Creatures - Millions of insects travel through a European pass.
Each year, a stunning event happens in the Pyrenees mountains: Over 17 million insects travel through a 30-meter-wide gap between two peaks on the French-Spanish border. Researchers from the University of Exeter revealed this fascinating fact in the "Proceedings B" journal of the British Royal Society. "Observing so many insects moving with purpose in the same direction is truly one of nature's wonders," commented Karl Wotton, a co-author of the study.
The majority of these migrating insects heading south at Puerto de Bujaruelo are common garden residents like the cabbage white butterfly (Pieris rapae), housefly (Musca autumnalis), and tiny 3-mm long grass flies (Chloropidae). "The experience was magical," said Will Hawkes, who led the study. "If I swung my net through the seemingly empty air, it would be packed with tiny flies, all participating in this incredible migration."
The migration likely involves over 10 billion insects crossing the Pyrenees annually and continuing on to Spain and maybe Africa. "These migrating insects, especially the flies, hold significant importance for our planet," added the University of Exeter. Nearly 90% of the captured insects were pollinators, transferring genetic material across plant populations, which benefits plant health.
Yet, with global warming, the number of insect migrants is expected to drop. But with resolute environmental and climate action, improvements might be possible: "Insects are resilient and can bounce back quickly," Hawkes remarked.
The team exploited the fact that the insects flying south during the day generally stay low above the pass, making counting easier. "The combination of high mountains and wind patterns converts an otherwise invisible high-altitude migration into a visible spectacle that can be viewed from below," explained Wotton. Roughly 90% of the insects captured over four years in the fall were flies. Species like butterflies and dragonflies only made up less than 2% total.
This incredible migration has been observed accidentally at Puerto de Bujaruelo, over 70 years ago. In October 1950, Elizabeth and David Lack, ornithologists, documented the migration of flies and other insects from the north to the south, marking the first recorded European case of fly migration.
The team returned to the same pass to examine if the migration still existed and gauge the quantity and types. They utilized video camera recordings, butterfly observation, and a flying trap to gather data. "The findings were really noteworthy," said Hawkes.
Several fly species, such as the Common Field Hoverfly (Eupeodes corollae), still have a high population density there: "There were days when the number of flies surpassed 3000 individuals per meter and minute." However, research in other European regions has indicated that the number of migrating hoverflies dropped by up to 90% since 1970. Hence, it is assumed that even more insects were on the pass decades ago.
Puerto de Bujaruelo, also identified as Port de Boucharo, lies at approximately 2270 meters elevation and is one of several passes in the mountainous area. It's not just popular among insects: Several well-traveled hiking trails also run through it.
