Scientists discover massive insect migration through 30-meter Pyrenees pass

Sunset over the Pyrenees. Credit: Will Hawkes

Every year, over 17 million insects migrate through a narrow 30-meter pass in the Pyrenees mountains, situated between France and Spain.

This remarkable discovery was made by researchers from the University of Exeter, who studied the insect migration at the Pass of Bujaruelo over four years.

Their findings, published in the journal Proceedings of the Royal Society B, highlight the importance of this pass for migrating insects.

The team monitored the pass each autumn, observing a vast number and variety of insects traveling south.

Their research suggests that billions of insects cross the Pyrenees annually, making this pass a crucial route for many species that start their journeys further north in Europe, including the UK.

“More than 70 years ago, ornithologists Elizabeth and David Lack first witnessed the incredible insect migration at the Pass of Bujaruelo,” said Will Hawkes from the Centre for Ecology and Conservation on Exeter’s Penryn Campus.

“They observed large numbers of marmalade hoverflies migrating through the mountains, marking the first recorded instance of fly migration in Europe. In 2018, we revisited the pass to see if this migration still occurred and to document the numbers, species, weather conditions, and ecological impacts of the migrants.”

The researchers used various methods to count and identify the insects, including video cameras for small insects, visual counts for butterflies, and a flight intercept trap for species identification.

“What we found was truly remarkable,” Hawkes continued. “Not only were vast numbers of marmalade hoverflies still migrating, but we also observed many other species. On some days, more than 3,000 flies per meter per minute were passing through the gap.”

Dr. Karl Wotton, the team leader, added, “Seeing so many insects moving in the same direction at the same time is one of nature’s great wonders.” Insect numbers peaked when the weather was warm, sunny, and dry, with low wind speeds and a headwind that kept the insects low and visible.

Flies made up 90% of the total insects observed, with butterflies and dragonflies comprising less than 2%. Many of the migrants were familiar garden insects such as the cabbage white butterfly and the house fly. Hawkes described the experience as magical, saying, “I would sweep my net through seemingly empty air and it would be full of the tiniest flies, all on this enormous migration.”

These migrating insects play essential roles in our ecosystem. Nearly 90% are pollinators, helping to spread genetic material between plants and improving plant health. Some insects are pest controllers, like the marmalade hoverfly, whose larvae eat aphids. Others help in decomposition and transport nutrients like phosphorus and nitrogen, which are vital for soil health and plant growth.

Due to climate change and habitat loss, these vital insect migrations are believed to be declining. Hawkes emphasized the importance of protecting these insects and their habitats, saying, “Insects are resilient and can bounce back quickly. By spreading knowledge about these remarkable migrants, we can foster interest and determination to protect them.”

The study highlights the incredible journey of these insects and underscores the need for conservation efforts to ensure their survival.