Pseudaletia unipunctata Haworth Lepidoptera Noctuidae

Natural History

Distribution. Armyworm is a native species occurring throughout North America. It is most common, however, in the eastern United States and Canada west to the Rocky Mountains. It does not overwinter at northern latitudes such as Canada and the northern states. Rather, armyworm disperses northward each spring, principally along the Mississippi River Valley, and then disperses southward during the autumn, principally along the east coast (McNeil, 1987). It is often called "true armyworm" to distinguish it from other armyworms such as fall armyworm, Spodoptera frugiperda (J.E. Smith), and yellowstriped armyworm, Spodoptera ornithogalli (Guenee). Army-worm also occurs in Central and South America, southern Europe, central Africa, and western Asia.

Host Plants. Armyworm generally prefers to oviposit and feed upon plants in the family Gramineae, including weedy grasses. Thus, such grain and grass crops as barley, corn, millet, oats, rice, rye, sorghum, sugarcane, timothy, and wheat may be consumed, as well as wild or weed grasses. During periods of abundance larvae feed more generally, damaging such vegetables as artichoke, bean, cabbage, carrot, corn, celery, cucumber, lettuce, onion, parsley, parsnip, pea, pepper, radish, sweet potato, watermelon, and others. Field crops damaged during such population outbreaks include alfalfa, dry bean, and sugarbeet in addition to the aforementioned grain crops. Adults feed on nectar of various flowers and sometimes feed on other sweet foods such as ripe and decaying fruit.

Natural Enemies. The importance of natural enemies, especially parasitoids, has been studied, though nearly all data are derived from the periods of high armyworm density, which is not typical for this insect. Krombein et al. (1979) listed 35 species of Hymenop-tera reared from armyworm, of which 19 species are braconids and 12 species are ichneumonids. Similarly, Arnaud (1978) listed 35 species of tachinids from armyworm. Breeland (1958) studied armyworm in Tennessee, and gave a long list of natural enemies known to affect armyworm around the world. He reported rates of parasitism to be 30-40% in Tennessee studies. The most important parasitoids found by Breeland during 1956 were Glyptapanteles militaris (Walsh) and Rogas terminalis (Cresson) (both Hymen-optera: Braconidae), which accounted for 27% and 5% of total parasitism, respectively; Winthemia rufo-picta (Bigot) (Diptera: Tachinidae), which accounted for 12% parasitism; and Eniscospilus merdarius Gravenhorst (Hymenoptera: Ichneumonidae), which accounted for 9% parasitism. In contrast, during 1957, W. rufopicta accounted for only 1.5% parasitism; parasitism by A. militaris and R. terminalis increased to 36% and 22%, respectively; and parasitism by E. merdarius dropped to 4.5%. Interestingly, Hyposoter sp. (Hymenoptera: Ichneumonidae) was not observed in 1956 but accounted for nearly 20% of total parasitism in 1957. Guppy and Miller (1970) provided keys to the immature stages of armyworm parasitoids.

Predators readily consume armyworm larvae. Ground beetles (Coleoptera: Carabidae) are especially effective because larvae spend most of their time in association with soil, but various predatory bugs (Hemiptera: various families), ants (Hymenoptera: Formicidae), and spiders (Araneae: Lycosidae and Phalangiidae) also feed on armyworm (Clark et al., 1994). Avian predators are often credited with destruction of armyworms. The bobolink, Dolichonyx ory-zivorus (Linnaeus), prospers during outbreak years and has sometimes been called the "armyworm bird." Other birds of note include the crow, Corvus brachyr-hynchos Brehm, and starling, Sturnus vulgaris Linnaeus.

Diseases commonly infect armyworms, especially during periods of high density. Bacteria and fungi, particularly the fungus Metarhizium anisopliae, are reported in the literature. In Arkansas, Steinkraus et al. (1993a) reported an epizootic caused by the fungus Furia virescens, and a significant incidence of mer-mithid nematodes, with mortality by the nematodes estimated at 13.5%. However, undoubtedly the most important diseases are viruses; several granulosis, cytoplasmic polyhedrosis, and nuclear polyhedrosis viruses often kill virtually all armyworms during periods of outbreak, especially when larvae are also stressed by lack of food or inclement weather (Tanada, 1959,1961).

Weather. Armyworm attains high densities irregularly, often at 5-20 year intervals. The exact cause is unknown, but outbreaks often occur during unusually wet years and are preceded by unusually dry years. Armyworm is not well-adapted for hot temperature; survival decreases markedly when temperatures exceed about 30°C. Consequently, at southern latitudes populations are higher early and late in the year, but at northern latitudes it is a mid-season pest.

Life Cycle and Description. Larvae apparently overwinter at least as far north as Tennessee, though they are unsuccessful in the northernmost states and in Canada, which are invaded annually by moths dispersing northward. In the south, all stages may be found during the winter months (Moran and Lyle, 1940). The number of generations varies among locations, but two generations occur in Ontario, 2-3 occur in Minnesota and New York, 4-5 are reported in Tennessee, and 5-6 in southern states (Knutson, 1944; Frost, 1955; Breeland, 1958; Guppy, 1961; Chapman and Lienk, 1981). In Tennessee, moth flights are observed in March-May, June, July-mid-August, September, and November. In New York, moths are common from March-September, but sometimes as late as November. A complete generation requires 30-50 days.

  1. Females deposit eggs in clusters consisting of 2-5 rows, in sheltered places on foliage, often between the leaf sheath and blade, especially on dry grass. Often females seem to deposit large numbers in the same vicinity, resulting in very high densities of larvae in relatively small areas of a field. Nevertheless, the eggs are very difficult to locate in the field. The eggs are white or yellowish, but turn gray immediately before hatching. They are spherical, and measure about 0.54 mm (range 0.4-0.7 mm) in diameter. The egg surface appears to be shiny and smooth, but under high magnification fine ridges can be observed. Their clutches are covered with an adhesive secretion that is opaque when wet but transparent when dry. As the adhesive material dries it tends to draw together the foliage, almost completely hiding the eggs. Mean duration of the egg stage is about 3.5 days at 23°C, and 6.5 days at 18°C, but the range is 3-24 days over the course of a season. Hatching rates are affected by temperature, with cool weather more favorable for embryonic survival. In Tennessee, about 98% egg hatch occurs in early spring and autumn, with hatching rates dropping to less than 30% during the summer; this probably accounts for the evolution of the dispersal behavior in this species.
  2. Larvae normally display six instars, though up to nine instars have been observed. Mean head capsule widths (range) are 0.34 (0.30-0.37), 0.55 (0.490.63), 0.94 (0.83-1.12), 1.5 (1.29-1.70), 2.3 (2.08-2.56), and 3.3 (3.04-3.68) mm, respectively, for instars 1-6. Head capsule widths increase slightly with increased temperature up to about 30°C (Guppy, 1969). Larvae attain a body length 4,6,10,15,20, and 35 mm, respectively, during instars 1-6. Except for the first instar, which is pale with a dark head, the larvae of army-worm are marked with longitudinal stripes throughout their development. The head capsule is yellowish or yellow-brown with dark net-like markings. The body color is normally grayish-green, but a broad dark stripe occurs dorsally and along each side. A light sub-spiracular stripe often is found laterally beneath the dark stripe. Development time varies with temperature. During summer larvae complete their development in about 20 days, but this is extended to about 30 days during the spring and autumn, and greatly prolonged during winter. Instar-specific development times recorded during early summer in Tennessee are 2-3, 2-3, 2-4, 2-3, 4-5, and 7-10 days for instars 1-6, respectively. The larvae tend to disperse upward following hatching, where they feed on tender leaf tissue. If disturbed, they readily extrude silk and spin down to the soil. Larvae in instars 3-6 are active at night, seeking shelter during the day on the soil beneath debris or clods of soil. (See color figure 39.)
  3. Larvae pupate in the soil, often under debris, at depths of 2-5 cm. Pupation occurs in an oval cell that contain a thin silken case. The pupa is moderate in size and robust, measuring 13-17 mm long and 5-6 mm wide. The pupa is yellowish-brown initially, but soon assumes a mahogany brown color. The tip of the abdomen bears a pair of hooks. Duration of

Armyworm larva.

Armyworm larva.

Head capsule of armyworm larva.

Armyworm pupa.

Armyworm pupa.

the pupal stage is 7-14 days during summer but longer early and late in the season, sometimes lasting 40 days.

Adult. The adult is a light reddish-brown moth, with a wingspan measuring about 4 cm. The forewing is fairly pointed, appearing more so because a transverse line of small black spots terminates in a black line at the anterior wing tip. The forewing is also marked with a diffuse dark area centrally containing 1-2 small white spots. The hind wings are grayish, and lighter basally. Adults are nocturnal. Mating commences 1-3 days after moths emerge from the soil, and usually 4-7 hours after sunset. Eggs are normally deposited within a 4-5 day period (range 1-10 days). Females produces an average of 4.9 egg masses (range 1-16 masses). Breeland (1958) reported that about 450 eggs are produced by each female (range 15-1350 eggs) in Tennessee, but Guppy (1961) found a mean fecundity of 1450 (range 250-1900). Feeding is necessary for normal oviposition. Mean longevity at warm temperature is about 9 days in males and 10 days in females (range 3-25 days) whereas at cool temperature mean longevity of males is 19 days and females 17 days. (See color figure 224.)

An excellent treatment of biology was given by Breeland (1958). Other informative publications include Davis and Satterthwait (1916b), Walton and Packard (1947), Pond (1960), and Guppy (1961). Devel-

Pseudaletia Unipunctata Haworth
Adult armyworm.

opmental biology was presented by Guppy (1969). A sex pheromone is described by Steck et al. (1982) and female response by Turgeon et al. (1983). Armyworm is included in the larval keys of Walkden (1950), Crumb (1956), Rings and Musick (1976), Oliver and Chapin (1981), and Capinera (1986). Larvae also are included in a key to armyworms and cutworms in Appendix A. Moths are included in the keys by Rings (1977a) and Capinera and Schaefer (1983), and pictured in Rockburne and Lafontaine (1976) and Chapman and Lienck (1981).

Damage

Larvae initially skeletonize foliage, but by the third instar they eat holes in leaves, and soon afterwards consume entire leaves. Larvae of armyworm are notorious for appearing out of nowhere to inflict a high level of defoliation. This occurs for several reasons: a highly clumped distribution of young larvae, with most of the crop uninfested until larvae are nearly mature and highly mobile; a tendency by larvae to feed on grass weeds preferentially, only moving to crops after the grass is exhausted; occurrence of a preponderance of feeding, about 80%, in the last instar; the nocturnal behavior of larvae, which makes them difficult to observe during the day; and the gregarious and mobile behavior of mature larvae, which form large aggregations or bands (hence the common name "army" worm). As earlier noted, grasses and grains are preferred, but as these plants are consumed larvae disperse, often in large groups, to other plants. During outbreaks, few plants escape damage.

Management

  1. Adults can be captured with blacklight traps. A sex pheromone has been identified and can be used for population monitoring (Kamm et al., 1982; Lopez et al., 1990). Light-colored pheromone traps capture more moths than dark-colored traps (Hendrix and Showers, 1990). It is advisable to examine crop fields for larvae, especially if moths have been captured in light or pheromone traps. Fields should be examined at dawn or dusk, because larvae are active at this time. If it is necessary to check fields during the day, it is important to sift through the upper surface of the soil and under debris for resting larvae.
  2. Larvae consume wheat bran or apple pomace baits treated with insecticide, but foliar and soil-applied insecticides are also effective, and used frequently (Musick and Suttle, 1973; Harris et al., 1975a; Harrison et al., 1980a).

Cultural Techniques. Cultural practices have limited effect on armyworm abundance due to their highly dispersive behavior. However, grass weeds are a focal point of infestation, and should be eliminated, if possible. Not surprisingly, no-till and minimum tillage fields experience greater problems with armyworm than conventional tillage fields (Harrison et al., 1980a; Willson and Eisley, 1992). Proximity to small grain crops is considered to be a hazard owing to the preference of moths for such crops, and the suitability of grains for larval development. In Virginia, destruction of winter cover crops by herbicide application is more favorable to armyworm survival than mowing of cover crops, apparently because predators are more disrupted by herbicide treatment (Laub and Luna, 1992). Before availability of effective insecticides, deep furrows with steep sides were sometimes plowed around fields to prevent invasion by dispersing armyworm larvae. Although this approach remains somewhat useful, but it is rarely practiced.

Biological Control. Some suppression of army-worm can be achieved with Bacillus thuringiensis, though it is not as effective as with some other caterpillars. Larvae also are susceptible to infection by the entomopathogenic nematode Steinernema carpocapsae, though neonate larvae are fairly resistant (Kaya, 1985).

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