The Tribute® Fly Control Block helps prevent the emergence of horn flies, house flies, face flies and stable flies from the manure of treated horses. The fly control ingredient in this block is ClariFly® (diflubenzuron), which is designed to pass through the horse and works by stopping the development of immature flies in manure (existing adult flies are not affected). The Tribute® Fly Control Block is safe for all horses, though should not be used with any other feed-through fly control supplements.