Stalk borers in corn
Order: Lepidoptera
Family: Crambidae
Description
European corn borer (Ostrinia nubilalis)
European corn borer is native to Europe and an introduced species in North America where it is distributed throughout most of southern Canada and regions of the Unite States east of the Rocky Mountains. 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. 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 red-brown head capsule. When larvae are fully developed, they are about 25 mm (1 inch) in length.
Southwestern corn borer (Diatraea grandiosella)
Southwest corn borer moths are white or pale yellow and roughly 19 mm (0.75 inches) long. When laid, the eggs are flattened, pale yellow, and measure 1mm in width. They develop three distinct red-orange stripes prior to hatching, which is a distinguishing feature of southwest corn borer eggs. 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) and are cannibalistic, with only a single larva developing in each plant. While mature larvae do spin a small amount of silk, pupation does not occur within a cocoon, and male pupae are typically smaller than female pupae.
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.
Larva of European corn borer. 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. Image credit: Frank Pearis, Colorado State University, Bugwood.org
Larva of southwestern corn borer. Image credit: Frank Pearis, Colorado State University, Bugwood.org
European corn borer egg mass. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Southwestern corn borer eggs in red-bar stage. Image credit: Phil Sloderbeck, Kansas State University, Bugwood.org
Adults of European corn borer. Image credit: Adam Sisson, Iowa State University, Bugwood.org
Adult of southwestern corn borer. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Corn leaf with shot-hole damage caused by European corn borer. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Southwestern corn borer larva in the rootcrown of a corn plant. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Damage to corn plant caused by European corn borer feeding. Image credit: Department of Plant Pathology, North Carolina State University, Bugwood.org
Life history and habits
European corn borer
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 within corn stalks where they pupate. After entering corn stalks, larvae are protected from exposure to chemical insecticides. European corn borer typically produces two generations per growing season in Colorado. The second-generation moths emerge from mid-July to early August and begin laying eggs of the second generation. The potential injury and management practices differ between the two generations since they are present at different stages of crop growth.
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. This species has a strong preference for corn but can develop on over 200 herbaceous plant species.
Southwestern corn borer
Larvae are the overwintering stage and do not survive temperatures that fall below -7°C (19.4 °F) on average for about 30 days. After overwintering, southwestern corn borer larvae line their tunnels with silk and chew an exit hole in the host plant. Pupation occurs in the spring and the moths 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 before pupating, 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. A single female can lay 100-400 eggs singly or in masses of three to five overlapping eggs.
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. This species produces two generations per year. This pest only develops on a few species of wild or cultivated grasses, with corn being the preferred host.
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. Yield losses up to 20% have been observed due to infestations of European corn borer. Additional losses can occur when injured corn becomes more susceptible to infected with the fungi Fusarium proliferatum and Fusarium subglutinans.
Southwestern corn borer
First generation larvae feed on plant whorls and can destroy the growing tip, while 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. In addition, feeding injuries on developing ears can lead to fungal infections of Aspergillus flavus, leading to the accumulation of aflatoxin which can drastically reduce the profitability of infested corn. Heavy infestations of southwestern corn borer can reduce plant height and grain yields by over 50%.
Monitoring
European corn borer
Five sets of 20 plants per 50 acres of corn can be inspected for feeding damage. In addition, five whorls can be checked for the presence of 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. Treatments should be considered 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
The use of light traps or pheromone-baited traps can be used to monitor adults. Once moths have been detected, corn should be checked weekly for egg masses and larvae from mid-May through June, with scouting for second generation borers from mid-July through August. If 25% of the scouted plants have egg masses or newly hatched larvae, chemical control should be considered.
Cultural control
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 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. The most severe yield losses are associated with late planting and harvesting, and 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. Corn hybrids of the inbred line MP496 are resistant to feeding of southwestern corn borer larvae and are less attractive hosts for oviposition. Destroying stalks in the fall and winter in the area can help reduce overwintering populations.
Biological control
The use of Bacillus thuringiensis (Bt) corn is recommended in some areas for both European corn borer and southwestern corn borer. This approach has been widely adopted throughout the corn belt in the United States. However, overreliance on Bt corn can promote resistance in the pest population. To combat this, insect resistance management (IRM) strategies have been developed to dilute resistance genes. This involves providing high dose alongside refuge areas, in which moths are exposed to a high dose of the Bt toxin and allowed to mate with individuals that feed on refuge plants that do not contain the Bt toxin. The success of this method relies on effective dispersal of moths between corn fields. In the corn belt, it is recommended that unsprayed refuges be 800 m (1/2 mile) or closer to Bt cultivation areas, or that sprayed refuges be 400 m (1/4 mile) or closer to Bt cultivation areas. At least 20% of the crop should be planted as refuge, and it is also recommended that growers minimize the interface between Bt and refuge corn. For more information on zoning and specific management recommendations in Colorado, visit the full factsheet.
Natural enemies include predators, parasitoids, and pathogens. 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. The fungus Beauveria bassiana, and the protozoan Nosema pyraustae, are pathogens that can kill European corn borers. Beauveria bassiana was found infecting 3-15% of diapausing southwestern corn borer in Texas and caused about 6% mortality.
Certain flies and wasps will parasitize European corn borer larvae. The parasitoid wasp Trichogramma minutum is an effective parasitoid of southwest corn borer eggs and can help suppress late-season populations. However, high parasitism rates of this wasp have not been documented in Colorado.
Chemical control
In states where these pests are prevalent, various insecticides are available for control. However, effectively timing the application is important, and Colorado registration and use restrictions should be reviewed before purchasing and applying insecticides.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Chippendale, M., and C. Sorenson. (n.d.). Biology and Management of the Southwestern Corn Borer. Radcliffe’s IPM World Textbook. Available https://ipmworld.umn.edu/chippendale
Giron-Calva et al. 2021. [beta]-carotene and Bacillus thuringiensis insecticidal protein differentially modulate feeding behaviour, mortality and physiology of European corn borer (Ostrinia nubilalis). PloS One. 16(2): 1-16.
Available https://colostate.primo.exlibrisgroup.com/discovery/fulldisplay?docid=cdi_gale_infotracmisc_A651981559&context=PC&vid=01COLSU_INST:01COLSU&lang=en&search_scope=MyCampus_FC_CI_PU_P&adaptor=Primo%20Central&tab=Everything&query=any,contains,european%20corn%20borer&facet=searchcreationdate,include,2000%7C,%7C2024&offset=0
Inglis, G., A. Lawrence, and F. Davis. 2000. Pathogens Associated with Southwestern Corn Borers and Southern Corn Stalk Borers (Lepidoptera: Crambidae). Journal of Economic Entomology. 93(6): 1619-1626. Available https://academic.oup.com/jee/article/93/6/1619/2217340
Przystalksi, M., and T. Lenartowicz. 2016. Comparing the resistance of mid-maturing maize varieties to European corn borer (Ostrinia nubilalis Hbn.)—results from the Polish VCU registration field trials. Plant Breeding. 136: 498-508. Available https://onlinelibrary.wiley.com/doi/epdf/10.1111/pbr.12494
Rice, K. 2022. Southwest Corn Borer Management in Missouri. University of Missouri – Extension. Available https://extension.missouri.edu/publications/g7111
Siegfried, B., and R. Hellmich. 2012. Understanding successful resistance management. GM Crops & Food. 3(3): 184-193. Available https://www.tandfonline.com/doi/full/10.4161/gmcr.20715
UNL. 2017. Insecticide Treatments for European Corn Borer in Field Corn. University of Nebraska-Lincoln: Department of Entomology. Available https://entomology.unl.edu/insecticide-treatments-european-corn-borer-field-corn
Williams, P., G. Windham, P. Buckley, and J. Perkins. 2005. Southwestern corn borer damage and aflatoxin accumulation in conventional and transgenic corn hybrids. Field Crops Research. 91(2): 329-336.
Available https://www.sciencedirect.com/science/article/pii/S0378429004002047