A native butterfly that most Californians can't name is having the kind of year monarchs no longer do: California tortoiseshells are erupting by the millions across the Sierra Nevada, coating Lassen Volcanic National Park and drifting through the Lake Tahoe basin in waves thick enough, in past outbreaks, to stop highway traffic.

The orange-and-brown insects — Nymphalis californica, or "torties" — are staging one of their irregular population explosions, a boom-and-bust phenomenon that scientists have documented in the Sierra since at least 1932 but still cannot predict from one year to the next. The 2026 surge has concentrated in the northern Sierra and southern Cascades, with the densest clouds massing around Lassen and thinner ribbons reaching south into the Tahoe basin. The spectacle is a study in contrast with the monarch, the tortoiseshell's famous orange cousin, whose overwintering population has cratered toward extinction even as this humbler native briefly blankets the mountains.

Anyone driving the mountain roads toward Lassen Volcanic National Park this week has been passing through clouds of butterflies. Regional biologists reported that California tortoiseshells are swarming the park, with smaller numbers reaching the Lake Tahoe area, SFGATE first reported on July 30.

"Driving up the mountain toward Lassen National Park, you just get these waves and waves of butterflies coming through, and it's just incredibly spectacular," Rachael Long, a retired entomologist and emeritus adviser for the University of California Cooperative Extension, told SFGATE. "It's just absolutely stunning to see thousands and thousands of butterflies flying past you." Long described the scene near Lake Almanor, a short drive from the national park.

The event is spectacular but not unprecedented. The California tortoiseshell is a native butterfly "known for having irregular population explosions," according to the Field Guide to California Insects, published by the University of California Press. In outbreak years, the swarms can reach almost incomprehensible scale. The National Park Service records that in late July to early August 2004, "a swarm of Tortoiseshells 40 to 50 miles long by 15 miles wide traveled south along the Sierra Nevadas." Comparable booms hit in 1932, 2019 and 2024, the agency notes — and in strong years, "unsuspecting hikers are stunned by clouds of tortoiseshells in Lassen Volcanic and especially on Lassen Peak."

What drives the cycle is the interplay between the butterfly and a single group of shrubs. Tortoiseshell caterpillars feed on Ceanothus — the wild lilac that carpets California's chaparral and mountain slopes. When weather favors both the host plants and larval survival, the population can multiply explosively within a season, then crash. Arthur Shapiro, a UC Davis emeritus professor who has monitored the state's butterflies for decades, cautioned that the outbreaks defy forecasting. "No two years are identical in terms of where they go precisely and the numbers involved," he told SFGATE.

The boom is a fleeting bright spot against a darker backdrop for California's more celebrated butterfly. The torties are frequently mistaken for monarchs, which they superficially resemble — both are orange with dark markings. But the two species are on opposite trajectories. Western monarchs, which cluster in eucalyptus and cypress groves along the coast each winter, have collapsed to a small fraction of their historic numbers and are candidates for federal Endangered Species Act protection. The tortoiseshell, by contrast, is a boom-bust generalist whose numbers can rebound in a single good year.

That resilience comes with its own hazards for the insects. Ravens have learned to exploit the swarms, plucking adult tortoiseshells out of the air during outbreak years, according to research published in Northwestern Naturalist. The butterflies that survive will disperse, mate and lay the next generation's eggs on the same mountain shrubs that fueled this year's eruption — setting up the possibility of another wave, on a schedule no one can reliably predict.

For now, the show is worth catching while it lasts. The densest swarms are concentrated around Lassen, a roughly three-and-a-half-hour drive from the Bay Area, with thinner ribbons of butterflies moving through the Tahoe basin. As Long put it: "It's an extraordinary feeling and sight to witness."