Every autumn, a narrow pass in the Pyrenees Mountains between southern France and northeastern Spain is overrun by a flood of marmalade hoverflies, pollinators with citrus-colored bodies and black stripes. The hoverflies avoid the mountain’s strong headwinds by flying close to the ground, making the sunlit flurries “almost look like a river of golden light,” says 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 wind and whirl through the pass by the thousands. “It almost looks like a snowstorm, with all the white and yellow,” Hawkes said.
The marmalade hoverflies, butterflies and other numerous insects all migrate south for the winter, some stopping in the warmer regions of Spain and others possibly going all the way to Sub-Saharan Africa. The Bujaruelo Pass, which sits at an altitude of almost 7,000 feet and is just under 100 feet wide, offers the insects a more hospitable gateway to Spain than the surrounding mountain peaks. But it is not a resting place, devoid of plants for the flies to feed on and bitterly cold at night. So on the busiest migration days, the buzzing hoverflies produce a buzzing sound – not the faint buzzing of a bee wandering around a garden, but a persistent note. “They have to get through it, so it’s a determined buzz,” 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 observe small birds crossing the steep peaks of the Pyrenees, which can reach heights of 3,500 metres. Although few researchers visited the pass in subsequent years, almost 70 years passed without any studies on the migration being published. In 2018, a group of researchers, including Hawkes, decided to change that. Their overview of four years of autumn migration was published in the journal on Wednesday Proceedings of the Royal Society B.
Hawkes had become fascinated by migrating insects while investigating such a migration through the Alps as a student. When his supervisor Karl Wotton, another author of the paper, told him about the Lacks’ paper in 2018, Hawkes jumped at the chance to do fieldwork that fall. The researchers wanted to do a systemic analysis of the migration to know how many and which types of insects were involved. For the large and slow butterflies, counting was somewhat easy. During the day, every two hours Hawkes sat on a rock on one side of the pass and counted how many butterflies flew past him in fifteen minutes: a stream of coal 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 waves, 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, all of which flew close to the ground to avoid headwinds, the researchers placed a smartphone camera in a waterproof housing and placed it facing a rock. Throughout the day, the phone took one-minute videos every 15 minutes.
While data collection was easy, data extraction was a headache. The researchers tried to design a computer software or AI model to distinguish the insects from the background, but nothing worked, Hawkes said. “In the end, it was most time efficient for me to sit for a month and count the number of flies individually as they moved through the frame,” he said. “These were millions of flies.” But even these images had some nice surprises, functioning as a camera trap to capture the various creatures inadvertently filmed by the camera: wandering stoats, curious birds, and the occasional tourist peeing near the rock.
Because the video footage was not clear enough to actually identify the flies flying by, they set up a fixed net trap on the side of the pass. Migrating insects flew into the net, got stuck, crawled to an opening and fell into a bottle of ethanol. This was the only way the scientists could collect a representative sample of their small specimens, which Hawkes would identify at night. “We recorded every type of insect that migrated through this mountain pass, which had never been done before,” he said.
To estimate the sheer number of insects moving through the pass, the researchers could distinguish the ratio of insect groups caught in the traps. If 20 percent of the insects in the trap were hoverflies, they assumed that 20 percent of the insects captured on camera were hoverflies. Overall, the researchers estimate that 17.1 million insects cross the Bujaruelo Pass each year, suggesting that billions of insects likely cross the Pyrenees Mountains each year.
At times during the researchers’ visits, it seemed that the Bujaruelo Pass was empty. The sky seemed clear, free of small migrants. But when Hawkes swung his net over the edge of the pass where the insects would emerge, it filled with tiny flies. And as the hoverflies disappeared after the sun set, they were replaced by squeaking death’s-head hawk moths that fluttered through the corridor, scented with the scent of honey they stole from beehives. Observing this relentless, single-minded journey of millions of insects always leaves Hawkes feeling humbled. “You feel like you’re observing something much bigger and more important than yourself,” he said.
The researchers only included insect groups in their analysis if they were observed more than 100 times. Some insects that didn’t make the cut included bumblebees, painted lady butterflies and hummingbird hawk moths. At night, the researchers also observed turnip moths being followed by their archenemies, small parasitic wasps known to lay their eggs in turnip moth caterpillars. Hawkes believes these rarer animals must still be migrants. “Why else would they be there?” he said.
Before Hawkes personally visited the pass, he expected butterflies and dragonflies to be the most common travelers, in part because the Lacks paper estimated that hundreds of butterflies passed through the pass every hour, accompanied by a dizzying stream of dragonflies. These showy insects also “steal the headlines about the illustrious insect migration,” Hawkes joked. But they constituted only 2 percent of the migration. But the highlights were clearly the flies, which made up 90 percent of all registered flyers. “That was the most exciting for me because it opens up a different world,” Hawkes said.
Most pollinator research focuses on bees, relegating other pollinating insects to the condescending group of “non-bee pollinators.” But hoverflies are extremely abundant pollinators known to visit at least 72 percent of global food crops, according to a 2020 article. The adults feed on nectar and pollen, which they can transport over extremely long distances during migration, even more than 100 kilometers over open water. Marmalade hoverflies hatch in late summer and begin flying south as temperatures drop, flying with the wind and using the sun as a compass, Hawkes said. About 75 percent of migrating marmelades are females, often carrying sperm to their final destination to lay their eggs, which will grow up to migrate back to the hoverflies’ northern home over a series of generations. Taken together, these generations of simple hoverflies transport nutrients, pollen, and elements around the world. “If we didn’t really think that flies migrate, then we’ve already missed their ecological impact on the planet,” Hawkes said.
Although the researchers could not directly compare the number of insects in the pass with the historical numbers more superficially recorded by the Lacks, a study in the mountains of southwestern Germany found that populations of migratory aphid-eating hoverflies have declined by an alarming 97 percent since 1970. had decreased. “We can assume that there will be a similar decline” in the Pyrenees, Hawkes said, adding that habitat loss, pesticide use and climate change all threaten populations of hoverflies and other insects.
These prospects are far from ideal. But Hawkes hopes people will take an interest in these surprising, remarkable migrants and make the world more hospitable to their journey by planting wildflowers or putting pressure on local governments to protect the species. He pointed out that insects such as marmalade hoverflies can lay thousands of eggs and reproduce all year round. “They could, if given the opportunity, if we provide them with a habitat, have a lot of children and then their numbers could increase again very quickly,” he said. “They are very resilient. We just have to give them the opportunity to do that.”