Diseases of insects are important factors reducing the number of insects in natural populations. They are density dependent and they act especially under conditions of outbreaks and close contact of susceptible hosts. They are im- portant allies of man in his effort to replace toxic chemicals with non-toxic, biological means in the conservation of the environment of mankind. The importance of diseases as reducing factors has often been disregarded in studies on insect ecology and population dynamics. Most cases of mortality in carefully watched experiments have simply been characterized as...