Spilosoma virginica Fabricius Lepidoptera Arctiidae

Natural History

Distribution. Yellow woollybear is a native insect, and is found throughout the United States and southern Canada. Yellow woollybear's range as a pest is generally restricted to the Great Plains region to the west coast. Even within this area, however, it infrequently is numerous enough to be damaging.

Host Plants. Yellow woollybear is a very general feeder, and reported from over 100 different plants. Yellow woollybear has been observed to damage such vegetable crops as asparagus, bean, beet, cabbage, cantaloupe, carrot, cauliflower, celery, corn, eggplant, lima bean, parsnip, pea, potato, pumpkin, radish, rhubarb, squash, sweet potato, Swiss chard, turnip, and watermelon. Other economic plants damaged include field crops such as alfalfa, peanut, and sugarbeet; fruits such as blackberry, cherry, currant, gooseberry, grape, and raspberry; and flowers such as canna, dahlia, geranium, hollyhock, hyacinth, and verbena. Among weeds fed upon are dandelion, Taraxacum officinale; dock, Rumex sp.; pigweed, Amaranthus spp.; lambsquarters, Chenopodium album; plantain, Plantago major; Russian thistle, Salsola kali; Spanish needle, Bidens bipinnata; and sunflower, Helianthus spp.

Natural Enemies. The number of parasitoids found in association with this insects is quite large, though the importance of these natural enemies has not been well-studied. Arnaud (1978) listed several tachinids reared from yellow woollybear, including Aplomya caesar (Aldrich), Blondelia hyphantriae (Tothill), Bombyliopsis abrupta (Wiedemann), Carcelia diacrisiae Sellers, C. reclinata (Aldrich and Webber), Compsilura concinnata (Meigen), Exorista mella (Walker), Gymno-carcelia ricinorum Townsend, Hubneria estigmenensis

(Sellers), Lespesia aletiae (Riley), L. frenchii (Williston), Mericia ampelus (Walker), Thelaira americana Brooks, and Winthemia datanae (Townsend) (all Diptera: Tachi-nidae). Wasps reared from yellow woollybear include Apanteles diacrisiae Gahan, A. scitulus Riley, (both Hymenoptera: Braconidae); Coccygomius sanguinipes (Vierick), Cratichneumon unifasciatorius (Say), Vulgich-neumon subcyaneus (Cresson), Therion morio (Fabricius), Hyposoter rivalis (Cresson), Enicospilus glabratus (Say) (all Hymenoptera: Ichneumonidae); Psychophagus omnivorous (Walker), Tritneptis hemerocampae Vierick (both Hymenoptera: Pteromalidae); Elachertus marylandicus Girault, E. spilosomatis Howard (both Hymenoptera: Eulophidae); Telenomus nigriscapus Ashmead, and T. spilosomatis Ashmead (both Hymenoptera: Scelioni-dae). The fungus Beauveria bassiana has been reported to cause low levels of mortality, and a granulosis virus has been observed (Boucias and Nordin, 1977).

Life Cycle and Description. There likely are three generations of yellow woollybear annually, despite the numerous reports of only two generations. The discrepancy is due to the overlapping flights of the moths from overwintering pupae with those of the spring generation. In the most complete study of yellow woollybear population dynamics, conducted in Iowa, the apparent spring flight of moths was shown to consist of two reproductive populations, each represented by separate peaks in abundance within the overall spring flight period. There also was a late summer flight which produced overwintering pupae (Peterson et al., 1993). Elsewhere, moths also are abundant in spring (April-June) and autumn (July-October), but there is considerable geographic variation in the timing of flights. For example, the early-season flight activity occurs in mid-April in Arkansas and North Carolina, but not until late May in Maine. Late-season flight occurs in August in Maine, September in North Carolina, and October in Arkansas. The pupal stage reportedly overwinters throughout this insect's range.

  1. The spherical eggs of yellow woollybear are yellow, 0.6 mm in diameter, and deposited in clusters of 50-200 both on plant foliage and on inedible substrata. Duration of the egg stage is about seven days. (See color figure 256.)
  2. Upon hatching, the larvae are hairy and bluish white. During the first two instars the larvae feed gregariously, and then disperse. As they mature, they retain their hairy characteristic, and develop both long, fine, soft hairs and shorter, stout bristles. The hairs are not so thick as to hide the larval body, which is quite variable in coloration. The most common color for larvae is yellow, which is the basis for the common name, but they also may be cream, light brown or dark brown. Also, there is a dark line along each side of the caterpillar, and the membrane between each body segment tends to be marked by dark pigment. The head is principally yellow, and this character is useful to distinguish it from saltmarsh caterpillar, Estigmene acrea (Drury), which tends to have a black head. Larvae attain a length of up to 5 cm at maturity. Larvae reared on bean foliage at 25°C required about 39 days to progress through nine instars (Capinera et al., 1987). Instar duration was 3.1, 2.4, 3.9,4.9, 3.6, 3.6, 3.6, 4.6, and 10.0 days for instars 1-9, respectively. Although Peterson et al. (1993) suggested that this long development time or large instar numbers might be indicative of a suboptimal host, Dethier (1987) similarly reported larval development times of 35 days on suitable host plants. Larvae often move to another plant after completing a meal, even though the plant is relatively suitable for growth and development. They spend less than 1% of their time in eating, 2.5% in wandering, and the remaining of their existence in resting (Dethier, 1987). (See color figure 88.)
  3. Pupation occurs in plant debris, under bark of trees, and in other sheltered locations. The pupal case is constructed from the larval hairs, which is held together loosely with silk. Duration of the pupal stage is 7-14 days, and the reddish-brown pupa measures about 15-16 mm long.
  4. Adults are medium-size moths measuring about 38-50 mm in wingspan. The wings are white, but the front wings bear a small black spot near the center, and the hind wings usually are with three black spots. The head and thorax are covered with white scales. The abdomen is yellow-orange with three rows of black spots, one row dorsally and one on each side.

The biology of yellow woollybear is not well documented. Brief treatment of yellow woollybear was provided by Riley (1871), Marsh (1912b), and Maxson (1948).

Damage

Larvae are defoliators. Young larvae are gregarious, and tend to feed together on the underside of foliage

Yellow woollybear larva.
Larva Noctidae
Adult yellow woollybear.

and skeletonize the plant tissue. Larger larvae disperse and feed sporadically, creating irregular holes in foliage. When larvae are particularly numerous, and succulent vegetation scarce, many of the large larvae remain on crops and inflict injury. This most often occurs in irrigated cropland when adjacent weedy vegetation senesces, or dries up due to drought. Typically it is only the late summer generation that attains densities adequate to inflict injury. Capinera et al. (1987) measured bean foliage consumption by each instar, and recorded over 300 sq cm of foliage consumed during the life of a caterpillar. Further, they estimated that 1.2-2.2 mature caterpillars per plant could inflict 20% defoliation, a level adequate to cause yield loss.

Management

Yellow woollybear is common among weeds growing along roadsides, fence rows, and irrigation ditches. Larvae disperse into crops only when native or weedy vegetation is depleted or otherwise unsuitable. Such sources of infestation should be monitored. Larvae are easily killed with foliar insecticides, though this is rarely warranted. Treatment of the source of infestation or the borders of crops is generally adequate to prevent damage. Burning of crop residues in the autumn is sometimes recommended to destroy overwintering larvae and pupae because they are located above-ground and very susceptible to fire. However, it is normally better to leave such organic matter on the soil surface, or tilled into the soil; the exception might be ditch banks or other small areas that are not tilled.

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