One of the weak points in using poisons is that some animals can apparently develop a resistance to them. This is not a question of a single individual becoming resistant during its lifetime. Within a large number of individuals, such as a population of flies, there will be numerous different combinations of hereditary material, and among these there will be some individuals with a genetic constitution that confers a greater degree of resistance to a given poison. There will be a tendency for such individuals to survive and to pass on this resistance to their offspring.

This ability to resist does not, however, mean that the animals are in other respects superior to the ‘old’ types. On the contrary, it has been shown that Dieldrin-resistant cockroaches, for example, have a reduced breeding capacity compared with that of the originally sensitive cockroach population.

Resistance is normally developed only to the particular poison that has been used, or to poisons of the same type, e.g. the chlorinated poisons. In such cases it should be possible to find a substance with a different composition which will prove effective.

There is no doubt that the resistance of certain pests to poisons is a serious problem, but in many cases of unsuccessful treatment it has been shown that this was due to the use of incorrect methods. Sometimes a treatment is thought to have been unsuccessful because insects have been seen to crawl away apparently unharmed by the poison. This frequently happens and it is due to the fact that synthetic insect poisons act very slowly, and it may be several hours before the animals die.