European Corn Borer

The European corn borer (Ostrinia nubilalis) was first discovered in North America in 1919 and in Manitoba in 1948. It has been an economic pest in Manitoba corn fields on a sporadic basis. The European corn borer will feed on sweet, grain and silage corn. In addition, the European corn borer has a wide variety of other host plants including potatoes, beans, wheat, millet and many species of large stemmed flowers and weeds.

In Manitoba there is only one generation of corn borer per year. In some years, as much as ten per cent of the population will attempt a second generation although this generation will be unsuccessful. Infestations in Manitoba are quite variable from year to year with some areas experiencing heavy infestations while other areas have little or no damage caused by the corn borer.

Life History

The European corn borer has four stages in its life cycle.

Adult

Adult moths are buff-coloured with brown markings on its wings (Figure 1). The male moths are darker with darker markings than the female moths. The adult moths emerge from the pupae in late June and early July. The adults are 0.5 inch (12 mm) long with a wingspan of 1.0 inch (25 mm) and are strong fliers. Newly emerged adults seek out dense areas of vegetation, preferring grassy ditches where they congregate and mate. After mating, females lay eggs during calm, warm summer evenings on the underside of corn leaves near the mid-rib.

 
Figure 1. Adult moths                                                         Figure 2. Egg masses

Egg

Each egg mass consists of 10 to 40 eggs. Each female can lay two egg masses per evening, for up to ten evenings. Newly laid eggs are white and overlap, resembling fish scales (Figure 2, upper egg mass). Just before the eggs hatch, they appear black. The dark appearance is caused by the dark heads of the young borers inside the eggs (Figure 2, lower egg mass). The eggs hatch within three to seven days, depending upon temperature.                                                   

Larvae

There are five larval instars of the corn borer. The first two instars (Figure 3) are whitish with black heads and feed within the whorl and cause shothole and window pane damage. The first two instars complete development in seven to ten days. The third instar larvae bores into the stalk. Once inside the stalk, it is too late to achieve effective chemical control. The later instars (third to fifth) feed within the stalk and ear shanks, disrupting the normal movement of nutrients, which results in decreased yield. Tunneling and boring may permit secondary infection and damage by rotting of the stalk and ear. Older larvae (Figure 4) are flesh coloured with black spots and may also feed on silks, kernels and cobs. Once they are finished feeding, the mature larvae overwinter in corn stalks, cobs and plant debris on the soil surface.

  
    Figure 3. Early instar               Figure 4. Older larva                                              Figure 5. Pupa
                                                                       

                              Figure 6. Scale for determining stage of European corn borer larvae.

Pupae

Damage

The European corn borer causes damage in several ways. The first sign of damage is the pinhole damage. If the larvae establishes within a plant it may cause several types of damage. The most severe damage is stalk breakage prior to harvest (Figure 7). Another type of damage is ear drop where cobs fall to the ground. The final type of damage may be the least obvious but can cause significant yield loss. By restricting nutrient flow in the plant, yield is affected by the production of smaller cobs (Figure 8).

Figure 7. Stalk breakage.                                     Figure 8. Smaller cobs.

Field Scouting

Management Worksheet For European Corn Borer

Economic Threshold (Corn borers/plant)

Control

Cultural Control

Stalk Management: Primary tillage such as chisel plowing or moldboard plowing in the fall can reduce overwintering populations. However, soil and moisture conservation must also be considered when managing insect populations. Mowing corn stalks after harvest can also reduce overwintering populations. 

In Manitoba, time of planting does not alter the severity of the corn borer infestation due the much longer oviposition (egg-laying) period.

Rotation will also help control populations.

Biological Control

There are many natural enemies that will feed on corn borer larvae. These include lady beetle adults and larvae, syrphid or hover fly larvae and green lacewing larvae. Minute pirate bugs will search for and feed on eggs of European corn borer and other caterpillars on corn. Natural enemies may not provide adequate control in outbreak situations. In non-outbreak situations it is important to not apply chemical control measures unless the economic threshold is reached so that the populations of beneficial insects will not be affected.

Birds may also prey on European corn borer larvae. Overwintering crows will perforate cornstalks and eat overwintering larvae of European corn borer. Red-winged blackbirds will also consume larvae of European corn borer, although they can also damage standing corn.

Resistant Cultivars

Cultivars of Bt corn (which express proteins from Bacillus thuringiensis) are available which are resistant to feeding by European corn borer, and for some cultivars feeding by certain other caterpillars. If planting cultivars of Bt corn, a refuge of non-Bt cultivars is required to be planted to reduce the odds of European corn borer developing resistance to Bt corn. There are 2 types of refuge. Some cultivars of Bt corn will be purchased containing an integrated refuge (sometimes referred to as refuge-in-a-bag), where seeds of a refuge cultivar have been pre-mixed with seeds of the Bt cultivar in the bag. This integrated refuge typically comprises 5% or 10% of the seeds in the bag. Other Bt cultivars will not have an integrated refuge blended in, in which case blocks or strips of a cultivar of corn susceptible to European corn borer needs to be planted within or adjacent to the Bt cultivar. This is called a structured refuge. Planting a non-Bt refuge is a requirement set by the Canadian Food Inspection Agency. Failure to comply with refuge requirements may lead to insect resistance, slow down the introduction of new Bt corn technologies, and affect individual grower’s access to these products. Growers of Bt corn are also required to monitor their crop for the presence of European corn borer and any feeding damage.

A table of currently commercialized Bt corn cultivars in Canada, and refuge requirements, is available at: http://www.cornpest.ca/bt-corn/index.cfm/

Chemical Control