It now seems evident that the use of non-specific insecticides, particularly in outdoor situations, presents an environmental hazard, indiscriminately killing harmful and beneficial organisms alike and leading to an accumulation of toxic residues. In any case insect resistance is a growing threat to their efficiency, and competitive economical substitute chemicals are becoming harder and harder to find. Existing methods of crop culture practices, the growing of resistant plant, environmental sanitation and biological control methods will undoubtedly have more important roles to play in the future but it is doubtful whether they can ever match the efficiency of some of the modern insecticides. New methods such as the use of specific attractants and juvenile hormones have not yet been adequately assessed. Genetic control methods as well as being species specific and non-contaminating have distinct potentialities in their own right. In their simplest form they involve the controlled exposure of insects to those sublethal irradiation doses or chemosterilant concentrations which have the least effect on their subsequent acitivity, competitiveness and longevity but which are sufficient to induce dominant lethal mutations in most of the developing gametes. These insects are then released into wild populations and when they mate with wild individuals, union of gametes may follow, but because one of them bears a dominant lethal, death occurs at an early stage in zygotic development. Lower irradiation doses (or chemosterilant concentrations, though they are seldom used in practice) result in translocations of broken chromosome parts without, in some cases, an effect on viability but in the heterozygous state always an effect on fertility. The release of such individuals results in a self-propagating form of partial sterility which seems more likely to lead to population replacement than population elimination and could provide a means of transporting genes advantageous to man.
