‘It Blindsided Me’: Surprise as Massive Butterfly Swarm Reaches Northern California

The front end of a massive migrating butterfly swarm reached Northern California late last week, and the so-called painted ladies are flying through some unexpected places, including San Francisco’s Presidio National Park. The butterfly species, which travels from the Mojave Desert to the Pacific Northwest every spring, is experiencing a population explosion this year. As millions filled the skies over Southern California, the winged insects made news headlines and appeared all over social media.

The butterflies typically follow an inland route, sweeping the Central Valley. A small number of strays might veer toward the coast, but this year, a larger group migrated west and there are confirmed reports of migrating butterflies along the Great Highway at Ocean Beach, in the Presidio and in Bolinas.

Jonathan Young, a wildlife biologist at the Presidio Trust, says there are resident painted ladies in the Presidio who are often spotted feeding on nectar-rich flowers, but the migrating insects are different. They move quickly, up to 25 mph, in a northerly direction, and they usually don’t stop.

Young hasn’t conducted an official butterfly count, but he has seen hundreds in the park and around the Bay Area since Friday. In the eight years since he has worked in the park, he has never seen anything like this.

“There has definitely been a big spike in the past couple days,” Young said. “It’s not the millions they’re seeing in Southern California, but it’s definitely a migration. I’ve never heard of the masses come through the city itself.”

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Arthur Shapiro, a professor of evolution and ecology at UC Davis, is an expert on painted ladies and he’s baffled by their migration this year.

“Damned if I know,” he said. “It blindsided me. This year, they went to the west for whatever reason. The weather patterns have been very different so that might have impacted it. We had an unusually cold February, and we’ve only seen some warm weather in the last few days.”

Shapiro has seen hundreds of migrating painted ladies around the Davis campus, and he thinks there are still hundreds, probably millions, more to come.

“The numbers have remained low until yesterday, when I counted over 300 here,” Shapiro told SFGATE in an interview Wednesday. “I went out twice. In the morning, I counted one every 90 seconds. That sped up to one per minute in the afternoon. I haven’t seen any today due to the bad weather. I expect the flight will not peak until early next week.”

Shapiro assumes when the main swam arrives, it will pass through Kern and Inyo counties before moving up through the Central Valley.

Painted ladies are reputedly the most widespread butterfly in the world, and due to their orange and black coloring, they’re commonly mistaken for monarch butterflies.

Their population has multiplied this year because thriving plant life and blooming flowers from a wet winter provided a healthy food supply for the butterflies in the desert.

This is the largest migration of painted ladies since 2005, and Shapiro says the insects totaled around a billion that year. Scientists suspect this year’s migration to be in the millions.