Helicoverpa zea Boddie Lepidoptera Noctuidae

Natural History

Distribution. Corn earworm is found throughout North America except for northern Canada and Alaska. It tends to be less abundant west of the Rocky Mountains, and is infrequently a pest in Canada's Prairie Provinces. It also occurs in Hawaii and the Caribbean islands. Corn earworm is common in South America, persisting to a southern latitude of about 40°. Its origin is uncertain, but likely is native to North America.

In the eastern United States, corn earworm does not normally overwinter successfully in the northern states. It is known to survive as far north as about 40° north latitude, or about Kansas, Ohio, Virginia, and southern New Jersey, depending on the severity of winter weather (Blanchard, 1942). However, it is highly dispersive, and routinely spreads from southern states into northern states and Canada (Hardwick, 1965b; Fitt, 1989; Westbrook et al., 1997). Thus, areas have overwintering, both overwintering and immigrant, or immigrant populations, depending on location and weather. In the relatively mild Pacific Northwest, corn earworm can overwinter at least as far north as southern Washington.

Host Plants. Corn earworm has a wide host range; hence, it is also known as "tomato fruitworm,'' "sorghum headworm,'' "vetchworm," and "cotton bollworm.'' In addition to corn and tomato, perhaps its most favored vegetable hosts, corn earworm also attacks artichoke, asparagus, cabbage, cantaloupe, col-lards, cowpea, cucumber, eggplant, lettuce, lima bean, melon, okra, pea, pepper, potato, pumpkin, snap bean, spinach, squash, sweet potato, and watermelon. Not all are good hosts, however. Harding (1976a), for example, studied relative suitability of crops and weeds in Texas, and reported that though corn and lettuce were excellent larval hosts, tomato was merely a good host, and broccoli and cantaloupe were poor. Other crops injured by corn earworm include alfalfa, clover, cotton, flax, oat, millet, rice, sorghum, soybean, sugarcane, sunflower, tobacco, vetch, and wheat. Among field crops, sorghum is particularly favored. Cotton is frequently reported to be injured, but this generally occurs only after more preferred crops have senesced. Fruit and ornamental plants may be attacked, including ripening avocado, grape, peaches, pear, plum, raspberry, strawberry, carnation, geranium, gladiolus, nasturtium, rose, snapdragon, and zinnia. In studies conducted in Florida, Martin et al. (1976a) found corn earworm larvae on all 17 vegetable and field crops studied, but corn and sorghum were most favored. In cage tests earworm moths preferred to oviposit on tomato over a selection of several other vegetables that did not include corn.

Such weeds as common mallow, Malva neglecta; crown vetch, Coronilla varia; fall panicum, Panicum di-chotomiflorum; hemp, Cannabis sativa; horsenettle, Sola-num spp.; lambsquarters, Chenopodium album; lupine, Lupinus spp.; morningglory, Ipomoea spp.; pigweed, Amaranthus sp.; prickly sida, Sida spinosa; purslane, Portulaca oleracea; ragweed, Ambrosia artemisiifolia; Spanish needles, Bidens bipinnata; sunflower, Helian-thus spp.; toadflax, Linaria canadensis; and velvetleaf, Abutilon theophrasti have been reported to serve as larval hosts (Ditman and Cory, 1931; Roach, 1975; Sudbrink and Grant, 1995). However, Harding (1976a) rated only sunflower as a good weed host relative to 10 other species in a study conducted in Texas. Stadelbacher (1981) indicated that crimson clover and winter vetch, which may be both crops and weeds, were important early season hosts in Mississippi. He also indicated that cranesbill, Geranium dissectum and G. carolinianum, were particularly important weed hosts in this area. In North Carolina, especially important wild hosts were toadflax and deergrass, Rhexia spp. (Neunzig, 1963). Gross and Young (1977) documented some of the differences in suitability among various natural hosts relative to development time, weight gain, and fecundity.

Adults collect nectar or other plant exudates from numerous plants. Lingren et al. (1993) studied adult host associations in Texas and Oklahoma, and reported that trees and shrub species were especially frequented. Among the hosts identified were Citrus, Salix, Pithecellobium, Quercus, Betula, Prunus, Pyrus and other Rosaceae, and Asteraceae. Callahan (1958) also presented a long list of adult hosts. The quality or quantity of nectar affects potential fecundity of moths, with such plants as alfalfa; red and white clover; milkweed, Asclepias syriaca; and Joe-Pye weed, Eupatorium purpureum proving especially suitable in Virginia (Nuttycombe, 1930).

Natural Enemies. Although numerous natural enemies have been identified, they usually are not effective at causing high levels of earworm mortality or preventing crop injury. For example, in a study conducted in Texas, Archer and Bynum (1994) reported less than 1% of the larvae were parasitized or infected with disease. However, eggs may be heavily parasitized (Oatman 1966a, Campbell et al., 1991). Tricho-gramma spp. (Hymenoptera: Trichogrammatidae), and to a lesser extent Telenomus spp. (Hymenoptera: Scelionidae), are common egg parasitoids. Also, natural control agents can affect populations late in the season (Roach, 1975). Exotic parasitoids and predators have been introduced to North America in hopes of gaining better natural control of H. zea, but thus far the imported beneficials have failed to establish successfully (King and Coleman, 1989).

Common larval parasitoids include Cotesia spp., and Microplitis croceipes (Cresson) (all Hymenoptera: Braconidae); Campoletis spp. (Hymenoptera: Ichneu-monidae); Eucelatoria armigera (Coquillett) and Archy-tas marmoratus (Townsend) (Diptera: Tachinidae). However, additional wasp and fly species have, on occasion, been reported from corn earworm (Arnaud, 1978; Krombein et al., 1979). In Mississippi, Lewis and Brazzel (1968) observed only M. croceipes to be abundant regularly.

General predators often feed on eggs and larvae of corn earworm; over 100 insect species have been observed to feed on H. zea. Among the common predators are lady beetles such as convergent lady beetle, Hippodamia convergens Guerin-Meneville, and Coleo-megilla maculata De Geer (both Coleoptera: Cocci-nellidae); softwinged flower beetles, Collops spp. (Coleoptera: Melyridae); green lacewings, Chrysopa and Chrysoperla spp. (Neuroptera: Chrysopidae); minute pirate bug, Orius tristicolor (White) (Hemiptera: Anthocoridae); and big-eyed bugs, Geocoris spp. (Hemiptera: Lygaeidae) (King and Coleman, 1989). Birds can also feed on earworms, but rarely are adequately abundant to be effective (Barber, 1942).

Within season mortality during the pupal stage seems to be slight (Kring et al., 1993), and though overwintering mortality is often very high the mortality is due to adverse weather and collapse of emergence tunnels rather than to natural enemies. A nematode, Chroniodiplogaster aerivora (Nematoda: Diplogasteri-dae), occurs naturally in midwestern states, but it is a weak pathogen (Steinkraus et al., 1993b). In Texas, Steinernema riobrave (Nematoda: Steinernematidae) has been found to be an important mortality factor of prepupae and pupae, but this parasitoid is not yet generally distributed. Similarly, Khan et al. (1976) found Heterorhabditis heliothidis (Nematoda: Heterorhabditi-

dae) parasitizing corn earwom in North Carolina, but it has not been found widely. Both of the latter species are being redistributed, and can be produced commercially; so in the future they may assume greater importance in natural regulation of earworm populations.

Epizootics caused by pathogens may erupt when larval densities are high. The fungal pathogen Nomur-aea rileyi, and the Helicoverpa zea nuclear polyhedrosis virus are commonly involved in outbreaks of disease, but the protozoan Nosema heliothidis and other fungi and viruses also have been observed.

Life Cycle and Description. This species is active throughout the year in tropical and subtropical climates, but becomes progressively more restricted to the summer months with increasing latitude. In northeastern states dispersing adults may arrive as early as May or as late as August due to the vagaries associated with weather; thus, their population biology is variable. The number of generations is usually reported to be one in northern areas such as most of Canada, Minnesota, and western New York (Knutson, 1944; Beirne, 1971; Chapman and Lienk, 1981), two in northeastern states (Prostak, 1995), two to three in Maryland (Ditman and Cory, 1931), three in the central Great Plains (Walkden, 1950) and northern California (Okumura, 1962), four to five in Louisiana (Oliver and Chapin, 1981) and southern California (Okumura, 1962), and perhaps seven in southern Florida and southern Texas. The life cycle can be completed in about 30 days.

  1. The eggs are deposited singly, usually on leaf hairs and corn silk. The egg is pale green when first deposited, becomes yellowish and then gray with time. The shape varies from slightly dome-shaped to a flattened sphere, and measures about 0.5-0.6 mm in diameter and 0.5 mm high. They bear 21-31 ridges radiating from the center. Fecundity ranges from 500-3000 eggs per female. The eggs hatch in about 3-4 days.
  2. Upon hatching, larvae wander about the plant until they encounter a suitable feeding site, normally the reproductive structure of the plant. Young larvae are not cannibalistic, so several larvae may feed together initially. However, as larvae mature they become very aggressive, killing and cannibalizing other larvae. Consequently, only a small number of larvae are found in each ear of corn. Normally, corn earworm displays six instars, but five is not uncommon and seven to eight have been reported. Mean head capsule widths are 0.29, 0.47, 0.77, 1.30, 2.12, and 3.10 mm, respectively, for instars 1-6. Larval lengths are esti mated at 1.5, 3.4, 7.0, 11.4, 17.9, and 24.8 mm, respectively. Development time averaged 3.7, 2.8, 2.2, 2.2, 2.4, and 2.9 days, respectively, for instars 1-6 when reared at 25°C. Butler (1976) reported that the cultured earworm on corn at several temperatures, reporting total larval development times of 31.8, 28.9, 22.4, 15.3, 13.6, and 12.6 days at 20°, 22.5°, 25°, 30°, 32°, and 34°C, respectively.

The larva is variable. Overall, the head tends to be orange or light brown with a white net-like pattern, the thoracic plates black, and the body brown, green, pink, or sometimes yellow or mostly black. The larva usually bears a broad dark band laterally above the spiracles, and a light yellow to white band below the spiracles. A pair of narrow dark stripes often occurs along the center of the back. The close examination reveals that the body bears numerous black thorn-like microspines. These spines give the body a rough feel when touched. The presence of spines and the light-colored head serve to distinguish corn earworm from fall armyworm, Spodoptera frugiperda (J.E. Smith), and European corn borer, Ostrinia nubilalis (Hubner). These other common corn-infesting species lack the spines and bear dark heads. Tobacco budworm, Heliothis virescens (Fabricius), is a closely related species in which the late instar larvae also bear micro-spines. Although it is easily confused with corn earworm, it rarely is a vegetable pest and never feeds on corn. Close examination reveals that in tobacco budworm larvae the spines on the tubercles of the first, second, and eighth abdominal segments are about half the height of the tubercles, but in corn earworm the spines are absent or up to one-fourth the height of the tubercle. Younger larvae of these two species are difficult to distinguish, but Neunzig (1964) gave a key to aid in separation. (See color figure 48.)

Helicoverpa Zea Spinule
Spinules associated with tubercles on abdomen of corn earworm larva. The minute spines are less than one-fourth the height of the tubercle.
Adult Helicoverpa Key
Head capsule of corn earworm larva.
  1. Mature larvae leave the feeding site and drop to the ground, where they burrow into the soil and pupate. The larva prepares a pupal chamber 510 cm below the surface of the soil. The pupa is mahogany-brown, and measures 17-22 mm long and 5.5 mm wide. Duration of the pupal stage is about 13 days (range 10-25 days) during the summer, but 250 days or more for the overwintering pupae. (See color figure 265.)
  2. Because pupation occurs rather deep in the soil, the moths have difficulty digging to the surface unless the tunnel created by the larvae as they dig into the soil remains intact. As with the larval stage, adults are quite variable. The front wings of the moths usually are yellowish-brown, and often bear a small dark spot centrally. The small dark spot is especially distinct when viewed from below. The forewing also may bear a broad dark transverse band distally, but the margin of the wing is not darkened. The hind wings are creamy white basally and blackish distally, and usually bear a small dark spot centrally. The moth
Corn earworm pupa.

measures 32-45 mm in wingspan. Adults are reported to live for 5-15 days, but may survive for over 30 days under optimal conditions. The moths are principally nocturnal, and remain active throughout the dark period. During the daylight hours they usually hide in vegetation, but sometimes they can be seen feeding on nectar. Oviposition commences about three days after emergence, and continues until death. Fresh-silking corn is highly attractive for oviposition but even ears with dry silk can receive eggs. Fecundity varies from about 500-3000 eggs, though feeding is a prerequisite for high levels of egg production. Females may deposit up to 35 eggs per day. (See color figure 235).

The biology of corn earworm was presented by several authors; among the most complete were Quain-tance and Brues (1905), Ditman and Cory (1931), Brazzel et al. (1953), Hardwick (1965b), and Neunzig (1969). An extensive bibliography was published by Kogan et al. (1978). Keys to Helicoverpa adults and larvae were provided by Hardwick (1965b). Keys to differentiate earworm from similar crop-infesting larvae were given by many authors, including Whelan (1935), Walkden (1950), Frost (1955), Okumura (1962), Oliver and Chapin (1981), and Capinera (1986). Corn ear-worm is also included in a key to armyworms and cutworms in Appendix A. Keys to moths can be found in Rings (1977a) and Capinera and Schaefer (1983). Artificial diet and rearing procedures have been developed (Burton, 1970; Singh and Moore, 1985).

Damage

Some consider corn earworm is the most costly crop pest in North America. It is more damaging in areas where it successfully overwinters, however, because in northern areas it may arrive too late to inflict extensive damage. It often attacks harvested portions of valuable crops. Thus, larvae often are found associated with such plant structures as blossoms, buds, and fruits. When feeding on lettuce, larvae may burrow

Helicoverpa Zea Identification

into the head. On corn, its most common host, young larvae tend to feed on silks initially, and interfere with pollination, but eventually they usually gain access to the kernels. They may feed only at the tip, or injury may extend half the length of the ear before larval development is completed. Such feeding also enhances development of plant pathogenic fungi, and is attractive to sap beetles (Coleoptera: Nitidulidae). If the ears have not yet produced silk, larvae may burrow directly into the ear. They usually remain feeding within a single ear of corn, but occasionally they abandon the feeding site and begin search for another. Larvae also can damage whorl-stage corn by feeding on the young, developing leaf tissue. Survival is better on more advanced stages of development, however (Gross et al., 1976). Young fields adjacent to favored, more mature plants are likely to experience low rates of egg deposition (Weisenborn and Trumble, 1988). On tomato, larvae may feed on foliage and burrow in the stem, but most feeding occurs on the tomato fruit. Larvae commonly begin to burrow into a fruit, feed only for a short time, and then move on to attack another fruit. Tomato is more susceptible to injury when corn is not silking; in the presence of corn, moths will preferentially oviposit on fresh corn silk. Other crops such as bean, cantaloupe, cucumber, squash, and pumpkin may be injured in a manner similar to tomato, and also are less likely to be injured if silking corn is nearby.

Management

Sampling. Eggs and larvae often are not sampled on corn because eggs are very difficult to detect, and larvae burrow down into the silks, out of the reach of insecticides, soon after hatching. Sampling protocols have been developed for larvae on corn, however (Hoffmann et al., 1996b). On tomato, eggs tend to be placed on the leaves immediately below the highest flower cluster (Zalom et al., 1983). Thus, sampling protocols, including both fixed and sequential sampling procedures, have been developed for this crop (Hoffman et al., 1991a). A common procedure is to examine the leaves beneath all flower clusters on 20-30 plants per field, but the sampling effort can be reduced significantly using sequential sampling. Similarly, sampling protocols for fruit damage have been developed (Wilson et al., 1983b).

Moths can be monitored with blacklight and pher-omone traps. Both sexes are captured in light traps whereas only males are attracted to the sex phero-mone. Both trap types give an estimate of when moths invade or emerge, and relative densities, but phero-mone traps are easier to use because they are selective. The pheromone is usually used in conjunction with an inverted cone-type trap. Trap designs and commercial source of pheromone lures affect trap catches (Gauthier et al. 1991, Lopez et al,. 1994). Generally, the presence of 5-10 moths per night is sufficient to stimulate pest control practices (Foster and Flood, 1995). Light traps have also been investigated for removal of moths from cropping areas. Some protection of small plantings can be attained, but this approach is ineffective for large areas or when moth densities are high (Barrett et al., 1971). Pheromone components also can be released in an area to confuse the moths and disrupt mating; this has been demonstrated experimentally (Mitchell et al., 1975) but has not yet come into commercial practice.

Insecticides. Corn fields with more than 5% of the plants bearing new silk are susceptible to injury if moths are active. Insecticides are usually applied to foliage in a liquid formulation, with particular attention to the ear zone, because it is important to apply insecticide to the silk. Systemic insecticides are not effective because the plant does not translocate insecticide effectively to reproductive tissues (Russell et al., 1993). Considerable economic benefit has been documented for chemical suppression of earworm on tomato in North Carolina (Walgenbach and Estes, 1992). Insecticide applications are often done at 2-6 day intervals, sometimes as frequently as daily in Florida. Because it is treated frequently and over a wide geographic area, corn earworm has become resistant to many insecticides (Fitt, 1989; Kanga et al., 1996). Susceptibility to Bacillus thuringiensis also varies, but the basis for this variation in susceptibility is uncertain (Stone and Sims, 1993). Mineral oil, applied to the corn silk soon after pollination, has insecticidal effects. Application of about 0.75-1.0 ml of oil 5-7 days after silking can provide good control in the home garden (Carruth, 1942; Barber, 1942). Corn earworm moths, like many moths, feed readily on baits containing sweet material such as sucrose. Some work has been conducted to demonstrate that baits containing insecticide can attract and kill moths (Ditman, 1937; Creighton and McFadden, 1976), but this has yet to be developed into a practical technique.

Cultural Practices. Cultural procedures have some application for corn earworm management. This dispersive species moves readily from weeds to crops, and among crops, as their host plants become more or less suitable. Thus, effective management is best considered on an area-wide basis (Graham et al., 1972). Trap cropping is often suggested for this insect; the high degree of preference by ovipositing moths for corn in the green silk stage can be used to lure moths from less preferred crops. Lima beans also are relatively attractive to moths, at least as compared to tomato (Pepper, 1943). However, it is difficult to maintain attractant crops in an attractive stage for protracted periods. In southern areas where populations develop first on weed hosts and then disperse to crops, treatment of the weeds through mowing, herbicides, or application of insecticides can greatly ameliorate damage on nearby crops (Snodgrass and Stadelbacher, 1994). In northern areas, it is sometimes possible to plant or harvest early enough to escape injury. Throughout the range of this insect, population densities are highest, and most damaging, late in the growing season. Tillage, especially in the autumn, can significantly reduce overwintering success of pupae in southern locations (Barber and Dicke, 1937).

Biological Control. Several insect pathogens have been evaluated for suppression of corn earworm. A nuclear polyhedrosis virus isolated from corn ear-worm is efficacious and has been sold commercially for larval suppression on non-food crops (Young and McNew, 1994). Application of virus to weed hosts over an area-wide basis early in the season has been shown to reduce earworm population increase (Bell and Hayes, 1994; Hayes and Bell, 1994). Several other viruses, isolated from other caterpillars, are possible biological control agents if they attain commercial development (Young and McNew, 1994). In addition to the nuclear polyhedrosis virus, the fungus Nomuraea rileyi, the bacterium Bacillus thuringiensis, and stei-nernematid nematodes all provide some suppression (Oatman et al., 1970; Ignoffo et al., 1978; Mohamed et al., 1978; Bartels and Hutchinson, 1995). Entomo-pathogenic nematodes, which are available commercially, provide good suppression of developing larvae if they are applied to corn silk; this has application for home garden production of corn but not commercial production (Purcell et al., 1992). Soil surface and subsurface applications of nematodes also can affect earworm populations because larvae drop to the soil to pupate (Cabanillas and Raulston, 1994, 1995, 1996). This approach may have application for commercial crop protection, but larvae must complete their development before they are killed, so some crop damage ensues.

Trichogramma spp. (Hymenoptera: Trichogramma-tidae) egg parasitoids have been reared and released for suppression of H. zea in several crops. Levels of parasitism averaging 40-80% have been attained by such releases in California and Florida, resulting in fruit damage levels of about 3% (Oatman and Platner, 1971). The host crop seems to affect parasitism rates, with tomato being an especially suitable crop for para-sitoid releases (Martin et al., 1976b).

Host-Plant Resistance. Numerous varieties of corn have been evaluated for resistance to earworm, and some resistance has been identified in commercially available corn varieties (McMillian et al., 1975; Story et al., 1983; Archer et al., 1994). Resistance is derived from physical characteristics such as husk tightness and ear length, which impede access by larvae to the ear kernels, or chemical factors such as may-sin, which inhibit larval growth (Douglas, 1947). Hostplant resistance thus far is not completely adequate to protect corn from earworm injury, but it may prove to be a valuable component of multi-faceted pest management programs. Varieties of some crops are now available that incorporate Bacillus thuringiensis toxin, which reduces damage by H. zea, and this likely will assume importance as a plant resistance mechanism (Benedict et al., 1996).

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Responses

  • kedija
    Why control Helicoverpa zea prepupae whereas the larvae stage has done damage?
    4 years ago

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