7 Basic Facts about Mayflies

I don’t always think about mayflies. But when I do, I usually catch more trout.

mayflies

Here are 7 things you need to know about Ephemeroptera — the insect order popularly known as mayflies. I’ve learned these from my friend, Bob Granger, and from the writings of Dave Hughes and Jim Schollmeyer. The insights have made me a better fly fisher:

1. All but one or two days of a mayfly’s 365-day life span is spent underwater.

This is the nymph stage. No wonder 85% of a trout’s diet comes from beneath the surface. It’s why fishing nymphs is almost always a sure bet.

2. Most mayflies hatch at mid-day.

This means that 11:00 a.m. to 2:00 p.m. is prime time — depending, of course, on wind and water temperature. Overcast, cool days are ideal, especially for Baetis flies and Blue-Winged Olives (BWOs).

3. Mayfly duns ride the surface until their upright wings are dry and hardened for flight.

Duns are the first of two adult forms of the mayfly. Their ride through the current typically lasts for ten to twenty feet. Obviously, this makes the duns vulnerable to rising trout. And these rising trout are vulnerable to your mayfly imitation.

4. If rising trout ignore the mayfly duns on the surface, they are feeding on emergers.

The emerger stage is the brief transition between the nymph stage and the dun stage. The child becomes an adult when the skin splits along the back of the nymph and the winged dun escapes. Wise anglers will put on an emerger pattern in these moments.

5. Once duns turn into spinners, they mate in the air and the females deposit their eggs.

At this point, the females are spent and fall to the water. This creates a “spinner fall” — another opportunity for a trout feeding frenzy. Anglers who see mayflies with flat wings like an airplane rather than with wings sticking up should switch to a spinner pattern.

6. Mayflies vary in size and in the time of year they appear.

In the western rivers, BWOs generally hatch from mid-March through May. Pale Morning Duns (PMDs) are more prominent from May through August. Then BWOs show up in force again in September. Typical sizes range from 14 through 18. But the brown and green Drakes in Henry’s Fork of the Snake River tend to be larger — from size 10 to 12.

7. Mayflies need cold, clean water.

Water pollution makes mayflies disappear. When mayflies disappear, the trout do too. So water conservation is vital to trout fishing.

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