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We were somewhere in the foothills east of Bakersfield when the first of the painted ladies showed up. 

This was the third week of March, on the way to Death Valley for what was being touted as the Bloom of the Century. Just before Route 58 started up the Kern River Canyon, Ron and I drove through a swarm of small orange butterflies. No way to identify them; I thought maybe California tortoiseshell, which were having a good flight year in the Coast Ranges. Next day at Furnace Creek, after encountering more of the same along the Badwater Road, I found a moribund painted lady (Vanessa cardui) in the parking lot. And then at Titus Canyon in the north end of the park, still more of them; a few stopped to nectar at a sweetbush with local checkerspots and blues, but most just kept moving. 

Then around the first of April, the wave of painted ladies hit the Bay Area. I got a secondhand report of orange butterflies in a residential neighborhood near Claremont Avenue. The East Bay Birders listserve erupted with painted lady sightings from El Cerrito, Castro Valley, Rossmoor, Martinez, Moraga, Mount Diablo. 

I spent most the morning of April 2 in a Zodiac in the Sacramento-San Joaquin Delta with a field biologist who was doing a songbird point-count in the marshes. From near dawn to noon the painted ladies streamed past, all headed north or northwest, sometimes pacing the boat. At one point we figured they were moving through at about one per second: mostly singles, once in a while two that seemed to be interacting.  

It rained the following day, a Sunday. But the sun came out on Monday, and the butterflies were still on the move. A friend spotted them in downtown Oakland; another called to say there were thousands moving through the UC campus. I saw them northbound along Martin Luther King in Berkeley, and the parallel side streets. When they came to an obstacle—a house, an apartment complex—they went up and over, not around it. That afternoon I watched a couple of them approach the south side of the Valley Life Sciences Building to within a foot or so, then fly straight up the face of the building and across the roof. 

Neither I or anyone I discussed it with had seen anything like this mass movement. But a little research determined that it wasn’t unprecedented. Painted ladies have a history of this kind of thing. 

In the spring of 1924, E. A. McGregor recorded a three-day flight through Southern California with densities of 300 butterflies per acre along a front at least 40 miles wide. Assuming 12 flight hours per day, McGregor calculated that at least 3 billion butterflies had moved through, all headed to the northwest. And I found references to similar flights in 1901, 1941, 1958, 1973, 1983, and 1998.  

Where were they coming from, and what had triggered the movements? Back in 1962 J. W. Tilden, who taught at what was then San Jose State College, provided tentative answers in an article in the Journal of Research on the Lepidoptera. He traced the 1958 outbreak to an origin in northern Mexico, where the cold-intolerant butterflies had spent the winter, and a final destination at least as far north as Oregon. What was going on appeared to be a multi-generational relay to the north, with no evidence of a southward return flight. Tilden thought the butterflies flew into the prevailing wind, although later research discounts that; they may orient by the sun and use polarized light to navigate. He couldn’t find a regular cycle to the movements but noted that “success in predicting these flights has been possible through knowledge of rainfall on the desert.” 

About 20 years later, M. T. Myres at the University of Calgary, along with reporting a major southbound movement through Alberta in the late summer of 1983, was the first to point out the coincidence between painted lady outbreaks and El Niño events. If heavy rainfall in the Mexican desert fostered the growth of vegetation that could feed a lot of painted lady caterpillars, that could explain the population peaks that seemed to set off the northward flights. The larvae aren’t too picky; they’ll consume thistles, fiddleneck, and a variety of plants in the mallow and pea families. Myres conceded that some flights, like those of 1901 and 1924, had occurred in non-El Nino years, but felt those could be explained by a phenomenon called the Namias-Sumner effect which could also dump a lot of rain on the deserts. 

Meanwhile, the British entomologist C. B. Williams had noticed that painted lady outbreaks in the British Isles often occurred in the same years as the North American movements. (The European flights originate in North Africa. Painted ladies are found on every continent except South America and Antarctica, and many oceanic islands. Stragglers have been recorded as far north as Iceland and Hudson Bay.) Myres suggested that the climatic triggers could be global. By 2002 enough observational data had been compiled—from stations at Berkeley and Mount Diablo, and elsewhere in the West—that Robert Vandenbosch at the University of Washington could crunch the numbers, and demonstrate a correlation between painted lady population cycles and both El Nino events and the longer-scale Pacific Decadal Oscillation.  

This was not supposed to have been a strong El Nino, but it was indisputably a wet one. And the rains that made the Southern California deserts bloom also seem to have produced that torrent of painted ladies. Iowa State University has a website (www.public.iastate.edu/~mariposa) where observers can post their sightings. It will be interesting to see how the rest of the year unfolds, how far north the travelers make it, and whether any of their descendants return to the deserts where it all began. ?