Great insect migration to the Pyrenees

Every fall, a sweeping stream of hovering marmalade flies, pollinators with acid-colored bodies and black stripes, flood a narrow pass in the Pyrenees mountain range between southern France and northeastern Spain. The hovercraft avoids the mountain’s harsh headwinds by flying close to the ground, making the sunlit flurries appear “like a river-like golden light,” said Will Hawkes, an insect migration scientist at the Swiss Ornithological Institute. Fleets of white and yellow butterflies flitting overhead are easily buffeted by the wind, swirling by the thousands across the pass. “It’s almost like a blizzard, with all the whites and yellows,” Hawkes said.

Marmalade hoverflies, butterflies and countless other insects migrate south for the winter, some stopping off in the warmer climates of Spain and others potentially heading to sub-Saharan Africa. At about 7,500 feet in elevation and less than 100 feet wide, Bujaruelo Pass offers insects a more welcoming entrance into Spain than the surrounding peaks. But it is not a place to rest, as it is devoid of vegetation that flies feed on, and it is very cold at night. So, on the busiest migration days, hoverflies make a buzzing noise—not the buzz of a wandering bee in the garden, but a sustained tune. “They have to get through it, so it’s a real determined grunt,” Hawks said.

A marmalade fly pollinates a flower. | Will Hawkes

This amazing migration of insects was the first registered In 1950 by married ornithologists David and Elizabeth Luck, who reached the pass honeymoon Species to observe how small birds cross the rocky peaks of the Pyrenees, which can reach heights of 11,000 feet. Although a few researchers visited the corridor in the following years, nearly 70 years passed without any published studies of the migration. In 2018, a group of researchers, including Hawkes, decided to change that. Their four-year survey of fall migrations was published Wednesday in the journal Proceedings of the Royal Society B.

Hawkes became fascinated by migratory insects when he researched one such migration across the Alps as a student. When his PhD supervisor, Carl Wootton, another author on the paper, told him about Lax’s paper in 2018, Hawks jumped at the opportunity to do fieldwork in the fall of that year. The researchers wanted to conduct a systematic analysis of the migration to find out how many and what types of insects were involved. For large, slow butterflies, the counting was fairly straightforward. Every two hours during the day, Hawks would sit on a rock on one side of the trail and count the number of butterflies that flew past him in 15 minutes—a rush of cabbage-white butterflies. Peres RabeAnd cloudy yellow butterflies Colias Crusius.

But it was impossible to count the vast majority of migrating insects by sight alone. They arrive in floating deluges, some only a few millimeters long. On some days, researchers observed more than 3,000 flies per meter per minute. To count these insects, all of which flew close to the ground to avoid headwinds, the researchers placed a smartphone camera in a waterproof case and placed it facing a rock. Throughout the day, the phone was taking 1-minute videos every 15 minutes.

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Footage of flies migrating to the right, over a gravel road against a rock
Screenshots from a smartphone. | Will HawkesHawkes, et al. (2024)

While data collection was easy, data extraction was a headache. Hawkes said the researchers tried to design a computer program or artificial intelligence model to catch the insects from the background, but nothing worked. “Ultimately, what was most efficient for me was to sit down for about a month and count the flies individually as they moved across the frame,” he said. “That was millions of flies.” But even these shots had fun surprises, acting like a camera trap to capture various creatures inadvertently caught on camera: a wandering stoat, curious birds, and the occasional tourist urinating near the rock.

Because the video wasn’t clear enough for the researchers to actually identify the flies passing through, they set up a fixed mesh trap on the side of the trail. Migrating insects will fly into the net, get stuck, crawl toward the hole, and fall into the ethanol bottle. This was the only way the scientists could collect a representative sample of their small specimens, which Hawks could identify at night. “We have recorded every type of insect migrating through this mountain pass, which has never happened before,” he said.

Two researchers wave dip nets back and forth in a mountain valley to catch small migrating flies
Researchers catch small hovercraft. | Will Hawkes

To estimate the sheer number of insects moving through the passage, researchers will determine what proportion of insect populations are caught in traps. If 20% of the insects in the trap were hoverflies, they assumed that 20% of the insects captured on camera were hoverflies. Overall, researchers estimate that 17.1 million insects cross the Pujaruillo Pass each year, suggesting that billions of insects likely cross the Pyrenees mountain range each year.

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Sometimes, during researchers’ visits, Bujaruelo Pass seems empty. The air seemed clear, devoid of any young migrants. However, when Hawks threw his net at the edge of the passage, where the insects would climb, it was filled with tiny flies. As the hoverflies disappeared after sunset, they were replaced by dead-headed moths, fluttering across the corridor, smelling of the honey they had stolen from the beehives. Observing this purposeful, relentless journey of millions of insects always humbles Hawks. “You feel like you’re observing something bigger and more important than yourself,” he said.

Thin cloud of migrating hovercraft in a rocky valley in the Pyrenees
Will HawkesHawkes, et al. (2024)

The researchers included insect groups in their analysis only if they were seen more than 100 times. Some insects that didn’t make the cut include bumblebees, painted butterflies, and bumblebee butterflies. At night, researchers also observed turnip moths tracked by their enemies, which are small parasitic wasps known to lay their eggs in turnip moth larvae. Hawkes believes that these scattered animals must still be migrating. “Why else would they be there?” He said.

Before Hawks visited the pass in person, he expected butterflies and dragonflies to be the most abundant travelers, in part because Lax’s paper estimates that hundreds of butterflies pass through the pass every hour, accompanied by a dizzying stream of dragonflies. Hawks joked that these flashy insects “steal the headlines about the migration of shiny insects.” But they accounted for only 2% of the migration. But flies were clearly the most prominent category, accounting for 90% of all recorded fliers. “That was the most exciting thing for me, because then this other world opens up,” Hawks said.

Most pollinator research focuses on bees, relegating other pollinating insects to the transcendent group of “non-bee pollinators.” But hoverflies are extremely abundant pollinators and are known to visit at least 72 percent of global food crops, according to a recent report. 2020 paper. Adults feed on nectar and pollen, which they can carry extremely long distances during migration, up to more than 62 miles across open water. Marmalade bugs hatch in late summer and begin flying south when temperatures drop, flying with the wind and using the sun as a compass, Hawks said. About 75 percent of migrating marigolds are female, often carrying sperm to their final destination to lay their eggs, which will grow up to migrate back to the hoverfly’s northern habitat over a series of generations. Together, these generations of humble hoverflies transport nutrients, pollen, and elements around the world. “If we don’t really believe that flies migrate, we’ve missed all of their ecological impact on the planet,” Hawks said.

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A marmalade fly perches on a rock overlooking the Pyrenees
What wonders the journey of this little fly holds! | Will Hawkes

Although researchers could not directly compare the numbers of insects in the corridor to the historical numbers superficially recorded in the Lax River, a study conducted in the mountains of southwestern Germany found that the numbers of aphid-eating hoverflies had declined by an alarming 97 percent. Since 1970. “We can assume there will be a similar decline” in the Pyrenees, Hawks said, adding that habitat loss, pesticide use and climate change are all threatening populations of hoverflies and other insects.

This view is far from ideal. But Hawkes hopes people will care about these surprising and remarkable migrants and make the world more welcoming of their journey, by planting wildflowers or lobbying local governments to protect these species. He pointed out that insects such as the orange fly can lay thousands of eggs and reproduce throughout the year. “They can, if they have the opportunity, if we provide them with the habitat, they can have a large number of babies and then their numbers can increase again very quickly,” he said. “They’re very resilient. We just have to give them the opportunity to do it.”

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