European corn borers feed on all types of corn, and also attack many other crop and weed species (e.g., peppers, apples, soybean, cotton, foxtails, pigweeds, ragweeds, smartweeds, etc.).
European corn borer is thought to have originated in Europe, where it is widespread. First detected in North America in 1917, the insect is found today in nearly all of the major corn-growing areas of the U.S and Canada.
- Full-grown larvae are 3/4 to 1″ long and tend to be light brown or pinkish gray in color with a brown head. Rows of light brown spots run the length of the body.
- Adult moths are quite small, with a wing span of about 1″; females are slightly larger than their male counterparts. Female moths are pale yellow-brown with irregular darker bands running in wavy lines across their wings. The male is darker in color – usually pale brown or grayish brown – but also sports dark zigzag lines and yellowish patches. Wings are usually folded over the body.
- Adult longevity is typically 18 to 24 days.
- The eggs are oval, flattened, and creamy white in color. The color darkens to beige or orangish tan color as hatching becomes imminent.
- The European corn borers overwinter as full-grown larvae in corn stalks and other plant residues.
- Overwintering larvae pupate in the spring, emerging as moths in late May and early June.
- Mating occurs in early to mid-June. Female moths generally lay their eggs on the underside of corn leaves and on ears in groupings of 15 to 30 eggs.
- Eggs hatch into larvae in about a week and commence feeding on leaves, making small holes and leaving a sawdust-like frass in their trail.
- As the larvae continue to grow, they bore into stalks, and some find their way to the ears.
- Mature larvae pupate within the host and emerge as adult moths to start the second generation in August. First-generation moths prefer early-planted corn, whereas second-generation moths target actively pollinating corn.
- The number of generations varies from one to four, dependent upon climate.
Damage and Detection
- During June and early July, young first-generation larvae feed on corn leaves and tassels and beneath husks.
- Older larvae tend to burrow into the corn stalk and the base of the ear or into the ear cob or kernels.
- By the time the first-generation borers are half-grown, they will have moved down the stalk and progressively bored into it, leaving behind sawdust-like excrement called frass at the stalk entry holes.
- The boring damage can weaken the plant to the point of causing stalk breakage or stunting, both resulting in yield reductions.
- After boring into the stalk, the larvae tend to feed inside the plant until they pupate. Stalk entry and tunneling by corn borer may make the host more susceptible to disease, which may lead to stalk rot or other maladies.
- Second-generation borers concentrate their attack in the ear zone, where they feed on the kernels and cob, resulting in yield losses. Regardless of the target of the attack, second-generation borer damage can result in grain and crop loss/yield diminishment and poor grain quality.
- In crops other than corn, the pattern of damage varies. European corn borer larvae damage the stem and fruit of beans and peppers. In celery, potato, rhubarb, and tomato, the stem is the primary target. In beet, spinach, and rhubarb, leaf tissue may be attacked.
Prevention and Control
Monitoring and Sampling
- Moths can be monitored using net traps baited with pheromone or blacklight lures to determine the degree of infestation.
- Suspended row covers erected while the corn is young can help detract the moths and eliminate/mitigate the need for insecticides.
- Destruction of stalks, where the larvae overwinter, is important. Plowing should be comprehensive (and deep), or the larvae will survive.
- Keeping fields free of weeds will also serve to control natural breeding grounds.
- There are a few natural enemies, such as certain wasps, which prey on eggs and small larvae of the European corn borer. Weighed against the impact, it is however not economically feasible to purchase and release these natural enemies. Biological insecticides based on Bacillus thuringiensis have, however, been found to be effective.
Host plant resistance
- Extensive breeding research has been conducted, and resistance has been bred into some grain corn, especially against corn borer populations with only a single annual generation. It has, however, proven to be difficult to successfully build up effective resistance without degradation of corn quality.
- Liquid formulations of insecticide are commonly applied to protect against damage to corn, particularly from the period of early tassel formation until the corn silks are dry.
- The critical periods are (1) the late whorl stage when the tassels just become visible inside the whorl, and (2) the fresh silk stage.
- In the late whorl stage, pesticides are aimed into the whorl, not elsewhere. During the fresh silk stage, pesticides are aimed at the silks.
- Intercrop Dill (Anethum graveolens), coriander (Coriandrum sativum), and buckwheat (Fagopyrum escuelentum) to prevent corn borer problems.
While every effort has been made to describe these plants accurately, please keep in mind that height, bloom time, and color may differ in various climates. The description of these plants has been written based on numerous outside resources.