Millions of insects migrate through the 30-meter pass of the Pyrenees

New research shows that over 17 million insects migrate every year over a single mountain pass on the border between France and Spain. Researchers from the University of Exeter studied migrating insects in the Bujaruelo Pass, a 30-metre gap between two high peaks in the Pyrenees.

Their work, published in the journal Proceedings of the Royal Society Bentitled “The Most Remarkable Migrants – A Systematic Analysis of the Western European Passage of Insects in a Pyrenean Mountain Pass.”

The team visited the pass every autumn for four years and observed the huge number and variety of diurnal flying insects heading south. The findings of this single flyby suggest that billions of insects cross the Pyrenees each year, making it a key site for many migratory species.

Migrating insects begin these journeys further north in Europe, including the UK.

“More than 70 years ago, two ornithologists – Elizabeth and David Lack – had the chance to witness an incredible insect migration at Bujaruelo Pass,” said Will Hawkes of the Center for Ecology and Conservation at Exeter’s Penryn Campus in Cornwall.

“They witnessed a remarkable number of marmalade hoverflies migrating over the mountains, the first recorded case of fly migration in Europe. In 2018 we went to the same pass to see if this migration was still happening and to record the numbers, species and weather conditions . and the ecological roles and impacts of migrants.”

The researchers used a video camera to count small insects, visual counts to quantify butterflies, and a flight trap to identify migrating species.

“What we found was really remarkable,” Hawkes continued. “Not only were huge numbers of marmalade hoverflies still migrating through the pass, but many more. These insects would begin their journeys further north in Europe and continue south to Spain and possibly beyond for the winter. There were days when the number of flies was well over 3,000 individuals per meter per minute.”

Team leader Dr. Karl Wotton said: “To see so many insects moving deliberately in the same direction at once is truly one of the great wonders of nature.”

Insect counts peaked when conditions were warm, sunny and dry, with low wind speeds and headwinds to keep insects low over the pass for counting.

Dr. Wotton continued: “The combination of high altitude mountains and wind patterns makes what is normally an invisible migration from high altitudes to this incredibly rare spectacle observable at ground level.”

A variety of insects were observed, but flies accounted for 90% of the total. Butterflies and dragonflies are well-known migratory insects, but make up less than 2% of the total.

Many of the migrants were well-known garden dwellers, such as the cabbage white butterfly (Pieris rapae), the house fly (Musca autumnalis) and even tiny grass flies (Chloropidae), barely 3 mm long.

Hawkes added: “It was magical. I would sweep my net through seemingly empty air and it would be full of the tiniest of flies that are all going on this incredibly huge migration.”

These migratory insects, especially flies, are extremely important to our planet.

Almost 90% of insects were pollinators, and through migration they transfer genetic material over long distances between plant populations, improving plant health. Some of the insects were pest species, but many were pest controllers, including marmalades and pansies, which eat aphids during their larval stage.

Many play a role in decomposition, and all transport nutrients such as phosphorus and nitrogen over long distances, which could be important for soil health and plant growth.

As a result of the climate crisis and habitat loss, these vital insect migrants are thought to be in decline.

Hawkes concluded: “By spreading knowledge about these remarkable migrants, we can spread interest and resolve to protect their habitat. Insects are resilient and can bounce back quickly. Together we can protect these most remarkable migrants of all.”