If you start Fly Predators on the recommended start date for your zip code, we expect fly populations to never build to a substantial level. If you are starting shipments late or already have a substantial fly problem, it could take up to 3 shipments of Fly Predators to bring everything under control. This is because Fly Predators are a fly preventative. Fly Predators kill developing flies, but do not affect adult pest flies that are already flying around.