs. There are numerous dark spots scattered over the sides and top of the body. The head is dark brown to black. These larvae or borers are the destructive stage.

The pupal stage of the corn borer is seldom seen. Pupae remain inside the host plant, but are present for only short periods of time in late spring and in July. They are smooth, light to dark brown in color, and 1/3-to 5/8-inch in length.

Life History

There are two generations per year of this insect in Pennsylvania. They pass the winter as fully grown larvae inside the stalks and residue of their host plants, and transform into the pupal stage from late April to early June. Moths emerge over a long period: from early May in southeastern areas to late June in northern counties. The majority of the eggs are deposited from mid-May to mid-June. The eggs hatch approximately 1 week after being laid. The newly hatched larvae chew small, round holes in the leaves and then move to the main stalk. They complete their growth inside the stalk, in about 3 weeks. Pupation takes place inside the stalk during July and early August.

The second generation moths emerge from late July to late August. At this time of year, most corn has passed the stage when it is attractive to the moths. A high proportion of these second generation eggs are deposited on plants other than corn. The larvae of this generation attack the tassel and shank areas of corn. Second generation larvae complete their growth before cold weather arrives, and then pass the winter in their burrows in the plants. Larvae that have not obtained full growth will usually die off during winter.

Damage

Corn borer larvae will feed on almost any weed or cultivated plant that has a stem or a fruit large enough for the larvae to bore onto. Besides corn, favored hosts include fruits of bell peppers, snap and lima beans, potato vines, fruits of tomatoes, and many flowers and weeds.

The first symptoms of corn borer infestation appear on the plants a few days after the first generation larvae hatch from the eggs. Small pin holes will be seen in the leaves and fine sawdust-like frass (excrement) will be scattered over the upper surface of the punctured leaves. Another typical symptom is a noticeable amount of frass and damaged leaves in the whorl of the plant. When the larvae enter the stalk, they leave visible, small, round holes with wet frass exuding from the holes. Feeding done inside the stalk may weaken the stalk to the point of breaking. Damage resulting from first generation corn borer larvae is seldom great enough on field corn to warrant insecticide application.

Activity of the second generation larvae, which appear from late July through August, is similar to the spring generation, but with several exceptions. Larvae commonly move to the tassel area. Infested tassels and the upper portion of the plant often break over. Some larvae enter the shanks and ears. Weakening of the shanks often results in the ears dropping to the ground where they cannot be gathered with the harvesting equipment.

Greatest field corn losses from second generation corn borer appear to occur on either late-planted or late-maturing varieties.

Control

It is seldom profitable to apply special control measures for European corn borer on field corn in Pennsylvania.

Cultural methods:

Select a hybrid that performs well in your area. Most hybrids carry some tolerance to corn borer damage.

Avoid late planting as much as possible. The second generation is apt to attack late-maturing plants.
Use good weed control programs to help reduce corn borers in the field. The adult moths hide in grass and weeds during the day, thus a good herbicide program will keep many of the moths out of the fields.
Keep weedy field edges and fence rows clipped to avoid a harbor for the moths.
Shred or bury old stalks to kill many of the overwintering larvae. However, the benefit for corn borer control is questionable.

Chemical control:

There is seldom appreciable economic loss from corn borer to corn harvested for ensilage. Thus, insecticides are not recommended to control this pest on silage corn in Pennsylvania. Grain losses from corn borer infestation have been relatively low in the state over the past decade. However, during this period, a number of fields, especially in the south central area of Pennsylvania, did suffer moderate stalk breakage.

Unfortunately, yield losses associated with stalk damage are not documented, so the value of insecticide application to control the pest is uncertain. There is some doubt as to the level of infestation above which it is profitable to apply an insecticide. This minimum level of infestation is seldom reached in Pennsylvania corn fields for the first generation corn borer larvae. Insecticide applications against first generation larvae seldom would be profitable.

Yield losses can be associated with the second generation larvae, but it is difficult to predict when an insecticide application will be profitable. Recommendations from several neighboring and mid-western states suggest applying insecticide when approximately 75 percent of the plants show pinhole feeding on the leaves and have sawdust-like frass in the whorl and tassel area.

Numerous insecticides are registered for corn borer control. Granular formulations are preferred. Aerial application is the most practical means of applying insecticides to corn for second brood corn borer. Check the Agronomy Guide or consult with your pesticide supplier or county agent for details of pesticide use.

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

*** Disclaimer ***

Authored by: Stanley Gesell, Extension Entomologist

Dennis Calvin, Professor of Entomology

Updated September 9, 2000

© The Pennsylvania State University 2002
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