Table of Contents
The corn earworm (Helicoverpa zea, formerly known as Heliothis zea) is a major pest of late-season sweet corn in Ontario and is responsible for a large percentage of grade-out corn. Earworms feed almost exclusively on the tips of the ears, leaving no visible damage on the husks or leaves. Fall armyworms - another late-season pest - feed extensively on the leaves, and often enter the ears through the side. European corn borers, which are present throughout the season, feed on all parts of the corn plant. Most farmers consider an intensive spray program necessary to control these pests.
The earworm has a wide host range, feeding on many cultivated crops and weeds. Elsewhere in its range, it is also known as the cotton bollworm, tomato fruitworm or tobacco budworm. In Ontario, however, it is a major problem only on sweet corn. Most seed corn fields are harvested before the worst of the earworm damage, and the damage to field corn is not considered economic.
The corn earworm is one of several pests that does not overwinter in Ontario. The entire population is killed by low winter temperatures. Each spring, the moths must reestablish themselves in Ontario, from the overwintering populations in the southern United States and Mexico. They begin their northward migration around May and usually reach Ontario sometime in August. Since moths are not strong enough to fly that distance on their own, they are carried by high-level winds. The warm southerly winds that blow into Ontario in the summer carry the moths long distances. These winds often bring rain, which commonly brings the moths back to earth.
Since their migration is so weather-dependent, the moths arrive in Ontario at different times each summer. Although they may arrive in late July some years, at other times they may not arrive until September.
Earworm distribution in Ontario also differs each year. While generally found in higher numbers in the counties that border Lake Erie and Lake Ontario, they may not infest parts of the province some years. For example, in 1994 there were no reports of earworms from any sites east of Toronto. Even when earworms are present in large numbers in one area, other corn fields that are only a few kilometres away may not be infested.
There are four stages in the corn earworm's life cycle. Earworms arrive in Ontario as moths. The moths lay eggs, which then hatch into larvae (caterpillars). After feeding for two to four weeks, the larvae become pupae. In Ontario, larvae are typically killed by frost before pupation. In warmer climates, adult moths emerge from the pupae to repeat the cycle.
The corn earworm adult is a buff- or tan-colored moth with a wingspread of 3.5 to 4 cm (1.25 to 1.5 inches) (Figure 1). The forewing may have several darker markings and always has a central brown dot, clearly visible on the underside of the wing and faintly visible from the top. The hindwings are very pale in color, with a darker brown border. Eyes of living moths are bright green, which fade to dull olive green or brown a few days after death. There is no easy way to differentiate male from female moths.
Figure 1. Corn earworm moth.
Although earworms may lay their eggs anywhere on the corn plant, almost all will be laid on fresh silks if these are available. Although eggs are laid individually, one female can lay over 100 each night, and over 1000 during her lifetime. Each egg is nearly spherical, and about the same color and diameter as a corn silk (Figure 2 and Figure 3). Most eggs are laid on the outer third of the silks and hatch in 2 to 10 days, depending on the temperature.
Figure 2. Corn earworm egg on fresh silk (actual size).
Figure 3. Egg magnified 10 times.
Upon hatching, young earworms (Figure 4) crawl down the silks toward the ear. After feeding on the silks inside the husk for a few days, they begin feeding on the kernels at the ear tip. The worms will grow up to 3.7 cm long (1.5 inches), with prominent stripes running the length of their bodies (Figure 5). The size and the presence of stripes differentiate earworms from European corn borers, while their head color (tan) differentiates them from fall armyworms. See Table 1 for more complete descriptions of these three pests.
Figure 4. Corn earworm larva, recently hatched. Note the stripes running longitudinally.
Figure 5. Fully grown larva, up to 3.7 cm (1.5 inches) long. Note the stripes and the tan-colored head.
When corn earworms are present, all sweet corn with exposed fresh silk is susceptible to damage. Corn in the tassel stage does not need to be protected from earworms but may be attacked by other pests. Where sequential plantings are located close together, the field with the most fresh silk will likely receive the bulk of the egg-laying; other fields, however, are not immune.
Earworms normally feed only on the kernels of sweet corn, beginning by feeding at the tip of the ear and moving down the ear as they grow. Feeding is almost always confined to the top third of the ear. Fecal matter is found as large moist pellets in the silk channel and at the ear tip. Earworms do not bore into the cob, as European corn borers and fall armyworms sometimes do.
Although earworms damage only a small percentage of the kernels, their presence and droppings are very distasteful to most consumers. Where control has been less than perfect in the field, growers are forced to check ears at harvest and cull the damaged ones. Earworm-infested ears can sometimes still be marketed if the tips are cut off, although this practice significantly reduces the shelf life of the corn. Sweet corn destined for the processing plant may be able to sustain some earworm damage at the ear tips, as the tips are not used in the finished product.
* A hand lens is the most reliable means of separating corn borers from the other two species.
In addition to direct damage, earworms can also predispose the crop to attack by other pests. Sap beetles will be attracted to the smell of fermenting sugars after earworms begin feeding on sweet corn kernels. In field corn, ear molds developing in the damaged kernels can cause toxicity problems for livestock.
The biology of the corn earworm makes it a very good candidate for an on-farm insect-monitoring program. The pest is present only at certain times of the year, its distribution is sporadic, and a cheap, effective monitoring method gives growers adequate time to implement a control program once the pest has been detected.
OMAFRA monitors corn earworms and other sweet corn pests at several sites across the province. Information about pest activity in your local area can be obtained by calling a regional vegetable agriphone. While regional information is helpful, it is not as reliable as insect counts from traps located on your own farm.
Corn earworms are monitored using a Heliothis trap, manufactured by Scentry Inc. (Figure 6) A pheromone lure (corn earworm Luretape manufactured by Hercon), which imitates the female earworm's sex attractant, is placed in the trap. Male earworm moths are attracted to the lure and are caught inside the trap. Since pheromone traps only catch males, they cannot be used to control an infestation. Their value lies in detecting the presence of the pest and in estimating the size of the infestation. (For information about ordering traps, call your local Pest Management Advisor.)
Figure 6. A Heliothis trap, baited with a pheromone lure, is a simple way to monitor for corn earworms
Sweet corn growers should use at least two Heliothis traps to monitor for earworms. Place one trap per field in the two corn fields that are farthest apart. If two traps are used in a single field, place them at opposite ends. Move traps as often as necessary to ensure that each trap is always next to a field with fresh silk. Establish traps in early July, then check them twice a week. Replace the pheromone lure every two weeks. (Store extra lures in the refrigerator or freezer.)
Most years, the first earworms will not be trapped until sometime in August. When moths are caught in the traps, compare them with Figure 1 to see if they are earworms and not a similar species. After the first earworms arrive, begin checking the traps three times a week. Continue monitoring until the last planting no longer has any fresh silk showing.
Before modern insecticides became available, corn growers applied mineral oil to the silk of each ear to prevent invasion by corn earworms. This method is still used occasionally by some organic farmers and home gardeners. It effectively controls the pest, but is time-consuming and the oil left at the ear tip may be distasteful to consumers.
Several predators and parasites, including ladybird beetles, lacewings, predatory bugs, and parasitic flies and wasps, attack earworm eggs and larvae. These beneficial insects, present naturally in the field, exert an ongoing influence in keeping the pest population in check. They are not, however, adequate for economic control. To date, no commercially available biological control agents will effectively control the earworm.
Those who want to avoid earworm damage without the use of insecticides must plant early. Sweet corn harvested before the middle of August is usually free of earworms. After that time, insecticides are necessary to protect the crop.
Corn earworms can be controlled with insecticide sprays applied every three to seven days while fresh silks are present. For best results, sprays should be based on trap counts and temperature. (See Table 2 for spray intervals.) Since there is more risk of damage, spray intervals are shortened as the earworm population increases. Intervals are also shortened as the temperature increases, because unexposed silks are growing faster and because the insecticides break down more rapidly.
The choice of insecticide is very important in controlling earworms, which have developed resistance to some insecticides in the carbamate family (such as Sevin and Furadan) and are no longer controlled by these chemicals. They can be controlled by synthetic pyrethroids; currently available products are listed in OMAFRA Publication 363, Vegetable Production Recommendations.
Since egg-laying occurs at dusk and since pyrethroids are more effective in cool temperatures, it is best to spray in the evening. Aim the nozzles at the ear zone and ensure that the silks are well covered.
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