Euxoa auxiliaris Grote Lepidoptera Noctidae

Natural History

Distrib├╝tion. This native insect is abundant in the Great Plains and Rocky Mountain regions in the United States and Canada. It has been recorded from all states west of the Mississippi River, and as far east in Canada as Ontario, but it attains high densities only in semiarid areas.

Host Plants. Army cutworm has been reported to feed on numerous plants. It is known principally as a pest of small grains, perhaps because these crops dominate the landscape where army cutworm occurs. Among vegetable crops, it has been reported to damage beet, cabbage, celery, corn, onion, pea, potato, radish, rhubarb, tomato, and turnip. Other crops injured include such fruit crops as apple, apricot, blackberry, cherry, currant, gooseberry, peach, plum, prune, raspberry, and strawberry, and such field crops as alfalfa, barley, clover, flax, rye, sanfoin, sunflower, sweet clover, timothy, vetch, and wheat. Army cutworm also feeds on noncultivated plants such as bluegrass, Poa spp.; bromegrass, Bromus spp.; buffalograss, Buchloe dactyloides; gramagrasses, Bouteloua spp.; field pennycress, Thlaspi arvense; dandelion, Taraxacum officinale; lambs-quarters, Chenopodium album; and lupine, Lupinus spp.

Natural Enemies. Many natural enemies have been found associated with army cutworm, and both hymenopterous parasitoids and disease have been documented to cause considerable mortality. Walkden (1950), working in the central Great Plains, reported mortality trends over a 20-year period and observed parasitism levels of up to 33% and disease incidence of up to 57%. Not surprisingly, incidence of disease was greatest at high armyworm population densities. Snow (1925) reported 30% parasitism in Utah. In a three-year study in Oklahoma, researchers found that less than 12% of larvae were parasitized, with most parasitism due to two species, Meteorus leviventris (Wesmael) and Apanteles griffini Viereck (both Hy-menoptera: Braconidae) (Soteres et al., 1984). A polyembryonic wasp, Copidosoma bakeri (Howard) (Hymenoptera: Encyrtidae), causes larvae to consume more food, to become larger, and live longer; this can result in the appearance of artificially high rates of parasitism, which sometimes exceeds 50% (Byers et al., 1993).

Among the other parasitoids known from army cutworm are such wasps as Cotesia marginiventris (Cresson), A. militaris Walsh, Chelonus insularis Cresson, Macrocentrus incompletus Muesebeck, Microplitis feltiae Muesebeck, M. melianae Viereck, Rogas sp., Zele melea (Cresson) (all Hymenoptera: Braconidae); Campoletis flavicincta (Ashmead), C. sonorensis (Cameron), Diphyus nuncius (Cresson), Exetastes lasius Cushman, and Spilichneumon superbus (Provancher) (all Hyme-noptera: Ichneummonidae). Flies known to parasitize this species include Bonnetia comta (Fallen), Euphoro-cera claripennis (Macquart), Mericia spp., Peleteria sp., Periscepsia cinerosa (Coquillett), P. helymus (Walker), and P. laevigata (Wulp) (all Diptera: Tachinidae).

Several viruses are known to infect army cutworm, including entomopox, granulosis, and nonoccluded viruses (Jackson and Sutter, 1985; McCarthy et al., 1975; Sutter, 1972, 1973). The relative importance of each is uncertain, but the granulosis virus is unusually pathogenic.

Life Cycle and Description. There is a single generation per year throughout the range of this insect. The eggs are deposited on soil in August-October.

They hatch in autumn or early winter, and larvae overwinter, feeding actively in the spring. Pupation occurs about a month before adults appear. Adults first become active in April-May in southern locations such as Kansas and Texas, whereas in more northern locations such as Alberta and Montana they may not appear until June-July. The moths migrate from the plains, where the larvae develop, to higher elevations in the Rocky Mountains, where the adults feed on nectar from flowers. The adults return to the plains in September- October.

  1. The eggs are deposited singly or in small clusters just beneath the soil surface on a solid substrate (Pruess, 1961). Soil particles adhere to the eggs so they are difficult to detect in the field. In shape, the eggs are a slightly flattened sphere, measuring about 0.6 mm in diameter and 0.5 mm in height. The egg is white to yellow initially, becomes gray to brown as the embryo matures. The egg is marked with about 18 very narrow ridges that radiate from the apex. Survival of eggs apparently is affected by moisture, and Seamans (1928) suggested that above-average rainfall in late summer and autumn assured good insect survival and damaging populations in the subsequent year; this concept appears not to have been independently confirmed, however. Field-collected females were reported by Pruess (1963) to produce 200-300 eggs, with the potential to produce about 500 eggs. However, Jacobson and Blakeley (1959) suggested that 1000-2500 eggs could be produced by a female based on laboratory studies in which larvae were fed dandelion, a highly suitable host.
  2. The eggs hatch in the autumn or early winter but the larvae are usually not noticed until spring when they increase in size and begin to consume considerable foliage. There are 6-7 instars, with head capsule widths of 0.26-0.30,0.40-0.45,0.65-0.72,1.04-1.21, 1.70-2.10, and 2.90-3.40 mm, respectively, for instars 1-6 among larvae with only six instars (Jacobsen and Blakeley, 1959). In comparison, head capsule widths of 0.25-0.30, 0.36-0.43, 0.55-0.70, 0.88-1.28, 1.40-1.90, 1.95-2.50, and 2.95-3.55 mm were reported for instars 1-7, respectively, in larvae with seven instars (Sutter and Miller, 1972). Additional instars apparently occur when larvae feed on less suitable host plants. Duration of the instars is estimated at 16-48,13-73, 4-70, 3-42, 6-11, 4-18, and 9-25 days for instars 1-7, respectively (Burton et al., 1980). The body color of the larvae is grayish-brown, but bears numerous white and dark brown spots. There usually is evidence of three weak light-colored dorsal stripes. Laterally, it tends to be a broad dark band, and the area beneath the spiracles

Army cutworm larva.

Army cutworm larva.

is whitish. The head is light brown with dark spots. Larvae attain a length of about 40 mm. They usually are found beneath the surface of the soil, emerging in late afternoon or early evening to feed. On cloudy days, however, they may be active during the daylight hours. Larvae assume a migratory habit when faced with food shortage, and numerous larvae proceed in the same direction, consuming virtually all vegetation in their path. It is this dispersive behavior that is the basis for their common name, and larvae are observed to disperse over 4 km. (See color figure 38.)

  1. Pupation occurs in the soil, in a cell prepared by the larva. The walls of the cell are formed with salivary secretion, which hardens and provide a degree of rigidity. The depth of pupation varies according to soil and moisture conditions, but it may be any depth up to 7.5 cm. The larva spends about 10 days in the cell before pupation. Duration of pupation is 25-60 days. The pupa is dark brown, and measures about 17-22 mm long and 6 mm wide.
  2. The adults measure 35-50 mm in wingspan. They are quite variable in appearance, with five named subspecies (Pruess, 1967), but moths generally assume two basic forms. One common form has the leading edge of the forewing marked with a broad yellowish stripe, and the remainder of the wing blackish but marked with white-rimmed bean-shaped and round spots, and a light transverse line. In another common color form the forewing is mottled brown, bearing bean-shaped and round spots but lacking bands and stripes. In all cases the hind wings are brownish with dark veins, and darker distally. The brown body of the moth is quite hairy. (See color figures 220 and 221.)

As earlier noted, the adults are migratory, dispersing from the plains to the mountains annually (Pruess, 1967; Pruess and Pruess, 1971). In transit and in the mountains they feed on nectar from flowering plants (Kendall et al., 1981). They are nocturnal, and seek shelter during the daylight hours. They have the habit of aggregating in houses, automobiles, and other sheltered locations where they become a nuisance, soil walls, and induce allergic reactions among some individuals (Storms et al., 1981). They also may aggregate in natural shelters in mountainous regions, where they become prey for bears (Chapman et al., 1955; Mattson et al., 1991). In the Rocky Mountain region they are commonly called "miller moths."

Millermoths
Adult army cutworm, light form.
Army Cutworm
Adult army cutworm, dark form.

An excellent summary of army cutworm biology was given by Burton et al. (1980). Rearing procedures using vegetation were provided by Blakeley et al. (1958) and using artificial diet by Sutter and Miller (1972). Sex pheromones have been identified (Struble and Swailes, 1977b; Struble, 1981b). Larvae are included in keys by Whelan (1935), Walkden (1950), and Capinera (1986), and are included in a key to armyworms and cutworms in Appendix A. Moths are included in pictorial keys of Okumura (1962), Rings (1977a), and Capinera and Schaefer (1983).

Damage

These insects principally are pests of small grain crops grown in arid regions, though many irrigated crops also are at risk. Larvae readily climb plants to consume foliage, eating holes in vegetation initially, and eventually destroying the entire plant. Although they burrow into the soil during the daylight hours, they do not normally feed below-ground. However, when succulent food is in short supply they follow the plant stem down into the soil. When food supplies are exhausted numerous larvae may disperse in search of additional food.

Management

  1. Adults can be captured in light and pheromone traps. However, males are attracted to the sex pheromone only during the autumn flight. Pheromone traps positioned at a height of 1 m or lower are more effective than those placed higher (Swailes and Struble, 1979). Larvae can be recovered from soil by raking through the top 5-7 cm.
  2. Persistent insecticides can be applied to vegetation to kill army cutworm larvae when they emerge from the soil to feed; Bacillus thuringiensis is not effective (McDonald, 1979; Bauernfeind and Wilde, 1993). Larvae also accept bran bait containing insecticide.

Cultural Practices. Cultural manipulations are not generally effective to prevent oviposition because moths deposit eggs on barren soil. Delayed planting of crops can be effective, however, as larvae complete their development on weeds or starve before crops are planted. If larvae are dispersing, creation of deep ditches with steep sides, or filled with running irrigation water, may prevent invasion of fields.

To protect plants grown in the home garden, barriers are occasionally used to decrease access by cutworms to seedlings. Metal- or waxed-paper containers with both the top and bottom removed can be placed around the plant stem to deter consumption. Aluminum foil can be wrapped around the stem to achieve a similar effect. Because larvae burrow and feed below the soil line, the barrier should be extended below the soil surface.

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