Vanessa cardui is having a big numbers year. Millions, perhaps billions of butterflies are migrating in multi-generational waves from the southern desert areas in Mexico and California northward as far as the Pacific Northwest.
The Painted Lady’s migratory cycle occurs each year. This year, because of a high volume of winter rain and plant growth in the western desert and in much of California, the migration continues to be remarkable with what appears to be multiples waves of butterflies.
The cycle begins with an overwintering population of adult butterflies in the desert. They wait for rain, mate, then the females lay eggs on sprouting host plants before they die. Lots of rain means more host plants, nectar plants and butterflies.
The heavy rains last winter nourished so much plant growth in the desert and along the butterfly’s migratory path that we have been hearing about, referring to and witnessing the wildflower superbloom since January. The butterflies are in a coinciding population boom. Abundant sustenance for the hatching larvae has made it possible for a countless number of them to develop into adults at different points along the migratory route.
As the butterflies pass through Ventura and Los Angeles counties, females nearing the end of their fat stores and lifespan mate and lay eggs on suitable plants in the mountains and open space. On recent hikes I have seen the Painted Lady caterpillars on several species of thistles, lupines and phacelia. All three are members of plant families that the caterpillars will eat. (The Painted Lady is a generalist when it comes to host plants. Some butterflies are much more specific. For example, the Monarch larvae will eat only milkweed.)
The next generation continues the migration northward until it reaches a northern destination. How does a migrating butterfly species know when to stop flying in a particular direction? Experts in the field are still conducting research to answer this question, so I will use a broad brush to venture my answer: Evolution. Millennia of practice, trial and error and natural selection for attributes that support the survival of a species.
In late summer, a future generation will begin the return trip southward, back to the southern points in the desert. In winter, the cycle begins again.
For more information about one of the most powerful and adaptive creatures in the world of Lepidoptera, visit Dr. Art Shapiro’s Vanessa cardui page. Dr. Shapiro is a professor of ecology and evolution at UC Davis. He has been studying butterfly population “trends” and migration for 34 years based upon data collected at specific monitoring sites in Central California.