Discestra trifolii Hufnagel Lepidoptera Noctuidae

Natural History

Distribution. Clover cutworm occurs throughout the United States except for the southeastern states. It also occurs throughout southern Canada and in Alaska. Apparently it is a native species, though it is also reported to occur in Europe and Asia.

Host Plants. This species is an occasional pest of several crops, preferring crops and weeds in the plant family Chenopodiaceae. Among vegetables attacked are beet, cabbage, lettuce, onion, pea, spinach, tomato, and turnip. Field crops consumed include alfalfa, cotton clover, flax, canola (rape), sugarbeet, and sunflower. The incidence of clover cutworm on sugarbeet explains the alternative common name, "striped beet caterpillar." However, weeds are the usual host. Weeds known to be suitable larval food plants include kochia, Kochia scoparia; lambsquarters, Chenopodium album; purslane, Portulaca sp.; and Russian thistle, Salsola kali.

Natural Enemies. Walkden (1950) reported that of field collected larvae, 12% died from parasitic Hyme-noptera, 16% from parasitic Diptera, and 22% from pathogens. Santiago-Alvarez and Federici (1978) reported Euplectrus sp. (Hymenoptera: Eulophidae) and Euphorocera tachinomoides Townsend (Diptera: Tachini-dae) attacking larvae in southern California. Other wasps parasitizing clover cutworm are Apanteles plathypenae Muesebeck, Meteorus leviventris (Wesmael) (both Hymenoptera: Braconidae), and Enicospilus mer-darius (Gravenhorst) (Hymenoptera: Ichneumonidae). Other parasitic flies include Euphorocera claripennis (Macquart) and Lespesia archippivora (Riley) (both Diptera: Tachinidae) (Arnaud, 1978). Both granulosis and a nuclear polyhedrosis viruses are known from clover cutworm larvae, and Federici (1978) reported the gran-ulosis virus to be especially important in regulating insect density. Federici (1982) also described an unusual rickettsia-like organism in clover cutworm larvae, but there is no indication that this pathogen occurs frequently.

Life Cycle and Description. There are three generations annually in Colorado and Kansas. Moth flights occur in late May, early July, and late August-early September. Pupae from the third generation overwinter. Knutson (1944) and Ayre et al. (1982a) suggested only two flights of adults in Minnesota and Manitoba, respectively, though the "second" flight period was protracted and may represent two overlapping periods of adult activity. Ayre et al. (1982a) suggested that the level of diapause induction in pupae was a critical determinant in overwintering survival in Manitoba; they speculated that if the last generation develops early in the season, diapause is not induced and the insects proceed with a generation that is not completed before the onset of winter.

  1. The eggs are deposited singly or in small clusters on the underside of leaves. They are white to pale yellow. They resemble a slightly flattened sphere and are equipped with ribs that radiate out from the top of the egg. The number of eggs produced by females is not well documented but seems to be in excess of 500 eggs, and is likely much greater. When reared on artificial diet, they generally produced 650-1700 egg per female (Santiago-Alvarez et al., 1979). Duration of the egg stage is 4-5 days.
  2. The larvae are dull green. There may be a weak white line dorsally along the length of the body. The most distinctive character, however, is the combination of a broad lateral yellowish or pinkish band below the spiracles, and black pigmentation surrounding the spiracles. The black pigmentation forms a

Clover cutworm larva.

Clover cutworm larva.

series of black spots immediately above the lateral band, and serves to distinguish this caterpillar from beet armyworm, Spodoptera exigua (Hubner), a species that is superficially quite similar (Capinera, 1986). Larvae attain a length of 35-40 mm at maturity. Duration of the larval stage is 16-22 days under warm conditions, but it may be extended to nearly 50 days by cool weather. (See color figure 47.)

  1. Mature larvae burrow into the soil to a depth of about 2-3 cm to pupate. The pupa is reddish brown and measures 13-14 mm long. Duration of the pupal stage is 10-20 days during the spring and summer generations, but about 150 days for the overwintering population.
  2. The adults have a wingspan of 31-35 mm. The front wings are yellowish brown, and heavily marked with darker and lighter spots. The hind wings are grayish basally with a diffuse darker brown band distally. The compound eyes of this moth bear hairs— a feature that is usefull for distinguishing it from some similar species. Females produce a sex pheromone that has been identified and synthesized (Struble and Swailes, 1977a). (See color figure 234.)

This insect is not well-studied. Marsh (1913) gave a brief account of its biology. Walkden (1950) gave additional notes and provided a key to noctuids in the central Great Plains. Crumb (1956) described the mature larva and provided a key to larvae. The larva was also included in a key by Capinera (1986), and in a key to armyworms and cutworms in Appendix A. The moth was included in pictorial keys developed by Rings (1977a) and Capinera and Schaefer (1983). Santiago-Alvarez et al. (1979) reported a suitable artificial diet.

Damage

The larval stage defoliates plants, though it appears to favor weeds over crops. It is not a ground-dwelling species and does not sever plants at the soil surface, but climbs plants to feed on leaves. During periods of great abundance it has caused significant damage, and has been reported to assume a gregarious, dispersive "armyworm" habit.

Management

Natural enemies, especially pathogens, generally serve to keep the population in check. Adult popula tions can be monitored with pheromone traps (Struble and Swailes, 1977a; Swailes and Struble, 1979; Ayre et al., 1982a). This species is reported to be easy to control with foliar applications of insecticides, though Bacillus thuringiensis is not often recommended. This species is a plant-inhabiting, climbing cutworm; therefore, the mechanical barriers recommended for protection of seedlings against ground-dwelling species are of little value. It is possible to deny access by ovipositing moths through use of netting and row covers, however.

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