Actually, the UV wavelengths might have a great effect on the feeding habits of trout at dawn and dusk. During these periods the percentage of available light shifts more to the ultraviolet wavelengths. So, since the ultraviolet markings on the mayflies differ by species and gender, the trout can select on those UV characteristics that are, alas, invisible to most of us.
We can meet this challenge by using body materials with some UV reflectivity, or , in the case of dubbed bodies, choose a UV reflective thread. The wings should, IMO, always have substantial UV reflective components. The mayflies themselves rely on these UV markers in order to see their potential mates in the dim light. With only one night of love, they want to be able to identify the partner of the proper sex and species (don't we all?).