Painted Lady mass migrations

I have just reported the first Painted Lady (Vanessa cardui) of the year to the Butterfly Conservation Painted Lady Survey. It settled today on my large buddleia, now covered in full bloom, and fed for a log time, fluttering from one bloom to the next (photo above). It has been a long wait considering the species has been on the news for a few months now. Painted ladies are strong flyers and migrate every year into nortern latitudes in search of foodplants and nectar, but migrations vary in their strength several orders of magnitude (see graph).

(From Stefanescu et al 2007)

Mass migration of butterflies occur every 6-10 years, as the last large migrations on this years scale in 1996 and 2003. The last couple of years have been quite poor. This year, in contrast, from a handful of sightings before May, thousands of migrating Painted Ladies were reported in the UK starting from southern England. This butterfly species overwinters in the semidesertic areas of North Africa but also of Mexico and Asia, as it is a cosmopolitan species. Depending of climate fluctuations such as El Nino, higher rainfall than usual in these regions make their larval foodplants - many, but mainly mallow and thistles - flourish, together with the butterflies. The newly emerged adults then start a northwards migration following their foodplants and nectar. Most records of migrating Painted Ladies are of individuals migrating in a given direction near the ground. However radar data of other migrant butterflies - the Monarch and the Red Admiral - documenting them migrating from 1000-3000 m high, sometimes rising with thermals and then gliding down with the aid of tail-winds towards their destinations, an energy-efficient and faster migration strategy than flying close to the ground. A recent study by Stefanescu and collaborators used an indirect approach to study how Painted Ladies migrate. They tested the idea that Painted Ladies migration from North Africa to southern Europe happens high up in the atmosphere aided by tailwinds. They tested whether butterfly peak records were associated to winds blowing from Africa - winds laden with Saharan dust are common in Spain in Spring. The data showed a clear effect of Northerly winds from North Africa in the sudden appearance of visibly migrating Painted Ladies in many localities of NE Spain. These results indicate that the butterflies use the wind to migrate more economically and doing so often flies high up over the ground. The flights close to the ground might reflect periods during the migration in which wind conditions are not favourable high up. The butterflies fly both by day and night and often over vast distances over the sea (there are records from fishing boats and oil rigs tens to hundreds of miles off the coast. Mysteriously, and contrasting with the spectacular spring migration, there is little evidence of the autumn return migration of the Painted lady -although the suden appearance of high numbers of butterflies in their winter grounds suggests that it does happen, as one could predict based on evolutionary grounds. There is hope that future analysis using radar might reveal if these butterflies return to Africa mostly flying high up, where their movements are difficult to observe and where they take advantage of fast winds. This is what Rebecca Nesbit, who is carrying research into the Painted Lady migration thinks could be the explanation.