‘Massive migratory flight of millions’: Rare butterfly swarm visits Tahoe, Lassen

A massive wave of California tortoiseshell butterflies is moving through the Sierra Nevada, creating a spectacle near Lassen Volcanic National Park and the Lake Tahoe area. Residents and visitors report thousands of the orange-and-brown insects fluttering through the region, with some accounts describing the swarm as thick enough to be heard against window blinds and doors.

Biologists classify these insects as an irruptive species. This status means the population size fluctuates based on environmental factors like food availability and climate. Experts suggest the current boom is a direct result of the Dixie Fire in 2021. The fire cleared vast tracts of land, which allowed wild lilacs to flourish. Since these plants serve as the primary host for the larvae, the increased vegetation provided the resources necessary for a massive surge in the butterfly population.

While the butterflies share a similar appearance to monarchs, they follow a different life cycle. They migrate to higher elevations during the heat of the summer to escape the temperatures of the valley. In the fall, they move back to lower elevations to hibernate. Researchers at the University of Nevada, Reno, and other institutions noted the influx at various survey sites, marking this year as one of the more active seasons for the species.

This phenomenon serves as a reminder of how ecosystems recover after major events. Though the sound of wings against residential homes can be surprising, experts confirm the event is a natural occurrence rather than a cause for concern. For now, those near the Sierra Nevada can expect to see the insects as they continue their seasonal migration through the mountain ranges.