One fly does not make a summer: The mass flight of mayflies on the River Enz

The White Mayfly

My immediate suspicion was confirmed when I saw the photo that was sent with it, in which countless insect corpses covered the roadsides on the bridge over the Enz River: Ephoron virgo, the European white mayfly! One must know: Each time the name “White Mayfly” is mentioned, a pleasant shiver runs down the spine of insiders, because this animal brings back memories of better times for insects. Ephoron virgo is a mayfly, which in past centuries was famous for its summerly mass flights at the lower reaches of larger European rivers.

In 1757, Jacob Schäffer reported in his treatise „Vom fliegenden Uferaas, wegen desselben am elften Augustmon. an der Donau, und sonderlich auf der steinernen Brücke, zu Regensburg ausserordentlich häufigen Erscheinung und Fluges“: On the 11th of the present month August, in the evening at around 11 o’clock, because it was very humid, and there was a strong thunderstorm in the sky, an enormous number of strange and unknown flies, or birds, as some had called it, rained down. The same would have fallen down, amongst various places of the Danube stream, especially so on the local stone bridge, at such a height that they would not only have covered all the stones completely, but would also have lain now and then 2 to 3 inches high atop one another. Whoever walked or stood on the bridge at this time would have been completely white, as if it snowed upon them.“

What Jacob Schäffer in Regensburg was astonished about was known to every local on the Elbe in the 19th century: fires were lit at regular intervals along the river to attract the hatching “whiteworms”, which rose from the middle of the water after sunset. Karl Russ wrote in 1887 in the then popular magazine “Die Gartenlaube”: “According to traditional custom, without quarrelling or bickering, people took possession of one place each on the riverbank, built a square stove about 3 meters in size, directly by the water and a little into the stream, built a small fireplace in the middle and laid an old wire netting on top of it. On top of it they put a wide, earthen pot without a bottom and light up in this pinewood. Soon the drakes swarm each fire by the millions like snowflakes, fall down with scorched wings onto sackcloth spread out all around, are swept together and poured into baskets… The drakes, which have dried in the air and been freed from their wings by shaking and blowing off, are now marketed as white worms.“ The animals were not only used as fish bait or food for cage birds. Farmers also fed the vast amounts of insects to the pigs or even spread them on the fields as fertilizer.

With light traps on the hunt

Such pictures in the head, one does as an entomologist simply what one must do: Light trap, insect net, collection tube, tweezers and camera and drive off! The evening hatching and swarming flight of the European White Mayfly is precisely timed and begins about one hour after sunset. Arriving in Vaihingen/Enz just in time, I saw the remains of yesterday’s swarming on the bridge over the Enz River in the middle of the city: A layer of the dead animals lay a centimeter high at the roadside, especially where street lamps were located.

On the Enz itself, the tension rose from sunset at 20:26. I started assembling the light trap in time not to miss the beginning of the flight. To attract the animals, we entomologists use battery-powered LEDs, which emit a particularly high proportion of ultraviolet light. These are placed in a tower made of white gauze so that they are clearly visible from all sides and the approaching animals can easily attach themselves to the gauze. The floor around the tower is covered with white linen. Since this light is irresistible to many species of insects, the operation of such installations outdoors is generally only allowed with a special permit. – By the way, insect traps based on the same attraction principle are often offered commercially to put mosquitoes out of commission in gardens. This does not work however: Mosquitoes are disinterested with UV light, instead searching based upon an increased CO₂ gradient brought upon by their victims breathing. Instead, numerous other completely harmless insects die in such automatic UV light traps. Therefore, outdoor operation is rightly forbidden (and useless indoors).

A cheerful rendezvous of species

What attraction such traps do have, I could experience again impressively that evening: Only a few minutes after switching on at 21:00 o’clock (CEST) hundreds of small mayflies flew in from the surrounding area and I began to register: Baetis scambus, Serratella ignita, Ephemera danica, Ecdyonurus dispar, Potamanthus luteus, Oligoneuriella rhenana… A cheerful rendezvous of species that had just finished their development as larvae in the water of the Enz and whose time for reproduction as winged insects had now come. Among them were species that develop in the lower reaches of larger rivers like the Yellow Mayfly (Potamanthus luteus) or the brushlegged mayfly Oligoneuriella rhenana. These species were on the verge of extinction in Germany a few decades ago because of the poorer water quality at that time.

Sad low point

In the course of the economic boom of postwar Germany, also Ephoron virgo, the European White Mayfly, had soon made acquaintance with laundry detergents in the rivers. Due to the increasing pollution of our rivers, the mass swarms became increasingly rare and were quickly a thing of the past. By the end of the 1970s, the species was considered lost throughout Germany – a sad low point. In this case the development was fortunately reversible: with the construction of sewage treatment plants, the water quality in our rivers also improved again. With the decrease of pollutants such as ammoniacal nitrogen and an improved oxygen content, the White Mayflies also returned. The first flights were observed in the Main as early as the beginning of the 1980s, followed by the first mass flights in the Rhine near Cologne and Bonn in the early 1990s. Since then, the White Mayfly has gradually reclaimed its old habitat. In past centuries, it was known in Baden-Württemberg not only from the Main and Tauber rivers, but also from the lower course of the Neckar and its tributaries Jagst and Kocher. And now it was time again: After the White Mayflies had arrived in Heidelberg and Neckargemünd in the mid-1990s, the species was first detected in the lower reaches of the Kocher in 2006. I had been expecting it further south for a long time, and now it had finally arrived: Over the past few years, the White Mayfly has made it upstream along the Neckar to the Enz.

Mass hatching ...

Punctually at 21:20 the first male flew to the light. Only 15 minutes later I had to switch off the lamp: According to a rough estimate 8,000 individuals had already landed there. This amount of insects is really hard to imagine if you have never seen and experienced them for yourself. With a high population density, you can actually watch a cloud of hatching animals rising from the river, flying towards you and being fluttered around by thousands and thousands of them the next moment. Certainly nothing for someone with entomophobia, but for me this produces an even stronger endorphin surge than Lübeck marzipan. Not only with me, but because the hatching in such masses is of course also a feast for all fish, birds, small mammals, and all other predators living in and around the river. And all fly fishermen can be sure that the trout will bite on their artificial fly bait. After hatching, however, the rod can be left in the corner, because then all the fish will have eaten up and will not bite for a while. Therein lies, by the way, also the biological reason of the mass-emergence: If all predators are simultaneously sated, then the surviving part of the population can devote itself safely to reproduction.

... shedding the skin ...

The entire life as adult insects lasts no longer than a few hours: The adult larva swims to the surface and sheds its skin. First, the males hatch, taking advantage of the extra time over the females to fly to shore and shed their skin again, including their wings – a unique process that only occurs in mayflies among winged insects. Immediately after moulting, the male flies back to the water in search of a female. The freshly hatched females do not shed their skin again, they are still mated as subimago while flying in the air.

... and spreading!

After mating, the females can still fly several kilometers to find a suitable place to lay their eggs. Females were found 10-20 km away from their original water. This shows the great potential of the species to spread. Of course, any light that deprives the animals of successful reproduction represents a danger. In our case, the huge swarms fluttering around the floodlight masts of the sports club in Vaihingen-Roßwag and on the lanterns of the Enzbrücke were hopelessly lost.

The air show has ended

One hour later, the spectacle was already over, so that I was able to pack up at 22:30 with several of the White Mayflies under my shirt and in my luggage. So for this evening the air show has ended, but there would be more chances in the coming nights. Ephoron virgo does not only hatch one night a year, but the hatching period lasts from the end of July to the beginning of September. Most of the time, however, far fewer animals hatch, mass swarms only appear at the peak of hatching season, lasting for only a few days.

By the way, the trigger for the hatching of the adults is the water temperature. After mating, females drop the fertilized eggs onto the water surface, and they immediately sink to the river bed to rest. Only at the end of April of the following year do the larvae hatch. In the interstitial system of the river bed, they live at first from biofilm, later, they dig U-shaped tubes in the river bed with their legs and horned-mandibles. The larvae then sit protected at the bottom of their tube and use their abdominal gills to create a permanent stream of water, bringing fine particles as food, which they filter out with their hairy front legs and mouth parts. Thus they grow up quickly until the water temperature triggers a new spectacle on a warm August night. Especially with all the bad news concerning the decline of insects in general: It is nice to be able to still experience something like this today.

Film Unterwegs mit Insektenforschern. Heute: Der synchrone Schlupf der Eintagsfliege “Ephoron virgo” [in German]