Fifth in a six-part series on pollution. All pesticides are poisonous in greater or lesser degrees. Most experts regard them as short-term solutions to pest control. They kill not only the pests, but also necessary beneficial organisms, such as the oxygen-producing phyto-plankton of the ocean, which renew approximately 70 per cent of the world's oxygen supply. The program investigates long-term effects of such substances as DDT, possible alternatives to chemical pesticides - including non-poisonous biological methods of pest control.