The Large Dark Olive (L.D.O.) is one of the first flies to hatch on the river in spring. Often you can get L.D.O.s hatching on the opening day of the new season if the weather is mild enough. Other times it can be as late as April before they start hatching. Weather conditions will dictate when they start hatching. It is the most important fly for the dry fly angler for the months of March and April and again in September. A must have fly for any dry fly angler in Ireland or the UK.