The short-lived nuptial dance of mayflies – The Rivers Tisza and the Danube in Bloom

The River Tisza is in flower again, said news reports in mid-June. This year, that information is of particular interest as in 2016 the Tisza mayfly and the blooming of the Tisza were added to the Collection of Hungarica, the Storehouse of Hungarian Specialities.

This swarming of a colourful, 8 to 12 cm long species of mayfly, Palingenia longicauda, is what causes this marvellous phenomenon lasting a few days. Those who wish to see the Tisza in bloom need to stay sharp, as the insects appear depending on weather conditions, first on the Lower Tisza, then along its middle section, and in a few days, the phenomenon is completed along the Upper Tisza as well.

Photo: Zsolt Czeglédi, MTI

The eggs of the Tisza mayfly take three years to develop into the mature insect. The nymphs moult up to over twenty times, then swarm after the last moulting beginning in the evening hours for a period of about three hours. The nymphs of males moult into a ’semi-finished insect’, a subimago, which first fly to the river’s banks, then moult again to become sexually mature, truly adult insets. The colourful male mayfly, with its long cerci flies low over the water in search of a mate.

The female is immediately sexually mature after she moults off her larval skin, and after successful mating over the river, she flies against the current at a height of 5-10 metres to find a place to lay her eggs. The eggs, which are laid on the surface of the water, sink to the river’s bottom, and the emerging nymphs dig into the banks of the riverbed. The protection afforded the Tisza mayfly extends to its habitat and all its forms, including the nymphs.

Angelic Little Brides – a video by the Cinema Film Workshop

Back in 2013, we reported on our website that the water quality of the Danube had increased sufficiently for this wonderful natural phenomenon to return to the river after long decades of absence. Previously, the insects were not only present in the tributaries of the Danube, but had swarmed in great masses over the Danube, too: the Danube also bloomed. The blooming of the Danube, which can be observed in evening lamplight, now returns every year. Depending on the weather, we can encounter the spectacularly intense evening nuptial dance of the approximately 3 cm mayflies for up to a week or two.

The mayflies, who fly over the Danube between half past eight and ten in the evening, primarily use optical cues to follow the river. Under natural conditions, the light reflected from the water’s surface is horizontally polarized, which, along with other aquatic insects, also attracts the Danube mayflies. “However, bridges, such as the one at Tahitótfalu, cast a shadow on the river, which confuses the Danube mayflies, who then begin to follow public lighting and the horizontally polarized light reflected from dark asphalt”, explained researcher György Kriska. The insects are unable to escape the trap made of light: they keep flying over the bridge until their energy is exhausted and they perish, dropping their eggs not in the water but onto the bridge. “The ecological value of a single Danube mayfly, prescribed by law, is ten thousand forints, and a single bundle of eggs contains 6 to 9 thousand eggs. This implies that assuming a swarm producing a million eggs, the ecological damage can be as high as ten billion forints”, the expert noted. Therefore understanding the behaviour of the insects and investigating the role of polarized light may serve the purposes of nature preservation (Source: kisoroszi.hu)

The Hungarian winner of the Sony World Photography Awards 2016 competition also records the Danube in bloom.

The photographer who took the winning photo, the researcher Imre Potyó from Göd, made the following statement when he accepted the award:

After decades of absence, the Danube mayfly (Ephoron virgo) has returned to the Danube thanks to improvements of water quality. For me, the swarming of the Danube mayfly is an extremely exciting phenomenon. I took the photo along an unlit, almost untouched section of the River Rába, with a long exposition time and a flashlight. Unfortunately, the lit bridges have a harmful impact on the natural swarming behaviour of the Danube mayflies... As a winner of the Hungarian national award, I can generate interest in these wonderful aquatic insects, and I am very happy that so many people will now admire these teeming mayflies in their natural, star-lit habitat.” – the FotoTV portal reported.