Insect Profiles: Isabella Tiger Moth (Pyrrharctia isabella)
Isabella tiger moth (Pyrrharctia isabella) is one of the most common insects in North America. It is recognized for its distinct coloration, and stiff bristles, which are soft to the touch. Isabella tiger moth can be observed throughout the growing season. It is considered a minor defoliator of many plants.
Distribution & Habitat
In the United States, Isabella tiger moth can found in nearly every state, except for Alaska and Hawaii. It is also common across much of Mexico, and southern Canada. Isabella tiger moth is occasionally found in the Arctic, where it may live through as many as fourteen winters. Isabella tiger moth is most prevalent in open dry to slightly moist habitats, where it prefers weedy fields, fallow cropfields, roadsides, and landscapes. It often appears in forested settings as well.
Isabella tiger moth consumes the leaves of asters, birches, clover, corn, elms, maples, and sunflowers, as well as a variety of grasses and weeds, including plantain, dandelion, and nettles. Adults drink nectar from wildflowers.
Isabella tiger moth is notable for its larval and adult stages. The larvae, often referred to as wooly bears, are usually black at the anterior and posterior, but red to orange-brown in the center. On some individuals, the black coloring may be relegated to the anterior end. When fully grown, wooly bears are more than two inches long, and covered with stiff bristles, as well as several longer soft hairs. Orange oblongs are visible between the bristles. These oblongs are entrances to the wooly bear’s respiratory system. The coloration of the wooly bear changes as the larvae progress through successive instars. The bristles generally become less black, and more red as they mature. Adult moths are a pale yellow to orange. Adults have a small head, and a furry thorax. Lines of dark spots on the top sides of the abdomen are conspicuous. Wings protrude out from the abdomen. They range from pale yellow to orange, and are speckled with black spots. Adults have three sets of legs, which are dark brown in color. The proximal segments on the first pair of legs are bright orange.
Isabella tiger moth produces two generations in its northern range. A third generation often occurs in the south. Adult females lay their eggs on a variety of hosts. The eggs hatch within two weeks, depending on the geographic location. As the larvae emerge, they navigate to the leaves of herbaceous plants, shrubs, and small trees, where they begin to feed. As the larvae feed, they pass through several instars, which alters their coloration. Environmental conditions influence the larvae’s coloration. Dry conditions cause the red bands to lengthen. Moist conditions cause the black bands to become more prominent. When the larvae of the second and third generations approach maturity, they enter a wandering state as they prepare to hibernate. In the insect’s northern range, this generally coincides with the first frost. During this stage, the larvae may be observed on roadsides, sidewalks, and lawns. If disturbed, the larvae will roll up in a ball. The larvae eventually settle in crevices, or under leaf litter and rocks, where they enter a dormant state. They overwinter in these locations, producing a cryoprotectant in their tissues that enables them to survive being frozen. The larvae break dormancy in early spring. Once active, they resume feeding for several nights. Soon, the larvae reach full maturity, and begin spinning a silken cocoon, within which they pupate. Adults emerge four weeks later. In the north, adult moths appear from April to June, with a second generation occurring two months later. The first generation pupates during summer. The second generation overwinters, and pupates in spring. In the south, the third generation overwinters as well, pupating in spring. During the evening, adult females extend a scent gland from their abdomen, which attracts the males. Males fly in zig-zag patterns, detect the scent, and locate their mates. Once they have mated, the females lay their eggs, and expire.
Several caterpillars appear similar to Isabella tiger moth, but differ in their coloration and patterning. These include the yellow bear, and the great leopard moth. Yellow bears are especially abundant before Isabella tiger moths appear. Yellow bears range in color from cream to dark gray. Their end segments are never darker than the middle. The great leopard moth is black with red between the end segments.
Effects on Trees
Isabella tiger moth feeds on the leaves of host trees, causing minor defoliation. While defoliation can temporarily reduce the ornamental value of host trees, a second flush of foliage is generally produced by the end of the growing season.
Isabella tiger moth is considered a minor nuisance. It does not cause extensive damage to trees. As such, management practices are not required. Isabella tiger moth has several natural predators that help limit populations. These include parasitic wasps, mantids, birds, and flies.
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Photo courtesy of Dwight Sipler/CC-by-A2.0