By Zoltán Fehér, Conservation Director, WWF-Hungary
Photocredit: © Zsolt Ujvári
On late summer evenings in Szentendre, a postcard-perfect town just north of Budapest, something extraordinary happens over the Danube. As the sun dips, millions of delicate white insects rise from the water and dance in the warm air. These are the Danube mayflies – and their dazzling display is more than a natural spectacle. It is a living barometer of the river’s health. Their presence tells a simple story: the water here is clean, and the river still flows as nature intended.
Life in Fast-Forward
The Danube mayfly spends most of its existence hidden. For up to a year, its larvae live buried in the gravel beds of the river. Then, in August, a silent alarm goes off. At dusk, they emerge en masse, mate, lay eggs – and die – all in the space of a single night.
Once, these insects were common along Europe’s great rivers. But decades of pollution and habitat loss drove them to the brink, even in the Danube. For 40 years they vanished entirely from the Hungarian Danube. Then, in 2012, they returned – thanks to improved water treatment and the fact that this section of the Danube still flows largely free of dams.
The adult mayflies brief airborne life is all about reproduction. After mating, the females fly upstream before dropping their eggs into the current. Those eggs drift back to where the mothers themselves emerged. Biologists call this the “compensatory flight” – a built-in insurance policy that helps keep the population steady.
Like many insects, mayflies navigate by light. But artificial lighting is a deadly distraction. They are drawn to the bright glare of streetlamps and vehicle headlights, which outshine the moon and the reflected light from the water. Asphalt and glass surfaces can mimic the river’s tell-tale shimmer, tricking them into laying eggs on roads instead of water – a fatal mistake. Bridges can cast deep shadows over the river, convincing the insects that their habitat has ended. In heavy swarms, their bodies can even make road surfaces dangerously slick.
Lighting the Way to Survival
Hungarian biologist György Kriska has studied mayflies for over a decade and discovered that they are most sensitive to blue and ultraviolet light. Working with colleagues, he designed a special bridge-lighting system to keep them over the water.
Photocredit: © Zsolt Ujvári
It was first trialled in 2019 on the Tildy Zoltán Bridge at Tahitótfalu, where lights are dimmed during the swarming season. In 2023, Budapest’s Árpád Bridge adopted the technology – a small but significant win for urban conservation.
Szentendre has no illuminated bridges, so locals came up with another idea: night-time flotillas of canoes, each carrying strong lamps. On swarm nights, volunteers paddle out and lure the insects away from streetlights, guiding them back to open water so they can lay eggs safely.
These community-led expeditions, organized by enthusiastic activists from local NGOs with the support of WWF-Hungary, are part rescue mission, part outdoor classroom. Before launching, participants learn about the mayflies’ role in the river ecosystem – and the bigger story they tell about the Danube’s health.
The mayflies’ return is a reminder that nature can rebound if given the chance. It shows that cleaner water, free-flowing channels and a little human ingenuity can restore even the most fragile rhythms of life. For now, on warm August evenings, the Danube still breathes. And for a few magical hours, its skies fill with wings.
LATEST UPDATE
This summer will likely look different in Szentendre: the traditional “mayfly rescue” canoe tours will hopefully not be needed. The local municipality – listening to the requests of experts and nature-loving residents – arranged for the power company to dim the riverside promenade’s lights during the swarming period. Reducing light pollution helps ensure that the mayflies are not lured away from the water and can lay their eggs safely in the river. In Szentendre, it seems the local stakeholders have already managed to join forces for the sake of the Danube mayfly – and this should serve as an inspiring example for other towns along the river. Read more here.
