Blue-Winged Olive

Telling it apart

• Small: size 16–20 in spring, 20–22 in autumn.
• Two tails only — the fastest thing to check on the water.
• Blue-grey wings, tall and rounded, with no mottling.
• Olive to olive-brown body, usually darker in autumn.
• A slender swimmer nymph that darts like a tiny minnow.

Several broods a year

Most of our mayflies run one generation a year: one hatch, one window, done. Baetis runs two or three. That is the whole reason you see them from ice-out to first snow, and why they reappear in September when nearly everything else has finished.

A useful side effect: the late broods are smaller than the spring ones. A box holding nothing but 16s will leave you short in October. Drop to 20, sometimes 22.

Why the bad days are the good ones

Everyone repeats that you want a cold grey day for a good Baetis hatch. True — and the mechanism is simple: in cold, humid air the dun takes far longer to dry its wings, so it rides much longer before it can fly.

The number of insects has not necessarily changed. What has changed is how long each one stays on the surface, which can double or triple. The apparent density goes up, trout move into feeding lies and commit. Bright May sunshine produces the same hatch, but each insect leaves in seconds and the fish never settle.

The spinner nobody fishes

Here is the detail worth the trip. In several Baetis species the female spinner does not lay by tapping the surface. She goes underwater — often crawling down a rock or a submerged branch — and glues her eggs to the substrate. She dies down there, then drifts back out, drowned.

So after every serious hatch there is a mass of drowned spinners drifting mid-water, and almost nobody fishes it. A small olive soft-hackle swung just under the surface at the tail end of a hatch is one of the better-kept secrets on our water.