The European corn borer, Ostrinia nubilalis, is a significant agricultural pest, particularly impacting maize. Originally from Europe, where it fed on millet, it was first reported in North America in Massachusetts in 1917. It has since spread across major corn-growing regions of the United States and Canada. Its feeding habits cause economic losses, historically exceeding one billion dollars annually due to decreased yield in corn and other susceptible crops.
Identification and Life Cycle
The European corn borer undergoes four distinct developmental stages: egg, larva, pupa, and adult moth. Female moths deposit their eggs in flat, overlapping masses on the underside of corn leaves. These masses contain 15 to 30 eggs, which are initially whitish or translucent light green and darken to a black-head stage just before hatching.
Larvae, the damaging stage, emerge from these eggs within three to seven days, depending on temperature. Young larvae are small, measuring about 1/32 to 1/16 inch, and are creamy-white to gray with a dark brown head and small, round, dark spots on each body segment. As they mature, larvae reach lengths of 0.75 to 1 inch, maintaining their flesh-colored or light tan appearance with rows of light brown spots.
Once mature, larvae transform into pupae, a reddish-brown stage that lasts about 12 days. Pupation occurs inside the plant’s stems or other host plant debris where the larvae overwinter. The adult moths emerge from these pupae, from late May to early July for the first generation and late July to early September for subsequent generations.
Adult moths are about 1 inch long with a wingspan ranging from 1 to 1.2 inches. Females are larger and lighter yellowish-brown with dark, irregular, wavy bands across their wings, while males are slightly smaller and darker with mosaic patterns. Climate and location determine the number of European corn borer generations per year; two generations are common in many corn-growing areas.
Signs of Infestation and Damage
Detecting a European corn borer infestation requires observing physical damage to the plant, as larvae spend much of their lives hidden inside stalks. Early signs of larval feeding include small, round “shot-hole” patterns on leaves, particularly as leaves unfurl from the whorl. This initial feeding may also appear as “window-pane” injury on foliage.
As larvae grow and bore into the plant, more damage becomes visible. Sawdust-like frass, which is larval excrement, can be found near entry holes in leaf midribs, tassels, or stalks. Infested tassels may break, and tunneling inside the stalk can lead to the plant collapsing or lodging, making harvest difficult.
The internal tunneling by larvae disrupts the plant’s ability to transport water and nutrients, resulting in stunted growth, wilting, and poor ear development. Significant yield losses can occur, with an estimated 5% reduction for each larva that completes development per stalk. Furthermore, the borings create entry points for secondary infections, increasing the plant’s susceptibility to stalk rot diseases and ear rots.
Management and Control Strategies
Managing European corn borer populations involves a combination of strategies to reduce their impact. Cultural control methods focus on modifying agricultural practices to disrupt the pest’s life cycle. Shredding corn stalks during or after harvest reduces the number of overwintering larvae by destroying their protective habitat. Adjusting planting dates can also influence infestation levels; extremely early or late planting of non-Bt corn hybrids may attract more egg-laying females.
Biological control utilizes natural enemies to suppress borer populations. Predatory insects like lady beetles, minute pirate bugs, and parasitic wasps such as Trichogramma species can feed on European corn borer eggs and young larvae, reducing pest numbers. Microbial insecticides containing Bacillus thuringiensis (Bt) are also applied, as these bacteria produce proteins that are toxic to the larvae when ingested.
Genetically modified crops, specifically Bt corn, represent a widely adopted control method in modern agriculture. These corn varieties are engineered to produce insecticidal proteins derived from Bacillus thuringiensis, providing built-in resistance against the European corn borer. Farmers are required to plant a non-Bt “refuge” alongside Bt corn to manage resistance development in the borer population.
Chemical control involves the application of insecticides, though precise timing is important for effectiveness. Insecticides are most effective when applied to young larvae before they bore into the plant stalks or ears, as once inside, they are protected from treatments. Scouting for egg masses and early larval feeding is therefore necessary to determine the optimal application window, 10-14 days after egg laying begins.