BWO Life Cycle
Blue Winged Olives undergo incomplete metamorphosis, meaning they have an egg, nymph, and adult stage. Mayflies lack the pupa stage that aquatic insects such as midges and caddis experience. BWO's hatch year round. However, there are two notable hatches, one in the Spring and one in the Fall. The baetis that hatch around these major events are pseudocleons better known as Pseudos. These tiny insects are just a pain in the ass and a different topic completely. So the Spring BWO hatch occurs late March - May and in the Fall late September - November, both depending heavily on weather. Cloudy with rain and or snow provides the best hatches and the happiest trout.
While true life stages are mentioned above, there are a few "stages" that really matter for anglers, nymph, emerger, dun, and spinner. Nymphs are available to trout when they blow little bubbles on their back and begin their ascent through the water column. Prior to this they can get washed off of rocks or plant life and find themselves bouncing along the bottom of the river before the hatch even commences. Once those nymphs hit the surface their skin splits on the back of their thorax and they must break out of the skin and through the surface of the water. Known as a mayfly Emerger. Dangerous time to be a BWO. Very susceptible to predation by trout. Now air breathing critters, or Duns, they drift along the surface of the water waiting for their wings to dry enough to take flight. Baetis fly to the nearest vegetation along the riparian corridor and wait to molt once more. Post this final molt, the blue wings are sexually ready to mate. Referred to as Spinners. Spinners can be identified by transparent wings and or a rusty colored body. The bugs mate and females hit the water to lay eggs and immediately become trout food, wings straight out to the sides, or Spent Spinners.
Crazy right? Even crazier is that all of this hatching happens within a few hours. Crazy.
Fishing The BWO Stages
Baetis nymphs need to be fished at a dead drift, and at the depth that the fish are holding at. They can be fished pretty much any time, starting September throughout the entirety of the Fall hatch as they are more active during this period. Match the size of naturals you may see drifting through the water column or find when digging through rocks. This will definitely be the way to go if you see zero rising trout. The most difficult part will be determining how deep to fish. Trial and error is the best way to determine depth. If no adults are seen at all, close to the bottom will get most eats. Tungsten beaded flies or split shot to get down in this scenario. Changes depths until you get an eat!
Flies to fish: Juju Baetis, olive Micro May, and Pearl Lightning Bug.
When trout are stuck eating emergers they can be difficult to trick and cause extreme frustration for the angler. A fin poking out of the water is a dead giveaway. Look for circles like a rising trout produces but rather than a mouth breaking the surface, a caudal or dorsal fin porpoising indicates a trout that is eating an emerger. Two methods can be used to approach this scenario. First, fish a weightless nymph or emerger pattern just behind a dry fly. Second, fish a pattern such as a Last Chance Cripple or Mole Fly with floatant only on the post. This allows part of the fly to ride in the film while the portion coated with Dry Magic will remain visible to the angler. Both methods must be dead drifted!
Flies to fish: pink CDC emerger, Smoke Jumper, BWO Bubbleback
NoTe: Fish stuck on emergers will not eat a dry fly!
Easiest to see, and most fun to fish. Watch for the white of the inside of a trouts mouth to open wide and take down the little sailboat BWO on the water. Fish a single dry fly that matches the size of the blue winged olives you see on the water or flying to the bushes at a perfect dead drift and you are good to go. If you feel certain that fish are on duns but they won't eat your fly, try a pattern that has CDC on it or a better drift.
Flies to fish: BWO Hi-vis Parachute, BWO Sparkle Dun, CDC Biot Comparadun
When fish are on spinners it can be more difficult to identify by trout rise form but is usually quite obvious when you factor in what you see on the surface of the water. The rise form is usually the tiniest little dimple on the surface of the water which is the result of a fish sucking down a spinner like a toilet flushes. Typically nothing breaks the surface of the water but you may see a tiny bump of displaced water from the trouts nose. The smaller the dimple the larger the trout. On the water you will see piles and piles of spinners with maybe a few duns mixed in between.
Fly to use: That's right, you only need one, Hi-Vis Rusty Spinner
If you want to see photos and learn more about aquatic insects, stop by our Bozeman fly shop to buy the book "Western Hatches" it is super helpful. Now that Fall is officially here, come gear up for the Baetis hatch!