According to the manufacturer, while they have not conducted any safety trials in poultry regarding its effects, they do state they don’t feel there would be any danger to the chickens.  The Farnam SimpliFly with LarvaStop Feed - Thru Fly Control is a fly growth regulator which helps prevent development of stable and house flies by breaking their life cycle to keep them away from the manure of treated horses. The chickens would be eating insects, not the manure directly, so if anything, they would only be ingesting very minute amounts of the product that should not cause any harm.