Baetis mayflies hatch throughout most of the year, which makes the blue-winged olive pattern an excellent trout fly to carry in your fly box year around. Trout can be fickle, so carry several "flavors" of baetis, including the blue-winged olive extended body, the blue-winged olive hatchmaster and the blue-winged olive thorax.
Hands down top 3 fly during ANY hatch. Something about this fly elicits lightning aggressive strikes, even when nothing else works. A few times, the trout have zapped this fly so hard, they've broken off before I had a chance to set the hook. Its this exact fly, not any similar pattern.