Flies cross mountains and continents in one of the largest migrations on Earth

One of the world’s largest migrations is made by some of its smallest inhabitants.

While not all insect migrations are as famous as the annual movements of monarch butterflies, their journeys are just as epic. New research has revealed the true scale of just one of these migrations by measuring species passing over the Pyrenees on their way to Africa.

The study estimated that around 17 million insects, including flies, bees and dragonflies, traverse the Bujaruelo pass each year. This suggests that billions cross the mountain range annually, with flies making up more than 89% of all migrants.

As Dr Will Hawkes, the lead author of the research, explains these numbers pale in comparison to the true scale of the invisible migrations happening above our heads.

“The insect migration that we saw going through the Bujaruelo pass was certainly impressive, but it is just a tiny sliver of something far bigger,” Will says. “In southern England alone, one study suggests that as many as 3.5 trillion insects migrate overhead each year!”

“In turn, this is just a fraction of the number of insects migrating in Europe, which must be truly huge.”

The findings of the research were published in the journal Proceedings of the Royal Society B.

Why do insects migrate?

Despite the sheer numbers of insects involved, their journey through the Bujaruelo pass was first spotted by chance. Ornithologists David and Elizabeth Lack identified the mass migration during a research trip in the 1950s, noting the ‘majestic phenomenon’ in a paper published at the time.

While their research wasn’t forgotten about, the concept of insect migrations wasn’t widely followed up on at the time. Decades later, more recent analysis has revealed that they’re much larger and more common than had ever been expected.

Although not involved in the study, the Natural History Museum’s Principal Curator of flies and fleas, Dr Erica McAlister, sheds further light on these journeys.

“Insects migrate in exactly the same way as other animals do,” Erica explains. “They move with the changing seasons and follow resources.”

“While people tend to pay attention to the monarch butterflies and dragonflies, they’re not the only insects that migrate. We now know that similar migrations take place all over the world, encompassing a huge biota of overlooked insects.”

Flies make up the vast majority of these migrants, but have historically been overlooked because of difficulties in identifying them. As a result, we’ve been missing out on finding out about some of the most important insects on our planet.

“Flies are the most ecologically diverse group of animals, and do everything from pollination to decomposition,” Erica explains. “One of the main reasons for this is the difference between the adult flies and their larvae, which have very different lifestyles.”

Two of the most important fly migrants are the hoverflies Episyrphus balteatus and Eupeodes corollae. Their larvae eat aphids, helping to control these pests, while the adults are important pollinators.

While a single fly might be able to make the whole journey from the UK down to northern Africa in autumn, they will die before making the return trip. Their descendants will follow the flowering of plants back north in spring, with their great grandchildren eventually making it back to northern Europe.

“Though no fly will complete the round trip, they make an important contribution by moving nitrogen and phosphorous around the world,” Erica says. “This is released into the environment when they die, where it contributes to soil health and helps plants to grow.”

“The individual legs of the journey are also important for ensuring gene flow between isolated groups. Flies returning to the UK are really important immigrants, increasing the genetic diversity and fecundity of local populations.”

Even as the importance of these migrants is increasingly recognised, it’s currently hard to precisely measure the impact of their journey. The new research hopes to start changing that.

Understanding insect migrations

To try and understand the insects travelling through the Bujaruelo pass, the team relied on a variety of different methods. As well as a tent-like structure known as a malaise trap to catch insects, they also used video cameras to record the species passing overhead.

Hoverflies were the most common family of insects traversing the pass, making up more than 20% of the entire sample. They were closely followed by the glass flies, fungus gnats and house flies, with the four groups representing around three-fifths of the total number of migrants.

The exact number of insects braving the pass changed daily, which seemed to be related to changes in the weather. Warm and sunny days saw more insects travelling through, while fewer insects were spotted when it rained.

The team have now turned to examining these questions on a broader scale, as well as trying to answer where these insects end up.

“I am currently using radars as part of research for the Swiss Ornithological Institute, and hope to quantify the total number of insect migrants across western Europe,” Will says. “I would also love to prove that the remarkable flies are travelling further south on their journey than expected, and even crossing the Sahara.”

Getting a better understanding of where these flies are travelling is also something of interest for Erica, who hopes to apply their findings to museum specimens.

“We’ve provided a lot of specimens from the Knepp rewilding site to the research group in Exeter, who plan to sequence them to see where they come from,” Erica explains. “In time, we hope that we’ll be able to go through our collections and compare it to historic records, allowing us to see how populations have changed over time and the impact of climate change.”

This research is more important than ever in light of dramatic declines in insects across Europe. Planting more nectar-rich plants and preserving natural habitats will be important to help stabilise their falling populations.

“While many flies are polyphagous, meaning they’re not tied to certain types of vegetation, they all need nectar,” Erica says. “However, there’s not as much as there used to be. We’re gradually starving our insects with our focus on ornamental species that have been bred for their looks rather than nectar.”

“Countries across Europe and Africa need to work together to ensure that nectar-rich habitats are available all along their migration route to give these insects the best chance of survival.”