Speyeria zerene sonomensis

Sonoma Zerene Fritillary

Family: Nymphalidae · Class: Insecta · Order: Lepidoptera

Conservation status: G5T1 S1

The Sonoma Zerene Fritillary (Speyeria zerene sonomensis) is a subspecies of the Zerene Fritillary butterfly endemic to a small region of northern California. Adults have a wingspan of 57-76 mm (2.2-3.0 inches), with males typically smaller than females. The dorsal wing surfaces are bright orange-red with black markings, including a distinctive pattern of black lines and spots. The ventral hindwings display the characteristic fritillary pattern of silver-white spots on a yellowish to reddish-brown background. Males can be distinguished from females by their narrower forewings and more pointed wing tips, while females are larger with broader wings and more extensive black markings. This subspecies has an extremely restricted range, historically occurring in the coastal prairie and grassland habitats of Sonoma County, California. The population is now confined to a few scattered locations in the southern portion of Sonoma County, representing a significant reduction from its historical distribution. The subspecies is distinguished from other Speyeria zerene populations by subtle morphological characteristics and its geographic isolation. Sonoma Zerene Fritillaries inhabit coastal prairie, native grasslands, and oak woodland edges at elevations typically below 300 meters (1,000 feet). The larvae feed exclusively on native violet species (Viola), particularly Viola adunca (hookedspur violet) and other indigenous violet species found in coastal California grasslands. Adults require open areas with abundant nectar sources and proximity to violet host plants for reproduction. Adults typically emerge in late spring to early summer, with flight periods generally occurring from May through July. Males establish territories and engage in patrolling behavior to locate females. After mating, females deposit eggs singly near violet host plants. The larvae feed on violet leaves and overwinter as partially grown caterpillars, completing development the following spring. Adults feed on nectar from various native wildflowers, including thistles, mints, and other composite flowers. The Sonoma Zerene Fritillary is ranked G5T1 S1, indicating it is critically imperiled in California and globally as a subspecies. The population has experienced severe decline due to habitat loss from agricultural conversion, urban development, and invasive plant species that have displaced native violets and nectar sources. Grazing practices and altered fire regimes have further degraded suitable habitat. The subspecies faces additional threats from climate change, which may shift suitable habitat conditions outside its current range. Conservation efforts focus on habitat restoration, invasive species control, and protection of remaining occupied sites. The small population size and restricted range make this subspecies particularly vulnerable to local extinctions and environmental stochasticity. Ongoing monitoring and habitat management are essential for preventing further population decline.

Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, and more.