Spodoptera exigua HUbner Lepidoptera Noctuidae

Natural History

Distribution. Beet armyworm is a tropical insect, and is native to Southeast Asia. It is now found around the world, except South America. It was first discovered in North America about 1876, when it was found in Oregon. It rapidly spread to Hawaii (1880), California (1882), Colorado (1899), Texas (1904), Arizona (1916), Mississippi (1920), and Florida (1924). It has since spread to Mexico and the Caribbean (Mitchell, 1979). As it is a tropical insect and lacks a diapause mechanism, it can overwinter successfully only in warm areas or in greenhouses. Daytime temperatures below 10°C are deleterious, and it rarely overwinters in areas where frost kills its host plants. Thus, overwintering is generally limited to Arizona, Florida, and Texas. Despite its inability to overwinter in most of the United States, beet armyworm nevertheless invades the southern half of the United States (Maryland to Colorado to northern California, and south) annually. Sometimes it is found as far north as New York and Ontario in the east, and British Columbia in the west. It is rarely viewed as a serious pest anywhere but the southern states, however, except sometimes in greenhouses.

Host Plants. This insect has a wide host range, occurring as a serious pest of vegetable, field, and flower crops; even trees are sometimes attacked. Among susceptible vegetable crops are asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip. Field crops damaged include alfalfa, corn, cotton, peanut, safflower, sorghum, soybean, sugarbeet, and tobacco.

Weeds also are suitable for larval development, including such common plants as lambsquarters, Chenopo-dium album.; mullein, Verbascum sp.; pigweed, Amaranthus spp.; purslane, Portulaca spp.; Russian thistle, Salsola kali; parthenium, Parthenium sp.; and tidestromia, Tidestromia sp. Although its host range is wide, there are significant differences in suitability even among hosts considered to be suitable. For example, in a comparison among diets consisting of sugar-beet, pigweed, or lambsquarters, sugarbeet-fed larvae had the shortest development time and the highest fecundity, lambsquarter had the longest development time and the lowest fecundity, and pigweed was intermediate (Al-Zubaidi and Capinera, 1986).

Natural Enemies. Numerous native natural enemies have adapted to this pest. Among the most common parasitoids are Chelonus insularis Cresson, Cotesia marginiventris (Cresson), and Meteorus autographae (Muesbeck) (all Hymenoptera: Braconidae), and the tachinid Lespsia archippivora (Riley) (Diptera: Tachini-dae) (Oatman and Platner, 1972; Tingle et al, 1978; Ruberson et al., 1994). Predators frequently attack the eggs and small larvae; among the most important are minute pirate bugs, Orius spp. (Hemiptera: Anthocor-idae); big-eyed bugs, Geocoris spp. (Hemiptera: Lygaei-dae); damsel bugs, Nabis spp. (Hemiptera: Nabidae); and a predatory shield bug, Podisus maculiventris (Say). Pupae are subject to attack, especially by the red imported fire ant, Solenopsis invicta Buren. Fungal diseases, Erynia sp. and Nomurea rileyi, and a nuclear polyhedrosis virus also inflict some mortality (Wilson, 1933,1934; Harding, 1976b; Ruberson et al., 1994). The important mortality factors vary among crops and geographic regions. None except the nuclear poly-hedrosis virus are highly specific to beet armyworm, which may explain why they are not especially effective. Virus is considered to be the most important mortality factor in Mexico (Alvarado-Rodriguez, 1987).

Life Cycle and Description. Seasonal activity varies considerably according to climate. In warm locations such as Florida, all stages can be found throughout the year, though development rate and overall abundance are decreased during the winter months (Tingle and Mitchell, 1977). The life cycle can be completed in just 24 days, and six generations have been reared during five months of summer weather in Florida (Wilson, 1934). However, generation times of 50-126 days have been observed, with a total of five generations annually, in southern California (Campbell and Duran, 1929).

Egg. Eggs are laid in clusters of about 50-150 per mass. Females may deposit over 1200 eggs during their life time, but normal egg production is about 300-600.

Morphology Armyworm Eggs
Beet armyworm egg.

They usually are deposited on the lower surface of the leaf, and often near blossoms and the tip of the branch. The individual egg is circular when viewed from above, but when examined from the side it is slightly peaked, tapering to a point. The eggs are greenish to white, and are covered with a layer of whitish scales that gives the egg mass a fuzzy or cottony appearance. They hatch in 2-3 days during warm, but the incubation period is extended to about four days in cool weather. The developmental threshold for eggs is estimated at 12.4°C.

Larva. Normally there are five instars, though additional instars are sometimes found. Duration of the instars under warm (summer) conditions is reported to be 2.3, 2.2, 1.8, 1.0, and 3.1 days, respectively (Wilson, 1932), and at constant 30°C instar development time was reported by Fye and McAda (1972) to be 2.5,1.5,1.2,1.5, and 3.0 days, respectively. Total larval development time is also influenced by diet quality (Al-Zubaidi and Capinera, 1984). The developmental threshold for larvae is estimated at 13.6° C. Only 1 mm long at hatching, the larvae attain a mean length of 2.5,5.8, 8.9,13.8, and 22.3 mm during instars 1-5, respectively (Wilson, 1932). Head capsule widths average 0.25, 0.45, 0.70, 1.12, and 1.80 mm, respectively.

The larvae are pale green or yellow during instars 1-2, but acquire pale stripes during instar three. During instar four, larvae are darker dorsally and possess a dark lateral stripe. Larvae during instar five are variable in appearance, and tend to be green dorsally, with pink or yellow color ventrally, and a white stripe laterally. A series of dark spots or dashes is often present dorsally and dorsolaterally. Sometimes larvae are very dark. The spiracles are white with a narrow black border. The body is practically devoid of hairs and spines. In the western states, the larva of beet armyworm is easily confused with clover cutworm, Discestra trifolii (Hufnagel), but beet armyworm lacks the black pigment adjacent to the spiracles that is so evident in clover cutworm. In the southern states, the larva of beet armyworm is easily confused with southern armyworm, Spodoptera eridania (Cramer), but southern armyworm can be distinguished by the presence of a

Spodoptera Eridania
Beet armyworm larva.

large dark spot laterally on the first abdominal segment that disrupts the lateral stripe. Beet armyworm occasionally bears a spot laterally, but if present it occurs on the mesothorax, not on the first abdominal segment.

Initially, the larvae of beet armyworm are gregarious, feeding as a group and skeletonizing plant foliage. As they mature, larvae become solitary and quite mobile, often traveling from plant to plant. Cannibalism may occur when larvae are at high densities or feeding on food low in nitrogen (Al-Zubaidi and Capinera, 1983). (See color figures 40 and 41.)

  1. Pupation occurs in the soil. The larva generally constructs a pupal chamber near the soil surface, digging only about 1 cm beneath the surface. The chamber is constructed from sand and soil particles held together with an oral secretion that hardens when it dries. The pupa is light brown and measures about 15-20 mm long. Duration of the pupal stage is 6-7 days during warm weather. Fye and McAda (1972) reported a pupal duration of 5.1 days at 30°C.
  2. The moths are moderately sized, the wing-span measuring 25-30 mm. The front wings are mottled gray and brown, and normally with an irregular banding pattern and a light colored bean-shaped spot. The hind wings are a more uniform gray or white color, and trimmed with a dark line at the margin. Mating occurs soon after emergence of the moths, and ovi-position begins within 2-3 days. Oviposition extends over a 3-7 day period, and the moths usually perish within 9-10 days of emergence. (See color figure 225.)

An overview of biology was given by Wilson (1932), and developmental biology by Wafa et al. (1969), Fye and McAda (1972), and Ali and Gaylor (1992). Brown and Dewhurst (1975) provided detailed description of all stages, and a comprehensive list of host plants. Rearing technology was discussed by many authors, including Cobb and Bass (1975) and Hartley (1990). A sex pheromone has been identified (Persoons et al., 1981; Mitchell et al., 1983). Effects of irradiation and

Adult beet armyworm.

potential for release of sterile insects has been investigated (Debolt and Wright, 1976). Larvae are included in keys by Okumura (1962), Oliver and Chapin (1981), and Capinera (1986), and are included in a key to armyworms and cutworms in Appendix A. Adults are included in a key by Capinera and Schaefer (1983). Heppner (1998) provided keys to the adults and larvae of North American Spodoptera.

Damage

Larvae feed on both foliage and fruit. Young larvae feed gregariously and skeletonize foliage. As they mature, larvae become solitary and eat large irregular holes in foliage. They also burrow into the crown or center of the head on lettuce, or on the buds of cruci-fers. As a leaf feeder, beet armyworm consumes much more cabbage tissue than diamondback moth, Plutella xylostella (Linnaeus), but it is less damaging than cabbage looper, Trichoplusia ni (Hubner) (East et al., 1989). This insect also is regarded as a serious pest of celery in California, and damage is directly correlated with abundance of late-instar larvae late in the season. However, damage to foliage and petioles (stalks) during the first half of the growing season is of little consequence because these plant parts are removed at harvest (van Steenwyk and Toscano, 1981). Tomato fruit is most susceptible to injury, especially near fruit maturity, but beet armyworm is not considered to be as threatening to tomato as is corn earworm, Heli-coverpa zea (Boddie) (Zalom et al., 1986a). Larvae not only damage tomato fruit, but may appear as contaminants in processed tomato (Zalom and Jones, 1994).

Management

  1. Pheromone traps can be used to detect the presence of adult beet armyworm (Trumble and Baker, 1984; Mitchell and Tumlinson, 1994). Visual sampling for damage and larvae, combined with an action threshold of 0.3 larvae per plant, was used successfully on cabbage in South Texas to determine the need for crop treatment with insecticides (Cartwright et al., 1987). A binomial sequential sampling program for armyworm-damaged tomato fruit was developed by Wilson et al. (1983b). Egg distribution on tomato was studied by Zalom et al. (1983). Regular monitoring of crops, probably about twice per week, is recommended because adults frequently invade from surrounding crops or weeds (Edelson et al., 1988).
  2. In the southeast and southwest, the relatively high abundance of beet armyworm has stimulated frequent application of insecticides. Chemical insecticides and Bacillus thuringiensis are commonly applied to foliage to protect against defoliation. Insecticide resistance is a major problem in management of this insect, possibly because it attacks crops such as flowers, cotton, and vegetables crops that are treated frequently with insecticides (Brewer et al., 1990; Brewer and Trumble, 1994). Beet armyworm abundance is favored by frequent insecticide use, and it is considered to be a secondary or induced pest in some crops (Eveleens et al., 1973). Also, intensive use of insecticides for beet armyworm control in vegetables such as celery has stimulated outbreaks of other pests, principally American serpentine leafminer, Liriomyza trifolii (Burgess).

Beet armyworm larvae are susceptible to management with other products such as neem formulations (Prabhaker et al., 1986). Eggs can be killed with petroleum oil (Wolfenbarger et al., 1970), and both eggs and young larvae can be controlled with foliar applications of 5% cottonseed oil, though this concentration is damaging to some plants (Butler and Henneberry, 1990).

Pheromones can also be used to disrupt mating and to inhibit or eliminate reproduction. Saturation of the atmosphere around beet armyworm-susceptible crops has been estimated to decrease mating by 97% (Waka-mura and Takai, 1992) and production of eggs and larvae by 57% and 95%, respectively (Mitchell et al., 1997).

Cultural Practices. Host-plant resistance in several crops has been studied for its contribution to beet armyworm pest management. In tomato, for example, resistance is correlated with total glycoalkaloid concentration in the fruit tissue. However, leaf tissue does not have any effective antibiotic chemistry, so larvae are able to develop on plants even if they have unsuitable tomato fruit (Eigenbrode and Trumble, 1994). The future for beet armyworm-resistant celery is most promising (Meade and Hare, 1991; Diawara et al., 1996).

Biological Control. Several insect pathogens may prove to be useful for suppression of beet armyworm. A nuclear polyhedrosis virus isolated from beet army-worm is fairly effective as a bioinsecticide under greenhouse conditions, where inactivation by ultraviolet light in sunlight is not a severe problem (Smits et al.,

1987). It is as effective as commonly used insecticides (Gelernter et al., 1986), but it is not commercially available. The fungus Beauveria bassiana has the same attributes and limitations (Barbercheck and Kaya, 1991). Entomopathogenic nematodes (Rhabditida: Steinerne-matidae and Heterorhabditidae) successfully infect both larvae and adults of beet armyworm, and infected adults can fly short distances, helping to spread the pathogens (Timper et al., 1988). Use of nematodes is similarly constrained by environmental conditions, but these biological control agents are available commercially.

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