Pseudoplusia includens Walker Lepidoptera Noctuidae

Natural History

Distribution. Soybean looper is native to the western hemisphere, where it is found from the United States to Argentina. It is a tropical or subtropical insect, and overwinters successfully only in warm climates. In the United States, it survives the winter in southern Florida and southern Texas. Not surprisingly, damaging populations are largely restricted to the southeastern and south-central states. The moths

are highly dispersive, however, and this insect has been captured throughout the United States, except for the Pacific Northwest. It is rare in Canada, but has been recovered from southern Ontario and Quebec, and from Nova Scotia.

Host Plants. This insect is known principally as a defoliator of soybean, but it reportedly has a fairly wide host range. Vegetables reported to be consumed are asparagus, bean, broccoli, celery, collards, corn, cowpea, garlic, lettuce, mustard, okra, pea, pepper, potato, sweet potato, tomato, watercress, and watermelon. Other host plants include field crops such as alfalfa, cotton, peanut, sunflower, and tobacco, and flower crops such as aster, begonia, calendula, carnation, chrysanthemum, geranium, and poinsettia. Weeds also are suitable hosts, including such common species as pigweed, Amaranthus sp.; cocklebur, Xanthium pennsylvanicum; horseweed, Egeron canadensis; wild sunflower, Helianthus spp.; pepperweed, Lepidium virginicum; and dock, Rumex spp. Herzog (1980) provided a long list of host plants.

Soybean looper has long been confused with cabbage looper, Trichoplusia ni; the larvae are difficult to separate accurately. Some of the aforementioned records may reflect such misidentifications, though this insect certainly does occur on vegetables, particularly tomato. It is invariably the dominant looper on soybean in the southeast, but cabbage looper is the dominant looper species on vegetables. Martin et al. (1976a) presented interesting data from Florida, collected both from field cages and crop fields, demonstrating the preference of soybean looper for field crops.

Natural Enemies. Soybean looper is host to numerous parasitoids, pathogens, and predators. In Georgia, it is common for 40% of larvae to succumb to a parasitoid or disease (Beach and Todd, 1985). In Louisiana, such natural mortality seems to be even higher, but the effectiveness of these biotic control agents varies considerably with time, location, and cropping system (Burleigh, 1972). Nearly all research has emphasized soybean, so results of some studies may not be applicable to vegetables. It is of interest to note that Martin et al. (1981b) reported higher levels of parasitism on soybean looper in vegetables than in soybean during studies conducted in northern Florida.

The most important parasitoids tend to be Copido-soma truncatellum (Dalman) (Hymenoptera: Encyrti-dae), Meteorus autographae Muesebeck, Apanteles scitulus Riley, and Cotesia marginiventris (Cresson) (all Hymenoptera: Braconidae). Several other wasp and fly (Diptera: Tachinidae) parasitoids exert only low levels of mortality (Burleigh, 1971 and 1972; Beach and Todd, 1985; Daigle et al., 1990).

Fungi are important mortality factors for soybean looper. Entomophthora gammae, Massospom sp., and Nomuraea rileyi are especially common. A nuclear polyhedrosis virus also is known. Often the pathogens are not effective until late in the season, or until looper densities are high (Burleigh, 1972; Harper and Carner, 1973).

Many general predators such as the lady beetle Coleomegilla maculata (Coleoptera: Coccinellidae); big-eyed bug, Geocoris spp. (Hemiptera: Lygaeidae); the damsel bug Nabis roseipennis Reuter (Hemiptera: Nabi-dae); and the ground beetle Calosoma spp. (Coleoptera: Carabidae) feed on soybean looper. All stages of development are subject to predation (Richman et al., 1980; Brown and Goyer, 1984).

Life Cycle and Description. Total generation time for soybean looper is estimated at about 27-34 days during the summer months. In most of Florida, moths remain active throughout the year, but in north the period of adult activity is restricted (Mitchell et al., 1975). Moths disperse northward annually, attaining Georgia in June and July, and South and North Carolina in August and September. There likely are 3-5 generations annually in the southern areas of soybean looper's range, and two per year in most northern latitudes such as North Carolina.

  1. The eggs are slightly flattened spheres, laid singly with a flattened side affixed to the foliage. They measure about 0.6 mm in diameter and 0.4 mm in height, and their close examination reveals numerous ridges radiating from the top of the egg. There are about 34 ridges per egg, but unlike many other noctuid eggs, the ridges are not sharply defined and easy to count. Initially, the egg is white, but turns light brown with maturity. Duration of the egg stage averages 4.1 days at 27°C. Females deposit about 300-600 eggs. Egg deposition is affected by adult food sources, cotton nectar being especially favorable (Jensen et al., 1974). Maximum egg production occurs at about 29° C (Mason and Mack, 1984).
  2. The larvae are green, and eventually attain a length of about 30 mm. The body is marked with thin white stripes dorsally and along the sides. In most respects soybean looper closely resembles cabbage looper, Trichoplusia ni, including the presence of only three pairs of prolegs and the presence of nipple-like vestigial prolegs on abdominal segments three and four. Soybean looper differs in having minute micro-spines, but this character is of marginal diagnostic value because the microspines are difficult to detect. The thoracic legs of soybean looper are often dark, and sometimes can be used to differentiate a specimen from cabbage looper. A reliable character for differentiation of these species is structure of the mandible (see Eichlin and Cunningham (1978) for an illustration of this character). The head is grayish-green, and may be marked with dark bands laterally. Generally there are six larval instars. According to Mitchell (1967), mean duration (range) of the instars is 3.7 (3-5), 2.1 (2-3), 2.2 (1-5), 2.6 (1-5), 2.9 (1-5), and 6.2 (4-11) days, respectively, for instars 1-6 when reared at 27°C. Head capsule widths for the six instars are about 0.25, 0.39, 0.60, 0.96, 1.45, and 2.18 mm, respectively. However, Shour and Sparks (1981) noted that the total number of instars could vary, and that total larval duration was 15.4, 16.8, or 20.7 days depending on whether there were 5, 6, or 7 instars, respectively. Strand

(1990) described the basis for instar variation in soybean looper. (See color figure 57.)

  1. Larvae spin a loose silk cocoon on the foliage and pupate within. Duration of the pupal stage is about seven days (range 3-11 days).
  2. The moth's front wings are marked with various shades of brown and gray, but the overall effect is dark. The silvery white marking at the center of the forewing, which is sometimes said to resemble a "dog leg,'' usually has a detached "foot." The hind wings are light brown basally and dark brown distally. The moth is readily confused with plantain looper, Autographa precationis, but the forewing of plantain looper is rusty red, whereas that of soybean looper has a brassy reflection. Wingspan of soybean looper moths is 30-39 mm. Moths begin oviposition 3-4 days after emergence, and adult longevity is typically 5-10 days. Moth activity is highest between 10 p.m. and 2 a.m. (Mitchell 1973).

A review of soybean looper biology was given by Herzog (1980). Rearing procedures were given by Hartley (1990). A multi-component sex pheromone was described by Linn et al. (1987). Keys to adults and larvae can be found in LaFontaine and Poole

(1991). Larvae also were included in the key of Crumb (1956), and adults in Eichlin and Cunningham (1978). Soybean looper is included in the key to vegetable-feeding loopers found in Appendix A.

Pseudoplusia Includens
Soybean looper larva.

Damage

Larvae feed principally on leaf tissue. Foliage consumption of soybean, which probably is similar for green bean, is 0.2, 0.8, 1.8, 7.9, 15.5, and 55.7 sq cm for the six instars, respectively (Reid and Greene, 1973). Larvae also sometimes feed on the pods of legumes, and the silk and kernels of corn (Janes and Greene, 1970).

Management

  1. Soybean looper moths can be captured with light and pheromone traps (Tumlinson et al., 1972), or combination of light and pheromone traps (Mitchell et al., 1975). Larval sampling has been studied extensively in soybean, but methodology for vegetables has not been developed. Herzog (1980) reviewed soybean looper sampling.
  2. Damage is usually prevented by application of insecticide to foliage. However, insecticide resistance has been reported to be a serious problem (Leonard et al., 1990). Population increases of soybean looper have been reported following foliar application of certain insecticides; it seems likely that destruction of beneficial insects by insecticides allows the loopers to attain elevated levels of abundance (She-pard et al., 1977). Soil-applied systemic insecticides have little effect on parasitism (McCutcheon et al., 1990).

Biological Control. Natural enemies are often adequate to maintain soybean looper at non-damaging densities on vegetable crops. If this level of control is not adequate, Bacillus thuringiensis can be applied. A nuclear polyhedrosis virus has been demonstrated to be effective under field conditions, but it is not available commercially (McLeod et al., 1982).

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