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California Tortoiseshell

Nymphalis californica

California Tortoiseshell

Adults Fly

March–May (overwintered adults) and late May–August (new adults); abundance varies dramatically year to year.

Eggs Laid

April–June (first brood); timing tied to ceanothus leaf-out.

Larvae Active

May–July (often in conspicuous groups on ceanothus).

Overwintering

Overwinters as an adult in bark crevices, rock outcrops, and other sheltered spots — same strategy as the Mourning Cloak.

Conservation (NatureServe)

G5 (Secure)
Widespread and generally secure; boom-and-bust cycles are natural and not a conservation concern. Depends on intact ceanothus shrub communities.

Caterpillar Host Plants (Willamette Valley natives)

  • Snowbrush ceanothus Ceanothus velutinus — Primary larval host; common on drier slopes and forest edges around the valley.
  • Redstem ceanothus Ceanothus sanguineus — Native ceanothus of valley foothills and forest openings; confirmed larval host.
  • Deerbrush Ceanothus integerrimus — Drier foothill sites; used where available.

Favorite Nectar Flowers (Willamette Valley natives)

  • Tall Oregon grape Mahonia aquifolium — Early spring bloom; important for overwintered adults emerging in March–April.
  • Red-flowering currant Ribes sanguineum — Early spring nectar; also used by overwintered adults.
  • Douglas aster Symphyotrichum subspicatum — Late-season nectar for summer adults.
  • Canada goldenrod Solidago canadensis — Late-summer nectar in open sites.
  • Showy milkweed Asclepias speciosa — Strong mid-summer nectar draw.

What the Caterpillars Eat

Leaves of native ceanothus shrubs (especially Ceanothus velutinus and C. sanguineus); larvae feed gregariously and can defoliate entire plants in outbreak years.

The California Tortoiseshell is one of the West’s most unpredictable butterflies. In quiet years, you might barely notice it. Then suddenly — in a boom year — these butterflies emerge by the millions, streaming over ridgelines and filling the air along foothill trails in a way that stops hikers in their tracks.

Its wings are a warm burnt orange with irregular black patches and a crisp dark border. The underside is mottled gray and brown, nearly bark-like — close the wings and this butterfly almost disappears against a log or rough surface. Like its close relative the Mourning Cloak, it overwinters as an adult, tucked into crevices and sheltered spots to wait out the cold.

Caterpillars are closely tied to native ceanothus, especially snowbrush on drier slopes and redstem ceanothus in the valley foothills. In big outbreak years, feeding caterpillars can defoliate entire hillsides of ceanothus — a dramatic sight, though the shrubs almost always recover.

Fun Facts

  • In outbreak years, California Tortoiseshells can migrate in enormous numbers — waves of thousands crossing roads and covering hillsides are well-documented in Oregon.
  • Like the Mourning Cloak, overwintering adults can survive nearly a year, making them among the longest-lived butterflies in the region.

Sources

Solitary wasp

A solitary wasp has no colony of workers helping raise her young. A female may stock her own nest cells with food, or find a host for her developing offspring. Solitary describes social life; parasitoid describes a feeding strategy. A wasp can be both.

Social wasp

Social wasps share a nest and cooperate in raising young. In yellowjacket and paper wasp colonies, reproductive females lay eggs and workers help feed the brood and maintain the nest. They can defend that nest, so observing a forager is different from approaching a colony.

Non-native Social wasp

This is a social wasp introduced beyond its natural range. In this case the European paper wasp was introduced to North America from Europe, probably hidden in shipping materials. Non-native describes its origin, while social describes colony life.