Manitoba Agriculture, Food and Rural Initiatives
European Corn Borer I
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In some years, this insect can be a concern to corn producers across
the Prairies, reducing yield of grain and silage corn and reducing
the quality of sweet corn.
Host Crops
Corn, beans, potatoes and various other plants.
Biology
Mature larvae are about 2.5-3 cm (1-1.2 in.) in length, white to tan-grey in colour, and covered with rows of small brown spots. The larva has a brown head and faint, reddish stripes that run the length of its body.
During the late-spring, the overwintered, full-grown larva chews an exit hole in its corn stalk and returns to its tunnel to spin a cocoon in which to pupate. Moths usually begin to emerge from corn debris in mid-June. They often gather in areas of dense, grassy vegetation bordering corn fields. This vegetation provides a moist, damp area, optimal for moth survival and egg laying. The resident moth population may be increased by moths flying in from the south in some years.
Peak egg laying and flight occur in early to mid-July, during calm, warm nights. Female moths will lay up to 500-600 creamy-white eggs, in batches of 15-25 eggs, on the undersides of corn leaves, near the midrib. The egg masses are about 6 mm (1/4 in.) in diameter.
The eggs hatch within a week (mid-July through early August). The young larvae disperse and feed on leaves and work their way to the whorl of the plant. Pinholes or shotholes are signs of borers already moving into the plant. During later stages of borer development, most larvae enter the tassel, stalk, and ear shank, but they may also feed on the silks, kernels and cobs. Nearly-mature larvae bore into the stalk by late summer to overwinter.
May June July August September Overwintering larvae pupate Moths emerge and egg laying begins Egg laying continues, larval feeding begins Larval feeding continues in stalks Larvae begin overwintering
Scouting Techniques
Beginning in early-July or when corn is about 0.5 m (2.5 ft. high), corn growers should check their fields on a weekly basis. Move well into the field before you begin. Check 20 plants in each of 5 different areas of the field to get a reliable idea of the infestation level.
Egg masses can be seen on the lower surface of the leaves, usually near the leaf mid-rib. The eggs are white when laid. Just before hatching, the egg mass darkens because of the darkening head capsules of the larvae. This sign of imminent hatching can be used to time subsequent treatments.
The first indication of larval corn borer activity is the socalled "windowpane" feeding injury to the foliage by the very early larval stages. Later, shot-holes appear in the leaves. They are caused by the early larvae tunneling through the emerging whorl.
As the larvae increase in size, they begin tunneling into the stalks or midribs of the corn foliage. The young larvae feed on leaves and usually can be found in the tassels, beneath the husks, and between the ears and stalks. The older larvae bore into the stalks and ears where they overwinter. Infested corn stalks may break or have weakened tassels that are easily broken by the wind. All of the above-ground parts of corn plants can be damaged by the larvae.
Pheromone baited traps can be used to monitor adult corn borer activity.
Economic Threshold (Corn borers/plant)
Control Costs1 ($/Acre) |
Crop Value ($/Acre) | |||||
150 |
200 |
250 |
300 |
350 |
400 |
|
6 |
1.00 |
0.75 |
0.60 |
0.50 |
0.43 |
0.38 |
9 |
1.50 |
1.12 |
0.90 |
0.75 |
0.64 |
0.56 |
12 |
2.00 |
1.50 |
1.20 |
1.00 |
0.86 |
0.75 |
15 |
2.50 |
1.88 |
1.50 |
1.25 |
1.07 |
0.94 |
18 |
3.00 |
2.25 |
1.80 |
1.50 |
1.29 |
1.13 |
21 |
3.50 |
2.63 |
2.10 |
1.75 |
1.50 |
1.32 |
24 |
4.00 |
3.01 |
2.40 |
2.00 |
1.72 |
1.51 |
27 |
4.50 |
3.38 |
2.70 |
2.25 |
1.93 |
1.70 |
1 Control cost = insecticide price ($/acre) and application costs ($/acre)
Lower infestation levels (ex, 15% of the plants shot-holed) may reduce the quality of sweet corn.
Control Tips
Stalk Management: Primary tillage such as chisel plowing or moldboard plowing in the fall can reduce the number of overwintering larvae. Mowing corn stalks after harvest can reduce overwintering populations up to 85%. Chopping corn stalks for silage will remove and destroy the larvae. Burning is not as effective because some borers will survive in stalks not completely burned.
Resistant cultivars: Cultivars of Bt corn are available that are resistant to feeding by European corn borer. If using cultivars of Bt corn, a refuge of at least 20% of the corn acres planted to non-Bt cultivars should be planted to reduce the odds of European corn borer developing resistance to Bt corn.
Control with insecticides: Insecticides may be effective if applied when larvae are young. Once larvae enter the stalks, insecticides are not effective.
For further information, contact your GO representative.
