Key to Sweet Corn Pests
Many pests of sweet corn go unnoticed because they are hidden within the soil, stalk, ear, or whorl. Few insects feed on exposed sites where they can be easily detected and controlled before damage occurs. As a result, much attention needs to be focused on early recognition of both pests and their injury.
A. Caterpillars that feed exposed and/or bore into whorl, stalk, or ear
- Corn earworm – Early instars; cream colored or yellowish-green with few markings; later instars: green, reddish, or brown with pale longitudinal stripes and scattered black spots; moderately hairy; up to 44 mm long; three pairs of legs, five pairs of prolegs (Figure 1); as leaves unfurl, ragged holes and soggy brown frass apparent; feeds on tip kernels of ear; round emergence holes in shuck
- European corn borer – Cream to light pink caterpillar with reddish-brown to black head; body up to 26 mm long with several rows of dark spots; has three pairs of legs near head and five pairs of prolegs (Figure 2); bores in stalk; tangled frass and silk near entrance hole; tassel and stalk breakage, ear dropping, small ears, and/or pin holes in leaves sometimes present
- Fall armyworm – Green, brown, or black caterpillar with black stripe down each side and yellowish-gray stripe down back; body up to 40 mm long (Figure 3A); head capsule with pale, but distinct inverted Y (Figure 3B); rarely found in North Carolina before July but may be common later; feeds on leaves, may attack ears near shuck; mature larva may bore into stalk
- Stalk borer – Brown worms with a dark brown band with some paler stripes; older worms are whitish or light purple (Figure 4); infested plants sucker excessively and do not tassel properly, stems sometimes lodge; plants past the "whorl" stage are resistant
B. Beetles that feed on foliage or silks
- Corn flea beetle – Black, oval insect 1.3 to 2.5 mm long, tinged bronze or bluish-green; yellow markings on legs; basal antennal segment orange (Figure 5); infested foliage with tiny round holes and bleached out spots or stripes
- Dusky sap beetle – Dark gray or brown, oval to oblong beetle with club-like antennae; body 3 to 4 mm long (Figure 6); feeds on ripening pollen; found on tassels or in leaf axils where pollen has collected
- Japanese beetle – Shiny metallic green, 13-mm-long beetle with coppery brown wing covers (Figure 7); feeds on silks and, in severe infestations, reduces pollination and kernel formation
C. Corn leaf aphid – Soft-bodied, pear-shaped, winged or wingless insect up to 2 mm long; body pale bluish-green with powdery coating and dark area around base or cornicles (Figure 8); feeds in colonies and causes leaf yellowing and deformation; cast aphid skins along with black moldy tassels, leaves and silks indicate high aphid population
D. Spider mites – Tiny pale or reddish spider-like arthropods feed on leaves, shucks and silks (Figure 9); heavily infested plants turn yellow or appear burned; sometimes silks are completely obscured by webs and mites (Figure GG)
Corn earworm / bollworm / tomato fruitworm, Heliothis zea (Boddie), Noctuidae, LEPIDOPTERA
Adult – The corn earworm moth has a wingspan of 26 to 38 mm and is usually light yellowish-olive in color. Each forewing has a dark spot near the center. Eyes are usually light green.
Egg – The dome-shaped egg, about 0.5 mm in diameter, is pale white when first laid and develops a reddish-brown band before hatching.
Larva – The 5 to 6 larval instars vary considerably in color. Newly hatched larvae are about 1.5 mm long and yellowish-white with dark head capsules. Second instars are yellowish-green and frequently have orange and brown longitudinal stripes; their head capsules are reddish-brown to brown. Up to 44 mm long, later instars are greenish-yellow, reddish or brown with pale longitudinal stripes, raised black spots (chalazae), and brown to orange heads. All instars have 5 pairs of fleshy prolegs.
Pupa – About 31 mm long and 6 mm wide, the pupa is reddish-brown to dark brown.
Distribution – This insect is found throughout most of the Western Hemisphere.
Host Plants – The corn earworm infests more than100 plants, but corn is the preferred host. The earworm is also found occasionally on wild hosts such as toadflax and vetch.
Damage – Though foliage is attacked early in the growing season, corn earworms prefer fruiting stages of their host plants. On seedlings, corn earworms devour leaves, buds, and tender new growth. On corn, first generation larvae feed in the tightly coiled blades. As a result, numerous ragged holes appear when the blades unfurl. Wet, tan to brown excrement lodges in the whorl and blade axils (Figure BB). This condition often is referred to as "shatterworm" injury.
As vegetable crops flower and produce fruit, larvae move to these plant parts. Entrance holes of small larvae may be barely detectable in tomatoes or completely undetectable in sweet corn ears. As infested tomatoes increase in size, small string-like scars may become apparent where the tiny larvae entered. In the corn ear, larval damage is usually confined to developing kernels in the ear tip area. Round holes (approx. 5 mm in diameter) through the shuck are usually emergence rather than entrance holes. On bean, okra, and tomato plants, large corn earworms may move from fruit to fruit feeding and leaving large gaping holes on the surface.
Life History – In North Carolina, corn earworms overwinter as "resting" (diapausing) pupae in soil at a depth of more than 5 cm. Adults emerge in early May, mate, and seek suitable oviposition (egg-laying) sites. A high percentage of first generation eggs are laid on the leaves of seedling corn when it is available. However, once corn begins silking, most eggs are deposited on silks. Later in the season, as corn silks dry up, oviposition again occurs on the leaves of other hosts, including many vegetable crops. Eggs are deposited singly, each female laying from 450 to 3,000. Within 2 to 5 days, the larvae emerge and begin feeding. Because this insect is cannabalistic, rarely does more than one larva develop from an ear or whorl. Larvae feed for 2 to 4 weeks during which time they develop through 5 or 6 instars. They then burrow in the soil to pupate. Two to 4 weeks later, a new generation of moths emerges. At least 3 generations occur each year in North Carolina.
Resistant sweet corn varieties are available. These varieties are not immune to attack. Although young corn may be damaged severely by corn earworms, insecticides generally are used only from silking until harvest to protect ears. When corn is harvested in the Coastal Plain from July 10 to July 20, the infestation in ears is relatively low, even if only a fair job of chemical control is carried out.
On tomato plants, damage is also low when the fruits are harvested before July 20. The first chemical treatment should be applied when tomatoes in the first cluster are about 13 mm in diameter. Followup treatments should be applied every 7 to 10 days as necessary. In the Coastal Plain, a 5 to 7 day schedule is needed after July 20. Cultivating fields of vegetable crops after harvest kills numerous pupae in the soil and exposes many to birds and other predators.
For recommended insecticides and rates, see the current North Carolina Agricultural Chemicals Manual.
Corn Flea Beetle
Corn flea beetle, Chaetocnema pulicaria Melsheimer, Chrysomelidae, COLEOPTERA
Adult – This oval black beetle is tinged with bronze or bluish-green, has yellow markings on its legs, and is 1.3 to 2.5 mm long. The basal segment of each antenna is orange.
Egg – Each white egg is about 0.35 mm long and pointed at one end. It gradually darkens before hatching.
Larva – The slender, white, cylindrical grub has a brown head and tiny legs. It may be 3.2 to 8.5 mm long when full grown.
Pupa – The white, soft-bodied pupa resembles the adult in size and shape and gradually darkens as it matures.
Distribution – The corn flea beetle occurs in most areas east of the Rocky Mountains. It infests corn all across North Carolina but appears to be more abundant in the Piedmont counties.
Host Plants – Although the corn flea beetle is a general feeder, its preferred hosts are grasses. However, sugar beets are periodically infested in other states.
Damage – Corn flea beetles attack foliage leaving small round holes and bleached out spots or stripes; larvae feed on roots of grasses (Figure CC). Direct loss caused by these injuries, however, is relatively insignificant. The overwintering beetles which carry bacterial wilt of corn (Stewart's disease) are responsible for most economic damage because of the spread of this disease. These beetles are usually most troublesome after a mild winter followed by a cold spring. Under such conditions, large numbers of beetles survive the winter and attack slowly growing corn over a prolonged period. Growth is retarded and leaves may wilt. Early maturing varieties in the middle Atlantic and southern states are most seriously affected.
Life History – Adults generally overwinter in litter and trash around fields. Mortality tends to be high during harsh winters. In early spring, beetles move to weeds and then to corn seedlings. Eggs are scattered on soil beneath host plants. In about 10 days, larvae emerge and begin feeding on and tunneling in underground stems, roots, or tubers. They feed for 3 to 4 weeks and develop through 3 instars before pupating in the soil. Seven to 10 days later, a new generation emerges. Three or more generations are completed each year.
Damaging corn flea beetle infestations can be prevented by plowing under crop residue and maintaining good weed control to eliminate overwintering sites. The use of wilt resistant hybrids also lessens the chances of excessive loss due to bacterial wilt. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Corn Leaf Aphid
Corn leaf aphid, Rhopalosiphum maidis (Fitch), Aphididae, HEMIPTERA
Adult – The oval wingless adult, about 2.0 mm long, is usually pale bluish-green with black antennae, legs, and cornicles and a dark area around the base of the cornicles. The head is marked with two longitudinal dark bands, and the abdomen with a row of black spots on each side. The body often seems to have a powdery coating. The winged form is about the same size.
Egg – Unknown.
Nymph – Similar to the wingless adult, the nymph is smaller and has no wings.
Distribution – The range of the corn leaf aphid extends throughout the tropical and temperate regions of the world. In the continental U.S. it occurs in all areas except the Rocky Mountain region. It is rarely a problem in the northern states.
Host Plants – The corn leaf aphid shows a preference for barley, sorghum, and corn, in that order. It also infests many other wild and cultivated grasses.
Damage – Feeding by colonies of these aphids causes mottling and discoloration of leaves. Heavily infested leaves turn red or yellow, shrivel, and die. The important damage usually occurs during and after flowering. At this time the aphid population peaks and feeds on corn tassels and silks. Areas fed upon become covered with sweet, sticky honeydew secretions. Black mold grows on the honeydew and may result in poor corn pollination, interference with photosynthesis and, in severe cases, reduced grain development. Entomologists have speculated that the honeydew attracts corn earworm moths and, therefore, induces heavy earworm egg deposition.
Life History – Little is known about the biology of this pest in North Carolina. Since the relationship between corn and this aphid is not well understood, it has been difficult to estimate damage and to determine thresholds. This aphid generally is not considered a serious threat.
Corn leaf aphid adults overwinter each year in southern states, including North Carolina. On warm winter days, females actively continue to feed and reproduce on winter grain crops or other grasses. The first spring adults are winged females which fly in search of suitable host plants, sometimes migrating far northward. Shortly thereafter, they give birth to live nymphs which usually develop into wingless females. Under favorable conditions, more winged females develop and migrate. Males are rarely found but females continue to reproduce without mating. No egg stage is known. Reproduction slows down in winter and summer and is most rapid during cool weather. Therefore, corn leaf aphids tend to be a problem on winter grains in spring and on late-planted corn in fall. The number of generations per year varies from 9 in Illinois to 50 in southern Texas.
Heavy corn leaf aphid infestations usually are restricted to late-planted corn; therefore, early planting and other cultural practices which hasten maturity help prevent aphid problems. Corn leaf aphids rarely require control in North Carolina because high temperatures and natural enemies reduce aphid populations in summer. Should damaging aphid populations develop, consult the current North Carolina Agricultural Chemicals Manual.
Dusky Sap Beetle
Dusky sap beetle, Carpophilus lugubris Murray, Nitidulidae, COLEOPTERA
Adult – This dark gray or brown beetle is oblong to oval in shape and has club-like antennae. It is between 3 and 4 mm long.
Egg – The white, sausage-shaped egg is pointed at one end. It is approximately 1.2 mm long and 0.25 mm wide.
Larva – When mature, the yellowish or pinkish-white grub is about 5.5 mm long. It has a translucent spiny body with 3 pairs of short legs near its head.
Pupa – The lightly colored pupa is approximately the same size and shape as the adult and darkens as it matures.
Distribution – The dusky sap beetle occurs from Brazil to New York and westward to Arizona, Utah, and Washington. In this country, it is particularly common along the Atlantic coast.
Host Plants – This beetle infests apple, corn, peach, tomato, pea, and yucca. It is also attracted to various tree stumps, wounds, acorns, Phylloxera galls, and dropped fruit or nuts.
Damage – Prior to 1950, sap beetles were considered "insignificant scavengers" and no threat to sweet corn. Today they are recognized as secondary pests which usually become problems after corn is damaged by other pests, such as the corn earworm.
Sap beetles are attracted to sweet corn as it tassels. They feed on ripening pollen and chew tassels. Later they move to leaf axils where pollen has fallen and collected. Should mature ears on the plant be damaged, dusky sap beetles eventually will oviposit in the kernels. As a result, ears become contaminated with larvae.
Life History – Dusky sap beetles overwinter as adults in soil or debris near the bases of trees or stumps. Resuming activity in April or early May, they are attracted to tree wounds. Approximately 13 days after emerging, first generation females deposit eggs in decomposing plant material. Later generations often oviposit in silk channels or kernels of sweet corn. Larvae feed on whatever is available when they emerge and eventually pupate in the soil. In summer, 28 to 30 days elapse between egg deposition and adult emergence.
Dusky sap beetles usually are scarce in spring and increase in numbers in fall. In sweet corn, however, sap beetle populations continue to decline after tasseling occurs (approx. June 15 in North Carolina). At least three or four generations occur each year. More generations are possible, especially if beetles develop in corn refuse.
Controls for this pest are primarily cultural. Plowing under crop debris destroys possible overwintering and breeding sites. Tight, long-husked corn varieties are more resistant to earworms and, therefore, are less likely to be infested by sap beetles. Varieties resistant to sap beetles include Country Gentleman, Golden Security, Tender Joy, Trucker's Favorite, Stowell's Evergreen, and Victory Golden. Whereas these varieties usually sustain less injury than susceptible varieties, they are not immune to attack.
An adequate spray program which controls other ear pests should prevent dusky sap beetles from becoming a problem. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
European Corn Borer
European corn borer, Ostrinia nubilalis (Hubner), Pyralidae, LEPIDOPTERA
Adult – The female moth has a robust body and a wingspread of about 25 mm. It is colored pale yellow to light brown. The outer third of the wings is usually crossed by dark zigzag lines. The male moth is smaller, more slender, and darker than the female. The outer third of its wings is usually crossed by two zigzag streaks of pale yellow, and often there are pale yellow areas on the forewings.
Egg – Each white egg is about half the size of a pinhead. The eggs change to pale yellow and darken just before hatching as the brown head of the borer inside becomes visible. Within the egg mass, the eggs overlap each other like fish scales. The masses of 20 to 30 eggs are covered with a shining waxy substance.
Larva – The newly hatched larva, about 1.5 mm long, has a black head, five pairs of prolegs, and a pale yellow body bearing several rows of small black or brown spots (Figure DD). It develops through 5 or 6 instars to become a fully grown larva about 25 mm long.
Pupa – The brown pupa is 13 to 15 mm long with a smooth capsule-like body.
Distribution – Introduced into the United States from Europe in 1909, the European corn borer has spread throughout the contiguous states and into Canada. In North Carolina, the largest populations of this pest occur in the Coastal Plain where 75% of the stalks in some fields have been attacked. However, 40% lodging of stalks due to borers has been observed in the Piedmont when harvesting has been delayed.
Host Plants – The European corn borer infests over 200 plants, but corn is a preferred host. Other vegetable crops likely to be injured include bean, beet, celery, potato, pepper, and tomato.
Damage – On most crops, borers begin feeding on the leaf surface. In corn, feeding first occurs in the whorl. Later the larvae bore down midribs of leaves into the stalk. Frass and silk near entrance holes are evidence of their presence. Borers weaken stalks, or stems, and interfere with the movement of plant nutrients. Consequently, yields are reduced. On infested corn plants, tassels and stalks may break, ears drop, or only small ears develop.
Potatoes and peppers are affected more adversely than corn by borer injuries which allow the entrance of bacterial and fungal pathogens. Reduced yields occur when disease organisms cause stems to collapse and tubers to rot. Potatoes that are produced on infested plants are often of poor quality due to the development of diseases such as black leg. Pepper fruit infested by European corn borers are commercially unacceptable.
Life History – Mature larvae overwinter inside tunnels in stubble, stalks, ears, or other protective plant material. They pupate in spring. During April and May, adult moths emerge and mate. Each female lays 500 to 600 eggs in small masses of 20 to 30 on the undersides of leaves. Eggs hatch in 3 to 12 days, depending upon temperature. Young larvae usually begin feeding on leaf surfaces and, as they mature, begin boring in the midribs of the leaves. Two to 3 days after eggs hatch, stalk or ear boring commences and continues until pupation. In Florence, South Carolina, the European corn borer completes four generations per year and may do so in parts of North Carolina too. If this is the case, eggs of the second generation are laid in mid- to late June, those of the third generation in late July, and those of the fourth generation in September. This last generation is not a threat to corn. First generation European corn borers are a threat to potatoes, and the second and third generations are pests of other vegetables.
Many natural parasites of this corn borer, mainly flies and wasps which have been introduced from Europe, exist in corn-growing areas. Other biological control agents such as ladybird beetles, predaceous mites, and the downy woodpeckers have also been responsible for some borer reduction. The bacterial insecticide, Bacillus thuringiensis however, shows some promise for borer control.
Chemical control for the European corn borer is difficult because sprays are effective only during the 2- to 3-day period after eggs hatch and before larvae bore into stems. Therefore, close attention must be paid to the presence of moths and eggs. The emergence of the first moths can be determined by using either light traps or screened cages. Treatments should begin 7 to 10 days after a moth flight or about 5 days after eggs are found. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Fall armyworm, Spodoptera frugiperda (Smith), Noctuidae, LEPIDOPTERA
Adult – The adult moth has a wingspan of about 38.5 mm. The hind wings are grayish-white; the front wings are dark gray mottled with lighter and darker splotches. Each forewing has a noticeable whitish spot near the extreme tip.
Egg – Minute, light gray eggs are laid in clusters and covered with grayish fuzzy scales from the body of the female moth. Each egg becomes very dark just before hatching.
Larva – About 30 to 40 mm long, the fully grown larva varies in color from light tan or green to nearly black (Figure AA). Along each side of its body is a longitudinal black stripe. Down the middle of its back is a wider yellowish-gray stripe with four black dots on each segment. The head of the fall armyworm often is marked with a pale but distinct inverted "Y."
Pupa – The pupa, approximately 13 mm long, is originally reddish-brown and darkens to black as it matures.
Distribution – The fall armyworm is a continuous resident of the Gulf states, the tropics of North, Central, and South America, and some of the West Indies. Each year it migrates as far northward as Montana, Michigan, and New Hampshire.
Host Plants – Field corn, sweet corn, sorghum, and grasses are preferred foods. However, the fall armyworm may also infest alfalfa, bean, peanut, potato, soybean, sweet potato, turnip, spinach, tomato, cabbage, cucumber, cotton, tobacco, all grain crops, and clover.
Damage – The fall armyworm is the second most important pest of sweet corn. It most frequently causes damage to the whorl of late pretassel corn. Larvae feed throughout the tightly coiled blades causing what is known as "shatterworm" injury. When the blades unfurl, the new leaves are riddled with numerous ragged holes. As with the corn earworm, wet tan excrement lodges in the remaining blades and blade axils.
In addition to defoliation, damage to corn may be three-fold. First, larvae feed on the undeveloped tassels of young plants. Secondly, immature ears are attacked near the shank. Lastly, large larvae may bore into maturing ears and stalks.
Life History – Fall armyworms overwinter in Florida and along the Gulf Coast in several life stages, but usually as pupae. Egg-laying moths appear in North Carolina about the middle of July. Each female lays about 1,000 eggs in masses of 50 to several hundred. Two to 10 days later, the small larvae emerge, feed gregariously on the remains of the egg mass, and then scatter in search of food. They usually are unnoticed until they are about 25.5 to 38.5 mm long, by which time, if abundant, they have consumed so much foliage that they create alarm. Larvae are most active early in the morning or late in the evening. When abundant, these caterpillars eat all the food at hand and then crawl in great armies to adjoining fields. After feeding for 2 or 3 weeks, the larvae dig about 20 mm into the ground to pupate. Within 2 weeks, a new swarm of moths emerges, usually flying several miles before laying eggs. Several generations may occur each year.
During favorable seasons, a number of parasitic enemies keep fall armyworm larvae down to moderate numbers. Cold, wet springs seem to reduce the effectiveness of these parasites and a population explosion often results.
Early planting is the most effective cultural control method in our state. For recommended chemical controls, consult the current North Carolina Agricultural Chemicals Manual.
Stalk borer, Papaipema nebris (Guenee), Noctuidae, LEPIDOPTERA
Adult – The forewings of this moth are basically reddish or grayish-brown marked with distinct white spots or obscure smoky areas. The outer third is paler and bordered by a thin white line. The hind wings are grayish-brown on the upper surface and fawn-gray below. The wingspan ranges from 25 to 40 mm.
Egg – The longitudinally ribbed egg may be spherical or slightly flattened and measures 0.4 to 0.6 mm in diameter. White when first deposited, it gradually turns brownish-gray or amber before hatching.
Larva – Early larval instars are brown and have a dark brown band around the middle and brown or purple longitudinal stripes on all but the first four segments (Figure FF). The mature larva is solid white or light purple and may reach a length of 31.8 mm.
Pupa – The light brown pupa gradually darkens as it matures and is about 16 to 22 mm in length.
Distribution – The stalk borer occurs in all areas east of the Rocky Mountains from southern Canada to the Gulf of Mexico. Highest populations are associated with fields and fence rows with large-stemmed weeds. In North Carolina, economic infestations are most common in the Piedmont, particularly in no-till plantings.
Host Plants – Stalk borers tunnel in almost any large-stemmed plant. Their host range encompasses at least 44 families and 176 species of plants. Some cultivated crops subject to infestation include corn, cotton, potato, tomato, alfalfa, rye, barley, pepper, spinach, beet, and sugarbeet. Although many weedy plants are infested, giant ragweed is preferred.
Damage – Stalk borers migrating from another host infest corn seedlings 6 to 60 cm (2 to 24 in) high causing 2 types of injury. Larvae that enter the plant through the lower stalk tunnel upwards severing the leaves from below - in this case, infested stalks are hollow, and apparently healthy green leaves wilt and die. Other larvae climb plants, enter from the top, and feed on buds and rolled leaves. As they unfurl, new leaves display ragged holes which increase in size as leaves develop. Both forms of injury result in destruction of tassels, production of suckers, and deformation of the upper plant. Soon after borers enter the seedlings, the stems often break. Frass is usually evident around the base of more mature infested plants. Once past the "whorl" stage, corn is somewhat resistant to stalk borer and recovers more readily from damage. Damage is sporadic but most commonly associated with border rows of conventionally planted corn and with no-till plantings.
Life History – Stalk borers overwinter as eggs on weedy plants. In May, newly emerged larvae feed as leafminers on broadleaf plants or as stem borers on grasses. On all hosts, larvae eventually bore into stems and feed until they kill or outgrow their host. When this occurs, they emerge at night and tunnel into new plants, including seedling corn. Developing through 7 to 16 instars, stalk borers mature in their second host. Late in July, borers emerge, construct individual cells in the soil, and begin a 4-week pupal period. Stalk borer moths emerge in late summer and deposit eggs singly or in masses between the leaf sheath and stems where they remain until the following spring. One generation occurs each year.
Stalk borers cannot be controlled once they have entered the plant; therefore, control measures are strictly preventative. Destruction of weeds in fields and along fence rows result in the elimination of many primary hosts from which the borers infest corn.
Publication date: Feb. 10, 2003
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