Mayfly time!

The mayfly is the iconic river fly, loved by trout and anglers, for whom fishing the mayfly’ is shorthand for having a lovely day by the river with fish rising everywhere and catching lots of trout on dry flies.

Jacobs Mayfly

The term duffers’ fortnight’ refers to the period when fishing is easy because trout are rising to gorge on mayfly and are much less likely to be spooked by a clumsy angler or poorly presented fly.

The reality is often a bit different. Avid anglers arrive at the river ready to fish by 10am, and wander the banks waiting for the hatch to start, which is usually mid-afternoon and often later. Once the hatch gets underway, the excitement builds as he waits for trout to rise. Waits. And waits some more. Sometimes it takes a few days for trout to switch on to rising and taking mayfly off the surface.

Oliver mayfly
A good mayfly imitation

The first sporadic rises are a window of opportunity to catch a trout, as soon the river is boiling with rises, the aggressive splashy rise that shouts mayfly’ to the angler. There are so many fish rising that it seems that he can’t fail to catch a fish. But with so many flies on the water the trout seem to be ignoring his fly, and when they do take the fly the temptation is to strike too soon. Eventually, with the right pattern and the right presentation, the first few trout are caught. Sometimes, it is so easy it becomes no challenge at all and perhaps a wee bit boring? Better to stop fishing now and admire the spectacle!

Now our angler looks up into the trees alongside the river and sees hundreds of mayfly spinners dancing, with swallows and gulls enjoying a feast. More excitement – there will be a fall of spinners perhaps this evening, another great opportunity to catch rising trout on dry flies. More waiting. Dusk, and all too often there are still no spinners and the unmated mayfly retreat to the trees to try again tomorrow and the angler goes home to try again tomorrow too. 

Mayfly final
Mayfly spent spinner

About Ephemera danica

The largest and best known of the upwinged flies, the most commonly occurring mayfly is Ephemera danica. The female mayfly dun or sub-imago is typically up to 25mm, with males about two thirds the size of the female. They have three tails, large hindwings, and a cream-yellow coloured body with distinct brown markings on the body segments. 

They generally live as a nymphs in the river or lake for two years (sometimes only one year) and emerge as sub-adults (dun or sub-imago) on the surface of the water during the day. They are very vulnerable to trout predation as they swim from their burrows to the surface, then stick in the surface film whilst their wings break free of their cases and they can fly. Damp and drizzly weather makes the lift off’ harder and so this is the best time to fish.

Mayfly and their relatives are unique in that they have two winged adult forms. The dun or sub-imago will seek shelter and fly into bankside vegetation or trees (this is one of the many reasons why bankside vegetation is crucial to a healthy river ecosystem). After a few hours they shed their sub-imago skin and become spinner or imago – ready to mate. The males will dance’ in the air as a swarm and the females fly into the swarm and are caught by a male. They mate and the female flies onto the river to deposit her eggs on the surface. The females then die on the surface, providing a second feast for trout. The males will die in the trees and bushes – a feast for birds. 

Backlit Mayfly
Mayfly life cycle
Mayfly life cycle

You can download a high resolution version of the mayfly lifecycle as a PDF document here.

Common questions about mayfly

When do mayfly hatch? 

This varies by river and can be anytime from early May to the end of June. Some rivers have reputations for very early mayfly hatches — the Dorset Stour and Loddon in Berkshire for example. Other rivers seem to have a trickle of mayfly coming off all through the summer, such as the Great Stour in Kent. The classic Wessex chalk streams such as the Avon, Test and Itchen have mayfly hatches starting at the end of May and carrying on to early June. 

What types of rivers have mayfly? 

Mayfly are generally associated with neutral or slightly alkaline rivers which have areas of fine gravel, sand and silt where nymphs can make burrows. Acidic rivers flowing over bedrock and boulders will generally not have mayfly, but they are more likely to have two other very large upwinged flies – March browns and brook duns – because their nymphs live under stones not in burrows. 

How do you tell the difference between a dun and a spinner?

Both behaviour and appearance will help you work out if you are seeing a dun or a spinner. Duns have dull coloured wings with brown veins and are seen emerging from the surface film or flying somewhat haphazardly above the river and towards the bank. Spinners have brilliantly translucent wings and longer tails than duns. The male spinners will be dancing’ in the air in swarms; females will be laying eggs by dipping onto the surface of the water or splayed out, dead on the water surface after egg laying. Duns can also be found on the water surface if they have failed to emerge properly or are damaged and unable to fly. 

Mayfly Dun f
Mayfly spinner

Mayfly colonising a new river?

It is sometimes said that a river that starts to have a mayfly hatch when historically it had no mayfly is a river that is dying. This may be because the river changes in the catchment have increased the quantity of sediment in the river or because low flows have prevented the scouring out of sediment. Whilst neither reason is causing the river to die’ it is probably not a sign of good health. 

Is it true that they only live for 24 hours?

Mayfly in the nymph stage usually live for a couple of years, but once they emerge as flying adults they have a have a short life. They can live longer than 24 hours if they manage to avoid being eaten by fish, birds and other insects but they die once they have mated and this is generally a short time but not precisely 24 hours. 

What prompts a hatch of mayfly?

The truth is that we don’t know for sure. It is thought to be largely a function of day length but short term conditions such as changes in water temperature and air pressure also may have an influence. Mayfly do tend to hatch from mid afternoon onwards, and once the mayfly season starts there will be a fairly consistent hatch every day for a couple of weeks or so. 

Further reading 

A Pictorial Guide to British Ephemeroptera by Craig Macadam, published by the Field Studies Council. 

In the Month of Mayfly’. A short article by Craig Macadam (PDF),

Matching the Hatch by Pat O’Reilly

First nature website on mayfly and mayfly imitations.

The Riverfly Partnership website