For example, the response of the bean to an attack is a smell which attracts plant's defenders.If the plant aggressor is the spider mite, then the plant's smell is alluring the predatory mites which are wipping out the spider mites. This smell is the indirect way of defence of the bean plant.
There is a problem though, when the plant suffers the simultaneous attack of the spider mites and the whiteflies, also. In this case, one of the important component of the smell, meaning betaocimene, is still produced, but in lower amounts, this fact leading to the plant inability of attracting predatory mites. In this case of dual attack, the plant is practically defenseless, the exposure to the spider mites and their progenies being totally and complete.
Once being revelated, this aspect led to the idea of a new method of fighting pests which are infecting the crops, the method being called "Integrated crop protection" and representing more methods combined in order to vanquish the various possible enemies.
Plants have several types of defence mechanism.Some plants have hairs on its body and leaves which prevents insects from reaching the surface of leaves. Some have spines that pierce into the mouths of insects.
Some insectivorous plants plants exude chemicals which consume insects.They change their shapes so that insects can be trapped inside and die.For example Nepenthes ,Drosera ,Utricularia ,Dionaea etc.