California tortoiseshell
Population booms send millions north in search of food, and in outbreak years these butterflies swarm mountain passes and even city streets.
| Where | Mountain chaparral and canyon shrubland |
|---|---|
| Size | 32 to 70 mm wingspan |
| When | February to October |
The California tortoiseshell is a small to medium butterfly with a wingspan of about 32 to 70 mm, its upperside bright orange with black spots and a black hindwing border, the underside striated brown for camouflage against bark and dead leaves.
Caterpillars feed only on wild lilac and can strip whole hillsides of leaves when populations spike. Adults overwinter as adults, and in some years move out of the mountains in huge numbers, turning up far outside their usual range.
Traits
Habitat
Heath
Colour
Orange, Black, Brown
Active
Day
To people
Can appear in huge outbreak swarms; harmless to people.
Activity through the year
Often confused with
Has a small white mark on the underside of each hindwing that the California tortoiseshell lacks, and its upperside shows a wider dark margin rather than bright orange.
Questions
Is the California tortoiseshell dangerous to people?
Can appear in huge outbreak swarms; harmless to people.
When can you see the California tortoiseshell?
The California tortoiseshell can be seen from February to October, by day.
Plate retouched from a photo by David Anderson, CC BY 4.0, iNaturalist Licence
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Butterflies and moths›Brush-footed butterflies›Nymphalis
Last updated 2026-09-27. Every entry is checked against published sources.