Santa Cruz is in the midst of a rare butterfly frenzy

A thick blanket of orange and brown butterflies arrived in Bonny Doon on Mother’s Day, following the 2020 CZU Lightning Complex fires that devastated the Santa Cruz mountains. Residents report the insects have not fully left, with numbers fluctuating. The butterflies resemble monarchs but are California tortoiseshells, a species that migrates seasonally and can surge in some years when conditions align. Local ecologists describe the irruption as episodic and unusual, noting spring migration from lower to higher elevations timed with wild lilac budding for egg-laying. Tortoiseshell populations can explode irregularly across the West, and adults emerge in late May to early June before quickly emigrating north or east and upslope.

"On Mother’s Day, the thick blanket of orange and brown butterflies arrived in Shannon Robbins' garden in Bonny Doon, a community in the Santa Cruz mountains devastated by the CZU Lightning Complex fires in 2020. The winged insects have not left yet, though their numbers seem to ebb and flow. "It's very uplifting," Robbins, a landscape designer who has lived in Bonny Doon for two decades, told SFGATE. "Everyone here is so excited.""

"The butterflies resemble monarchs, a species on brink of extinction that shelters along California's coast every winter. Yet the visitors are in fact a more prosperous insect called California tortoiseshells (Nymphalis californica), whose sudden appearance in Santa Cruz came as a surprise to local ecologists, too. "The irruption of California tortoiseshell butterflies in Santa Cruz County is episodic and very unusual in my experience from 1986 to present," Grey Hayes, an ecologist who lives just north of Santa Cruz, wrote to SFGATE."

"Although residents have not seen swarms of tortoiseshells in Santa Cruz for years, spring always marks a seasonal migration for the species from lower to higher elevations. They time their travels with the budding of wild lilacs, where they lay their eggs. Tortoiseshell populations, found throughout the West, are known to explode some years when the conditions are just right."

""This is a mass migrant that makes news at irregular intervals by tying up traffic!" Arthur Shapiro, an emeritus professor at UC Davis who specializes in butterflies, wrote about tortoiseshells. He added: "Adults emerge in late May to early June and almost immediately emigrate, going north or east and upslope. Breeding localities in summer vary widely from year to year - sometimes in the high southern Sierra, sometimes in the""

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