Sweet corn
Sweet corn: Corn Earworm, Helicoverpa zea (Boddie), Lepidoptera: Noctuidae
Biology: Adults of the corn earworm, also known as the cotton bollworm, are light tan in color and are about 35 mm long. Moths generally have green eyes. Adults that are active in late winter arise from two sources, i.e., overwintering pupae in soil and flights of moths from southern areas. Adults are attracted to many host plants but flowering plants are favored. Eggs are initially deposited on foliage of seedling corn. Eggs are near white when laid but darken just prior to larvae emergence. Later generations deposit eggs on sweet corn ears and silks and emerging larvae chew down the silk channel to the developing ears. Larvae initially are minute, about 2mm, but at maturity can reach 45 mm in length (Photo 141). Three pair of true legs occur on the thorax and four pair plus an anal pair are found on the abdomen. Color of larvae varies greatly. Mature larvae that have developed on foliage are mostly green while those developing on ears are reddish brown with longitudinal lines. The pupal stage occurs in soil and color ranges from light tan shortly after pupation to dark brown just prior to moth emergence. All stages of the corn earworm can be found throughout Arkansas during the production season and resulting damage to ears is often severe. In Arkansas, three or four generations occur each year.
Management: On seedling sweet corn damage is generally very low and management is not needed (Photo 142). When sweet corn begins to silk, however, management of corn earworm, even in small home gardens, is generally required. Unfortunately, the tolerance is zero for corn earworm damage and the presence of larvae in sweet corn produced for processing and for many fresh markets. The producers of this sweet corn must rely heavily on insecticides applied directly to the ear silk. Because corn silk grows very rapidly, insecticide deposits are continually pushed away from the silk channel and newly emerging silk lacks the insecticide and is susceptible to earworm attack. Thus, insecticide sprays may be required at almost daily intervals. In fields where very low levels of earworm presence in sweet corn are tolerated, the number of insecticide applications can be greatly reduced. Also, earworm populations generally increase throughout the summer and early planted sweet corn may be less severely attacked than later planted sweet corn. Some hybrids possess some level of resistance to the corn earworm. Finally, the use of transgenic Bt sweet corn likely has some impact of reducing damage to foliage and ears. Future transgenic lines may possess much stronger toxicity to the corn earworm.
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Sweet corn: Corn flea beetle, Chaetocnema pulicaria Melsheimer,Coleoptera: Chrysomelidae
Biology: The corn flea beetle is a very small (2 mm long) black beetle found on the foliage of several plants including grasses and sweet corn (Photo 143). As the adult beetle is approached, its common defensive behavior is to use its enlarged hind femurs to jump; hence the name flea beetle. Little is known about the corn flea beetle in Arkansas. Surveys have detected low numbers on field corn at Marianna and Des Arc, however, it likely occurs throughout the state. Adults damage sweet corn by feeding on the epidermis of the leaf. Damaged foliage has light colored streaks that may give the corn a silver appearance. In addition to foliar damage, the corn flea beetle is capable of transmitting Stewart’s wilt disease. The corn flea beetle adult overwinters on grasses including weeds and grass crops like wheat in Arkansas. With warm temperatures in late winter, adults begin to feed, mate and lay eggs in the soil near grasses and corn seedlings. Larvae develop on roots and stems within the soil, pupate and adults emerge to begin the process again. Multiple generations occur each year. Flea beetle problems are greatest following mild winters.
Management: The threshold for corn flea beetle is not clearly defined but more than two adults per seedling (5-leaf or smaller), may warrant foliar insecticide application. In fields with soil applied insecticides at planting it is unlikely that high number of flea beetles will develop. Many of the current field corn hybrids possess some resistance to Stewart’s wilt.
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Sweet corn: Aphids including the corn leaf, Rhopalosiphum maidis (Fitch) and bird cherry-oat aphids, Rhopalosiphum padi Linnaeus, Aphididae: Hemiptera
Biology: Aphids found on Arkansas sweet corn are minute (<3 mm long) bluish-green insects (Photo 144). Clear membranous wings may be present but wingless forms are more common. Aphids occur in colonies that contain different size nymphs and adults. As the newly born nymphs increase in size, molting occurs and the white exoskeleton is left on the leaf surface.
Aphid species that attack sweet corn occur throughout the state and are often found in the whorl of the plant during mid to late summer. Late planted corn is generally more susceptible to damage from aphid feeding. Aphids feed by inserting their stylet or beak into the plant tissue and removing plant sap. Large amounts of sap are removed and the partially digested contents are excreted onto the plant surface in the form of a clear sticky honeydew. A dark grey mold may later form on the honeydew. The level of injury in Arkansas sweet corn appears to be minimal and generally does not require management. Aphids also transmit viral diseases but aphid control is not always effective in viral disease management. Aphids are capable of overwintering on alternate host plants in Arkansas. Also, winged aphids are carried into the state on winds coming from more southern areas each spring. Adults colonize grassy hosts and sweet corn seedlings and reproduce asexually through the summer. Development of nymphs is rapid and many generations occur each season. As temperatures peak during mid to late summer, reproduction rate declines. Thus, early planted corn that matures in mid summer generally is less susceptible to aphid population increase. Later planted corn, however, may experience large aphid populations in early fall.
Management: Early planted corn rarely harbors large aphid populations. In late planted corn, aphid populations may be very high, but when this population buildup occurs on maturing corn, aphid management will likely have little economic benefit. Excessive aphid populations on actively growing corn may be managed with foliar insecticides but the benefits may be very limited. Numerous beneficial organisms affect aphids including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Some corn hybrids possess some level of resistance to the aphid.
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Sweet corn: Cutworms including the black cutworm, Agrotis ipsilon (Hufnagel), Lepidoptera: Noctuidae
Biology: Cutworm larvae are dark grey to black caterpillars that can generally be found just below the soil surface feeding on seedling sweet corn. Although larvae are minute, <3 mm long, at hatching, they are not likely to be detected until they are at least 10 mm long. At maturity larvae are almost 35 mm in length (Photo 145). The caterpillars have three pair of true legs on the thorax and five pair of fleshy “prolegs” on the abdomen. Cutworms occur throughout the U.S. and throughout Arkansas. Recent surveys have detected large populations near Stuttgart and Des Arc. Larvae feed on emerging seedlings and often cut off the plant near or below the soil surface (Photo 146). Several adjacent plants within the drill line can be killed by a single cutworm. Cutworms generally are capable of overwintering as pupae in soil in Arkansas especially in southern counties. In addition to adults emerging in late winter from the overwintered pupae, adult moths fly into Arkansas from more southern states. Moths lay eggs on many weed hosts and crops including corn. Newly hatched larvae can produce “shot holes” in sweet corn foliage. Larger larvae often cut the seedling and feed below the soil surface. Where damage occurs, larvae can be detected by removing the upper layer of soil near a damaged plant. The number of generations per year varies from 1 to 3 depending on cutworm species.
Management: The first step in cutworm management is proper crop rotation. Corn that follows corn or planting corn in recently turned pastures tends to have more damage from cutworms. Also, adults are attracted to fields with weeds on which eggs are laid. Thus, early seedbed preparation prior to planting reduces the likelihood of cutworm damage. Providing a minimum of two weeks of host free time prior to planting should reduce the attractiveness of the field to cutworm adults. No or reduced till fields are more susceptible to economic losses from cutworms. In areas with histories of cutworm problems, treated seed or soil insecticides applied at planting may be justified. Foliar insecticide sprays may be used to reduce cutworm populations but early scouting for damage is critical. Foliar insecticide sprays should only be used when the damage levels exceed the threshold of 6-8% of the seedlings with cutworm damage above the surface of the ground or 2-4% of the plants cut below the surface. Finally, insecticide success may be reduced when late stage larvae are targeted as most of their time is spent underground.
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Sweet corn: European Corn Borer, Ostrinia nubilalis (Hubner), Lepidoptera: Pyralidae
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Photo 147 |
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Photo 148 |
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Photo 149 |
Biology: The European corn borer is a major pest of Arkansas field corn. However, its impact on sweet corn is minimal. European corn borer adults are small, about 20 mm in length, and fragile moths. Wings are yellow and brown and exhibit a zigzag pattern (Photo 147). Males are slightly smaller and darker than females. In Arkansas European corn borer adults may be confused with adults of the garden webworm, Achyra rantalis (Guenee), a much more common but slightly smaller moth. Mature European corn borer larvae overwinter in corn stubble and other host plant material in Arkansas. In April and May, European corn borer adults begin to emerge from overwintering sites and mate. European corn borer eggs are laid on corn foliage in a mass of up to about 25 cream colored eggs in an overlapping fish scale arrangement. Upon hatching European corn borer larvae are about 1 mm in length. Shortly after hatching, first generation larvae wander over the leaf surface and begin to feed on unfolding leaves. This feeding removes most of the leaf surface leaving only a thin layer of the upper or lower epidermis. Damage often appears as elongated “window panes” parallel to the leaf veins. Young larvae also feed completely through leaves still rolled in the whorl. As these leaves unfold, a “shot-hole” appearance may be observed. The tubercles of the European corn borer larvae are pronounced and the upper integument is darker than the lower integument (Photo 148). After feeding for about 3 weeks, larvae pupate within the plant (Photo 149). European corn borers can be found throughout the Arkansas River Valley, north central, northeastern, and eastern Arkansas. None have been found in southwestern or southeastern Arkansas during recent surveys. Second generation larvae attack most structures on the corn plant. Ear shank and stalk tunneling reduces kernel size and yield.
Management: Corn planted early in the season may reach maturity before the second generation corn borer larvae can affect ears. Stalk destruction, particularly in field corn, can reduce overwintering corn borer populations. From the insect management view, the most effective method of stalk destruction is to mow and disc stalks shortly after harvest in late summer or early fall. This leaves the overwintering larvae on or near the soil surface where the effects of low temperatures and rainfall have their greatest effect on producing larvae mortality. Natural enemies including beneficial insects, rodents and birds also contribute to larvae mortality in stalks left on soil surfaces. In late winter, prior to corn borer pupation, the stalk residue should be turned under as much soil as possible. Corn borers are susceptible to several insecticides applied to corn foliage. Difficulties with foliar insecticide effectiveness are associated with corn borer biology. First generation corn borers penetrate corn structures within about 10 days of hatching and second generation within about 7 days. After entering the plant, corn borers are very difficult to target. Thus, foliar applications must be timed precisely to be effective. While the most effective method of timing insecticides is by scouting for corn borer eggs, it is a very difficult task. Use of pheromone trap collections may provide a good indication of when to initiate scouting. Another method of monitoring adult flight is to search for infested corn stalks. By visually inspecting the ground for insect frass and the lower portion of corn stalks for damage, corn borer larvae can be easily detected. Once found, the stalk can be cut with use of a large knife and carefully split. Both larvae and pupae can be detected. When pupae color changes from tan to dark brown, the moth will emerge within a day or two. Mating and egg laying begins one or two days after moth emergence and eggs will hatch in about five days. Thus foliar sprays should be applied approximately seven days from first detection of dark corn borer pupae. Another method of foliar insecticide application that should be successful is through center pivot irrigation where available. Use of genetically modified sweet corn hybrids with the gene that codes for the Bacillus thuringiensis (Bt) is highly effective against European corn borer. However, vegetable processors are generally unwilling to accept Bt sweet corn due to public concern with genetically modified food crops.
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Sweet corn: Fall Armyworm, Spodoptera fugiperda (Smith), Lepidoptera: Noctuidae
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Photo 150 |
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Photo 151 |
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Photo 152 |
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Photo 153 |
Biology: Fall armyworm adults are large bodied moths with dark grey forewings that have lighter banding. A light spot occurs near the apex of the forewings. Young larvae on sweet corn are similar to the corn earworm and are often green (Photo 150). Mature larvae are up to 40 mm long and are dark brown in color with numerous black spots (Photo 151). Setae or hairs are less pronounced on fall armyworm than on corn earworm. The head capsule on fall armyworm larvae has a distinct light colored inverted “Y”. Fall armyworm occurs throughout Arkansas and its impact on sweet corn is similar to that of the corn earworm. This impact occurs in several forms including foliar damage to young corn (Photos 152 and 153), especially with late planted sweet corn; damage to tassels and silks; and direct damage to kernels. In early spring, adults migrate into Arkansas from more southern states, mate and seek suitable host plants for egg laying. Eggs are laid in masses that contain up to a few hundred eggs. Emerging larvae feed for two to three weeks and then pupate just below the soil surface. Multiple generations occur each year.
Management: In Arkansas, early planted sweet corn is usually not greatly affected by fall armyworm. Numerous beneficial organisms affect larvae including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Later planted sweet corn, however, may be severely affected and foliar insecticides generally offer the only approach at management. When fall armyworm larvae are detected in the whorl of seedling corn, spray volume should be increased and the spray directed into the whorl. Surfactants will likely increase effectiveness. The use of transgenic Bt corn likely has some impact of reducing damage to foliage and ears. Future transgenic lines may possess much stronger toxicity to the fall armyworm.
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Sweet corn: Seedcorn maggot, Delia platura (Meigen),Diptera: Anthomyiidae
Biology: Adult seedcorn maggots resemble house flies but the body is somewhat slimmer and at rest, wings are held more backward directly over the abdomen. The damaging form is the larval stage. Larvae can be found attacking germinating seed, roots and stems and at maturity are creamy white and about 6 mm long (Photo 154). A distinct head and legs are lacking. Seedcorn maggots occur worldwide and throughout Arkansas. Many crops are attacked. Most problems in sweet corn occur in fields high in organic content. Freshly plowed fields with decaying grass and weeds are especially attractive to adult flies that lay eggs near the decaying plant material. When corn is seeded into this ground, the developing maggots often move to the corn seed and can reduce germination, seedling vigor and stand. In northern states, the seedcorn maggot overwinters as a pupa within the soil. All stages can be found during mild winters in Arkansas. As spring nears, adults seek decaying organic matter for egg deposition. After hatching larvae feed for 1 to 3 weeks, they pupate within the soil. Adults emerge and renew the cycle. Multiple generations occur each year in Arkansas.
Management: Planting in soils with fully decomposed plant material should reduce the attractiveness of the field to adult flies. Damage also is more severe in fields where seed are slow to germinate and where seedling growth is retarded. Thus, delaying planting until soil is warmer will reduce the impact of the seedcorn maggot. Treated seed and soil insecticides applied at planting are effective tactics. Use of insecticide sprays after maggot problems are detected on germinating seed or seedlings is of no benefit.
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Sweet corn: Wireworm, Coleoptera: Elateridae
Biology: Several species of wireworms occur in Arkansas and descriptions of the different species vary. In general, wireworm adults, also known as click beetles, are dark brown hard bodied beetles. The term “click” comes from the ability to snap the hinge between the thorax and abdomen resulting in a flip that rights the upturned insect. Size varies but adult wireworms in sweet corn are about 30 mm long. Larvae occur in the soil. Although the larvae of some species are white, the most common in Arkansas is tan and at maturity about 20 mm in length (Photo 155). True legs are evident and the head is somewhat flattened. Wireworms occur throughout the state but the major damage resulting from their feeding has been observed in Clay County in northeast Arkansas and near Paris in Logan County. Larvae feed on newly planted seed, emerging seedlings, and can be found infesting the lower stems of larger corn plants (Photo 156). Damage in some fields has been substantial and at times fields have been replanted due to stand loss. The biologies of wireworms are also quite variable. Some species complete two generations per year while some require up to 5 years for a single generation. In general, adults emerge from overwintering larvae in the spring and search for grassy fields. Eggs are laid in the soil usually where grasses are available for the larvae to feed. When corn is planted the seed and emerging seedlings are burrowed into. Seedlings can be weakened and killed. Wireworm larvae can be found by carefully digging up weak seedlings and searching the soil. In dry conditions, wireworm larvae move deep into the soil and are difficult to detect.
Management: In Arkansas, avoidance of recently plowed grassy fields including pastures will reduce the likelihood of damage from wireworms. Crop rotations, i.e., sweet corn following a broadleaf like soybean, will reduce damage but with wireworms that require multiple years for development, damage may be substantial. Damage also is more severe in fields where seed are slow to germinate and where seedling growth is retarded. Thus, delaying planting until soil is warmer may reduce the impact of the wireworms. Although the use of treated seed and soil insecticides applied at planting may provide some control, wireworm problems persist in Clay County despite insecticide application. Foliar insecticide application to seedlings is of no benefit.
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