About 100 years ago, it was dicovered that insects transmit grave, mainly tropical diseases - very much to the astonishment of the physicians of that time, who saw this as a new, possibly easy, way of eradicating the diseases by exterminating the vectors. For some years, DDT and other then new insecticides achieved great improvements, but subsequently serious setbacks emerged: insecticide-resistance and anxiety about toxicity to man and animals. In the past twenty years, serious efforts have been made tofind alternatives, with varying degrees of success.