Pests can develop resistance against the poisons that we use. It is not the individuals that become resistant.

Resistance occurs because the control removes the pests that can be killed with the agent. In this way you make room for those individuals that do tolerate the poison best, and when they later become numerous you have a resistant population. Resistance is a relative term and means in most cases simply that pests can withstand larger and larger doses of a particular insecticide. Control attempts that fail do not necessarily mean that the pests were resistant to the poison, although it is often said. Resistance can be viewed only by laboratory tests under standardised and controlled conditions. Animals can seem resistant when they keep moving after taking in the poison, but the explanation is usually that synthetic insecticides act quite slowly, but safely. In Northern Europe, brown rats, house flies and cockroaches have proven to be resistant to some poisons.