Selective effects of insecticides on within-species variation: The lessons to be learnt when considering the environmental effects of pollutants

The use of insecticides has created environments which have favoured the development of resistance in many species that have been exposed to them. Exposure to insecticides has also brought about the development of resistance in some non-pest species (Section 1). However, because of the suddenness of the spread of resistance in many instances, it may bring about a concomitant erosion of genetic variability (Section 2); this process has been called the hitch-hiking effect. The extent of the erosion of variability that may be expected in a particular instance has been shown to depend on the extent and distribution of exposure to the pollutant, the genetic parameters of the species population and the genetic nature of the resistance factor (Section 3).

Genetic analyses have been carried out in a number of cases, and pest resistance to insecticides seems usually to be attributable to a gene at one locus. If the cases of resistance which have been investigated are typical, then hitch-hiking effects may be expected in some instances and not in others (Section 4).

The development of the technique of gel electrophoresis has enabled estimates to be made of the genetic variability of species populations. It is therefore now possible to investigate directly whether pollution has eroded genetic variability in particular instances. This seems to have been investigated in only one case, although estimates of genetic variability have been made for over 250 species populations and instances of pollution could easily be investigated using available techniques (Section 5).

It is suggested that electrophoretic analyses of genetic variability should be incorporated into pollution monitoring programmes and carried out in polluted and control environments on the same individuals for which measurements of pollutant concentrations are taken (Section 6).