The Baetis, or Blue Winged Olive (BWO) is a Mayfly commonly found hatching on most western waters both spring and fall. On rivers that close for the winter the BWO hatch is often the anglers last chance at rising fish before the seasons end. On the flip side, BWO's are often the first Mayfly hatch of spring which frequently helps fly fishers emerge from the depths of the past winter.
Baetis tend to be small, #16's are considered huge, and most hatches are #18's and smaller. On a recent float down a western water I encountered a prolific Baetis hatch and caught a little video of the micro-Mayflies as they drifted down the river. View it here:
Baetis, like most Mayflies, swim to the surface, break free of their nymphal shell and emerge on the rivers' surface into an adult. At this point they drift on the surface while their wings dry. Once dry, they fly away, temporarily escaping feeding Trout. The warmer the weather, the less time they spend on the surface and the less likely the Trout are to take advantage of the vulnerable Mayflies. In cooler weather (especially overcast or rainy days) the tiny Mayflies spend extra time on the surface waiting for their wings to dry before they can fly away. This causes large numbers of Mayflies to be drifting on the surface for extended periods which in turn offers Trout an all you can eat lunch. These are the days many anglers stay home because it isn't bluebird weather. Don't make that mistake, watch for the cloudy days, toss your Gore-Tex jacket in the gear bag and hit the water.
Fall is a great time of year, enjoy the changing leaves and Trout packing on weight to survive the winter ahead. It won't be long before old man winter is here!