When a pesticide is used repeatedly the species may develop a resistance to it. People then develop more powerful pesticides to kill the resistant pests. These in turn cause a development of populations resistant to the new pesticides. It seems that man cannot stay ahead of the pests. Although the use of pesticide has greatly increased over the last decades, the damage caused by pests is about the same as it was before the use of chemical pesticides.
Many pesticides remain in soil or water long after they are used or needed.
Invertebrates, resistant to the pesticide, take it in, and when they are eaten, pass the pesticide along. If an animal eats enough invertebrates containing a pesticide, it may eventually die or be unable to reproduce. The presence of this pesticide in the falcon's body caused the eggs of the bird to be infertile or thin-shelled. These thin-shelled eggs brake before the young could hatch. DDT was banned in Alberta when it was discovered that the pesticide was causing the disappearance of peregrine falcons and other birds of prey.