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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
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.

Dorsal view of a Amphizoa (Amphizoidae) Beetle Larva from Sears Creek in Washington
This is the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Species Baetisca laurentina (Armored Mayflies)

This Midwestern species is responsible for all my Baetisca fishing experiences. Read the section on the genus Baetisca for most of the details.

Where & when

This species is very important in parts of northern Wisconsin. In Mayflies of Michigan Trout Streams, the Leonards report that the nymphs are locally abundant in Michigan, but they did not find the adults in fishable numbers.

In 20 records from GBIF, adults of this species have been collected during June (45%), May (20%), July (15%), April (10%), and August (10%).

In 3 records from GBIF, this species has been collected at elevations of 3, 20, and 26 ft.

Species Range

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Body length: 8 mm
Wing length: 10 mm

Abdominal tergites of male imago deep brown, sternites paler brown; longitudinal veins brown.

Head and thorax deep brown; lateral flange-like edge of mesonotum paler; thoracic sternum deep brown. Legs dull amber, the fore legs deeper in color than the second and third pairs. Wings hyaline; longitudinal veins brown; cross veins pale, almost invisible except in the anal region. Abdominal tergites deep brown: sternites paler brown. Forceps dull dirty amber colored; tails pale brownish, the joinings darker.


The nymph possesses both dorsal and lateral spines on the mesonotal shield; the frontal projections are reduced to very small blunt tubercles; the genae are very slightly produced only. The dark longitudinal veins separate this species from Baetisca obesa, in which the veins are largely pale.

Specimens of the Mayfly Species Baetisca laurentina

2 Male Duns
1 Female Dun
2 Male Spinners
2 Female Spinners
11 Nymphs

1 Underwater Picture of Baetisca laurentina Mayflies:

Discussions of Baetisca laurentina

An important hatch
2 replies
Posted by Troutnut on Jun 26, 2006
Last reply on Apr 8, 2013 by Willy
Based on reports from several sources and my own experiences, I'm beginning to think it is significantly more important than it has been credited for. The duns emerge by crawling out onto land, so they aren't important, but some of my most memorable fishing nights of 2005 were due to Baetisca spinner falls.

It is a tricky hatch to detect. I haven't seen more than a couple of their spinners in the air at a time, though some of my friends report spotting their swarms. Normally for me they just showed up on the water from unseen swarms upstream. They were mixed with spinners from Ephemerella invaria and Maccaffertium vicarium, among others, but the fish were relentlessly selective to the Baetisca laurentina spinners.

I wasted the better part of an hour flinging a sulphur imitation the first time I encountered a Baetisca fall. Like Ephemerella spinners, they can be hard to spot on the water, and they were much more sparse. I finally captured one, noticed the very different body profile, and since I didn't have anything remotely imitating it I continued to catch no fish. I returned the next night with an imitation with a robust, opaque body, and the fish went crazy for it.

I just finished reading through the account by Caucci and Nastasi in Hatches II about how the extremely important Ephemerella invaria sulphur species went unnoticed for decades because it was confused with Ephemerella dorothea. The maddening difficulty of some dorothea hatches was partially explained away once people understood this difference.

Although Baetisca is much less prominent than Ephemerella invaria, I suspect it has similarly been confused with well-known sulphur species in the rare locations and occasions where it is important.
Can't wait to hit this hatch
Posted by Troutnut on Jun 12, 2006
Last reply on Jun 12, 2006 by Troutnut
It sounds like the Baetisca hatch is really going on here in northern Wisconsin now. I've heard reports from more than one source about fishable Baetisca laurentina hatches on two of my favorite rivers. I bought some imitations and hopefully I'll be using them tomorrow!

Start a Discussion of Baetisca laurentina


Mayfly Species Baetisca laurentina (Armored Mayflies)

Species Range
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