Millions of insects migrate through the 30 meter high Pyrenees Pass

More than 17 million insects migrate every year through a single mountain pass on the border between France and Spain, new research shows. Scientists from the University of Exeter have studied migrating insects in the Pass of Bujaruelo, a 30-metre gap between two high peaks in the Pyrenees.

Their article, published in the magazine Proceedings of the Royal Society Bis entitled “The Most Remarkable Migrants – Systematic Analysis of the Western European Insect Flight Route at a Pyrenean Pass.”

The team visited the pass every fall for four years and observed the large number and variety of day-flying insects moving south. The findings from this single pass suggest that billions of insects cross the Pyrenees every year, making it an important location for many migratory species.

The migrating insects begin these journeys further north in Europe, including Britain.

“More than seventy years ago, two ornithologists – Elizabeth and David Lack – came across an incredible display of insect migration at Bujaruelo Pass,” says Will Hawkes of the Center for Ecology and Conservation at Exeter’s Penryn Campus in Cornwall.

“They witnessed remarkable numbers of marmalade hoverflies migrating through 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 taking place, and to determine the numbers, species and weather conditions and ecological roles and impacts of the migrants.”

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

“What we found was truly remarkable,” Hawkes continued. ‘Not only were huge numbers of marmalade hoverflies still migrating through the pass, but many more. These insects would have started their journey further north in Europe and continued south to Spain and perhaps beyond during the winter. There were some days when the number of flies was well over 3,000 individuals per meter, per minute.”

Team leader Dr. Karl Wotton said: “It is truly one of the great wonders of nature to see so many insects all moving purposefully in the same direction.”

Insect numbers peaked when conditions were warm, sunny and dry, with low wind speed and headwinds to keep the insects low over the pass so they could be counted.

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

A range of insects were seen, but flies made up 90% of the total. Butterflies and dragonflies are well-known migratory insects, but they make up less than 2% of the total.

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

Hawkes added: “It was magical. I swept my net through seemingly empty air and it was full of the smallest flies, all traveling on this incredibly huge migration.”

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

Nearly 90% of insects were pollinators, and by migrating they move genetic material over large distances between plant populations, improving plant health. Some insects were pest species, but many were pest controllers, including the marmalade and colorful hoverflies that eat aphids during their larval stages.

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.

Due to the climate crisis and habitat loss, these vital insect migrants are thought to be in decline.

Hawkes concluded: “By spreading knowledge of these remarkable migrants, we can spread interest and determination to protect their habitats. Insects are resilient and can recover quickly. Together we can protect these most remarkable migrants of all.”