Long-distance autumn migration across the Sahara by painted lady butterflies: exploiting resource pulses in the tropical savannah

Abstract

The painted lady, Vanessa cardui , is a migratory butterfly that performs an annual multi-generational migration between Europe and North Africa. Its seasonal appearance south of the Sahara in autumn is well known and has led to the suggestion that it results from extremely long migratory flights by European butterflies to seasonally exploit the Sahel and the tropical savannah. However, this possibility has remained unproven. Here, we analyse the isotopic composition of butterflies from seven European and seven African countries to provide new support for this hypothesis. Each individual was assigned a geographical natal origin, based on its wing stable hydrogen isotope (δ 2 H w ) value and a predicted δ 2 H w basemap for Europe and northern Africa. Natal assignments of autumn migrants collected south of the Sahara confirmed long-distance movements (of 4000 km or more) starting in Europe. Samples from Maghreb revealed a mixed origin of migrants, with most individuals with a European origin, but others having originated in the Sahel. Therefore, autumn movements are not only directed to northwestern Africa, but also include southward and northward flights across the Sahara. Through this remarkable behaviour, the productive but highly seasonal region south of the Sahara is incorporated into the migratory circuit of V. cardui .

Back to Africa: autumn migration of the painted lady butterfly Vanessa cardui is timed to coincide with an increase in resource availability

“…Each year, the painted lady butterfly Vanessa cardui (Linnaeus) undertakes a multi‐generational migration between Africa and Europe, which enables it to track and exploit abundant but ephemeral resources over a vast latitudinal gradient (Stefanescu et al ., ; Talavera & Vila, ). The migratory circuit in the Western Palaearctic consists of a general northward movement from northern Africa to Europe in spring, followed by a reverse southward movement of the population at the end of summer and in autumn, as has been confirmed by field and radar observations, experimental data, and stable isotope markers (Nesbit et al ., ; Chapman et al ., ; Stefanescu et al ., , , ; Hu et al ., ). From September–October to March–April, V. cardui is recorded continuously in northwest (NW) Africa and the Canary Islands, where it produces up to three consecutive generations (Wiemers, ; Stefanescu et al ., ).…”

Matching geographical assignment by stable isotopes with African non-breeding sites of barn swallows Hirundo rustica tracked by geolocation

“…Today this approach is frequently used especially in the study of long-distance migratory animals whose distribution during parts of the annual cycle remains unknown so far. Prominent examples include diverse animal classes like insects [13, 14], mammals [15] and most frequently birds [16]. …”

Migration distance as a selective episode for wing morphology in a migratory insect

“…Since stable isotopes in the chitin of wing tissue of adult flying insects is metabolically inert after formation [15, 19, 20], stable isotope measurements can be used to identify the geographic natal origin of adults regardless of how far they have flown from the natal site [21]. Stable isotope measurements have been used successfully to determine natal origin of migratory insects such as butterflies [22–24] and dragonflies [21]. …”