California tortoiseshells (Nymphalis californica) are found in the conifer forests of Western North America. These butterflies overwinter as adults. In the northern, colder parts of their range the adults emerge in late May or June and have only one generation. Those that overwinter at lower elevations or warmer habitats, such as California’s Central Valley, have three or more generations in one season and are migratory. When California tortoiseshells in warmer areas emerge they often have a generation in place then move north, east and upslope for the summer, returning downslope in late September.
California tortoiseshell wings are yellow-orange with white bars and black bars, spots and borders dorsally. Ventrally the wings are a dark, mottled brown. The California tortoiseshell resembles a dead leaf when its wings are closed.
Caterpillars feed on ceanothus shrubs, especially snowbrush. They can occur in such large numbers that ceanothus plants over many acres are defoliated.
Adult California tortoiseshells visit many different flower species for nectar. They also eat aphid “honeydew”, sap and over-ripe or damaged fruit. Often, huge groups of these tortoiseshells can be seen at muddy areas or lake shores sipping water and minerals.
These specimens were part of a swarm of California tortoiseshells “feeding” on a wet shore at Manzanita Lake in Lassen Volcanic National Park.