Stalk Borers in Corn
Description
European corn borer, Ostrinia nubilalis
European corn borer eggs are scaly and glossy white in appearance and are laid in masses of 15 to 30 eggs. Upon hatching, larvae are cream or pink in color with small brown spots and a dark or reddish brown head capsule. When larvae are fully developed, they are about 25 mm (1 inch) in length.
Male moths are visibly darker and slightly smaller than the pale yellow female moths. Adults have an average wingspan of about 25 mm (1 inch). The forewings of both genders have dark wavy lines.
Southwestern corn borer, Diatraea grandiosella
Southwestern corn borer eggs are laid in groups of three to five. They are creamy white initially but develop three distinct reddish stripes prior to hatching.
After emerging, larvae are dull white with noticeable black or dark brown spots along the body. However, the spots are not usually present on overwintering larvae. Mature larvae reach lengths of 25–32 mm (1-1.25 inches). Southwestern corn borer moths are white or pale yellow and roughly 19 mm (0.75 inches) long.
Quick Facts
• Both corn borers species are moths that tend to produce two generations per growing season.
• Management of both corn borer species involves a combination of biological, cultural and chemical control.
Frank Pearis, Colorado State University, Bugwood.org
There are two stalk borer pests of significance in Colorado; the European corn borer and the southwestern corn borer. Both are species of moth with similar life history.
Frank Pearis, Colorado State University, Bugwood.org
Frank Peairs, Colorado State University, Bugwood.org
Phil Sloderbeck, Kansas State University, Bugwood.org
Adam Sisson, Iowa State University, Bugwood.org
Frank Peairs, Colorado State University, Bugwood.org
Frank Peairs, Colorado State University, Bugwood.org
Frank Peairs, Colorado State University, Bugwood.org
Department of Plant Pathology, North Carolina State University, Bugwood.org
Life History and Habits
European corn borer
European corn borers typically have two generations per growing season in Colorado. The potential injury and management practices differ between the two generations since they are present at different stages of crop growth.
European corn borer larvae overwinter in stalks of corn and other plants. They pupate in early spring and moths emerge in late May or early June. Female moths lay eggs in June in weedy edges around corn fields. Eggs are laid on the underside of corn leaves near the midrib. After hatching, larvae develop over four to six weeks and pupate inside of the corn stalk. Second generation moths emerge from mid-July to early August and begin laying eggs of the second generation.
Second generation moths tend to prefer laying eggs on tasseling corn in the green silk stage. Fields that mature later in the season are more attractive to egg-laying moths than fields nearing maturity. Larvae of the second generation tend to overwinter and do not pupate until the following spring.
Southwestern corn borer
Like the European corn borer, the southwestern corn borer produces two generations a year and has a similar life history.
After overwintering, southwestern corn borer larvae pupate in the spring and emerge about 10 days later. These adults lay first generation eggs, usually on the upper surface of leaves. After hatching larvae feed for four to six weeks in whorls and plant stalks. They then pupate, which results in another flight of second generation moths. The second generation moths tend to lay eggs on the upper surface of leaves in the ear zone of tasseled plants. Unlike the European corn borer, southwest corn borer larvae will attack any stage of the plant.
When fully developed, second generation larvae prepare for overwintering by tunneling to the base of the plant and form a groove inside of the stalk several inches above the soil surface. They then tunnel into the crown of the corn plant, molt, and plug up the entrance of the hibernation cell where they overwinter.
Injury
European corn borer
Feeding of first generation larvae causes shot-hole formation in emerging leaves (see images). As the larvae grow, they bore into several plant structures including ears, tassels, shanks, and stalks.
Symptoms of feeding injury include dropped ears, broken shanks, broken stalks, sawdust-like castings on leaves, and holes in stalks.
Southwestern corn borer
First generation larvae feed on plant whorls and can destroy the growing tip. Second generation larvae feed on the ears or leaf sheathes. The larvae of both generations bore into stalks when they are about halfway through their development.
Boring of second generation larvae weakens the structural integrity of the plant and makes them more susceptible to lodging.
Monitoring & Chemical Control
European corn borer
Examine five sets of 20 plants per 50 acres of corn for feeding damage. In addition, check five whorls for live larvae at each sampling site. Taking fewer samples increases the risk of making incorrect treatment decisions.
When feeding injury is present on 25% of the plants, and the whorls contain live larvae, chemical control is economically justifiable. Control is not possible after larvae enter the stalk, and second generation larvae show high survival during pollination. Consider treating crops when scouting shows a total of 25% of plants containing egg masses. After pollination, this threshold can be raised to 50%.
A second treatment may be justified if egg laying continues after the first treatment.
Southwestern corn borer
Corn should be checked for egg masses and larvae from mid-May through June, with scouting for second generation borers from mid-July through August.
If 25 percent of the scouted plants have egg masses or newly hatched larvae, chemical control should be considered.
Cultural Control
The use of Bt corn is recommended in some areas for both European corn borer and southwestern corn borer. For more information on zoning and specific management recommendations, consult the full factsheet.
European corn borer
Planting earlier in the season, and planting of shorter season hybrids, tend to attract first generation female moths who are attracted to taller plants. Later plantings with longer season hybrids will be more attractive to second generation female moths since they are more attracted to flowering plants.
Heavily infested fields should be harvested first in order to minimize losses due to lodging and ear drop, which can outweigh early harvest and drying expenses.
Southwestern corn borer
Crop rotation is recommended to reduce yield loss.
Early plantings can help prevent second generation damage by the southwestern corn borer.
Early harvesting of infested crops can help minimize yield loss due to lodging and ear drop. Also, short season varieties can escape damage of second generation southwestern corn borer larvae.
Destroying stalks in the fall and winter in the area can help reduce overwintering populations.
Biological Control
The fungus, Beauveria bassiana, and the protozoan, Nosema pyraustae, can kill European corn borers.
Certain flies and wasps will parasitize European corn borer larvae.
Predators of European and southwestern corn borer include lady beetle larvae and adults, and green lacewing larvae that feed on eggs and newly emerged larvae.