Plant and insects have co-existed for more than hundred million years. This long term co-existence makes them set up a complex interaction system about foraging and defensing. Since almost a millions years, plant defense evolved, and at the same point of time, the insects foraging mode evolved further resulting to widen their diet. These phenomena showed strong proof that insect-plant interactions are developed by co-evolution of both plant and insect. Also, insects are one of the richest organisms group in nature, which have more than 800,000 species, and nearly half of them feed on plants. Yet, the plants can be seen covered nearly all across the planet.
Plants could be attacked by a large amount of herbivores and diseases as carried from different species. It is important to get high level defense to have a better life. Speaking of plants defense, the definition would be to describe plants evolved a series of adaptions to improve their survival by reducing the impact of herbivores. There are different defensive mechanisms, which contribute to a negative influence on host insects. For the target of the defense, two types can be drawn, direct defense and indirect defense. Direct defense make the herbivores target, and indirect defense means to enhance the attractive level of the plant to the nature enemies of herbivores.
For different pathways of defense, following two catalogues can be concluded: 1. Preexisting or constitutive defenses are referred to the first guard of defense system. It consist of defensive components, for example: thorns, waxy layers, trichomes, some toxic secondary metabolites (terpenoids, phenolics and nitrogen-contained compounds like alkaloids, cyanogenic glycosides), gum-produced and etc. 2. Inducible defenses, result of evolution, only can be activated upon attack. For direct inducible defenses, they have ability to change the structure of associated components such as protein, and they have highly plastic trait, which can change phenotype flexibly based on different stimulation coming from the herbivores. For indirect inducible defenses, they can release volatiles as chemical information to help the natural enemies of herbivores working more efficient.
By long- term co- existence, the plant defense evolved, and at the same point of time, the insects as one of the most diverse groups evolved by changing foraging behavior. Symbiotic association is one of the co-evolution results. Associations between different organisms can be concluded as symbiosis, parasitism and predator-prey system. During natural selections, symbiosis sparked for it is a kind of associations that both of the participating organisms get benefits. The entire organism like insect microorganism, and plant, will more or less have symbiosis associations with each other. For instance, some of the plant have ancient bacteria for photosynthetic. Even we human-beings, there are bacteria and virus exists in our body system like mitochondria. Insects as multicellular are seems to have the highest stand ability for foreign organisms, which can live with the organisms inside and outside the body in large amount of forms. Symbiotic associations will probably change the host organism’s regular metabolic pathways by involving in as a function unit. It has been known for a long time that symbiotic bacteria can help the insects expending their range of diets and facilitating colonization of new feeding niches.