Interactions between Cassava and Arthropod Pests

Study of the interactions between plants and arthropods and especially of the resistance of plants is an essential component of integrated pest management. In the context of interactions between cassava and its three main pests in Africa (cassava green mite, variegated grasshopper and cassava mealybug), it is observed that knowledge in this filed is not very advanced except for the cassava-mealybug model. The study of this began in 1987 and has revealed the following features: (i) the resistance to mealybug developed by cassava is partial and is expressed according to the three categories of resistance, i.e., non-preference, antibiosis and tolerance. The results indicate horizontal, polygenic resistance; (ii) the behavioural characteristic of the insect and the sensorial equipment of its antennae and labium (the site of olfactory and gustatory chemoreceptors) suggest that the chemistry of the surface of the plant probably plays a determinant role in the success of the plant recognition phase; (iii) cassava mealybug principally feeds on phloem sap of which the main component is sucrose and which has a low amino acids content, a high cyanide glycosides content and also contains glycosylated flavonoids including rutin. Comparison of cassava phloem sap and honeydew excreted by the insect indicates that all these compounds are ingested and metabolized; (iv) the strong positive correlation established between the degree of antibiotic resistance of various cassava genotypes and their phloem rutin contents suggests that this secondary compound contributes to plant resistance to the mealybug; (v) considerable fluctuations in the pest numbers observed each year in the field were linked with variations in phloem rutin contents and these variations are affected by cultural practices. It was concluded that; (i) research on plant-insect interactions is complex as it requires a multi-disciplinary approach involving entomologists, biochemist, plant physiologists and plant breeders; (ii) there is a need to develop such studies on the other cassava pests while deepening those on the cassava-mealybug model.