European Corn Borer After 20 Years of Bt Corn
Crop Insights written by Mark Jeschke, Ph.D., Agronomy Information Manager
- European corn borer was once one of the most destructive pests of corn in North America; however, it’s impact has declined with the widespread adoption of Bt corn..
- Despite the reduced importance of European corn borer as a pest in corn, populations are still present and outbreaks can still occur and cause significant yield losses in unprotected corn.
- European corn borer can produce one generation per year (univoltine) or multiple generations (bivoltine or polyvoltine) depending on growing season length.
- The major damage caused by European corn borer is due to tunneling in stalks, ear shanks and ears. Tunneling disrupts water and nutrient transport in the plant and increases risk of stalk lodging and ear drop.
- Population levels of European corn borer can vary greatly from year to year, and outbreaks are difficult to predict.
- Scouting to determine infestation levels and timing of larvae activity is necessary for effective management of European corn borer in non-Bt corn.
- Insecticides can be an effective tool for managing European corn borer; however, proper timing of applications is critical.
European corn borer (Ostrinia nubilalis) is an insect pest of corn native to Europe and western Asia. Its first documented occurrence in North America was near Boston, Massachusetts in 1917, where it is believed to have entered the continent via broom corn imported from Italy and Hungary. Populations spread west across the U.S. and north into Canada. By the 1920’s, European corn borer had become an important pest of corn in the Midwestern U.S. and today it can be found throughout corn production areas east of the Rocky Mountains.
For many years, European corn borer was one of the most destructive and economically important pests of corn in North America, with total crop losses averaging over $1 billion per year (Mason et al., 1996). However, it’s importance as a pest has declined with the widespread adoption of Bt corn over the past 20 years. Following initial introduction in 1996, Bt corn adoption has gradually increased and now accounts for nearly 80% of corn acres planted in the United States (Figure 2). Nearly all Bt corn in the U.S. includes one or more traits for European corn borer protection.
Figure 1. A late-stage European corn borer larva.
Figure 2. Adoption of Bt corn in the United States, 1996-2016 (USDA-ERS, 2017).
The reduced importance of European corn borer as a pest in corn has been a result of both the high proportion of corn acres protected by Bt traits and the reduction of overall population levels that this has caused. The potential for widespread planting of Bt corn to suppress population levels of European corn borer was initially unclear, particularly due to its relatively wide host range. In addition to corn, European corn borer has over 200 other host plants, including several crop and weed species. However, research has shown suppression of European corn borer population levels associated with Bt corn use in several Midwestern states (Hutchison et al., 2010). The greatest beneficiaries of the lower European corn borer population levels have been growers planting non-Bt corn, since they have realized a lower risk of yield loss due to crop damage without incurring the additional cost of planting Bt corn.
Figure 3. Side-by-side comparison of European corn borer damage in non-Bt (left) and Bt corn (right) in Nebraska.
Bt hybrids have continued to provide effective protection against European corn borer damage, and the overall threat to corn yield posed by European corn borer is generally lower now than it was prior to the introduction of Bt corn. However, populations are still present and outbreaks can still occur and cause significant yield losses in unprotected corn. A Pioneer research location in eastern Iowa in 2016 included a block planted with corn hybrids lacking European corn borer protection. This site experienced high European corn borer pressure in 2016 resulting in extensive damage to the non-Bt corn. Yield in these plots was reduced by around 60 bu/acre relative to the rest of the site, which was planted with Bt hybrids (Figure 4).
This site provided a clear demonstration that European corn borer is still capable of causing large yield losses in nonprotected corn. Given the location of this trial in eastern Iowa, a high percentage of the corn in surrounding area would likely have had Bt protection for European corn borer, indicating that significant damage is still possible even in areas with high Bt corn adoption. In areas where Bt adoption is lower or has declined, the risk of damage and yield loss could be greater. Several recent anecdotal reports from agronomists have noted localized increases in European corn borer populations in areas where growers have switched away from Bt corn to reduce costs (Begemann, 2017; Potter and Ostlie, 2017; Unglesbee, 2016).
The potential for European corn borer to cause economic damage in corn will likely never be eliminated. The ability of populations to subsist on a wide range of host species other than corn has allowed, and will continue to allow, populations to persist in corn-producing areas, albeit at lower levels than during the pre-Bt corn era. It is therefore important to maintain familiarity with European corn borer lifecycle, identification, crop damage, and management options; particularly for growers planting non-Bt corn.
Figure 4. Plot yield map of a Pioneer research location in eastern Iowa in 2016. Yield was reduced by approximately 60 bu/acre on average in plots lacking European corn borer protection. Each rectangle represents a 5-ft x 17.5-ft plot (1.5 m x 5.3 m). Click here (JPG 158 KB) or on the image above for a larger view.
Number of Generations
European corn borer can produce one generation per year (univoltine) or multiple generations (bivoltine or polyvoltine) depending on the length of the growing season (Figure 5). Following its initial introduction in North America, European corn borer populations produced only one generation per year. A two-generation per year population emerged in the eastern and north-central U.S. in the 1930s and spread rapidly through the central and western Corn Belt in the 1940s. This two-generation lifecycle predominates in most of the Corn Belt today. The vast majority of corn acres in North America lie within the region affected by two generations of European corn borer annually. Univoltine populations are most common in the northern U.S. and southern Canada. In the southern U.S., the longer growing season allows three and, in the far south, four generations each year.
Northern regions of the Corn Belt may be affected by both univoltine and bivoltine populations. The proportion of univoltine and bivoltine individuals in a population can vary from season to season based on growing conditions. A longer growing season with a warm spring and extended fall season will likely result in a higher proportion of bivoltine individuals the following season. Conversely, a shorter growing season with a cool spring and early fall will tend to result in more univoltine corn borers the following season.
Figure 5. Approximate geographic range of univoltine, bivoltine, and polyvoltine populations of European corn borer in the United States.
European corn borers overwinter in corn stalk residue as full-grown larvae in suspended development (diapause). When temperatures reach 50ºF (10ºC) in the spring, development resumes. Larvae pupate and emerge as adult moths, usually in late May or early June in the central Corn Belt. The pupal stage of the corn borer is rarely observed, as the pupae remain inside the cornstalk. The pupae are smooth, typically dark brown in color, and 1/3 to 5/8 of an inch in length. Adults fly to grassy areas to mate and then to selected corn fields to lay their eggs. These first-generation moths target the tallest corn fields for egg deposition.
European corn borer eggs are laid in an overlapping cluster, resembling fish scales, usually on the underside of leaves. Eggs are white when first laid. Each egg is about half the size of a pin head. After 3 to 5 days, the eggs change from white to a yellowish color. The appearance of a black spot in each egg indicates that the larvae are nearing hatch (Figure 6). Hatch typically occurs 7 to 10 days after eggs are laid but is temperature-dependent, so timing can vary due to weather conditions.
Figure 6. European corn borer eggs are laid in an overlapping cluster, resembling fish scales. Eggs are white when first laid (left). As the European corn borer larvae grow within the eggs, the presence of their black heads indicates that the larvae are nearing hatch (right).
Figure 7. During the larval stage there are five developmental stages called instars. The first two feed on leaf tissue, the third will bore into the stalk, and the fourth and fifth feed only on the stalk
Newly-hatched larvae will migrate toward the whorl to feed. First instar larvae will feed on young, developing leaf tissue without eating all the way through the leaf, resulting in injury patterns referred to as “window paning” (Figure 8).
Figure 8. Larvae will feed on young, developing leaf tissue without eating all the way through the leaf. The black head and lack of distinctive spots or stripes helps distinguish European corn borer from other corn caterpillars.
As larvae develop, their feeding will penetrate completely through the leaf, leaving a random “shot hole” feeding pattern (Figure 9). These holes become visible as the leaves grow out of the whorl. Leaves that are fed upon while still in the whorl will often emerge with multiple holes in a transverse pattern across the leaf.
Figure 9. "Shot hole" feeding pattern caused by European corn borer in South Dakota.
Third instar larvae will begin feeding in the whorl before boring into the stalk. Larvae will go through the fourth and fifth instars inside the stalk, completing their growth about three weeks after hatching. Fifth instar larvae are around one inch in length. Upon reaching the fifth instar, univoltine larvae will enter diapause (developmental inactivity). Bivoltine larvae will pupate inside the stalk during July and early August (Figure 10).
Figure 10. European corn borer larvae transform into the pupal stage inside the cornstalk. Pupae are dark brown and the outline of the head, wings, and abdomen can be seen.
Second generation adult emergence and egg laying begins during late July and continues through the end of August. Eggs of the second generation usually hatch in 5 to 7 days depending on weather conditions. Newly-hatched second generation larvae generally feed on the leaf axil, closer to the stalk, rather than the blade of the leaf. The larvae also feed on pollen that has collected in the leaf axil. Second generation larvae do not begin feeding on the stalk of the corn plant until the fourth instar, due to the hardness of the maturing stalks. Second generation larvae feed on the tassel and ear shanks which can result in ear drop. In the fall, fifth instar larvae will enter diapause and overwinter inside the stalks.
Population Levels and Outbreaks
One constant in European corn borer history is the difficulty of predicting outbreaks. This is because infestation levels in one year have much less impact on the following year’s numbers than do conditions during moth flights, mating, egg laying, and hatch. When inclement weather accompanies these European corn borer activities, larval survival may be greatly reduced. Under optimal conditions, each female moth can produce over 400 eggs and spread them over many plants and fields, allowing European corn borer populations to swell rapidly.
Historically, European corn borer has exhibited a tendency for population cycling in which population levels spike every 6 to 8 years. This pattern has been attributed to Nosema pyrausta, a pathogen that infects European corn borer and has the effect of regulating populations (Lewis et al., 2009). This pattern of population cycling has persisted to some extent during the era of Bt corn, although at generally lower population levels (Hutchison et al., 2010).
European corn borer larvae can be distinguished from other corn caterpillars by their dark brown or black head and lack of distinctive spots or stripes. Early instar larvae are dull white. Mature larvae are about ¾ to 1 inch (19 to 25 mm) long, dull white to grayish in color, and have small brown halo-shaped spots running the length of the body. Their skin is smooth and free of hairs and they have four prolegs on their 3rd, 4th, 5th, 6th and 10th abdominal segments. Female moths are pale yellow-brown and typically around 1 inch (25 mm) in length. Male moths are smaller with darker bands than the female. Both have front wings with jagged bands or lines across the wings (Figure 11).
European Corn Borer
- Young larvae are dull white; older larvae have darker halo-shaped spots
- Dark brown or black head
Western Bean Cutworm
- Head is solid orange
- Two dark brown stripes behind the head
- Larval color is highly variable
- Alternating dark and light stripes running the length of the body
- Usually found in leaves
- Accompanied by slight webbing
Southwestern Corn Borer
- Southern areas of U.S. only
- Dark spots on white body or pure white in late fall
Lesser Corn Stalk Borer
- Purple bands
- Found sporadically, rarely a significant pest of corn
Figure 11. European corn borer adult male (left) and female (right).
Damage and Impact on Corn Yield
The major damage caused by European corn borer is due to tunneling in stalks, ear shanks, and ears. Tunneling disrupts water and nutrient transport in the plant and increases risk of stalk lodging and ear drop. In addition, damage may allow higher levels of stalk rots and ear molds. The magnitude of the yield reduction due to corn borer tunneling depends primarily on the growth stage of the corn plant when attacked, the growing environment, and hybrid tolerance or resistance. Larval feeding during mid- to late vegetative (V6- V16) and early reproductive stages (VT-R3) can reduce yield more than larval feeding in later reproductive stages. Environmental stresses such as drought or disease affect borer-damaged plants more than undamaged ones.
To help determine yield loss levels due to European corn borer, Pioneer researchers tested hybrids with Bt insect protection for European corn borer vs. their non-Bt counterparts in replicated research trials. These studies were conducted in 119 locations over six years. Plots were evaluated for corn borer damage by examining 10 consecutive plants in each plot.
Figure 12. Relationship of European corn borer tunnels/plant to the yield advantage of Pioneer® brand corn products with a Bt trait for European corn borer protection over hybrids without insect protection, 119 environments, 6 years.
Stalks were split on these 10 plants and the number and total length of tunnels were recorded. Plots were harvested for yield, moisture, and test weight measurements. Results showed that hybrids with Bt insect protection for European corn borer had an average yield advantage of approximately 7% for every corn borer cavity per plant (Figure 12). This relationship was demonstrated at low as well as high infestation levels.
Scouting and Management in Non-Bt Corn
Scouting to determine infestation levels and timing of larvae activity is critical for effective management of European corn borer in non-Bt corn. With normal temperatures, the ideal “window” of treatment will only be about 4 to 6 days and, once larvae are in the stalk, insecticide treatments will be ineffective.
Corn borer moths mate in grassy areas and fly into corn fields to lay their eggs. Female moths in flight are attracted to the tallest corn in an area, so earlier-planted fields are at greater risk for infestations of first-generation larvae. Trapping of European corn borer moths during mating activity can be a helpful tool to guide field scouting. Black light and pheromone traps can both be used to monitor moth activity. However, moth trapping is not predictive of infestation levels or crop damage. It can help guide scouting efforts but cannot serve as a substitute for scouting.
Corn of less than 18 inches extended leaf height is safe from corn borer feeding, because of DIMBOA (2,4-dihydroxy-7- methoxy-1,4-benzoxazin-3-one), a naturally occurring antibiotic present in corn, wheat, and other grass species. The concentration of DIMBOA is greatest in seedlings and decreases as the plant ages. As corn grows above 18 inches, the natural resistance of the plant is diminished, and feeding damage can occur. Begin scouting fields for signs of shot holes in the leaves about 200 heat units after corn reaches 18 inches in extended leaf height.
Good random samples taken throughout the field are needed to accurately estimate European corn borer populations. Moths and egg laying may be concentrated along field edges, grass waterways, or access roads, so sampling along these edges will not provide an accurate estimate of the field population. Female moths are selective about where in the field they deposit their eggs; consequently, infestations tend to be clustered, rather than uniform across the field. The first sample should be collected at least 100 feet in from the edge of the field. Plants should be sampled across the entire field, using care to sample areas that may have different plant heights, age, or density. If more than one hybrid is planted in the same field, consider each hybrid as a separate field for scouting purposes. Scout 20 random plants at each of five locations in the field for feeding. Pull out and unroll at least two whorls at each of the five locations to estimate borers per plant.
If a majority of the larvae found are less than ¼ inch long, then wait 3 to 5 days for additional larvae to hatch before treating. However, be sure to treat before larvae are 11/16-inch long (about the length of a dime). After this stage, larvae leave the whorl and tunnel into the stalk where an insecticide application will not kill them. Wet excretory material protruding from entry holes indicates that stalk boring has begun.
Egg mass counts are the preferred method of scouting for second generation European corn borer. Begin scouting for egg masses in corn when corn borer moths are being collected in light or pheromone traps. Continue scouting every 3 to 5 days, especially during the early part of the moth flight period. Egg laying may extend over a period of 3 to 4 weeks. Concentrate sampling efforts on fields with the highest likelihood of infestation – late planted fields and/or those that are green, succulent, shedding pollen, or have green silks in late July and early August.
Scout for egg masses on a minimum of 50 randomlyselected plants from several different parts of the field. Look for egg masses on the underside of leaves above and below the ear leaf. Egg masses are usually laid on the underside of the leaves near the midrib on the middle 1/3 of the plant. Count the number of plants with egg masses and the number of egg masses per plant. Multiply the number of infested plants by 2 to get the percent infestation. Insecticide treatments should be applied when the majority of eggs are in the black head stage or hatching. If eggs have already hatched, look for entry holes and frass on the stalks. Split stalks if necessary to determine if larvae have entered the stalk where they will not be affected by an insecticide treatment.
Figure 13. European corn borer larva tunneling in corn stalk.
Insecticide Treatments and Economic Thresholds
Properly timed insecticide applications can provide effective control of first generation European corn borer. Managing the second generation is more difficult; an insecticide treatment will likely provide around 65% control. Economic thresholds for insecticide treatment vary based on several factors:
- Percent of plants with whorl feeding damage or egg masses or larvae
- Corn growth stage
- Cost of treatment
- Expected value of the crop
Corn yield loss per borer will be greater with earlier infestations relative to the crop growth stage (Table 1). Additional stress factors such as drought and foliar disease can exacerbate yield losses from European corn borer damage. Numerous universities have economic threshold worksheets that can provide an estimate of the potential value of an insecticide treatment.
Table 1. Yield losses caused by European corn borer damage at various corn growth stages (Boyd and Bailey, 2001).
Bt corn has been and continues to be a very effective management tool providing protection against European corn borer. The widespread adoption of Bt corn in the Midwestern U.S. over the last 20 years has suppressed population levels and reduced the importance of European corn borer as a pest of corn. However, the wide host range of European corn borer allows it to persist as an ever-present threat to corn production. Outbreaks and yield loss can still occur in unprotected corn. It is important that growers planting non-Bt corn maintain familiarity with European corn borer lifecycle, identification, crop damage, and management options in order to avoid significant yield losses in the event of an outbreak.
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