NorCal, get ready for a rare mass migration of painted lady butterflies triggered by recent abundant rainfall
Hundreds of butterflies flew by Brian Brown in a matter of moments as he was standing in his front yard in Monrovia, Calif., on Tuesday. The steady stream of winged insects moved as a swarm.
“They were all going in a single direction and moving in a determined manner,” Brown said.
The butterfly species, which migrates from the Mojave desert to Oregon every spring, are experiencing a population explosion this year, and their fast-moving and massive swarm is expected to reach Northern California any day now.
“In 2005, we had a similar outbreak,” said Arthur Shapiro, a professor of evolution and ecology at UC Davis. “They arrived here on March 11. I thought it would have been great fun if they arrived here on March 11 again, but they didn’t. They should in theory get here this week.”
When they arrive, Shapiro says some might flutter over to the Bay Area, but he expects the main group to sweep the Central Valley, generally following 1-5 to Oregon.
Painted ladies are reputedly the most widespread butterfly in the world, says Brown, 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.
“I like to think of them as being like the plagues of locust that used to be around the world, except they eat plants we don’t care about,” said Brown. “They’re looking for what we consider weeds, and as they feed, they build up big populations.”
The butterflies move quickly, as fast as 25 mph, says Shapiro, and can go for days without stopping. Researchers once tracked a swarm that travel 350 miles from Lone Pine in the Eastern Sierra to Davis, and the butterflies made the journey in three days.
This is the largest migration of painted ladies since 2005, and Shapiro says the insects totaled around a billion that year. Shapiro suspects this year’s population is similar in size.