Blue Wing Olives (BWO) is the common name used to describe a very prolific Wisconsin mayfly hatch that occurs on trout streams all over the state. The Blue Wing Olives are small mayflies that many fly fishermen use to describe several different mayfly species. Although there are totally different mayfly species they are all small, have blueish-gray wings and olive to brownish-olive bodies. By the way, the wings can vary from light to dark and gray to bluish gray depending on the species.
The BWOs I know all come from the Baetidae family, which include Acentrella, Baetis, Plauditus, Pseudocloeon and many more. These mayflies often have multiple broods during the trout season and usually hatch in great numbers which makes them a hatch to be ready for. Blue Wing Olives generally range in size from around size #16 down to size #22 depending on the species and time of year. On the Wisconsin northwood trout streams I generally find them hatching in big numbers on really cloudy and rainy days when you sort of least expect it.
My preference is a pheasant tail nymph (PT nymph) for imitating the nymphs, in sizes #16 to #22. Several different types of emerger patterns are useful to imitate the different transition stages to a dun. These tiny mayflies seem to float a long way while trying to escape their shuck, dry their wings and finally fly away, which makes them extremely vunerable to the trout. The dun imitations I prefer are Comparaduns with either fine deer hair or CDC wings. For spinners a small red spinner with white poly wings works great.