Distribution. Black swallowtail, also known as "American swallowtail," is found in southern Canada and the eastern United States as far west as the Rocky Mountains. Its range also extends south into Mexico and northern South America. Anise swallowtail, also known as "western swallowtail," inhabits southwestern Canada and the western United States from the Rocky Mountains to the Pacific coast. The ranges of these native species overlap in the Rocky Mountain region.
Host Plants. Larvae of these species are commonly known as "parsleyworm" or "celeryworm" because they feed on plants in the family Umbelliferae. Vegetable crops fed upon by larvae include carrot, celery, fennel, parsley, and parsnip. The herbs anise, caraway, and dill also are eaten. Numerous umbelliferous weeds serve as suitable hosts, including angelica, Angelica spp.; cow parsnip, Heracleum spp.; lovage, Ligusticum spp.; water hemlock, Cicuta maculata; and wild carrot, Daucus carota. There are some chemical similarities between Umbelliferae and Rutaceae, the plant family containing citrus. Thus, occasionally these swallowtails are observed feeding on citrus, particularly orange trees, or other Rutaceae. This has occurred in California with greater frequency since fennel has been planted widely at lower elevations. Nowhere are these swallowtails considered to be a serious pest on citrus, however. Adults visit a variety of flowers to obtain nectar, they seem particularly fond of milkweed, Asclepias spp.; thistle, Cirsium spp.; and red clover, Trifolium pratense. Relative suitability of some plant hosts for black swallowtail larvae was provided by Finke and Scriber (1988).
Natural Enemies. Avian predators are an important source of mortality for adults. Resting butterflies are especially susceptible to predation, and during inclement weather they spend more time roosting, thus incurring higher levels of predation (Lederhouse et al., 1987). Larvae are attacked by several predatory insects such as shield bugs (Hemiptera: Pentatomi-dae), damsel bugs (Hemiptera: Nabidae), and assassin bugs (Hemiptera: Reduviidae). A wasp parasitoid, Trogus pennator (Fabricius) (Hymenoptera: Ichneumo-nidae) attacks the larvae of both swallowtail species; black swallowtail, and likely anise swallowtail, are attacked by the fly parasitoids, Buguetia obscura (Coquillett), Compsilura concinnata (Meigen), and Lese-siafrenchii (Williston) (all Diptera: Tachinidae).
Life History and Description. These two species are very similar in life history, differing principally in the appearance of adults, as described below. There are two generations annually in northern regions of New York, but three generations in warmer climates. Overwintering occurs in the pupal stage, and in warm climates some pupae from the second generation, as well as those from the third generation, enter diapause and emerge the following spring.
Black swallowtail larva.
posterior of each body segment is edged in green, and orange or yellow spots are located near the center of each body segment. The fifth instar is predominantly green, with black restricted to the center of each segment; this stage also has the orange spots found on the earlier instar. Larval development time is 10-30 days, depending on temperature. Larvae have an orange, eversible gland that resembles horns or antennae when extended. Located dorsally behind the head, the gland is exposed only when the caterpillar is disturbed. The gland, called an osmeterium, releases volatile chemicals that deter predation by some, but not all, insect predators (Berenbaum et al., 1992). (See color figure 98.)
The adults of anise swallowtail may greatly resemble black swallowtail, but more commonly they have the interior row of yellow spots greatly expanded, filling the central region of the wings with yellow. The net result is that the western species, anise swallowtail, is predominantly yellow, whereas the eastern species, black swallowtail, is predominantly black.
The adults of both species are quite large, the wing-span measuring over 7 cm. Adults are active during most of the day, and males are territorial, defending areas against other males. They often perch on elevated objects to maintain a good view of their territory, or they may patrol an open area, looking for females with which to mate. Males often frequent hilltops, and virgin females fly to hilltops to seek mates.
The biology of these species was given by Chittenden (1912d), Fisher (1980), Scott (1986) and Opler and Krizek (1984). Culture techniques were described by Carter and Feeny (1985).
Larvae are large and can consume considerable quantities of foliage toward the end of their development. This should be of concern only in the home garden, because the butterflies are not sufficiently abundant ever to threaten commercial cultivation of an umbelliferous crop.
Owing to their infrequent occurrence, control of larvae should not be necessary. For at least the past 100 years, hand picking has been recommended in the home garden, and this recommendation remains valid. In instances where plot size is too big to make this practical, larvae can be killed easily with foliar insecticides, either chemical or Bacillus thuringiensis.
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