The cause of the strange 2019 butterfly invasion in Europe

For much of 2019, Europe was filled with migrating butterflies. Specifically, millions of butterflies of the species Vanessa cardui, known by popular names such as the cardera butterfly, generating an extraordinary natural spectacle that impacted the collective memory of many countries.

Now, an international team of scientists has managed to decipher the origin of this population explosion and how these butterflies spread across several continents.

The study, led by researchers from the Barcelona Botanical Institute (IBB), a joint center of the Higher Scientific Research Council (CSIC) and the Barcelona Natural Sciences Museum Consortium (CMCNB), analyzed the DNA of pollen grains that hundreds of butterflies they carried on their bodies, in 10 different countries and for 7 months in 2019.

Using massive sequencing or metabarcoding techniques, the research team, headed by Johanna Luise Gorki of the IBB, identified the pollen of up to 398 different species of plants, of which many were not found where the butterflies were collected, being these native to regions distant By studying the distributions of these plants, the origins and routes of the butterflies were identified. The presence of endemic plants from the Near East revealed the original focus of migrations in this region. This finding coincided, in turn, with hundreds of observations by citizens who described clouds of butterflies migrating from the eastern Mediterranean to eastern Europe.

To understand the possible mechanisms that led to a massive explosion of millions of butterflies, the researchers used satellite images. In the words of Roger López-Mañas, co-author of the study, “analyses of satellite images taken over the last 20 years allowed us to detect that vegetation growth levels at the beginning of 2019 were exceptionally high in the Middle East, covering green large expanses of desert. The butterflies must have taken advantage of this anomaly to lay eggs on these plants: they arrived at the right place at the right time.”

The cardinal butterfly (Vanessa cardui) migrates annually through several successive generations, completing migratory cycles between tropical Africa and Europe up to 15,000 kilometers away. The population explosion of 2019 had a cascading effect that was noticeable throughout Europe. In May, mass migrations were already observed in Scandinavia and Russia, and at the end of June, they were noticeable as they passed through the United Kingdom and the Pyrenees. Pollen analyzes showed that butterflies collected in Scandinavia had visited typically Mediterranean or Eastern European plants, while those collected in the Iberian Peninsula showed pollen from northern Europe. With this data, and through statistical and ecological modeling techniques, the researchers were able to generate probabilistic maps of the routes that the different generations took across the continent.

A newly emerged cardigan butterfly. (Photo: Gerard Talavera)

Tracking the migratory movements of organisms as small as insects entails great technical difficulty. However, these have great ecological importance as pollinators, for the transfer of biomass to soils, as food for other species, as possible agricultural or forestry pests, or as possible transmitters of parasites. However, these movements usually go unnoticed, and there are no established protocols to systematically monitor them.

This discovery exemplifies the potential impact that insect migrations can have, involving ecological processes across very broad regions, and even several continents. “Our results demonstrate the interconnectivity between our borders and ecosystems. The demographic explosions of insects that occur in one place can have consequences in other regions and times, due to their possible migration,” explains Gerard Talavera, co-author of the study, and adds: “The extraordinarily favorable conditions that the carder butterflies found for breeding in the Middle East at the beginning of 2019 had consequences in the Canary Islands in the month of October, receiving an exceptional migratory flow of these butterflies after several generations having traveled throughout Europe.”

The work also highlights the importance of citizen science, that is, the contributions of hundreds of amateur observers in many countries who recorded their findings on digital platforms. These data, analyzed as a whole, corroborated the researchers’ results obtained through pollen analyses, demonstrating the complementary value of the scientific method and citizen participation.

With this objective, the research team at the Barcelona Botanical Institute coordinates a citizen science program to collect data from observations of larvae and adult butterflies of migratory species worldwide through the Butterflymigration.org website. “Only through collective observation, coordinated between observers from different countries, will it be possible to obtain a complete vision of the phenomenon of migration,” concludes Gerard Talavera.

The study is titled “Pollen metabarcoding reveals the origin and multigenerational migratory pathway of an intercontinental-scale butterfly outbreak.” And it has been published in the academic journal Current Biology. (Source: IBB / CSIC / CMCNB)