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.
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.
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).
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.
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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