Every Year, 17 Million Insects Stream Through One Narrow Mountain Pass

In a majestic and rarely observed sight, millions of migrating insects hum through a 30-meter-wide pass in the Pyrenees every autumn.

The massive flying convoy fascinated the University of Exeter ecologists who recently witnessed it. It also indicated that the broader region likely hosts billions of the migrating pollinators each year, a boon for ecosystems across Europe.

In the event, marmalade hoverflies (Espisyrphus balteatus) stream through the narrow Puerto de Bujaruelo like a hose on full blast.

“What we found was truly remarkable,” Will Hawkes, with the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall, said in a press release. “There were some days when the number of flies was well over 3,000 individuals per meter, per minute.”

The narrow conduit has also reportedly facilitated human travel since the Roman Empire. These days, it’s a pollinator superhighway at an altitude of 2,273m.

an arched bridge in pristine wilderness

The San Nicolas de Bujaruelo bridge, Monte Perdido National Park, Aragon, Spain. Photo: Shutterstock

Normally ‘invisible’

Hoverflies accounted for 90% of the flying species. Butterflies and dragonflies added up to 2% of the total — enough for Hawkes’ team to count by visual observation. Netting the bugs was the only way to come close to an accurate estimate of their overall numbers.

“You sweep your net, and it’s full of the tiniest of flies,” Hawkes told The Guardian, “all journeying on this unbelievably huge migration.”

The fall migration originated mainly from northern Europe, in locales as far north as the United Kingdom, the team said. It was anybody’s guess where the insects spent the winter. They could be headed to Valencia, Spain — where dozens of the continent’s butterfly species amass — or even further south.

butterfly perched on orange flower

A colorful Mediterranean fritillary butterfly (Argynnis pandora). Photo: Shutterstock


In the grand scheme of insect migrations, the action in the Puerto de Bujaruelo is fairly routine. But it’s usually impossible to experience at a close enough distance to make a perceptible impression. One 10-year study found 3.5 trillion aphids, moths, flies, and other insects migrated at high altitude over the southern United Kingdom every year.

But unless you can reach that high altitude yourself, through some means that allow you to take a look around, you’ll never scope the tiny travelers.

At the Puerto de Bujaruelo, on the other hand, conditions are perfect.

“The combination of high-altitude mountains and [low-speed] wind patterns render what is normally an invisible high-altitude migration into this incredibly rare spectacle observable at ground level,” said lead researcher Karl Wotton. “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.”

Insects on decline?

To select Puerto de Bujaruelo as a study location, Wotton’s team took cues from decades-old observations. Two bird biologists first recorded the sweeping migration in the 1950s. Curious whether it still existed despite reports of worldwide insect decline, the group carried out research over a four-year span.

They found species that play key roles in not only decomposition and pollination, but also pest control. (Hoverflies prey on the larvae of aphids, which can harm plants if their numbers get out of control). Every insect in the migration transports nutrients thaat soil and plants rely on, such as phosphorus and nitrogen.

A 2021 article in the journal PNAS likened global insect decline to “death by a thousand cuts.” CNN reported habitat loss is creating an “insect apocalypse” in some areas.

Wotton’s research team, meanwhile, said that other areas in the Pyrenees could see migrations similar to the one in the Puerto de Bujaruelo. If that’s the case, it would make the region a keystone for migratory insects across the continent — and even highlight their recovery.

Insects are resilient and can bounce back quickly,” Wotton said. “Together, we can protect these most remarkable migrants of all.”

Sam Anderson

Sam Anderson spent his 20s as an adventure rock climber, scampering throughout the western U.S., Mexico, and Thailand to scope out prime stone and great stories. Life on the road gradually transformed into a seat behind the keyboard, where he acted as a founding writer of the AllGear Digital Newsroom and earned 1,500+ bylines in four years on topics from pro rock climbing to slingshots and scientific breakthroughs.