LAKE TAHOE — Lake Tahoe is always beautiful, but if you visit right now, you may get an unexpected bonus: thousands upon thousands of bright orange butterflies gliding gracefully through the air.
This summer, scientists and delighted hikers are noting millions of California Tortoiseshell butterflies, black and orange butterflies native to the Sierra Nevada region that resemble monarch butterflies. The largest numbers are reported around Lassen Volcanic National Park, while smaller swarms are reported around the Lake Tahoe area, entomologist Rachael Long told SFGATE.
"It's just absolutely stunning to see thousands and thousands of butterflies flying past you. It's an extraordinary feeling and sight to witness," she said.
The current explosion, which Long estimates comprises millions of butterflies, is due to several favorable conditions. Tortoiseshell caterpillars eat ceanothus shrubs, more commonly known as the California lilac. Large fires like the CZU Lightning Complex, the Dixie Fire, and the Angora Fire created burn scars, where ceanothus flourishes
Earlier this spring, large numbers of California tortoiseshell butterflies were reported throughout the Santa Cruz Mountains and Marin County. That boom of butterflies benefited from warm, dry weather and abundant food, allowing many butterflies to successfully reproduce. Their offspring have matured and flown uphill into the Sierra, where cooler temperatures and more abundant ceanothus support the next stage of their life cycle.
This year has seen above-average numbers, but the last major invasion occurred in 2019, when several visitors noticed exceptional numbers of butterflies across Lake Tahoe. Researchers say the Angora Fire burn scar has since become rich in ceanothus, creating ideal habitat for tortoiseshell caterpillars, according to the Tahoe Daily Tribune.
These butterfly population explosions tend to be unpredictable.
"Although the rhythm of seasonal migration is constant, the number of individuals vary dramatically and not in a predictable cycle," according to the National Park Service. "Some years, unsuspecting hikers are stunned by clouds of tortoiseshells in Lassen Volcanic and especially on Lassen Peak. Other years, the butterfly is no more prevalent than other species found within the park."
More than 20 years ago, the region saw one of its largest populations in years.
"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," the National Park Service noted.
The park service asks visitors to take it slow to see and protect the butterfly migration.
"In the event you are lucky enough to witness a population explosion or mass migration, you can enjoy the sight and protect the butterflies by traveling under 30 mph. A slower speed allows the butterflies to drift over your vehicle instead of decorating your vehicle's grill or adding a large, yellow splat to your windshield."
A swarm of insects is not often positive news, but scientists say this is nothing to worry about. "It's not a problem. It's not, you know, strange. But it's impressive, and it's cool to see," University of Nevada biologist Matthew Forister told SFGATE.
The butterflies will soon begin a new stage.
In late September, the butterflies will migrate downslope aback to the foothills to hibernate. In late winter, they will awaken to start the cycle again.
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