Compton Tortoiseshell (Nymphalis vaualbum) – Wildlife of Japan

Introduction

The Compton Tortoiseshell (Nymphalis vaualbum), known in Japanese as Erutateha, is a striking forest butterfly found in northern Japan. It belongs to a Holarctic species group that ranges across the temperate Northern Hemisphere. In Japan, the subspecies samurai lives in cool deciduous forests filled with elm and birch. Because it overwinters as an adult, it often reappears early in spring when the forests are still quiet.

Appearance

This medium to large butterfly has a wingspan of 50–70 mm. Its upperside shows deep orange to orange-brown tones with dark wing bases and bold black spots. In addition, the outer margins are strongly scalloped, creating a rugged outline that is typical of tortoiseshell butterflies.

A single white spot on the leading edge of the forewing provides an important identification clue.

The underside is mottled gray-brown and mimics the texture of tree bark or dead leaves. On the hindwing underside, a pale L-shaped mark gives the butterfly its Japanese name, “Erutateha.” Combined with the notched wing edges, this mark makes the species easy to recognize.

Habitat & Distribution

Globally, Nymphalis vaualbum occurs across Eurasia and North America.
In Japan, the subspecies samurai appears mainly in:

• Hokkaido – from lowlands to mountain zones
• Honshu – especially from the Chubu region northward, mainly above ~1,000 m
• Rare and local west of Kinki

The butterfly prefers cool mixed and deciduous forests where elm (Ulmus) and birch (Betula) are common. It is often observed along forest edges, mountain roads, streamside habitats, and sunny gaps within the forest.

During winter, adults hide in tree hollows, cracks, or sheltered buildings where snow does not accumulate.

Behavior

In Japan, the Compton Tortoiseshell is univoltine, producing one generation each year.
New adults emerge in July and August. As autumn approaches, they search for overwintering shelters. When temperatures rise in early spring, the overwintered adults resume activity and often survive into May.

The species shows several characteristic behaviors:

• It basks with wings fully open in sunny spots.
• It rests with wings closed against tree trunks, blending seamlessly with the bark.
• It flies quickly along forest edges and mountain trails.

Unlike many butterflies, it does not rely only on flowers. Instead, it feeds on:

• Tree sap
• Fermenting or fallen fruits
• Moist soil and animal droppings
• Flowers of shrubs such as hydrangea relatives and rowan species

Consequently, it can thrive even when nectar sources are limited. Overwintered adults commonly appear faded and worn after surviving the long cold season.

Diet

Larval host plants include:

• Elms (Ulmus) — such as Harunire
• Birches (Betula) — including Shirakanba, Dakekanba, and Udaikanba

Adult food sources include:

• Tree sap
• Fermenting fruits
• Moist soil and puddles
• Animal droppings
• Flowers of forest shrubs

This broad diet allows the species to adapt to various forest conditions, especially in early spring when flowers are scarce.

Reproduction

After overwintering, females mate in spring and lay eggs on young leaves or buds of elm and birch. The reproductive cycle continues as follows:

• Eggs are placed on twigs or new foliage.
• Larvae feed on leaves in early summer and may produce light silk webbing.
• Pupae form on sheltered parts of the tree.
• Adults emerge from July to August, feed through late summer and autumn, and then seek overwintering shelters.

Conservation

The Compton Tortoiseshell is not listed on Japan’s national Red List. Even so, several prefectures classify it as a species of regional concern:

• Tokyo – Vulnerable
• Niigata – Near Threatened

Because the species depends on intact deciduous forests, habitat loss poses a significant risk. Converting natural forests into conifer plantations reduces the availability of host plants such as elm and birch. Therefore, preserving mixed broad-leaved forests is crucial for maintaining healthy populations.

Author’s Impression

Since the Compton Tortoiseshell overwinters as an adult, it sometimes appears very early in spring. Seeing one glide into a sunny clearing feels like the first sign of the new season. If you wait quietly in a bright, warm spot, the butterfly may land nearby to bask, opening its warm orange wings in the soft spring light.