Small Mayfly Nymphs

As we move into and through Spring most of cold-water streams will have emergences of Little Olive Mayflies. These bugs fall into several families, but the most common is Baetis. Please remember that trout do not speak Latin and don't care if you do either!

The bugs are often referred to as Blue Winged Olives or BWO's. Have you heard the terms and wondered what they meant?

Here is basic information that will help us understand more about BWO nymphs and emergers. We'll talk about the adults and spinners in another article!
Identification: BWO nymphs are slender, tapering from the end of their thin abdomen, the back half of the bug, to their boxy thorax, the front half of the bug, with three sets of legs. BWO nymphs have three tails and their gills are always on the abdomen. Any insect with three tails and gills on the abdomen is a mayfly nymph!

When the nymphs are ready to begin emerging they will get very dark, especially on the top of the thorax where the wings are forming and preparing to pop open.

Location: BWO nymphs are found in slower moving water. They are swimmers and wriggle through the water from one place to another. As you can see by our picture of the nymph, they are thin and don't look very sturdy. They live under rocks and other cover and eat detritus that is on the bottom of a river. The nymphs often break loose and drift with the current, becoming trout chow.

Life Cycle: BWO's have two broods a year, which means the nymphs emerging in March and April were eggs in October and November of last year. The eggs that successfully hatch and grow now will emerge again in late September, most of October and early November. What we learn now will be useful twice! BWO's hatch from an egg, grow into a larva and then emerge into an adult.

The day the bugs emerge is the happiest of their short lives. The nymph's body is ripe; the wing pad is black and ready to pop. The bug will let go of its lodgings and drift with the current toward the water's surface. When it arrives the exoskeleton, which is the outside of the bug, will split open and the adult will emerge onto the surface of the water. The entire time the bug is drifting with the current.

As the adult struggles out of the shuck it begins to dry and breaths air through breathing tubes rather than assimilating oxygen through gills. Amazing. When the adult is completely out of the shuck and the wings are dry, the adult flies off the water into a bush where it molts, becoming sexually mature. It then finds a mate. The female lays eggs and dies, drifting once again along the surface of the water.

The entire emergence process from nymph, to spent adult probably takes a few hours at most.

Sizes: The BWO's we'll see earliest will be tiny. Sizes from 20-24 will not be uncommon. As the BWO season continues the bugs may get larger, all the way to size 16-20.

Fakes: BWO patterns are abundant. Start with the basics: Pheasant Tail Nymphs - with or without a bead, and with or without flash; RSII in gray, black and chocolate; Olive or Black Hare's Ear Nymphs; locally popular patterns! Small Soft Hackle patterns are also excellent!

Fishing: I'd encourage you to rig up with two nymphs. The first one should be a size 14-16 Copper John or Bead Head Prince nymph; this will give weight to the operation and help get the BWO imitation to the bottom where the fish are feeding. To the bend of the first fly attach about eighteen inches of tippet and tie on your BWO nymph imitation. Fish along the edges of riffles where they enter into runs, or in slower water. Fish the nymph patter early in the morning and keep your eyes peeled for rising fish. That indicates the bugs are on top and it is time to switch to an adult. I'll talk about them in an ensuing article!