On Utila and Belize, we haven't been to Roatan yet but are going in June, no one in my family of 7complained more than occasionally about the sand fly bites. I however, have a terrible reaction to them. I have decided I must be allergic to the bites. Deet helps but only minimally, not worth it. They find me no matter where I go and bite any skin that is exposed. I rarely see them or feel them biting. It is only after a while I find a terrible desire to scratch. I try to ignore it. Occassionally if I dont, it helps but most often, it is impossible not to scratch, then once I do, the misery starts. I lay awake for hours every night, rubbing, not scratching, and going absolutely insane.
This next trip, I am taking cactus juice to try and Listerine, which I believe saved my life on Tobacco Caye in Belize. I also plan to take Benydryl to see if it will help with my reaction. I hope it does.
My best key to survival is to stay anywhere there is a breeze. I think the small critters struggle to maneuver in the wind. I spend my topside time on any deck I can find over the water. There is almost always a nice breeze there, a comfy hammock or chair and sitting over the water is not a bad way to spend vacation.