Climate Change

Low Danube Waters Bring Huge Mayfly Swarms in Budapest

Reuters

17:22 JST, August 8, 2025

BUDAPEST (Reuters) — Mayflies on the Danube River in Budapest are swarming earlier and in greater numbers than usual this year due to low water levels and an unusually hot summer, filling the air along the riverbanks as they are drawn to the city lights.

Hundreds of thousands, sometimes millions, of mayflies engage in a frantic rush to mate and reproduce before they perish within just a few hours. Mayflies live for less than a day after they hatch, and their abundance is closely connected to the health of the Danube.

Reuters
Pedestrians watch the swarming mayflies near the Danube River.

Reuters
Mayflies swarm around street lamps with the Hungarian Parliament building in the background near the Danube River.

Reuters
Mayflies lie on a sidewalk-embedded street lamp near the Danube River.

Danube mayflies usually start swarming in August, but this year it began in mid-July, and is expected to last for weeks. Researchers say their early appearance in huge swarms could be linked to higher temperatures.

“We have observed that massive mayfly swarms occur in years when the water levels are particularly low,” said Gyorgy Kriska, associate professor at Eotvos Lorand University in Budapest.

Last month, unusually low water levels disrupted shipping along the Danube, Europe’s second longest river.

Low water means more sunlight reaches the riverbed, helping algae grow, which provides food for the mayfly larvae, Kriska said.

Danube mayflies did not swarm for about four decades as the river was too polluted. They returned in 2012 when the Danube became cleaner due to the construction of wastewater treating plants, he added. The mayfly is a protected species, with the notional value of a single specimen being 10,000 forints ($30).

As mayflies swarm upriver after mating, they can be confused by city lights or the shadows of bridges, causing them to crash on the riverbank instead of laying their eggs in the water.

To help them as much as possible, Hungarian researchers invented and installed special blue lights on two bridges.

“Our lights simply keep them here … they continue to fly in front of the lamp … and crash into the water,” Kriska said.

“This way, we can ensure they stay above the water, and the next generation survives.”