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.

Adult moths are buff-coloured with brown markings on its wings
Figure 1

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.                                                   


Figure 2.

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 (Figure 4). 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 5) 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.

   shothole and window pane damage
    Figure 3                                                    Figure 4                                                                     Figure 5
                                                                       

European Corn Borer Larval Size Scale

Pupae

In the spring, mature larvae turn into pupae to complete the life cycle (Figure 6).

Mature larvae turn into pupae to complete the life cycle
Figure 6 

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).

The most severe damage is stalk breakage prior to harvest. By restricting nutrient flow in the plant, yield is affected by the production of smaller cobs.

Figure 7                                                                                   Figure 8

Field Scouting

 Field Scouting for Egg Masses and Young Larvae: Field scouting should begin in early July. Fields should be prioritized by planting date and relative maturity. Older fields should be scouted first. In each field, check 10 locations (10 plants at each location) for egg masses and young larvae. Females are attracted to corn silks and deposit egg masses predominately on leaves near the ear. The larvae are most commonly found in the midrib and leaf axil. At each location pull open the whorl to check for larvae feeding within the whorl. If the majority of larvae have bored into the stalk, do not apply insecticides as they are ineffective once the larvae have entered the stalk. If no larvae or egg masses are found, repeat scouting every five to seven days. Continue scouting until larvae start to tunnel into the stalk or the end of July if no egg masses/larvae are found.
 
Thresholds
 
For grain corn, the average grain weight reduction when stalk feeding was initiated during the 10-leaf, 16-leaf, blister and dough stages were 5.94, 5.01, 3.13, and 2.41 percent per larvae per plant, respectively.
 
Use the Management Worksheet and Economic Threshold table below to determine whether chemical control measures are necessary.

Management Worksheet For European Corn Borer

_____ % of 100 plants infested x _____ average # borers/infested plant = _____ Borers/Plant
_____ borers/plant x 5% yield loss/borer = _____ % Yield Loss
_____ % yield loss x _____ expected yield (Bu/A) = _____ Bu/A Loss
_____ Bu/acre loss x _____ price/Bu = $_____ Loss/A
$_____ loss/A x _____ % control = $_____ Preventable Loss/A
$_____ preventable loss/acre - $_____ cost of control/A = $_____ Gain (+ or -)/Acre if treatment applied

Economic Threshold (Corn borers/plant)

Control Costs 1 ($/Acre) Crop Value ($/Acre)
200 250 300 350 400 450 500 550 600
6 0.75 0.60 0.50 0.43 0.38 0.34 0.30 0.27 0.25
8 1.00 0.80 0.67 0.57 0.50 0.45 0.40 0.37 0.34
10 1.25 1.00 0.83  0.71 0.63 0.56 0.50 0.46 0.42
12 1.50 1.20  1.00 0.86  0.75  0.67 0.60 0.55 0.50
14 1.75 1.40  1.17 1.00  0.88  0.78 0.70 0.64 0.59
16   2.00 1.60  1.33 1.14  1.00  0.89 0.80 0.73 0.68
18   2.25 1.80 1.50 1.29  1.13 1.00 0.90 0.82 0.75
1Control costs = insecticide price ($/acre) and application costs ($/acre)

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. Natural enemies do not provide 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.

Resistant Cultivars

Cultivars of Bt corn (which express proteins from Bacillus thuringiensis) are resistant to feeding by European corn borer. 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. 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/index.cfm/news-archive/currently-registered-bt-corn-traits-in-canada-april-2015/

Chemical Control

Insecticides should only be applied when economic thresholds (ET) have been surpassed. To determine whether the population in a given field has reached the economic threshold, it is important to scout the field. Insecticide recommendations for European corn borer in field corn can be found in Manitoba Agriculture, Food and Rural Development's Guide to Field Crop Protection. Insecticide recommendations for European corn borer in field and sweet corn can be found in the WCCP Guide to Integrated Control of Insect Pests of Crops: http://www.westernforum.org/Documents/WCCP/WCCP_documents/WCCP_Guidelines/WCCP_15/WCCP%20Guide%20-%20Cereal%20Grains%20-%202015.pdf
 

Revised: July 2015
For more information contact: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Development, Carman, Manitoba, Canada, R0G 0J0.