Distribution. Fall armyworm is a native to the tropical regions of the western hemisphere from the United States to Argentina. It normally overwinters successfully in the United States only in southern Florida and southern Texas, but during warm winters it may survive along the Gulf Coast and in southern Arizona. It is commonly found in the Caribbean, including Puerto Rico. Fall armyworm is a strong flier, and disperses to long distances annually during the summer months. It is recorded from virtually all states east of the Rocky Mountains, from Arizona and California, and from southern Ontario. As a regular and serious pest, its range tends to be mostly the southeastern states, though it is feared by sweet corn growers as far north as the New England states.
Host Plants. This species seemingly displays a very wide host range, with over 80 plants recorded, but clearly prefers grasses. The most frequently consumed plants are field corn and sweet corn, sorghum, Bermudagrass, and grass weeds such as crabgrass, Digitaria spp. When the larvae are very numerous they defoliate the preferred plants, acquire an "armyworm" habit and disperse in large numbers, consuming nearly all vegetation in their path. Many host records reflect such periods of abundance, and are not truly indicative of oviposition and feeding behavior under normal conditions. Vegetables, other than sweet corn which is frequently at risk, are only occasionally damaged, but include a wide range of crops such as asparagus, bean, beet, cabbage, chickpea, cowpea, corn, cucumber, kale, onion, pea, pepper, potato, rutabaga, spinach, sweet potato, tomato, turnip, and watermelon. Field crops are frequently injured, including alfalfa, barley, Bermudagrass, buckwheat, cotton, clover, corn, oat, millet, peanut, rice, ryegrass, sorghum, sugarbeet, sudangrass, soybean, sugarcane, timothy, tobacco, and wheat. Other crops occasionally injured are apple, grape, orange, papaya, peach, strawberry and many flowers. Among the weeds known to serve as hosts are bentgrass, Agrostis sp.; crabgrass, Digitaria spp.; Johnsongrass, Sorghum hale-pense; morningglory, Ipomoea spp.; nutsedge, Cyperus spp.; pigweed, Amaranthus spp.; and sandspur, Cen-chrus tribuloides. In studies conducted in Honduras, fall armyworm larvae preferred Amaranthus foliage over both corn and sorghum (Portillo et al., 1996b). Pencoe and Martin (1981) measured development on several grasses found in Georgia, and determined that large crabgrass, Digitaria sanguinalis; goosegrass, Eleusine indica; vaseygrass, Paspalum urvillei; and coastal Bermudagrass, Cynodon dactylon were very suitable hosts whereas yellow nutsedge, Cyperus esculentus; the sedge, Cyperus globulosus, and Texas panicum, Pani-cum texanum, were relatively poor hosts.
There is some evidence that fall armyworm strains exist, based primarily on their host plant preference. One strain feeds principally on corn, but also on sorghum, cotton and a few other hosts if they are found growing near the primary hosts. The other strain feeds principally on rice, Bermudagrass, and Johnsongrass. Some reproductive isolation exists between the strains, even when both occur in the same area (Pashley, 1988).
Natural Enemies. Cool, wet springs followed by warm, humid weather in the overwintering areas favor survival and reproduction of fall armyworm, allowing it to escape suppression by natural enemies. Once dispersal northward begins, the natural enemies are left behind. Therefore, though fall armyworm has many natural enemies, a few act effectively enough to prevent crop injury.
At least 53 species of parasitoids are known from throughout the range of fall armyworm, mostly in the families Braconidae and Ichneumonidae (both Hymenoptera), and Tachinidae (Diptera), but not all occur in North America. The known parasitoids, nearly all of which attack the larval stage, were listed by Ashley (1979). The parasitoids most frequently reared from larvae in the United States are Cotesia mar-giniventris (Cresson) and Chelonus texanus (Cresson) (both Braconidae), species that are also associated with other noctuid species. In a study conducted in Georgia, for example, C. marginiventris was the most abundant parasitoid collected in 1990, attaining up to 34% parasitism early in the season, but Chelonus insularis Cresson (Hymenoptera: Braconidae) assumed dominance late in the season (Riggin et al., 1992). During the second year of the study, however, the most abundant parasitoid was Archytas marmoratus (Townsend) (Diptera: Tachinidae), followed closely by Ophion flavidus Brulle and Aleiodes laphygmae (Gahan) (both Hymenoptera: Ichneumonidae). Also, in a subset of the study, the same authors found Aleiodes laphygmae and Ophion flavidus to be the dominant parasitoids (Riggin et al., 1993). Luginbill (1928) and Vickery (1929) described and pictured many of the fall army-worm parasitoids.
The predators of fall armyworm are general predators that attack many other caterpillars. Among the predators noted as important are various ground beetles (Coleoptera: Carabidae); the striped earwig, Labi-dura riparia (Pallas) (Dermaptera: Labiduridae); the spined soldier bug, Podisus maculiventris (Say) (Hemi-ptera: Pentatomidae); and the insidious flower bug, Orius insidiosus (Say) (Hemiptera: Anthocoridae). Vertebrates such as birds, skunks, and rodents also consume larvae and pupae readily. Predation may be quite important, as Pair and Gross (1984) demonstrated loss of pupae to predators at 60-90% in Georgia.
Numerous entomopathogens, including viruses, fungi, protozoa, nematodes, and a bacterium are associated with fall armyworm (Gardner et al., 1984), but only a few cause epizootics. Among the most important are the S. frugiperda nuclear polyhedrosis virus (NPV), and the fungi Entomophaga aulicae, Nomuraea rileyi, and Erynia radicans. Incidence of NPV reached 50-60% in Louisiana, but disease typically appears too late to alleviate high levels of defoliation. A most interesting pathogen is the ectoparasitic nematode Noctuidonema guyanense (Nematoda: Aphelenchoidi-
dae). This nematode is a weak pathogen, having a debilitating effect on its host. Although fall armyworm is the principal host, it is associated with many other Lepidoptera (Rogers et al., 1991; Simmons and Rogers 1996).
Life Cycle and Description. The life cycle is completed in about 30 days during the summer, but 60 days in the spring and autumn, and 80-90 days during the winter. The number of generations occurring in an area varies with the appearance of the dispersing adults. The ability to diapause is not present in this species. The population often spreads northward at about 480 km (300 miles) per generation. In Minnesota and New York, where fall armyworm moths do not appear until August (Knutson, 1944; Chapman and Lienk, 1981), there may be but a single generation. The number of generations is reported to be one to two in Kansas (Walkden, 1950), three in South Carolina (Luginbill, 1928), and four in Louisiana (Oliver and Chapin, 1981). In coastal areas of north Florida, moths are abundant from April to December, but some are found even during the winter months (Tingle and Mitchell, 1977). With the aid of certain weather patterns, dispersal northward may be much faster than just described. For example, during 1973 a redistribution of moths from Mississippi to Ontario occurred in about two days, a distance of 1600 km (1000 mile), with the aid of strong surface winds (Rose et al., 1975).
Egg. The egg of fall armyworm is dome shaped; the base is flattened and the egg curves upward to a broadly rounded point at the apex. It is well-marked with 47-50 ridges that radiate outward from the apex. The egg measures about 0.4 mm in diameter and 0.3 mm in height. They are deposited on hosts and non-hosts; in the latter case the larvae disperse, often with the help of a strand of silk, which allows them to be blown a considerable distance by wind. The female typically produces several egg masses during her oviposition period, with deposition occurring at night, and on larger plants if provided a choice between large and small. The number of eggs per mass varies considerably but it is often 100-200, and total egg production per female averages about 1500, with a maximum of over 2000. They are sometimes deposited in layers, but most are spread over a single layer, and are attached to foliage. The female also deposits a layer of grayish scales between the eggs and over the egg mass, imparting a furry or moldy appearance. Initially the eggs are grayish green, but they soon turn brown. Egg masses are deposited beneath leaves when the moth density is low, but oviposition becomes indiscriminate at high densities. The period of incubation is only 2-3 days during the summer months.
Fall armyworm egg.
Larva. There usually are six instars in fall army-worm. Head capsule widths are about 0.35, 0.45, 0.75, 1.3, 2.0, and 2.6 mm, respectively, for instars 1-6. Larvae attain lengths of about 1.7, 3.5, 6.4, 10.0, 17.2, and 34.2 mm, respectively, during these instars. Young larvae are greenish with a black head, the head turning orangish in the second instar. In the second, but particularly the third instar, the dorsal surface of the body becomes brownish, and lateral white lines begin to form. In the fourth to the sixth instars the head is reddish-brown, mottled with white, and the brownish body bears white subdorsal and lateral lines. Elevated spots occur dorsally on the body; they are usually dark, and bear spines. The face of the mature larva also is marked with a white inverted "Y" and the epidermis of the larva is rough or granular in texture when examined closely. However, this larva does not feel rough touch, as does corn earworm, Helicoverpa zea (Boddie), because it lacks the microspines found in the similar-appearing corn earworm. In addition to the typical brownish form, its brown dorsal coloration may be replaced with green. In the green form, the dorsal elevated spots are pale rather than dark. Larvae are most active in the morning, late afternoon, and evening, and tend to conceal themselves during the brightest time of the day. Duration of the larval stage tends to be about 14 days during the summer and 30 days during cool weather. Mean development time was determined to be 3.3,1.7,1.5,1.5, 2.0, and 3.7 days for instars 1-6, respectively, when larvae were reared on corn at 25°C (Pitre and Hogg, 1983). However, total larval development time was extended when larvae were fed less suitable hosts: from 13.5 days on corn, to 18.9 days on soybean, and 22.3 days on cotton. As development time increased, pupal weights and survival rates decreased. (See color figure 51.)
Fall armyworm larva.
Fall armyworm larva.
Fall armyworm pupae.
ning. Females may mate repeatedly, but only once per night. After a pre-oviposition period of 3-4 days, the female normally deposits most of her eggs during the first 4-5 days of life, but some oviposition occurs for up to three weeks. The oviposition period tends to be shorter under warm conditions, sometimes as short as one day, and longer under cool conditions. Duration of adult life is estimated to average about 10 days (range about 7-21 days). (See color figures 238, 239, and 240.)
A comprehensive account of the biology of fall armyworm was published by Luginbill (1928), and an informative synopsis by Sparks (1979). Ashley et al. (1989) presented an annotated bibliography. Fall armyworm was included in many larval identification guides, such as the keys by Whelan (1935), Walkden (1950), and Crumb (1956), and the pictorial keys by Okumura (1962), Rings (1976), Oliver and Chapman (1981), and Capinera (1986). It also was included in a key to armyworms and cutworms in Appendix A. Fall armyworm was included in the keys to moths by Rings (1977a) and Capinera and Schaefer (1983), and pictured by Chapman and Lienk (1981). Heppner (1998) provided a key to the adults of North American Spodoptera. A sex pheromone has been described (Tumlinson et al., 1986). Culture of this insect was easily accomplished with a bean-based diet (Perkins, 1979).
A highly visible form of damage by larvae is consumption of foliage. Young larvae initially consume leaf tissue from one side, leaving the opposite epidermal layer intact. By the second or third instar, larvae begin to make holes in leaves, and eat from the edge of the leaves inward. Feeding in the whorl of corn often produces a characteristic row of perforations in the leaves, though the larvae quickly produce a ragged appearance as they grow and feed. Larval densities are usually reduced to 1-2 per plant when larvae feed in close proximity to one another, due to cannibalistic behavior. Older larvae cause extensive defoliation,
Adult fall armyworm.
Fall armyworm pupae.
often leaving only the ribs and stalks of corn plants, or a ragged, torn appearance. Total leaf consumption by larvae exceeds 100 cm. The proportion of defoliation is estimated at 0.1%, 0.6%, 1.1%, 4.7%, 16.3%, and 77.2% during instars 1-6, respectively. As over three-fourths of the defoliation occurs during the last instar, the presence of larvae is easily overlooked and damage occurs suddenly. Marenco et al. (1992) studied the effects of fall armyworm injury to early vegetative growth of sweet corn in Florida. They reported that the early whorl stage was least sensitive to injury, the mid-whorl stage intermediate, and the late whorl stage was most sensitive to injury. Further, they noted that mean densities of 0.2-0.8 larvae per plant during the late whorl stage could reduce yield by 5-20%.
Larvae also burrow into the growing point (bud, whorl, etc.), destroying the growth potential of plants, or clipping the leaves. In corn, they sometimes burrow into the ear, feeding on kernels in the same manner as corn earworm, Helicoverpa zea. Unlike corn earworm, which tends to feed down through the silk before attacking the kernels at the tip of the ear, fall army-worm feed by burrowing through the husk on the side of the ear. Ear damage is of greater concern to sweet corn growers than foliage feeding.
Cultural Techniques. The most important cultural practice, employed widely in southern states, is early planting and/or early maturing varieties. Early harvest allows many corn ears to escape the higher armyworm densities that develop later in the season (Mitchell, 1978). Reduced tillage seems to have little effect on fall armyworm populations (All, 1988), though delayed invasion by moths of fields with extensive crop residue has been observed, thus delaying and reducing the need for chemical suppression (Roberts and All, 1993).
Host-Plant Resistance. Partial resistance is present in some sweet corn varieties, but it is inadequate for complete protection. Resistance is largely due to non-preference by larvae, but some antibiosis is present (Wiseman et al., 1981, Wiseman and Widstrom, 1986).
Biological Control. Although several pathogens have been shown experimentally to reduce the abundance of fall armyworm larvae in corn, only Bacillus thuringiensis now is feasible, and success depends on having the product on the foliage when the larvae first appear. Natural strains of Bacillus thuringiensis tend not to be very potent, and genetically modified strains improve performance (All et al., 1996). An interesting and unusual approach to biological control involves the application of mass-produced parasitoid larvae, Archytas marmoratus (Diptera: Tachinidae). The fly larvae are mechanically extracted from the female flies, suspended in aqueous solution, and sprayed onto plants (Gross and Johnson, 1985; Gross et al., 1985).
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