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Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Mayfly Species Stenacron interpunctatum (Light Cahill)

This is the best of the sporadic summer hatches known to anglers as Light Cahills. It was formerly known as Stenonema canadense and is considered by most angling historians as the model for that classic Catskill pattern.

Where & when

Time of year : Mid-May through late August, peaking in late June and early July

Stenacron interpunctatum emergence is quite sparse, but it can be important because there is so little competition on the water on late-season evenings.

The best hatching in the East is in late June an early July, and in the Midwest this species peaks about a month later.

Hatching behavior

Time of day : Sporadically all afternoon and evening in cool streams; concentrated in the evening in warm weather

This species emerges more gracefully than the related duns of Maccaffertium and Stenonema, and emerger patterns take on less importance. They drift for a while as duns, and that is the stage to imitate, although the nymphs preparing for emergence can also be of value.

Spinner behavior

Time of day: Twilight or after dark; may be earlier on cloudy days

Habitat: Riffles

Stenacron interpunctatum flies return to the stream as spinners a couple days after hatching.

The females oviposit while laying on the water.

Nymph biology

Current speed: Medium to fast

Substrate: Gravel, detritus

Environmental tolerance: Tolerant of warm temperatures, but not pollution

Specimens of the Mayfly Species Stenacron interpunctatum

1 Male Spinner
1 Nymph
1 Female Dun
Female Stenacron interpunctatum (Light Cahill) Mayfly Dun
This is a fairly small late-season female Stenacron dun.

1 Streamside Picture of Stenacron interpunctatum Mayflies:

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Species Range
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