TRUCKEE - At first, John White thought nothing of the colorful butterflies fluttering by him. But then they kept coming in waves over his backcountry camp north of Donner Summit.
"It's a show stopper. They're everywhere," said the Healdsburg man who's visiting the high country with his wife and two children.
Millions of California Tortoiseshell butterflies are heading south along the Sierra Nevada crest in what scientists are calling one of the insect's largest mass migrations in three decades.
Arthur M. Shapiro, biology professor at the University of California, Davis, said most embarked on the long journey from the Warner Mountains near the Oregon border and began reaching the Lake Tahoe area last week.
The insects - featuring orange-brown wings with large black spots - should reach Yosemite and Sequoia national parks in another two weeks, then descend to the Sierra foothills in late September, he said.
"It's one of the top five Tortoiseshell migrations in the last 33 years. There may be tens of millions of them," Shapiro said while scooping some with a net along the Pacific Crest Trail near Donner Summit.
"I've had people tell me it's a life-changing experience to see so many of them. They felt they were in heaven or some enchanted place," he added.
But while the butterflies are delighting nature lovers, they're annoying mountain motorists for a simple reason: They smear like butter and obscure vision when they hit windshields.
"Everybody has been getting hit. They splatter pretty good," said Carla Port of the Truckee Shell station.
"They're calling them suicide butterflies because they keep hitting the windows. We have plenty of squeegees and they've been getting a lot of use," she added.
In the 1970s, Tortoiseshell swarms prompted a closure of Interstate 80 in the Sierra due to safety concerns.
But Shapiro said he's unsure whether the density will be high enough to stop traffic this year.
"It's very hard to get a number for them because they're so unevenly distributed through space and time," he said. "We'll have to wait and see what happens."
Brian Olson, a California Highway Patrol dispatcher in Truckee, said the butterflies have caused inconveniences but no hazards so far.
"Our officers are seeing a lot of them but not of the magnitude to where they obscure vision," he said. "They're getting cars dirty if they go through the wrong area. But so far it hasn't endangered anyone."
Dennis Murphy, biology professor at the University of Nevada, Reno, said scientists think the region's dry weather could be a factor in this year's population boom of the insects.
"It's not predictable but it is an expected phenomenon," Murphy told the North Lake Tahoe Bonanza newspaper.
Other California butterflies that gather in large numbers are monarchs and the painted lady.
The Tortoiseshell ranges on the West Coast from British Columbia to Mexico, and to Montana, Wyoming, Colorado and New Mexico in the east.
Adults fly until fall, then hibernate over winter in the foothills. They spend most of their time in the summer above tree line to stay cool and conserve energy.
"The higher you go the more you see," Shapiro said, noting large concentrations of them on Castle Peak and Mount Lola north of Donner Summit.
"It was like being in the midst of a rust-colored snowstorm," his student, Sarah Thrasher, said after a weekend hike to Mount Lola.
