that overwinters in corn stubble.
in ear shank.
H.R. Willson
Associate Professor
J.B. Eisley
Research Associate
The European corn borer (ECB), Ostrinia nubilalis (Hubner), is a common pest of corn in Ohio that may cause economic losses during a growing season. European corn-borer infestations differ over time and among geographic regions in the state. Where ECB is active, the development of borers in corn stalks interferes with the flow of nutrients in the host plant, enhances infection by stalk diseases, causes stalk breakage and ear drop, and reduces corn yield.
ECB overwinters as late-instar larvae in corn stalks. In the early spring, the overwintering larvae pupate and then emerge as moths that prefer to deposit their egg masses on the underside of leaves of mid-whorl stage corn. Each egg mass appears like a small mass of fish scales and may include 15 to 20 eggs. The eggs hatch into early-instar larvae that initially feed on foliage, causing window-pane injury, and on the tender central whorl, which subsequently leads to shot-hole injury in the emerging foliage. As the larvae reach the third- and fourth-instar stages (about 1/2-in. long), they will tunnel into the mid-ribs and stalks. There they will complete their larval development as fifth-instar larvae and transform into pupae from which adult moths will emerge.
The larval generation, which occurs in the spring on whorl-stage corn, is called the first brood. The period of time required for larval growth from egg hatch to pupation is about 29 to 33 days, depending on prevailing temperatures. During the period of larval development, significant proportions of larvae perish due to natural elements such as heavy rains or predation by beneficial insect predators. Although 15 to 20 larvae may emerge from a single egg mass, it is rare to find one or more mature larvae in a corn stalk or adjacent stalks to which larvae may have migrated. Mortality of ECB larvae occurs primarily during the early first-, second-, and third-larval stages when the larvae are most susceptible to adverse weather and predators. The late fourth- and fifth-instar stages are less likely to be killed by weather or predators, but they are susceptible to parasitism by beneficial parasitic wasps.
Larvae of the first brood pupate in the stalks and later emerge as adult moths in late July and early August. These adults of the summer flight prefer to deposit their eggs on late-planted corn. The larvae hatching from this generation are referred to as the second brood; they will overwinter as late-instar larvae in corn stubble. Injury caused by the second brood often includes stalk breakage, ear drop due to infestation of shanks, and infestations of the ear.
|
|
| Fifth-instar corn-borer larva that overwinters in corn stubble. |
Second-brood ECB late-instar larva in ear shank. |
Larvae of the second brood are also susceptible to adverse weather, predators, and parasites. The overwintering generation of late second-brood larvae is very susceptible to insect, avian, and mammalian predators that become active during the late fall, winter, or early spring. A cold winter with a good snow pack may favor survival of overwintering ECB larvae more than a mild winter when predators are active.
Yield losses due to corn borer are due to a combination of stalk injury by first- or second-brood larvae, ear drop due to second-brood injury to shanks and ears, and enhancement of stalk rot due to microbial infection of injured stalks. In general, if one larva tunnels and completes its development per stalk, the assumption is that a 5% yield reduction may be expected. The severity of ear drop and stalk breakage depends on the incidence and location of borer cavities and environmental conditions favoring plant infections by microbial agents.
Sampling for first-brood injury should be initiated when corn is in the whorl stage and early shot-hole foliar injury is evident. Sampling should be based on inspection of a series of 20 plant samples from five or more locations in a field. Notes should be taken on the proportion of the stand exhibiting whorl injury, and the whorls of at least two plants from each sample of 20 plants should be dissected to determine the number of larvae per injured plant and the predominant stage of ECB larvae development.
 |
 |
| Mid-whorl-stage corn exhibiting shot-hole injury by first-brood ECB. | ECB entry hole indicates larvae are no longer susceptible to treatment. |
If an infestation is significant, 75% or more of a stand will exhibit whorl-feeding injury, and early larvae will be readily found in infested whorls. It should be recognized that early larvae in a whorl may have a high level of mortality due to climatic conditions; thus, it is important to determine whether larvae are present or not when estimating stand infestation. (Larvae will most likely be found in whorls exhibiting foliar injury on the central whorl.)
If rescue treatment is under consideration, the treatment should be applied before larvae begin tunneling into the stalk, and an average of one or more larvae per stalk must be prevented from causing stalk injury. Applications of granular treatments tend to be more effective than liquid treatments.
Sampling second-brood corn borer is more difficult than first brood, since detection of early larvae on corn in the tassel stage or later is complicated by the size of the stand. However, in a year of heavy second-brood activity, the problem may be forecasted by observation of an abundant summer flight of adults. Assessment of second-brood activity by counting egg masses is commonly recommended, but observation of one or more egg masses per plant is a rare event in Ohio.
 |
| Corn stalk with multiple cavities due to second-brood ECB infestation. |
Assessment of second-brood injury in early September with an emphasis on evaluation of stalk quality and ear shank infestation will provide relevant information on the need for scheduling timely harvest of corn stands that may be susceptible to significant ear drop or stalk breakage if harvest is delayed. Corn-borer management has recently shifted to the use of preventive practices such as planting Bt-corn hybrids, which provide virtually 100% protection against ECB infestation, or using a systemic insecticide at planting, which provides effective control of first- brood ECB. Where investment in preventive measures is considered, annual assessment of the incidence of corn-borer stalk injury and yields obtained per hybrid have become important factors in making decisions on whether investment in a preventive practice is warranted over time.
J. B. Eisley, Research Associate
Department of Entomology
The Ohio State University
1991 Kenny Road
Columbus, OH 43210
Phone: 614-292-3851
E-mail: eisley.1@osu.edu
Click here to view the PDF of this Fact Sheet.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868