Western Yellowstriped Armyworm Spodoptera praefica Grote Lepidoptera Noctuidae

Natural History

Distribution. Yellowstriped armyworm, Spodoptera ornithogalli (Guenée), is common in the eastern United States as far west as the Rocky Mountains, and occurs in southern Canada. However, it also is reported from southwestern states, including California. The distribution of this native insect includes Mexico, Central and South America, and many Caribbean islands. As a pest, however, its occurrence is limited principally to the southeastern states. Western yellow-striped armyworm, Spodoptera praefica (Grote), is known only from the western states, principally California and Oregon. It also is a native insect. In California, S. praefica is much more important than S. ornithogalli.

Host Plants. These species are very general feeders, reportedly damaging many crops. Among vege table crops injured are asparagus, bean, beet, cabbage, cantaloupe, carrot, corn, cucumber, lettuce, onion, pea, potato, rhubarb, rutabaga, salsify, sweet potato, tomato, turnip, and watermelon. Other crops damaged include alfalfa, blackberry, cotton, clover, grape, lentil, peach, rape, raspberry, sorghum, soybean, sugarbeet, sweetclover, sunflower, tobacco, wheat, and several flower crops. Some of the weed species known to be suitable hosts are castorbean, Ricinus communis; dock, Rumex sp.; gumweed, Grindelia sp.; horse nettle, Sola-num carolinense; horseweed, Erigeron canadensis; jimson-weed, Datura sp.; lambsquarters, Chenopodium album; morningglory, Ipomoea sp.; plantain, Plantago lanceolata; prickly lettuce, Lactuca scariola; and redroot pigweed, Amaranthus retroflexus.

In California, western yellowstriped armyworm develops for 1-2 generations on rangeland plants, preferring storksbill, Erodium sp.; and foxtail, Setaria sp. early in the year. However, as these plants senesce the armyworms move to irrigated areas, where they prefer alfalfa and morningglory. Alfalfa is a very suitable host, and such fields become heavily infested and serve as important sources of armyworms for other crops later in the season. A similar phenomenon was reported in Washington, where larvae developed on thistle, Cirsium spp.; wild lettuce, Lactuca spp.; wild mustard, Brassica spp.; and goosefoot, Chenopodium spp.; and then moved to crops as the weeds senesced (Halfhill, 1982).

Natural Enemies. Several wasp parasitoids affect S. ornithogalli including Rogas laphygmae Viereck, R. terminalis (Cresson), Zele mellea (Cresson), Chelonus insularis Cresson and Apanteles griffini Viereck (all Hymenoptera: Braconidae). Also, Euplectrus plathype-nae Howard (Hymenoptera: Eulophidae) attacks larvae and causes a cessation of feeding within two days (Parkman and Shepard, 1981). Thus, this parasi-toid is particularly valuable at minimizing damage. Numerous flies have been found to parasitize these armyworms including Archytas spp., Choeteprosopa hedemanni Brauer and Bergenstamm, Euphorocera omissa (Reinhard), E. tachinomoides Townsend, Lespesia aletiae (Riley), L. archippivora (Riley), Omotoma fumifer-anae (Tothill), Winthemia quadripustulata (Fabricius), and W. rufopicta (Bigot) (all Diptera: Tachinidae). A nuclear polyhedrosis virus is highly pathogenic to larvae, and survivors that do not succumb exhibit reduced fecundity (Hostetter et al., 1990; Young, 1990). Undoubtedly, predators are important, but unlike the situation with western yellowstriped armyworm, their effect has not been quantified.

Not surprisingly, western yellowstriped armyworm also is host to many parasitic insects. Among wasps associated with this caterpillar are Chelonus insularis

Cresson, Cotesia marginiventris (Cresson) (both Hyme-noptera: Braconidae), Hyposoter exiguae (Viereck), and Pristomeres spinator (Fabricius) (both Hymenoptera Ich-neumonidae). Flies parasitizing western yellowstriped armyworm include Archytas californiae (Walker), Euce-latoria armigera (Coquillett), Euphorocera claripennis (Macquart), and Lespesia archippovora (Riley) (all Diptera: Tachinidae). Despite the numerous parasitoids, the first three species listed above, all wasps, account for 95% of recorded parasitism in alfalfa. The relative importance of the individual parasitoid species varies geographically (Miller, 1977). Of great interest, however, is the impact of predation. Bisabri-Ershadi and Ehler (1981) reported that over 96% of total mortality occurred in the egg and early larval stages, and most was attributed to predation. The most important predators were minute pirate bug, Orius tristicolor (White) (Hemiptera: Anthocoridae); big-eyed bugs, Geocoris spp. (Hemiptera: Lygaeidae); and damsel bugs, Nabis spp. (Hemiptera: Nabidae). The plant bug Lygus hesperus Knight (Hemiptera: Miridae) was a facultative predator, often feeding on armyworm eggs. A nuclear polyhedrosis virus is found in western yellowstriped armyworm, and when larvae occur at high densities it may be a significant mortality factor.

Life Cycle and Description. There apparently are 3-4 generations annually, with broods of adults present in March-May, May-June, July-August, and August-November. Some of the latter brood of yel-lowstriped armyworm and all members of the latter brood of western yellowstriped armyworm overwinter as pupae rather than emerging as adults. Although eggs, larvae and adults of yellowstriped armyworm may be present in autumn or early winter they cannot withstand cold weather, and perish. Development time, from egg to adult, is about 40 days.

  1. The eggs are greenish to pinkish brown and bear 45-58 small ridges. In shape, the egg is a slightly flattened sphere, measuring 0.46-0.52 mm in diameter and 0.38-0.40 mm in height. Females typically deposit clusters of 200 -500 eggs, usually on the underside of leaves. Total fecundity was determined to be over 3000 eggs under laboratory conditions. They are covered with scales from the body of the adults. Duration of the egg stage is 3-5 days at warm temperature.
  2. The larvae initially are gregarious in behavior, but as they mature they disperse, sometimes spinning strands of silk upon which they are blown by the wind. There usually are six instars, though seven instars have been reported. Head capsule widths are about 0.28, 0.45, 0.8-1.0, 1.4-1.6, 2.0-2.2, and 2.83.0 mm, respectively, for instars 1-6. The larva grows from about 2.0 to 35 mm long over the course of its development. In California, mean development times for instars 1-7 of S. praefica were 2.5,2.8,3.0,3.4,4.5,5.2,6.9 days, respectively, during the second generation. There was a total larval development time of 26.8 days during the second generation, but it was shortened to 21.3 days during the third. Coloration is variable, but mature larvae tend to bear a broad brownish band dor-sally, with a faint white line at the center. More pronounced are black triangular markings along each side, with a distinct yellow or white line below. A dark line runs laterally through the area of the spiracles, and below this is a pink or orange band. Dark subdorsal spots are found on the mesothorax of yellowstriped armyworm, and the triangular shape of these spots aids in distinguishing this insect from sweetpotato armyworm, Spodoptera dolichos, and velvet armyworm, S. latifascia, in eastern states. The head is brown but has extensive blackish markings. In western yellowstriped armyworm the blackish markings on the head form an arc on each side that extend from the mouth to the back of the head; in yellowstriped armyworm the band is less definite dorsally. Duration of the larval stage is 14-20 days, with the first three instars requiring about two days each and the last three instars requiring about three days each. (See color figure 65.)
  3. Larvae pupate in the soil within a cell containing a thin lining of silk. The reddish brown pupa measures about 18 mm long. Duration of the pupal stage is 9-22 days, normally averaging 12-18 days.
  4. The moths measure 34-41 mm in wing-span. The front wings are brownish-gray with a complicated pattern of light and dark markings. Irregular whitish bands normally occur diagonally near the center of the wings, with additional white coloration distally near the margin. The hind wings of yellowstriped armyworm are opalescent white, with a narrow brown

Yellowstriped armyworm larva.

margin. In western yellowstriped armyworm the hind wings are similar but tend to be tinted with gray, and the underside bears a dark spot centrally. In S. praefica, both sexes are similar in appearance, whereas in S. ornithogalli the sexes are dimorphic. Under laboratory conditions average longevity of adults is 17 days, with most egg production completed by the tenth day (Adler et al, 1991). (See color figure 252.)

The most complete description of S. ornithogalli and its biology was given by Crumb (1929), with additional comments by Crumb (1956). Blanchard (1932) decribes S. praefica. van den Bosch and Smith (1955) and Oku-mura (1962) described both species, and their biology in California. Keys for identification are also found in these references. Keys for separation of Spodoptera adults can be found in Todd and Poole (1980) and Heppner (1998). Larvae can be distinguished using the keys of Passoa (1991) and Heppner (1998). Rearing on artificial diet was described by Adler and Adler (1988).

Damage

Larvae damage plants principally by consumption of foliage. The small, gregarious larvae tend to skeletonize foliage but as the larvae grow and disperse they consume irregular patches of foliage or entire leaves. However, they also feed on the fruits of tomato, cotton, and other plants. Larval consumption of soybean was estimated by King (1981) to total 115 sq cm; this is an intermediate value relative to some other lepidopter-ous defoliators.

Management

Insecticides. Insecticides are applied to foliage to prevent injury by larvae. The microbial insecticide Bacillus thuringiensis can be applied to kill armyworms, but should be applied when the larvae are young, as they become difficult to control as they mature. Larvae consume bran bait containing insecticide.

Cultural Methods Proximity of crops to range-land-containing weed hosts, or to alfalfa, may be important factors predisposing vegetable crops to injury.

Adult yellowstriped armyworm.

At high densities, especially if alfalfa hay is mowed, larvae sometimes disperse simultaneously and invade nearby vegetable fields. Physical barriers such as trenches can be used to deter such dispersal.

Was this article helpful?

+1 0

Post a comment