The term “natural enemies” is used for organisms that kill or injure other animals. For example predators and parasitoids are natural enemies of pest insects. Spider are natural enemies of stem borers. Also pathogens are natural enemies.
In IPM, natural enemies are often called “defenders” because they are defending the crop against pests.
Natural enemies play a very important role in keeping pest populations under control. Insects can multiply very rapidly. In the absence of natural enemies, insect populations “explode” (see example in the box below). But in a healthy crop this will rarely happen.
1 female moth → 200 eggs → 200 caterpillars → 200 pupae → 100 male & 100 female adults
100 females → 20,000 eggs → 20,000 caterpillars → … → 10,000 males & 10,000 females
10,000 females → 2,000,000 eggs → 2,000,000 caterpillars … etc.
Many parasites (egg parasites, larval parasites), predators and pathogens will each take their share in reducing the pest population. Also abiotic factors such as temperature, wind and rain will contribute to this regulating process.
In a healthy crop, pest populations are thus maintained at a fairly low and steady level. However, this balance can be easily disrupted. Sprays of pesticides, which are aimed at killing pests, will also kill the natural enemies. This creates an unbalanced situation where the next generations of the pest can develop rapidly in the absence of their enemies. This process is called resurgence.