Alderflies

Sialis lutaria

- Macro of Alder-fly Sialis lutaria with its eggs on stem. The larvae commonly live in ponds.

Description/identification

This is because they are as happy in the acid upland forest pools of the Spey Valley in Scotland as they are in the rich floodplain ponds of the Thames Valley.

Alderfly larvae are predators of  silty, vegetation rich, environments. The dark brown adult alderflies, which carry their wings folded in the shape of a tent over their backs, emerge from ponds, rivers and lakes in spring and early summer. They are easy to see because they fly by day and are rather tame. Eggs are laid on plants overhanging the pond, and when they hatch the tiny larvae drop into the water or onto the ground and then crawl into the water.

Like many aquatic insects, the adult life of alderflies is short – just a week or two from the time they emerge in late April to the end of June. By contrast the larvae live underwater for around 2 years, or occasionally three.

Which alderflies will you find in your pond?

There are three different kinds of alderfly in Britain, and the commonest – the Mud Alderfly (Sialis lutaria) – is the one that is usually found in ponds. The two other species are rarely observed in garden ponds.

However, of all the animals associated with good quality ponds, alderflies are the scarcest and most difficult to attract. Only one in 10 of the 2009 Big Pond Dip surveyors found them in their gardens.

We know that alderflies can travel at least 250 m between waterbodies, and can probably go considerably further than this, so it is unlikely to be a problem with finding ponds. They have also been seen visiting garden ponds in the spring where they do not subsequently breed, so clearly not all ponds are equal as far as these animals are concerned.

Encouraging alderflies to your pond

We don’t know exactly how to attract alderflies to a pond although there can’t be much doubt that the better the quality the pond, the more likely they are to come – so once again, clean water, plenty of plants and natural edges will help. But you will probably also need the things normally associated with more mature ponds – accumulated sediment and emergent plants on which the animals can lay their eggs, although trees and bushes overhanging the water might do as well. Reed Sweet-grass and Common Reedmace are both used for egg laying, although these plants are often labelled ’too invasive’ and people advised not to encourage them.

Alderflies are a sign of a good quality pond. Where ponds are suffering from pollution or other damaging impacts alderflies are less likely to be found.