Insect-induced plant galls of India: unresolved questions

With about 2000 different galls (implies that almost the same number of inducing-insect species exists), the Indian subcontinent displays a rich variety in gall flora. Gall-inducing insects of peninsular India are endemic, whereas those in the temperate Himalayan slopes and in the Indo-Gangetic plains show affinity to Central Asian and European gall-inducing elements. Fossil records indicate that galls existed in India from the late Cenozoic period. Throughout the Indian subcontinent, species richness in gall midges (Diptera: Cecidomyiidae) is almost uniform, inducing different types of galls, including the incredible 'cylinder-piston' gall, which, however, has been recorded only in the natural areas around Coimbatore. In the light of the global pattern, cecidomyiid-induced galls should be the major component in Indian gall flora, although much needs to be known about the identities of midges and the nature of galls they induce. Gall-inducing cynipids (Hymenoptera: Cynipidae) and aphids (Hemiptera: Aphididae) are restricted to the Himalayan slopes, whereas gall-inducing thrips (Thysanoptera) are confined to peninsular India. Against such a confusing but interesting distributional pattern, this review brings into focus several biological, ecological, and evolutionary questions that remain unanswered in the background of what a gall is, how galls are initiated and what factors trigger gall growth, what designs are evident among gall-inducing insects in terms of host dependence, and biogeographical patterns in the distribution of gall-susceptible plants vis-a-vis gall-inducing insects, referring specifically to the gall flora of the Indian subcontinent.