Rudd).
The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. The World Health Organization estimates that during the period of its use approximately 25 million lives were saved. DDT seemed to be the ideal insecticide. However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and DDT was also discovered to have a high toxicity toward fish.
The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolized very rapidly by animals; instead, it is deposited and stored in the fatty tissues. The biological half-life of DDT is about eight years; that is, it takes about eight years for an animal to metabolize half of the amount it assimilates. If ingestion continues at a steady rate, DDT builds up within the animal over time.
Pesticides, for the most part, are intended to kill selected organisms. All pesticides are alike in blocking some metabolic process; however, how they do this-that is, their mode of action- is sometimes very difficult to determine, and in many instances we still do not know. In rare instances a pesticide may even exhibit more than one mode of action, making classification complex. At present, we have only fragments of knowledge concerning the great array of commonly used pesticides. Even in the well-established field of molecular pharmacology, a similar but somewhat less extreme situation exists (Pesticides theory and application-George W. Ware).
Resistance to insecticides by insects dramatically exemplifies the selection principle of evolution. The susceptible insects are killed, leaving behind only those that are genetically resistant to the toxicant. Resistant individuals make up an increasingly large part of the pest population and pass their resistance on to the next and future generations.
