Spodoptera eridania Cramer Ledidoptera Noctuidae
Natural History
Distribution. This insect is native to the Americas, occurring widely in North, Central and South America and the Caribbean. In the United States, southern armyworm is found principally in the southeastern states. Although its range extends west to Kansas, New Mexico, and California, this species is of little consequence in western states.
Host Plants. Southern armyworm has a very broad host range. Some of the vegetables injured are beet, cabbage, carrot, collards, cowpea, eggplant, okra, pepper, potato, squash, sweet potato, tomato, and watermelon. Other crops injured include avocado, citrus, peanut, sunflower, velvet bean, and tobacco. Numerous weeds are consumed, but pigweed, Amar-anthus spp.; and pokeweed, Phytolacca americana; are especially favored, and grasses are rarely eaten. There are several reports of armyworm infestations beginning with these two weeds, and adjacent crops experience damage only after the more favored weeds are consumed. (See color figure 1.)
Natural Enemies. Several wasp parasitoids commonly associated with caterpillars of other species, including Cotesia marginiventris (Cresson), Chelonus insularis Cresson, Meteorus autographae Muesebeck, and M. laphygmae Viereck (all Hymenoptera: Braconi-dae) also attack southern armyworm. Meteorus auto-graphae was the dominant parasitoid in a Florida study (Tingle et al., 1978). Also reared from southern armyworm are Campoletis flavicincta (Ashmead) and Ophion flavidus Brulle (both Hymenoptera: Ichneumo-nidae); Euplectrus platyhypenae Howard (Hymenop-tera: Eulophidae); Choeteprosopa hedemanni Braeur and Bergenstamm, Euphorocera claripennis (Macquart), Gonia crassicornis (Fabricius), Winthemia quadripustulata (Fabricius), and W. rufopicta (Bigot) (all Diptera: Tachi-nidae). Predators certainly must be an important factor in southern armyworm biology, but this aspect seems undocumented. Larvae are susceptible to infection by the fungus Beauveria bassiana (Gardner and Noblet, 1978).
Life Cycle and Description. The number of generations is estimated at four annually in Florida. Insects may be present throughout the insect's range from March until October, but they are most commonly observed in late summer and autumn. In northern Florida, moths can be found throughout the year, withstanding several days of freezing weather (Mitchell and Tumlinson, 1994). About 30-40 days are required for a complete generation.
- The shape of the egg is a flattened sphere. Eggs measure about 0.45 mm in diameter and 0.35 mm in height. They bear about 50 slender ribs which radiate outward from the center. The eggs are greenish initially, turning tan as they age. They are laid in clusters, and covered with scales from the body of the moth. Duration of the egg stage is 4-6 days.
- The larvae display six instars as they grow to attain a length of about 35 mm. The head capsule widths are about 0.25-0.30, 0.40-0.50, 0.60-0.80, 0.95-
Southern armyworm larva.
1.15, 1.35-1.85, and 2.35-2.85 mm, respectively (Redfern, 1967). Larvae are green or blackish-green with a uniform light brown or reddish-brown head throughout their development. Larger larvae bear a narrow white line dorsally, and additional stripes laterally. Each side normally bears a broad yellowish or whitish stripe that is interrupted by a dark spot on the first abdominal segment, though in some cases this spot is weak. A series of dark triangles is usually present dorsolaterally along the length of the body. Larvae usually are found on the lower surface of leaves, and are most active at night. Duration of the larval stage is normally 14-20 days. (See color figure 56.)
Pupa. The larvae pupate in the soil, usually burrowing to a depth of 5-10 cm. The pupa is mahogany brown and measures about 16-18 mm long and 56 mm wide. Duration of the pupal period is 11-13 days.
Adult. The moth measures 33-38 mm in wing-span. The front wings are gray and brown, with irregular dark brown and black markings. The wing pattern is highly variable. Some individuals bear a pronounced bean-shaped spot near the center of the wing, whereas others lack the spot or instead bear a broad black band extending from the center of the wing to the margin. The hind wings are opalescent white.
The biology of southern armyworm is poorly documented, but Chittenden and Russell (1910) gave key features. Additional description was given by Crumb (1929). The larval keys developed by Levy and Habeck (1976), Passoa (1991), and Heppner (1998) are useful to distinguish southern armyworm from related species. It was also included in a key by Oliver and Chapin (1981). Heppner (1998) provided a key to the adults of North American Spodoptera. Rearing techniques were provided by Redfern and Raulston (1970). A sex pheromone has been identified and evaluated in the field (Mitchell and Tumlinson, 1994).
Damage
This is one of the most common pests of southeastern vegetable gardens. Larvae are defoliators and feed gregariously while young, often skeletonizing leaves. As they mature they become solitary, and also bore readily into fruit, often damaging tomato. When rfliAultJ \ X l„«S»J life
Adult southern armyworm.
stressed by lack of food they eat the apical portions of branches, bore into stem tissue, and attack tubers near the surface of the soil. High densities and lack of food sometimes prompt larvae to move in great number, the basis of the "armyworm" designation, feeding on all vegetation in their path. Southern armyworm is more damaging to cassava than black cutworm, Agrotis ipsilon (Hufnagel), given equal numbers of insects (Pena and Waddill, 1981).
Management
Insecticides. Southern armyworm is best controlled with foliar insecticides when larvae are small. Insecticides vary considerably in their toxicity to larvae (Aziz, 1973). This insect is difficult to control with botanical insecticides (Valles and Capinera, 1993). Berger (1920) reported some success at southern armyworm suppression by application of bran bait containing insecticide. However, this is useful principally for large, mobile larvae that have left the plant and are on the soil surface.
Host-Plant Resistance. There is limited information that suggested differences in susceptibility among sweet potato cultivars to armyworm damage (Habeck, 1976). Conventional sources of insect resistance in corn have little effect on southern armyworm (Manuwoto and Scriber, 1982).
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