This swimming mayfly is the dominant mayfly in lakes. It can also occur in slow sections of some rivers. Hatches begin as early as April or May and continue until the end of September or early October.
When the water is warm enough, eggs grow to mature nymphs in about six weeks. This means there are several generations within one fishing season; the insects get progressively smaller with each generation. In spring, duns are usually matched with a size 14 dry fly, but by fall, a size 16 or even 18 is needed. Also, the duns get darker as the season progresses.
Hatches usually start in mid-morning or early afternoon. While trout take the duns during the hatch, they also take the nymphs beginning a few hours before the onset, and continue taking them during and after the hatch. A Pheasant Tail is a passable imitation of a Callibaetis nymph, but in my experience the Flashback Pheasant Tail outfishes a plain Pheasant Tail by three to one.
When fishing nymphs, use an intermediate line, a long leader, and an excruciatingly slow, steady retrieve. When using a dry fly, trim the hackle off the bottom so it is flush with the dubbing. A cripple pattern is usually a better choice when trout are feeding on the surface. When the wind blows, however, it an be difficult to get a drag-free float of a cripple or dun imitation, so you might want to switch to a nymph when it's breezy.
Spinners can be important. There are no tricks to fishing the spinner fall: just chuck your imitation out there with a thin, long tippet and wait for something to turn up; try to stay awake.