Agricultural scientists have also looked at controlling fruit flies with parasitic wasps. But the most common ones kill off only about one fly in 20, leaving plenty of survivors to go on the rampage. Lethal traps baited with fly-attracting pheromones are another option. But they, too, are expensive. Instead, most farmers simply harvest their fruit early, when it is not yet fully ripe. This makes it less vulnerable to the flies, but also less valuable.