Butterfly Migration Mystery Solved
Painted Ladies are a small group of lovely butterflies (of the genus Vanessa) with a very wide distribution in much of the Northern Hemisphere. I would wager that many, if not, most of the yards in North America have been visited by at least one of the Vanessa species. The common Red Admiral also is in the genus. “The” Painted Lady (V. cardui) , found in North America, Europe and Asia may be the most widely distributed of all butterflies yet the details of its life cycle are still being worked out. It’s one of the relatively few butterfly species that undergo long-distance migration, a phenomenon more widely associated with the Monarch Butterfly.
In the United Kingdom, recent study has found that millions of V. cardui undergo a two-way migration across the English Channel to and from wintering grounds in Africa. Formerly, the northward trip to Great Britain was well known, but glimpses of the butterflies heading south over the Channel were rare. In the spring of 2009, however, with the help of radar monitoring of insect movements, millions of painted ladies were observed crossing the Channel and reaching the shores of the United Kingdom. It turns out that the butterflies fly south across the Channel at altitudes around 500m (1650 ft) where favorable winds help them in their migration. The numbers involved are impressive- an estimated 11 million painted ladies reached the United Kingdom via Channel overflights and about 26 million departed for Africa. Thus the U.K. is a net source of these butterflies rather than a net “sink” as some previously opined.
In North America, the same species occurs year-round in warm areas like the Sonoran Desert but only in the warm months in colder regions. Migration of the Painted Lady is more sporadic than that of its congener the Red Admiral (V. atalanta) and that of the Monarch which have more or less regular seasonal migrations. In fact, Painted Ladies may not migrate every year and the number of butterflies involved fluctuates greatly. Variable climatic conditions typify their habitats and climatic events like el Nino are associated with larger migrations. During an el Nino event, (which occur every 3 to 7 years and lasts 12 to 18 months), arid areas where most Painted Ladies reside are wetter than average. The deserts respond with heavy plant growth and flowering which stimulates butterfly reproduction and caterpillar development. However food supply can be depleted, driving outward migration. Although this is risky and many butterflies perish, the broad range of food plants suitable for the larval diet assures that many butterflies will reach adulthood. Southward migration occurs at a level sufficient to maintain the migratory cycle and get enough adults back to the source areas to maintain adequate numbers of butterflies. This return migration often is not as conspicuous to observers as are the northward movements. Note that the other widely distributed Vanessa species, the American Painted Lady (V. virginiensis) is the most cold-hardy of the group and can overwinter in the northern states. Its migration is smaller than the other two species. There are additional species in the genus that occur in smaller areas- including V. annabella, which is restricted to the Pacific Slope of North America and V. tameamea in Hawaii.
The following were among the sites reviewed for this post: www.vanessa.ent.iastate.edu; scim.ag/PaintedLady