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Mayfly

Blue-winged Olive

Baetis · genus

A small, dependable mayfly hatch that keeps Idaho trout looking up in cool-weather windows.

Overview

About Blue-winged Olive

Baetis is a genus of mayflies in the family Baetidae, commonly called Blue-winged Olives by anglers. With at least 150 described species worldwide and strong variety in North America, this is a broad hatch group rather than a single bug.

Also known as: BWO, Baetis

Life Cycle

Eggs

Female mayflies deposit hundreds to thousands of tiny eggs either in or near the water. These eggs then sink to the stream bottom to develop.

nymph

Nymphs reside on the stream bottom, where they feed and grow through a series of molts, spending the majority of their lives underwater.

emerger

The nymph actively swims to the water's surface, where it begins the crucial process of emerging into its adult form.

dun

After emerging, the freshly hatched adult, known as a dun, rests briefly on or just above the water's surface before flying off.

spinner

Matured duns molt into spinners, which are the sexually reproductive adult stage. These spinners take to the air for mating.

Egg Laying Spinner

After mating, the female spinner returns to the water's surface to deposit her eggs, initiating the next generation.

Spent Spinner

Having completed their reproductive cycle, the adult mayflies die, often floating dead on the water's surface, a common sight for anglers.

Stages: Anglers commonly fish Baetis as nymph, emerger, dun, and spinner. Like other mayflies, they develop underwater, emerge at the surface, drift briefly as duns, then return as spinners to mate and fall spent.
Emergence cues: Most important during cool to moderate conditions, often on overcast or unsettled days, with activity building when water temperatures are favorable for mayfly emergence and during softer light periods.
Behavior: Nymphs drift in the current before emergence; emergers can hang in or just under the film; duns ride the surface as they dry; spinners fall flush and helpless, often creating selective feeding.

How to Tie Blue-Winged Olive Patterns

nymph

Fly Tying a Blue Winged Olive Nymph (Mayfly Trout Pattern)

Savage Flies

Materials

Hook#14-181x long nymph
ThreadBrown
TailFlank fibers, wood duck
RibBrown thread or floss
BodyRabbit, light olive
WingcaseBrown or natural duck
ThoraxRabbit, light olive
LegsPartridge, brown or natural

emerger

Barr's Emerger BWO

tightlinevideo

Materials

Emerger Hook2X-short, sizes 18-24Dai-Riki 125
Veevus Thread16/0dark brown
Hackle FibersBrown or ginger
Beaver Dubbingolive
FlashabouOne strand
Wood Duck
Beaver FurNatural

dun

The Easiest Parachute BWO You’ll Ever Tie!

Fly Fish Food

Materials

Hook18TMC 100 Dry Fly Hook
Thread18/0SemperFli Nano Silk Ultra Fine 30D 18/0 - Olive
Tail / PostEP Fibers - Minnow
BodyFulling Mill Tactical Dry Fly Dub - Grey Olive
HackleWhiting Bronze Cape - White dyed Medium Dun

spinner

Blue Wing Olive Spinner, Holsinger's Fly Shop

Holsinger's Fly Shop

Materials

Firehole 419Size 18
Thread70 DenierOlive
Coq De Leon
DubbingX-tremely Fine, Blue Winged Olive
EP FibersTrigger Point, Blue Winged Olive

Egg Laying Spinner

Diving BWO Egg Layer

East Rosebud Fly & Tackle

Materials

Firehole 633#14Hook
Veevus10/0Thread, Olive
Coq de Leon Tailing PackTail, Medium Dun
Hare-Tron DubbingBody, Olive
Sparkle Emerger YarnWing, Clear White
Coq de Leon Tailing PackLegs, Medium Dun

Spent Spinner

Life Cycle #6: Blue Winged Olive / Baetis - Fly Tying Tutorial

troutcastz

Materials

Hook#14Ahrex FW501
TailCoq De Leon fibers
BodySuper Fine Dubbing - Blue Winged Olive
WingSwiss CDC - Dark Grey Khaki
ThoraxSuper Fine Dubbing - Blue Winged Olive

Where It Hatches

Blue-winged Olive

One species, multiple rivers.

River

Fly Patterns

Baetis Emerger#18-22
Parachute BWO#18-22
Pheasant Tail Nymph#12-22
RS2#18-24
Soft Hackle#12-18

Angler Notes

Why it matters: BWOs are one of the most dependable small mayfly groups trout feed on, and they often trigger selective rising fish when little else is happening.
Imitation tips: Carry slim nymphs, sparse emergers, low-riding duns, and spent spinners in olive and gray-olive shades. Keep patterns trim, avoid overdressing, and match size carefully.
Key life stages: nymphemergerdunspinner
Presentation: Start with a dead-drift nymph or emerger through likely seams, then switch to a drag-free dry when fish begin showing. During surface feeding, long leaders, fine tippet, and exact lane control usually matter more than fancy pattern details.

Identification

Key features: Slim mayfly profile with compact bodies, upright wings on duns, and a generally delicate look; anglers usually key on the small size and olive-toned body more than exact species.
Size: Small; commonly imitated in the #16-22 range, sometimes smaller.
Coloration: Olive to gray-olive bodies with dusky to slate-blue wings; nymphs are usually olive-brown to dark olive.
Similar bugs: PMDMahogany dunCallibaetisSmall mayfly spinner

Habitat

Water types: RiversStreamsSpring creeksTailwaters
Substrate: Common around mixed stream bottoms with gravel, cobble, and aquatic vegetation or other current-sheltered feeding lanes.
Conditions: Most relevant in moving water with healthy insect life, especially moderate flows and clear to slightly stained conditions where trout can key on small drifting mayflies.