Seventy years on from a chance encounter in the Pyrénées with winged minibeasts, millions of insects have been recorded migrating through a 30-metre wide gap among 3,000- metre mountains.
A team from the University of Exeter, including insect migration scientist Doctor Will Leo Hawkes, recorded the number and the species of insects flying through the Bujaruelo Pass on their migration journey, counting up to 17 million insects in what was described as a “magical experience” and “a living carpet”.
In the 1950s, researchers had come to the same pass to study bird migration but had been surprised to see millions of insects migrating too. No further research had been published until ‘The most remarkable migrants-systematic analysis of the Western European insect flyway at a Pyrenean mountain pass’ by the University of Exeter team in June 2024.
They were surprised to find that flies accounted for about 90% of insects migrating. “This was fascinating because we had not known this before and they play vital ecological roles, far more than butterflies. They are pollinators, leave nutrients and are good at controlling our pests,” said Dr Hawkes.
The insects will generally live for about two months, despite having a year-long migration. “They have this multi-generation migration. A bird completes its own migration loop, going down south in the autumn and returning in the spring.
Migration from Sweden
“These insects will leave in autumn, and one insect will make the journey down to southern Spain and probably even sub saharan Africa. Then, four or maybe even five generations of that single insect will make the journey back to the UK, France, even as far north as Sweden.”
This means that adults will arrive in an area on the way back, lay their eggs and die off. The larvae will then take up the migration without ever meeting their parents or having any experienced insects to follow and learn from.
“Their whole migrationary behaviour is from this instinctual, genetic basis and there is so much we do not know about it.”
They migrate in order to have more children. By avoiding winter, they can reproduce throughout the year.
It is currently estimated that only about 2% of insects migrate, although the figure could be higher, but the number of individuals that migrate is very high because of how many are born.
As insects are difficult to track, it is not exactly clear where they come from but Dr Hawkes hopes that in future years, some studies will be performed to work out where they come from. “I expect them to be coming from all over Europe.”
The journey from England to the Pyrenees is only expected to take around 50 hours. Because of their size, they cannot fly that far and instead are adept at making the most of the wind.
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'A living carpet'
“Watching it was magical. A lot of these flies are quite beautiful and nice-tempered, happy to sit on your finger. Some days, the ground was a living, moving carpet.”
There were significant shifts over the four years, with numbers varying from a few million insects to millions and millions. This was mostly due to weather conditions but four years was not enough to ascertain trends.
“They would stay stable year on year if we were not affecting them, but due to climate change and pesticides, we are impacting insect numbers. Although we saw 17 million insects crossing this 30-metre pass in one year, this number may have been way higher in the past, we do not know.
“That is why creating baselines like this is very important, to be able to compare in 50 years in the future. Fortunately, these insects are very resilient and will return to usual numbers if given the chance and have all their useful ecological impacts, such as fuelling garden beds.”
Interest in insects
Dr Hawkes highlights the importance of educating people on these insects so that they can learn to love them. “If you do not know them, you cannot love them and care about them.”
For his part, he had always been interested in insects and butterflies: “When I was little, I used to be paid one penny for every caterpillar I would take off the cabbages in the garden. I always imagined that these cabbage white butterflies would stay around the garden, especially because I would find some chrysalises and cocoons in the winter.
“When I went to the mountains, stood looking down the valley, there were thousands or even tens of thousands of cabbage white butterflies flying en masse like a snowstorm. A lot of these cabbage white butterflies that you see in your garden will migrate huge, huge distances.”
They recorded the insects thanks to cameras and identified them with traps. Counting the insects was a painstaking process which involved counting the insects, frame by frame, for a minute’s worth of footage every fifteen minutes. “It took forever and I clicked through over four million frames each season. Luckily it was a labour of love,” said Dr Hawkes.
During their migration, the insects will generally fly very high, using the sun as a compass and the winds as a propeller, which is why people do not know about these insect migrations. However, in the mountains, they are forced to fly along the ground due to different weather conditions, hence being recordable.
“There will be insects migrating above your head right now, you just cannot see them.”
A study in England found that around three trillion insects migrated in a certain area.
Dr Hawkes believes that there are probably only around 50 people worldwide working on insect migrations, but he says that studying that is more important than studying bird migrations because of the massive ecological impact. “It is so important to protect these lovely little things,” he said.
