Since your gelding is scratching both his mane and tail, it is not likely due to an internal parasite. The most likely cause of the scratching this time of the year is some type of skin allergic reaction, possibly to an insect (Culicoides) or a plant allergen. To determine the cause, have your veterinarian evaluate your horse's skin and environment. The gelding may also require skin testing or blood testing to determine the most likely allergens involved.