Heliothis virescens Fabricius Lepidoptera Noctuidae

Natural History

Distribution. Tobacco budworm is a native species and is found throughout the eastern and southwestern United States. Like corn earworm, Helicoverpa zea (Bod-die), it generally overwinters successfully only in southern states. However, it occasionally survives in cold climates in greenhouses and other sheltered locations. Tobacco budworm disperses northward annually, and can be found in New England, New York, and southern Canada during the late summer. It also occurs widely in the Caribbean, and sporadically in Central and South America.

Host Plants. Tobacco budworm is principally a field-crop pest, attacking such crops as alfalfa, clover, cotton, flax, soybean, and tobacco. However, it some times attacks such vegetables as cabbage, cantaloupe, lettuce, pea, pepper, pigeon pea, squash, and tomato, especially when cotton or other favored crops are abundant. Tobacco budworm is a common pest of geranium and other flower crops such as ageratum, bird of paradise, chrysanthemum, gardenia, geranium, petunia, mallow, marigold, petunia, snapdragon, straw-flower, verbena, and zinnia. Weeds serving as a host for larvae include beardtongue, Penstemon laevigatus; beggarweed, Desmodium spp.; bicolor lespedeza, Lespe-deza bicolor; black medic, Medicago lupulina; cranesbill, Geranium dissectum; deergrass, Rhexia spp.; dock, Rumex spp., groundcherry, Physalis spp.; Japanese honeysuckle, Lonicera japonica; lupine, Lupinus spp.; morningglory, Ipomoea spp.; a morningglory, Jacque-montia tamnifolia; passionflower, Passiflora sp.; prickly sida, Sida spinosa; sunflower, Helianthus spp.; toadflax, Linaria canadensis; and velvetleaf, Abutilon theophrasti (Brazzel et al., 1953; Neunzig, 1963; Graham and Robertson, 1970; Roach, 1975; Harding, 1976a; Stadelbacher, 1981; Pair, 1994; Sudbrink and Grant, 1995). In Georgia, Barber (1937) determined that tobacco bud-worm developed principally on toadflax during April-May for 1-2 generations, followed by one generation on deergrass during June-July and 2-3 generations on beggarweed during July-October. In Mississippi, cranesbill was identified as the key early season host plant (Stadelbacher, 1981). In southern Texas, cotton is the principal host, but such weeds as wild tobacco, Nicotania repanda; vervain, Verbena neo-mexicana; ruellia, Ruellia runyonii; and mallow, Aubiti-lon trisulcatum, are important hosts early or late in the year (Graham et al., 1972). In cage tests and field studies conducted in Florida and which did not include cotton, tobacco was more highly preferred than other field crops and vegetables, but cabbage, collards, okra, and tomato were attacked (Martin et al., 1976a).

Natural Enemies. Numerous general predators have been observed to feed upon tobacco budworm. Among the most common are Polistes spp. wasps (Hymenoptera: Vespidae); big-eyed bug, Geocoris punc-tipes (Say) (Hemiptera: Lygaeidae); damsel bugs, Nabis spp. (Hemiptera: Nabidae); minute pirate bugs, Orius spp. (Hemiptera: Anthocoridae), and spiders.

Several parasitoids also have been observed, and high levels of parasitism have been reported (Lewis and Brazzel, 1968; Tingle et al., 1994). The egg parasi-toid Trichogramma pretiosum Riley (Hymenoptera: Tri-chogrammatidae) can be effective in vegetable crops. Other important parasitoids are Cardiochiles nigriceps Viereck in vegetables and Cotesia marginiventris (Cresson) in other crops (both Hymenoptera: Braconidae). Effectiveness of the parasitoids varies among crops (Martin et al., 1981). Other species known from tobacco budworm include Archytas marmoratus (Townsend) (Diptera: Tachinidae); Meteorus autographae Muese-beck (Hymenoptera: Braconidae); Campoletis flavicincta (Ashmead), C. perdistinctus (Viereck), C. sonorensis (Cameron), Netelia sayi (Cushman) and Pristomerus spi-nator (Fabricius) (all Hymenoptera: Ichneumonidae).

Pathogens are also known to inflict mortality. Among the known pathogens are microsporidia, Nosema spp., fungi such as Spicaria rileyi, and nuclear polyhedrosis viruses. In a study conducted in South Carolina, Spicaria was a more important mortality agent than natural incidence of virus, and was considered to be one of the most important natural mortality agents (Roach, 1975).

Life Cycle and Description. Moths emerge during March-May in southern states, followed by 4-5 generations through the summer, with overwintering commencing in September-November. Four generations have been reported from northern Florida (Chamberlin and Tenhet, 1926) and North Carolina (Neunzig, 1969), and at least five from Louisiana (Brazzel et al., 1953). Moths have been collected in New York in July-September, but at such northern latitudes it is not considered to be a pest (Chapman and Lienk, 1981). This species overwinters in the pupal stage.

  1. The eggs are deposited on blossoms, fruit, and terminal growth. They are spherical, with a flattened base. They measure 0.51-0.60 mm wide and 0.50-0.61 mm long. They initially are whitish to yellowish white, turning gray as they age. Narrow ridges radiate from the apex of the egg, and number 18-25. Eggs of tobacco budworm are nearly indistinguishable from those of corn earworm. At high magnification, however, the primary ribs of tobacco budworm eggs can be observed to terminate before they reach the rosette of cells surrounding the micropyle; in corn ear-worm at least some primary ribs extend to the rosette (Bernhardt and Phillips, 1985). Females normally produce from 300-500 eggs, but fecundity of 1000-1500 eggs per female have been reported from larvae cultured on artificial diet at cool temperature (Fye and McAda, 1972).
  2. Tobacco budworm larvae have 5-7 instars, with five or six most common. Head capsule widths for larvae that develop through five instars measure 0.26-0.31, 0.46-0.54, 0.92-0.99, 1.55-1.72, 2.382.87mm for instars 1-5, respectively. Larval lengths are 1.1-4.0, 4.2-8.0, 8.7-14.7, 18.5-25.6, and 23.335.6 mm for these same instars. Head capsule widths for larvae that develop through six instars measure 0.26-0.31, 0.36-0.53, 0.72-0.85, 1.12-1.25, 1.60-1.72, and 2.40-2.82 mm for instars 1-6, respectively. Larval lengths are 1.4-4.1, 3.0-7.0, 7.5-9.2, 12.0-15.8, 19.524.3, and 25.5-36.0 mm for these same instars. Development time was studied by Fye and McAda (1972) at various temperatures. When cultured at 20°C, development required about 4.6, 2.6, 3.1, 3.7, 10.1, and 9.8 days for instars 1-6, respectively. At 25°C, larval development times were 3.1, 2.0, 1.9, 2.1, 5.7, and 2.5 days, respectively. Young larvae are yellowish or yellowish-green with a yellowish-brown head capsule. Later instars are greenish with dorsal and lateral whitish bands, and with a brown head capsule. Many of the bands may be narrow or incomplete, but a broad, lateral subspiracular band is usually pronounced. Body color is variable, and pale green or pinkishforms, or dark reddish or maroon forms are sometimes found. Larvae are very similar to corn earworm. As in corn earworm, its body bears numerous black thorn-like microspines. These spines give the body a rough feel when touched. Early instars are difficult to separate from corn earworm; Neunzig (1964) gave distinguishing characteristics. Starting with the third instar, close examination reveals tubercles with small thorn-like microspines on the first, second, and eighth abdominal segments that are about half the height of the tubercles. In corn earworm the microspines on the tubercles are absent or up to one-fourth the height of the tubercle. Larvae exhibit cannibalistic behavior starting with the third or fourth instar, but are not as aggressive as corn earworm. (See color figure 61.)
  3. Pupation occurs in the soil. Pupae are shiny reddish brown, become dark brown before emergence of the adult. The pupa averages 18.2 mm long and 4.7 mm wide. Duration of the pupal stage is reported to be about 22 days at 20°C, 13.0 days at 25°C, and 11.2 days at 30°C. Diapause is initiated by either low temperature or short day length (Henneberry et al., 1993; Henneberry, 1994).
  4. The moths are brownish, and lightly tinged with green. The front wings are crossed transversely by three dark bands, each of which is often accompanied by a whitish- or cream-colored border. Females tend to be darker. The hind wings are whitish, with the distal margin bearing a dark band. The moths measure 28-35 mm in wingspan. The pre-oviposition per-

Tobacco budworm larva.

Tobacco budworm larva.

1953 Busdworms

iod of females is about two-days long. Longevity of moths is reported to range from 25 days when held at 20°C, to 15 days at 30°C. A sex pheromone has been identified (Tumlinson et al., 1975).

The biology of tobacco budworm was given by Neunzig (1969) and Brazzel et al. (1953). The larva was included in keys by Okumura (1962) and Oliver and Chapin (1981); the latter publication also pictured the adult stage. Tobacco budworm also is included in a key to armyworms and cutworms in Appendix A. Larvae are readily cultured on bean-based rearing media or other diets (King and Hartley, 1985).

Damage

Larvae bore into buds and blossoms (the basis for the common name of this insect), and sometimes the tender terminal foliar growth, leaf petioles, and stalks. In the absence of reproductive tissue, larvae feed read-

Sex Pheromone Heliothis Virescens
Spinules associated with tubercles on abdomen of tobacco budworm larva. The minute spines are about one-half the height of the tubercle.

ily on foliar tissue. Neunzig (1969) infested tobacco with both tobacco budworm and corn earworm, and observed very similar patterns and levels of injury by these closely related species. In California, both budworm and earworm burrow into the heads of developing lettuce. Entry of larvae into fruit increases frequency of plant disease. Research in southern Arkansas tomato fields indicated that though tobacco budworm was present from May-July, they were not nearly as abundant or damaging as corn earworm (Roltsch and Mayse, 1984).

Management

  1. Large cone-shaped wire traps baited with sex pheromone lures are commonly used to capture tobacco budworm moths (Hartstack et al., 1979). Smaller bucket traps can be used to capture these moths, but they are not very efficient (Lopez et ah, 1994).
  2. Foliar insecticides are commonly used in crops where tobacco budworm damage is likely to occur. However, destruction of beneficial organisms often results, and this is thought to exacerbate budworm damage. Also, resistance to insecticides is widespread, particularly in crops where pyrethroid use is frequent (Kanga et ah, 1995; Greenstone, 1995). Larvae also consume bait formulated from cornmeal and insecticide (Creighton et ah, 1961).

Cultural Techniques. Early season destruction of weeds with herbicide or mowing, or destruction of larvae on the weeds by treatment with insecticides, can reduce tobacco budworm population size later in the year (Bell and Hayes, 1994; Snodgrass and Stadelbacher, 1994).

Biological Control. The microbial insecticide Bacillus thuringiensis is effective against budworm (Johnson, 1974; Stone and Sims, 1993). Heliothis nuclear polyhedrosis virus has been used effectively to suppress tobacco budworm on field crops (Andrews et al., 1975) and on early season weed hosts (Hayes and Bell, 1994). Tobacco budworm also is susceptible to nuclear polyhedrosis virus from alfalfa looper, Autographa californica (Speyer) (Vail et al., 1978; Bell and Romine, 1980). Release of Trichogramma egg parasitoids has been shown to be beneficial in some vegetable crops (Martin et al., 1976b).

Host-Plant Resistance. Although there is little evidence for natural resistance to tobacco budworm among many crops, cotton is being genetically engineered to express resistance (Benedict et al., 1996). Enhanced resistance to larval survival by cotton should result in lower insect pressure on nearby vegetable crops.

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