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Variegated cutworm

Peridroma saucia (Hiibner) (Lepidoptera: Noctuidae)

Natural History

Distribution. Variegated cutworm is found in many areas of the world. It occurs throughout the western hemisphere, and in Hawaii, and also portions of Europe, Asia, and North Africa. The origin of this insect is uncertain, but is thought to be Europe, where it was described in 1790. First observed in North America in 1841, it is now abundant in southern Canada and the northern United States, where it is often considered to be the most damaging cutworm pest of vegetables.

Host Plants. This cutworm has an extremely wide host range. Unlike some cutworms that expand their dietary range only when confronted with overpopulation and starvation, variegated cutworm feeds readily on numerous plants. Among vegetables attacked are asparagus, bean, beet, Brussels sprouts, cabbage, cantaloupe, carrot, cauliflower, celery, Chinese cabbage, col-lards, corn, cowpea, cucumber, garbanzo, globe artichoke, kale, lettuce, lima bean, mustard, onion, pea, pepper, potato, sweet potato, Swiss chard, radish, rhubarb, rutabaga, spinach, squash, tomato, turnip, and watermelon (Rings et ah, 1976b). Based on frequency of reports, variegated cutworm is most likely to be found damaging beet, cabbage, lettuce, potato, and tomato. Variegated cutworm also is known to damage fruit trees, including apple, apricot, avocado, cherry, currant, gooseberry, grape, lemon, mulberry, orange, plum, raspberry, and strawberry. Other crops injured include alfalfa, barley, clover, corn, cotton, flax, hops, mint, sunflower, sugarbeet, sweet clover, tobacco, wheat, and many flower crops. Weeds are occasionally consumed, but seem not to be preferred. Some of the weeds eaten are jimsonweed, Datura sp.; dock, Rumex sp.; dogfennel, Eupatorium capillifolium; plantain, Plantago sp.; ragweed, Ambrosia sp.; and shepherdspurse, Capsella bursa-pastoris.

Natural Enemies. Numerous natural enemies are known for variegated cutworm. Walkden (1950) reported mortality of variegated cutworm over a 20-year period in the central Great Plains, and frequently observed 20-75% mortality, with wasp and fly parasitoids accounting for most of the deaths among larvae. In a study of cutworms in Oklahoma, Soteres et al. (1984) reported six species of Braconidae, three species of Ichneumonidae, and one species of Eulophidae, but among the Hymenoptera only Ophion sp. (Ichneumonidae) accounted for more than 5% mortality. In the same study, 12 species of Tachinidae were observed, but among the Diptera only Archytas apicifer (Walker) and Peleteria texensis Curran (both Tachinidae) accounted for more that 5% mortality. In Oregon, Coop and Berry (1986) reported eight species of Hymenop-tera from variegated cutworm larvae. The parasitoid, Meteorus communis (Cresson) (Braconidae), was recovered from about 35% of the intermediate age larvae. Not only did the parasitism affect abundance of cutworm in subsequent generations, but caused a 93% reduction in foliage consumption by parasitized larvae. A study of variegated cutworm in Hawaii demonstrated 33-80% parasitism, with most of the parasitism due to Hyposoter exiguae (Viereck) (Ichneumonidae), Cotesia marginiventris (Cresson) and Meteorus laphyg-mae Viereck (both Braconidae) (all Hymenoptera) (Hara and Matayoshi, 1990).

Other Hymenoptera known to parasitize variegated cutworm include Apanteles xylinus (Say), Chelonus insu-laris Cresson, C. militaris (Walsh), Meteorus autographae Muesebeck, M. leviventris (Wesmael), Microplitis feltiae Muesebeck, Rogas perplexus Gahan, and R. rufocoxalis Gahan (all Braconidae); Campoletis sonorensis (Cameron), Enicospilus merdarius (Gravenhorst), Nepiera fuscifemora Graf, and Ophion flavidus Brulle (all Ichneumonidae); and Dibrachys canus (Walker) (Pteromali-dae). Other Diptera parasitizing this cutworm are Archytas aterrimus (Robineau-Desvoidy), A. cirphis Curran, Bonnetia comta (Fallen), Carcelia spp., Chaeto-gaedia monticola (Bigot), Clausicela opaca (Coquillett), Eucelatoria armigera (Coquillett), Euphorocera claripennis (Macquart), E. omissa (Reinhard), Gonia longipulvilli Tothill, G. porca Williston, G. sequax Williston, Lespesia archippivora (Riley), Madremyia saundersii (Williston), Peleteria texensis (Curran), Periscepsia helymus (Walker), P. laevigata (Wulp), Voria ruralis (Fallen), Winthemia leucanae (Kirkpatrick), W. quadripustulata (Fabricius), and W. rufopicta (Bigot) (all Tachinidae).

The fungus Metarhizium anisopliae and viruses can inflict mortality, but incidence is often low. Both a granulosis (Steinhaus and Dineen, 1960) and a nuclear polyhedrosis virus (Harper, 1971) are known to affect this cutworm.

Life Cycle and Description. There are 2-4 generations annually, with two generations common in northern states and southern Canada, and 3-4 in southern areas. Flights of moths are protracted, and generations are difficult to discern based on adult populations. Overwintering may occur in the pupal stage, or perhaps the larval stage, but there also is evidence that moths migrate into northern areas from southern latitudes each spring, and return to the south in the autumn. In Iowa, 3-9% of moths collected during the spring flight were considered to have emerged from overwintering pupae, with the balance migrating from southern latitudes (Buntin et ah, 1990). The April-May spring flight in Iowa tends to be followed by generations and adult flights in June-July, August-September, and October-November. Total duration of the life stages is usually 35-70 days. Overwintering in Canada is unlikely (Ayre, 1985) except in relatively warm areas such as coastal British Columbia.

Egg. The egg of variegated cutworm is hemispherical; the egg is flattened at the point of attachment to a leaf or plant stem. The surface of the egg is marked with ridges, about 42 in number, radiating from the center. The egg measures about 0.55-0.58 mm in diameter and 0.40-0.45 mm in height. Initially, they are white, but soon turn brownish. The developmental threshold for the egg stage is estimated at 3.0-6.0?C by various authors. Duration of the egg stage is 4-6 days in warm weather (20-30?C), but 10 days when held at 15?C. Eggs are deposited in clusters, often numbering several hundred per egg mass. Females may deposit 1200-1400 eggs during their lifespan.

Larva. There normally are six instars. The developmental threshold for the larval stage was estimated at 2.6-6.7?C by various authors. Mean duration of the instars is reported to be 6.5, 4.6, 4.8, 4.7, 6.7, and 16.8 days for instars 1-6, respectively, at 15?C. When reared at 25?C, instar durations are reduced to 3.1,1.9,2.2,2.2, 2.9, and 8.4 days, respectively (Shields, 1983). Head capsule widths are 0.30-0.35, 0.46-0.62, 0.80-1.00, 1.20-1.65,1.9-2.6, and about 3.0-3.2 mm, respectively. Body lengths are estimated at 2.0-3.0, 3.6-6.5, 5.3-9.0, 12-16, 25-28, and 35-46 mm for instars 1-6, respectively. Body color is brownish gray to grayish black. The most distinctive character is a dorsal yellow or whitish spot, which is present on each of the first four abdominal segments, often on the first six segments, though this character may be absent in early instars. Less distinctive is the black "W"-shaped mark on the eighth abdominal segment of the last instar, and light brown or tan on the posterior end of the body. An inconspicuous black line is often present laterally above the spiracles. An orangish-brown line may connect the spiracles, and below the spiracles there usually is some irregular yellowish or orangish coloration. The head is orangish-brown and marked with darker spots. (See color figure 63.)

 

Pupa. The larva forms a cell in the soil and pupates near the soil surface. The pupa is mahogany brown, and measures 15-23 mm long and 5-6 mm wide. The developmental threshold of the pupal stage was estimated at 4.3-8.5?C by various authors. Duration of the pupal stage is about 33 and 13 days at 15? and 25?C, respectively.

Adult. The adult is fairly large in size, measuring 43-50 mm wingspan. The front wings normally are grayish brown, tinged with reddish and shaded centrally and distally with darker brown. The background color varies, however, from dark brown to yellowish-brown. A bean-shaped spot and a smaller round spot are usually evident centrally. The hind wings are iridescent or pearly white with brown veins and brown shading marginally. The head and thorax are dark brown, whereas the abdomen is lighter brown. Females produce a sex pheromone to attract males (Struble et al., 1976). Oviposition commences 7-14 days after emergence of the adults. (See color figure 250.)

Description and biology of variegated cutworm was given by Chittenden (1901), Crumb (1929), and Walk-den (1950). Developmental biology was given by Sim-onet et al. (1981) and Shields (1983). Rearing methods were described by Finney (1964) and Harper (1970). A bibliography was published by Rings et al. (1976a). The larva was included in keys by Whelan (1935), Crumb (1956), Okumura (1962), Rings (1977b), Oliver and Chapin (1981), and Capinera (1986). It is also included in a key to armyworms and cutworms in Appendix A. The moth was included in pictorial keys by Rings (1977a) and Capinera and Schaefer (1983).

Damage

The larvae cause considerable mortality to seedlings by cutting off the plant at the soil surface. Larvae also are defoliators, and though they commonly frequent low-growing herbage, they readily climb trees to feed on buds and foliage. Young larvae may remain on the foliage during the daylight hours, but feed principally during the evening hours. Large larvae often hide in the soil or other sheltered locations during the daytime hours, moving to exposed areas of foliage in the evening to feed. Larvae may burrow into tomato fruit and the heads of cabbage and cauliflower. At high densities larvae may assume a gregarious, dispersive "armyworm" habit, but this is uncommon. It also invades greenhouses frequently.

 

 

The larvae consume about 125 sq cm of sugarbeet or 160 sq cm of potato foliage during their larval development (Capinera, 1978c; Shields et al., 1985). Potato and many other plants can tolerate some defoliation without significant yield decrease. Late in the season, after tuber formation is nearly complete, potato can withstand up to 75% leaf loss without yield decrease. At this time up to 40 variegated cutworms per plant can be tolerated. However, at an earlier period in the season such as at full bloom, plants may be able to tolerate only three cutworms per plant (Shields et al., 1985).

Management

Sampling. The adult populations can be monitored with blacklight and pheromone traps. Captures by the two types of traps are correlated, but pheromone traps capture larger numbers from the spring generation, whereas blacklight traps capture larger numbers at other times of the year (Willson et al., 1981). Pheromone traps, though not completely species-specific (Ayre et al., 1983), provide considerable selectivity relative to blacklight traps, thereby reducing labor requirements associated with population monitoring.

Larval populations are difficult to sample. Young larvae may be found clustered on foliage, but older larvae tend to hide in sheltered locations or burrow beneath the surface of the soil during the daylight hours. If plants are severed at the soil surface, or have disappeared, it is important to rake the soil surface and search for cutworm larvae.

Insecticides. Insecticides are commonly recommended to protect young plants from cutting damage, and older plants from defoliation and fruit injury. Insecticide applications are directed at the foliage or soil, the latter because the larvae often seek shelter there. Insecticides differ greatly in their effectiveness, and larger larvae are considerably more difficult to kill (Harris et al., 1977). Insecticide-treated bran baits are effective against variegated cutworm. Bacillus thurin-giensis is not recommended. Variegated cutworm larvae are sensitive to neem products, which act as feeding deterrents and disrupt larval growth and survival (Koul and Isman, 1991; Isman, 1993).

Cultural Practices. Cutworm problems often develop in weedy fields or portions of fields infested with weeds. It is advisable to till, or otherwise destroy weeds, 10-14 days in advance of planting, as this should cause small larvae to starve. If seedlings are to be transplanted into a field or garden, larger plants are preferred, because they are less likely to be irreparably damaged by cutworms. Transplanted plants derive considerable protection if surrounded by a barrier such as a can or waxed paper container with the bottom removed. Aluminum foil wrapped around the base of the seedling also deters cutting by larvae. Biological Control. Variegated cutworm is susceptible to infection by entomopathogenic nematodes (Nematoda: Steinernematidae and Heterorhabditidae) (Morris and Converse, 1991), but demonstration of practical use under field conditions is lacking.