The bloodworm nymph is an important part of the trouts diet. Fish expect to see them and will consume in large numbers all year. Imitating  the larvae of the chironomid midge which make up a large part of a trouts diet, the bloodworm flies are very effective indeed. Probably best results in slow waters, see the apps bloodworm for another variation. All season long the bloodworm can catch fish and is an option when fishing droppers.