This migration was first recorded more than 70 years ago, but this new research is the first to estimate the numbers and types of insects that pass through this gap.
At their peak, there were more than 3,000 insects per meter per minute, according to the study published in the journal Proceedings of the Royal Society B.
“It was the most amazing thing I've ever seen,” said Hawkes, an insect migration researcher at the University of Exeter in Britain.
The British team of researchers was inspired by ornithologists Elizabeth and David Lack, who first recorded the phenomenon in 1950, describing the insects as “a most remarkable migrant”.
The team has visited the mountain pass every autumn since 2018, and set up video cameras and traps to count and identify the insects.