Massive butterfly migration sparked by a surge in vegetation

In 2019, Europe witnessed an extraordinary natural phenomenon: a massive butterfly migration involving millions of painted lady butterflies (Vanessa cardui) migrating across the continent. This impressive sight left a lasting impression on many countries.

Now, an international team of researchers has unraveled the origin of this demographic explosion and how these butterflies spread across several continents.

Mystery of butterfly migrations

Researchers from the Botanical Institute of Barcelona (IBB), a joint center of the Spanish National Research Council and the Cerdanyola del Vallès Campus of the Autonomous University of Barcelona, led a remarkable study analyzing the DNA of pollen grains carried by hundreds of butterflies across 10 different countries over seven months in 2019.

Using mass DNA sequencing techniques, known as metabarcoding, the team identified pollen from up to 398 different plant species. Many of these plants were native to distant regions, revealing the butterflies’ origins and migration routes.

The presence of plants endemic to the Middle East indicated that the migrations began in this region. The theory aligned with numerous citizen observations of butterfly clouds migrating from the eastern Mediterranean to Eastern Europe.

Role of satellite imagery

To understand the mechanisms behind this massive butterfly migration, the researchers turned to satellite imagery.

“The analysis of satellite images taken over the past 20 years allowed us to detect that vegetation growth levels in early 2019 were exceptionally high in the Middle East, covering large desert areas in green,” noted Roger López-Mañas, co-first author of the study.

“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.”

Painted lady’s epic journey

The painted lady butterfly migrates annually through successive generations, completing migratory cycles between tropical Africa and Europe, covering up to 15,000 kilometers.

The demographic explosion of 2019 had a cascading effect throughout Europe. By May, massive butterfly migrations were observed in Scandinavia and Russia, and by the end of June, they reached the United Kingdom and the Pyrenees.

Pollen analysis showed that butterflies collected in Scandinavia had visited flowers from Mediterranean or Eastern European plants, while those collected in the Iberian Peninsula showed pollen from Northern Europe.

Using this data, the researchers generated probabilistic maps of the butterflies’ routes across the continent.

Importance of monitoring insect migration

Tracking the migratory movements of insects poses significant technical challenges. However, these movements are ecologically important as insects act as pollinators, biomass transfer agents to soils, food for other species, potential agricultural or forest pests, and possible transmitters of parasites.

Despite their importance, migratory movements often go unnoticed, and there are no established protocols for systematically monitoring them.

This discovery highlights the ecological impact of insect migrations, involving processes across vast regions and even continents.

“Our results demonstrate the interconnectedness of our borders and ecosystems. Insect population explosions in one place can have consequences in other regions and times due to their migration,” explained study co-author Gerard Talavera.

“The extraordinarily favorable conditions found by the painted lady butterflies for breeding in the Middle East in early 2019 had repercussions in the Canary Islands in October, with an exceptional migratory flow of these butterflies after several generations having traveled across Europe.”

Citizen science: A key to understanding migration

The study also underscores the importance of citizen science. Contributions from hundreds of observers in many countries, who recorded their findings on digital platforms, corroborated the researchers’ results obtained through pollen analysis.

The team from the Botanical Institute of Barcelona coordinates a citizen science program to collect data on observations of larvae and adult migratory butterflies worldwide through the website www.butterflymigration.org.

“Only through collective observation, coordinated among observers from different countries, will it be possible to obtain a complete understanding of the migration phenomenon,” said Talavera.

The remarkable 2019 butterfly migration

The 2019 butterfly migration was a remarkable event that showcased the intricate connections between ecosystems and highlighted the importance of monitoring and understanding insect migrations.

The collaborative efforts of researchers and citizen scientists have provided valuable insights into this natural spectacle, emphasizing the need for continued observation and study.

The full study was published in the journal Current Biology.

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