Galls arise as the result of an inter-specific association between a host plant and an insect’ (Rohfritsch 1992). ‘Gall morphology is influenced by two genotypes: that of the insect, which provide the stimulus, and that of the plant, which determines the growth response. Gall inducers are distinctive in that they actively manipulate the host plant through mechanical and or chemical stimuli to form a structure which provide the inducer with both nutrition and shelter’ (Stone & Scho??nrogge 2003). The various agents that have been reported to act as biological incitants of tumorous growth in plants can be classified as follows:
i. Virus: Abnormal plant growths induced by viruses. Sweet clover root tumor is a well studied example (Kelly et al. 1949).
ii. Bacteria: Bacteria induce plant tumors and most common example is crow gall tumor induced by Agrobacterium tumefaciens. It is one of the experimental models in the field of plant tumor physiology (Stafford 2000).
iii. Fungus: Fungi also induce galls on various plant organs. Root galls on Brassica campestris Linn. (Mani 2000) is an example.
iv. Nematoda: Many round worms were also reported as gall inducers especially root galls. Root galls on Abelmoschues esculentus (Linn.) Goodey (Mani 1948) is one example.
v. Mites: The gall inducing acarines mostly belong to the family Eriophyidae and lesser extent to the Tenuipalpidae (Channabasavanna 1966). Leaf gall on Pongamia glabra Vent is a common mite gall observed in south India (Sundar-Raman 1924)
vi. Insects: Gall inducing capacity exists in diverse insect orders including Thysanoptera, Hemiptera, Diptera, Hymenoptera, Lepidoptera and Coleoptera (Mani 2000; Raman 2007a).
a. Thysanoptera: Thrips are mostly phytophagous (feeding on plants) or mycophagous (feeding on fungal mycelia or spores) (Raman & Ananthakrishnan 1983a). Thrips induced galls vary from irregular proliferation of plant tissues to highly organized structures (Ananthakrishnan & Raman 1989). Austrothrips cochinchinensis Karny causing bud gall on the axillary buds of Getonia floribunda Lamarck (Combretaceae) (Ayyar 1928; Mani 1948) is a common example from south India.
b. Diptera: ‘The family Cecidomyiidae (gall midges) is one of the most species rich families of Diptera and also contains the largest group of gall inducing insects in the world’ (Skuhrava & Skuhravy 2010). A total of 394 species belonging to 125 genera under three subfamilies are reported from India (Sharma 2009). About 45% of the total galls on Indian flora belong to gall midges (Mani 1948). ‘On a global scale in addition to the family Cecidomyiidae, few gall inducers were also reported from family Tephritidae and Chloropidae’ (Dreger-Jauffret & Shorthouse 1992).
c. Hemiptera: The gall induction property in Hemiptera can be seen in scale insects, aphids, and psyllids. Psyllids induce diverse galls ranging from simple leaf roll to highly complex galls (Yang & Raman 2007). Gall inducing taxa in Psylloidea occur in all of the families including Triozidae, Phacopteronidae and Calophyidae (Burckhardt 2005). Gall inducing coccid family, Beesonidae is associated with host plants of the family Dipterocarpaceae (Raman & Takagi 1992). ‘Aphids induce very less number of galls globally; no more than 10% of the 4401 species are confirmed gall inducers’ (Wool 2004). Several species of non-galling aphids occur commonly in peninsular India, whereas no gall-inducing aphid is known in this region (Raman 2007b). Some are known from the Himalayan region of India and they are confined only to the temperate Himalayan slopes (Chakrabarti 2007).
d. Hymenoptera: Gall inducing hymenopterans (gall wasps) include cynipid wasps (Cynipidae), fig pollinating wasps (Agaonidae), saw flies (Tenthredinidae) and few members from Chalcidoidea. In India, saw fly and cynipid induced galls are confined to the slopes of Himalaya (Raman 2007a). Although they are mostly entomophagous parasites, many gall making species are met with among the families Agaonidae, Eulophidae, Eurytomidae, Pteromalidae, Tanostigmatidae, and Torymidae. (Narendran et al. 2007)
e. Lepidoptera: Relatively few lepidopterans induce galls. A total of 179 lepidopteran gall inducers are identified globally belonging to 20 families. Fifteen gall inducing lepidopterans are reported from India (Mani 2000). Betousa stylophora (Swinhoe) causing stem galls on Emblica officinalis Gaertn. (Mani 1948; Nayar 1948) is a common gall found throughout India.
f. Coleoptera: Very few insects under this order have gall inducing capacity. The only taxon of Coleoptera recognized as gall inducing species is the subfamily Ceutorhynchinae of Curculionidae. Little is known about the gall inducing coleopterans of the Orient (Korotyaev et al. 2005). Baris cordiae Marshall is an example for coleopteran galls, on the leaf and petiole of Cordia myxa Linn. (Mani 2000).
Close to 15,000 vascular plant species host galls induced by a range of diverse arthropods (Meyer 1987). Vascular plants that host gall inducing insects range from Gymnosperms to Dicotyledons. Among flowering plants the natural orders Leguminosaceae, Moraceae, Lauraceae, Combretaceae, and Anacardiaceae bear great number of galls in the Indian subcontinent (Raman 2007a). The natural orders Fagaceae, Saliaceae, Rosaceae, Euphorbiaceae, and Pinaceae host the maximum number of gall inducers in temperate parts of the world (Abrahamson & Weis 1987). Among Gymnosperm, one species of Coniferales and a species of Gnetum host gall inducers (Raman 2007a). Gall incidence is rarely reported from Monocotyledons. An exceptional example for a gall on Monocotyledon is the ‘silver shoot’ induced by Orseolia oryzae (Wood-Mason) (Grover 1988) which occurs extensively throughout the tropical regions in the Orient, but hosted by a limited number of species of Poaceae including Oryza sativum.
Based on the plant organ infected, galls are classified as leaf galls, stem and twig galls, bud and flower galls or root galls. Leaf galls appear on the leaf blade or petioles. These are the most common galls and may appear as leaf curls, blisters, nipples, or erineums on the upper or lower surface of the leaf. Stem and twig galls are deformed growth restricted to stems and twigs ranging from slight swellings to large knot like growths. Bud and flower galls are deformed tumorous growth on bud or flower. Root galls are the neoplastic growth that originates on the root of the host plant. Leaf is the most susceptible plant organ for gall development and fewer galls occur on shoot axes, root, vegetative and floral buds (Dreger’Janffret & Shorthouse 1992). About 60% of the galls from the oriental realm are leaf galls, 15% are stem galls, 12% bud galls and the flower and fruit galls represent less than 6% (Mani 1964).
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