European Corn Borer in Field Corn

Agdex#:	111/612
Publication Date:	12/97
Order#:	97-021
Last Reviewed:	05/98
History:	Revision of "European Corn Borer in Field Corn," December 1997
Written by:	Paul Hagerman - Horticultural Crops Advisor/OMAFRA

Table of Contents 

1. Introduction
2. Biology
   • Adult
   • Egg
   • Larva
3. Types of Damage
4. Management of Corn Borers
   • Natural Factors
   • Cultural Management
   • Biological Control
   • Choice of Hybrid
   • Chemical Control
5. Scouting Procedures
   
   

Introduction

The European corn borer Ostrinia nubilalis (Hubner) is a major pest of grain corn in Ontario, causing losses in both yield and quality. It is an introduced species which arrived in the Great Lakes area in the early 1900's. It is now found throughout east and central North America, including most parts of Ontario.

Within Ontario, there are two strains of corn borers - bivoltine and univoltine (Figure 1). In the southwestern counties of Essex, Kent and Elgin the bivoltine strain completes two generations in most summers and can go on to a partial third generation in unusually warm years. In the rest of the province, the univoltine strain normally completes only one generation per year but may begin a second generation in warm years. Both strains exist in significant numbers in a broad area of overlap including Lambton, Middlesex, Oxford, Brant, Haldimand-Norfolk, Hamilton-Wentworth and Niagara Counties.

Figure 1. Map of Southern Ontario, showing range of univoltine and bivoltinecorn borer populations with overlap zone

The bivoltine and univoltine strains are very similar. There are no visible differences and they respond to control measures in the same way. Both are attracted to the same pheromone lure. They differ in their response to temperature and day length. Under similar environmental conditions, the bivoltine strain emerges earlier in the spring and enters diapause (the inactive over-wintering stage) later in the summer or fall. The risk of crop damage by and the timing of control strategies for corn borers depends partly upon which strain is present.

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Biology

The corn borer has four stages in its life cycle - adult (moth), egg, larva (caterpillar), and pupa. The winter is spent as a fully grown caterpillar in or near last year's host plant. While most corn borers probably over-winter in corn stalks and cobs, they can also be found in other host plants such as large-stemmed grasses and various vegetables. This Factsheet will look at the adult, egg and larva stages of the corn borer.

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Adult

In the spring, the corn borer caterpillar changes to a pupa in its over-wintering site, and then a few weeks later emerges as an adult moth. Males usually emerge a few days before females. While emergence begins around the third week of May in the southern-most part of the province, moths do not usually appear until mid-June in eastern Ontario.

Corn borer moths (Figure 2) are 1.5 - 2 cm long and about 1 cm wide when the wings are folded at rest. Their colour varies from pale yellowish-brown to medium grey. Their forewings have wavy dark lines running across them. Males have darker wings and are a little smaller than females.

Figure 2. Corn borer moths (Male is darker than female)

Corn borer moths spend most of their time in "action sites" - unmowed grassy areas near corn fields where dew accumulates. Females in action sites emit a pheromone (sex attractant) to attract males for mating. After mating, females fly to the crop to lay eggs, then return to the action site. Females normally lay one or two egg masses per night, containing five to 50 eggs each, and they can live for up to two weeks in good weather conditions. Egg laying is increased by warm, calm evenings and overnight dew. Favourable weather during egg laying is often a major factor leading to crop damage from corn borer in the fall.

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Egg

Corn borer eggs are laid in a creamy white mass which resembles overlapping fish scales. In hot weather eggs can hatch in as little as three days, but in cooler weather they may take up to nine days to hatch. Each egg develops a black centre (blackhead stage) about one day before hatching (Figure 3).

Figure 3. Corn borer egg masses (Creamy-white when laid with black heads becoming visible shortly
before hatching)

Corn borer eggs may be laid on any part of the corn plant - leaves, stem or husk. Most will be found on the underside of the leaves, close to the midrib. Eggs laid after tasselling are most often located within three leaves above or below the ear.

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The larvae (caterpillars) that hatch from eggs are about 3 mm (1/8") long with a dark head and a spotted, dirty white body (Figure 4). On whorl-stage plants, the larvae move down in the whorl shortly after hatching to feed on the soft, rolled-up leaves. If they hatch after tasselling, they normally spend the first day or two in the leaf axils feeding on pollen.

Figure 4. Newly hatched larva

Figure 5. Windowpanes and shot hole damage from early larval feeding

Most caterpillars do not survive more than a few days, but succumb to desiccation, predatory insects, drowning in rainwater, etc. For young corn in the early whorl stage, when corn plants are up to 40 cm (16") tall, the corn plant itself may increase their mortality rate using a chemical called DIMBOA. This natural plant chemical reduces corn borer feeding and increases the tendency of small caterpillars to leave the plant. DIMBOA is one of the factors responsible for the so called "natural resistance" of certain corn hybrids. Unfortunately, it dissipates as the plant grows and is of no real value by the late whorl stage.

As the corn borer grows, its feeding preferences change. After pollen, the young larvae feed on leaves creating "window panes", "pinholes", or "shot holes" (Figure 5). Usually they make several small feeding holes close together and they rarely feed at leaf margins. (Large feeding holes at leaf margins are typical of armyworms, fall armyworms and grasshoppers.)

Corn borers prefer feeding in concealed locations so they are not usually found on the outside of the plant. In whorl stage corn, they are often deep in the whorl. In tasselling corn, they may move up to the top of the plant and tunnel into the tassel. This damage leads to broken tassels, especially if winds are heavy (Figure 6).

Figure 6. Broken tassel with corn borer frass

Figure 7. Stalk entry with frass

Many borers will move to the ear and feed on the kernels or tunnel into the stalk, often entering at a leaf axil and leaving a pile of frass (sawdust-like droppings) beneath the entry hole (Figure 7). Once inside the stalk, borers tunnel up or down in the internode. The entry holes are common invasion sites for stalk rot organisms.

Corn borer caterpillars go through five instars (growth stages) and reach a total length of about 2.5 cm (1") when fully grown. The colour may vary from pale greyish-brown to dirty white to pale pink, with a medium to dark brown head capsule (Figure 8). Table 1 outlines the differences between corn borers and other caterpillars commonly found on corn.

After fifth instar, the first generation of the bivoltine strain turns to a pupa inside the stalk, tassel, etc. and emerges a few weeks later as an adult to begin the second generation. Corn borers that reach fifth instar in the fall enter an inactive phase called diapause, in which they spend the winter. They will not pupate until spring.

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Types of Damage

Corn borers feed on all parts of the corn plant except the root. Their damage leads to physiological yield loss, harvest yield loss, loss of quality and loss from secondary pests.

Harvest yield loss results when stalk or shank breakage leads to less efficient harvesting. The grain is there but the combine doesn't pick it up. Harvest yield loss is greatest with weak-stemmed hybrids and increases when late harvesting and/or adverse weather lead to more lodging in the field.

The plant produces less grain due to feeding damage to the leaves (loss of photosynthetic area) and from tunnelling in the stalk and ear shank (reduction in transport efficiency of plant carbohydrates). This is called physiological yield loss. Several studies attempting to quantify physiological yield loss have shown that it varies tremendously with choice of hybrid, weather and time of corn borer infestation. An average figure of 5% yield loss per borer per plant is often used, with a range of 2 û 7% possible under different conditions. The greatest yield loss occurs when tunnelling begins as corn enters the blister stage. Physiological yield loss is hidden ù it is not as easy to see as harvest yield loss but it can often be greater.

Corn borers cause loss of grain quality by damaging individual kernels while feeding in the ear. They can also reduce the size of all kernels by feeding elsewhere in the plant (physiological loss). Further loss of quality may occur if secondary pests are present.

The most important secondary pests are ear molds and stalk rot organisms. These diseases can enter through corn borer entry holes. Ear molds may cause the grain to be downgraded as it is less suitable as livestock feed (Figure 9). Some ear molds produce mycotoxins which cause adverse reactions in livestock at very low levels. Stalk rot organisms (Figure 10) accelerate stalk deterioration, leading to reduced carbohydrate transport during grain fill (more physiological yield loss) and more lodging. Other secondary pests which may follow a corn borer infestation include sap beetles and birds. The latter can potentially cause extensive grain losses.

Figure 9. Ear mold which often develops after corn borer damage to the ear

Figure 10. Stalk rot developing in corn borer tunnel

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Management of Corn Borers

Natural Factors

Most corn borers are killed by natural factors. A brief explanation of these illustrates how some can be influenced as part of an overall management strategy.

• Weather

  The weather conditions that occur while female corn borers are laying eggs are probably the greatest factor in determining the size of the population in a given year. For most of Ontario, peak egg laying occurs in a two-week period, usually in late July. In the bivoltine area there are two peaks usually in late June and again in late August. Warm calm nights with dew will maximize egg laying and increase the risk of damage in the field.

  Over-wintering weather also influences corn borer survival to a small extent. The diapausing larvae are able to survive very cold temperatures but mortality is increased by freeze and thaw cycles. Many borers are also killed over the winter by insect-eating mammals and birds.
  
  

• Natural Enemies

  Like all insects, corn borers are attacked by several parasites, predators and pathogens. Of the parasitic insects, some are native, but others were introduced to North America by corn borer control programs beginning in the 1920's and 30's. Most attack over-wintering larvae and result in higher winter mortality. There are also some parasites which attack corn borer eggs. Overall, parasites have a low impact on the population.

  Besides various birds and mammals which feed on corn borers, many predatory insects are present in corn fields. Those which feed on corn borers include minute pirate bugs, ladybird beetles and larvae, lacewing adults and larvae, ground beetles and spiders.

  There is some evidence that the level of predatory insects in corn fields can be influenced by farm management practices. The use of insecticides, including those applied at planting, reduces the predator population. In no-till systems the level of predators has been reported to increase.

  A few insect pathogens (diseases) can become important mortality factors, especially during prolonged periods of wet weather. Corn borers killed by these pathogens will often have mold growing on the body.

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Cultural Management

Clean plowing in the fall can kill a large percentage of over-wintering corn borers and it was once a recommended control practice. However, it is now believed that the reduction in corn borers achieved by plowing is less important than the soil conservation that could be achieved by reduced tillage.

Chisel plowing can kill 30 - 40% of the over-wintering larvae and chopping stalks before plowing can increase that to about 95%. Cultural management on a field by field basis has relatively little impact on next year's population because moths can disperse to new fields in the spring. In the univoltine area, chopping stalks has been shown to be effective as an area-wide management technique, as it killsmost corn borers and greatly reduces the number of eggs laid. In the bivoltine area, the relationship between killing overwintering larvae and the size of the second generation is poorly understood.

Good weed control can help minimize the number of corn borers in a field. Moths normally rest and mate in grassy areas outside the corn field, but if a field is very weedy they may spend more time in that field resulting in proportionately more egg laying.

Early harvesting can help reduce harvest yield loss due to corn borer. The longer a crop stands in the fall and the more wind it is exposed to, the greater the potential for lodging, especially where corn borer populations are high.

Biological Control

The presence of natural biological controls has already been mentioned. It may also be possible to manage corn borers by purchasing and releasing biological control agents in the field. Currently, the most promising agents are tiny parasitic wasps called Trichogramma, which kill corn borer eggs. Ongoing research is looking at the efficacy and cost-effectiveness of these wasps in corn production.

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Choice of Hybrid

Many corn hybrids have some natural resistance to corn borer feeding. Prior to 1996, all Ontario corn hybrids promoted as "corn borer resistant" were of this type. They are attacked by corn borers but do not suffer much yield loss. Natural resistance involves many plant characteristics, such as levels of DIMBOA, stalk strength, ear shank strength, etc. While natural resistance has been steadily improving it does not eliminate corn borer damage.

Beginning in 1996, several new hybrids were introduced with almost complete resistance to the corn borer. These "Bt corn" hybrids have been produced using biotechnology and traditional breeding methods. A gene from Bacillus thuringiensis, a soil bacterium, was inserted into the genetic material of the corn plant. It causes the plant to produce a certain protein which is toxic to corn borers when they eat it. As a corn borer begins to feed on a Bt corn plant, it picks up a toxic dose, stops feeding and eventually dies. The insect gut is alkaline and it is unable to digest the protein. Mammalian guts are acidic so the protein is digested as a nutrient and has no ill effect.

Extensive field trials on Bt corn hybrids have found almost complete control of leaf feeding, ear and stalk tunnelling, and reduced levels of ear molds and stalk rots (Figure 11). This should lead to improved yields in areas where corn borer pressure has been heavy.

Figure 11. Two stalks, with and without stalk rot

Just as insects can develop resistance to insecticides they may also be able to overcome the barriers they face in resistant hybrids. Bt corn hybrids are a recent introduction and no field resistance has been found in corn borers at this time. However, the widespread planting of Bt corns would exert tremendous selection pressure on corn borers and could lead to resistant populations in the future. If this happens then Bt corn hybrids will suffer corn borer damage just like any other.

To minimize the risk of corn borers developing resistance to Bt corn, it is important to ensure they have alternative foods without the Bt gene. Some of these foods will be supplied through other hosts, such as vegetables and weeds, but each farm should also ensure that at least 25% of their corn acreage is in non-Bt corn. This should help to keep the overall population of corn borers susceptible to Bt and keep Bt corn hybrids as a useful tool for corn borer management.

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Chemical Control

In some cases, it may pay to control a corn borer infestation with an insecticide spray. The spray decision will depend on the stage at which the corn was attacked, the severity of the infestation, the anticipated yield and value of the corn, and the cost and effectiveness of control. Sprays are more likely to be beneficial when a high-yielding field suffers a heavy corn borer infestation and the price of corn is up.

To determine whether an insecticide spray will be worthwhile, perform the following calculations:

1. Calculate the Preventable Yield Loss (PYL) in bu/ac or tonne/ha.

   PYL = [anticipated yield x infestation level (from scouting)] x [percent yield loss (Table 2) x anticipated percent borer control*]

    

   *For anticipated borer control, assume 50 (50%). This could rise to 67 (67%) if most eggs have hatched and most larvae have not yet tunnelled, and if the spray is applied in the evening in temperatures less than 25 °C. However, a poorly timed spray could result in virtually no mortality.
   
   

2. Calculate the benefit of spraying, in $/ac or $/ha.

   Benefit = preventable yield loss x market value of crop
   
   

3. Calculate the cost of spraying in $/ac or $/ha.

   Cost = insecticide cost + application cost

   In most cases, custom application is preferred as high-clearance ground equipment or aerial equipment is needed for thorough spray coverage.
   
   

4. Subtract cost from benefit to see if the spray will make money.

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Scouting Procedures

To determine the infestation level, it is essential to scout fields in the early stages of corn borer feeding. The best time for scouting can be judged by monitoring the flight of corn borer moths.

Heliothis traps, baited with European corn borer pheromone lures of the "Iowa" strain, can be used for on-farm monitoring. Place traps in grassy areas near corn fields beginning in late May (bivoltine areas) or mid-June (univoltine areas) and replace the lures weekly. For sources of trapping supplies, contact your nearest OMAFRA Crop Advisor. OMAFRA maintains a province-wide monitoring program for corn borers and reports are available on many regional agriphones.

Normally trap counts start low in the spring and then increase as more corn borers emerge. Fields should be scouted when trap counts are on the rise. Corn borer eggs, larvae, and some of their damage can only be seen close up, so examination of individual plants is necessary. For a field of eight hectares (20 acres), checking 100 plants (five groups of 20 consecutive plants) is sufficient. For a larger field, check 200 plants.

Look for corn borer eggs under the leaves and small larvae in leaf axils or in the whorl. Look for feeding damage as windowpanes, pinholes, or shot holes in the leaves, broken tassels, eartip or husk feeding or entry holes into the stalk above leaf axils. In each group of 20 consecutive plants, select two plants with damage and pull the whorls out, then unroll the leaves and check for live larvae.

Calculate the percentage of infested plants, the percentage with live larvae, and the number of larvae per plant. Also compare the number of plants with live larvae (these can likely be killed by a spray) with the number that have unhatched eggs or have larvae already tunnelled into the stalk (these will likely escape a spray). If a high percentage of plants show damage but very few have live larvae, then it is likely that most have already tunnelled into the stalk and it is too late to spray.

Sprays are most effective when applied in the late afternoon or evening. Corn borers are more active at night, the insecticides do not break down as quickly in the cooler night temperatures, and there is less chance of killing honey bees. The risk of bee kill can also be reduced by avoiding any sprays when the corn is shedding pollen and taking care not to allow drift onto field edges.

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