Plant Defenses Against Insect Herbivory

Abstract

Herbivory, the act of consumption of plant biomass by specialist animals, regulates the cycling of biotic and abiotic ecosystem components, through a complex process transferring materials among various trophic levels. Herbivores include insects and mammals of varying sizes, the former being most important due to their high diversity. Insects consume the biomass in varying proportions, depending on their size and density. Apparent checks and balances between prey and predators or hosts and parasites are chemically governed functions. Plants and herbivores receive and send signals to each other as well as to organisms in higher trophic levels (predators) through volatile chemicals. Besides several morphological defence mechanisms, plants evolved specific chemical defences against insects. Among herbivores, insects also co-evolved mechanisms to overcome the volatile chemical arsenals of plants. In this review the role of plant defense against insect herbivory is discussed. The plant responses to repel insects and the synthesis of volatile chemicals to attract predatory insects or parasites are reviewed. Plants evolved genes (activated on insect attack) inducing the secretion of volatile chemicals. Such signalling attracts predators or parasites and is absent in plants when they are experimentally injured. Signalling is caused by the reaction with elicitors contained in the oral secretions of herbivorous insect. Through chemically operated keys, plants and insects regulate ecosystem functioning, allowing co-existence in wild and natural ecosystems.