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Mayflies 101

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July 12, 2016
Category:   Fishing Articles

Mayflies 101

Michael Alberghini / Sportsmens Compass Team Member

Mayflies are a group of aquatic insects that make up a large percentage of most trout diets. For a fly fisherman, mayfly activity is a long awaited and carefully studied phenomena. Mayfly emergences can create spectacular fishing all day, with trout gorging themselves on bugs. Some species can be found from East to West coast, while others can have extremely small geographical ranges.


A group of male Baetis tricaudatus dry their wings

The first thing to understand is that there are many different species of mayflies each with their own differences. Each insect has its own specific time of the year and day that it will hatch, with many other variables coming into play. Most species have a nickname, often based on their color or a fly pattern first developed to imitate the insect. For most rivers you will fish, someone has made what we refer to as a “Hatch Chart.” A hatch chart shows you what time periods a given insect will emerge. This allows you to determine what insects are possible to see during a given time period and allow you to be sure you have the right flies. Some charts go more in depth than time of the year, including time of day, size of fly, etc.

You may be wondering why all species don’t just hatch at once? Most mayflies decide it is time to hatch from a combination of water temps and photo periods. Small variances in the population are responsible for early and late bloomers, and this is what allows an individual species to hatch for days, weeks or even months.

For the northeast, the first major hatch of mayflies are Ephemerella subvaria. Their more common names are Hendrickson and Red Quill, referring to the females and males, respectively. When the water temperature hits 50 degrees farenheit, E. subvaria is in its’ full swing of emergence. The nymphs, or immature insects, rise up from the streambed through the water column up to the surface. From there, the subimago (often referred to by anglers as a “dun”) breaks out of the nymphal shuck and begins to spread its wings.


Paraleptophlebia adoptiva subimago

At this time, the insect is extremely vulnerable to trout because of its inability to use its wings. Once the mayfly has sufficiently dried its wings it attempts to break free of the surface tension and eventually flies away. Mayflies will spend the day in the trees, hanging out and getting ready to mate. Winged mayflies only live about one or two days maximum. The last stage of its life involves a final molt into an imago, often referred to as a spinner due to their flight pattern during mating. Spinners lose their mouthparts, their wings generally become translucent and their bodies often turn a pale, rusty color.

In general, spinners mate above the river, usually at the head of a riffle. When the spinners finish mating, they fall to the surface of the water dead, where they become easy targets for trout. This often occurs for most mayfly species 1 or 2 hours before dark, or sometimes just after dark. This is not the case for every species, however, and knowing these differences can turn an ok day into a spectacular day. For example, after mating, the females of some species dive down beneath the surface of the river to deposit an egg sack. During these spinner falls, imitating these diving spinners rather than a traditional floating spinner can up your catch rate.


Isonychia bicolor subimago

Each mayfly species has its own quirks and subtle nuances that are important to learn if you want to be a successful trout angler. While a general knowledge of mayfly activity is enough to get it done, a deeper understanding will significantly up your catch ratio. For example, Isonychia bicolor are a species of lightning fast swimmers that almost exclusively swim out to the banks in their nymphal stage to hatch. So in this case a normal emerger pattern floating on the surface would not be effective, even though fishing emergers to most people is standard practice. Instead, nymphing with and Iso pattern and letting it swing at the end of the drift, or even giving it a few quick strips will be a vastly more effective method of fishing during an Iso emergence.

So read up on your local species and tie up or buy some flies to match them. The more you pay attention to the bugs, the more fish you will catch!



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