Every fall, a narrow pass in the Pyrenees between southern France and northeastern Spain is inundated with a rush of marmalade anemones, pollinators with citrus-colored bodies and black stripes. Warblers avoid the mountain’s harsh headwinds by flying close to the ground, so the sun-drenched flurries look “almost like a river like golden light,” said Will Hawkes, an insect migration scientist at the Swiss Ornithological Institute. Fleets of white and yellow butterflies hovering above the flies are more easily struck by the winds that swirl in thousands through 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 in the warmer climates of Spain and others potentially heading as far as sub-Saharan Africa. At nearly 7,500 feet and just under 100 feet wide, the Bujaruelo Pass offers a more hospitable gateway to Spain for the bugs than the surrounding peaks. But it is not a resting place, devoid of plants for flies to feed on and bitterly cold at night. So on the busiest days of the migration, the swirling hovercraft make a buzzing sound—not the idle hum of a bee roaming the garden, but a sustained tone. “They have to pass, so it’s a really determined hum,” Hawkes said.
This spectacular insect migration was first recorded in 1950 by married ornithologists David and Elizabeth Lack, who came to the pass on their honeymoon to watch the tiny birds cross the rocky peaks of the Pyrenees, which reach heights of up to 11,000 feet. Although few researchers visited the pass in the years that followed, nearly 70 years passed without any published studies on the migration. In 2018, a group of researchers, including Hawkes, set out to change that. Their survey of four years of fall migration was published Wednesday in the journal Proceedings of the Royal Society B.
Hawkes became fascinated with migrating insects when he studied one such migration across the Alps as a student. When his supervising doctoral student Karl Wotton, another author of the paper, told him about the Lacks’ work in 2018, Hawkes jumped at the opportunity to conduct fieldwork at the pass that fall. The scientists wanted to do a systematic analysis of the migration to know how many and what kinds of insects were involved. For the large and slow butterflies, the counting was rather simple. 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 flurry of white butterflies. Pieris rapaeand cloudy yellow butterflies Colias croceus.
But it was impossible to count the vast majority of migrating insects by sight alone. They arrived in floating floods, some only a few millimeters long. On some days, the researchers observed more than 3,000 flies per meter per minute. To count these insects, which flew close to the ground to avoid headwinds, the researchers put a smartphone camera in a waterproof case and placed it facing a rock. All day, the phone took minute-long videos every 15 minutes.
While data collection was easy, data extraction was a headache. Researchers have tried to design computer software or an AI model to pick the insects out of the background, but nothing has worked, Hawkes said. “In the end, it was the most time-efficient for me to sit for maybe a month and count the number of flies individually as they moved through the frame,” he said. “It was millions of flies. But even these shots yielded fun surprises, acting as a camera trap to capture various creatures inadvertently caught on camera: wandering weasels, inquisitive birds, and the occasional hiker peeing by a rock.
Since the video wasn’t clear enough for the scientists to actually identify the flies whizzing around, they set up a stationary net trap on the side of the pass. Migrating insects would fly into the net, get stuck, crawl to the opening and fall into the ethanol bottle. This was the only way for scientists to collect a representative sample of their small specimens that Hawkes identified at night. “We recorded every single species of insect migrating through this mountain pass, which had never happened before,” he said.
To estimate the sheer number of insects moving through the pass, researchers would discern the ratio of groups of insects caught in the traps. If 20 percent of the insects in the trap were moths, they assumed that 20 percent of the insects caught on camera were moths. Overall, the researchers estimate that 17.1 million insects cross the Bujaruelo Pass each year, suggesting that billions of insects likely cross the Pyrenees each year.
At times, during the researchers’ visits, the Bujaruelo Pass appeared to be empty. The air seemed clean, without the little migrants. Yet when Hawkes swung his net over the edge of the pass where the insects were supposed to be, it filled with tiny flies. And while the pansies disappeared after sunset, they were replaced by screeching hawks, flying down the corridor, scented with the honey they stole from the hives. Observing this relentless, purposeful journey of millions of insects always humbles Hawkes. “You feel like you’re observing something much bigger and more important than you,” he said.
The researchers only included groups of insects in their analysis if they were spotted more than 100 times. Insects that didn’t make it were bumblebees, painted butterflies and hummingbirds. At night, the researchers also observed turnip moths being tailed by their enemies, tiny parasitic wasps known to lay their eggs in turnip caterpillars. Hawkes believes these rarer animals must still be migrants. “Why else would they be up there?” he said.
Before Hawkes personally visited the pass, he expected butterflies and dragonflies to be the most abundant travelers, in part because Lack’s paper estimated that hundreds of butterflies would pass through the pass every hour, accompanied by a dizzying stream of dragonflies. These flamboyant insects are also “stealing their famous insect migration headlines,” joked Hawkes. But they made up just 2 percent of the migration. However, the most striking were clearly the flies, which made up 90 percent of all recorded fliers. “That was the most exciting for me because then it just opens up this other world,” Hawkes said.
Most pollinator research focuses on bees, relegating other pollinating insects to the condescending group of “non-bee pollinators.” But pansies are very abundant pollinators, known to visit at least 72 percent of the world’s 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 hoverflies 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 migrating marmalades are females, often carrying the sperm to their destination, where they lay eggs that grow and migrate back to their northern home over a series of generations. All together, these generations of humble houseplants transport nutrients, pollen and elements around the world. “If we really didn’t think flies were migrating, then we missed all of their ecological impact on the planet,” Hawkes said.
Although the researchers couldn’t directly compare insect numbers in the pass to historical numbers more superficially recorded by the Lacks, a study in the mountains of southwestern Germany found that populations of migrating aphids have declined by an alarming 97 percent since 1970. “We can expect a similar decline” in the Pyrenees, Hawkes said, adding that habitat loss, pesticide use and climate change are threatening populations of fireflies and other insects.
This view is far from ideal. But Hawkes hopes that people will take an interest in these surprising, remarkable migrants and make the world more welcoming to their journey – by planting wildflowers or pressuring local governments to protect the species. He pointed out that insects like marmalade moths can lay thousands of eggs and reproduce throughout the year. “They could, if given the opportunity, if we provide them with habitat, have lots 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.”