Every fall, a narrow pass in the Pyrenees mountain range between southern France and northeastern Spain is flooded with a torrential stream of marmalade butterflies, pollinators with citrus-colored bodies and black stripes. The hoverflies avoid harsh mountain winds by flying close to the ground, making the sunlit streams look “almost like a river of golden light,” said Will Hawkes, an insect migration scientist at the Swiss Ornithological Institute. Flotillas of white and yellow butterflies hovering above the flies are more easily buffeted by the winds, swirling by the thousands across the walkway. “It’s almost like a storm, with all the whites and yellows,” Hawkes said.
Jam butterflies, butterflies and countless other insects all migrate south for the winter, some stopping in the warmer climes of Spain and others potentially going as far as sub-Saharan Africa. Bujaruelo Pass, which sits at an elevation of nearly 7,500 feet and is just under 100 feet wide, offers insects a more hospitable gateway to Spain than the surrounding peaks. But it is not a place of rest, without plants on which flies can feed and cold at night. So on the busiest days of migration, flying butterflies produce a humming noise—not the faint hum of a wandering bee in a garden, but a steady note. “They’ve got to go through, so it’s a real buzz set up,” Hawkes said.
This spectacular insect migration was first recorded in 1950 by married ornithologists David and Elizabeth Lack, who had come on a sort of honeymoon to observe how the little birds crossed the craggy peaks of the Pyrenees, which could reach 11,000 feet. Although few scholars visited the passage in the following years, nearly 70 years passed without any published study of the migration. In 2018, a group of researchers, including Hawkes, decided to change that. Their survey of four years of fall migrations was published Wednesday in the journal Proceedings of the Royal Society B.
Hawkes had become fascinated with migrating insects when he researched such a migration across the Alps as a student. When his PhD supervisor, Karl Wotton, another author on the paper, told him about Lacks’ paper in 2018, Hawkes jumped at the chance to do fieldwork in the passage that fall. The researchers wanted to do a systematic analysis of the migration to know how many and what kind of insects were involved. For large, slow butterflies, counting was somewhat straightforward. Every two hours during the day, Hawkes sat on a rock on one side of the pass and counted how many butterflies flew past him in 15 minutes – a swarm of cabbage white butterflies. Pieris rapaeand sullen yellow butterflies Colias croceus.
But it was impossible to count the vast majority of migrating insects by sight alone. They arrived in rippling floods, some of them only a few millimeters. On some days, the researchers observed more than 3,000 flies per meter per minute. To count these insects, all of which flew close to the ground to block the headwind, the researchers placed a smartphone camera in a waterproof box and placed it against a rock. Throughout the day, the phone made a one-minute video every 15 minutes.
While collecting the data was easy, extracting the data was a headache. Researchers tried to design a computer software or AI model to distinguish the insects from the background, but nothing worked, Hawkes said. “It ended up being the most efficient thing for me to sit down for a month and count the number of flies individually as they moved across the frame,” he said. “That was millions of flies.” But even this footage had fun surprises, acting as a camera trap to catch the various critters inadvertently caught on camera: wandering birds, curious birds and the occasional tourist urinating near the rock.
Since the video footage was not clear enough for the researchers to identify the flies moving forward, they set up a stationary net trap on the side of the passage. Migrating insects would fly into the net, get stuck, crawl to an opening, and drop into a bottle of ethanol. This was the only way for the scientists to collect a representative sample of their small samples, which Hawkes would identify at night. “We have recorded every single species of insect that migrates through this mountain pass, which has never been done before,” he said.
To estimate the large number of insects moving through the passage, the researchers would distinguish the ratio of groups of insects caught in the traps. If 20 percent of the insects in the trap were hoverflies, they assumed that 20 percent of the insects caught on camera were flies. Overall, researchers estimate that 17.1 million insects cross the Bujaruelo Pass each year, suggesting that billions of insects likely cross the Pyrenean mountain range each year.
Sometimes during the researchers’ visits, the Bujaruelo Pass seemed empty. The air seemed clean, without a single little immigrant. Still, when Hawkes waved his net over the edge of the passage, where the insects would come, it was filled with tiny flies. And as the butterflies disappeared after sunset, they were replaced by death’s-head hawks screeching through the corridors, carried away by the smell of honey they stole from the hives. Watching this relentless and purposeful journey of millions of insects always makes Hawkes feel humbled. “You feel like you’re observing something bigger and more important than yourself,” he said.
The researchers included groups of insects in their analysis only if they were seen more than 100 times. Some insects that didn’t make the cut included wasps, painted lady butterflies and hummingbird hawk moths. During the night, the researchers also observed turnip-tailed moths from their enemies, small parasitic wasps known to lay their eggs in turnip moth caterpillars. Hawkes believes that these rarer animals must still be migrants. “Otherwise why would they be up there?” he said.
Before Hawkes visited the pass in person, he expected butterflies and dragonflies to be the most frequent travelers, in part because Lacks’ paper estimated that hundreds of butterflies passed through the pass every hour, accompanied by a dizzying stream of dragonflies. These conspicuous insects “steal their prominent insect migration headlines,” joked Hawkes. But they accounted for only 2 percent of migration. But the standouts were clearly the flies, which made up 90 percent of all registered flyers. “That’s what was most exciting to me, because then it just opens up this other world,” Hawkes said.
Most pollinator research has focused on bees, relegating other pollinating insects to the condescending group of “non-bee pollinators.” But butterflies are extremely abundant pollinators known to visit at least 72 percent of global food crops, according to a 2020 paper. Adults feed on nectar and pollen, which they can carry extremely long distances during migration , even more than 62 miles across open water. Marmalade butterflies hatch in late summer and begin flying south when temperatures drop, flying with the winds and using the sun as a compass, Hawkes said. About 75 percent of migratory marmalades are female, often carrying sperm to their final destination to lay their eggs, which will grow to migrate back to the butterflies’ northern home over a series of generations. All together, these generations of humble flies transport nutrients, pollen and elements around the world. “If we didn’t think that flies migrate, then we’ve lost their entire ecological impact on the planet,” Hawkes said.
Although researchers could not directly compare the number of insects in transit with historical numbers more superficially recorded by absences, a study in the mountains of southwestern Germany found that populations of aphid-eating migratory flies had declined by an alarming 97 percent since 1970.” We can assume there will be a similar decline” in the Pyrenees, Hawkes said, adding that habitat loss, pesticide use and climate change all threaten populations of flies and other insects.
This view is far from ideal. But Hawkes hopes people will take an interest in these surprising and extraordinary migrants and make the world more welcoming on their journey – by planting wildflowers or putting pressure on local governments to protect the species. He noted that insects such as marmalade butterflies can lay thousands of eggs and reproduce throughout the year. “They can have a lot of babies, if they’re given the chance, if we provide the habitat for them, and then their numbers can grow again very quickly,” he said. “They are very resilient. We just have to give them the opportunity to do that.”