Alder Fly

The Alder fly is a member of the Sialidae family. It is a large, sluggish insect often found on vegetation near to both running and still water. Its larvae are aquatic carnivores that live in the silt at the bottom of ponds and slow-flowing rivers. Adults are seen in early summer (April in the South of the UK and early May in the more northern parts). They emerge in vast numbers and live for just a couple of days in order to mate. The females lay their eggs on overhanging vegetation. The larvae hatch and drop into the water where they develop over one or two years. The Alder can be identified by its black veined wings, which fold up like a tent over its back.

Please Note: You will often see these flies referred to as Caddis or Sedge flies; technically they are not of the same species and are quite different (as you will see below).

Where To Find Them

In the British Isles, the Alder fly is one of the most widespread, due to its ability to tolerate multiple different environments - from acid to alkaline water ph levels, to flowing and stillwater.

There are three different kinds of alder fly in Britain, and the commonest – the Mud Alderfly (Sialis lutaria) – is the one that is usually found in and around our rivers and stillwaters. The two other species are rarely observed, so the main thrust of this email is regarding Sialis lutaria.

When hatching, the Alder fly is the main diet of most stillwater trout (especially once they have gone native). 

When To Find Them

Alder flies usually start hatching from April and continue through until the end of June.

What To Look For

Alder flies are pretty easy to identify, the have wings which fold up (like a tent) over their back. These wings have dark black veins.
The only time a trout will encounter the Alder is in its larvae form (Remember: the adults lay their eggs on overhanging leaves). Once you see masses of the adults in the air, that's the time to switch to nymphs. 
Unlike Caddis flies, the Alder is more active during the day, only mating at night. They are also very tame, they will often land on you whilst you're fishing if they do - take a close look at them. If it is windy, the adults sometimes get blown onto the water where the trout can mop them up (think and target these you would any other terrestrial).


Alder flies mate in the evenings and the hours of darkness. The females then lay a vast number of eggs on grass stems and overhanging leaves near water. When the larvae hatch they drop into the water or onto the ground nearby, and make their way into their new aquatic home. The larvae live their entire time underwater and are fierce predators, armed with strong sharp mandibles. Their favourite habitats are silty riverlake beds. The larvae will stay submerged for up to two years eating decaying vegetation. Once they reach full size (around 14mm - a size 12 or 14 hook), they leave the water and spend their pupal stage on land before metamorphosis into the sexually mature insect. Adult alderflies stay near to the water in which they had lived when they were younger. Once in their adult stage, they tend to live only 2 to 3 weeks, which they spend mainly in reproducing (lucky insects).

Due to the fact trout very rarely see the adult fly - from a fly fishing perspective the stage which we must try to imitate are:

  • Nymphs
  • Adults (it's always worth having a nice imitation should any be blown onto the water)


The most popular imitations for Alder nymphs are either of flies below. Don't worry if you don't have these exact patterns, as anything which has the same general colour, profile and size will do. These flies should be fished as you would any other nymph, and they should be used as soon as you start to see the adults in the air; this can be anytime during daylight hours.

Alder Nymphs

Copper-Head Mary Tungsten Nymph

Adult Alders

It is always worth having an adult Alder pattern in your fly box (just in case). Here's the best one we have found:

Alder Fly


This email is brought to you with the sole intent to spread the information around so we can all maybe learn something. If you would like to buy any of the flies contained within this email, we do hope that you will consider us and see what flies we have to offer. If you would like any further details on any of the above flies, they can be found using these links:

Alder Nymphs:
Copper-Head Mary Tungsten Nymph
Amber Flame Czech Nymph
Olive Czech Nymph

Adult Alder:
Alder Fly

Previous issues of Hatch Chat, can be found on our blog, here: