Plants produce a broad range of secondary metabolites that are toxic to insect herbivores, either as part of their normal program of growth and development or in response to biotic stress (insect infestation). These secondary compounds that may act as signals, i.e. allelochemicals, mediating interactions between insects and plants. Secondary metabolites are present in all plants generally as mixtures that can be highly diverse. These mixtures have been shown to protect plants against insects; thus, secondary metabolite diversity has been suggested to be a resistance trait against plant consumers. Secondary metabolites are an extremely diverse group of natural products synthesized by plants. Most of secondary metabolites, such as terpenes, phenolic compounds and alkaloids are classified based on their biosynthetic origin.