Hypena scabra Fabricius Lepidoptera Noctuidae

Natural History

Distribution. This native species is found widely in eastern North America. It is recorded from all states east of the Great Plains, and there are occasional records from the Rocky Mountain region. As a pest, however, it is best known from the soybean-growing areas of the midwest and southeast where it has an abundance of suitable host material. In Canada, green cloverworm is known from southern Ontario, but it rarely causes serious damage.

Host Plants. Larvae of green cloverworm develop successfully only on plants in the family Leguminosae. They have been observed to feed on weeds and crops from other plant families, but this occurs only after legumes have been consumed. Vegetable crops eaten include bean, cowpea, faba bean, lima bean, and pea. Field crops suitable for development include alfalfa, alsike clover, crimson clover, red clover, white clover, lespedeza, birdsfoot trefoil, velvet bean, and soybean. Pedigo et al. (1973) indicated that the most common food plants are soybean, alfalfa, clovers, field bean, lima bean, and pea, in that order. Adults feed on the nectar from blossoms.

Because of the preference for soybean, most of this insect"s biology and management recommendations have been derived from soybean-based research, but in large measure the findings should be applicable to related crops.

Natural Enemies. Many natural enemies are known, with their significance varying according to cloverworm population density. In Iowa, research has shown that during low (endemic) densities parasi-toids, and to a lesser extent predators, are relatively important. During outbreak (epidemic) densities, resulting from invasion by many migrating moths early in the year, the entomopathogenic fungus Nomuraea rileyi becomes a key factor. The effectiveness of the fungus is principally dependent on presence of high densities of larvae in the second generation. The fungal disease, but not the other mortality factors, is capable of controlling the cloverworm population (Pedigo et al., 1983; Thorvilson and Pedigo, 1984).

Among the parasitoids commonly attacking green cloverworm are several wasps (Hymenoptera: mostly Braconidae and Ichneumonidae) and flies (Diptera: Tachinidae) (Lentz and Pedigo, 1975; Mueller and Kunnalaca, 1979; Bechinski and Pedigo, 1983; Bechi-nski et al., 1983a; Daigle et al., 1988). The most abundant larval parasitoids are Cotesia marginiventris (Cresson) and Rogas nolophanae Ashmead (both Hyme noptera: Braconidae). Egg parasitism is infrequent, but predation of eggs and young larvae by Nabis americo-ferus Carayon and N. roseipennis Reuter (both Hemi-ptera: Nabidae) is documented (Sloderbeck and Yeargan, 1983b). Predation assumes greater importance in the pupal stage, when such predators as ground beetles (Coleoptera: Carabidae), field crickets (Orthoptera: Gryllidae), and rodents inflict heavy mortality. In addition to the aforementioned entomopatho-genic fungus, a granulosis virus sometimes occurs (Carner and Barnett, 1975; Daigle et al., 1988)

Life Cycle and Description. There normally are three generations annually in Iowa, with four flights of moths present in May, June-July, August, and September. The fourth flight may not be evident in some years. Green cloverworm fails to overwinter successfully in cold climates such as Iowa, and reinvades the northern states each spring. The green cloverworm is reported to overwinter in the pupal and adult stages in the south, and as far north as southern Ohio. The overwintering of this species has not been intensively studied in southern states, but it remains reproduc-tively active throughout the year along the Gulf Coast. It is thought to overwinter in the south as far north as southern Virginia, Kentucky, southern Missouri, and most of Texas. In the spring, when sustained winds blow from the south-central states northward, green cloverworm moths are carried into northern areas (Wolf et al., 1987). The length of the life cycle is about 40 days during the summer months.

  1. Females normally deposit 200-230 eggs, but up to 670 eggs have been recorded from a single female. They are deposited singly. The egg initially is greenish, becomes speckled with orange or red, and then purplish gray just before hatching. The egg is a slightly flattened sphere in shape; the base, in particular, is flattened. The egg measures about 0.51 mm in diameter and 0.35 mm in height, and bears 14-19 readily discernible ridges. Hatching occurs 2-5 days after oviposition.
  2. There are 6-7 instars, the larvae growing from about 1.5 mm to over 30 mm long as they mature. The larvae are green throughout their development. Larvae bear a pair of longitudinal white stripes along each side, with a less distinct along the back, but they are fairly indistinct until about the third instar. During the terminal instar the white stripes fade, the insect appearing almost entirely green. One of the most distinctive features is the presence of only four pairs of prolegs. The larva walks with a looping motion. Mean head capsule widths (range) for the larvae are 0.23 (0.13-0.28), 0.35 (0.32-0.43), 0.57 (0.48-0.70), 0.89

Green cloverworm larva.

Green cloverworm larva.

(0.66-1.00), 1.24 (0.70-1.50), 1.69 (1.35-2.00), and 1.88 (1.73-2.30) mm for instars 1-7, respectively. Duration of the instars was estimated by Stone and Pedigo (1972) to be about 3.1,1.4,1.9,2.1,2.5,3.7, and 5.5 days, respectively, for a total larval duration of about 19 days when reared on soybean. Hill (1925) reported an average larval development period of 22.8 days when fed alfalfa. Larvae are solitary in their feeding behavior.

  1. As the larvae are near completion of their development they spin a loose web in preparation for pupation. Pupation may occur in the plant canopy, whereby larvae usually web together a leaflet and pupate within a leaf fold. Most larvae, however, drop to the soil and pupate at the surface or just beneath the surface. The pupa is brown to brownish black, and measures 11-15 mm long. Duration of the pupal stage is 9-12 days.
  2. The adults are mottled grayish-brown with black and silver markings. The male and female differ somewhat in appearance, however. The male has less distinctive markings, bearing about three irregular, transverse black lines across the forewing. The female also has transverse black markings but with greater contrast, and silver and reddish brown areas distally. The hind wings are blackish brown. Wingspan measures 27-34 mm. The mouthparts of both sexes protrude, forming a distinctive snout. Moths hide in vegetation during the daylight hours. They become active at dusk, and reportedly feign death if disturbed, suddenly dropping to the soil with their wings folded. Once they take flight they are strong fliers, and their flight may continue until the early morning hours. Oviposition commences about 4-5 days after adult emergence, and continues for 10 days or longer. Ovi-position may occur on both the upper and lower leaf surfaces. Apparently moths prefer to oviposit on leaf surfaces that contain leaf hairs, and deposit eggs preferentially on the lower leaf surfaces of alfalfa and clover because of the greater pubescence. On soybean, which is hairy on both surfaces, the females do not discriminate between locations. Moths preferentially deposit eggs on pubescent varieties of soybean relative to glabrous varieties (Pedigo, 1971). (See color figure 244.)
Adult green cloverworm.

A good summary of green cloverworm biology was given by Pedigo et al. (1973), but a more detailed description, particularly of insect morphology, was found in Hill (1925). The larva was included in keys by Crumb (1956), Oliver and Chapin (1982), Capinera (1986), and in a key to loopers in Appendix A. The adult occurred in a key by Capinera and Schaefer (1983).

Damage

Larvae feed principally on the leaf tissue between the main veins of leaves. Most authors indicate that pods, blossoms, or stem tissue are rarely consumed. Larvae usually feed from the lower leaf surface, and instars 1-2 or 1-3 do not eat completely through the leaf tissue, but leave the upper epidermis intact. Each larva eventually consumes over 100 sq cm of bean leaf tissue, with about 90% occurring in the last two instars. As the beans are very tolerant of defoliation, withstanding about 30% leaf tissue loss before yields are depressed, at least 5-6 mature larvae likely are necessary to inflict damage.

In Delaware, green cloverworm larvae were frequently observed to feed on small pods of lima bean (Burbutis and Kelsey, 1970). However, the beans were very tolerant of injury, and larval densities of up to 8 per plant did not suppress yield.

Management

  1. The adult populations can be monitored with blacklight traps, though more males than females are captured. Eggs are deposited principally on the upper surface of leaves. Egg dispersion is random. A sequential sampling plan for eggs was developed by Buntin and Pedigo (1981). Sweep nets are usually used to sample larvae. Larval dispersion is aggregated, and a sequential sampling protocol was presented by Bechinski et al. (1983b).
  2. Green cloverworm rarely attains pest status in vegetable crops, but can be controlled easily with foliar insecticides. Also, insecticide-containing baits are effective (Morgan and Todd, 1975). Bacillus thuringiensis products are not usually recommended.

Cultural Practices. Cloverworm is most abundant late in the season, in late-maturing cultivars, and in narrow-row plantings (Buschman et al., 1981). Planting date apparently has little influence on damage (McPherson et al., 1988). Tillage practices similarly have few consistent effects on green cloverworm populations (Sloderbeck and Yeargan, 1983a; Thorvil-son et al., 1985a).

Green cloverworm oviposits readily in alfalfa, and the first generation often occurs in this crop or clover before soybean or bean are available. Alfalfa is harvested frequently, however, and harvesting results in mortality of most larvae. Thus, alfalfa acts as a "sink" for the cloverworm population, causing a decline in abundance. It is the presence of soybean that generally leads to the great abundance of green cloverworm late in the season (Buntin and Pedigo 1983). Alfalfa also acts as an early season source for parasitoids and disease (Thorvilson et al., 1985b).

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