Bio-inoculants to boost resistance in crop plants against insects: Field success and struggles in India
Abstract: In addition to exploiting the inherited resistance in crop plants against the infesting insects, temporary induction of resistance against insect pests mediated through nutrient amendments is being explored now. Organic and inorganic sources of major and minor nutrients, when supplied to crop plants, influence and/or induce resistance against insect pests. In lieu of synthetic fertilizers bio-inoculants when amended to crop plants, offer resistance or at least tolerance to insect pests besides enhancing yield parameters. Preliminary attempts were made to explore the role of bio-inoculants in inducing resistance in tomato and sesame against selected insect pests. Three bio-inoculants viz., Arbuscular Mycorrhizal (AM) fungi, Azospirillum and phosphobacteria were evaluated in comparison with Farm Yard Manure (FYM) and inorganic fertilizers for their influence on the incidence of the shoot webber and capsule borer, Antigastra catalaunalis Dup. (Pyraustidae: Lepidoptera) on resistant (IVTS 2001-26) and susceptible (IVTS 2001-26) sesame accessions under field conditions in Tamil Nadu, a southern peninsular state in India. In another semi-field study with tomato, these bio-inoculants were evaluated for their influence on incidence of tomato fruit borer, Helicoverpa armigera (Hubner). In a subsequent in-depth study, four species of AM fungi namely Glomus fasciculatum, G. mosseae, Acaulospora laevis and Gigaspora margarita were found to enhance resistance in tomato against the noctuid caterpillars, H. armigera Hübner and Spodoptera litura Fab., through biochemical alterations especially by elevating the phenol content of the foliage. Similarly, Azospirillum amended sesame plants resisted the damage by shoot webber, A. catalaunalis which was mediated through increased levels of phosphorus and potassium. The levels of resistance induced by these bio-inoculants were found to be varying in each crop. The results of these investigations envisage further analysis of precise biochemical and molecular aspects of the interaction of microbial inoculants, crop plants and the infesting insect pests.