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% …
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