The LDO is the first upwing of the season that is of importance to the fly fisher. In March you can expect the hatch anytime from 10am till 5pm, so it can be worth waiting around. A burst of LDO can come out of nowhere and equally disappear just as quick so it pays to be ready to fish hard for as long as the hatch lasts.
Large Dark Olive Emerger.
I was shown this pattern by my good friend and top dry fly angler Jim Lees. This has been my go to fly now for a number of years. Super simple and super effective.