European Corn Borer in Sweet Corn and Other Horticultural Crops
Table of Contents
The European corn borer Ostrinia nubilalis (Hubner), a major pest of sweet corn, can also damage peppers, snap beans, potatoes, tomatoes, apples and other horticultural crops. It is an introduced species which arrived in the Great Lakes area in the early 1900's. It is now found throughout eastern 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 bivoltine corn borer populations and overlap zone.
The bivoltine and univoltine strains appear identical, are attracted to the same pheromone lure, and respond to control measures in the same way. 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.
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 field corn, 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.
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. The 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 eggs are laid in a creamy white mass which resembles overlapping fish scales. In hot weather eggs may hatch in as little as three days, but in cooler weather they may take up to nine days to hatch. Each viable egg develops a black centre (blackhead stage) about one day before hatching (Figure 3).
Figure 3. Corn borer eggs masses (Creamy white when laid with black heads visible about one day before hatching).
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).
They go through five instars (growth stages) and reach a total length of about 2.5 cm (1") when fully grown. Larval colour may vary from pale greyish-brown to dirty white or pale pink; head capsules range in colour from medium to dark brown (Figure 5).
Figure 4. Newly hatched corn borer caterpillar.
Figure 5. Fully grown corn borer caterpillar.
Control strategies for the European corn borer are most effective when timed to the most sensitive stage in the insect's life cycle. Traps that catch moths are used to determine when peak flights occur and when egg laying will occur.
Corn borer moths can be trapped using a Heliothis trap (Figure 6) baited with a pheromone lure. Traps should be set up in unmowed grassy areas near corn fields. In all parts of Ontario, corn borers are attracted to the "Iowa strain" of lure. Check traps in your fields once or twice a week and replace lures weekly. OMAF also maintains several corn borer trapping sites across the province with reports available on regional agriphones.
Figure 6. Heliothis peromone trap for monitoring corn borer moths (Note location in grassy area).
Sweet corn is susceptible to corn borer damage from the late whorl stage until harvest. Although the borers feed on all above-ground parts of the plant, the greatest economic damage occurs when borers feed on the ears (Figure 7). They may enter the ear through the tip, shank or husk and cause extensive feeding damage to the kernels. Ears infested with caterpillars or their frass (droppings) or ears with damaged kernels are unmarketable.
A corn borer infested field may also attract blackbirds, initially searching for insects, but later damaging the corn.
Figure 7. Corn borer damage on ears.
Fortunately, most corn borers in a field are killed by natural factors. Heavy rain will drown many small larvae and very dry weather will dessicate them. Many eggs and small larvae are eaten by predators, especially minute pirate bugs (Figure 8) and lady beetle adults (Figure 9) or larvae (Figure 10). Several parasites attack corn borers in the fall and increase over-wintering mortality. Unfortunately, beneficial insects do not usually achieve economic control of corn borers.
Figure 8. Minute pirate bug (Actual size: 2 - 3 mm).
Figure 9. Lady beetle adult.
Figure 10. Lady beetle larva.
Early planted field corn can effectively reduce corn borer damage in small sweet corn plantings by acting as a trap crop. The moths will lay most of their eggs on the taller field corn, lowering the borer population in the sweet corn.
It may also be possible to manage corn borers by releasing tiny wasps called Trichogramma into the field several times during the season. The Trichogramma become parasites on the corn borer eggs and prevent them from hatching. The wasps are harmless to humans. Of the several species of Trichogramma that are commercially available, Trichogramma brassicae and T. evanescens are most effective against corn borers.
When pheromone traps or local agriphone messages indicate that corn borer moths are flying, growers should begin to scout fields to help time insecticide sprays. Spraying according to results of weekly or biweekly scouting ensures optimal use of sprays. Field trials have shown that scouted fields require fewer sprays to achieve a similar level of control as calendar-sprayed fields. While many growers may choose to scout their own fields, consultants offer this service in some parts of the province.
When scouting your fields, look for egg masses, small caterpillars or feeding damage on the corn plants. Corn borer egg masses are usually found on the undersides of the leaves, near the midrib (Figure 2). Since most egg masses are laid on the central part of the plant, examine three leaves above and three leaves below the ear. Look for small caterpillars on the leaves, in leaf axils or in the silks. Feeding damage may be found:
Corn borer damage is, however, sometimes confused with that of other pests. To aid in identification, Table 1 below compares the European corn borer with other caterpillar pests that cause damage to sweet corn.
Figure 11. Window pane / shot hole damage
Figure 12. Broken tassel from corn borer tunneling
Sequential sampling is a fast and accurate method to determine if a spray is needed in a field. If the population of corn borers in a field is either very high or very low, then a spray/no spray decision can be made after about 15 - 20 minutes of scouting. If the population is moderate, then continue counting until an accurate decision can be made.
When scouting, examine plants from all parts of a field. Start by looking at five plants near the field edge. Then walk 10 paces into the field and 10 rows to the left or right and examine another five plants. Continue across the field in this zig-zag fashion, checking five plants at each site. Keep a running total of the number of plants infested with corn borer eggs, larvae or fresh damage. Compare the total with the "Spray" and "No Spray" columns in the sequential sampling chart (Table 2). Continue sampling until a decision is reached. For recommended sprays, see the current issue of OMAFRA Publication #363, Vegetable Production Recommendations.
This scouting method was developed to assess corn borer levels and it is also effective for fall armyworm, but it will not detect corn earworms. Timing sprays by field scouting is most effective in the early part of the season before earworms arrive in August or later. Earworm arrival can be detected with Heliothis traps baited with corn earworm lures. Local agriphones also report on their arrival. More information on biology and control is available in the OMAFRA Factsheet, Corn Earworm (Order No.95-065).
*ND - No Decision
Using biotechnology, scientists have recently developed sweet corn varieties with genetic resistance to the European corn borer. A gene from a soil bacterium called Bacillus thuringiensis (Bt) has been incorporated into corn plants, enabling the plants to produce a protein which causes corn borers to stop feeding and die. This protein may also control corn earworms and fall armyworms but it is harmless to humans, wildlife and most other insects. In the field, these "Bt-sweet corn" varieties should control corn borer and other caterpillar pests without applying any additional insecticides.
The European corn borer is the major insect pest of Ontario peppers, attacking both hot and sweet varieties. They enter the pepper underneath the cap and feed extensively inside the fruit (Figure 13). Since there is very little leaf feeding and often no external symptoms on the fruit, an infestation may go undetected until harvest. Neither infested nor damaged peppers are acceptable in fresh or processing markets. Corn borer entry holes in the fruit may also lead to premature rotting of peppers.
In 1996, corn borers attacked the stems of peppers, tomatoes and potatoes early in the season. In peppers, corn borer feeding partially girdled the stems, weakening the plants for the rest of the season. In potatoes and tomatoes, the larvae tunneled into the stems, causing rapid wilting and death (Figure 14).
Figure 13. Corn borer damage in pepper fruit.
Figure 14. Corn borer tunnel in potato stem.
Weather conditions causing late planting of field corn after a cool spring may have accounted for this unusual damage. Corn borer moths, unable to find suitable egg laying sites on corn, dispersed to other, less desirable crops. This type of stem damage will not likely be an annual problem. However, it would be wise to scout these crops in early to mid-July for signs of stem damage. Look for individual wilted stems in potatoes and tomatoes, and for girdled or broken stems in peppers.
While no pepper variety is resistant to corn borer, some varieties suffer less injury than others. Hot peppers are generally more resistant than sweet peppers. Varieties having the fruit cap pressed tightly against the top of the fruit tend to suffer less damage than those with an open cap.
Insecticides are necessary in most cases to control corn borers in peppers. Heliothis traps in grassy areas may help in assessing times of greatest risk. Field scouting has not been effective in detecting eggs or young larvae.
Peppers are susceptible to corn borers from the time the fruits are walnut-sized until harvest. A regular spray schedule effectively protects the crop, with shorter spray intervals needed when corn borer populations are high. The addition of a surfactant helps the spray material to adhere to the waxy leaves and fruit. See OMAFRA Publication 363, Vegetable Production Recommendations for a list of recommended products.
Corn borers cause problems in snap beans used for processing as larvae feeding on pods may show up in the finished product. The risk of damage increases when the corn borer population is high and other host plants are not available - most likely either early in a cool summer before field corn is attractive to corn borers or late in a warm summer after field corn has dried down.
In snap beans, corn borer larvae feed briefly on the leaves, but soon begin tunneling into the stems and pods (Figure 15). Since much of the pod damage is internal, with only a small entry hole visible on the outside, grading out infested pods is very difficult.
Figure 15. Corn borer damage on snap bean pods.
To reduce the risk of corn borer infestation, avoid fields near corn or fields that had corn nearby in the previous year. If rotation is not feasible, protect the snap beans with regular insecticide sprays, starting when the beans begin flowering. See OMAFRA Publication #363, Vegetable Production Recommendations, for a list of products.
European corn borers have been an occasional problem in apple orchards, mostly in Essex County. The larvae tunnel into fruit, usually entering at the calyx end and feeding inside until early fall. The damage is similar to that of the codling moth. Problems develop in late August and September, as corn fields dry down and orchard spray programs wind down. The damage is usually confined to the border rows. Since it is difficult to predict which orchards will be damaged, it is wise to check the fruit for damage as harvest nears.
In New York state, corn borers have also been found to tunnel into the shoots of young, non-bearing trees, causing dieback. This problem has not been reported in Ontario.
Where corn borers have been a problem in apples, maintain insecticide spray programs later in the season, especially in the border rows of the orchard.
Vegetable Production Recommendations, OMAFRA Publication #363
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