Callosamia promethea

Description

The Promethea Moth (Callosamia promethea) is a large silk moth in the giant silkworm family (Saturniidae), with a wingspan of 7.6-10.2 cm. It exhibits pronounced sexual dimorphism: males are dark brown to black with tan wing margins and eyespots on the forewings only; females are larger and more brightly coloured in shades of rust, cream, and reddish-brown, with eyespots on all four wings. Males closely resemble the toxic pipevine swallowtail butterfly in a form of Batesian mimicry. Also known as the Spicebush Silkmoth, it is ranked Secure (G5) globally by NatureServe.

This species is unique among Saturniidae for its temporal separation of the sexes: males are diurnal (active by day) while females are nocturnal, overlapping only during a narrow evening window for mating. This unusual arrangement allows females to lay eggs at night without trading off mating opportunities.

Lifecycle

Promethea Moths produce one generation per year in northern regions. Females lay eggs in clusters of 4-10 on the underside of host plant leaves at night. The eggs are white, flat, and elliptical. Caterpillars hatch and feed solitarily, consuming leaves from the edge inward.

Early-instar larvae are yellow with black longitudinal stripes. As they mature through successive moults, they become blue-green and develop four prominent red protuberances near the head and a single yellow protuberance at the rear. After the fifth instar, the caterpillar spins a remarkable cocoon: it wraps itself inside a living leaf, securing it with silk, and the leaf remains attached to the branch while the caterpillar pupates within. The dead leaf cocoon hangs conspicuously through winter, suspended from thin branches that are inaccessible to most predators.

Adults emerge in early summer (May-July in Ontario). Pupae are the overwintering stage in the north. Adults have vestigial mouthparts and do not feed; their brief adult life is devoted entirely to reproduction.

Ecology

Larvae are generalist feeders on a variety of deciduous trees, with sassafras (Sassafras albidum), spicebush (Lindera benzoin), and tulip tree (Liriodendron tulipifera) as primary hosts. Additional host families include Rosaceae (cherries, apples) and Oleaceae (ashes, lilacs). Despite this breadth, larvae show no physiological disadvantage when raised on a host different from their parent's host — nutritional quality of the individual plant matters more than species fidelity.

Adult males rely on Batesian mimicry of the pipevine swallowtail for protection from birds and other visual predators. Females remain camouflaged on vegetation while releasing pheromones during the late afternoon calling period. Males navigate upwind along the pheromone gradient — one male was documented locating a female from 37 km away.

The cocoon's position on thin, dangling branches provides effective protection: the branches are too slender to support the weight of mice, and the tough silk casing resists woodpeckers. Flies and parasitic wasps are the primary natural enemies, attacking the pupal stage.

Host Plants

Larvae feed on a broad range of deciduous trees and shrubs. Key hosts in Ontario include:

• Sassafras (Sassafras albidum)
• Spicebush (Lindera benzoin)
• Tulip Tree (Liriodendron tulipifera)
• Various Rosaceae (Wild Black Cherry, Apple)
• Various Oleaceae (White Ash, Lilac)

Habitat

The Promethea Moth inhabits deciduous forests throughout eastern North America, from the Atlantic coast west to the Great Plains. In Ontario, it occurs within the Carolinian Forest zone and extends into the Great Lakes-St. Lawrence forest region where its host plants are present. The species is not considered rare or threatened, and its caterpillars do not cause noticeable damage to host trees even at high densities.