Hatch Guide: Spring Blue Wing Olives

Hatch Guide Spring Blue Wing Olives (2)

There is a large group of mayflies from the family Baetidae that we commonly refer to as BWO’s or blue-winged olives. Some other names include blue duns, little olives, and Baetis. Whatever you call them, these little mayflies are an important part of a fly fisher’s yearly playbook and often they steal the show.

That’s can be the case in spring and fall when these smallish bugs come off in masses. During spring the trout really key on them because it’s the first mayfly hatch of the year and the trout seem to appreciate an option after feeding on midges, scuds and sowbug all winter. In addition, BWO hatches often are strongest on the cold and sometimes wet days. That means these little mayflies must ride the currents for a significant time before their wings dry. That gives trout plenty of time to find these olives and the fish enjoy this easy meal. Anglers treasure the opportunity, too—once the bugs emerge, often in masses, anglers can easily match them and consistently take trout during a nice three-to four hour window.

I won’t try to provide a scientific name to describe the olives that come off on my home river, Montana’s Missouri, or any other water. One problem with common names however, is they can lead to common misconceptions. Are all of these BWOs blue winged? No. In general, they are gray winged, with the potential for a very light bluish or dun cast. Are the bodies all olive? Nope. But they probably exhibit a shade of olive or a brownish cast on top with a lighter colored belly. Sizing is also variable, with BWO hook sizes ranging from #24 up to #16 with #18 and #20 being the most common.

For a laymen such as myself, I prefer to call these small mayflies by their most popular common name—BWOs, even though their size and color vary widely wherever they are found.


Hatching can happen as early as February and continue through May. On most waters, the bulk of the hatch occurs when water temps are in the 40’s on up to the lower 50’s. BWOs fade off in warmer summer flows, only to pick up again as temps cool down in the fall. The only time they disappear entirely is during the coldest months of winter. On some temperate spring creeks, BWOs can hatch year-round.

Life Cycle

When it comes to matching the BWO, the key is understanding their life cycle and behavior, so you can fish your imitations the way fish expect to see them. We will focus on the nymphs, emergers, adults and spinners. All of those stages are important to the angler.


Nymphs are always available to fish. They are good swimmers, though they prefer moderate currents. BWO nymphs often exhibit behavioral drift, which means they travel short distances in the current to find new homes. Sunrise and sunset periods feature the strongest movements.


When an emergence kicks in, the nymphs swim to the surface. As the nymph becomes an adult it uses surface tension to break free of its old skin, which is also called a nymphal shuck. Emerger patterns can be deadly when a hatch begins and they continue to work into the peak of the emergence. Hatches often come off like clockwork around 1:30 p.m. most days, though there may be emergences earlier and later.

Hatch Guide Spring Blue Wing Olives (1)


Hatches take place regularly during their seasonal windows, though damp, low pressure weather seems is a catalyst for strong hatches. As mentioned, overcast and drizzly days provide the best surface action with lots of adults floating for extended periods before their wings dry and they can fly away from the surface. These conditions bring lots of fish to the top.

During bright days and drier weather, it does not take the adults long to dry their wings and fly away. Under these conditions, emergences often seem brief.

When imitating adults, cripple patterns are always an excellent choice. Emergence is a difficult process and some adults fail to break free of their shucks or sometimes manage to break free but have deformed or broken wings from the process. Fish quickly learn that the cripples are glued to the water and will not fly away. This makes cripples the easiest bug for these fish to take. Cripple patterns are particularly important during a heavy hatch when naturals cover the surface. A trout may not choose your adult dun imitation over the masses of actual insects floating on the surface, but they may single out your cripple pattern for he reasons just stated.


BWO spinnerfalls are not as notable as those of larger mayflies, such as the PMD. Many anglers don’t even pay attention the BWO spinnerfall. That said, most dry-fly fanatics carry some pattern to match spinners. Spinners can show up on the water almost any time of day, but are definitely a key focus in the afternoons. If the trout are conditioned to feed on top by some strong daily hatches, they will often keep looking up for any spinners that roll along.

Of note, most BWOs are underwater egg layers. That means the females crawl down plants or rocks to get under the surface and lay their eggs. Many will be carried along, sub-surface, after they expire. Dead-drifting a sunken wet fly pattern can be effective before and after an emergence.


BWOs, or whatever you want to call them, are big players in a western trout’s diet. Many anglers focus primarily on the adult phase of the life cycle. While we all love it when the fish eat dries, hardcore fly-fishers prepare for every stage of the BWO life cycle. If you want to crack the code, you’ll need a variety of flies to match various stages of this hatch, at any given time of the day. When you get it right you’ll be a happy angler because spring BWO hatches are strong, predictable, and the trout really get on them. That means plenty of trout in your net.

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I fly fish to live (25+ years guiding). I live to fly fish (obsession). At the age of two, I captured my first Bluegill in Southern Michigan. Since then, I have never stopped looking into waters for fish. My first wild trout came from the waters of Glacier NP a few years later. I spent much of my youth chasing fish in Wisconsin, the Great Lakes and throughout central Canada. I went to Alaska in 1989, where I met my wife, started a family and spent 26 seasons guiding anglers. Great Falls and the North 40 Fly Shop are now home base. Stop by and lets talk fish, bear encounters or even my experience with Bigfoot.

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