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If you were paying attention last month, you may have noticed a number of small pale-orange butterflies flying in a northwesterly direction. That would have been the painted lady migration, a not-quite-annual phenomenon that sometimes blankets the state. In peak years, numbers have been estimated in the billions. 

The painted lady (Vanessa cardui) also stages mass migrations in Europe. Two other closely related species, the American lady (V. virginiensis) and the West Coast lady (V. annabella), are strictly North American, and non-migratory. Distinguishing the three can be tricky, but the painted lady tends to be larger with a pinkish tinge on the upper wing surface. Another form, the Kamehameha lady (V. tameamea), is endemic to Hawaii.  

The ladies that were moving through the Bay Area originated in the deserts of southeastern California and northern Mexico. The great migratory flights appear to coincide with El Nino events, when heavy rainfall promotes the growth of the caterpillar’s food plants—a broad range including mallows, pea relatives, thistles, and fiddlenecks. Some outbreaks have occurred in non-El Nino years, though. 

There’s no way to get an accurate count of the fliers, but we have a back-of-the-envelope calculation from E. A. McGregor based on the migration of 1924. During a three-day passage in southern California, McGregor saw densities of 300 ladies per acre along a 40-mile front. Assuming twelve flight hours per day, he estimated a total of 3 billion butterflies. 

According to Art Shapiro at UC Davis, the last big movement, in the spring of 2005, may have been that large. He saw them moving through Sacramento at the rate of three per second. I encountered the ladies first in Death Valley, where Ron and I had gone to see the Big Bloom. By the time we got back to Berkeley they were beginning to show up in the Bay Area, and kept coming through for at least a week. 

This year I started seeing them locally in late March. Shapiro got reports of significant numbers at Mammoth Lakes in the Sierra on March 15. That wave reached the Davis-Sacramento area on St. Patrick’s Day. A second front, probably also trans-Sierran migrants, was observed at San Jose beginning March 24. 

By the end of the month, there was more traffic along the coast than inland. Then another surge came up the Central Valley around April 2. About the time the coast/valley flights began to taper off, there was a passage along the east slope of the Sierra.  

Some migrants may get as far as Oregon. But Shapiro says numbers dropped out of the migration in the Valley, having burned through the fat reserves they stockpiled as larvae. As long as the fat lasts, painted ladies don’t eat or mate; they just keep moving.  

The dropouts begin nectaring, and males set up courtship territories, typically in west- or southwest-facing sites with a vertical backdrop. Several males may share a display site, a behavior called lekking. Mating takes place up in a tree and is rarely observed.  

As with monarchs, the generation that breeds here will go no further north. But their offspring may. At the end of summer there’s a more leisurely movement south, often stronger east of the Sierra, culminating in a new desert generation.  

The whole butterfly developmental thing is quietly mind-boggling. Not long ago I found myself having to explain complex metamorphosis to a third-grader. “Why do they do that?” he asked. And caught me flat-footed. There must be an evolutionary advantage to going through larval, pupal, and adult stages, because the most successful groups of insects all do it.  

But think about the genetics that have to be involved. An aphid hatches into the world as a recognizable aphid. If it changes at all, it’s just to become a bigger aphid. So it’s intuitive to think of a genetic program for making an aphid. But there can’t be just one recipe for a painted lady. Each life stage would have to have its own set of instructions, switched on at the appropriate time. 

And then consider how evolution would work in such a creature. We think of natural selection as affecting reproductive adults—the stage that can produce more copies of itself. For butterflies (or beetles or bees), though, traits that promote survival would be selected at every stage, even though only the final stage can reproduce. Think of the stinging hairs of some moth caterpillars, or the cryptic appearance of some butterfly chrysalids. If the insect doesn’t survive through its nonreproductive stages, it’s never going to have a chance to demonstrate its Darwinian fitness. 

I hope some of the evo-devo folks are working on all this. In the meantime, it’s good to remember that there’s nothing simple about a butterfly.