Fly Fishing Entomology 101 – Blue-Winged Olive

Veteran fly fisher Dave Hughes claims that Blue-Winged Olives are the most important mayflies for fly fishing. I believe he is right. Trout seem to feed on them with the same intensity that kids (and adults!) eat popcorn. Here is a quick profile of this species:


  • “Blue-Winged Olive” is commonly abbreviated as “BWO.”
  • BWOs are also known as “Little Olives.”
  • The Latin name for BWOs is Baetis. Technically, the BWO is a sub-species of Baetis, but many fly fishers use “BWO” and Baetis as synonyms.

The Basics

  • These flies are ubiquitous. You will find them in slow, medium, and fast currents. They live in freestone rivers, spring creeks, and tailwaters.
  • Although BWO hatches happen every month, they are most prolific in April-May and again in September-October.
  • The best time of day for BWO hatches is late morning to early afternoon — the warmest part of the day. Cloudy, rainy conditions intensify and lengthen these hatches.

Nymph Stage

  • While BWOs in the nymph stage are excellent swimmers, they tend to drift with little or no movement.
  • BWO nymphs have slender, tapered bodies which some fly fishers describe as “torpedo-shaped.” Their color ranges from olive to dark brown.
  • BWO nymphs have two long antennae and three tails—with the center tail considerably shorter than the outer two.

Adult Stage

  • The most prominent feature of a BWO dun (newly hatched adult) is its large wings in comparison with the rest of its body. The wing color varies from a pale gray to a dark gray with a bluish tint — hence the name “Blue Winged Olive.”
  • BWO duns ride the surface of the current for up to twenty feet until their wings dry and they can fly. Also, some BWOs get stuck in an “emerger” phase while they are trying to scape their nymphal shuck.
  • A fully mature BWO adult is called a “spinner.” Within twelve hours of emerging to the surface and flying to streamside bushes or brush, the sexually mature BWOs mate in swarms near the edge of a river or stream. So trout typically feed on BWO spinners in slower water near the river’s edge.

Effective Patterns

  • The classic BWO nymph pattern is a Pheasant Tail (or some variation of it).
  • One of the best emerger patterns is Craig Matthews’ Little Olive Sparkle Dun.
  • For the dun stage, a Parachute Adams will often work as well as a Parachute BWO. If the trout are not hitting one of these standard patterns, then switch to a Red Quill Spinner or a Blue Quill Spinner.
  • Hook sizes for BWOs will range between 16 and 24. However, a size 18 or 20 usually does the trick.

Sources: Bob Granger, Dave Hughes, Craig Matthews, Jim Schollmeyer

5 Replies to “Fly Fishing Entomology 101 – Blue-Winged Olive”

  1. I appreciate the information in Entomology 101, this is helping me understand the particular fly better, these articles are to the point and priceless. Thanks

  2. Since I am going to Yellowstone in the middle of August this year, what flies do you recommend that I tie for this occasion?

    1. Terrestrials should be starting–hoppers, beatles, ants. I’d consider some smaller hopper patterns too–in a size 10 or 12 (as well as the standard size 8). Elk Hair Caddis patterns work well (14, 16), and you might even tie some Spruce Moth patterns. We’ve had success on those in August in the Yellowstone ecosystem. Streamers are still a good bet, especially during mid-day. Brown, black, and olive are good colors.

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