No matter whether you have been fly fishing before or not, you are probably familiar with what a mayfly looks like. They are synonymous with the sport of fly fishing. Often found on the covers of books, postcards, fishing magazines, and in fishing photography, mayflies play an important role in the art of fly fishing and the fly fishing world.
While it could be argued that mayflies are not the most important aquatic insects in the river (caddis are more prolific and can tolerate greater environmental conditions), they are critical to summer fly fishing. Understanding aquatic insects, their place in the food chain, and their life cycle, will result in more fish caught, a deeper enjoyment of the sport of fly fishing, and better understanding of the complex ecosystems of rivers and streams.
Here's everything you need to know about mayflies in fly fishing.
Table of contents
What is a Mayfly?
Mayflies are small aquatic insects that belong to the order Ephemeroptera. There are over 3000 different species of mayflies. Don't feel overwhelmed. We will cover some of the most common mayfly hatches that an angler needs to know in order to maximize your time on the water.
The Mayfly Lifecycle
Mayflies are aquatic insects, which means that they live in water. Mayflies hatch in a process that involves 4 stages: egg, mayfly nymph, sub-imago, and imago. The female mayfly forms an egg sack on her abdomen. Her eggs are dropped in the water (4,000-8,000 in a single egg sack) and the eggs sink to the bottom of the river. They stick to rocks or debris and will remain in the egg stage from one day to several weeks. Eventually, they change into the nymphal stage.
Mayfly larvae or mayfly nymphs, as fishermen call them, can grow anywhere from 3 to 25 mm. Characteristically, a mayfly nymph possesses a long slender body with three distinct parts: the head, thorax and abdomen. Most mayfly nymphs have two or three long tails. In addition, mayflies have smaller gills that are attached to their abdomens. The gills are extremely sensitive to silt and water pollution, so they typically occupy healthy bodies of water.
Left with few means to mislead predators Mayflies spend the majority of their lives in the nymph stage. In addition, they are typically unavailable to fish as a food source for most of their lives. The reason for this is that mayflies can be separated into 4 categories: swimmers, burrowers, clingers, and crawlers. The chosen method of development keeps them out of harm's way of predators.
Mayflies emerge when environmental factors reach an optimal tipping point. The main factor to affect mayfly hatches is the water temperature. Temperature for hatches varies greatly depending on the species. In most rivers, the temperature needs to be at a minimum of 46 degrees. In the summer 60 degrees is an optimal temperature for most mayflies.
Hatches are the most prolific on cool cloudy days. This is because cool moist air makes it difficult for mayflies to dry their wings which must sit vertically for them to take flight. The more damp and cold, the longer it takes for their wings to dry, which means mayflies are drifting longer downstream to hungry trout.
Burrowers, Crawlers, Clingers, and Swimmers
Many mayfly nymphs will burrow into the stream bed floor. This provides ample opportunity for the nymphs to grow and be safe from predators. Clingers, swimmers, and crawlers are usually found occupying the underside of subsurface rocks. This is obvious when flipping rocks over in your local river. Almost immediately small squirming aquatic insects can be seen on the underside of the stones. Not only does this provide shelter from predators, but the rock provides a valuable food source for the nymphs. Using their specially adapted jaws, they can scrape algae from the underside of the rocks.
The main point to understand is that for most of the insect's and fish's life, they are unavailable to one another. It is at the very moment when time, water temperature, the time of the year, and many other environmental factors reach a tipping point. The nymph must transition from nymph to sub-imago.
Mayfly Nymphs and Emergers
This precipitates the moment when the mayfly nymph crawls out from under the rock or burrows out of the stream bed and makes it way to the surface of the river. This the first opportunity for a fish to eat the insect. In order for the nymph to emerge to the surface of the river, it must swim vertical in the water column. To help aid in the process, mayflies will trap air bubbles between their skin and nymphal shuck. This small air bubble acts as life jacket and helps them on their way to the surface. In addition, fish key in on this as well. This is the reason that many flies are tied with a small piece of flash or a clear glass bead. These added touches simulate the air bubble during emergence.
This portion of mayfly hatches are known as the emerger phase. Often time this is most important phase of the hatch for fly fisherman. Emergers can be eaten by trout anywhere in the water column, deep subsurface or in the actual surface film. In my opinion the emerger phase of the hatch is more important then adult mayflies. After all, fish live underwater, so the majority of the food eaten will also be underwater. Furthermore, you can nymph emerger patterns deep, mid water column, or a with a more traditional dry fly pattern just at the surface.
In addition, the mayfly emergers in the surface film can offer anglers some of the easiest dry fly fishing. Trout are looking for an easy meal. Mayflies have the most difficult part of their journey when they meet the meniscus at the surface of the water. This strong barrier of water is the result of surface tension created by water molecules pulling against each other in a three dimensional way. Getting through the meniscus is a laborious process. Many mayflies don't make it through, or hang in the surface for long periods of time. This is easiest meal for trout. An immobile prey, helplessly struggling to break free the menscus and it's nymphal shuck make for an easy target.
The Mayfly Subimago Phase
Mayflies that make it out of their nymphal shuck onto the surface of the river, successfully make it the sub imago phase (or dun phase as they are known to fisherman) of the hatch lifecycle. This is essentially the adolescent portion of a adult mayflies life. Immature mayflies are not sexually mature, mayflies spend long amounts of time floating on the surface water. They must dry their wings before they take flight from the water. Mayflies have 2 sets of wings intact at maturity. The hind wings are usually very small and often hard to see. This makes them easy to identify.
The wings are held upright in a vertical fashion. To a fly fishermen, depending on what kind of mayfly nymphs are hatching, mayflies appear similar to small sailboats floating on the rivers surface. This is the exact opposite of caddisfly larvae or stone flies hatching. Both of those insects have wings that lay down in a horizontal fashion.
This is the second opportunity of vulnerability that both trout and fly fisherman can take advantage of. In addition, environmental factors, such as: cloud cover, cool air, and moisture in the air can all prolong the process. This means longer time spent on the water and thus more feeding fish and mayfly hatches with greater intensity.
The Adult Mayfly
Once the mayflies' wings are fully formed and dried, they leave the surface of the water and fly to nearby bushes. Once in the streamside bushes, they will occupy this space for 24-48 hours, molting one last time, reaching sexual maturity and transitioning into the adult stage. It should be noted that an mature mayfly is unique in that unlike other insects, it is the only insect that molts after it's wings are fully formed. This is the final phase of the lifecycle, and the adult mayfly or the imago phase, and eventually finding a mate.
Any angler that has spent time on a river during mayfly hatches will recognize this last portion of the lifecycle by the large clouds of mating mayflies that gather in swarms over the bushes of the river. This can happen anytime of the day, but most often it happens in the evening. Fly fishermen, often refer to the mayfly adults as spinners.
The female, with her egg sack fully formed on the end of her abdomen, will return to the river's surface and drop her egg sack. In one last moment of exhaustion, she falls flat to surface of the water fully "spent" of life and dies. This is the final moment that fish and will be able to capitalize on the hatch. Lifeless morsels of food floating down the river are too good to pass up.
How to Fish a Mayfly Hatch
Deciphering what exactly is happening during the hatch can sometimes be difficult. In addition, when the fish are up and eating, there is no doubt you will not remember the intricacies of this article. Therefore, to help you remember the the different portions of the hatch and how to capitalize on them I'll make it as easy as possible.
Phase 1: Egg - Not important to anglers
Phase 2: Nymph - Not super important, most of their lives are spent burrowed or under rocks out of the way from predators. These are opportunistically knocked free in the water current and will be eaten by fish.
Phase 3: Emerger
- If no fish are present on the surface of the water, nymph weighted flies or emerger nymph flies under an indicator deep in the water column
- If fish are present high in the water column or you notice fish rising at the surface similar to a porpoise arcing through the water, then fish a emerger dry fly in the surface film, or a small unweighted nymph emerger on a greased piece of tippet. This allows the small nymph to hang just under the surface of the water, simulating the final phase of a nymph emergers journey to the surface.
Phase 4: Duns
- This is very obvious. If you see fish heads popping out above the surface of the river and open mouths eating small sailboat like insects on the surface, present a true dry fly pattern matching the size and color. This is dry fly fishing at it's finest.
Phase 5: Spinner Fall
- This is probably the most difficult phase to decipher as an angler. A spinner fall will often look similar to fish eating emergers. Many trout will be poirpoising on the surface of the river, but they will not eat emergers. Close observation is needed to decipher if there are adult mayflies splayed out flush to the water's surface. This is the time to cast a spinner patterns that lay flush to the surface of the river.
What Mayflies are Important to Fly Fishing
Depending on where you fish, there are different mayfly hatches that are going to be important to you. I would suggest consulting your local fly shops or local hatch charts, to familiarize yourself with the many species in your area. In addition, the common names of mayflies often change depending on the geographic region and fly fishing tradition. This can make it confusing when trying to decipher which mayfly hatches are occurring. Below are some that anglers should universally know and they typically are important in a trout's diet.
Blue Wing Olive
Names: B.W.O., Blue Wings, Baetis, Little Blue Winged Olive, Small Blue Winged Olive
Size: Small usually 6mm-14mm
Color: Olive, grey, dark grey
Hatch Time: Early and Late Spring. A coldy day can really make this hatch pop.
Flies: Choose flies in sizes #16-#22.
Dries: Small parachute adams, klinkhammers, B.W.O. foam para emerger, baetis cripple
Nymphs: WD40 emerger, small RS2, simple baetis, flash back pheasant tails, RS2 mercury BWO
Blue-winged olives are a mayfly within the genus Baetis and part of the Batidae family. This species of mayfly are extremely widespread across North America, and there is a good chance that if you fly fish they are hatching near you.
Blue-winged olives are often a favorite hatch for anglers. This is usually one of the first mayfly hatches in the spring. In addition, they hatch in large numbers, and the duns ride on the surface of the water for long periods of time. The B.W.O. dun is a developing immature mayfly. Duns ride for long periods of time on the water’s surface to develop their upright wings. This time period makes them ideal targets for trout looking for an easy meal. It also gives the fly fisherman an excellent chance to dry fly fish.
Pale Morning Duns
Names: P.M.D.'s, Summer Fly, Little Yellow Mayfly
Size: 5mm-9mm
Color: Yellow, Pink, Light Green
Hatch Time: Summer - late June, July and August
Flies: Choose flies in sizes #14-20.
Dries: small parachute adams, PMD thorax, light cahill, para PMD, light cahill parachute, CDC comparadun, Harrop's P.M.D. cripple
Nymphs: Newburry's purdy john, split cased PMD, frenchie pheasant tail, CDC jig head pheasant tail, Bighole PMD, PMD perdigon, quill jig perdigon
P.M.D.'s are some of the most important mayflies hatching in the summer months. This is one of my favorite hatches. The nymphs love to live in shallow gravely riffles. When the time is right they make their way to the surface of the river to hatch. This will often result in large numbers of trout willing to move out of deeper water and into the shallow water to feed.
When this mayfly hatches is can last a few hours, and make for some exciting top water fishing. This is especially true on cloudy days. One thing that can be confusing is the color of the insect. While most people think they are yellow, they can also be pink and even light green. If fish start to refuse your dry flies, change to a cripple or comparadun fly pattern. This will usually result in additional eats.
Green Drakes
Names: Greater Green Drakes, Western Green Drakes, Eastern Green Drakes, Lead Wing Olives, Great-Lead Wing Olives
Size: 13 mm-16 mm
Color: Bright green to Dark Green, There are also undertones of yellow bands
Hatch Time: Summer - late June, July and August
Flies: Choose flies in sizes #6-#10.
Dries: Green Drake Paradrake, Green Drake Foam Para Emerger, Harrops Green Drake Cripple, CDC Green Drake Emerger
Nymphs: Newbury's dirty hipster green drake, Olive Hares Ear
The green drake hatch is considered by many to to be the pinnacle of dry fly fishing. I have personally experienced epic dry fly fishing during green drake hatches on the Henry's Fork, Taylor River, and South Fork of the Snake, and on the Roaring Fork. It is a un-reliable and sporadic hatch on many rivers, so If you are lucky enough to catch the hatch at the right time, it is an amazing experience.
It should be noted that the nymph and spinner stages of this hatch are not super important. Although trout will eat both phases, fish mostly key into the emerger and dun stage of the green drake hatch. When this hatch is on, most often every big fish in the river will be up and eating. It can turn into an absolute feeding riot. You cannot miss it, as green drakes are large, and it is easy to spot heads annihilating insects at the surface.
In this scenario it is best to take your time, pick out specific fish, and make well timed presentations with dry flies. Flies will need to presented into specific feeding lanes. I prefer to fish a two dry rig with a proper dry fly as my point fly and 24" of tippet tied to the bend of the hook with a CDC emerger pattern. This covers both bases well. Extending your leader to get some extra dead drift out of the fly will help too.
Flavs
Names: Lesser Green Drakes, Small Wester Green Drakes, Small Lead Wing Olives,
Size: 7 mm-12 mm
Color: Dark Green or Dark Grey
Hatch Time: Summer - late July, August, and into early Fall
Flies: Choose flies in sizes #10-#16
Dries: Rusty Spinner, Parachute Adams, Foam Para Emerger in Olive, Extended Body Para Emerger Drake, Harrop's Cripple Flav, Jak's Flav
Nymphs: Olive Hare's Ears, Pheasant Tails,
Flavs are part of the same genus as green drakes. They appear very similar in shape. They are much smaller then Green Drakes and this is what set them apart. While the green drake is probably the more famous hatch, the flav hatch will typically follows the green drake hatch. It last longer, is more predictable, and puts trout into a similar feeding frenzy. Although it is not as famous as the green drake hatch, for anglers it is probably more important.
Flavs spend most of their lives in medium to fast water and then migrate to slower water prior to hatching. This means that they will often be found floating on slick flat water with lots of large fish up and eating them. This the perfect dry fly fishing scenario. As with green drakes, take your time, and pick your fish. While the emerger and dun portions are the most important times of the flav hatch, fish will eat the nymphs and spinner portion of this hatch. Carry a wide range of flies for this hatch. Stocking up on a robust number of dun and emerger patterns with some common nymphs like pheasant tails and hare's ears. Larger rusty spinners are wise to carry too.
Tricos
Names: Trikes, white winged curse, and snowflake mayflies
Size: 3mm-6mm
Color: Male dark brown, female light green abdomen dark brown thorax. Wings are whitish grey
Hatch Time: Summer - late July, August, and into early Fall
Flies: Choose flies in sizes #18-#22
Dries: Unlike other mayfly hatches where often one fly will do the trick, you will need to carry two colors of dries for both the dun and spinner phase of the hatch. The males are dark brown or black and the females are best represented with the color olive. Small parachute adams, small cdc dries, and other generic dry fly patterns in the same color and size will work. For the spinners, small poly wing spinners in colors representing both sexes are effective. Midge patterns can also work if you are in a pinch.
Nymphs: Almost small generic pattern will work. Small pheasant tails and small B.W.O. nymphs patterns will work well. RS2 patterns in black, brown, and olive are effective.
The dun and spinner phase are the most important part of this hatch. Tricos are the smallest mayflies on the river, and a fishes level of selectiveness can seem maddening. Males and females are different colors. In addition, they are multi brood insects, meaning multiple generations hatch at one time. This is why it can seem like hatches go on and on.
Fish will key in on a specific size, color and sex. This makes for frustrating fishing, as every fish in the river can be up but you will receive nothing but refusals. This is why they are named the white wing curse. It is best to carry multiple colors of dries representing both sexes. Start with the male first and if that doesn't work, then switch to the female. If you are catching fish and then suddenly stop catching trout, but fish are still rising...switch to the different sex mayfly.