The Brown mayfly begins hatching when the water temperature hits about 42 degrees - which coincidentally happens around the end of March in northern climates, hence the name. The hatch is usually in the afternoon, and a spinner fall occurs later on that day. The March Brown fly pattern imitates this adult mayfly very well. These are generally big mayflies - they are not your BWO. Being the first large mayfly hatch of the year, trout feed voraciously on this hatch coming clear out of the water for your offering!