There are 51 different kinds of mayflies found in Britain. Even though mayflies are called by anglers, and some biologists, ‘riverflies’, about half the British species can be found in still waters and 10 are commonly seen in ponds.
Some species, like the Pond Olive, are found in ponds where the water is alkaline or neutral, whereas others, such as the Sepia Dun, depend on more acid conditions.
Which mayflies will you find in your pond?
By far the commonest mayfly in ponds is the Pond Olive (Cloeon dipterum). Pond Olives are fast colonizers of new ponds, including ponds in gardens. In Freshwater Habitats Trust’s detailed garden pond surveys Pond Olives were recorded in just over half of all ponds, and they were the second most widespread animal after the Common Water Slater (Asellus aquaticus).
Over the whole of Britain, Pond Olives are found in about two-fifths of all ponds. They are commoner in the south of the country, where we have found them in 70% of ponds. Although they are mainly still-water animals, Pond Olives also live in some running waters and are found in about 10% of rivers.
In more acidic waters – both in the north and south of the country – you may find the Sepia Dun (Leptophlebia marginata) or the Claret Dun (Leptophlebia vespertina) mayflies in your pond. They are much less widespread than the Pond Olive, being in only around 5% of ponds in the countryside. So far neither species has been recorded in garden ponds – but they must be out there somewhere!
Most people have been taught that adult mayflies only live for a day once they emerge. But not the Pond Olive – it lives for up to a fortnight. Pond Olives are also unusual in that the female lays eggs which hatch immediately on contact with water. The young mayflies (called either nymphs or larvae) then live underwater for several months grazing the microscopic algae which grow on sand, gravel and submerged plants. The larvae often overwinter in this form. The adults hatch in the spring, with a second generation in the summer.
Encouraging mayflies to your pond
To make good habitat for Pond Olives all you need is clean sand and gravel – on these substrates in new ponds they can be hugely abundant. In some garden ponds, for example, we have found 3000 larvae with our standard 3 minutes of pond netting. This is roughly equivalent to a mayfly in every 2.5 x 2.5 cm (1 inch square) of pond bottom. Pond Olives also live happily on submerged plants, and in the grassy edges of ponds.
The Pond Olive is quite tolerant of pollution and lives in both good and bad ponds. In winter, at low temperatures, they can survive long periods with little oxygen, although in summer low oxygen levels will kill the larvae.