'Magical sight': Mass insect migration in European mountain pass

Scientist Will Hawkes said the first sign he was about to witness one of nature's great migrations was being enveloped by a "blizzard of butterflies, cabbage whites and clouded yellows, like a storm of petals".

Then, standing on a mountain pass on the border of France and Spain during a hot day in September, he heard a "purposeful hum".

When he looked down, there were so many insects moving around his feet that it looked "like a living carpet," Hawkes told AFP.

More than 17 million insects migrate through the 30 metre (100 foot) Pass of Bujaruelo in the Pyrenees mountain range every year, according to a study led by Hawkes published on Wednesday.

The migration was first recorded more than 70 years ago, but the new research is the first to estimate the number and types of insects buzzing through the gap.

At its peak, there were more than 3,000 insects per metre every minute, according to the study in the journal Proceedings of the Royal Society B.

"It was the most wonderful thing I've ever seen," said Hawkes, an insect migration researcher at the UK's University of Exeter.

The British team of researchers was inspired by ornithologists Elizabeth and David Lack, who first recorded the phenomenon in 1950, and described the insects as "the most remarkable migrants of all".

The team has visited the mountain pass every autumn since 2018, setting up a video camera and traps to count and identify the insects.

Butterflies, dragonflies, house flies and other insects were detected migrating south from mainland Europe during autumn.

But most -- 90 per cent -- were marmalade hoverflies, the researchers found.

These "gorgeous" orange and black-striped hoverflies eat aphids, so are great for ridding crops or gardens of the pests, Hawkes said.

Because they traverse hundreds of kilometres, they might even be better pollinators than bees, he added.

But the insect migration may have been far greater in the past.

Marmalade hoverfly numbers are believed to have declined due to climate change, pesticides and habitat loss.

Hawkes pointed to a 2020 German study which found that the number of aphid-eating hoverflies -- such as marmalade hoverflies -- had declined by 97 per cent.

But he emphasised that anyone can still go and see this "amazing insect migration event".

The pass is easiest to reach from the French mountain town of Gavarnie. People can drive close to the pass, and it is a few hours walk from there.

Hawkes recommended a sunny day in September or October -- particularly if the wind is blowing from the south.

"It's just the most magical sight."