Insect-plant interactions: The gall factor

Gall-inducing capability is confined to species of Thysanoptera, Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera in Insecta, and to those of Eriophyoidea in Acarina. Consequent to gall induction, plants experience a modest level of stress, whereas the inducing organisms gain shelter and nutrition. The capability to induce galls is a specialized habit within the broad context of insect phytophagy. This habit in insects is deemed to have arisen from either the leaf-mining or the tunneling insects. Gall-inducing insect taxa remain scattered among unrelated natural orders, and induce galls on host-plant species belonging to unrelated families. They are considered to have arisen many times and through different evolutionary routes. Physiological differences in the behavior of the inducing insect and susceptibility level of the plant have also contributed to the variety of galls we see today. A majority of gall-inducing insects are specific to particular plant species, and even to specific plant organs; however, a few have been found, in recent times, to be capable of inducing galls on plant species that are closely related to their preferred hosts. A gall is an organized, usually an elegant geometrical entity, which arises as a sequel to the feeding (occasionally, oviposition) stimulus of insects. No gall-inducing insect inflicts any serious damage to the host plants. The behavioral modifications displayed in both the plant and the insect are fascinatingly revealing, which involve highly complex levels of interactions. Gallinducing insects display a sophisticated biology and physiology and utilize their hosts more efficiently and resourcefully than their nongall-inducing relatives.In such a context, this chapter elucidates aspects of species richness and patterns of adaptive radiation, what a gall is and what orchestrates its induction, processes that underscore gall initiation and growth with examples of a fewexquisite galls, besides offering an explanation to the question: why galls develop? Patterns of adaptations in the host plant in organizing nutrition for the insect, and how a rapid differentiation of the tissue of nutrition can be an indicator of the level of susceptibility and resistance in different subspecific variants of plants are also discussed.