The Baetis mayfly, also known as the blue-winged olive, little olive, little blue dun or BWO mayfly typically hatch in the spring and the fall, but are available as trout food all year around, and are one of the most abundant mayflies, making appearances in nearly every moving body of water. These small, olive bodied flies with grey to slate wings range in size from #16 - #22, and typically, but not exclusively, hatch on overcast days.
Baetis Nymph: The nymph lives under rocks, stream plants and in decomposing vegetation. They are good swimmers, so can move around by crawling or swimming.
Baetis Emerger: When mature, the nymph lets go of the river bottom, swimming upward in short bursts. Water’s surface presents a barrier that is difficult and time consuming for the nymph to penetrate. Trout recognize this stage of the Baetis hatch as an effortless meal, so fishing a Baetis emerger just below the surface is often very productive. Once the emerging nymph breaks through the water surface, their exoskeleton splits apart, and the adult or dun struggles out. A decent percent of these emergers get stuck trying to crawl out of the exoskeleton, and are called cripples. The emerger and cripple are completely defenseless, and are effortless meals for trout.
Baetis Adult: After exiting the exoskeleton, the adult rides on the water and dries its wings, then flies off as a dun or adult mayfly. The dun or adult mayfly usually fly toward vegetation along the bank, and only hours later, molt into a spinner, or a sexually mature mayfly. Spinners return above the river, flying in swarms and mate. Some species of females will return to the water, lay eggs on submerged objects in the river, while others will fly along the water surface while depositing eggs into the water. Once finished, the female dies with its wings flat on the water, and drifts down the river.