‘One of the Great Wonders of Nature’: Migration of 17.1 Million Insects Through Pyrenees Mountain Pass Witnessed by Scientists

Birds, fish and caribou are all known for their long migrations, but did you know insects migrate too?

According to a new study, each year 17.1 million insects move through a 98.4-foot gap in the Pass of Bujaruelo between two Pyrenees peaks on the border between Spain and France.

For four years, a team of scientists from the University of Exeter visited the pass to monitor the extensive number of species of day-flying insects on their way south.

“In autumn 1950 David and Elizabeth Lack chanced upon a huge migration of insects and birds flying through the Pyrenean Pass of Bujaruelo, from France into Spain, later describing the spectacle as combining both grandeur and novelty. The intervening years have seen many changes to land use and climate, posing the question as to the current status of this migratory phenomenon,” the authors wrote in the study. “Numbers at this single site hint at the likely billions of insects crossing the entire Pyrenean mountain range each year, and we highlight the importance of this route for seasonal insect migrants.”

The insects migrating along this well-established route start their journeys in the United Kingdom and other parts of northern Europe.

“[Elizabeth and David Lack] witnessed remarkable numbers of marmalade hoverflies migrating through the mountains, the first recorded instance of fly migration in Europe. In 2018, we went to the same pass to see if this migration still occurred, and to record the numbers, species, weather conditions and ecological roles and impacts of the migrants,” said Will Hawkes, lead author of the study and a postdoctoral researcher with the University of Exeter’s Centre for Ecology and Conservation, in a press release from the University of Exeter.

17 MILLION insects migrate through a 30m Pyrenean mountain pass every autumn!

They fly through the mountains on their remarkable journeys from northern Europe (including the UK) into Spain and beyond for the winter!

Our new paper is out right now: https://t.co/yQOEzoWxGC

🧵 pic.twitter.com/MFPf59XCDm

— Dr Will Leo Hawkes (@WillLeoHawkes) June 12, 2024

The researchers did counts of the small insects using a video camera and used visual counts for butterflies. To identify migrating species, they used a flight intercept trap.

“What we found was truly remarkable,” Hawkes noted. “Not only were vast numbers of marmalade hoverflies still migrating through the pass, but far more besides. These insects would have begun their journeys further north in Europe and continued south into Spain and perhaps beyond for the winter. There were some days when the number of flies was well over 3,000 individuals per metre, per minute.”

The study, “The most remarkable migrants — systematic analysis of the Western European insect flyway at a Pyrenean mountain pass,” was published in the journal Proceedings of the Royal Society B.

“To see so many insects all moving purposefully in the same direction at the same time is truly one of the great wonders of nature,” said Dr. Karl Wotton, team leader and co-author of the study, in the press release.

Insect numbers peaked during sunny, dry and warm conditions when wind speed was low and there was a headwind. This kept the insects flying low over the pass, making them easier to count.

“The combination of high-altitude mountains and wind patterns render what is normally an invisible high-altitude migration into this incredibly rare spectacle observable at ground level,” Wotton explained.

Most of the total number of insects — 90 percent — were flies, though a range of species was observed. While dragonflies and butterflies are well-known migratory species, they accounted for less than two percent. Many of the migrating insects were known garden dwellers like house flies, tiny grass flies and the cabbage white butterfly.

“It was magical. I would sweep my net through seemingly empty air and it would be full of the tiniest of flies, all journeying on this unbelievably huge migration,” Hawkes observed.

Almost 90 percent of the migrators were pollinators who carry genetic material long distances by migrating between plant populations, which improves plant health. They also transport nitrogen, phosphorus and other nutrients and assist with decomposition.

Many of the insects were “pest controllers,” such as pied and marmalade hoverflies who feed on larval aphids.

These important migrating insects are believed to be declining due to habitat loss and climate change.

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