Millions of insects migrate through the Pyrenees each year

Over 17 million insects migrate annually through a single mountain pass on the France-Spain border in the Pyrenees, according to new research from the University of Exeter. 

Scientists studied insects in the Pass of Bujaruelo, a narrow 30-meter gap in the Pyrenees, during autumn over four years. They found a variety of day-flying insects heading south, suggesting billions cross the region each year.

Incredible spectacle of insect migration 

“More than 70 years ago, two ornithologists – Elizabeth and David Lack – chanced upon an incredible spectacle of insect migration at the Pass of Bujaruelo,” said lead author Will Hawkes, an expert in ecology and conservation at Exeter. 

“They witnessed remarkable numbers of marmalade hoverflies migrating through the mountains, the first recorded instance of fly migration in Europe.”

“In 2018, we went to the same pass to see if this migration still occurred, and to record the numbers, species, weather conditions and ecological roles and impacts of the migrants.”

Capturing the insect migration 

The researchers used various methods, including video cameras, visual counts, and flight intercept traps to identify and quantify the insects that migrate through the Pyrenees.

“What we found was truly remarkable,” Hawkes said. “Not only were vast numbers of marmalade hoverflies still migrating through the pass, but far more besides.” 

“These insects would have begun their journeys further north in Europe and continued south into Spain and perhaps beyond for the winter. There were some days when the number of flies was well over 3,000 individuals per meter, per minute.”

“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,” added senior author Karl Wotton, an expert in animal migration at the same university.

Peak of insect migration 

Insect migration peaked under warm, sunny, and dry conditions with low wind speeds and a headwind that kept the insects low over the pass, making them easier to count. While flies made up 90% of the total, butterflies and dragonflies, though well-known migratory insects, comprised less than two percent.

Among the insects that migrate through the Pyrenees, the scientists identified familiar garden species like the cabbage white butterfly (Pieris rapae) and the house fly (Musca autumnalis), as well as tiny grass flies (Chloropidae), only 3mm long.

“I would sweep my net through seemingly empty air and it would be full of the tiniest of flies, all journeying on this unbelievably huge migration,” Hawkes said.

Crucial ecological roles of the insects

These insects play crucial ecological roles. Nearly 90% were pollinators, moving genetic material between plant populations, enhancing plant health. 

Some insects were pests, but many were pest controllers like hoverflies, which consume aphids during their larval stage. Others contributed to decomposition and nutrient transport, impacting soil health and plant growth.

Due to climate change and habitat loss, these essential insect migrants are believed to be in decline. 

“By spreading the knowledge of these remarkable migrants, we can spread interest and determination to protect their habitats. Insects are resilient and can bounce back quickly. Together, we can protect these most remarkable migrants of all,” said Hawkes.

More about insect migration 

Insect migration is a fascinating natural phenomenon involving the large-scale movement of various insect species from one location to another, often over considerable distances. 

This behavior is driven primarily by the need to find suitable habitats for feeding, breeding, and avoiding adverse environmental conditions such as extreme temperatures or scarcity of food. 

Monarch butterflies 

Among the most well-known insect migrants are monarch butterflies, which undertake an extraordinary journey of thousands of miles between North America and central Mexico, where they overwinter.

Influencing factors

Insect migration can be seasonal, with many species moving in response to changing weather patterns and the availability of resources. 

For instance, locusts are infamous for their migratory swarms, which can travel vast distances and devastate crops along the way. 

These migrations are typically triggered by population density and food shortages, leading to synchronized movements that can cover hundreds of miles.

Multiple generations 

Other insects, like certain dragonflies, also migrate, often following similar routes year after year. These journeys can span continents and involve multiple generations to complete, with the offspring continuing the journey their parents started. 

Ecological balance 

Insect migration plays a critical role in maintaining ecological balance, as these movements help with pollination, pest control, and the distribution of nutrients across different ecosystems. 

Despite the challenges and dangers they face, migratory insects are remarkably adapted to their arduous journeys, showcasing the resilience and adaptability of these small but vital creatures.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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