Possible reversal of resistance.

The author discusses the factors influencing reversal of resistance to DDT, BHC and diazinon, as revealed by the experience of the last 15 years in the control of the housefly on farms in Denmark. Supplementary data from the literature on the stability of resistance in field populations of other insects are also taken into consideration and the genetic mechanisms involved in the development and reversal of resistance are examined. The author concludes that reversal often results in a heterozygous population with few highly resistant insects. If the heterozygotes are sufficiently susceptible, resumption of the use of the insecticide may give good control for a time, but resistance tends to return quickly. The earlier the development of resistance is detected and the insecticide withdrawn, the more rapidly and completely does reversal take place. Regular surveys of susceptibility are therefore necessary, preferably by methods distinguishing between homozygous and heterozygous resistance. When resistance occurs and another insecticide is substituted, reversal may be promoted by choosing an insecticide that selects against the resistant genotypes. In practice, this is difficult to achieve and there is more often a positive correlation between the two insecticides, so that the substitute insecticide slows down or prevents the reversal of resistance. The author refers briefly to methods of counteracting this effect and to the possible influencing of reversal by using methods of control other than insecticides.