Scientists have found that a native British parasitoid wasp (Aphaereta debilitata) has been found to be very effective at controlling the shore flies that infest lettuce and celery greenhouses, damaging crops and annoying farmers reports PlanetEarth Online. The experiment showed promising results: In all three units where the wasps were introduced, the number of shore flies was significantly reduced, along with the amount of crop damage observed, says Tilley.