Pyrrharctia isabella J E Smith Lepidoptera Arctiidae

Natural History

Distribution. A native species, banded woolly-bear is found throughout the United States and southern Canada. Banded woollybear is the best known of the woollybears, because in American folklore its color pattern is said to foretell the severity of forthcoming winter weather. Banded woollybear, though common, is more of a curiosity than a pest.

Host Plants. Banded woollybear consumes a wide breadth of flora, though damage to economic plants is infrequent, and therefore poorly documented. It is recorded from beet, corn and pea, and probably nibbles on nearly any garden vegetable, but its normal hosts are weeds and wild native plants. The normal foods are asters, Aster sp.; dandelion, Taraxacum officinale; dock, Rumex spp.; goldenrod, Solidago spp.; plantain, Plantago spp.; sweetclover, Melilotus; and some grasses. In the only broad study of banded woollybear feeding, a no-choice test, over 90 plant species from over 50 plant families were consumed to some degree (Shapiro, 1968).

Natural Enemies. Several natural enemies are known, but they have not been the subject of much study, so their relative importance are to be determined. Fly parasitoids associated with banded woollybear include Euexorista futilis (Osten Sacken), Exorista mella (Walker), Hubneria estigmenensis (Sellers), Mysta-cella spp., Parachaeta fusca Townsend, Thelaria spp., and Winthemia datanae (Townsend) (all Diptera: Tachini-

dae). Among wasps known to attack this woollybear are Apanteles flavovariatus Muesebeck (Hymenoptera: Braconidae); Coccygomius pedalis (Cresson), Dusona crassicornis (Provancher), and Enicospilus glabratus (Say) (all Hymenoptera: Ichneumonidae). A cytoplasmic polyhedrosis virus has been isolated from banded woollybear larvae; infected larvae displayed significantly slower rates of development (Boucias and Nordin, 1978).

Life Cycle and Description. The phenology of banded woollybear is not well known, but there appears to be at least two generation in the southern United States, and only a single generation in the northern states and southern Canada (Goettel and Philogene, 1978a). Thus, there are reports of a single generation in Ontario, Quebec, the New England states, and New York, but two generations in Illinois.

  1. Banded woollybear deposits spherical eggs in clusters of 50-100 eggs. Duration of the egg stage is about six days at 27°C.
  2. Larval development is rather variable in this species. When reared on artificial diets, the larvae display 7-10 instars, with development times of about 3, 3, 3.2, 3.9, 4.9, 6.9, 14, 21 (and if present) 21 days, respectively for instars 1-9 (Goettel and Philogene, 1978b). These same authors (Goettel and Philogene, 1979) also provided head capsule measurements for larvae with 7-10 instars. Head capsule widths for larvae with eight instars, a common number, are 0.4, 0.5, 0.8, 1.2, 1.7, 2.3, 2.9, and 3.7 mm, respectively. Instar number is influenced by photoperiod but not by temperature; exposure of larvae to greater than 14 h photophase results in an increased number of instars (Goettel and Philogene, 1978a).

The banded woollybear is thickly clothed with stout bristles. The head and body are black, with black or reddish-brown spines covering the body. Typically the caterpillar's bristles are brown at the middle of the body, and black at both the anterior and posterior ends of the caterpillar. Young larvae tend to be about two-thirds black, with the amount of black dissipating as larvae mature. Sometimes only the head end remains black, and in California caterpillars are sometimes uniformly brown. The larva overwinters and is often observed in the autumn as it disperses in search of a suitable overwintering shelter. If cold weather commences early in the year, the larvae are relatively immature, and thus possesses a disproportionately large amount of brown coloration as compared to years with long summers, when the larvae are more mature. Thus, there is some meteorological basis to the American belief that banded woollybear foretells the weather. Banded woollybear larvae tend to have mostly one type of hair, of equal length, and have a rather short-cropped look as compared to some other long-haired species such as yellow woollybear, Spilo-soma virginica (Fabricius). There may be a few longer hairs at both the anterior and posterior ends of the caterpillar, but this does not detract from the overall impression of uniform hair length. The mature larva attains a length of about 30 mm, and has a tendency to roll into a ball if disturbed.

  1. Banded woollybear overwinters as a larva in leaf debris, under loose bark and similar shelter, and in the spring it feeds briefly before pupating. The pupal case is constructed principally from hairs from the caterpillars body, spun loosely together with silk. The coccoon is dark because it consists mainly of black and brownish body hairs. The pupa is light to dark brown. Pupation usually requires 14-21 days.
  2. The moth is yellowish-orange to light brown, with one to several dusky spots on the front wings and two on each hind wing, and has a wingspan of about 45 mm. In males, the hind wing tends to be paler. The abdomen is marked with three rows of black spots. Banded woollybear courtship is unusual in that it involves acoustical signals produced by
Banded woollybear larva.
Forest Defoliators Lifecycle

females in response to male production of pheromone (Krasnoff et al, 1987; Krasnoff and Yager, 1988).

The biology of this species is poorly documented. Banded woollybear biology was discussed by Saun-ders (1873) and a method of culture was given by Goettel and Philogene (1978b).


Larvae are defoliators, but they feed principally on weeds and other noncrop plants. Young larvae are gregarious, feeding together on the underside of foliage and skeletonizing the plant tissue. Larger larvae disperse and feed sporadically, creating irregular holes in foliage.


It is highly unusual to experience banded woollybear in enough abundance to warrant concern. However, they are easily killed with foliar insecticides if it becomes necessary.

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  • asphodel
    Is Pyrrharctia isabella a pest?
    9 years ago

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