Citheronia regalis

Description

The Regal Moth (Citheronia regalis), also known as the Royal Walnut Moth, is a giant silk moth (Saturniidae, subfamily Ceratocampinae) and the heaviest moth by body mass found north of Mexico. The adult wingspan ranges from 9.5-15.5 cm. The forewings are greyish to olive with prominent orange veins and rows of yellow spots; the hindwings are orange with a darker margin. The body is stout and orange-brown, banded with cream. Males have large, plumose antennae for detecting female pheromones. Like all Saturniidae, the adult mouthparts are vestigial — the moth does not feed and lives approximately one week, sustained entirely by fat reserves accumulated during the larval stage.

The caterpillar is far better known than the adult — and for good reason. The Hickory Horned Devil, the final larval instar, is the largest caterpillar in North America, reaching 15 cm in length. It is brilliant green with enormous, curving, black-tipped red horns behind the head and smaller horn-like spines along the body. Despite its terrifying appearance, which has inspired its common name, the caterpillar is entirely harmless: the horns are not venomous, do not sting, and the larva is one of the more easily handled of the Saturniidae. Earlier instars are quite different — small, dark, and curled in a J-shape during the day, resembling bird droppings. The fifth instar is so dramatically different that it can be mistaken for a different species entirely.

The species was first described by Danish entomologist Johan Christian Fabricius in 1793. Its range is centred on the deciduous forests of eastern North America, from the Deep South — where it remains common — northward to Pennsylvania, New Jersey, and historically into southern New England. It is considered rare and sporadic north of the Ohio Valley and is listed as extirpated from Connecticut.

Lifecycle

A single generation is produced each year throughout the range, though records from the deep southern United States suggest an extended emergence period during the longer growing season. Adults are typically on the wing from late June through August, with larvae peaking from August through October.

Females deposit yellowish, ovular eggs approximately 2 mm in diameter, singly or in groups of up to four, on the upper surface of host plant leaves — preferentially on walnut and hickory. Eggs hatch after 7-10 days. The tiny yellow larvae darken rapidly and remain solitary throughout development. Early instars feed at night and rest curled in a defensive posture during the day. Later instars feed openly. The caterpillars progress through five instars over approximately 37-42 days. At each molt, the appearance changes substantially. The final instar — the Hickory Horned Devil — is unmistakable: bright leaf-green with an enormous pair of thoracic horns that are black at the base, red in the middle, and tipped with black.

Just prior to pupation, the fully fed larva expels its gut contents and its colour shifts from green to turquoise. It then descends from the host tree and burrows 15-20 cm into the soil, where it forms a smooth earthen chamber. Unlike most Saturniidae, Citheronia regalis does not spin a silk cocoon — it pupates in direct contact with the soil. The pupa is dark brown to black, with a short cremaster. Some pupae enter extended diapause, overwintering for two seasons — a strategy that may buffer the population against adverse conditions such as fire, flooding, or a poor host-plant year, and that helps maintain genetic diversity across generations.

Adult emergence is synchronized with the first spells of humid mid-summer weather. Emerging moths pump hemolymph into their wings over 2-3 hours before they are capable of flight. Females emit pheromones at night; males, equipped with broad, plumose antennae, detect these chemical signals from kilometres away and fly upwind to locate mates.

Ecology

The Regal Moth is a specialist of walnut and hickory — the larvae feed primarily on members of the Juglandaceae, including Black Walnut (Juglans nigra), Butternut (J. cinerea), and various hickories (Carya spp.). Regional host shifts occur where these preferred trees are absent: in the South, larvae use Sweetgum (Liquidambar styraciflua) and Persimmon (Diospyros virginiana); elsewhere, sumacs (Rhus spp.) are accepted. Larvae are solitary and rarely occur in numbers sufficient to cause defoliation, though a single individual in its ravenous fifth instar can strip several branches of leaves before descending to pupate.

Like other walnut-feeding Saturniidae, the Regal Moth has evolved physiological tolerance to juglone, the allelopathic compound that renders walnut foliage toxic to most herbivorous insects. This biochemical adaptation allows the species to exploit a host-plant niche with relatively little competition from other Lepidoptera. The adults, having no mouthparts, do not feed and play no role in pollination. Their sole ecological function is reproduction — the brief, pheromone-driven adult phase is among the most compressed life-history strategies of any temperate insect.

The species has declined significantly in the northeastern portion of its range. While still common in the Deep South — from the Carolinas to Texas — it is now rare and sporadic north of the Ohio Valley. The causes of decline are not fully understood but likely include habitat fragmentation, reduction in walnut and hickory abundance in second-growth eastern forests, and possibly the same introduced parasitoid fly (Compsilura concinnata) that has affected Luna Moth and other native Saturniidae populations.

Host Plants

Larvae feed primarily on members of the walnut family, with regional alternatives:

• Black Walnut (Juglans nigra) — preferred northern host
• Butternut (Juglans cinerea)
• Hickories (Carya spp.) — including pecan (C. illinoinensis)
• Sweetgum (Liquidambar styraciflua) — used in southern range
• American Persimmon (Diospyros virginiana)
• Sumac (Rhus spp.)

Habitat

Regal Moths are inhabitants of eastern North American deciduous forests, with a strong association with walnut- and hickory-rich woodlands. In Ontario, they would be expected in the Carolinian Forest region, where Black Walnut and Butternut occur naturally in rich bottomlands and riparian corridors. The species' range extends from the Deep South northward through the Mississippi and Ohio Valleys to Pennsylvania and southern New England, with Ontario populations at the extreme northern limit of the range. Adults are nocturnal and are most frequently encountered at lights.