Animal migrations are marvels of the natural world. People are entranced by mass movements that span continents, as creatures travel thousands of kilometers in the pursuit of food, mates and ideal weather.
Monarch butterflies trek across North American to high-altitude forests in Mexico, where they transform trees into shimmering black and orange sculptures. Waves of caribou travel some 3,000 kilometers to reach summer feeding grounds in the Arctic. More than half a million sandhill cranes trace a route spanning most of North America. Each of these species have been the subject of breathless nature documentaries.
Here’s one migratory miracle you probably haven’t seen on screen: Eupeodes americanus,the American hoverfly. The centimeter-long fly, whose black-and-yellow stripes make it look like a tiny wasp, has been found in Alabama with ratios of isotopes indicating they spent time in northern Canada. That would translate to a migration of as much as 3,000 kilometers.
It turns out this is just one of hundreds of flies that show signs of migrating each year, an ebb and flow of species that has gone largely unnoticed, despite their ecological importance. While not as charismatic as bigger, more photogenic creatures, these peripatetic flies can pollinate plants, break down carrion, consume pests and transport both nutrients and diseases across vast distances.
“Migrating flies play an unparalleled range of ecological roles—making them a major force in ecosystems and the economy,” said Will Hawkes, who studies insect migrations at the University of Essex in the United Kingdom.
Hawkes and several colleagues are now trying to give migratory flies a scientific “glow-up.” In a new paper in Biological Reviews, they spell out what’s known—and not known—about these underappreciated globetrotters.
The American hoverfly, it turns out, is not alone. The scientists’ survey of past studies turned up 592 fly species that showed signs of being migratory. Nearly half were hoverflies—notable for their ability to seemingly freeze in midair and suddenly change direction, thanks to their blindingly fast wingbeats. They were followed by mosquitoes (29 species), non-biting midges (24), and houseflies (22).
Their migratory routes can be mind-bendingly long and forbidding, often involving multiple generations to complete a single circuit. Migratory flies have turned up on oil rigs in the middle of the Gulf of Mexico, a remote island sandwiched between Antarctica and the southern tip of Africa, and mountain passes high in the Swiss Alps and the Himalayas. At least one species is thought to venture across the forbidding sands of the Sahara. Hawkes has tracked millions of bean seed flies, a pest whose larvae ruin crops of peas and beans, as they make a springtime trip from the Middle East to the island of Cyprus, which involves an ocean crossing of more than 100 kilometers. They are crisscrossing every continent on the planet except Antarctica, unless you count Marion Island in the southern Indian Ocean.
These tiny bugs can travel at speeds rivaling much larger winged creatures. Hoverflies in the United Kingdom have been clocked migrating at 40 kilometers per hour (kph), more than 80% of the speed of songbirds. Much like birds, they show an aptitude for catching a lift on winds, and some have been found to use the sun as a kind of compass to steer by.
As they move about the planet, these flies bring with them myriad benefits and a few headaches. Around 62% of the potential migratory species are known pollinators. Bits of pollen have been found on the heads of migrating flies in the Alps and eastern Mediterranean. By one estimate, two species of hoverflies transported as many as 8 billion grains of pollen from southern England to mainland Europe in the spring and up to 19 billion grains back again in the fall.
Some of their services are a bit less appetizing, if no less valuable. Fly larvae can feast on detritus and carrion, speeding along decomposition. Fifty houseflies, for instance, can lay enough eggs that their larvae will convert nearly half a metric ton of pig waste into compost. Around a tenth of the migratory flies will also feed on agricultural pests. The larvae of two hoverflies that arrive in southern England in the spring are thought to consume as many as 10 trillion aphids there every year.
Such benefits, however, are countered by the toll some flies take on people and their harvests. Approximately a sixth of migratory flies are thought to transmit diseases. Most notably, mosquitoes carry diseases such as malaria that kill over 500,000 people every year.
Like insects more broadly, there are signs that human activities are taking a toll on these species. While migrating flies might be more adaptable to the effects of climate change because they can move about, scientists in Germany recently documented a 97% decline in migratory hoverflies in one region.
Those kinds of results underscore the importance of preserving migration corridors for even these tiny flies, said the researchers. “Factors including urbanization, intensive agriculture and destruction of wetlands have made large areas unfriendly to insects—potentially creating damaging gaps on insect migration routes,” warned Karl Wotton, another Exeter scientist involved in the research.
So the next time you think about swatting that pesky fly, maybe you should think twice. It might have come a long way to buzz around your head.
Hawkes, et. al. “Lords of the flies: dipteran migrants are diverse, abundant and ecologically important.” Biological Reviews. April 1, 2025.
Image: Pixabay
