The Marvel of the Tisza Bloom – A Life Born for a Day
Mayflies, short-lived insects known for their mass mating dances, have been known since ancient times. Aristotle (384–322 BC) referred to them as ephemeron, meaning “living for a day.” In Hungary, the first written record of the Tisza mayfly phenomenon comes from Marsigli, who documented this unique event on the Tisza River.
The Tisza mayfly (Palingenia longicauda) is Europe’s largest mayfly species, with a body length of 2.5–3.8 cm and a total length of up to 12 cm including its long white tail filaments. It has two pairs of delicate, lightly veined, translucent brown wings. Females are larger than males, with broader wings and bodies but shorter tails, which allows them to fly faster.
Each female lays between 7,000 and 8,000 eggs on the water’s surface. These sink to the riverbed, where the larvae hatch and burrow into the clay-rich substrate, feeding on decaying organic matter. Their development takes about three years, during which they molt approximately 20 times. Growth is highly dependent on water temperature.
Once development is complete, the emergence—known as the Tisza bloom—begins, usually in warm, calm weather. Males first appear as subimagoes, an intermediate flying form, which fly to the riverbank and molt one final time into full adults. Females emerge slightly later, skipping the subimago stage and transforming directly into adult mayflies. They do not feed during this stage; their sole focus is reproduction.
The mass emergence occurs in stages. During the early phase, only a few hundred mayflies are seen. The peak phase is a breathtaking sight, with millions of insects swarming above the water. In the final phase, only a few dozen individuals remain. The phenomenon usually takes place between 6 and 9 PM.
Beyond being a spectacular natural event, the Tisza bloom is ecologically vital. Mayflies serve as a rich food source for many fish, bird, and amphibian species. In the past, farmers even used the abundance to feed their livestock. The larvae, through their burrowing activity, also help create microhabitats for other bottom-dwelling aquatic organisms.
Sadly, the Tisza mayfly disappeared from Western Europe by the early 20th century and declined sharply in Central Europe. Today, they occur in large numbers only in the Tisza River, though they were once widespread across Europe, from Scandinavia to the Mediterranean (excluding only the extremes). This drastic decline is due to climate change but is mostly the result of human impacts such as river regulation, dredging, artificial riverbank protection, and water pollution.