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BWO Hatches 101

8 mins read
By Spencer Durrant

Understanding blue-winged olive (BWO) hatches is a huge key to becoming an angler who can succeed on the water year-round. BWO hatches are the first bug to hatch in spring – aside from midges – and they’re often the final hatch before winter sets in and trout shut down. BWOs are vital to trout, and if you want to be a great angler, understanding these little bugs should be vital to you, as well.

So, let’s get started with a crash course in BWO hatches.

close up of blue-winged olive mayfly on a dark background

What is a BWO?

A blue-winged olive is a mayfly. You’ve probably heard the term “baetis” used before when folks talk about mayflies. Baetis is a genus of mayfly, and tons of different mayfly species will hatch with olive or dark brown bodies, and blue wings. The term “blue-winged olive” is a collective phrase us fly anglers use to refer to any mayfly with an olive or dark brown body, and blue wings.

The BWO Life Cycle

graphic of a blue-winged olive mayfly life cycle depicting all life stages of this aquatic insect.

The BWO life cycle is pretty simple. These bugs hatch from an egg and form their nymph stage. These nymphs are easy to spot when turning over rocks in a river, and mayfly nymphs are one of the most commonly-imitated bugs by different fly patterns.

After the nymph stage is the emerger, where the BWO is getting ready to unfurl its wings and fly off to make more BWOs. This is the key stage of the hatch when BWOs are in a vulnerable position. Emergers don’t have quite the dexterity that nymphs or duns do, and since they live in the surface film, they’re an easy target for trout.

If a blue-wing survives to become an adult, or dun, they’ll fly off to mate and start the entire process over again.

When fish are keyed in on BWO hatches, they’re usually snacking on emergers or crippled adults that aren’t able to quickly leave the river.

When BWOs Hatch

Mayflies hatch year-round, including the little ones we collectively refer to as blue-winged olives. But the biggest hatches of BWOs occurs twice a year – in the spring, then again in the fall. The spring hatches tend to start in early April and go through May. Fall hatches occur in September and go through November, depending on your exact location.

graphic that shows when blue-winged olive hatches are at their peak, highlighting the periods of april to may and september to october

Where BWOs Hatch

BWOs hatch on just about every trout river in the world. Per the National Wildlife Federation, most BWO “nymphs develop in streams and rivers that are relatively clean.”

So, that means you’re likely to find BWOs hatching in your local river if that river has a healthy population of trout.

As far as where specifically you’re likely to see them in a river – they’ll hatch in just about any water type where you typically see bugs on the surface. Riffles, pools, and eddies are places to really look out for when searching for trout rising to BWOs. Some of the best BWO hatches I’ve ever fished were in shin-deep riffles. Obviously, you shouldn’t expect to see them hatching in rapids.

Why Trout Love BWOs

Blue-wings are such an integral food source because they’re such a prevalent aquatic insect. The big BWO hatches in the spring and fall are thick enough on some rivers that the bugs form a floating mass that almost looks like carpet. Trout are known to gulp mouthfuls of dozens of bugs at the height of a BWO hatch, instead of picking them off one-by-one.

Think of BWOs like pumpkin spice lattes. They’re only around once a year for a few months, but during that time, it’s impossible to not start craving something pumpkin spice flavored. Once BWOs start popping up, trout are going to key in on them specifically.

close-up of a trout eating blue-winged olive mayflies

Best Flies for BWO Hatches

While BWOs hatch on virtually every trout river in the world, the patterns we use to imitate these bugs at various life stages don’t change all that much. To be successful in a blue-wing hatch, you need to have a few different flies in your box.

Emergers

Hatching bugs that are emerging from their larval stage into a dun are aptly named emergers. These are must-have patterns, because they’re an easy food source for trout. Emergers often get stuck in the surface film, making them an easy target for fish to snack on. You’ll generally want to fish emerger patterns at the beginning of a hatch.

Sparkle duns, Barr’s emergers, and a good ol’ BWO emerger are great options to use during the first stage of a BWO hatch.

Both the Barr’s Emerger & BWO Emerger are included in the Emerger Baker’s Dozen Pack.

Duns

An adult mayfly is called a dun, and these bugs have the classic silhouette you probably associate with blue-winged olives. While there are a ton of different dun patterns out there for BWOs, I’ve found that a parachute is often the best choice. It sits right on the water’s surface, but has the right shape and color to blend into just about any BWO hatch.

Both Blue Wing Olive patterns are included in the Blue Wing Olive Baker’s Dozen Pack.

Cripples (or Spent Spinners)

Crippled mayflies are just that – duns that have deformed wings, or other defects which present them from taking flight and mating. Crippled bugs are an ideal food source for trout because they can’t escape a hungry fish. Often, even if there are a ton of duns on the water, fish will opt to eat a crippled mayfly instead. In fact, I usually fish through a BWO hatch with a cripple pattern instead of a dun, because it tends to be more effective.

There are a lot of ways to tie crippled BWO patterns, but the most productive one I’ve found is the Last Chance Cripple. Developed in Last Chance, Idaho, for use on the Henry’s Fork, the Last Chance Cripple isn’t a fly you should leave home without.

Here is a video on how to tie this pattern.

Wrapping Up

BWOs are a hugely important food source for trout across the world. They’re an abundant aquatic insect, which is why understanding their hatches is key to becoming a successful, well-rounded angler. Once you understand their life cycle, it’s easy to pick a fly pattern based on what fish are likely eating at the time.

Now, it’s just a matter of getting out on the water and putting this knowledge to use.

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